General Scientific Meeting 2016 of the Belgian Physical Society
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Welcome to the General Scientific Meeting 2016 of the Belgian Physical Society, which is hosted by the UGent. This conference brings together physicists in Belgium of different sectors and different scientific subdisciplines. A diverse program is offered including plenary talks, a young scientist oral presentation competition, and a best poster competition. Parallel sessions and poster sessions will be organized on:
- Astrophysics, Geophysics & Plasma Physics
- Atoms, Molecules, Optics & Photonics
- Biophysics, Medical , Statistical & Mathematical Physics
- Condensed Matter & Nanophysics
- Fundamental Interactions, Particle & Nuclear Physics
- Physics and Education
Confirmed plenary speakers:
- Prof. Bernard Bigot / Prof. Jean Jacquinot (ITER)
- Prof. Sergio Bertolucci (CERN)
Organizing / Scientific Committee:
- Maarten Baes, UGent
- Wim Cosyn, UGent
- Jan Danckaert, VUB
- Mieke De Cock, KULeuven
- Gilles De Lentdecker, ULB (Vice-President BPS)
- Christophe Detavernier, UGent
- Jozef Ongena, RMA (President BPS)
- Linda Schepens, UGent (Secretary)
- Michael Tytgat, UGent (Chair)
- Xavier Urbain, UCLouvain
- Michael Wübbenhorst, KULeuven
Important dates:
- April 10th: deadline for young speaker and poster contest applications
- April 17th: closing of call for abstracts
- April 24th: deadline for early registration
- May 15th: registration closing date
- May 18th: conference date
We would like to acknowledge the following sponsors of our event:
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08:15
Registration
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Plenary Session - 1Convener: Dr Jozef Ongena (ERM-KMS, Plasma Physics Lab)
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1
Welcome addressSpeakers: Jozef Ongena (ERM-KMS, Plasma Physics Lab), Michael TYTGAT (UGENT)
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2
Invited talk: Progress of ITER and the way forward
The goal of ITER is to demonstrate the scientific and technical feasibility of fusion energy, building on several decades of worldwide research on the physics and technology of magnetic confinement. ITER is the world’s largest and most complex energy research project undertaken with the prospect of an inexhaustible energy respectful of the environment. To meet this challenge an international collaboration of seven partners (EU, China, India, Japan, South Korea, Russia and the United States) has been established in order to build, operate ITER and share the scientific results. The presentation will summarise the motivation, the objectives, and the status of the project. The ITER project benefitted during last year from bold organizational changes. These changes, aimed at creating a focused Project Team, helped greatly to accelerate the pace of the construction. The presentation will give examples of major achievements obtained recently. Among these, an outstanding result obtained collectively is the progress in the manufacture of the superconducting cables: 90% of the needed cable-in-conduit have been produced fully complying with the technical requirements.
The presentation will also address the physics issues which will be most important during the commissioning and the various operation phases of ITER. Many of these issues are included in the experimental programs of existing (or about to start) experiments in the world physics community where European teams and facilities play a major role. The importance of collecting data in a number of innovative physics subjects will be stressed.
Speaker: Prof. Jean JACQUINOT (ITER) -
3
Invited talk: CERN now and in the futureSpeaker: Prof. Sergio BERTOLUCCI (INFN)
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1
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10:50
Coffee
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Plenary Session - 2
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4
The European Physical Journal (EPJ): Introduction and OverviewSpeaker: Christian CARON
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5
Imprinting superconducting vortex footsteps in a magnetic layerSpeaker: Mr Jérémy BRISBOIS
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4
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12:50
Lunch
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Astrophysics, Geophysics and Plasma PhysicsConvener: Prof. Maarten BAES (UGent)
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6
Magnetic Condensation in space, astrophysics and laboratory.
We start from a recent breakthrough, Park, J. et al., Phys. Rev. X 5.2 (2015): 021024: the first experimental demonstration of the transition of a high pressure plasma to a state of magnetic condensate. A magnetic condensate is a system where the magnetic field is completely expelled from a plasma, forming a sharp transition: the plasma behaves like a drop of liquid with a definite surface. This is in sharp difference with regular low pressure plasmas that behave like diffuse clouds. The critical advantage of a magnetic condensate is that particles attempting to leave are reflected back by the boundary: the magnetic field condensate act as a true wall. The confinement is improved by several orders of magnitude, compared to the diffuse state. The experiment mentioned above proved this transition to a better confinement but left a number of point for future investigation:
1) Was the improved confinement seen in the experiment truly caused by magnetic condensation? We want to prove in computer simulation the development of a magnetic condensate as a response of a sufficiently high pressure plasma;
2) We want to measure how good the confinement is by measuring the loss in computer simulations under different conditions;
3) Can we sustain a steady state magnetic condensate via injection of particles to replenish the losses?
All three points above have never been demonstrated in experiment or in simulation. There are only approximate theories based on order of magnitude arguments. We report on our iPic3D simulation effort to address those questions and address the pratical relevance to space, astrophysics and laboratory plasmas.
[1] Park, J., Krall, N. A., Sieck, P. E., Offermann, D. T., Skillicorn, M., Sanchez, A., Davis, K., Alderson, E., Lapenta, G. (2015). High-Energy Electron Confinement in a Magnetic Cusp Configuration. Physical Review X, 5(2), 021024.
Speaker: Prof. Giovanni LAPENTA (KULeuven) -
7
Identification of edge-localized instabilities in nuclear fusion plasmas using pattern recognition techniques
At sufficiently high heating powers, fusion plasmas undergo a transition to a high confinement (H-mode) regime, which is the reference scenario for ten-fold power multiplication inductive operation in the next-step fusion device ITER. H-mode is characterized by steep pressure gradients in the edge region which leads to magnetohydrodynamic (MHD) instabilities called edge-localized modes (ELMs). ELMs are intense, short duration, repetitive events that result in sudden expulsion of energy and particles from the plasma edge. On the one hand, large type I ELMs pose a serious concern as they can cause high transient heat loads on the plasma-facing components (PFCs). On the other hand, controlled ELMs are crucial for regulating the core concentration of impurities, in particular tungsten (W), which is produced by interaction of the plasma with the PFCs. A key challenge is to provide a quantitative distinction between the different observed classes of ELMs and to relate this classification to the physical processes responsible for them. In this work, a pattern recognition-based classification scheme is developed for the characterization and automatic classification of ELM types, with the aim to distinguish ELM classes in a practical, fast and standardized way. The first step entails the construction of probability distributions of ELM properties, namely waiting times, energy losses, as well as global plasma parameters. In the next step, we employ the mathematical framework of information geometry, which allows the calculation of the geodesic distance (GD) as a natural and theoretically well-motivated similarity measure between probability distributions. The developed distance-based classifier is then applied for the classification of type I and type III ELMs with success rates up to 96%. The presented system demonstrates the advantage of considering complete probability distributions of plasma quantities in contrast to mean values and at the same time provides a fast, high-accuracy classification of ELM types which significantly reduces the effort of ELM experts in identifying ELM types. Further, the classification results can potentially contribute to physical understanding of ELMs and optimization of control and mitigation schemes.
Speaker: Ms Aqsa SHABBIR (UGent) -
8
3D model of a reverse-vortex flow gliding arc plasmatron
3D model of a reverse-vortex flow gliding arc plasmatron
G. Trenchev, St. Kolev, A. Bogaerts
PLASMANT research group, Department of Chemistry, University of Antwerp, Universiteitsplein 1, Wilrijk, Belgium Faculty of Physics, Sofia University, 5 James Bourchier blvd, 1164 Sofia, Bulgaria
georgi.trenchev@uantwerpen.be
This study employs a comprehensive computational model for a 3D gliding arc plasma in COMSOL. The plasma arc is stabilized in the reverse-vortex gas flow of a gliding arc plasmatron. The modelling gas is argon, with a reduced reaction set. The gas flow is modeled with a RANS turbulent model. The model covers different flow rates and cathode currents. Results for the arc plasma density and gas temperature are presented. The model is based on a gliding arc plasma reactor envisaged for CO2 conversion.
Gliding arc (GA) plasma reactors are well-known atmospheric plasma sources [1, 2, 3]. They are realiable, and in general, simple to build and maintain. When used for gas conversion applications, they face an importantissue – a significant amount of gas passes outside the discharge zone, which decreases the overall efficiency. A new method of solving these common GA problems, is to stabilize the gliding arc in the center of a reverse-vortex flow. A vortex flow is produced when the gas enters a cylindrical tube through a tangential inlet. If the outlet is on the opposite side with respect to the inlet, a forward vortex is produced. If it is on the same side, a secondary reverse-vortex will result (fig. 1). The reactor geometry used in our model is illustrated in figure 2.
Fig. 1: Schematical comparison between forward (a) and reverse (b) vortex flows. [1] Fig. 2: Geometry used in the model, with a radius of 6.35 mm.
The gas flow is computed as a stationary solution with the RANS k-ε turbulent model. Flow rates between 20 and 40 L/min are considered and the gas pressure is atmospheric.
Fig. 3: Gas flow streamlines. Fig. 4: Gas velocity magnitude (top view). [m/s]
Fig. 3: Gas flow streamlines. Fig. 4: Gas velocity magnitude (top view). [m/s]
In figure 3, the gas flow streamlines are presented, and the formation of the reverse-vortex in the reactor centre can be seen. The velocity magnitude of the flow (fig. 4) ranges from 50 to 100 m/s, depending on the flow rate. The model uses a fluid plasma description within a quasi-neutral approximation, as a full fluid plasma model requires very intensive computations 3, especially in 3D. The electron impact reaction set is also significantly reduced compared to the set presented in 3, in order to lower the computation time even further. Only 3 different species are considered in the model – electrons, Ar ions and excited atoms. The average electron energy is also computed. Figure 5 shows the electrical circuit of the reactor.
Fig. 5: Electrical circuit of the reactor. A voltage of 1000V is applied between the cathode and the anode, and the current is limited by a ballast resitor. The capacitor forms a filtering circuit.
Fig. 5: Electrical circuit of the reactor. A voltage of 1000V is applied between the cathode and the anode, and the current is limited by a ballast resitor. The capacitor forms a filtering circuit.
In figures 6 and 7, the resulting arc is represented by semi-transparent isosurfaces. The arc glides in the reactor until it stabilizes in the centre, spinning around its own vertical axis.
Fig. 6: Plasma arc at initial stage (100 µs). Fig. 7: Plasma arc at a later stage (1.1 ms).
Fig. 6: Plasma arc at initial stage (100 µs). Fig. 7: Plasma arc at a later stage (1.1 ms).
In figures 8 and 9, the resulting plasma density and gas temperature in the arc are presented, at a late stage of the arc development, when it is attached to the outer edge of the outlet (1.5 ms).
![Fig. 8: Plasma density at 1.5 ms [1/m3] Fig. 9: Gas temperature at 1.5 ms[K]]5
Fig. 8: Plasma density at 1.5 ms [1/m3] Fig. 9: Gas temperature at 1.5 ms[K]
The plasma density is within the typical range for low-temperature gliding arcs at atmospheric pressure [1, 2]. The gas temperature is also typical for a low-temperature plasma source 1. As the gas flow in the reactor takes places from the walls to the centre (see fig. 3), the arc is thermally insulated from the walls, which improves the reactor efficiency.
References 1 A. Fridman, Plasma Chemistry, p. 187-207, Cambridge University Press, New York, US, 2008 2 A. El-Zein, G. El-Aragi, M. Talaat, A. El-Amawy, Discharge characteristics of gliding arc plasma reactor with argon/nitrogen, Journal of Advances in Physics, 7(1), 1316-1323, 2015 3 St. Kolev, A. Bogaerts, A 2D model for a gliding arc discharge, Plasma Sources Sci. Technol., 24, 015025, 2015
Speakers: Prof. Annemie BOGAERTS (University of Antwerp), Mr Georgi TRENCHEV (University of Antwerp) -
9
Generation of anisotropic turbulence in drifting proton-alpha plasmas
Solar wind plasma temperature and solar wind turbulence at ion and electron scales often show anisotropic features, with different temperature and fluctuation power in parallel and perpendicular direction with respect to the orientation of the background magnetic field. The ratio between the power of the magnetic field fluctuations in parallel and perpendicular direction at the ion scales may vary with the heliospheric distance and depends on various parameters, such as the plasma compressibility, waves properties and the non-thermal plasma features, such as temperature anisotropies and relative drift speeds. In this work we have performed 2.5D hybrid simulations to study the importance of relative drifts and a gradual solar wind expansion in a multi-species plasma, consisting of fluid electrons, kinetic (particle-in-cell) protons and a drifting population of He ++ ions. At the beginning of the simulations we impose a turbulent spectrum of paralllel propagating Alfv\'en-cyclotron waves, co-existing with the drifting multi-species plasma. In the course of nonlinear evolution of the system we observe substaintial anisotropic cascade of the magnetic field power spectra towards perpendicular wave numbers. The nature of the anisotropic turbulent cascade depends on the differential streaming between the different ion populations and is affected by the solar wind expansion. In the case of sub-Alfv\'enic differential streaming the perpendicular wave power is enhanced and the anisotropic cascade is shifted towards smaller wave numbers.
Speaker: Dr Yana MANEVA (KU Leuven) -
10
The Flemish Mercator Telescope
The Mercator telescope is a 1.2m semi-automatic telescope placed at the Roque de los Muchachos observatory at the Canary Islands. The permanent availability allows us to occupy a specific niche in observational astrophysics: the possibility to make long-term time-series. To exploit this fully we developed a modern instrument programme. I will start this contribution with a review of our world-class instruments HERMES (a high-resolution optical spectrograph with a high throughput) as well as MAIA (3-arm camera with large fame-transfer CCDs). By continuous innovation, we maintain our instrument suite and telescope as state of-the art facilities. In the main part of the contribution I will illustrate our scientific research based on Mercator data by deploying several themes focussed on our research on stellar structure and evolution of single and binary stars.
Speaker: Prof. Hans VAN WINCKEL (KU Leuven) -
15:00
Break
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11
Meet your neighbour: a game of light and shadows in Andromeda
The closest large galaxy to our own Milky Way is Andromeda (M31). Its proximity allows observations of superb spatial resolution at all wavelengths. We have combined observations of M31 from Earth and space telescopes, ranging from the ultra-violet to millimeter wavelengths. This wavelength range covers the emission from all kinds of stars in this galaxy, but also the dark shadows caused by interstellar dust extinction. Additionally, we can see the thermal emission of the interstellar dust in the far-infrared domain. In this presentation, I will guide you through the many faces of the Andromeda galaxy and explain the underlying interplay between starlight and dust. I will then proceed to outline the 3-D model I made of this system, and what this can learn us about the complex absorption and radiative heating processes that define a galaxy's appearance and internal evolution.
Speaker: Mr Sébastien VIAENE (UGent) -
12
Dust and molecular cloud, a shocking divorce
Molecular clouds (MC) are the birthplace of stars. This made them a prime object of interest for many observers. But the gas they are mostly composed of is more often than not, hard to observe directly. As a proxy, we look for the dusts present in these clouds. Even if they represent only a fraction of the mass of the MC (up to 1%), they have a strong radiation field associated to them.
An common assumption is that these dusts are locked in the dynamics of the gas of the MC, and that observing the spatial distribution of the dusts gives direct information on the distribution of the gas. But this assumption is more and more challenged in recent years, with simulations pointing out to the fact that strong dynamics, like shocks, may dissociate gas and dust.
In our present work we have study the destruction of a dusty molecular cloud by shocks. We show how the dust, in the expected concentration found in MC, does not play a significant role for the dynamics. The dust is nonetheless redistributed and the picture we obtain is drastically different from one that would result from observing the gas directly. MCs are simulated with MPI-AMRVAC and emissions from the dust are obtain with SKIRT.
Speaker: Dr Rémi MONCEAU-BAROUX (KULeuven) -
13
Dust Obscured Blazars as Sources of High-Energy Neutrinos
Active Galactic Nuclei (AGN) are believed to be among the most promising sources of the ultra-high-energy cosmic ray flux. A hadronic component which is accelerated in the high energy environment of an AGN immediately implies the production of high-energy neutrinos. Nevertheless, no clear correlation between AGN and the high-energy cosmic-neutrino flux obtained by IceCube has been found so-far, putting strong limits on the neutrino production at AGN. We discuss a specific type of AGN for which an enhanced neutrino production is expected. This specific sub-set is given by AGN with their high-energy jet directed toward Earth, which is obscured by surrounding dust or gas, defining Dust Obscured Blazars. This type of AGN is predicted to have an enhanced neutrino emission due to the interaction of a possible hadronic component inside the AGN-jet with the surrounding matter. From two different galaxy catalogs, we have selected a sample of nearby sources with the characteristics of Dust Obscured Blazars. This selection is based on observations in the X-ray and radio bands. The data is consequently used to investigate the column density of the surrounding matter, providing an estimate for the neutrino production enhancement due to the nucleon-matter interactions in a beam dump scenario for various dust or gas compositions.
Speaker: Mr Giuliano MAGGI (Vrije Universiteit Brussel) -
14
Advanced hydrodynamics with the moving mesh code Shadowfax
The current leading model for cosmology is the ΛCDM model, which assumes a Universe filled with Cold Dark Matter (CDM), and a dark energy component (Λ). The model succesfully predicts the non-linear growth of cosmic structures from the initial density perturbations observed in the Cosmic Microwave Background, to the large scale web of galaxies in the present-day Universe. The ΛCDM predictions agree very well with observations at large cosmic scales but is difficult ot falsify the theory at these scales. We therefore turn to small cosmic scales, the regime where dwarf galaxies live.
Since dwarf galaxies are relatively faint, detailed observations of these systems are limited to dwarf galaxies in our own Local Group, a population which consists mostly of satellites of our own Milky Way and Andromeda. Modelling these systems is extremely challenging, as the properties of dwarf galaxies satellites will be affected by the presence of their parent galaxy. A full model therefore should take into account enough large scale information to accurately model the growth of the parent halo, and should at the same time have a high enough resolution to resolve the small scale physics that governs the evolution of the satellite. Furthermore, it is crucial that these models include an accurate treatment of the hydrodynamics of the interstellar and intergalactic medium, as the complex interplay between star formation and stellar feedback with the surrounding gas is responsible for the properties of the resulting satellite.
To enable future model predictions for the properties of low-mass Local Group dwarf galaxies, I have developed a new simulation code, Shadowfax, which uses a novel moving-mesh hydrodynamical integration scheme. This scheme is more accurate than traditional hydrodynamical schemes used for simulations of galaxy formation and evolution, and is Lagrangian in nature, making it ideally suited for simulations with a high dynamic range in masses and sizes. During my presentation, I will explain the basics of the moving-mesh scheme, and demonstrate its validity with results obtained using Shadowfax. I will conclude with a short note on scaling, and will explain how I plan to make Shadowfax ready for simulations on high-performance computing systems.
Speaker: Mr Bert VANDENBROUCKE (UGent) -
15
Mapping 3D Climates in the habitable zone of M dwarfs
We will show the results of two large parametric climate state studies for tidally locked terrestrial planets around M dwarf stars. We investigated for an Earth-like atmosphere and thermal forcing, 3D climate states for rocky planets with sizes between 1-2 Earth radii, orbital periods between 1-100 days, and also for different surface friction scenarios. We identified distinct climate state transitions that occur for faster and faster orbital periods, that is, towards the inner edge of the habitable zone. In addition, two regions of climate state degeneracies were found. The different climate states - in particular at the inner edge of the habitable zone - show large differences in surface temperatures, circulation and wind systems. Furthermore, different climate states are favoured in our model for different planet sizes and for different surface friction efficiencies. Our studies provide the first "map" for the 3D circulation system to be expected on tidally locked rocky planets around M dwarfs. They will help to better identify potentially habitable planets with relative short orbital periods.
Speaker: Dr Ludmila CARONE (KU Leuven, Centre for mathematical Plasma-Astrophysics)
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Atoms, Molecules, Optics and PhotonicsConveners: Prof. Jan Danckaert (Vrije Universiteit Brussel), Prof. Xavier Urbain (Université catholique de Louvain)
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16
Dynamical Studies of Ultrafast Charge Migration in Diatomic and Modular Molecules
The recent developments in the generation of optical attopulses suggest that it will soon become experimentally feasible to induce and subsequently directly probe ultrafast charge transfer between the end moieties of modular molecules. One ultrafast pulse creates a non-stationary state of the neutral or of the cation, that can be probed by a second pulse. Such experiments would allow characterizing a purely electronic time scale, before the coupling to the nuclei takes place. This is a pre Born-Oppenheimer regime where the electronic states are not stationary. 1 Our next goal is to investigate how the onset of nuclear motion and subsequently the fate of a chemical reaction can be controlled through the non equilibrium electronic density resulting from the interaction with a strong ultra short excitation pulse.
We will report on the simulation of realistic pump probe experiments that monitor the ultrafast electronic dynamics in LiH,[2,3] in the medium size bifunctional molecule PENNA (C10H15N)[4], C60[5] and other medium size molecules using a coupled equation scheme that includes the ionization continua and field effects. We will further illustrate how time and angular resolved photoelectron distributions provide an accurate probe of the electronic dynamics. We will then discuss the role of nuclear motion in the diatomic molecule LiH using full electron–nuclei quantum dynamics computation.[6] These recent computations show that the ultrafast beatings of the electronic coherences in space and in time are modulated by the different periods of the nuclear motion but survive for a large number of vibrational period[6]. Our results also show that dissociation to specific asymptotes can be controlled through the spatial localization of the non equilibrium density at the end of the excitation by a strong atto pulse.
Acknowledgments: This work has the support of FRFC 2.4545.12 (FR and BM), of DoE AMOS DE-SC0012628 (FR-RDL) and of the Einstein Foundation (RDL)
References 1 F. Remacle and R. D. Levine Proc. Natl. Acad. Sci. USA 2006, 103, 6793-6798. [2] B. Mignolet, R. D. Levine, F. Remacle Phys. Rev. A 2014, 88, 021403(R). [3] B. Mignolet, R. D. Levine and F. Remacle, J. Phys. Chem A 2014, 118 ,6721-6729. [4] B. Mignolet, R. D. Levine, F. Remacle J. Phys. B 2014, J. Phys. B: 47, 124011. [5] H. Li et al, Phys. Rev. Lett. 2015, 114, 123004. [6] A. Nikodem, R. D. Levine, F. Remacle, J. Phys. Chem. A, 2016, submitted.
Speaker: Prof. francoise REMACLE (University of Liege Address) -
17
Measuring Chern numbers in Atomic Gases: 2D and 4D Quantum Hall Physics in the Lab
Optical-lattice experiments have recently succeeded in probing the geometry of 2D Bloch bands with cold neutral atoms. Beyond these local geometrical effects, which are captured by the Berry curvature, 2D Bloch bands may also display non-trivial topology, a global property captured by a topological invariant (e.g. the first Chern number). Such topological properties have dramatic consequences on the transport of non-interacting atoms, such as quantized responses whenever the bands are uniformly populated. In this talk, I will start with the first experimental demonstration of topological transport in a gas of neutral particles, which revealed the Chern number through a cold-atom analogue of quantum-Hall measurements. I will then describe how this Chern-number measurement could be extended in order to probe the topology of higher-dimensional systems. In particular, I will show how the second Chern number – an emblematic topological invariant associated with 4D Bloch bands – could be extracted from an atomic gas, using a 3D optical lattice extended by a synthetic dimension. Finally, I will describe a general scheme by which optical lattices of subwavelength spacing could be realized. This method leads to topological band structures with significantly enhanced energy scales, offering an interesting route towards the experimental realization of strongly-correlated topological states with cold atoms.
Speaker: Prof. Nathan GOLDMAN (Physics of Complex Systems -- ULB) -
18
Power-laws in the dynamic hysteresis of quantum nonlinear photonic resonators
We theoretically explore the dynamic hysteresis behavior of a driven-dissipative photonic resonator with a Kerr-type nonlinearity [1]. In the regime where the semiclassical approach predicts bistability, the exact steady-state density matrix is well known to be unique and a statistical mixture of two states. A direct consequence is that the full quantum treatment predicts no static hysteresis cycle of the excited population as a function of the driving intensity. We predict that in the quantum regime a dynamic hysteresis with a rich phenomenology does appear when sweeping the driving amplitude in a finite time. The hysteresis area as a function of the sweep time reveals a double power-law decay, with a behavior qualitatively different from the mean-field predictions. We show that the dynamic hysteresis can be understood as due to a non-adiabatic response region with connections to the celebrated Kibble-Zurek mechanism for dynamic phase transitions. These theoretical predictions can be explored in a broad variety of physical systems, e.g., circuit QED superconducting resonators and semiconductor optical microcavities.
[1] W. Casteels, F. Storme, A. Le Boité, and C. Ciuti - Phys. Rev. A 93, 033824 (2016)
Speaker: Dr Wim CASTEELS (Laboratoire Matériaux et Phénomènes Quantiques, Paris Diderot - Paris 7, France) -
19
Ion pair dissociation of highly excited carbon clusters
Carbon-chain molecules have been detected in many astrophysical environments such as interstellar medium (ISM) and circumstellar envelopes. These molecules can be formed in different processes, such as neutral-neutral, ion-neutral or ion-ion collisions and the demand for accurate molecular data concerning these processes is very strong.
We will present measurements of ion pair dissociation of carbon clusters formed in high velocity collisions of Cq+n projectiles with atoms in the AGAT set-up [1], situated nearby the Tandem accelerator in Orsay (France).We propose a theoretical interpretation to the ion pair relaxation experiment of C+2 into C++/C−. Ion pair dissociation is a relaxation process in which anionic and cationic fragments are simultaneously emitted. Even if it is a minor process which requires a high excitation of the molecule, the strongly varying probabilities found in the literature (from 10−4 to a few %) is not completely understood.
Using multi-configurational ab initio methods (CASSCF/MRCI + Davidson correction) with different basis sets, we were able to obtain the potential energy curves for the fifty low-lying 4A2 electronic states arising from various molecular dissociation channels and to characterize the ionic channels C++(1S)/C−(4S∘) which interact with valence electronic states resulting in a large number of avoided crossings.
Based on that information and with a statistical approach depending on the dipole excitation selection rules, we propose an interpretation of the measured branching ratio.
Reference
[1] Béroff, K. et al 2013 J. Phys. B At. Mol. Opt. Phys. 46 015201.
Speaker: Mr Thibaut LAUNOY (Université Libre de Bruxelles) -
20
Dopant induced CO poisoning tolerance of Pt clusters
The development of efficient fuel cells is a promising strategy to diminish fossil fuel consumption. A major drawback of state-of-the-art proton exchange membrane fuel cells (PEMFCs) is CO poisoning of the platinum catalyst.[1] CO molecules present in the fuel preferentially adsorb to Pt nanoparticles, thereby blocking the active sites and degrading the cell’s performance. Several Pt alloys are known for an enhanced tolerance to the CO poisoning, improving the fuel cell performance. [2] The physical mechanism responsible for the tolerance remains a subject of debate, being ascribed either to an alteration of the Pt electronic structure upon alloying or to a bi-functional mechanism, in which OH groups interact with CO and form CO2 and H2 thereby regenerating the active sites.[2] In this contribution we discuss the influence of Nb, Mo, Sn and Ag dopant atoms on the CO adsorption on PtN+ (N=13-23) clusters. Clusters can be studied in molecular beams and high vacuum conditions, allowing a perfect control over the particle’s mass and excluding reactions with unknown molecules.[3,4] Combined mass spectrometric experiments and density functional theory calculations show a significant reduction in the reactivity for Nb and Mo doped clusters, which is attributed to electron transfer from the dopant to the Pt atoms and the concomitant reduction of the CO binding energies. On the other hand Sn and Ag doping has a limited effect on the CO adsorption. Analysis of the density of states demonstrates a correlation of dopant induced changes in the electronic structure with the enhanced tolerance to CO poisoning.
[1] J. Baschuk, and X. Li, Int. J. Energy Res. 2001, 25, 695. [2] S. Ehteshamia and S. Chan, Electrochim. Acta 2003, 93, 334. [3] S. M. Lang et al., Angew. Chem. Int. Ed. 2010, 49, 980. [4] V. Kaydashev, E. Janssens and P. Lievens, Int. J. Mass Spectrom. 2015, 379, 133.
Speaker: Mr Ferrari PIERO (KUL) -
21
Information propagation and equilibration in long-range Kitaev chains
The Lieb-Robinson bound has been a milestone in our understanding of the nonequilibrium dynamics of nonrelativistic short-range interacting quantum systems. In essence it states that the effect of a perturbation at a point A cannot be felt at another point B until a time t=r/v, with r the distance between A and B and v a characteristic velocity in the system. Even though causality is not strictly imposed on the model, it emerges as a mere consequence of the short-range nature of interactions, thereby giving rise to an effective light cone.
Recent advances in in cold-atom and trapped-ion experiments have made it possible to also study the behavior of long-range interacting quantum systems. As in these systems the Lieb-Robinson theorem ceases to hold, there is not much general that can be said about the rate at which correlations can travel through the system. New bounds on information propagation have been proposed, but they seem too loose to provide any significant insight into models of interest. Thus, is it really possible to transmit information superluminally in these systems? Or is some notion of causality still preserved, despite the long-range nature of interactions?
With this work we aim at providing some insight into these elementary questions by considering an exactly solvable fermionic model with pairing interactions that decay as 1/rα, known as the long-range Kitaev chain. We study the dynamics after an abrupt change of vacuum and evaluate the mutual information between two disconnected subsystems in the chain. Surprisingly, we find that by far most of the information in this model still propagates inside a well-defined light cone, even for very long-range interactions (α<1). Moreover, the crucial difference with short-range interacting models lies in the fact that, counterintuitively, the dynamics can be slowed down significantly, rather than sped up. We illustrate that, thanks to the integrability of the model, the distribution of quasiparticle group velocities is sufficient to explain these remarkable observations.
[1] M. Van Regemortel, D. Sels, M. Wouters, Phys. Rev. A 93, 032311 (2016)
[2] E. H. Lieb and D. W. Robinson, Comm. Math. Phys. 28, 251 (1972)
[3] P. Calabrese and J. Cardy Phys. Rev. Lett. 96, 136801 (2006)
[4] P. Richerme, Z.-X. Gong, A. Lee, C. Senko, J. Smith, M. Foss-Feig, S. Michalakis, A. V. Gorshkov, C. Monroe, Nature (London) 511, 198 (2014)
[5] M. Foss-Feig, Z. X. Gong, C. W. Clark, A. V. Gorshkov, Phys. Rev. Lett. 114, 157201 (2015)
Speaker: Mr Mathias VAN REGEMORTEL (Universiteit Antwerpen) -
22
The geometric aspect of weak and modular values
*Mirko Cormann¹² and Yves Caudano¹²
¹Research Centre in Physics of Matter and Radiation (PMR) ²Namur Center for Complex Systems (NaXyS) University of Namur, rue de Bruxelles 61, Namur, Belgium*
In the last decade, there has been considerable advancement in the study of weak quantum measurements. The experimental observations resulting from weak quantum measurements of pre- and post-selected sub-ensembles can usually be described using weak and modular values [1,2]. Weak values are a form of generalization of the expectation value of an observable for pre- and post-selected sub-ensembles. Taken as expectation values, weak values are very unusual, though: they can be complex numbers or outside the range of the eigenvalues of the observable. Modular values were less reported in the literature. They are related to unitary operators. In a sense, they are counterpart of the weak value for unitary operators: weak and modular values have similar expressions, but applied to observables and unitary operators, respectively.
Weak and modular values of two-level quantum states possess a geometric representation in terms of three dimensional vectors on the Bloch sphere. In this case, the complex values are most usefully expressed using their polar form (modulus and argument) rather than their cartesian representation (real and imaginary components) [3]. The argument of the polar form has a topological origin that depends on a solid angle intercepted on the Bloch sphere during the system evolution from its initial to final state, i. e. from the pre- to the post-selected state.
In this work, we express weak and modular values of three-level and higher-level quantum systems by their polar form. By considering the Majorana representation of qudits, we describe complex weak and modular values of N-level systems using a purely geometrical approach on the Bloch sphere. We show that the modulus of these values is determined by the product of N-1 square roots of probability ratios. We find that their argument is deduced from a sum of N-1 solid angles. We use this theoretical approach to examine the well-known discontinuous effects around singularities of the weak value for a three-level system. Furthermore, we discuss the feasibility to measure experimentally the polar components of the modular value by encoding the three-level state in a bipartite qubit state. This is a preliminary step in the experimental realization of weak and modular value measurements of higher-level systems.
[1] Y. Aharonov, D. Z. Albert, and L. Vaidman, Phys. Rev. Lett. 60, 1351 (1988).
[2] Y. Kedem, and L. Vaidman, Phys. Rev. Lett. 105, 230401 (2010).
[3] M. Cormann, M. Remy, B. Kolaric, and Y. Caudano, Phys. Rev. A 2016, in press, arXiv:1508.01353.
Speakers: Mr Mirko CORMANN (UNamur), Dr Yves CAUDANO (UNamur) -
23
Seeing (Ultra-)Sound through Mechanoluminescence
Please see the attachment.
Speaker: Mr Simon MICHELS (UGent) -
24
Soliton-core filling in superfluid Fermi gases with spin imbalanceSpeaker: Mr Wout VAN ALPHEN (University of Antwerp)
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Biological, Medical, Statistical and Mathematical PhysicsConvener: Prof. Michael Wübbenhorst (KU Leuven, Dept. Physics & Astronomy)
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Rheology of complex macromolecules: relating their composition to their viscoelastic properties
Understanding and predicting the viscoelastic response of polymer melts or concentrated solutions from the knowledge of molecular architecture represents a very active field of research with important challenges. Our first objective is to develop a general coarse-grained model for predicting the viscoelastic properties of linear and branched polymers. Based on the tube theory proposed by de Gennes [1] and by Doi and Edwards [2], as well as on the ability to synthesize well-defined polymers of complex architectures, we have studied more and more complex chain architectures towards randomly branched polymers [3]. This model can now be extended to many other applications [4]. In particular, we investigate the inverse problem of predicting molecular weight distribution from the relaxation moduli, based on a parametric approach, which allows us to face the ill-posedness of this problem. Taking advantage of the high sensitivity of rheology for branched architectures, we also use this approach for the detection of long chain branching. Our present objective is to extend this approach to describe the rheology of macromolecular self-assemblies (such as telechelic polymers) exhibiting reversible structural changes during deformation and thermorheological complexity.
References
[1] M. Doi and S. F. Edwards, The Theory of Polymer Dynamics; Oxford University Press: New York (1986).
[2] P. G. de Gennes, Reptation of a polymer chain in the presence of fixed obstacles, J. Chem. Phys., 55, 572-579 (1971).
[3] E. van Ruymbeke, C. Bailly, R. Keunings, D. Vlassopoulos, A general methodology to predict the linear rheology of branched polymers, Macromol., 39: 6248-6259 (2006).
[4] E. van Ruymbeke, H. Lee, T. Chang, A. Nikopoulou, N. Hadjichristidis, F. Snijkers, D. Vlassopoulos, Molecular rheology of branched polymers: decoding and exploring the role of architectural dispersity through a synergy of anionic synthesis, interaction chromatography, rheometry and modeling, Soft Matter, 10: 27, 4762-4777 (2014).
Speaker: Prof. Evelyne VAN RUYMBEKE (Université Catholique De Louvain) -
26
Characteristic length scales of Brownian relaxation in nanoparticle suspensions
Brownian relaxation in colloidal suspensions has been observed in different susceptibilities such as shear moduli, viscosities or diffusion coefficients [1-6]. We present Brownian relaxation times obtained by Small Angle Oscillatory Shear (SAOS) measurements on suspensions consisting of two types of a low-molecular weight glass former (Diglycidyl Esther of Bisphenol A) filled with silica nanoparticles [7, 8]. In the description of the Brownian relaxation process, the radius of the colloidal particles is often chosen as a characteristic distance to describe the diffusion process. The related time, i.e. the time that colloidal particles in the suspensions need to travel this distance is then equal to the classical Peclet time. In the present contribution we will discuss the use of a concentration dependent characteristic length scale and examine the validity of this concept by comparing it to results from own measurements as well as to data reported in literature.
[1] Cichocki, B., and Hinsen, K., Physica A: Statistical Mechanics and its Applications, 187, 133 (1992).
[2] Shikata, T., and Pearson, D. S., Journal of Rheology, 38, 601 (1994).
[3] Sohn, I., and Rajagopalan, R., Journal of Rheology (1978-present), 48, 117 (2004).
[4] van der Werff, J., de Kruif, C., Blom, C., and Mellema, J., Physical Review A, 39, 795 (1989).
[5] Watanabe, H., Yao, M.-L., Yamagishi, A., Osaki, K., Shikata, T., Niwa, H., and Morishima, Y., Rheologica Acta, 35, 433 (1996).
[6] Weeks, E. R., and Weitz, D., Physical Review Letters, 89, 095704 (2002).
[7] Dannert, R., Sanctuary, R., and Baller, J., Journal of Rheology, 59, 391 (2015).
[8] Dannert, R., Sanctuary, R., Thomassey, M., Elens, P., Krüger, J. K., and Baller, J., Rheologica Acta, 53, 715 (2014).
Speaker: Dr Jörg BALLER (University of Luxembourg) -
27
Feasibility of microdosimetry for hadron therapy using mini Tissue-Equivalent proportional counters
Hadron therapy is increasing worldwide for treating some radio-resistant tumours due to its more advantageous depth dose deposition, less lateral spread and better sparing of healthy tissues close to the irradiated target as compared to photon or electron beams. Besides these favourable physical properties, ion beams offer potential biological advantages over protons making them even more suitable for the treatment. The enhancement of the relative biological effectiveness (RBE) of fast carbon ions is due to the high ionization density along the penetration depth. Furthermore, the ion interaction with tissue causes the fragmentation of the projectile and of the target along the penetration path, which results in a complex radiation field. Therefore the radiation quality (i.e. particle type and energy of the radiation field) varies significantly with depth of the irradiated volume. A characterization of the radiation biological effectiveness of the clinical beam in terms of measurable physical quantities at the subcellular scale could be useful for optimizing treatment plans. Microdosimetry can be useful for this purpose, because it studies the probability distributions of the imparted energy when a single ionizing particle crosses a micrometre sized site (the size of a chromosome). Tissue-Equivalent gas proportional counters (TEPCs) are the reference microdosimeters in experimental microdosimetry. The microdosimetric measurements can be used to assess the RBE of the radiation by linking the physical parameters with the corresponding biological response [1]. Due to the high particle fluence rate of therapeutic beams only miniaturized TEPCs of the order of 1 mm3 can be employed to minimize the signal pile-up effects when exposed to these high intensity beams. The mini TEPC, designed and built at INFN Legnaro laboratories, has successfully measured in low-LET therapeutic proton beams in the past [2]. However, the assumption that this device could also be used in carbon beams is not straightforward since high-LET radiation can give rise to large electronic avalanches, which can produce some distortions on the measured microdosimetric spectra. The aim of this research, in collaboration with the group of INFN-LNL, is to contribute to the development of mini-TEPCs for measuring the radiation quality of charged particle beams namely carbon ions. To this end, the response of the mini TEPC was characterized mainly with experimental measurements in known radiation fields but also with the general-purpose Monte Carlo code FLUKA. All the physical parameters affecting the measured microdosimetric spectrum such as gas multiplication characteristics, the calibration procedure, the detector's geometry, the simulated site size and the gas filling type have been carefully studied during this project [3]. Then, the first microdosimetric measurements with the mini TEPC at the Italian therapeutic carbon-ion beam (Centro Nazionale di Adroterapia Oncologica, CNAO) were performed with monoenergetic carbon ions [4] proving its feasibility to measure the radiation quality at various depths in a water phantom.
Loncol, T et al. Radiobiological effectiveness of radiation beams with broad LET spectra: Microdosimetric analysis using biological weighting functions. Rad. Prot. Dosim. 52(1-4):347-352 (1994). De Nardo, L. et al. Microdosimetric investigation at the therapeutic proton beam facility of CATANA. Rad. Prot. Dosim 110 (1-4): 681-686 (2004). Chiriotti, S., Microdosimetry of hadron therapy beams using mini tissue-equivalent proportional counters. PhD thesis. Université catholique de Louvain (Belgium) (2015). Conte, V., Colautti, P., Chiriotti, S., Moro, D., Ciocca, M., Mairani, A. Mini TEPC Microdosimetric Study of Carbon Ion Therapeutic Beams at CNAO. New Journal of Physics (2016). Under preparationSpeaker: Dr Sabina CHIRIOTTI ALVAREZ (Belgian Nuclear Research Centre (SCK•CEN)) -
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Phase locking of spiral waves in a rotating electrical field
Many natural systems including the oscillating Belousov—Zhabotinsky chemical reaction and electrical signalling in the heart can be effectively described by a set of coupled reaction-diffusion equations. It is well known that in a constant external field (e.g. electrical field), spiral waves start drifting.
Recently, we showed by numerical and analytical methods that an electrical field that rotates at a frequency close to the natural rotation frequency of the spiral wave can phase-lock the spiral wave, forcing it to rotate at a new frequency. In two spatial dimensions, this technique enables selection of a unique spiral wave frequency. In three dimensions, the rotating external field can restabilise scroll wave turbulence induced by negative filament tension.
Speaker: Dr Hans DIERCKX (UGent) -
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Heat-transfer method-based cell culture quality assay through cell detection by surface imprinted polymers
In this work, we describe a novel tool for monitoring the quality of cell cultures in terms of contamination and genomic stability in real time. The proposed platform, the so-called heat-transfer method (HTM), enables to monitor the heat-transfer resistance at solid-liquid interfaces in real-time.1 Previously, it was shown that it is possible to detect cells in buffer solution in a fast, low-cost, highly selective and specific manner by combining HTM with surface imprinted polymers (SIPs).2 SIPs are synthetic cell receptors that can be made via various routes including the stamping method that was used here.3 To this extent, aluminum chips were coated with semi-cured polyurethane by spin coating. In parallel, a monolayer of target cells (the cell type one wishes to detect) was applied onto a PDMS stamp that was pressed onto the polyurethane-covered chip. The polymer layer was cured overnight, enabling the cross-linking polymer to interact with the template cells. After removal of the stamp and cells, microcavities were left behind on the stamp that are complementary to the template cells in size, shape and the distribution of functional groups.3 When cells are kept in culture for a prolonged period of time, the quality of these cell lines is often compromised.4 The lack of tools to effectively monitor cell culture quality in real time forces researchers to discard cell lines after a limited number of cell passages. Although the gold standard cell culture quality assay, STR DNA profiling allows researchers to uniquely identify cells and compare their cell cultures to the original cell line in a relatively straightforward manner,5 its use for routine screening of cell cultures is limited. The technique is typically slow, labor-intensive, the data interpretation requires some expertise and it needs to be used in a lab environment. In this work, we offer a label-free, fast, low-cost and user-friendly alternative that can be used on-site.
To assess the platform’s potential for cell culture quality screening, the breast cancer cell line ZR-75-1 was cultured for a prolonged period (25 passages). In parallel, a descendant, morphologically identical cell line that differs from the original cell line only in its growth pattern (steady adherent growth versus fast suspension growth) was cultured in the same lab. This cell line was acquired after the original cell line was cross-contaminated in a previous experiment, further stressing the need for a user-friendly tool to monitor cell culture quality in real time.
During the prolonged culturing phase of the ZR-75-1a cell line, a sample of the cell culture was analyzed with HTM at given time intervals. Initially the rise in thermal resistance (Rth) upon exposure to a cell culture sample remained constant. However, at higher passage numbers the signal started decaying, indicating that the cells had changed. Visually it could be seen that a number of cells started growing in suspension, possibly caused by a cross-contamination of the ZR-75-1 cell culture with the faster growing descendant cell line, which would explain the drop in Rth. These findings were confirmed by classical STR DNA profiling, indicating that our platform is indeed capable of monitoring the quality of cell cultures in real-time.
van Grinsven, B.; Eersels, K. et al. ACS Appl. Mater. Interfaces 2014, 6, 13309-13318. Eersels, K. et al. ACS Appl. Mater. Interfaces 2013, 5, 7258-7267. Hayden, O. and Dickert, F.L. Adv. Mater. 2001, 13, 1480-1483. Hughes, P. et al. Biotechniques 2007, 43, 575-586. Reid, Y. et al. Assay Guidance Manual 2004.Acknowledgement: Funding by the Interreg Project “MicroBioMed” and the KU Leuven project “Smart Cellular Scaffolds” is greatly appreciated.
Speaker: Dr Kasper EERSELS (KULeuven) -
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Surprisingly strong thermal insulation effect of self-assembling thiol monolayers
Thiol self-assembled monolayers (SAMs) are popular due to their utility for various applications. In biosensing platforms SAMs are often used as linkers to tether receptors to biochip surfaces [1]. To get a full understanding of SAM-properties, it is important to study also their formation kinetics and thermal-transport properties. This latter point was not addressed before. However, it is already known that DNA and adsorbed cells at solid-liquid interfaces show a heat-blocking effect, which is utilized in the biosensing technique HTM (heat transfer method) [2].
In this work we employed HTM to study the formation kinetics of thiol SAMs on gold surfaces using ethanol as a solvent. The thiols were 1-dodecanethiol and 11-mercaptoundecanoic acid (11-MUA) differing in their specific head groups, being -CH3 for 1-dodecanethiol and -COOH in case of 11-MUA. Complementarily, the layer formation was monitored with quartz crystal microbalance, Fourier-transform infrared spectroscopy, and atomic force microscopy. The results indicate that presence of a SAM on the gold surface leads to a surprisingly strong increase of the interfacial heat-transfer resistance Rth, given the fact that thiol molecules are only 1.5 nm in length. Results show a concentration-dependent jump in Rth for concentrations higher than 0.5 mM. The thermal resistance displays a two-step evolution for concentrations below 0.5 mM: Initially, the thermal resistance decreases in comparison to a blank gold substrate. In a second phase, Rth increases gradually, eventually reaching a stable plateau value. This behavior can be attributed to a transition from a lying-down to a standing-up conformation of the thiols. The results demonstrate that a nanometer thin ‘thiol carpet’ causes an unexpectedly strong heat-blocking effect at gold/ethanol interfaces. Furthermore, the absolute increase of the thermal-boundary resistance depends on the used head group. This observation points to an interface effect, which can possibly be explained by the mismatch between the phonon frequencies of gold and the vibration frequencies of ethanol and thiol molecules in the THz regime.
Acknoledgments: FWO project G.0B62.13N “Exploration of heat-conducting effects for applications in bio- and chemosensors” is gratefully appreciated.
References
[1] Vericat, C.; et. al, Chem. Soc. Rev. 2010, 39, 1805–1834.
[2] van Grinsven, B.; et. al, ACS Appl. Mater. Interfaces 2014, 6, 13309–13318.
Speaker: Mehran KHORSHID (KULeuven, Soft Matter Physics and Biophysics Section, Celestijnenlaan 200D, B-3001 Heverlee, Belgium) -
31
nvestigation of nonlinear optical properties of opsins: towards opsin probes for cell membrane voltage
Unraveling the immense complexity of the human brain requires the ability to selectively interfere and record neural activity. ‘Optogenetics’ describes a variety of techniques for specifically controlling neural activity with light, using light sensitive membrane proteins (opsins).1 Given the significant advantages of nonlinear optics, activating opsins by two-photon absorbance is already an emerging field.2 Activity read-out by means of membrane potential sensing still relies on voltage sensitive fluorescent probes or microelectrode recording. However, it has been proven that retinal analogues can serve as voltage sensing probes by means of second harmonic generation (SHG), when inserted into the cell membrane.3 Since opsins use this retinal chromophore as their photoswitch, we are investigating the potential of opsins to act as voltage reporters. In this contribution we report the molecular first hyperpolarizability characterization of several light sensitive membrane proteins. This parameter is indicative for their potential SHG voltage sensitivity.
Fiala, A., Suska, A. & Schlüter, O. M. Optogenetic approaches in neuroscience. Curr. Biol. 20, R897–903 (2010). Oron, D., Papagiakoumou, E., Anselmi, F. & Emiliani, V. Two-photon optogenetics. Progress in brain research 196, (Elsevier B.V., 2012). Theer, P., Denk, W., Sheves, M., Lewis, A. & Detwiler, P. B. Second-harmonic generation imaging of membrane potential with retinal analogues. Biophys. J. 100, 232–42 (2011).Speaker: Mr Yovan DE COENE (Laboratory of Soft Matter and Biophysics) -
32
Linear irreversible thermodynamics for periodically modulated systems
We show how to formulate linear thermodynamics for periodic driving including small scale systems. In particular, we derive generalized Onsager-Casimir relations.
Speaker: Mr Karel PROESMANS (UHasselt) -
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Self-organisation of stochastic oscillators in a predator-prey model
A predator-prey model of dual populations with stochastic oscillators is presented. A linear cross-coupling between the two populations is introduced following the coupling between the motions of a Wilberforce pendulum in two dimensions: one in the longitudinal and the other in torsional plain. Within each population a Kuramoto type competition between the phases is assumed. Thus, the synchronization state of the whole system is controlled by these two types of competitions. The results of the numerical simulations show that by adding the linear cross-coupling interactions predator-prey oscillations between the two populations appear which results in self-regulation of the system by a transfer of synchrony between the two populations. The model represents several important features of the dynamical interplay between the drift wave and zonal flow turbulence in magnetically confined plasmas, and a novel interpretation of the coupled dynamics of drift wave-zonal flow turbulence using synchronization of stochastic oscillator is discussed.
Speaker: Dr Sara MORADI (ULB) -
34
Indeterminism in Newtonian physics: a quest for probabilities
Norton's dome is an example of indeterminism in Newtonian physics, based on a differential equation involving a non-Lipschitz continuous function [1]. It involves a gravitational field, in which a mass is placed with velocity zero at the apex of a dome (x=0,v=0), which has the following shape:
y(x)=−2/3(1−(1−3/2|x|)2/3)3/2.
Besides the trivial, singular solution, r(t)=0 (where r is the arc lenght measured along the dome), there is a one-parameter family of infinitely many solutions to this Cauchy problem, which can be represented geometrically as a `Peano broom':
r(t)=0(if t≤T); 1144(t−T)4(if t≥T),
where parameter T is a positive real number (representing the time of the onset of the movement).Similar examples have been discussed by Poisson and other 19th century physicists [2]. We analyze and two conflicting intuitions about such cases using discrete models. In particular, we present an alternative model using difference equations and an infinitesimal hidden variable. (We use `infinitesimal' in the sense of non-standard analysis, which is close to Leibniz's formulation of the calculus as well as to physical praxis [3].) Our hyperfinite model for the dome is deterministic. Moreover, it allows us to assign probabilities to the variable in the indeterministic model.
References
[1] J. D. Norton. The dome: An unexpectedly simple failure of determinism. Philosophy of Science (2008) 75:786–798.
[2] M. van Strien. The Norton Dome and the Nineteenth Century Foundations of Determinism. J Gen Philos Sci (2014) 45:167–185.
[3] S. Albeverio, J. E. Fenstad, R. Hoegh-Krøhn, and T. Lindstrøm. Non-Standard Methods in Stochastic Analysis and Mathematical Physics. Pure and Applied Mathematics. Academic Press, Orlando, FL, 1986.
Speaker: Prof. Sylvia WENMACKERS (KU Leuven)
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Condensed Matter and Nanostructure PhysicsConvener: Prof. Christophe Detavernier (UGent)
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Colouring atoms in 3 dimensions
The properties of materials are determined by the positions of the atoms, their chemical nature and the bonding between them. Therefore, reaching atomic resolution in 3D has been the ultimate goal in the field of electron tomography for many years. One of the possibilities to perform electron tomography with atomic resolution is by applying reconstruction algorithms based on compressive sensing [1,2]. The methodology was applied to high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images acquired from defect-free Au nanorods [1]. Also nanostructures that include defects can be investigated in this manner. Going further than determining the positions of atoms is the aim to determine the type of individual atoms in hetero-nanoparticles. Again using a combination of HAADF-STEM and compressive sensing, we were able to distinguish individual Ag from Au atoms in core-shell Au@Ag nanorods, even at the metal-metal interface [2]. Also energy dispersive X-ray (EDX) mapping can be combined with electron tomography and the use of the Super-X system is hereby very beneficial. EDX tomography was applied to obtain qualitative information concerning a galvanic replacement reaction in AuAg nanoparticles and to investigate the 3D composition of Fe and Co in nanodumbbells [3,4]. Quantification 3D EDX results can be considered as the next crucial step. Finally, by combining electron tomography with electron energy-loss spectroscopy at high energy resolution, we were able to determine the valency of the Ce ions in CeO2-x in 3D [5]. We kindly acknowledge Prof. L. M. Liz-Marzán and Prof. K. Soulantica for the provision of the samples.
[1] B. Goris, S. Bals, W. Van den Broek, E. Carbo-Argibay, S. Gomez-Grana, L. M. Liz-Marzan, G. Van Tendeloo, Nature Mater. 11 (2012) 930
[2] B. Goris, A. De Backer, S. Van Aert, S. Gómez-Graña, L. M. Liz-Marzán, G. Van Tendeloo, S. Bals, Nano Lett. 13 (2013) 4236
[3] N.Liakakos, Ch. Gatel, Th. Blon, Th. Altantzis, S. Lentijo-Mozo, C. Garcia-Marcelot, L.-M. Lacroix, M. Respaud, S. Bals, G. Van Tendeloo, K. Soulantica Nano Lett.14 (2014) 2747
[4] B. Goris, L. Polavarapu, S. Bals, G. Van Tendeloo, L. M. Liz-Marzán Nano Lett. 14 (2014) 3220
[5] B. Goris, S. Turner, S. Bals, G. Van Tendeloo ACS Nano 8 (2014) 10878
Speaker: Prof. Sara BALS (EMAT-University of Antwerp) -
36
Lab on-chip for testing thin film materials: extraction of mechanical properties at the nanometer scale
Thin films constitute the building blocks of a large number of modern technologies including protective coatings, microelectronic devices, bio-responsive membranes and photovoltaic cells. These nano-layers exhibit vastly different mechanical properties from their bulk counterpart. New measurement techniques are therefore required to accurately and reliably measure the mechanical properties of thin films.
At UCL, a simple lab-on-chip concept has been created to assess the mechanical properties of submicron freestanding thin films. It relies on the use of internal stresses generated in an “actuator layer” to apply a deformation to a “specimen layer” attached to it owing to the release of an underneath “sacrificial layer”. The simplest test structure configuration gives access to one point of the stress - strain curve of the specimen material while photolithography enables to reproduce this elementary tensile test structure thousands of times to generate the full stress – strain behaviour up to fracture.
This simple idea gives access to several extensions including different loading configurations, relaxation tests, TEM observation of deformation mechanisms and piezoresistance measurements.
This lab-on-chip concept has already proven to be suitable for extraction of mechanical properties of ductile materials (Pd, AlSi, Ni, Cu, Pt, etc.) as well as brittle materials (monocrystalline Si, polycrystalline Si, oxides, nitrides, metallic glasses). The piezoresitance of silicon nanowires under high tensile stress has also been assessed through this technique.
Speaker: Dr Michael COULOMBIER (UCL) -
37
Multivariate analysis in ellipsometry data processing. A review with examples of applications
For more than 40 years now, single wavelength and spectroscopic ellipsometry have been intensively used to determine optical properties as well as thickness of multilayered materials (see e.g. [1]). Principal components analysis (PCA) and hierarchical cluster analysis (HCA) are multivariate analysis methods respectively used for dimension data reduction and classification, respectively [2, 3]. They are now widely used to help in the processing of the surface analysis data, mainly XPS [4] and ToF-SIMS data [5]. On the contrary, they have only been applied to ellipsometry data in a (very) limited number of cases: characterization of gold nanocolloids [6] and identification of the optical response of microorganisms thin films [7].
In this contribution, we will briefly review PCA, HCA and support vector machines (SVM) to present new applications to ellipsometry data processing. Examples will be given from three original topics : (a) the characterization of the resonance plasmon parameters in nanocomposite thin films embedding silver nanoparticles, (b) the processing of imaging spectroscopic ellipsometry data cubes and (c) the synthesis of metal nanoparticles by laser annealing of polymer films.
Acknowledgments: This research was financially supported by the F.R.S.-FNRS (Grant 1926111).
[1] H. Tompkins and E. Irene, Handbook of Ellipsometry (William Andrew Pub., 2005).
[2] H. Grahn and P. Geladi, Techniques and applications of hyperspectral image analysis (J. Wiley, 2007).
[3] J. A. Hartigan, Clustering Algorithms (John Wiley & Sons, Inc., New York, NY, USA, 1975).
[4] P. Rouxhet and M. Genet, Surface and Interface Analysis 43, 1453 (2011).
[5] C. May, H. Canavan, and D. Castner, Analytical Chemistry 76, 1114 (2004).
[6] E. M. Brouwer, E. S. Kooij, H. Wormeester, M. A. Hempenius, and B. Poelsema, Journal of Physical Chemistry B: Materials, surfaces, interfaces, & biophysical 108, 7748 (2004).
[7] E. Garcia-Caurel, J. Nguyen, L. Schwartz, and B. Drévillon, Thin Solid Films 455-456, 722 (2004).
Speaker: Prof. Michel VOUE (Université de Mons) -
38
About the vortex core profile in superfluid Fermi gases
A characteristic property of superfluidity and -conductivity is the presence of quantized vortices in rotating systems. To study the BEC-BCS crossover the two most common methods are the Bogoliubov-De Gennes (BdG) theory and the usage of an effective field theory (EFT). In order to simplify the calculations for single vortices, it is often assumed that the hyperbolic tangent yields a good approximation for the vortex structure. The combination of a variational vortex structure, together with cylindrical symmetry yields analytic (or numerically simple) expressions.
The focus of this contribution is to investigate to what extent this analytic fit truly reflects the vortex structure throughout the BEC-BCS crossover at finite temperatures. The vortex structure will be determined using the effective field theory presented in [Eur. Phys. Journal B 88, 122 (2015)] and compared to the variational analytic solution. By doing this it is possible to see where these two structures agree, and where they differ. This comparison results in a range of applicability where the hyperbolic tangent will be a good fit for the vortex structure.
Speaker: Mr NICK VERHELST (Universiteit Antwerpen (UA)) -
15:15
Break
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Metal-Organic Frameworks: When the whole is more than the sum of its parts.
Metal-Organic Frameworks (MOFs) are a versatile class of crystalline materials showing great promise in a wide range of applications (e.g. gas sensing and storage, luminescence, pressure sensors, catalysis, …). In addition, their very structure puts them at several cross-sections: classical solids and molecules, surfaces and bulk,… Furthermore, since they combine properties either intrinsic to solids or molecules, these materials also provide ample opportunities to investigate fundamental materials properties from both physical and chemical perspective. In short, they are a dream-come-true playground for the material scientist.
However, the size of their crystalline unit cell and their complexity makes them far from trivial systems to study. Despite this last aspect, high quality first principles calculations on these systems are nowadays feasible, albeit computationally very demanding. Luckily the Flemish HPC facilities bring the numerous rewards from such computational studies within grasping reach.
In this contribution, we will discuss the results obtained for several types of MOFs, showing how computational results can be linked to experimental observations, and how these lead to a deeper fundamental understanding of the system under study.
The breathing MIL-47: How does the magnetic configuration of the MIL-47 MOF link to its flexibility, and how does it relate to the experimentally observed pressure induced breathing.[MIL47] The functionalized luminescent UiO-66: What is the influence of linker-functionalization on the light absorption properties? And how do the functional groups modify the electronic structure?[UIO66] The recently discovered COK-69: Can the local structure of the titanium-oxide node be deduced from calculations and how does the blue color upon irradiation fit into the story of this flexible luminescent MOF? [COK69](left) Spin density of anti-ferromagnetic MIL-47(V) with ferromagnetic chains. (right) Electronic band structure and density of states.
References[MIL47] “Quasi-1D physics in metal-organic frameworks: MIL-47(V) from first principles”, Danny E. P. Vanpoucke, Jan W. Jaeken, Stijn De Baerdemacker, et al., Beilstein J. Nanotechnol. 5, 1738-1748 (2014),
[UIO66] “Understanding intrinsic light absorption properties of UiO-66 frameworks”, Kevin Hendrickx, Danny E.P.Vanpoucke, Karen Leus, et al., Inorg. Chem. 54(22), 10701-10710 (2015),
[COK69] “A Flexible Photoactive Titanium MOF based on a [TiIV3(µ3-O)O2(COO)6]-Cluster”, Bart Beuken, Frederik Vermoortele, Danny E.P.Vanpoucke, et al., Angew. Chem. Int. Ed. 54(47), 13912-13917 (2015),
Speaker: Danny VANPOUCKE (Hasselt University) -
40
Superconductivity in strontium titanate and SrTiO3-based structures due to the electron – LO-phonon interaction
Recently, superconductivity in strontium titanate and at the SrTiO3-LaAlO3 interface became a subject of renewed interest because of specific properties of SrTiO3. The superconducting phase transition in these systems occurs at very low temperatures combined with low carrier densities. Strontium titanate is a strongly polar crystal where the Fröhlich electron – LO-phonon interaction dominates in the electron response [1]. Therefore we suggest that the same interaction provides superconductivity in these systems.
Superconductivity in bulk strontium titanate and at the SrTiO3-LaAlO3 interface is analyzed discussing different approaches and paradigms. Because the LO-phonon frequencies are high with respect to both the thermal energy and the Fermi energy, the superconducting system is strongly non-adiabatic. The dielectric function method [2, 3] is shown to be the most consistent for non-adiabatic electron-phonon systems.
We have applied the dielectric function method to treat superconductivity at the SrTiO3-LaAlO3 interface [4] and in bulk doped strontium titanate using recent results for the band structure of SrTiO3 [5]. The critical temperatures are calculated without fitting. The obtained critical temperatures are in line with experimental data. The present method explains the observed multi-dome shape of the critical temperature in bulk strontium titanate as a function of the electron concentration due to multiband superconductivity.
This research was supported by the Flemish Research Foundation (FWO-Vl), project nrs. G.0115.12N, G.0119.12N, G.0122.12N, G.0429.15N, by the Scientific Research Network of the Research Foundation-Flanders, WO.033.09N, and by the Research Fund of the University of Antwerp.
References
J. T. Devreese, S. N. Klimin, J. L. M. van Mechelen, and D. van der Marel, Phys. Rev. B 81, 125119 (2010). D. A. Kirzhnits, E. G. Maksimov, and D. I. Khomskii, J. Low Temp. Phys. 10, 79 (1973). Y. Takada, J. Phys. Soc. Jpn. 45, 786 (1978); 49, 1267 (1980). S. N. Klimin, J. Tempere, J. T. Devreese and D. van der Marel, Phys. Rev. B 89, 184514 (2014). J. L. M. van Mechelen, D. van der Marel, C. Grimaldi, A. B. Kuzmenko, N. P. Armitage, N. Reyren, H. Hagemann, and I. I. Mazin, Phys. Rev. Lett. 100, 226403 (2008).Speaker: Serghei KLIMIN (TQC, Universiteit Antwerpen) -
41
Optical gain in solution processable 2D CdSe nanoplatelets
Colloidal quantum dots are nanometer sized semiconductor crystalllites obtained by solution-based synthesis. They exhibit unique optical and electronic properties that depend on their size, shape and composition. In combination with their suitability for solution processing, many applications are found for these novel materials, ranging from lasing, lighting, photovoltaic devices to bio-imaging. Due to their tunable photoluminescence, colloidal quantum dots are promising optical gain materials for lasers. Moreover, the low cost and ease of processing provide substantial benefits over classical, epitaxially grown semiconductors. However, high pumping thresholds for photo-excitation and low material gain remain limiting factors.
Here, we have investigate optical gain in CdSe nanoplatelets, also called ‘solution processable quantum wells’ using ultrafast pump-probe spectroscopy. As platelets are 2D materials, they have advantages like large absorption cross section, slow non-radiative recombination rates and narrow emission linewidth. Recent studies have shown gain in these platelets under continuous wave optical pumping, but it remains unclear under what conditions – single/bi-exciton or multi-excitons – optical gain is achieved. We address this point in a quantitative transient absorption study on CdSe platelets of different thickness. We observe optical gain in 3 monolayer (MLs) thick CdSe platelets, yet we find a gain threshold that corresponds to approximately 160 excitons per platelet, a number substantially higher than the bi-exciton regime. These results question the interpretation of the optical properties of CdSe nanoplatelets in terms of quantum-dot like models.
Speaker: Ms Renu TOMAR (Physics and Chemistry of Nanostructures Group, Department of Inorganic and Physical Chemistry, Gent University) -
42
Phase Formation in High-Entropy Alloy Thin Films
Since the beginning of the 21st century, a novel approach to design alloys with promising properties has been introduced [1]. High-Entropy Alloys (HEAs) are composed of five or more principal elements in equimolar ratios. The high mixing entropy significantly reduces the Gibbs free energy of the solid solution, hence single-phase multi-component solid solutions are thermodynamically stable, especially at high temperature. In contrast to conventional alloys, the large number of elements does not lead to complex systems but rather simple fcc and/or bcc crystalline structures or amorphous phases are formed. HEAs have excellent properties such as high hardness and strength, corrosion resistance and thermal stability, which makes them interesting materials for a range of applications. Although bulk HEAs have already been studied, the formation and properties of thin film HEAs have not yet been studied in great depth due to the alloy’s complexity. This work investigates the interplay between different elements and their chemical, topological, and thermodynamic properties on the phase formation in HEA thin films. As sputter deposition is a momentum- and energy-driven process, also the deposition conditions influence the film properties [2,3]. The 5-element CoCrCuFeNi alloy was used as a starting point and the influence of three solute elements (Nb, In and Ge) on the film growth and phase formation was studied. In this way it is possible to make a cautious set of guidelines for future alloy design.
Keywords: High-entropy alloys, multi-element thin films, sputter deposition, phase formation
References [1] J. W. Yeh et al., Adv. Eng. Mater. 6 (5) (2004) 299, [2] B. R. Braeckman et al., Thin Solid Films 580 (2015) 71, [3] B. R. Braeckman, D. Depla, J. Alloys Compd. 646 (2015) 810
Speaker: Mr Bert BRAECKMAN (Vastestofwetenschappen, UGent) -
43
Thermal and Plasma Enhanced Atomic Layer Deposition on Powders and Particles
Surface engineering of micro- and nanoparticles is of great importance in fields such as catalysis, energy, sensing and additive manufacturing. For many of these applications particles are required with different bulk and surface properties. A popular technique to achieve this is to coat the particle surface with a nanometer thick layer. Atomic layer deposition (ALD) is known as a reliable technique for covering complex 3D objects with ultrathin conformal coatings. However, to perform ALD on large quantities of powders, the individual particles need to be fluidized or agitated. Fluidized bed reactors are most often used for ALD on particles, but this reactor concept does not seem to be compatible with plasma enhanced ALD, which is advantages for e.g. coating on temperature sensitive polymer particles or deposition of metals and metal nitrides.
At UGent, a rotary reactor was developed to agitate particles, enabling the deposition of conformal coatings by thermal and plasma-enhanced ALD. Particles ranging from nanometer size to millimeter size were successfully coated with layers of Al2O3, TiO2, SiO2, AlN and TiN. The ALD processes were characterized in-situ by means of mass spectroscopy (MS) and optical emission spectroscopy (OES). The composition and conformality of the coatings were evaluated by X-ray fluorescence (XRF), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM).
These ultrathin conformal coatings have, for instance, been applied to polymer particles for improving rheology (flowability) or altering the hydrophobicity, and to metallic particles for corrosion prevention.
Our results prove that the developed rotary reactor enables conformal deposition on nano- and micropowders by thermal and plasma enhanced ALD. In this way, surface engineering of such particles can be achieved.
Speaker: Dr Geert RAMPELBERG (Ghent University)
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35
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Fundamental Interactions, Nuclear and Particle PhysicsConvener: Wim Cosyn (UGent)
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44
Nuclear Structure Studies with Radioactive Ion Beams
Our understanding of the structure of the atomic nucleus, previously based on the information collected on stable nuclei, has been severely challenged in the last thirty years, since systematic research on nuclei far from stability was started. The unusual neutron-to-proton ratio of those systems revealed special features of the underlying nucleon-nucleon interaction, reflected in rearrangements of nuclear shells.
In the last few years the improvements in the range and quality of available radioactive ion beams (RIBs), together with the development of refined detection methods, have allowed the detailed experimental investigation of a number of key nuclei. The nuclear physics groups at the KU Leuven employ a number of complementary techniques to access information on ground- and excited-state properties of short living nuclei produced at RIB facilities.
In this talk a selection of recent results and the perspectives in the short-term future will be presented.
Speaker: Prof. Riccardo RAABE (KU Leuven, Instituut voor Kern- en Stralingsfysica) -
45
Beyond integrability for collective pairing systems
Pairing is an essential ingredient to understand the low-energy structure of atomic nuclei [1]. Whereas the Bardeen-Cooper-Schrieffer (BCS) Ansatz has proven very successful in capturing the collective nature of the nuclear pair excitations, there remain some important deficiencies related to finite-size effects of the nuclear many-body problem [1]. Richardson and Gaudin (RG) have shown that the finite-size reduced (s-wave) BCS Hamiltonian, the simplest yet most insightful model for nuclear pairing, is integrable and solvable using a Bethe Ansatz technique [2,3]. Moreover, the rapidities encoding the Ansatz allow for a clear-cut physical interpretation of the dominant pairing modes in the system [4]. However insightful, the effective applicability of the RG systems is limited due to the integrability constraints of the model. However, turning limitation into opportunity, the on-shell (integrable) Bethe Ansatz can also be used as an ideal starting point that already includes the collective pairing correlations in a qualitative sense, and in which the more realistic pairing correlations can be built in via conventional quantum many-body techniques.
In the present presentation, I will illustrate how the RG Bethe Ansatz offers insight into the pairing dynamics of nuclear systems, and show how one can go beyond integrability (either via variational [5] or coupled-cluster approaches [6]) to describe realistic pairing correlations in atomic nuclei
[1] Ring P and Schuck P 2004 The Nuclear Many-Body Problem 3rd ed (Berlin: Springer)
[2] Richardson R W 1963 Phys. Lett. 3 277
[3] Gaudin M 1976 J. Phys. (Paris) 37 1087
[4] De Baerdemacker S 2012 Phys. Rev. C 86 044332
[5] De Baerdemacker S et. al. (in preparation)
[6] Henderson T M, Scuseria G E, Dukelsky J, Signoracci A and Duguet T 2014 Phys. Rev. C 89 054305
Speaker: Dr Stijn DE BAERDEMACKER (UGent) -
46
Three-Dimensional Structure of the Nucleon and Quantum Chromodynamics
We address the state-of-the-art and prospects of the study of three-dimensional intrinsic structure of the nucleon, one of the hot topics in modern high-energy hadron physics. We give an overview of the relevant experiments at current and planned facilities and discuss the most urgent issues in the theory of string interaction - Quantum Chromodynamics - related to the 3D-imaging of the nucleon.
Speaker: Igor CHEREDNIKOV (Universiteit Antwerpen) -
47
Search for new heavy resonances decaying in di-lepton or di-photon pairs at CMS
In the quest for new physics beyond the Standard Model, decay channels in leptons or photons are particularly interesting, since they arise in clean final states, allowing quick and well defined strategy of analysis.
Resonances in the dielectron and dimuon decay channels arise in many well established theories beyond the standard model, like grand unified theories (GUT) or models proposing extra spatial dimension(s). The first part of the talk presents the search of heavy neutral resonances decaying in di-electron or di-muon pairs, where up to 2.8/fb of proton-proton (pp) collision data with √s = 13 TeV collected by the CMS experiment at the LHC were analyzed. In absence of a significant deviation from the standard model predictions, 95% confidence level limits are calculated.
The second part of the talk is focused on the search for heavy neutral resonances decaying in di-photon pairs, whose existence is predicted in two classes of models: ADD (Arkani-Hamed, Dimopoulos e Dvali) and Randall-Sundrum (RS) models. The search employs 3.3/fb of pp collision data collected by the CMS experiment in 2015 at a center-of-mass energy of 13 TeV. It is aimed at spin-0 and spin-2 resonances of mass between 500 and 4500 GeV and relative width up to 5.6 × 10−2 . The results of the search are combined with those obtained by the CMS collaboration in similar searches at √s = 8 TeV. The observed mass distribution was found to be consistent with the expectations from the SM. The largest excess in the combined analysis (13 TeV + 8 TeV) is observed for m = 750 GeV and relative width = 1.4 × 10−4 . The local p-value corresponds to approximately 3.4 standard deviations, which are reduced to approximately 1.6 standard deviations after taking into account the effect of searching for several signal hypotheses.
Speaker: Mr Giuseppe FASANELLA (ULB and Sapienza) -
48
Measurement of the cross section of top quark pair production in association with a Z boson in pp collisions at 13 TeV
The measurement of the cross section of top quark pair production in association with a Z boson, using proton-proton collisions at a center-of-mass energy of 13 TeV. The data sample used corresponds to an integrated luminosity of 2.7 fb−1. The measurement is performed in three- and four-lepton final states where the jet and b-jet multiplicities were exploited to enhance the signal over background ratio.
Speaker: Mrs Deniz POYRAZ (UGent) -
49
Short-range correlations in neutrino-nucleus scattering
We present a detailed study of charged-current quasielastic neutrino-nucleus scattering and of the influence of short-range correlations on one- and two-nucleon knockout processes. The short-range correlation formalism is implemented in the impulse approximation by shifting the complexity induced by the correlations from the wave functions to the operators. The model is validated by confronting (e,e′) cross section predictions with electron scattering data in the kinematic region here the quasielastic channel is expected to dominate. Further, the 12C(ν,μ−) cross sections relevant for neutrino-oscillation experiments are studied. Double differential 12C(ν,μ−) cross sections, accounting for short-range correlations in the one-particle emission channel and the two-particle two-hole channel, are presented for kinematics relevant for recent neutrino-nucleus scattering measurements.
Speaker: Mr Tom VAN CUYCK (UGent) -
50
How non-perturbative physics can resurge from perturbation theory
Important features of physical systems; non-perturbative phenomena, cannot be captured by the most wildly used tool in physics: perturbation theory. Perturbation theory is indeed inherently not complete answer. In QCD for example, the growing number of diagrams implies that perturbation theory does not give a finite result to physical quantities or cannot explain the appearance of a mass scale.
Quiet surprisingly there is growing evidence that non-perturbative effect are actually captured by perturbation theory. This talk will introduce the audience to how non-perturbative effect can ‘resurge’ from perturbative data. This technique of resurgence can be used to investigate the non-perturbative sector of exactly solvable models in two dimensional Quantum Field Theory.
Speaker: Mrs Saskia DEMULDER (Vrije Universiteit Brussel) -
51
Flavor changing neutral currents of top quarks at the LHC, a probe for new physics
When one observes the phenomena in our universe, one notices that the Standard Model (SM) fails to explain several outstanding problems, such as dark matter or dark energy. These shortcomings can be addressed by new physics extensions including new particles and symmetries at higher energy scales. At present, the hope is that unambiguous signals for such new physics will be discovered in the next runs of the LHC at CERN. Direct searches for new particles at the LHC, with distinct signatures such as missing energy final states, might be beyond the kinematic reach of the LHC. However, these particles can still contribute to the observed final states at loop-level altering the expected kinematics in the SM. In general, these processes are suppressed, typically at the order of the inverse mass squared of the new particle. Therefore, this suppression could be so high that the process may still missed at the LHC. In both cases, direct or indirect observation of new physics would yield a breakthrough towards understanding the completion of the SM. Flavor changing neutral currents (FCNC) processes, fall in the latter case and are an important probe for new physics. These processes contain at least two generations of fermions in the initial and final states and are suppressed in the SM by the Glashow-Iliopoulis-Maiani (GIM) mechanism. By extending the SM, one can build new physics models where the contributions of these processes are three orders of magnitude larger than those of the SM (e.g. 2HDM) and thus observable at the LHC. Thus, even if direct detection of new particles fails at the LHC, one may still expect indirect contributions in the SM sector. FCNC processes involving top quarks have been sought in the decays of top quarks to an up or charm quark and a scalar boson (e.g. H boson) or in the decay of a top quark into an up or charm quark and a vector boson (e.g. Z boson). In my talk I shall discuss the current state of these searches at the CMS experiment and will compare these to other experiments.
Speakers: CMS COLLABORATION (CERN), Isis VAN PARIJS (VUB) -
52
On the feasibility of RADAR detection of high-energy neutrino-induced showers in ice
We discuss the RADAR detection technique as a probe for high-energy neutrino-induced particle cascades in ice. We show that this technique, if one is able to scatter efficiently, will be able to probe neutrino energies even larger than those covered by the IceCube neutrino observatory which is sensitive up to PeV energies. A detailed model for the RADAR reflection off of the ionization plasma induced by a high-energy particle cascade in a dense medium like ice will be presented. It is shown that the scatting efficiency depends strongly on the (unknown) properties of the induced ionization plasma. To get a better handle on these properties we performed a beam test experiment at the Electron Light Source facility of the Telescope Array collaboration in Utah, USA. First promising experimental results will be presented in combination with an outlook to future experimental work.
Speaker: Krijn DE VRIES (VUB/IIHE) -
53
NEW PROMPT FISSION GAMMA-RAY SPECTRAL DATA IN RESPONSE TO A HIGH PRIORITY REQUEST FROM OECD/NUCLEAR ENERGY AGENCY
The study of fission fragment de-excitation is important for both nuclear applications and fundamental nuclear physics. For modelling innovative nuclear reactor (GEN-IV) cores, a better understanding of the released heat during fission of the major isotopes (235U,239Pu) is crucial. Present knowledge on the released heat states that around 10% of all energy released in fission comes from gamma-rays, and about 40% of these are prompt, i.e. within a few ns after fission. Since currently evaluated data from the 1970s show a deviation from benchmark calculations of up to 28%, an urgent request for new prompt fission gamma -ray spectra (PFGS) measurements was put high on the OECD Nuclear Energy Agency (OECD/NEA) Nuclear Data High Priority Request List (HPRL) targeting at an uncertainty of 7.5%.
In response to the HPRL, our group executed a dedicated measurement program on prompt fission gamma-rays employing state-of-the-art lanthanide halide detectors (cerium-doped LaBr3 and LaCl3 as well as CeBr3) with superior timing and good energy resolution. These detectors allow clean discrimination between prompt gamma-rays and other gamma-rays (both isomeric gamma-rays and from inelastically scattered neutrons). Our results from 252Cf(SF) , 235U(nth,f), 241Pu(nth,f) ,240,242Pu(SF), and, most recently, 239Pu(nth,f) provide PFGS characteristics (average number of photons per fission, average total energy per fission and mean photon energy) with uncertainties as low as 2%.
Prompt fission gamma-ray spectral data provide also important information about the nuclear de-excitation. The present challenge in nuclear modelling is the description of the competition of prompt neutron and gamma emission during the de-excitation process. Since fission is a unique tool for producing very neutron-rich isotopes, far from the valley of stability, we measured fragment-mass resolved prompt fission gamma-ray spectra and also investigated the time dependence of prompt gamma-ray emission to allow benchmarking nuclear theory.
Speaker: Mrs Angelique GATERA (European Commission - Joint Research Centre - Institut for Reference Materials and Measurements, Geel, Belgium and Ghent University, Gent, Belgium)
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44
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Physics and EducationConvener: Prof. Mieke De Cock (KU Leuven - departement Natuurkunde en Sterrenkunde)
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Poster Session
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54
3D DESIGN AND PERFORMANCE ANALYSIS OF NANO-SIZED SQUID-ON-TIP
SQUID-on-tip (SOT) is the most sensitive detector of small magnetic moments to date [1]. We analyze the performance of such nano-sized SOT (Fig. 1) in the presence of the magnetic field, using the state-of-the-art three dimensional (3D) simulations within the phenomenological Ginzburg-Landau (GL) theory. Based on the observed behavior of the superconducting order parameter in the SOT, the distribution of the Cooper-pair density and the behavior of the circulating supercurrents, we propose engineering solutions at the nanoscale to improve the sensitivity of the device. By introducing the constriction in the arms of the SQUID loop (Fig.1b), the gradient of phase of the order parameter is largely enhanced in the constriction of the SOT in the Meissner and first vortex state. This is expected to facilitate the interference pattern of the critical current of the SOT and reduce the noise. Furthermore, wedge shaped three junction loop of SOT (3JSOT) (Fig. 2) enables the tunability of the device and its selective response to both the in-plane and out-of-plane components of magnetic field [2]. We further discuss the other realizations of the SOT with engineered 3D shapes of the SQUID itself [3] and show the relation of those geometries to sensitivity to 3D magnetic field.
[1] D. Vasyukov, Y. Anahory et al., A scanning superconducting quantum interference device with single electron spin sensitivity, Nature Nanotech 8, 639-644 (2013). [2] Y. Anahory, J. Reiner et al., Three-Junction SQUID-on-Tip with Tunable In-Plane and Out-of-Plane Magnetic Field Sensitivity, Nano Letters 14, 648-6487 (2014). [3] C. Granata, & A. Vettoliere, Nano Superconducting Quantum interference device: A powerful tool for nanoscale investigations, Physics Reports, In press (2015).
Speaker: Mr Abul HASNAT RUBEL (University of Antwerpen, Jagannath University) -
55
A New Measurement for the Electron Impact Ionization of He(1s2sˆ3S)
The electron impact ionization of the metastable state 1s2s3S of helium is particularly important in the modelling of plasmas, and, as a simple system, is also a benchmark for theories. Indeed, there are many theoretical studies devoted to the calculation of the ionization cross section [2, 3], ranging from Born calculations to sophisticated close-coupling calculations. In contrast, only a few absolute measurements exist, a fact explained by the difficulty to produce metastable helium in a controlled manner. The latest and most comprehensive measurement, by Dixon et al. [1], lies about a factor of two higher than the most up-to-date calculations and therefore another absolute measurement is highly demanded by the community.
We designed a novel source for the production of a fast, intense beam of metastable helium atoms in the 1s2s3S state. A fast beam of He+ ions, produced in a duoplasmatron source, is sent into a sodium vapour cell where double charge exchange converts He+ into He− with an efficiency of 1%. A continuous wave CO2 laser is then used to photodetach a large fraction of He− ions, leaving the remaining neutral helium atoms in the 1s2s 3S state exclusively. The beam of neutral, metastable atoms is injected downstream in a setup designed to measure cross sections for electron-ion collisions [4] using the animated-crossed-beam method.
The present results lie significantly lower than the previous experiments. On the other hand, they are in very good agreement with the theoretical results of Fursa and Bray [2], who performed a set of calculations including the effect of doubly excited states. The reference values reported more recently by Ralchenko et al. do not include doubly excited states [3], and the agreement with our data is poorer at higher electron energies. This highlights the influence of such autoionizing states in the ionization process. We also measured for the first time, to our knowledge, the double ionization of He− and He(1s2s3S) by electron impact. The analysis of the data is currently under way.
References
[1] A. J. Dixon, M. F. A. Harrison and A. C. H. Smith, J. Phys. B 9, 2617 (1976).
[2] D. V. Fursa and I. Bray, J. Phys. B 36, 1663 (2003).
[3] Y. Ralchenko, R. K. Janev , T. Kato, D.V. Fursa, I. Bray and F.J. de Heer, At. Data Nucl. Data Tables 94, 603 (2008).
[4] J. Lecointre, S. Cherkani-Hassani, D.S. Belic, J.J. Jureta, K. Becker, H. Deutsch, T.D. Märk, M. Probst, R.K. Janev and P. Defrance, J. Phys. B 40, 2201 (2007).
Speaker: Mr Matthieu GÉNÉVRIEZ (Université Catholique de Louvain) -
56
A new tool for modelling ion cyclotron resonance heating wave propagation and damping in non-axisymmetrical magnetic confinement fusion machines
Ion cyclotron resonance heating (ICRH) is a routinely used method to bring plasmas to fusion relevant temperatures in magnetic confinement fusion machines (see e.g. [1]). Properly modelling the dielectric response is a challenge both from the physics and the computational point of view. More often than not simplifications are made, first of all by truncating the dielectric tensor at the leading order terms in finite Larmor radius corrections and secondly by simplifying the geometry. A new solver is under development at LPP-ERM/KMS to help understand the ICRH wave propagation and damping in stellarators such as Wendelstein-7X. It is based on the philosophy pioneered by Jaeger [2] for the AORSA code, and intends to complement the physics described in the SCENIC code (see e.g. [3]) with a rigorous computation of the wave polarisation at higher cyclotron harmonics. The key idea here is to assess the importance of higher order finite Larmor radius corrections while keeping both the zero order distribution functions as well as the geometry simple and given. In anticipation of its exploitation for the 3D fully inhomogeneous plasmas of W-7X, a focal point of the present work is to develop and test a technique for solving the relevant wave equation with as minimal as possible CPU requirements while keeping the full richness of the kinetic corrections. Preliminary results of this code are provided and discussed.
References
[1] E. Lerche et al., Nuclear Fusion 54 (2014) 073006
[2] E.F. Jaeger et al., Physics of Plasmas, 8 (2001) 1573
[3] M. Jucker et al., Computer Physics Communications 182 (2011) 912
Speaker: Dr Dirk VAN EESTER (LPP-ERM/KMS) -
57
Accelerated Aging Testing of Phosphors in Remote wLED Configuration
Since the introduction of high brightness and high efficiency light emitting diodes (LEDs), solid state lighting is ready to take over the lighting and display market. While the original concept of white LEDs (wLEDs) is based on a blue-emitting chip on which a light-converting phosphor is coated, a new approach has gained interest in the last decade [1] (see figure 1). In this so-called remote approach, a phosphor plate is placed at a certain distance from the LED chip. This lowers the phosphor temperature since the conductive heating by the LED chip is eliminated. Moreover the lowered excitation flux on the phosphor results in less self-heating during the downshifting of the blue light by the phosphor. Due the reduced reabsorption of light by the LED chip, the inherent scattering properties of the phosphor particles and the freedom to optimize the light mixing chamber, the remote approach yields improved light out-coupling and a more uniform light distribution.
Despite the benefits described above, research on the stability and durability of remote phosphors is still scarce and inconsistent, while of most importance from a commercial point of view. Standard accelerated aging methods are available for lumen depreciation in on-chip LED lifetime prediction [2], but no established standard methods exist for the remote- approach.
This work investigates the influence of three acceleration factors for aging: (1) heat, (2) humidity and (3) irradiance, ultimately providing a way to investigate the aging of phosphors in a remote configuration. Phosphor plates are mounted on teflon mixing chambers with a blue pumping LED at the bottom and placed in a climate chamber. Integrating sphere measurements, XRD and SEM(-EDX) studies are performed at predefined aging-times to retrieve the influence of each acceleration factor on the phosphor stability with respect to photoluminescence (PL), particle crystallinity and morphology respectively. As a proof of concept, commercial phosphors are tested first, providing optimized accelerated aging conditions. The approach is then transferred to investigate in-house prepared phosphors. Summarizing we present a technique which allows us to study the degradation of phosphors in the remote wLED configuration.
Speaker: Mr Reinert VERSTRAETE (LumiLab, Department of Solid State Sciences, Ghent University, Ghent, Belgium) -
58
Advanced structural and elemental characterization of energy materials and solid-electrolyte interfaces: From fundamental corrosion reactions to Li-ion batteries
Our society is using ever more complex materials which often involve nanometre-scale morphology. From impurities or minor alloying additions at grain boundaries to nanostructured (composite) materials or the naturally nanostructured electrochemical interfaces: The detailed knowledge of structural and elemental distributions in the bulk as well as on/along the surfaces or interfaces becomes increasingly crucial for important technical developments. Our group aims to gain mechanistic insight into reactions of materials and interfaces related to energy materials. Research focusses on electrochemical interfaces, in particular related to positive electrodes of Li-Ion Batteries (LIB), but also on electrodeposition and corrosion including alloying and dealloying processes. We use a wide range of advanced characterization techniques to explore structural and elemental changes during reactions. Here we present a selection of examples of employing techniques such as synchrotron-based in-situ x-ray diffraction, Atom Probe Tomography (APT), or an electrochemical Surface Force Apparatus (SFA) for studies on Li-ion battery materials and fundamental corrosion.
Speaker: Mr Boaz MOEREMANS (Hasselt University) -
59
AFM characterization of micro-contact printed self-assembled monolayers of alkanethiols
Self-assembled thiol monolayers (SAMs) are molecular assemblies of organic constituents formed spontaneously by the adsorption process of thiol molecules in liquid or vapor phase on metal or metal oxide surfaces [1]. On the one hand, chemisorption of the adsorbate head group onto metal surfaces at the metal-SAM nano-interface leads to a strong sulfur-metal bond, on the other hand, interactions including van der Waals and hydrophobic forces exist among backbone chains, ensuring an efficient packing of the monolayer and contributing to stabilize the structures with increasing chain length. Thiol SAMs can be prepared with a wide variety of terminal groups (i.e. the group opposite to the sulfur moiety) yielding surfaces with tailored hydrophilic- or hydrophobic properties that can be used as compact platforms for bio- or chemo-sensors [2] or for corrosion inhibition [3,4]. In this work we present a systematic atomic force microscopy (AFM) characterization of well-defined thiol SAM patterns formed onto an ultraflat Au (111) by micro-contact printing [5]. We assess the difference in thickness of different chain length alkanethiol monolayer patterns and evaluate how molecules behave when more complex configurations are used. Concretely, we analyze the organization of alkanethiol molecules in overlapped double patterns, as well as the changes of simple monolayer micropatterns when exposed to a different alkanethiol solution. The work prepares further alloy corrosion studies using well-controlled molecular inhibitor films [1] Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology, Chem. Rev. 2005, 105, 1103. [2] Jans, K. Stability of mixed PEO-thiol SAMs for biosensing applications, Langmuir 2008, 24, 3949. [3] Pareek, A.; Ankah, G. N.; Cherevko, S.; Ebbinghaus, P.; Mayrhofer, K. J. J.; Erbe, A.; Renner, F. U. Effect of thiol self-assembled monolayers and plasma polymerfilms on dealloying of Cu–Au alloys, RSC Adv. 2013, 3, 6586. [4] Renner, F. U.; Ankah, G. N.; Bashir, A.; Ma, D.; Biedermann, P.; Shrestha, B. R.; Nellessen, M.; Khorashadizadeh, A.; Losada-Pérez, P.; Duarte, M. J.; Raabe, D.; Valtiner, M. Star-shaped crystallographic cracking of localized nanoporous defects, Adv. Mat. 2015, 33, 4877. [5] Whitesides, G. M.; Kumar, K. Features of gold having micrometer to centimeter dimensions can be formed through a combination of stamping with an elastomeric stamp and an alkanethiol “ink” followed by chemical etching, App. Phys. Lett. 1993, 63, 2002.
Speaker: Mrs ShovaHasselt University NEUPANE -
60
Aharonov-Bohm oscillations of bosonic matter-wave beams in the presence of disorder and interaction
We study the one-dimensional (1D) transport properties of an ultracold gas of Bose-Einstein con- densed atoms through Aharonov-Bohm (AB) rings [1–3]. Our system consists of a Bose-Einstein condensate (BEC) that is outcoupled from a magnetic trap into a 1D waveguide which is made of two semi-infinite leads that join a ring geometry exposed to a synthetic magnetic flux φ. We specifically investigate the effects both of a disorder potential and of a small atom-atom contact in- teraction strength on the AB oscillations. The main numerical tools that we use for this purpose are a mean-field Gross-Pitaevskii (GP) description and the truncated Wigner (tW) method [4, 5]. The latter allows for the description of incoherent transport and corresponds to a classical sampling of the evolution of the quantum bosonic many-body state through effective GP trajectories. We find that a correlated disorder suppress the AB oscillations leaving thereby place to weaker amplitude, half period oscillations on transmission, namely the Aronov-Al’tshuler-Spivak (AAS) oscillations [6, 7]. The competition between disorder and interaction leads to a flip of the transmission at the AB flux φ = π. This flip could be a possible preliminary signature of an inversion of the coherent backscat- tering peak [8, 9].
References
[1] R.A. Webb, S. Washburn, C.P. Umbach and R.B. Laibowitz, Phys. Rev. Lett. 54, 2696 (1985).
[2] A. Levy Yeyati and M. Büttiker, Phys. Rev. B 52, R14360 (1995).
[3] H. Sigurdsson, O. V. Kibis, and I. A. Shelykh, Phys. Rev. B 90, 235413 (2014).
[4] C. W. Gardiner and P. Zoller, Quantum Noise, Springer (2004).
[5] J. Dujardin, A. Argüelles, and P. Schlagheck, Phys. Rev. A 91, 033614 (2015).
[6] B.L. Al’tshuler, A.G. Aronov, and B.Z. Spivak, JETP Lett. 33 (1993).
[7] D. Yu. Sharvin and Yu. V. Sharvin, JETP Lett. 34 (1981).
[8] M. Hartung, T. Wellens, C. A. Müller, K. Richter, and P. Schlagheck, Phys. Rev. Lett. 101, 020603 (2008).
[9] T. Wellens and B. Grémaud, Phys. Rev. Lett. 100, 033902 (2008).
Speaker: Mr Renaud CHRÉTIEN (ULG) -
61
Anticoherence of spin states in the Majorana representation
We investigate multiqubit permutation-symmetric states with maximal entropy of entanglement. Such states can be viewed as particular spin states, namely anticoherent spin states. Using the Majorana representation of spin states in terms of points on the unit sphere, we analyze the consequences of a point-group symmetry in their arrangement on the quantum properties of the corresponding state [1]. We focus on the identification of anticoherent states (for which all reduced density matrices in the symmetric subspace are maximally mixed) associated with point-group-symmetric sets of points. We provide three different characterizations of anticoherence and establish a link between point symmetries, anticoherence, and classes of states equivalent through stochastic local operations with classical communication. We then investigate in detail the case of small numbers of qubits and construct infinite families of anticoherent states with point-group symmetry of their Majorana points, showing that anticoherent states do exist to arbitrary order.
[1] D. Baguette et al., Phys. Rev. A 92, 052333 (2015).
Speaker: Mr Dorian BAGUETTE (Institut de Physique Nucléaire, Atomique et de Spectroscopie, Université de Liège, Bât. B15, B - 4000 Liège, Belgium) -
62
Breathing effect in V-doped Metal Organic Framework MIL-53(Al) studied by Electron Paramagnetic Resonance (EPR)
Metal-Organic Frameworks (MOFs) are crystalline porous materials constructed of metal ions connected by organic linkers. These materials possess many interesting features, like well-defined pore size, pore shape and ultra-high porosity. A characteristic example of MOFs with one dimensional pores is Al-MIL-53 ([Al(OH)(BDC), BDC = terephthalate or 1,4-benzenedicarboxylate]. The 3D framework of as-synthesized Al-MIL-53 (Al-MIL-53as) is built up of infinite chains of corner-sharing AlO4(OH)2 octahedra. The chains are connected by the organic BDC linkers creating one-dimensional rhombically shaped porous channels. In the Al-MIL-53as the channels are filled with uncoordinated terephthalic acid molecules. These, together with residual solvent or water molecules, can be removed by calcination or solvent extraction, which is referred to as activation of the MOF. The activated Al-MIL-53 structure exhibits breathing, the structure can reversibly change from a large open pore (LP) to a narrow pore form (NP) by changing the temperature and/or pressure conditions.
The breathing effect triggered by temperature was investigated in Al-MIL-53 doped with V. Since the V4+ dopant ions have one unpaired electron (3d1 configuration) they exhibit an Electron Paramagnetic Resonance (EPR) spectrum. EPR spectroscopy is a nondestructive analytical technique very sensitive to local environment of the paramagnetic ion. Spectrum analysis often allows chemical identification of the central paramagnetic ion and of its nearest environment. The breathing effect in Al-MIL-53 was monitored with in situ EPR, recording spectra as a function of temperature in air and in vacuum. In Figure 1 EPR spectra of V4+ in the NP form (A) and the LP form (B) are presented together with the NP and LP structures (1 and 2 respectively). We show that VIV centers can be used as local probe to detect these phase transitions in the Al-MIL-53 framework.Speaker: Ms Irena NEVJESTIC (Ghent University) -
63
Chaotic Bohmian trajectories for stationary states
We study the possibility of chaos for the Bohmian dynamics when the wave function is stationary. Examples of stationary wave functions are given for which there is chaos, as demonstrated by numerical computations, for one particle moving in 3 spatial dimensions and for two and three entangled particles in 2 dimensions. What is important for the amount of chaos is the overall complexity of the wave function. Some simple measures that partly capture the complexity of the wave function are considered: the participation ratio and different measures of entanglement. We find that these measures often tend to correlate to the amount of chaos. However, the correlation is not perfect, because the measures do not depend on the intrinsic complexity of the states of a given basis.
Speaker: John MARTIN (Institut de Physique Nucléaire, Atomique et de Spectroscopie, Université de Liège, Bât. B15, B - 4000 Liège, Belgium) -
64
Characterization of iron oxide nanoparticles by magnetometry: temperature deviation from Langevin law
Iron oxide nanoparticles (NP) are of great interest in nanomedicine. They are used in hyperthermia, tumor targeting, drug delivery therapy and in Magnetic Resonance Imaging (MRI) as negative contrast agent. Because NP’ size and magnetization play a key role in their behavior as MRI contrast agents, these parameters need optimal characterization methods.
In this work, we explored the characterization by magnetometry, in particular the effects of temperature, size distribution and anisotropy on magnetization of different-sized magnetite NP (Fe3O4). With a Vibrating Sample Magnetometer (VSM), we carried out magnetization as function of the magnetic field (MH curves) at 100, 200, 275, 300 and 315 K. Transmission electron microscopy (TEM) was also performed to measure directly the size distribution and agglomeration of the NP.
By fitting the MH curves with Langevin law, we observed that the NP’ size parameter was decreasing with the decrease of temperature in both simple Langevin and size distribution model. We attempted to explain this effect : The first hypothesis was that, at low magnetic fields, Néel relaxation intensity varies with the temperature. This would explain the differences in curves’ slope and so the size parameter. Nevertheless, the size decrease also occurs above 273 K where Brown relaxation should totally erase Néel relaxation effect. To test the second hypothesis we verified that the MH curves follow well the Langevin law by plotting the normalized magnetization (M/Msat) as function of the magnetic field normalized by the temperature (B/T). If they follow well the Langevin law, all the curves have to be superimposed[1], which is the case. Except for the 100 K MH curve which is the only one to diverge slightly from the other curves. Finally, we tried to fit the MH curves with a non-zero uniaxial anisotropy model used by Respaud and al[1] but this model didn’t succeeded to fit our MH curves.
In conclusion, we have to test others hypotheses like the stability of the fit itself and/or to test others anisotropy model like cubic anisotropy to understand completely this phenomena. It is difficult to characterize nanoparticles by magnetometry. Because the size obtained is sensitive to the temperature, it is touchy to arbitrary choose one specific temperature. However, MH results at room temperature are in good agreement with TEM measurements, especially for the size distribution width.
[1] M. Respaud and al., Surface effects on the magnetic properties of ultrafine cobalt particles, Physical Review B, 57, 5: 2925-2935 (1998)
Speaker: Mr Daniel HENRARD -
65
characterization of iron oxide particles with MR relaxometry: good and bad news
Superparamagnetic iron oxide particles find their main application as contrast agents for cellular and molecular Magnetic Resonance Imaging (1). The contrast they bring is due to the shortening of the transverse relaxation times T1 and T2 of water protons (2). In order to understand their influence on proton relaxation, different theoretical relaxation models have been developed, each of them presenting a certain validity domain, which depends on the particle characteristics and proton dynamics (3). In this work, relaxation properties of suspensions of iron oxide particles in different solvents and at different temperatures, corresponding to different proton diffusion properties, were evaluated thanks to the measurement of nuclear magnetic relaxation dispersion (NMRD) curves (4). These latter represent the evolution of the relaxation times with the magnetic field. The fitting of T1 NMRD profiles by the suited theory constitutes an interesting tool of characterization of the nanoparticles. The Roch theory, developed in the Motional Averaging Regime, was successfully used to fit T1 NMRD profiles, even completely outside the MAR validity range, and provided a good estimate of the particle size. In order to refine the characterization of the particles, we tried to perform a simultaneous fitting of T1 and T2 NMRD data, which was unfortunately impossible. This occurrence constitutes a clear limitation of the Roch model. Finally, the theory was shown to fit satisfactorily the deuterium T1 NMRD profile of superparamagnetic particles suspensions in heavy water, usually dominated by quadrupolar interactions, providing good estimates of the size and magnetization of the particles.
Bulte J W M and Kraitchman D L 2004 Iron oxide MR contrast agents for molecular and cellular imaging NMR Biomed. 17 484–99 Gossuin Y, Gillis P, Hocq A, Vuong Q L and Roch A 2009 Magnetic resonance relaxation properties of superparamagnetic particles Wiley Interdiscip. Rev. Nanomed. Nanobiotechnol. 1 299–310 Roch A, Muller R N and Gillis P 1999 Theory of proton relaxation induced by superparamagnetic particles J. Chem. Phys. 110 5403–11 Gossuin Y, Orlando T, Basini M, Henrard D et al NMR relaxation induced by iron oxide particles: testing theoretical models Nanotechnology 27 155706Speaker: Yves GOSSUIN (umons) -
66
Composition dependent self-organization in Au-Ag core shell nanostructures
Bimetallic AuxAg1-x clusters with various compositions (x = 0.9, 0.7, 0.4 and 0.2) grown in the gas phase and deposited on amorphous SiO2 wafers and TEM grids were characterized by a combination of aberration corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and grazing-incidence small-angle X-ray scattering (GISAXS). Clusters with an average diameter of 3.0 ± 0.6 to 3.7 ± 0.7 nm were produced using dual-laser ablation of pure Ag and Au metal targets under UHV conditions1,2. Control over the bimetallic clusters composition was achieved in the gas-phase prior to deposition by fine tuning of the laser energy densities and the time delay between the laser pulses. Composition was monitored by reflectron time of flight mass spectrometry using a newly developed binomial combination model. Clusters were then deposited in soft landing mode with a low coverage of 0.1 atomic monolayer (ML) to minimize aggregation on SiO2 wafers and TEM grids for GISAXS and HAADF-STEM measurements respectively. HAADF-STEM and GISAXS data analysis3,4 show, in excellent agreement, core-shell structures for all the investigated compositions with a systematic composition-dependent inversion of the element in the core and the shell. Less abundant species always form the core of the clusters: a gold rich core is observed in Au0.9Ag0.1 and Au0.7Ag0.3 clusters while a silver rich core is observed in Au0.4Ag0.6 and Au0.2Ag0.8 clusters. Despite the low surface coverage fraction, GISAXS proved to be extremely sensitive to measure the cluster size and structural organization. The composition-dependent core-shell structure of bimetallic AuxAg1-x clusters is expected to affect significantly their catalytic properties that are currently being tested for CO oxidation reactions.
References
1 Bouwen, W. et al. Production of bimetallic clusters by a dual-target dual-laser vaporization source. Rev. Sci. Instrum. 71, 54-58 (2000).
2 Verschoren, G. et al. Electron scattering in Au films containing Co clusters. Thin Solid Films 516, 8232-8239, doi:10.1016/j.tsf.2008.02.055 (2008).
3 Li, Z. et al. Structures and optical properties of 4–5 nm bimetallic AgAu nanoparticles. Faraday Discuss. 138, 363-373 (2008).
4 Lazzari, R. IsGISAXS: a program for grazing-incidence small-angle X-ray scattering analysis of supported islands. J. Appl. Crystallogr. 35, 406-421, doi:Doi 10.1107/S0021889802006088 (2002).
Speaker: Mr Ting-Wei LIAO (KU Leuven) -
67
Computational Multi-Fluid Model for Partially Ionized and Magnetized Plasma
In the present work, we present a computational model for studying reactive and partially ionized plasmas in thermo-chemical nonequilibrium under the effect of electromagnetic fields, such as in astrophysics or fusion-related applications. In order to tackle the non-equilibrium effects present in such a plasma, we adopt a multi-fluid formulation including electromagnetic effects. Multi-fluid equations consider each species within the mixture as behaving as different fluids, interacting among each other by means of collisions and chemical reactions. In the present poster, we will present a two-fluid model, accounting for ions and neutral species. The model includes chemical reactions for characterizing ionization, recombination and charge exchange collisions and accurate transport fluxes, obtained through the Grad’s method and considering the anisotropy introduced by the magnetic field in the ionized species. The resulting equations are coupled to the full Maxwell’s equations in order to capture phenomena such as electromagnetic waves or charge separation effects, neglected in standard magnetohydrodynamics (MHD) simulations. Our computational model will be used to simulate magnetic reconnection testcases which have been proposed in recent literature and for which reference numerical simulations are available with more simplified models (including our recently published two-fluid model).
Speaker: Mr Alejandro ALVAREZ LAGUNA (VKI - KU Leuven) -
68
Coordinate-based manipulation of guided waves with metamaterial waveguides
The confinement of light to material interfaces and thin layers, i.e., the propagation of surface waves along metal-dielectric interfaces or guided waves along dielectric waveguides, enables a multitude of photonic applications in (bio)sensing, optical circuitry and optical actuation. Several research groups try to enhance our control on the propagation of confined light by making use of artificial materials, e.g., metamaterials with tailored optical properties determined by subwavelength structures. To efficiently determine the appropriate geometry, spacing and composition of metamaterial building blocks, one often relies on a geometrical design tool known as transformation optics. Transformation optics determines the specific metamaterial properties that impose desired light flows inside a metamaterial device―based on coordinate deformations of initially straight trajectories―leading to extraordinary applications such as invisibility cloaks, optimized Cherenkov detectors and efficient beam steering devices. Unfortunately, the conventional application of transformation optics to slab waveguides leads to impractical and bulky designs, implementing metamaterials both inside and outside of the waveguide’s core. In this contribution, we restore the two-dimensional nature of guided modes by introducing new relations between two-dimensional coordinate deformations of confined light flows and the waveguide’s properties. In particular, our designs consist of metamaterial waveguide cores of varying thickness without need for metamaterials outside of the core. We verify the effectiveness and versatility of our design with three proof-of-concept devices: a beam bender, a beam splitter and a conformal lens. We anticipate that the seamless design of multifunctional metamaterial waveguides, combining benders, splitters and lenses with one consistent coordinate transformation, opens up new opportunities for guiding confined light along optical chips.
Speaker: Ms Sophie VIAENE (Vrije Universiteit Brussel) -
69
Design of an ICRF system for plasma-wall interactions and RF plasma production studies on TOMAS
Ion cyclotron wall conditioning (ICWC) is being developed for ITER as a baseline conditioning technique in which the ion cyclotron heating and current drive system will be employed to produce and sustain the current-less conditioning plasma. The TOMAS project (TOroidal MAgnetized System, operated at the FZ-Juelich, Germany) proposes to explore several key aspects of ICWC. This project stands on two pillars featuring plasma and material studies: (a) plasma-induced material modification and optimization of the wall conditioning efficiency via exposure of probes made of real PFC and the use of tracers; (b) detailed research on ICWC plasma production and optimisation to benchmark codes.
The ICRF system requirements to fulfill the above aims are: (a) ability to couple op to 6 kW of RF power to low density and low temperature plasma (1011/cm3, 3-10 eV) (b) ability to initiate plasma in broad frequency range (15 to 45MHz) for plasma production studies.
For this purpose we have designed an ICRF system made of a single strap antenna within a metallic box, connected to a feeding port and a pre-matching system. We discuss the design work of the antenna system with the help of the commercial electromagnetic software CST Microwave Studio. The simulation results for a given geometry provide input impedance matrices for the two-port system. These matrices are afterwards inserted into various circuit models to assess the accessibility of the required frequency range.
The sensitivity of the matching system to uncertainties on plasma loading and capacitance values is notably addressed. With a choice of three variable capacitors we show that the system becomes resilient to such uncertainties. We also demonstrate that the system can cope as well with the high reflected power levels during the short breakdown phase of the RF discharge.
Speaker: Dr Tom WAUTERS (aboratory for Plasma Physics - Royal Military Academy) -
70
Development of a hydrogen maser in the TE111 mode
We present the recent progress in the development of a hydrogen maser in the TE111 mode. Hydrogen masers use the transition at 1420.405 MHz between the two hyperfine levels F = 0 and F = 1 of the 1s1/2 ground state of the hydrogen atom [1]. Standard hydrogen masers are heavy devices which are based on the use of a TE011 cylindrical cavity with dimensions of the order of 27 cm [1]. In contrast, the TE111 mode is the lowest frequency mode of a cylindrical cavity in the usual regime D/L<0.985 where D and L are the diameter and the length of the cavity, respectively [2]. In comparison with the standard masers, the TE111 mode makes thus possible to reduce dimensions significantly to obtain resonance at 1420.405 MHz, which is very interesting for space applications and in particular in the context of the global positioning system.
The design of the upper und lower parts of the maser were studied in order to obtain a compact model. The cavity is made of aluminum and is composed of two halves which clamp a thin Teflon FEP sheet (0.125 mm). This sheet is used as a septum in order to create two storage regions in the cavity. This is compulsory because the TE111 mode exhibits two regions with opposite directions of the magnetic field. The measured frequency of the cavity at room temperature is 1420.610 MHz. Therefore in order to obtain the resonant frequency in vacuum, the working temperature of the maser should be around 40 °C. The frequency of the cavity is tuned by using a varactor diode, which allows a tuning range of 60 kHz. The loaded quality factor of the cavity with the teflon sheet is 13600. The cavity is surrounded by a thermal screen, a solenoid, three magnetic shieldings and a vacuum bell. Four ovens are used for the temperature control of the maser and a temperature stability of 10−4 K is expected.
[1] D. Kleppner, H. C. Berg, S. B. Crampton, N. F. Ramsey, R. F. C. Vessot, H. E. Peters, and J. Vanier, Phys. Rev. A 138, 972 (1965).
[2] J. D. Jackson, Classical electrodynamics, Wiley, New York, 1962.
Speaker: Ms Emeline VAN DER BEKEN (Université de Liège) -
71
Drift waves in the solar corona
The heating of the solar corona is still an unanswered puzzle for solar physicists. Magnetic reconnection and wave heating models relying on MHD simulations are generally invoked to explain the coronal heating, which ignore some of the physics that occurs due to the interaction between ions and electrons. In the present work we turn to drift waves as a new candidate for the plasma heating mechanism in the solar atmosphere. We employ a two-fluid model (COOLFluiD) to the simulate the solar corona environment and provide a driving mechanism for the drift waves, i.e. a density gradient perpendicular to the ambient magnetic field. The model includes ions and electrons as separate species. We explore the effect that the scale of the density inhomogeneity presents in driving the waves. Most importantly we are interested in the heating effect of the plasma that the drift waves generate, the growth rate of the density modulation, and the transport effects that occur in the plasma due to the drift wave instability.
Speaker: Nataly OZAK (KU Leuven) -
72
Exploring Models of Solar Dark Matter with PINGU Events
While attempting to identify the particle content of dark matter, one often assumes that Dark Matter is a particle arising from a supersymmetric theory. When a population thereof becomes gravitationally bound in the Sun, mutual annihilations can produce a flux of neutrinos. By estimating how this flux would be recognised by a neutrino telescope one can then make conclusions about whether the proposed model of supersymmetry will be possible to exclude or discover.
I describe studies which include PINGU, a future extension of the neutrino telescope IceCube, in such an analysis. These efforts are based on a likelihood method first presented by P. Scott et al., JCAP11 (2012) 057. The interest in PINGU stems in one regard from its decreased energy threshold, which opens up lower WIMP masses for discovery or rejection than in IceCube. On the other hand, the technical advancements made with PINGU since the previous studies of this type may also be exploited. Based on a relatively event selection, PINGU is characterised by its signal detection efficiency, including systematic uncertainties, energy resolution, and angular resolution in a fast, parametric fashion.
This method has been tested both on phenomenological scans, extracting sensitivity curves, and on importance-sampled scans of the NMSSM (Next-to-Minimimal Supersymmetric extension of the Standard Model), including astrophysical uncertainties. I show these first results, which we are preparing for publication.
Speakers: Prof. Carlos PÉREZ DE LOS HEROS (Uppsala Universitet), Christoph RAAB (Université Libre de Bruxelles) -
73
Fabrication and characterization of a single cluster transistor
Small atomic objects such as molecules and atomic clusters play a key role in bottom up approaches to tailor properties of matter and constitute fundamental building blocks for the synthesis of new advanced materials. Due to quantum confinement effects, small clusters with a countable number of atoms show entirely new physical phenomena without equivalent in bulk materials [1]. The strong size dependence of the properties of atomic clusters opens up a so-called third dimension for all elements of the periodic table [2]. This represents a tremendous space of exploration with important interdisciplinary potential. For clusters study we propose a novel fabrication method to create the first SET based on a single cluster with a countable and controllable number of atoms (Fig. 1). The main motivation to create such device is to investigate intrinsic properties of size selected clusters, in particular to extract their entire discrete electronic energy level spectrum. For the purpose of SET tailoring the electromigration technique was improved in means of controllability and reproducibility. The developed technique was also successfully applied to study the superconducting properties of nanosized aluminium junctions [3].
[1] J. A. Alonso, Structure and Properties of Atomic Nanoclusters. Imperial College Press, 2005. [2] A. W. Castleman and S. N. Khanna, “Clusters, Superatoms, and Building Blocks of New Materials.,” J. Phys. Chem. C, vol. 113, no. 7, pp. 2664–2675, Feb. 2009. [3] X. D. A. Baumans, D. Cerbu, O.-A. Adami, V. S. Zharinov, N. Verellen, G. Papari, J. E. Scheerder, G. Zhang, V. V Moshchalkov, A. V Silhanek, and J. Van de Vondel, “Thermal and quantum depletion of superconductivity in narrow junctions created by controlled electromigration.,” Nat. Commun., vol. 7, p. 10560, Jan. 2016.
Speakers: Prof. Alajandro SILHANEK (Département de Physique, Université de Liège, B-4000 Sart-Tilman, Belgium), Prof. JORIS VAN DE VONDEL (INPAC - Institute for Nanoscale Physics and Chemistry, Department of Physics and Astronomy, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium), Dr Thomas PICOT (KU Leuven), Prof. Victor MOSHCHALKOV (INPAC - Institute for Nanoscale Physics and Chemistry, Department of Physics and Astronomy, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium), Mr Vyacheslav ZHARINOV (INPAC - Institute for Nanoscale Physics and Chemistry, Department of Physics and Astronomy, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium), Mr Xavier BAUMANS (Département de Physique, Université de Liège, B-4000 Sart-Tilman, Belgium) -
74
Far-infrared and dust properties of present-day galaxies in the EAGLE simulations
The EAGLE cosmological simulations reproduce the observed galaxy stellar mass function and many galaxy properties to unprecedented levels of agreement. In this work, we study the dust-related properties of present-day EAGLE galaxies through mock observations in the far-infrared and submm wavelength range obtained with the 3D dust radiative transfer code SKIRT. Specifically, we select a set of redshift-zero EAGLE galaxies that matches the K-band luminosity distribution of the galaxies in the Herschel Reference Survey (HRS), a volume-limited sample of about 300 normal galaxies in the Local Universe. We find overall agreement of the EAGLE dust scaling relations with those observed in the HRS, and we investigate the effect of adjusting certain assumptions in our post-processing procedure. As a result, we are able to constrain the important dust-related parameters in our method, informing the calculation of dust attenuation for EAGLE galaxies in the UV and optical domain.
Speaker: Mr Peter CAMPS (UGent) -
75
Frequency combs via plasmonic resonances in time-dependent graphene lattices
Recently, frequency comb generation using time modulated planar graphene sheets was introduced by Ginis et al. Here we show that this process is more e?fficient in a graphene ribbon lattice than in a planar structure. To do so we exploit the plasmonic resonances of the lattice, which are very sensitive to the graphene doping level. By dynamically changing this doping the transmission becomes time-dependent and allows for eff?ective frequency comb generation in the infrared range.
Speaker: Mr Galaad ALTARES MENENDEZ (UMons) -
76
From BEC polaron to BCS polaron
Ever since the polaron concept was introduced by Landau in 1933 to describe the quasiparticle arising from the interaction between an electron and the polarization cloud it drags along while moving in a polar crystal, a wide variety of physical systems have been mapped on the polaron problem. Among these realizations, one that has been the focus of much attention in the recent years is the BEC polaron: a quasiparticle arising from the interaction of an impurity with the Bogoliubov excitations of a Bose-Einstein condensate. In this presentation we consider the interaction of a single impurity atom with the collective excitations of a fermionic superfluid by mapping it on the same Hamiltonian used in the BEC polaron case. This ansatz is in principle valid only in the extreme BEC side of the Feshbach resonance where the Fermi superfluid becomes in fact a molecular BEC. In the framework of a recently developed effective field theory [1] this molecular condensate is described by a macroscopic wavefunction. The description in terms of a macroscopic wavefunction remains valid also when moving away from the BEC limit and towards unitarity, provided the coefficients of the field equation are properly adapted. This allows to study how the properties of the BEC polaron change when the underlying condensate no longer consists of pointlike bosons, but of Cooper pairs. The polaron problem is then studied in the weak coupling limit by employing the well known T=0 perturbative treatment and the behavior of effective mass and polaronic coupling constant is examined as function of the impurity-boson interaction and of the fermion-fermion interaction in the underlying superfluid [2].
[1] S. N. Klimin, J. Tempere, G. Lombardi, J. T. Devreese, Eur. Phys. J. B 88, 122 (2015).
[2] G. Lombardi, J. Tempere, arXiv:1604.00776 [cond-mat.quant-gas]
Speaker: Giovanni LOMBARDI (Universiteit Antwerpen) -
77
High-resolution, 3D radiative transfer modeling of nearby DustPedia galaxies
The DustPedia project aims at conducting a definitive study of interstellar dust in the Local Universe, by gathering multi-wavelength imaging data of nearby galaxies, and modelling them with state-of-the-art modeling tools. As part of the DustPedia project, we create 3D models for a representative set of nearby galaxies using the radiative transfer code SKIRT. We simultaneously derive the 3D distribution and spectral properties of the stellar populations and the interstellar dust in each galaxy, by fitting radiative transfer models directly to imaging data from UV to submm wavelengths. We fully take into account the effects of absorption, multiple scattering and thermal re-emission by the dust in our models.
We present preliminary modelling results on two well-known face-on spiral galaxies, M51 and M81, and focus on the dust heating mechanisms in these galaxies. By using radiative transfer, we take into account the effects of non-local heating, as opposed to for example pixel-by-pixel modelling techniques. We exploit our knowledge of the internal radiation field to investigate the contribution of the evolved and young stellar populations to the heating of the dust at every position in the galaxy. Our results indicate that young stellar populations are not always the dominant heating agent, and that the contribution of evolved population is important as well.
Speaker: Mr Sam VERSTOCKEN (UGEnt) -
78
ICRH on the stellarator Wendelstein 7-X: Challenges in the Construction of the system, Physics and Applications
The main objective of the stellarator Wendelstein 7-X (W7-X) is to show the potential of optimized stellarators. The main heating system is ECRH up to 10 MW steady-state at 140 GHz. NBI is also foreseen in pulses up to 10 s. An important aim of W7-X is to demonstrate fast ion confinement at volume averaged beta values up to 5%, corresponding to plasma densities above 1020 m-3. Mimicking the behaviour of alpha particles in a future stellarator requires the presence of energetic ions in the core of W7-X plasmas with energies in the range 50-100 keV. Given the expected high plasma densities, this is a challenging task. It can however be done using Ion Cyclotron Resonance Heating (ICRH) and currently an ICRH system is being constructed for W7-X, aiming at delivering RF power levels up to ~1.5 MW in the frequency range 25-38 MHz with pulse lengths up to 10 s using a two strap antenna. To ensure optimal coupling in the various magnetic configurations of W7-X the antenna surface is carefully matched to the standard magnetic configuration of W7-X, resulting in a complex 3D shape of the antenna requiring state of the art CNC machining equipment for its fabrication. In addition the antenna can be radially moved (over 35cm) and a gas puffing system is implemented to improve local coupling whenever needed. The system is intended to be ready in the operational phase 1.2b of W7-X.
Speakers: André MESSIAEN (Laboratory for Plasma Physics, Ecole Royale Militaire-Koninklijke Militaire School, 1000 Brussels, Belgium, TEC Partner), Dirk VAN EESTER (LPP-ERM/KMS), Fabrice LOUCHE (LPP-ERM/KMS), Jozef Ongena (ERM-KMS, Plasma Physics Lab), Mr Michel VERVIER (ERM-KMS), Dr Yevgen KAZAKOV (ERM-KMS) -
79
Identification of b-jets at CMS
In high-energy elementary particle physics many searches for new physics phenomena, as well as precision measurements in the top quark and Higgs boson sector, rely on the identification of jets originating from b-quarks. This process is also known as b-tagging. The hadrons produced in the decay of b-quarks have characteristics which, within the framework of a detector such as the CMS-detector, can be used to differentiate b-originated jets from other jets. The CMS collaboration has developed various b-tagging algorithms, using multivariate techniques among others, exploiting these characteristics in order to optimize b-tagging. Using 13 TeV proton-proton collision data from CMS, the b-tagging performance has been measured.
Speaker: Mr Kevin DEROOVER (VUB) -
80
Identification of c-quark jets at the CMS experiment
Accurate identification of jets originating from b quarks has been of prime importance for many measurements and searches in CMS. The development of a charm tagger, a tool to identify jets initiated by charm quarks, will be no different, allowing analysts to broaden the spectra of their research. We detail the technique that was used to train an MVA–based discriminator to tag charm jets, its expected performance on simulations, and finally its calibration on top quark pairs. The datasets used for the calibration of the algorithm have been selected starting from the proton–proton collisions at the center–of–mass energy of √s= 13 TeV recorded by the CMS experiment during the start of LHC Run II in 2015.
Speaker: Mr Seth MOORTGAT (Vrije Universiteit Brussel (IIHE)) -
81
Impedimetric and thermal detection of the peanut allergen Ara h1
About 1% of the world population is affected by an allergy for peanuts which is the most common cause of fatal-food-related anaphylaxis. Due to the high risk of exposure and the fact that doses of a few milligrams can cause such an allergic reaction, a lot of research has been done towards the detection of these immunogenic proteins. The protein Ara h1 was identified as the culprit in 95% of all allergic reactions to peanuts. To date the assays to detect the Ara h1 allergen rely on the ELISA assay, lateral flow assays and mass spectroscopy all of which are neither cheap nor fast. Aptamers, which are single DNA or RNA oligonucleotides, offer a cheaper and more stable receptor element. Amino (NH2)-terminated Ara h 1 aptamers were covalently attached to carboxylated gold surfaces and to nanocrystalline diamond with hydrogen termination using the chemical EDC coupling route. Subsequently, the functionalized surfaces were used in a setup that combines both electrochemical impedance spectroscopy and heat transfer measurements. The sensor surfaces were placed onto a copper lid which serves as a heat provider. Such an assembly was mounted onto a transparent Perspex flow cell with an inner volume of 110 ml, sealed with a miniature O-ring and fixed with screws. Two miniature thermocouples were placed at the copper backside and at 1.7 mm above the surface of the substrate in order to monitor the temperatures of the copper, T1, and of the fluid, T2, respectively. The heat flow was generated with a power resistor with a fixed resistance. For impedance spectroscopy measurements, gold electrodes served as counter electrode within a range of 100 Hz to 100 kHz. Liquids can be exchanged with a syringe-driven flow system. In this work, the aptamer functionalized surfaces were exposed to increasing amounts of Ara h1 in order to acquire response curves. Non-specific binding of the various contaminants, that a sample originating from food might bring along, are avoided by orienting the sensor face down to avoid sediments on the sensor while BSA washing is used to block any non-specific binding sites. Electrochemical impedance spectroscopy and the heat transfer method can both be used to detect Ara h1 concentrations in liquids downward to a detection limit of 3 nM. This allows the detection of Ara h1 in a dilution of 50 mg peanut butter in a 20.000 times larger volume of buffer solution.
References
[1] M. Peeters et al. Journal of Biosensors and Bioelectronics 5 (3), 155 (2014). [2] M. Peeters et al,. ACS AMI Mater. Interf. 2015, 7: 16-23.
Acknowledgements FWO project “Diamond-based impedimetric and nanophotonic biosensors for the detetection of proteins” (G.0997.11N)
Speaker: Mr Wouter STILMAN (Hasselt University and Department of soft matter physics and Biophysics KU Leuven) -
82
IShTAR: An international facility for studying the interactions between ICRF waves and plasma
IShTAR (Ion Sheath Test ARrangement), a facility located at the Max-Planck Institut für Plasmaphysik in Garching (IPP-MPG Germany), is dedicated to the investigation of the interactions between an ICRF antenna and a plasma [1, 2]. It is an international collaboration with three main partners: University of Lorraine (France), Ghent University (Belgium), and IPP-Garching and other universities. In contrast to a tokamak, a dedicated test stand provides more experimental time and freedom to impose the parameters, gives better access for the instrumentation and antennas and can be set up to have a simple geometry more easily amenable to comparison with theory. The main aim of the project is indeed to provide the experimental data on RF sheaths in order to validate and improve theoretical predictions. The operating conditions are representative of the plasma edge of a magnetic confinement fusion machine. The test bed is composed of magnetised (Bmax = 0.24 T) plasma column of about 0.4 m diameter in a cylindrical vacuum vessel (1 m diameter, 1.1 m length). The plasma column is created by the expansion of an external cylindrical magnetized plasma source (0.4 m diameter, 1 m length) generated by a helical antenna that has been designed to excite the m = 1 helicon mode. An optimisation of the plasma source in order to get the highest density and most radially uniform plasma is presented. A single strap RF antenna has been designed, to excite an RF sheath, the plasma-facing surface of which is aligned to the cylindrical plasma to ease the modelling [3]. The plasma parameters it faces will be varied in the ranges relevant for a tokamak edge plasma. The installed and planned diagnostics are designed to measure the plasma wave interactions. With a 2D probe array and probes mounted on a moving manipulator [4], we can measure simultaneously the density and the potential on magnetic field lines in the neighbourhood of the antenna. With this information also directly in front of the RF antenna, a detailed study of the density modifications induced by RF sheaths can be made. Measuring the electric field directly inside the sheath will require the development of specific diagnostic methods, different options are looked into [5, 6, 7].
[1] R. D’Inca et al, AIP Conf. Proc. 1689, 050010 (2015) [2] K. Crombe et al, AIP Conf. Proc. 1689, 030006 (2015) [3] F. Louche et al, AIP Conf. Proc. 1689, 070016 (2015) [4] E. Faudot et al, Review of Scientific Instruments 86, 063502 (2015) [5] L. Chérigier-Kovacic et al, Review of Scientific Instruments 86, 063504 (2015) [6] E. Martin et al, AIP Conf. Proc. 1689, 030011 (2015) [7] U. Czarnetzky et al, Phys. Rev. Lett. 81, 4592 (1998)
Speaker: Dr Kristel CROMBE (UGent) -
83
LiGa5O8:Cr3+ as NIR Persistent Phosphor for In Vivo Imaging
The use of infrared emitting persistent phosphors for medical imaging is an exciting application in the field of persistent luminescence [1]. Current research focuses on host materials doped with Mn2+ or Cr3+, as these transition metals show emission at wavelengths larger than 600 nm, which falls in the tissue transparency window [2]. In this work, we focus on Cr-doped LiGa5O8 (LGO), which is one of the most promising dopant-host combinations for near infrared emission [3].
LGO has an inverse spinel crystal structure, where half of the Ga3+ cations in the host occupy tetrahedral lattice sites and the other cations occupy octahedral sites. Owing to this inverse structure, LGO has a large amount of intrinsic defects, making the host highly suitable as persistent phosphor candidate. Cr dopants substitute for Ga in octahedral sites, leading to NIR emission from the 4T2 and 2E states, after excitation with UV or VIS light. Persistent luminescence is induced by UV excitation and lasts for about 20 min, before the afterglow intensity drops below 0.01 mW cm-2 sr 1. Partial substitution of Ga in the lattice with Ge4+ cations induces additional lattice defects that enhance the persistent luminescence and increases the afterglow time of the Cr3+ emission.
[1] Q. le Masne de Chermont et al., PNAS 104, 9266 (2007).
[2] Y. Zhuang et al., Opt. Mater. 36, 1907 (2014).
[3] F. Liu et al., Sci. Rep. 3, 1554 (2013).
Speaker: Mr Olivier Q DE CLERCQ (Lumilab, Dept. Solid State Sciences, Ghent University, Ghent, Belgium) -
84
Lithiation Mechanism Study of Si/Ti4Ni4Si7 (STN) alloy
Lithium-ion batteries are considered as power source for electric vehicles (EV), as well as off-grid energy storage for power plant. These specific applications drive the research on high energy density, cost-effective, safe and environmental friendly battery materials. Silicon, the earth crust’s 3rd abundant material, shows very promising properties for use in battery anodes, such as the highest known Li-alloy capacity, safety, or environmental friendliness. Yet, its lithiation mechanism remains unclear. One reason is the electrode’s amorphous structure indcuced by Li insertion. Also, there are a series of side reactions which happen at the surface forming the solid electrolyte interface (SEI). We consider Atom Probe Tomography (APT) [1] as a powerful tool to understand the phase changes at anatomic level. Aided by Focus Ion Beam (FIB), we can address the element distribution in any local structure. On the other hand, Hard X-Ray Photoelectron Spectroscopy (HAXPES) is adopted for a detailed SEI study. The employed high energy X-ray beams can probe deeper information of core level photoemission. In this study, the silicon alloy Si/Ti4Ni4Si7 (STN) has been studied [2] for both, its phase change and SEI composition during lithiation.
[1] Diercks et al., Microscopy of Chemical and Mechanical Heterogeneities in Lithium Cobalt Oxide, Microscopy and Microanalysis 21 (2015), 523.
[2] Son et al. “A Highly Reversible Nano Si Anode Enabled by Mechanical Confinement in an Electrochemically Activated LiXTi4Ni4Si7 Matrix.” Advanced Energy Materials 2 (2012), 1226.
Speaker: Yueming ZHENG (Hasselt University) -
85
Measurement of the top quark pair production cross section using dilepton events at 5 TeV
At hadron colliders, top quarks - the heaviest elementary particles known- are dominantly produced in pairs (ttbar), a production mechanism having been discovered more than twenty years ago at Tevatron, Fermilab. Although the ttbar process has already entered the domain of 'precision' Physics, especially with the advent of the multi-TeV energies at the CERN Large Hadron Collider, there still remains room for dedicated experimental measurements of the top quark in the 'gap' of energies between Tevatron and LHC. Hence, the study presented here measures the total inclusive cross-section for pair production at a center-of-mass energy of 5 TeV having utilized a low-pileup proton-proton (pp) collision dataset whose recorded luminosity by CMS amounts to 26 pb-1. This work has considered only leptonic W-boson decays, the latter characterized by the presence of an energetic opposite-sign lepton pair plus missing energy. The current measurement can be used for stronger constraints to the poorly known gluon distribution (PDF) inside the proton at large longitudinal parton momentum fraction, while it paves the way for the very first observation of this elementary particle in nucleus-nucleus (AA) collisions.
Speaker: Mr Georgios Konstantinos KRINTIRAS -
86
Merged-Beam Study of Mutual Neutralization of $Li^+ + D^-$
Modelling of stellar atmospheres requires various detailed and accurate data on different processes such as the mutual neutralization (MN) of cation-anion pairs that can affect atomic species of interest. Indeed, neutralization reactions play an important role in atmospheric and astrophysical processes. Furthermore, there is a strong demand from the astrochemical community for information about the low-energy cross section of processes involving H− (or D−).
Our merged-beam setup [1] was modified in order to be able to study the mutual neutralization of 7Li++2D− ⟶ Li∗(nl)+D(1s) and determine which states of the lithium neutral atom are predominant in the total cross section. Preliminary measurements at ∼ 7 meV have been performed and compared with theoretical calculations [2]. Our apparatus gives access to the branching ratio among accessible neutral channels of the lithium atom and could discriminate the Li(3p) and Li(3d) channels separated by only 44 meV.
The measured total cross section of this study agrees with the results of Croft et al. However, our measured branching ratio suggests higher proportions of the 3d (by approximately 6%) and 3s (by approximately 5%) channels at the expense of Li(3p).
Further experiments at higher collision energies are still needed to completely compare our results with the experimental study of Pear and Hayton [3] which was restricted to higher collision energies in the range of 0.7 to 316 eV or with theoretical calculations.
References
[1] Nkambule, S. M., Elander, N., Larson, Å., Lecointre, J. and Urbain, X., Differential and total cross sections of mutual neutralization in low-energy collisions of isotopes of H++H−, Phys. Rev. A 93, 032701 (2016).
[2] Croft, H., Dickinson, A. S. and Gadéa, F. X., A theoretical study of mutual neutralization in Li+H collisions. J. Phys. B: At. Mol. Opt. Phys. 32, 81 (1999).
[3] Peart, B. and Hayton, D. A., Merged beam measurements of the mutual neutralization of He+/H− and Li+/D− ions. J. Phys. B: At. Mol. Opt. Phys. 27, 2551 (1994).
Speaker: Thibaut LAUNOY (Université Libre de Bruxelles) -
87
Modeling exchange bias with MuMax3
Since the discovery of the exchange bias phenomenon, which is a shift in the hysteresis loop when coupling a ferromagnet (FM) to an antiferromagnet (AFM), a lot of scientific efforts were made in order to explain this effect. Although exchange bias is widely used in spin valves, it’s origin is not yet completely understood and sometimes obscured by the disorder in the antiferromagnetic grains and the various mechanisms that contribute to this effect. This makes it difficult to study these systems, using only theoretical models. Here we show that this complex interaction can be modeled within the standard micromagnetic software MuMax3[1].
Apart from a shift of the hysteresis loop, also a training effect in several polycrystalline FM/AFM bilayers can be found, i.e. the bias field and coercivity of the system decrease for an increasing number of hysteresis cycles n. This effect can be subdivided into 2 types: a thermal and an athermal component. The latter contains the largest contribution and is only present for n=1.
A common way to model these 2 effects is by considering the presence of pinned and rotatable uncompensated AFM spins at the FM/AFM interface. We are able to simulate these 2 effects in a polycrystalline Co(30nm)/CoO(3nm) bilayer taking into account the local granular structure of the AFM. We can also proof that the athermal component of the training effect can be attributed to the reorientation of rotatable uncompensated AFM spins during the first hysteresis loop. This leads to a reduction of the coercivity and as well as the bias field, as has been confirmed by experimental data[2].
[1] Arne Vansteenkiste, Jonathan Leliaert, Mykola Dvornik, Mathias Helsen, Felipe Garcia-Sanchez, and Bartel Van Waeyenberge. The design and verification of mumax3. AIP Advances, 4(10), 2014.
[2] T. Dias, E. Menéndez, H. Liu, C. Van Haesendonck, A. Vantomme, K. Temst, J. E. Schmidt, R. Giulian, and J. Geshev. Rotatable anisotropy driven training effects in exchange biased co/coo films. Journal of Applied Physics, 115(24), 2014.
Speakers: Prof. Bartel VAN WAEYENBERGE (Ghent University), Mr Jonas DE CLERCQ (Ghent University) -
88
Modeling neutrino-nucleus scatterings: From very low energies to the quasielastic peak
In recent years, there has been substantial development in accelerator-based neutrino-oscillation experiments. The quest for more precise measurements of the neutrino mass-squared differences and mixing angles in these experiments, faces a number of challenges. These are related to the large systematic uncertainties associated with the basic underlying neutrino-nucleus signal in the detector. Major issues arise from the fact that the neutrino energy-flux in experiments is distributed over a wide range of energies from very low to a few GeV. Hence a number of nuclear effects over a broad kinematical range (from low-energy nuclear excitations to multinucleon emission) simultaneously come into play. The simulation codes used in the analysis of the experimental results are predominantly based on relativistic Fermi gas (RFG) models. RFG can describe the quasielastic (QE) cross section sufficiently accurate for medium momentum (q ≈ 500 MeV/c) transfer reactions, but its description becomes poor for low momentum (q < 300 MeV/c) transfer processes, where nuclear effects are prominent. For the broad neutrino energy-flux used in the experiments, more realistic nuclear models are required.
We present a self-consistent continuum random phase approximation (CRPA) approach to inclusive quasielastic neutrino-nucleus scattering. The description of the nucleus starts from a mean field (MF) potential, where long-range correlations are added by means of a continuum random phase approximation (CRPA) based on a Green's function approach using an effective Skyrme interaction as residual interaction. We validate our formalism by confronting our cross-section predictions with high-precision inclusive electron-scattering data for a variety of nuclear targets (12C, 16O, 40Ca), in the kinematic region where quasielastic scattering is expected to be the dominant process. We examine the separate longitudinal and transverse contributions to 12C (e,e′) and compare them with the available data. We report on cross sections calculations for charged-current quasielastic (anti)neutrino scattering off 12C in the energy range of interest for the MiniBooNE and T2K experiments. We compare our results with the MiniBooNE (neutrino and antineutrino) and T2K (neutrino) cross-section measurements. The CRPA predictions reproduce the gross features of the measured cross sections. We pay special attention to the low-energy excitations which appear to account for the non-negligible contributions in the signal of MiniBooNE, T2K and other similar experiments, and require a microscopic nuclear investigation beyond the Fermi gas model.
Speaker: Dr Vishvas PANDEY (UGent) -
89
Modelling of plasma screening effects in relativistic multiconfiguration Dirac-Fock calculations. Application to the atomic structure and inner-shell transitions of some iron ions
X-ray emission lines from accreting sources, most notably the K_alpha- and K_beta-lines from iron ions, have observed widths and shifts which imply an origin very close to the compact object in many cases [1]. The inferred line origin can be near either the innermost stable circular orbit or the event horizon in the case of a black hole. The intensity of these lines can provide insight into the amount of gas and other properties, including the effects of special and general relativity in the emitting region, and this information is not available from other observational techniques.
Much of what we can learn from these K-emission lines depends on the use of reliable atomic parameters that allow us to infer the rate at which ions emit or absorb in line transitions under various conditions. In the case of iron ions, for example, these atomic parameters allow us to derive the number of iron ions responsible for line emission observed from different objects, which in turn gives information on the fractional abundance of iron relative to other elements (i.e. hydrogen). However, the rates and assumptions employed in these model calculations are all based on isolated iron ions. They do not account for the true situation which is a dense plasma in which the effects of nearby ions and electrons can have significant effects on the processes affecting line emission and the survival or destruction of iron ions. Although dynamical models for black hole accretion flows appear to support the existence of rather high densities, up to 1E20 – 1E21 cm-3 [2,3], their effect on line emission has not been explored so far.
The main goal of the present work is to estimate the effects of plasma environement on the atomic parameters associated with the K-vacancy states in highly charged iron ions. In order to do this, multiconfiguration Dirac-Fock computations have been carried out for these ions by considering a time averaged Debye-Hückel potential for both the electron-nucleus and electron-electron interactions using a combination of the GRASP92 code [4] for obtaining the wavefunctions and the RATIP code [5] for computing the atomic parameters. A first set of results related to the ionization potentials, the K-thresholds, the transition energies and the radiative emission rates for some iron ions will be presented during the conference.
C.S. Reynolds and M.A. Nowak, Phys. Rep. 377, 389 (2003). R.C. Reis and J.M. Miller, Astrophys. J. 769, L7 (2013). J.D. Schnittman, J.H. Krolik and S.C. Noble, Astrophys. J. 769, 156 (2013). F.A. Parpia, C. Froese Fischer and I.P. Grant, Comput. Phys. Commun. 94, 249 (1996). S. Fritzsche, Comput. Phys. Commun. 183, 1523 (2012).Speakers: Mr Jérôme DEPRINCE (Université de Mons), Dr Pascal QUINET (Université de Mons & Université de Liège) -
90
Monte Carlo Simulations of Atomic Layer Deposition on 3D large surface area structures
Atomic Layer Deposition (ALD) is a technique for the deposition of uniform thin films with a thickness control on the atomic scale. Due to the self-limited nature of the surface reactions, it is possible to grow uniform thin films with an excellent conformality. Therefore ALD has become a key method for coating and functionalizing 3D large surface area structures such as anodized alumina (AAO), silicon pillars, nanowires and carbon nanotubes. Those structures are of high interest for different applications in electronic devices such as fuel cells, batteries, dye-sensitized solar cells and photocatalytic surfaces. These large surface area substrates often consist of arrays of quasi-1D holes (into which the precursor gas needs to penetrate, e.g. for AAO), or ‘forests’ of pillars (where the precursor gas can reach the surface through the empty 3D space surrounding the pillars). Using a full 3D Monte Carlo model, we compared deposition onto an array of holes versus a forest of pillars. During a simulated TMA cycle, we calculated the TMA exposure dose that is required to achieve ~90% coverage as a function of the height H and the center-to-center distance D for both hole-type (Fig. 1, top) and pillar-type features (Fig. 1, bottom). The required precursor exposure is indicated as a logarithmic color scale. The geometry is expressed in dimensionless numbers H/d and D/d, by taking the ratio of the height H and array spacing D to the diameter d of the hole/pillar. The color plots clearly indicate that the required exposure for an array of holes is determined by the aspect ratio H/d of the individual holes, and is independent of their spacing D in the array (as expected). For the pillars, the required exposure dose increases with decreasing inter-pillar spacing D and converges in the limit of very small D to the exposure of an array of holes (as the remaining space in between nearly touching pillars starts to act as 1D hole-like structures). The black lines in Fig. 1 indicate structures that share the same surface area enhancement factor (SAE). When targeting a specific SAE (e.g. 20x), the pillar geometry is clearly much more ALD friendly, in particular for tall features (large H/d value, e.g. 80) that can then be spaced reasonably far from each other (e.g. D/d ~ 4). Fig. 2 shows a cross section through the color plots in Fig. 1 at D/d = 3, and clearly illustrates that ALD of pillar-type arrays requires 10-30x less exposure dose as compared to hole-type arrays.
Speakers: Christophe DETAVERNIER (Ghent University), Filip GEENEN (UGent), Dr Jolien DENDOOVEN (UGent), Véronique CREMERS (Ghent University) -
91
Mutual Neutralization Studies at Subthermal Collision Energies
Anions play important roles in the chemistry of various astrophysical environments ranging from planetary and stellar atmospheres to interstellar clouds [1-3]. A key reaction for the ionization balance in those media is the Mutual Neutralization (MN) of atomic or molecular anions and cations [3]: A+ + B− → A + B.
MN studies with atomic ions have so far mainly been limited to collision energies down to a few eV, which is higher than the range of energies relevant for cold astrophysical environments. Moreover, these experiments could not resolve the electronic, angular momentum, and spin states of the neutral products [4].
We recently upgraded the merged beam setup in Louvain-la-Neuve to reach 5 meV collision energies, and incorporated three-dimensional momentum imaging using two position sensitive detectors located 3.25 meters downstream from the 7 cm long region where the beams overlap. Besides providing clear coincidence signals, this technique gives unambiguous identification of LS-terms of the products via the measurement of the Kinetic Energy Release (KER). The KER-resolution (50 meV FWHM at 1 eV) is mainly limited by the finite length of the interaction region. Measuring the momentum distribution of both products yields the total and the energy and angular differential cross sections. The absolute cross section scale is defined by the simultaneous measurement of associative ionization A+ + B− → AB+ + e− cross sections by means of a 180∘ bending magnet, a 30∘ electrostatic deflector, and a channeltron detector.
We present absolute MN cross sections for N+ colliding with O− in the 0.005-10 eV energy range and KERspectra at subthermal energies for C+ on P−, C−, Si−, O− and S−. The latter leads to identifications of the capture states (n, L, and S) of the products. While several of the A+ + B− systems that we have studied give results well accounted for by means of simple multi-channel Landau-Zener calculations, there are striking exceptions. For the C+ + O− and C+ + P− systems, there are final states populated that require much stronger couplings to the initial channel than expected.
The present study will also serve to benchmark merged ion beams studies at the double electrostatic storage ring DESIREE now in operation at Stockholm University [13]. With DESIREE it will be possible to study MN between molecular ions with very low internal energies (down to 10 K).
References
[1] D. Smith and P. Spanel, Mass Spectrom. Rev. 14, 255 (1995).
[2] P. Chaizy, H. Rème, J. A. Sauvaud, C. d’Uston, R. P. Lin, D. E. Larson, D. L. Mitchell, K. A. Anderson, C.W. Carlson, A. Korth, and D. A. Mendis, Nature 349, 393 (1991).
[3] M. Larsson, W. D. Geppert, and G. Nyman, Rep. Prog. Phys. 75, 066901 (2012).
[4] M. Terao, S. Szücs, M. Cherkani, F. Brouillard, R. J. Allan, C. Harel, and A. Salin, Europhys. Lett. 1, 123 (1986).
[5] R. D. Thomas, H. T. Schmidt, G. Andler, M. Björkhage, M. Blom, L. Brännholm, E. Bäckström, H. Danared, S. Das, N. Haag, et al., Rev. Sci. Instrum. 82, 065112 (2011).
Speaker: Mr Arnaud DOCHAIN (Université Catholique de Louvain) -
92
Nuclear magnetic relaxation induced by gel-suspended cells labelled by iron-oxide nanoparticles: experimental and simulation studies
Iron-oxide nanoparticles are used as negative contrast agents in Magnetic Resonance Imaging (MRI)1. Their great magnetization induces magnetic inhomogeneities which shorten the relaxation times. Their efficiency can be quantified by their relaxivities, i.e. their relaxation rates normalized by the iron concentration.
Cells loaded by iron oxide nanoparticles are commonly used to track tumor cells in vivo: cells are labelled in vitro and then injected to the animal. MR images allow following these cells in a non-invasive way and should ideally lead to their quantification in vivo. Unfortunately, when particles are internalized in cells, their associated relaxation times decrease – inducing a less efficient contrast on the MR image and making the labelled cells more complicated to interpret or to quantify2. An accurate model predicting the relaxivities associated to cells loaded by iron-oxide nanoparticles would make their quantification feasible.
The decrease of the relaxation times is usually attributed to the nanoparticle agglomeration, which is well known to modify the relaxation rate3. Water exchange seems also to influence the contrast4. However, no complete study has been made to identify the impact of these mechanisms on the relaxation times. In this work, we present experimental and simulation NMR results on samples containing iron-oxide nanoparticle loaded cells suspended in an agarose gel. Nuclear Magnetic Relaxation Dispersion measurements were performed for different iron or cell concentrations. Simulations reproducing the transverse relaxation at high magnetic field and modelling the labelled cells suspended in gel were also performed and are compared to experiments.
Speaker: Quoc Lam VUONG (UMONS) -
93
On the existence of a double S-shaped process curve during reactive magnetron sputtering
Reactive DC magnetron sputtering is a common technique to deposit compound films. Starting from an elemental metal target, the composition on the substrate can be altered by adjusting the reactive gas flow. However, at given sputtering conditions, a transition in operating conditions is noticed at critical reactive gas flow rates inducing the well-known hysteresis effect during reactive sputtering. These inherent instabilities restrict the achievable substrate compositions or decrease the deposition rate substantially. Reactive sputter deposition models simulating the hysteresis effect predict an S-shaped process curve of the discharge voltage and the reactive partial pressure as a function of the reactive gas flow. It can be demonstrated that this S-shape process curve causes the inherent instability under typical operating conditions. Therefore, modelling the S-shaped process curve yields important information on the influence of the operating variables on the hysteresis and on the parameters of interest.
Up till now, the experimentally and simulated S-shaped process curves were assumed to be unique for each specific system. However, we report the existence of a double S-curve during reactive sputtering implying that there are at least two different reactive gas flows corresponding to each reactive gas pressure at constant operating conditions.
Speaker: Mr Roeland SCHELFHOUT (Department of Solid State Sciences, Ghent University) -
94
Out-of-plane optical anisotropy in organic layers
We consider the in-plane anisotropy of thick organic layers obtained by Langmuir-Blodgett technique. As the optical axis of theses Y-type structures lays in the incidence plane, the optical properties can be obtained from standard spectroscopic ellipsometry (SE) measurements [1]. Ordinary (no) and extraordinary (ne) complex refraction indexes are characterized for Cd2+-stabilized arachidic acid films whose thickness ranges from two-layers (2L) up to 200-layers (200L). Both no and ne are appropriately modeled by a Cauchy dispersion law. The Cauchy law parameters as well as the layer thickness are described as a function of the number of layers in the structure (Nlayers). In ellipsometry data analysis, multivariate methods have only been used in a very limited number of cases : Brouwer and coworkers linked the thickness of thin layers of colloidal gold nanoparticles to the principal components [2] and Garcia-Caurel performed hierarchical classification of FTIR-SE responses of bacterial films to classify microorganisms[3]. We recently used support vector machines to analyze the silver nanoparticles resonance peak parameters in plasmonic nanocomposites [4]. In this contribution, we use Complex principal Components Analysis (CPCA) of the complex SE data [5] and compare the results with standard PCA techniques applied to real and imaginary parts of the ellipticity ρ.
[1] R. Azzam and N. Bashara, Ellipsometry and polarized lightNorth-Holland personal library (North-Holland Pub. Co., 1977).
[2] E. M. Brouwer, E. S. Kooij, H. Wormeester, M. A. Hempenius, and B. Poelsema, Journal of Physical Chemistry B: Materials, surfaces, interfaces, & biophysical 108, 7748 (2004).
[3] E. Garcia-Caurel, J. Nguyen, L. Schwartz, and B. Drévillon, Thin Solid Films 455-456, 722 (2004).
[4] C. Guyot and M. Voué, Applied Surface Science (2016), arXiv:1506.01581 [cond-mat.mtrl-sci].
[5] J. Horel, J. Climate 23, 1660 (1984).
Speaker: Corentin GUYOT (UMONS) -
95
Particle-in-cell Monte Carlo collision simulations of ICRF discharge initiation in tokamaks and stellerators
M. Tripsky1,2, T. Wauters1, A. Lyssoivan1
1Laboratory for Plasma Physics-ERM/KMS, 1000 Brussels, Belgium
2Ghent University, Department of Applied Physics, 9000 Ghent, Belgium
The RFdinity1d3v particle-in-cell Monte Carlo collision (PIC-MCC) model is used to study discharges produced and sustained by ion cyclotron range of frequency (ICRF) waves in absence of plasma current for applications for wall conditioning (ICWC, Te=3−5eV, ne<1012cm−3) in superconducting fusion machines, for RF-assisted start-up in tokamaks and for target plasma production (ne=1013cm−3) in stellarators [1]. The model examines the breakdown phase of ICRF discharges, and its dependency on the RF discharge parameters (i) antenna input power Pi, (ii) RF frequency, (iii)~shape of the electric field and (iv) the neutral gas pressure (pH2).
The RFdinity1d3v PIC-MCC model (1D in displacements and 3D in velocity space) follows the motion of both electrons and ions in a narrow bundle of magnetic field lines close to the antenna straps, by the numeric Leapfrog schema. The charged particles are accelerated in the parallel direction with respect to the magnetic field BT by the Lorentz force resulting from the sum of two electric fields: (i) the vacuum RF electric field in front of the ICRF antenna ERFzERFz and (ii)~the electrostatic field EPzEPz determined by the solution of Poisson's equation. The toroidal profile of ERFz for each strap is in the present study approximated by the sum of two Gaussians with opposite sign centered around the two gaps between the strap and the antenna box. Earlier reported work employed ERFz profiles obtained by 3D CST Microwave Studio simulations with the actual design of the ICRF antenna [2]. The Poisson equation is solved for the toroidal charge density ϱ using the Fast Fourier Transform method respecting periodic boundary conditions. The charge density ϱ is obtained by a PIC approach, weighting the charges of each simulated particle on a toroidal cell-grid by b-spline functions of first order. For practical MC simulations of relevant electron densities, each of the simulated super-particles represents a number of real particles (both electrons or ions). In this stage of development, the model applies the Monte Carlo collision schema (MCCS) only on collisions between electrons and neutral hydrogen molecules (e−H2). The model includes 12 collisions grouped into 6 collision types (\textit{ionization, vibrational excitation, excitation, dissociation, dissociative ionization and elastic scattering}) [2]. The collision cross-sections are taken from [3]. We used the standard "null-collision" method applied in the PIC-MCC models with a constant collision frequency $\nu' = max(n_{H_2} \sigma^{e-H_2}T \upsilon)[5],toestimateafractionoftheelectronsthatundergocollisionswithinatimestep\Delta tas P{null}~=~1~-~\exp(- \nu' \Delta t)$ [5].
The most important improvement of the present model compared to our earlier work [2,4] concerns the acceleration of charged particles in the plasma-produced electric potential. The effect of EPz becomes important upon reaching intermediate densities ne≈1011m−3 that are still much lower than threshold densities for wave excitations (nSWe∼1013m−3). In contrast to the antenna vacuum RF field, ERFz which locates in the antenna area (LANT≈0.8m for one strap antenna) and oscillates with fixed shape and amplitude (E0≈0−100kV/m), EPz has a varying shape (resulting from the collective motions of the charged particles) and a varying amplitude (effect of the increasing density of the charged particles in time). Consequently, in the very early stage of discharge production when EPz is small, only a fraction of the electrons can access the area in front of the antenna to be accelerated by ERFz, most of the electrons (low energetic electrons) are reflected by the ponderomotive force at the edge of the antenna box. The amplitude of the EPz is below 1V/m for electron densities up to 1010m−3, and locates in the vicinity of the antenna area. As the magnitude of this potential increases in time, more electrons enter the antenna area for further acceleration by the ERFz. This effect is both visible by a dramatic increase in the electron multiplication rate and by a strong shift in the electron velocity distribution towards Maxwell distribution.
[1] A. Lyssoivan et al., Plasma Phys. Control. Fusion 54, 074014 (2012).
[2] M. Tripsky et al., Proceedings of the 21st Topical Conference 1689, California, USA (2015).
[3] D. Reiter, HYDHEL, Atomic and Molecular Data for EIRENE, Tech. rep., Fz-Juelich GmbH (2002).
[4] M. Tripsky et al., in European Conference Abstracts (ECA), 2014, vol. 38F, Berlin, Germany.
[5] V. Vahedi, M. Surendra, Computer Physics Communications 87 (1995).
Speaker: Mr Matej TRIPSKY (Laboratory for Plasma Physics-ERM/KMS, 1000 Brussels, Belgium) -
96
Phase 2 Upgrade of the CMS Muon System with triple-GEM detectors
The Compact Muon Solenoid (CMS) detector installed at the CERN Large Hadron Collider (LHC) has an extensive muon system which provides information simultaneously for identification, track reconstruction and triggering of muons. The extreme particle rates and high integrated charge expected during the High Luminosity phase of the LHC (HL-LHC) impose the need to upgrade several parts of the CMS detector, in particular also its muon detection system. The CMS GEM collaboration proposes to extend the high-eta region of the muon system with several different stations of Gas Electron Multiplier (GEM) detectors. GEMs have the ability to provide robust and redundant tracking and triggering functions with an excellent spatial resolution of order 100 micron and a high particle rate capability, with a close to 100% detection efficiency. In this contribution, the present status of the CMS GEM project will be reviewed, and the significant achievements from the start of the R&D in 2009 will be emphasized. The design of the triple-GEM detectors proposed for installation in different stations of the CMS muon system will be described along with the envisaged front-end electronics and data-acquisition system. Results from the detector tests will be reported.
Speaker: Ms Sinem SALVA (UGent) -
97
Plasma enhanced Atomic Layer Deposition of zinc sulfide
The increasing world energy demand in combination with the dependency on limited fossil fuels results in a lot of stress on global climate and the geopolitical situation. Most energy scenarios emphasise the importance of innovations not only on the generating side e.g. by renewable energy, but also on the consumer side e.g. by more energy efficient lighting or personal electronics devices. Recently, there has been much interest in zinc sulfide (ZnS) due to its applications as a buffer layer in thin film photovoltaic (PV). Being a II-VI semiconductor with a wide band gap which can be doped n- and p-type, ZnS is also an interesting candidate for transparent conducting films (TCF)s which are needed for light emitting diodes (LEDs). Since both these targeted devices require uniform coatings with precise thickness control, Atomic Layer Deposition (ALD) is an ideal deposition technique. ALD is a self-limited deposition method that is characterized by alternating exposure of the growing film to chemical precursors, resulting in the sequential deposition of (sub)monolayers over the exposed sample surface. The main advantages of ALD are atomic level control over layer thickness, low deposition temperatures and conformal and pin-hole free coating. One of the earliest reported ALD processes was in fact ALD of ZnS from elemental Zn and S.
In this work we report on a plasma enhanced ALD process for ZnS. Thin films were deposited in a home-built pump-type ALD reactor by using diethylzinc (DEZ) in combination with H2S or H2S/Ar-plasma as reactants. The substrates were Si(100) wafers covered with 100 nm thermally grown SiO2 or glass slides. Argon diluted H2S-Plasma was used instead of a pure H2S-Plasma in order to minimize the exposure of the ALD reactor to the highly reactive S radicals. The plasma was generated remotely from the substrate by RF inductive coupling at 200 Watt. The substrate temperature was varied from 60°C to 300°C. Thin film growth rate was monitored in-situ by spectroscopic ellipsometry while the structural and optical properties were characterised ex-situ using X-ray diffraction (XRD), X-ray fluorescence (XRF), X-ray reflectivity (XRR), X-ray photoelectron spectroscopy (XPS) and UV/Vis spectroscopy.
Saturation was observed for both H2S and H2S/Ar-plasma pulse times longer than 3s proofing that it is a true ALD process. Comparing the ZnS film thickness as a function of ALD cycles, it can be seen that the Ar/H2S-plasma enhanced process nucleates slightly earlier than the thermal process. The growth per cycle (GPC) of the thermal ALD process decreases by more than 75% when increasing the deposition temperature from 60°C to 300°C. The GPC of the plasma enhanced ALD process is much less dependent on substrate temperature. This expansion of the ALD temperature window can lead to a better device integration or better matching of temperature windows for ALD of ternary compounds. Both thermal and plasma enhanced ALD leads to crystalline ZnS films with similar XRD patterns, indicating mainly the cubic zinc blende phase.
Speaker: Mr Jakob KUHS (Ghent University, Department of Solid State Sciences, CoCooN Research Group) -
98
Preliminary Design of an ICRF Traveling-wave Comb-line Antenna for Fusion Devices
The preliminary design of an Ion Cyclotron Range of Frequency traveling-wave antenna, based on a comb-line structure in a resonant ring, is presented. The design maximize the coupled power to the inhomogeneous plasma giving an operation band characterized by an almost perfect matching to the generator(s) with a simple feeding system. The antenna system is suitable for the operation in a fusion device.
Speaker: Riccardo RAGONA (Laboratory for Plasma Physics, LPP-ERM/KMS) -
99
Quantum Chromodynamics at Modern High-Energy Facilities
We overview the current and planned experiments in high-energy hadron physics, which are aimed to investigate unresolved issues in our understanding of the intrinsic structure of the strong interacting particles. Special emphasis is put on the spin- and transverse momentum- dependent correlations of the fundamental constituents of the nucleons - quarks and gluons.
Speaker: Igor CHEREDNIKOV (Universiteit Antwerpen) -
100
Robust analysis of trends in noisy data
Detection and quantification of trends of key quantities in terms of a set of ‘predictor’ variables is a common task in fusion data analysis, as in many other areas of science, for model building and experimental planning. In fusion science, the standard way to handle the corresponding regression analysis problem is by means of a linear or power-law regression function and ordinary least squares (OLS) to perform the fit. There are essentially two issues with this common approach that we intend to address in the present work. On the one hand, OLS is a very simple technique that is not suitable in the presence of complex uncertainties on the measured data. Its assumptions can be overly simplifying, e.g. when the measurements originate from multiple diagnostics or experiments, when the predictor variables are affected by considerable uncertainty, or when the data contain outliers. This often leads to erroneous estimates of the regression parameters, which, moreover, greatly depend on the adequateness of the proposed regression function. On the other hand, the measurements used in the regression analysis are often averages over a time window or over multiple occurrences of the phenomenon under study (e.g. certain plasma instabilities). Effectively, this means that potentially valuable information in the data is discarded. Whenever a measured quantity is subject to considerable fluctuation or measurement noise – a common situation in fusion – it can be very beneficial to consider the probability distribution of the quantity instead of its average. Indeed, whereas the average provides a first-order summary of the distribution, the actual distribution contains information about the typical spread of the measurements, symmetry around the mean, frequency of extreme values, etc. We have developed the method of geodesic least squares regression (GLS) that does not depend on the overly simplifying assumptions of OLS, by exploiting the full probability distribution of the regression variables. In the present contribution, the method is applied to regression analysis of plasma energy confinement and energy of edge-localized modes (ELMs). We demonstrate the strongly improved robustness of GLS compared to conventional OLS, with respect to both uncertainty in the data set and in the regression model. We show that this is because GLS performs regression between probability distributions. Finally, we highlight the significant potential and ease of use of GLS for general-purpose analysis of trends in fusion science.
Speaker: Geert VERDOOLAEGE (UGent) -
101
Search for a heavy scalar boson decaying into a pair of Z bosons in the 2l2v final state
I will present a generic search for a heavy scalar boson using 13 TeV proton-proton collision data acquired by the CMS experiment in 2015. Events in the decay channel H->ZZ->2l2v are selected from data corresponding to an integrated luminosity of 2.3 fb^-1. An analysis of the reconstructed transverse mass and missing transverse energy is performed in different event categories, and limits are set on the production cross section of a heavy scalar of variable width in the gluon-fusion and vector-boson fusion production modes. The limits are interpreted as constraints on the production of a heavy scalar in the electroweak singlet model. They are stronger than the Run 1 analysis results in the mass range around 1 TeV and above. Constraints are also set in the tan beta - mH parameter space of specific Type-I and Type-II two-Higgs doublet models.
Speaker: Mr Hugo DELANNOY (on behalf of the CMS collaboration) -
102
Search for dark matter with jets and missing transverse energy at 13 TeV at CMS
Dark matter is necessary to explain multiple astrophysical observations that appear to be the result of the presence of mass which cannot be seen using light or other electromagnetic waves. One way to search for dark matter is through production at colliders, such as the LHC at CERN. A search for new physics has been performed at the CMS experiment using events having large missing transverse momentum and one or more jets with high transverse momenta in a data sample of proton-proton interactions at the centre-of-mass energy of 13 TeV. The data correspond to an integrated luminosity of 2.1 fb−1 collected by the CMS detector at the LHC. Results are presented in terms of limits on dark matter production based on simplified models.
Speakers: CMS COLLABORATION (CERN), Isabelle DE BRUYN (Vrije Universiteit Brussel) -
103
Search for SUSY with multileptons in proton proton collisions in $\sqrt{s}$ = 13 TeV data using CMS detector
A search for new physics is performed using events with multileptons ( ≥ 3 electrons or muons) in the final state using the CMS detector. Results are based on a sample of proton-proton collisions at a centre-of-mass energy of 13 TeV at the LHC corresponding to an integrated luminosity of 2.3 fb−1. Search regions have been defined by the number of b-tagged jets, missing transverse energy, hadronic transverse energy, and the invariant mass of opposite-sign, same-flavor dilepton pairs in the events. No excess above the standard model background expectation is observed.
Speaker: Mr Illia KHVASTUNOV (Gent University) -
104
Shaping Time Impact on 500 mm3 CdZnTe Detector Spectra Quality
In recent years there has been an increased interest in the use of a medium resolution semiconductor CdZnTe (CZT) detector for gamma-ray spectroscopy and in safeguards applications. However, due to the different mobility and lifetime of electrons and holes in the crystal’s sensitive volume, a CZT detector shows an asymmetrical peak characterized by low-energy tailing.
The type and settings of the electronics used to process the signal can have an impact on the observed peak shape, e.g. the slope of low-energy tail and the resolution. In this work, we study impact of the shaping time parameter on the slope of low-energy tail and the resolution using two different types of signal processing instrumentation.
A 500 mm3 CZT detector was tested with different settings of the shaping time parameter on analogue and digital instrumentation. The best setting was chosen upon the impact on the slope of low-energy tail and the resolution. The best results on both types of instrumentation were compared. Digital MCA-527 instrumentation with 1.2 µs shaping time setting showed the lowest slope of low-energy tail and the smallest impact on resolution.
Speaker: Mr Iaroslav MELESHENKOVSKII (SCK-CEN) -
105
Signatures of the phase transition in a topological superconductor coupled to a bath
We study a topological superconductor capable of exchanging particles with an environment. The isolated superconductor is known to undergo a phase transition, which vanishes when the system is coupled to an environment. However, we show how signatures of the phase transition can still be observed, playing an important role in the particle-exchange mechanism.
These results are obtained by modelling the system with an integrable Hamiltonian, building on the class of Richardson-Gaudin models, and the mean-field description of the phase transition is linked with the exact solution of the model in order to obtain an accurate description of this mechanism.
Speaker: Mr Pieter CLAEYS (UGent) -
106
Signatures of Unsteady Asymmetric Magnetic Reconnection at the Magnetopause
With this work we aim at studying with more details the highly multiscale kinetic process of magnetic reconnection occurring at the dayside magnetopause. This process is partially responsible for geomagnetic substorms and capable of producing highly energetic particles. In particular, we present results on the electron dynamics from fully kinetic Particle-in-Cell (PIC) simulations. The ultimate aim is to produce simulation insights for the observations registered by the recently launched Multiscale MagnetoSpheric NASA mission, which features unprecedented space and time resolution instruments.
In a previous work we have studied the electrons behavior during rapid magnetic island coalescence in absence of a guide field. We have ultimately identified three different reconnection regions marked as X-, D- and M-regions according to their local anisotropic and agyrotropic properties (Cazzola et al. 2015).
Here, we extend the analysis to the case with guide field, where the same type of regions are also observed. The electron velocity distributions characterizing these regions are additionally provided for a straightforward identification with observations. Finally, different rendering methods to highlight agyrotropic regions from PIC simulations are addressed, revealing important discrepancies in some relevant areas, such as the separatrices and the inner islands.
Speaker: Mr Emanuele CAZZOLA (Center for mathematical Plasma Astrophysics, Department of Mathematics, KULeuven, Leuven, Belgium) -
107
Simulating dust scattering polarization in spiral galaxies
Radiative transfer simulations that describe the propagation of light from and through astronomical objects are gaining more and more importance when interpreting observational data. One observational signature that has not been fully exploited is polarization, which is mainly due to scattering off electrons and dust grains. Recently, this potential is being realized, and a growing number of radiative transfer codes are capable of calculating and predicting polarimetry from astronomical objects.
We have developed an elegant reference-frame-free description of scattering polarization and implemented it into the Monte Carlo radiative transfer code SKIRT. We have validated the accuracy of our implementation using simple test cases and a detailed comparison to other codes. Using our new implementation, we simulate imaging and polarization maps of a realistic spiral galaxy model, with the aim to investigate the possibility to use polarimetry to detect spiral arms in edge-on galaxies. We find that we can easily identify the signatures of spiral structure using polarization information at 1 micron, complementing the information obtainable from regular imaging data.
Speaker: Mr P. Christian PEEST (Ugent) -
108
Size-dependent penetration of gold nanoparticles through a defect-free NaCl membrane
Membranes and their size-selective filtering properties are universal in nature and their behaviour is exploited to design artificial membranes suited for, e.g., molecule or nanoparticle filtering and separation. Exploring and understanding penetration and transmission mechanisms of nanoparticles in thin film systems may provide new opportunities for size selective deposition or embedding of the nanoparticles.
Here, we demonstrate an unexpected finding that metal nanoparticles can be size-selectively sieved through atomically thin nonporous alkali halide films on a metal support and that this sieving effect can be tuned via the film thickness. Specifically, relying on scanning tunneling microscopy and spectroscopy techniques, combined with density functional theory calculations, we find that defect-free NaCl films on a Au(111) support act as size-selective membranes for deposited Au nanoclusters. The observed sieving ability is found to originate from a driving force towards the metal support and from the dynamics of both the nanoparticles and the alkali halide films.
Speaker: Dr Zhe LI (KU Leuven) -
109
SoLid technology and SM1 prototype
This poster will cover the novel technology used in the SoLid experiment and will discuss the ?first results of the SubModule1 prototype.
Speaker: Ms Celine MOORGAT (Ugent) -
110
Space dosimetry with luminescent detectors
Ionizing radiation exposure represents one of the most important health risk for astronauts in space. Radiation dose rates in space are typically more than two orders of magnitude higher than on Earth. Therefore, it is of primordial importance to monitor the astronauts’ radiation doses. However, space dosimetry is very challenging due to the high complexity of the space radiation field consisting of protons, neutrons, electrons and heavy charged particles (HCPs) with energies from few keV up to TeV. Consequently, it is necessary to perform dose mapping experiments inside spacecrafts and to establish individual dose monitoring protocols for astronauts’ radiation protection. Dose mapping and individual dose monitoring in spacecrafts require passive, small, safe and light detectors which are able to measure doses for the whole range of particles and energies. With thermoluminescent (TL) and optically stimulated luminescent (OSL) detectors it is relatively easy to measure the low linear energy transfer (LET) part of the spectrum. The measurement of the high LET part is done typically using track etch detectors, but the dose assessment process with this kind of detectors is very time consuming and requires specialized equipment and personnel. However, it seems that information on the high LET part of the spectrum can be obtained also with more convenient TL and OSL detectors by careful analysis of the TL glow curve’s or OSL decay curve’s shape. The main goal of my PhD is to thoroughly investigate the possibility of determining total radiation doses in space with TL and OSL detectors only. This is being done by combining irradiations of the detectors in space and at different calibrated HCP accelerator facilities with simulations of the space radiation field and its interaction with the spacecraft and the detectors. As a first step of this work, the thermoluminescent proprieties of LiF:Mg,Ti and LiF:Mg,Cu,P detectors exposed to different radiation qualities were investigated and the influence of the dose assessment method on the final results was evaluated. Furthermore, the first detectors coming back from the International Space Station were analyzed as well.
Speaker: Mr Alessio PARISI (SCK•CEN, Université de Mons - Faculté Polytechnique) -
111
Structural assignment of small silver clusters
Johan van der Tol, Dewei Jia, Yejun Li, Valeriy Chernyy, Joost Bakker, Minh Nguyen, Ewald Janssens
Silver clusters composed of a few atoms are very interesting for photography and redox catalysis. This is mainly because of their size dependent optical properties and a strong interplay between their geometric and electronic structure, which has a discrete density of states.
Despite the wide interest in small silver clusters, there is so far, besides for Ag₃ and Ag₄ [1], no definitive (experimental) assignment of their geometry.
Ion-mobility measurements on cationic silver clusters in the gas phase are available [2], but those measurements only provide cross sections. In addition, optical absorption spectra of Ag₄ to Ag₁₄ have been recorded, mapping their electronic excited states [3].
We have recorded the infrared multiphoton dissociation (IR-MPD) spectra of Ag₃⁺ to Ag₁₂⁺, by using a Free-electron laser in the far infrared (100-200 cm⁻¹ spectroscopic wavenumbers). The silver clusters were produced in a laser vaporization cluster source and tagged with weakly bound argon, with Arₙ up to n=4, which is released after resonant photon absorption.
Comparison of calculated vibrational spectra for different structural isomers by density functional theory with the experimental IR-MPD spectra allows to determine the structures of the silver clusters. Meanwhile, the effect of Ar attachment on the smallest clusters( Ag₂⁺,Ag₃⁺, Ag₄⁺) was measured and calculated.
[1] A. Fielicke et al. J. Phys. Chem. A 110, 8060 (2006)
[2] P. Weis et al. Chem. Phys. Lett. 355, 355(2002)
[3] Harb et al. J. Chem. Phys. 129, 194108 (2008)
Speakers: Mr Johan VAN DER TOL (KU Leuven), Mrs dewei JIA (KU Leuven) -
112
Structure identification of Transition Metal Doped Silicon Clusters
Metal doped silicon clusters have attracted a lot of interest the past decades. Fundamental understanding of small metal doped silicon clusters and their bonding is relevant for nanoscale silicon components in optoelectronic and semiconductor devices. Here, we have explored the geometric, electronic and magnetic properties of cationic doped silicon clusters (Si_nAg^+ (n = 6−15), Si_nAu^+ (n = 2−15), Si_nCo^+ (n = 5−8) and Si_nCo_2^+ (n = 8−12)) using infrared multiple photon dissociation (IR-MPD) and of neutral Si_nCo (n = 10−12) clusters by tunable IR-UV two-color ionization, both in combination with density functional theory computations. Based on the comparison of experimental and calculated IR spectra for the identified low energy isomers, structures are assigned. It is found that the coinage metal dopants (Ag and Cu) prefer adsorption positions with a low coordination number up to n = 15, which is different from dopants with unfilled d shells (V, Mn, and Co). Endohedral caged structures are found for Si_nCo with n = 10−12. In particular, interesting magnetic properties are found for the doubly doped Si_nCo_2^+ (n = 8−12) clusters.
Speaker: Mr Yejun LI (Solid State Physics and Magnetism Section, KULeuven) -
113
Study of the surface stability of the topological insulator Bi2Te3 in different environments using scanning probe microscopy.
Abstract
We present the results of the characterization of the topological insulator (TI) Bi2Te3 in four different environments using scanning probe microscopy (SPM) based techniques. Upon exposure to air at room temperature the cleaved surface of the pristine Bi2Te3 is observed to be strongly modified during scanning tunneling microscopy (STM) measurements. Remarkably, there is no surface alteration observed when probing Bi2Te3 samples in ultrahigh-vacuum by STM, both at low temperature (4.5 K) and at room temperature. STM experiments in an organic solvent environment show that the Bi2Te3 surface at the liquid/solid interface is modified as well, yet an enhanced stability was observed when using solutions containing perylene tetracarboxylic diimide (PDI). Finally, in a nitrogen gas environment (0% relative humidity) the Bi2Te3 surface demonstrated an increased surface stability and only very limited changes during STM scanning when compared to ambient conditions (30% relative humidity). We believe that the reduced surface stability upon exposure to ambient conditions is triggered by a combination of effects that are discussed in our poster. Only a limited number of studies investigate the behavior of the surface of the TI Bi2Te3 under ambient conditions when using SPM techniques 1-3. Controlled surface modification can pave the way towards new possibilities for surface patterning at nanometer scale. Our findings are crucial for further in depth studies of the intrinsic properties of the 3D TI Bi2Te3 and for potential applications that include room temperature TI devices operated under ambient conditions.
References
[1] P. Ngabonziza, R. Heimbuch, N. de Jong, R. A. Klaassen, M. P. Stehno, M. Snelder, A. Solmaz, S. V. Ramankutty, E. Frantzeskakis, E. van Heumen, G. Koster, M. S. Golden, H. J. W. Zandvliet, and A. Brinkman, In-situ spectroscopy of intrinsic Bi2Te3 topological insulator thin films and impact of extrinsic defects, Physical Review B 92, 035405 (2015).
[2] Rita J. Macedo, Sara E. Harrison, Tatiana S. Dorofeeva, James S. Harris, and Richard A. Kiehl, Nanoscale Probing of Local Electrical Characteristics on MBE-Grown Bi2Te3 Surfaces under Ambient Conditions, Nano Letters 15, 4241 (2015).
[3] Guolin Hao, Xiang Qi, Yundan Liu, Zongyu Huang, Hongxing Li, Kai Huang, Jun Li, Liwen Yang, and Jianxin Zhong, Ambipolar charge injection and transport of few-layer topological insulator Bi2Te3 and Bi2Se3 nanoplates, Journal of Applied Physics 111, 114312 (2012).
Speaker: Ms Asteriona-Maria NETSOU (KU LEUVEN) -
114
Super- and subradiance from indistinguishable atoms with quantized motion
We derive and solve a markovian master equation for the internal dynamics of an ensemble of indistinguishable two-level atoms including all effects related to the quantization of their motion [1]. Our equation provides a unifying picture of the consequences of recoil and indistinguishability of atoms beyond the Lamb-Dicke regime on both their dissipative and conservative internal dynamics, and is relevant for experiments with ultracold atoms. We give general expressions for the decay rates and the dipole-dipole shifts for any motional states, and we find analytical formulas for a number of relevant states (Gaussian states, Fock states and thermal states). We show that dipole-dipole interactions and cooperative spontaneous emission can be modulated through the motional state of atoms. As an application, we study the impact of the quantized atomic motion on Dicke super- and subradiance [2]. In particular, we compute the radiated energy rate up to 30 atoms and provide analytical predictions for large number of atoms based on a mean-field approach [3,4].
[1] F. Damanet, D. Braun, and J. Martin, Phys. Rev. A 93, 022124 (2016).
[2] R. H. Dicke, Phys. Rev. 93, 99 (1954).
[3] M. Gross and S. Haroche, Phys. Rep. 93, 301 (1982).
[4] H.-P. Breuer and F. Petruccione, The Theory of Open Quantum Systems (Oxford University Press, 2006).
Speaker: François DAMANET (Institut de Physique Nucléaire, Atomique et de Spectroscopie, Université de Liège, Bât. B15, B - 4000 Liège, Belgium) -
115
Systematics in the luminescence of ns$^2$ ions
Luminescent materials or phosphors play an important role in many everyday applications such as lighting and displays. These phosphors consist of a host compound doped with luminescent ions called activators. Almost all the commercial phosphors used today are activated by lanthanide ions[1]. However, a few years ago the rare-earth market was struck by a crisis when China, the leading exporter of rare-earth oxides, imposed export quotas. This led to a renewed interest into possible alternatives for the lanthanide-based phosphors. The purpose of this work is to investigate some of these possible alternatives: ions featuring an ns2 electronic ground state such as Pb2+, Sn2+, Bi3+ and Sb3+.
Phosphors which are used for different applications have to meet different requirements [2]: while conversion phosphors used in fluorescent lamps are excited by ultraviolet line emission, those used in light-emitting diodes (LEDs) are often excited by near ultraviolet or blue band emission. It can readily be seen that, in the absence of a model, designing a new phosphor that satisfies all these requirements would be very time consuming and would essentially come down to a trial and error process of combining several hosts with different activators.
While a systematic investigation of the luminescence of lanthanides in different hosts has already been carried out, this is still lacking for the ns2 ions [3,4]. Therefore, after a short introduction about the optical properties of these ions, a model describing the systematics of their luminescence will be presented. This model is based on data collected from the available archival literature and its validity is tested by applying it to a case study where these dopants are embedded in CaS and SrS.
[1] P. Pust, P. J. Schmidt, W. Schnick, Nature Materials 2015, 14, 454-458.
[2] P. F. Smet, A. B. Parmentier, D. Poelman, J. Electrochem. Soc. 2011, 158, R37-54.
[3] P. Dorenbos, Journal of Solid State Sciences and Technology 2003, 2, R3001-3011.
[4] J. J. Joos, D. Poelman, P. F. Smet, Phys. Chem. Chem. Phys. 2015, 17, 19058-19078.
Speakers: Mr David VAN DER HEGGEN (LumiLab, Department of Solid State Sciences, Ghent University, Gent, Belgium), Mr Jonas J. JOOS (LumiLab, Department of Solid State Sciences, Ghent University, Gent, Belgium) -
116
Taking Phosphor Investigation to the Next Level with SEM-CL/EDX
Luminescent materials, also known as phosphors, are an essential component of white light emitting diodes (LEDs). White LEDs consist of a blue emitting LED chip and one or more phosphor materials which convert part of the blue light to longer wavelengths. In order to mimic the spectrum of a black body radiator as closely as possible, luminescent materials showing broad emission bands are used. For these phosphors there is a strong correlation between their composition and structure on the one hand and their optical properties on the other hand [1].
A perfect means for microscopic investigation of these materials is provided by SEM-CL/EDX. A scanning electron microscope (SEM) is capable of producing images with high spatial resolution, showing the morphology and average grain size of the phosphor powders. With the addition of an energy-dispersive X-ray detector (EDX) local composition can be studied simultaneously. Since dopant concentration is often limited to 0.1% to 1%, EDX analysis only allows a qualitative interpretation for presence and dispersion of the dopants. Studying cathodoluminescence (CL) in the same microscope strongly extends the analytical power of the measurement system: by coupling a CCD-based spectrometer to the SEM, it is possible to match a local emission spectrum to a nanometer sized area (Fig. 1). Since small variations in dopant concentration and particle morphology have an influence on intensity and shape of the CL spectrum, differences in local spectra show where to look for particularities. Linking these spectra to the EDX results can confirm these local variations in dopant concentration. Another advantage of SEM-CL/EDX, in addition to its high spatial resolution, is its ability to obtain more detailed depth-resolved information by varying the electron-beam energy [2].
Correlating structural, compositional and luminescent information allows for identification of non-uniform doping or impurity phases, which can be used to improve synthesis methods and possibly also the overall performance of the phosphors [3].
[1] D. Poelman, and P.F. Smet, Physica B. 439, 35-40 (2014). [2] B.G. Yacobi, and D.B. Holt, Journal of Applied Physics. 59, R1-R24 (1986). [3] P.F. Smet, J. Botterman, A.B. Parmentier, and D. Poelman, Optical Materials 35, 1970–1975 (2013).
Speaker: Lisa MARTIN (LumiLab, Department of Solid State Sciences, Ghent University) -
117
The CMS Resistive Plate Chamber setup at the CERN Gamma Irradiation Facility
The CERN engineering and physics departments designed and built the new Gamma Irradiation Facility (GIF++) which is fully operational since March, 2015. The GIF++ is motivated by strong needs from the LHC detector and accelerator communities to perform long term ageing studies. It is a unique facility where high energy charged particles beam (mainly muons) are combined with a flux of photons (662 keV) from a 13.9 TBq 137Cs source. The CMS Resistive Plate Chamber (RPC) community made and installed several different RPC prototypes to be tested at the GIF++. In this contribution we will present the GIF++ infrastructure and the tools used for the CMS RPCs studies.
Speakers: Mr Muhammad Gul (UGent), Mr Nicolas ZAGANIDIS (UGent) -
118
The ESA Virtual Space Weather Modelling Centre – Part 2
The goal of the ESA ITT project AO-1-8384-15-1-NB VSWMC-Part 2 is to further develop the Virtual Space Weather Modelling Centre (VSWMC), building on the Phase 1 prototype system and focusing on the interaction with the ESA SSA SWE system. The objective and scopes of this project include:
The efficient integration of new models and new model couplings, including a first demonstration of an end-to-end simulation capability. The further development and wider use of the coupling toolkit and the front-end GUI which will be designed to be accessible via the SWE Portal. Availability of more accessible input and output data on the system and development of integrated visualization tool modules.The consortium that took up this challenge involves: 1) the Katholieke Universiteit Leuven (Prime Contractor, coordinator: Prof. S. Poedts); 2) the Belgian Institute for Space Aeronomy (BIRA-IASB); 3) the Royal Observatory of Belgium (ROB); 4) the Von Karman Institute (VKI); 5) DH Consultancy (DHC); 6) Space Applications Services (SAS); 7) British Antarctic Survey (BAS). The VSWMC-Part 2 project started on 17 February 2016, and spans a period of two years. At the time of the COSPAR meeting, Part 2A will be almost finished. Therefore, the updated architectural design of the full VSWMC system will be ready, . Furthermore, the detailed design of the Part 2 prototype, based on the to-be-performed requirements analysis, will be almost ready. It will thus be more clear what models and model couplings the Part 2 VSWMC will comprise. The VSWMC system is being developed under ESA's Space Situational Awareness (SSA) Programme and is intended to become an operational system as part of the ESA SSA SWE system
Speaker: Stefaan POEDTS (KU Leuven) -
119
The Giant VCMA effect for novel MRAM applications
The Voltage Control of Magnetic Anisotropy (VCMA) effect allows to control a magnetic bit by means of an electric field [1] instead of high currents, enabling a much lower power consumption. State-of-the-art Magnetic Tunnel Junction (MTJ) material stacks using mostly MgO as a dielectric have been screened for VCMA applications [1,2]. The MgO VCMA effect is currently too weak for use in logic and memory applications. We aim to obtain an electronic VCMA effect rather than an ionic effect, which would be too slow for nanoelectronics applications. The use of high-κ dielectrics promises to allow a giant electronic VCMA effect, opening the door to low power voltage-controlled nanoelectronics.
We aim at investigating high-κ dielectrics for giant VCMA applications. The first step for obtaining VCMA on Atomic Layer Deposition (ALD) high-κ dielectrics is tailoring the magnetic anisotropy, for which no work has been done. The use of ALD dielectrics is key for obtaining good insulating and reliability properties to sustain electric fields. Our work so far has focused on HfO2 in combination with different ferromagnet\nonmagnet bilayers, such as Co\Pt, CoFeB\Ta and CoFe\Ta. We show that Perpendicular Magnetic Anisotropy (PMA) can be obtained in all three multilayers.
[1] Maruyama, T., Y. Shiota, T. Nozaki, K. Ohta, N. Toda, M. Mizuguchi, A. A. Tulapurkar, T. Shinjo, M. Shiraishi, S. Mizukami, Y. Ando, Y. Suzuki . Nature nanotechnology 4, 158 (2009) [2] Kita, Koji, David W. Abraham, Martin J. Gajek, and D. C. Worledge. J. Appl. Phys 112, 033919 (2012)
Speakers: Mr Bart VERMEULEN (KU Leuven), Dr Koen MARTENS (imec) -
120
The High-Throughput approach to Computational Materials Design
The field of materials design occupies itself with the search for materials that exhibit specific properties. Such materials can be obtained by finetuning promising candidates using defects or by synthesizing entirely new materials based on experience from nature. This requires a detailed understanding of the nanoscale features which gives rise to these properties, as well as considerable experimental effort in preparing and characterizing each attempt. As a result material design is often cumbersome and expensive. An alternative approach to obtain this knowledge by performing quantum-mechanical simulations. These have now reached the robustness and accuracy required to reliably predict many material properties solely from first principles. Computational materials design is not hindered by experimental difficulties and is highly generic, enabling the simulation of hypothetical materials with great ease. In addition, the advancement of computing has made it possible to perform such calculations relatively inexpensively in large numbers. This has lead to the so-called high-throughput approach where, rather than focusing on the details of a single system, large databases of hypothetical materials are created. By computationally characterizing each of these materials large-scale screening procedures can be performed, reducing the experimental work to the most promising candidates. Large databases also offer another advantage: the observation of global trends which would never become visible if only a few materials were synthesized. This can be taken even further by introducing techniques from machine learning where non-intuitive descriptors can be directly linked to experimental quantities even if the underlying mechanism is not yet known. This has the potential to guide us through entirely unexplored regions of materials space and with it greatly increase our knowledge of condensed matter physics.
Speaker: Mr Michael SLUYDTS (UGent) -
121
The oblique Hanle effect in graphene: A novel approach to determine spin lifetime anisotropy
Jeroen E. SCHEERDER1 , Bart RAES2, Marius V. COSTACHE2, Frederic BONELL2, Juan F. SIERRA2, Jo CUPPENS2, Sergio O. VALENZUELA2,3 and Joris VAN DE VONDEL1
1 INPAC - Institute for Nanoscale Physics and Chemistry, Department of Physics and Astronomy, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium
2 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, Campus UAB, Bellaterra, 08193 Barcelona, Spain
3 Institució Catalana de Recerca i Estudis Avançats (ICREA), 08070 Barcelona, Spain
Identifying the main microscopic process for spin relaxation in graphene stands as one of the most fascinating puzzles for the graphene and spintronic communities. A key property to solve this puzzle is the dependence on the carrier concentration of the spin lifetime anisotropy, which is quantified by the ratio ζ between the in-plane and out-of-plane spin lifetimes [1]. However, the only reported measurements of the out-of-plane spin lifetime, and ζ, required large (>1T) magnetic fields applied perpendicular to the graphene plane [2], thereby rendering the method unusable for low carrier densities.
Here, we demonstrate a conceptually new approach that overcomes this limitation because it does not require such large out-of-plane magnetic fields, thus making it reliable for both low and high carrier densities [3]. The approach relies on spin precession measurements under perpendicular and oblique magnetic fields (~0.1T) with respect to the graphene plane (i.e. the oblique Hanle effect). This allows to generate in- and out-of-plane spin populations and to evaluate the spin lifetime anisotropy.
In our experiments we probe the spin lifetime anisotropy in graphene based non-local lateral spin valve devices on silicon oxide. We demonstrate that in our samples ζ is independent of carrier density and temperature down to 150K, and much weaker than previously reported. Indeed, ζ is equal to one within the uncertainty of our measurements, indicating scattering dominated by random magnetic impurities or defects. These results open the way for systematic anisotropy studies on graphene with a controlled number of impurities and on different substrates. Such information is crucial to find a route to increase the spin lifetime towards the theoretical limit and, therefore, has important implications for both fundamental science and technological applications.
References
[1] W. Han et al, Nature Nanotech. 9, 794 (2014)
[2] N. Tombros et al, Phys. Rev. Let. 101, 046601 (2008) and M.H.D. Guimarães et al, Phys. Rev. Lett. 113, 086602 (2014)
[3] B. Raes, J.E. Scheerder, M.V. Costache, F. Bonell, J.F. Sierra, J. Cuppens, J. Van de Vondel and S.O.Valenzuela, Determination of the spin lifetime anisotropy in graphene using oblique spin precession, accepted in Nature Communications
E-mail: jeroen.scheerder@fys.kuleuven.be
Speaker: Mr Jeroen SCHEERDER (INPAC—Institute for Nanoscale Physics and Chemistry, Department of Physics and Astronomy, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium) -
122
The One-Photon Detachment of $\mathrm{O}^-$: theory and experiment
The photodetachment of the negative ion of oxygen, although a seemingly simple process, is the subject of a long lasting discrepancy between theory and experiment. On the experimental side, the reference values for the photodetachment cross section are those of Smith and Branscomb et al. [1, 2], which have been widely used to put other relative measurements on an absolute scale. On the theoretical side, computing the photodetachment cross section for an open-shell ion remains a challenging task. However, even the latest ab-initio calculation significantly differs from the experiments, both in shape and magnitude [3].
We present a joint experimental and theoretical study of the one-photon detachment of O−. The experiment was performed using the animated-crossed-beam method [4], which relies on repeatedly sweeping the laser beam across the ion beam while recording the ion current, the laser power and the number of neutral atoms produced. The cross section is then direclty retrieved from these easily measurable quantities, without unnecessary assumptions on the interaction volume. The theoretical work is twofold. The residual oxygen atom is described by a configuration interaction (CI) expansion that is voluntarily restricted in order to keep the photoionization calculations simple, yet including enough correlation to reproduce the electron affinity of the O−(1s22s22p52Po) initial state and the polarizability of the O(1s22s22p43P) ground state with sufficient precision. The CI description is then used to perform ab initio, non-perturbative R-Matrix Floquet calculations.
The present theoretical and experimental results agree within error bars. They lie some 15% lower than the extensive calculation of Zatsarinny and Bartschat [3]. They are also significantly higher than the previous measurements, the difference reaching 20% above 2.2 eV, where we do not reproduce the plateau previously observed. A long-standing discrepancy between theory and experiment is thus resolved. The present values may also question other photodetachment cross sections that were calibrated using the value of Smith, e. g., for the C− ion [5].
References
[1] S. J. Smith, Proceedings of the 4th International Conference on Ionization Phenomena in Gases (ICPIG) (1959).
[2] L. M. Branscomb, S. J. Smith and G. Tisone, J. Chem. Phys. 43, 2906 (1965).
[3] O. Zatsarinny and K. Bartschat, Phys. Rev. A 73, 022714 (2006).
[4] M. Génévriez and X. Urbain, Phys. Rev. A 91, 033403 (2015).
[5] M. L. Seman and L. M. Branscomb, Phys. Rev. 258, 1602 (1962).
Speakers: Matthieu GÉNÉVRIEZ (Université Catholique de Louvain), Xavier Urbain (Université catholique de Louvain) -
123
The phase-2 upgrade of the CMS Resistive Plate Chamber system
During the High Luminosity phase, the CERN Large Hadron Collider (HL-LHC) will produce particle collisions at intensities about 10 times higher than its design value. In turn, this will induce much higher particle background rates in the accelerator and the effects integrated over a long period of time will present a new challenge for the LHC detectors such as the Compact Muon Solenoid (CMS). In particular, the high pseudo-rapidity region of its muon system is covered only by Cathode Strip Chambers (CSC) and lacks redundant coverage despite the fact that it is the most challenging region for muon detection in terms of backgrounds and momentum resolution. In order to maintain the present muon detection and triggering performance during the HL-LHC, additional new muon detectors will be installed in this region. In particular, additional Resistive Plate Chambers (RPCs) are planned to be installed in the endcap disks, in the region close to the beamline. Preliminary CMS RPC performance and stability results at the new GIF++ facility will be reported. The GIF++ is a new gamma irradiation facility to test detectors for the HL-LHC program; it is a unique place at CERN where high energy charged particle beams (muon beam with momentum up to 100 GeV/c) are combined with a 14 TBq 137 Cesium source. Plans for ageing studies at the GIF++ for the CMS RPC system will be also discussed.
Speaker: Mr Alexis FAGOT (UGent) -
124
The semi-digital hadronic calorimeter (SDHCAL) for future leptonic colliders
The first technological SDHCAL prototype having been successfully tested, a new phase of R&D, to validate completely the SDHCAL option for the International Linear Detector (ILD) project of the ILC, has started with the conception and the realization of a new prototype. The new one is intended to host few but large active layers of the future SDHCAL. The new active layers, made of GRPC with sizes larger than 2 m2 will be equipped with a new version of the electronic readout fulfilling the requirements of the future ILD detector. The new GRPC are conceived to improve the homogeneity with a new gas distribution scheme. Finally the mechanical structure will use the electron beam welding. The progress realized will be presented and future steps will be discussed.
Speaker: Mr Antoine PINGAULT (UGent) -
125
The target material dependence of HiPIMS discharges
The current-voltage characteristics used in DC sputtering are inadequate to describe high power impulse magnetron sputtering (HiPIMS). A comparative study of materials (Mg, Cu, Ti, Nb, Cr, V, Zn, Al,...) and material conditions for HiPIMS deposition under Ar, and Ar + O2 ambient is undertaken. A comparison is made in the voltage-current-time diagrams using a Melec sipp 2000 powerbank combined with a ADL dc powersuply in constant voltage mode. Conditions such as duty cycle, working gas pressure, and frequency are searched where the different target materials are operational under same pressure, and pulse conditions. This makes comparison between the different current-voltage-time diagrams possible. These voltage scans allow us to search a voltage at which a large amount of the different materials run stable under two pulse conditions: one where the on time is too short for rarefaction to occur and one where the on time is considerably longer allowing typical HiPIMS behavior such as rarefaction, and self sputtering to occur. We can now investigate the difference in amplitude of these current-time characteristics with only the target material related values as changing parameters.
Speaker: Mr Filip MOENS (UGent) -
126
The variation of the dust attenuation curve in the nearby Universe
Interstellar dust absorbs and scatters nearly half of the starlight in the Universe, heavily influencing our view on galaxies. Furthermore, it plays a crucial role in several astrophysical processes. A full understanding of the dust properties and the interplay between dust and starlight is essential to probe the current and past star formation activity and to constrain the cosmic star formation history.
In extragalactic studies, dust attenuation is often simplified to the Milky Way attenuation law or the Calzetti relation. However, there is growing evidence for strong deviations from a universal dust attenuation law.
We have initiated an ambitious program to study the variation of the dust attenuation curve and the dust properties in the nearby Universe. It is based on the SINGS/KINGFISH sample, a unique local galaxy sample spanning a wide range of morphological galaxy types, metal abundances and star formation activity. We are gathering multi-wavelength imaging data for this sample, covering the ultraviolet to the submillimeter wavelength range.
In this poster, we will present the first results of our analysis of the variation of the dust attenuation curve on spatially resolved scales of about 200 pc in the spiral galaxy NGC 628.
Speaker: Mrs Marjorie DECLEIR (Ghent University) -
127
Thin, low roughness Ru films deposited by thermal and plasma enhanced atomic layer deposition using RuO4 and H2 at low temperatures
Atomic layer deposition (ALD) is a thin film deposition technique in which the growing film is alternately exposed to typically a chemical precursor and a gas (reactant), each reacting with the surface in a self-limited way. This results in the sequential deposition of mono or sub-monolayers of material and enables the deposition of thin films with precise thickness control and excellent conformality. Due to these interesting properties, metal oxide ALD is currently used in nanoelectronics to fabricate the features with the smallest dimensions. [1]
Though ALD is traditionally perceived as a layer-by-layer technique, and this is true for ALD of metal oxides, a nucleation controlled growth mode is generally observed for ALD of noble metals. After a certain incubation period, during which the actual nucleation takes place, growth is initiated in localized islands spread across the surface. Only in a later growth stage, the islands coalesce into a continuous layer which, depending on the density of nuclei, may have a rough surface morphology. This prevents the application of noble metal ALD in nanoelectronics as very thin (~ 1-3 nm), continuous and smooth metal films are required.
Ruthenium atomic layer deposition (ALD) has been identified by the International Technology Roadmap for Semiconductors 2011 as a potential candidate to replace the currently used TiN capacitor electrode of the Metal-Insulator-Metal capacitor in Dynamic Random Access Memory and to use it as a nucleation layer for Cu electroplating in the deepest levels of metalization. [1] Ru ALD is usually achieved by using metalorganic precursors in a combustion chemistry with O2 gas or NH3 plasma. [2] Recently, our group has developed a new Ru ALD process, using the inorganic RuO4-precursor in a reduction chemistry with H2 gas [3] or H2 plasma [4]. The inorganic nature of the precursor results in the absence of C-impurities in the deposited films, while the use of a reducing agent as the reactant leads to a control over the amount of O-impurities (<5 at.%). The RuO4/H2 and RuO4/H2-plasma processes have a high saturated growth rate of 0.1-0.12 nm/cycle. The RMS roughness and the electrical resistivity of the films are both low (0.1-0.3 nm and 18-27 Ω.cm), even for very thin films of 3-5 nm. The thermal process has a narrow ALD temperature window near 100°C, while plasma enhancement allows deposition at lower temperatures with a temperature window between 50°C and 100°C. Although the properties of this process are very promising for its application in nanoelectronics, an incubation period was observed during deposition on certain substrates, and hence this needs further investigation.
In this work, the novel RuO4-based processes were monitored by in situ grazing incidence small angle x-ray scattering (GISAXS) and x-ray fluorescence (XRF) to obtain in-depth information about the morphological evolution during the nucleation and growth. In GISAXS, a monochromatic beam of x-rays is reflected at grazing angles of the sample surface, and a 2D detector is used to monitor the pattern of diffuse reflection that is surrounding the specular reflected beam. The measured GISAXS pattern can be considered as the representation of the surface morphology in reciprocal space. Hence one can extract information such as surface roughness or nuclei shape, size and distribution from a GISAXS pattern. Using in situ XRF one can quantify the amount of material deposited during an ALD-process, i.e. the growth rate at any time during deposition. The combination of these two techniques yields a strong tool to investigate nucleation during metal ALD [5]. Inherently, the low surface coverage during nucleation means that one needs synchrotron radiation to reach sufficient sensitivity with these techniques.
In figure 1, in situ XRF results are shown for a selection of experiments conducted at the SIXS beamline of the Soleil synchrotron. One can see that the thermal process has a large incubation period on oxide surfaces, which is due to nucleation, and this period can be reduced by using the plasma enhanced process. From the in situ GISAXS data it was derived however that for the plasma enhanced process on SiO2 the film still nucleates as islands, and the spacing between the islands was found to increase from 15 nm to 30 nm. This increase in particle spacing could be explained by a cluster/atom surface diffusion mechanism. (Figure 2) For the thermal process on Si-H, the films start to grow as a layer rather than islands, which corresponds to the absence of the nucleation period found by XRF. (Figure 2) Furthermore the low overall scattered intensity found by GISAXS means that the film has a low roughness, which was confirmed by Atomic Force Microscopy.
The conclusion is that Ru thin films deposited by the RuO4/H2 (PE)ALD processes nucleate as a layer on Si-H, and are therefore smooth and continuous even for a low thickness. Although incubation times on oxide surfaces are high for the thermal process, these can be decreased by using the plasma enhanced process, but in this case one observes island growth.
References:
[1] ITRS 2011, 2013 (http://www.itrs.net): Front End Processes and Interconnect chapters.
[2] J. Hämäläinen, M. Ritala, M. Leskelä, Chem. Mater. (2014) 26, 786.
[3] M. M. Minjauw, J. Dendooven, B. Capon, M. Schaekers, C. Detavernier, J. Mater. Chem. C (2015) 3, 132.
[4] M. M. Minjauw, J. Dendooven, B. Capon, M. Schaekers, C. Detavernier, J. Mater. Chem. C (2015) 3, 4848.
[5] K. Devloo-Casier, K. F. Ludwig, C. Detavernier, J. Dendooven, J. Vac. Sci. Technol. A (2014) 32, 010801.
Acknowledgements:
This research was supported by the European Research Council (Starting Grant No. 239865), by the Flemish Research Foundation FWO (Project G.0209.11), GOA no. 01G01513. Matthias M. Minjauw and Jolien Dendooven acknowledge funding by FWO Vlaanderen.
Speaker: Mr Matthias MINJAUW (Ghent University) -
128
Towards a catheter based sensor for the electronic detection of histamine in the intestinal tract
Irritable bowel syndrome (IBS) is a disease affecting up to 15% of the population in the developed world. Symptoms such as abdominal pain, diarrhea, constipation and psychological problems negatively affect the patient’s life leading to an economical burden on society due to an increased absence from work and costs related to medical care. The exact cause of IBS is still up for debate but over the years it has become clear that an elevated histamine concentration in the gastrointestinal (GI) tract can be both an indicator and cause of IBS [1]. The current state of detection requires extraction of gastrointestinal fluids by means of endoscopy and biopsy of the GI tract to exclude certain other diseases. Currently the histamine concentration can be measured by relatively expensive techniques such as high pressure liquid chromatography (HPLC). An alternative method of detection utilizes molecularly imprinted polymers (MIPs) which are biomimetic receptor elements that can be produced at low cost and offer superior resistance to detoriation by heat, pH and time. In the past these MIPs have been used in combination with electrochemical impedance spectroscopy and the heat-transfer method (HTM) to detect histamine, serotonin and nicotine in biological fluids [2-3]. These detection methods rely on the fact that water filled cavities in the MIPs show a lower thermal- and electrical resistance than cavities that are filled with bound target molecules.
In our research we are working towards integration of a MIP based biosensor into a catheter such as the ones that are already used inside hospital settings and in parallel work is done on an external measurement cell for in vitro reference measurements. The intestinal, catheter based measurement, will allow for more accurate histamine measurements because the patient experiences less stress compared to a fullscale endoscopy. Along with more patient comfort an on-site measurement does not suffer from sample detoriation due to ambient oxygen which is hard to avoid in the time gap between extraction and analysis in a hospital lab. The external measurement cell can be connected to currently existing GI fluid-aspiration catheters in order to measure the histamine concentration simultaneously right next to the patient during extraction. Impedance spectroscopy is the proposed read-out method since this is already commonly performed on patients for medical purposes. Acknowledgements: FWO project G.0B25.14N. Monitoring of gut functions and inflammation processes with biomimetic sensors based on molecularly imprinted polymers.
[1] G. Barbara, V. Stanghellini, R. De Giorgio, C. Cremon, G.S. Cottrell, D. Santini, G. Pasquinelli, A.M. Morselli-Labate, E.F. Grady, N.W. Bunnett, S.M. Collins, R. Corinaldesi, Gastroenterology, 2004, 126, 693-702.
[2] G. Wackers, T. Vandenryt, P. Cornelis, E. Kellens, R. Thoelen, W. De Ceuninck, P. Losada-Pérez, B. van Grinsven, M. Peeters, P. Wagner, Array Formatting of the Heat-Transfer Method (HTM) for the Detection of Small Organic Molecules by Molecularly Imprinted Polymers, 2014, 14(6), 11016-11030.
[3] M. Peeters, P. Csipai, B. Geerets, A. Weustenraed, B. van Grinsven, R. Thoelen, J. Gruber, W. De Ceuninck, T. J. Cleij, F. J. Troost, P. Wagner, Heat-transfer-based detection of L-nicotine, histamine, and serotonin using molecularly imprinted polymers as biomimetic receptors, 2013, 405(20), 6453-6460
Speaker: Gideon WACKERS (KULeuven) -
129
Turbulence analysis in the reconnection exhaust of 3D PIC numerical simulation of magnetic reconnection
Turbulence is the physical phenomenon responsible for the cascade of energy from large scales, where energy is injected, to small scales, where the energy can eventually be dissipated. Turbulence is ubiquitous in space plasmas and its features in the different regions of the heliosphere (like solar wind, magnetosphere, solar corona ...), along with the mechanisms at small scale responsible for dissipation, are still subject of studies. An interesting topic is understanding the relationship between turbulence and magnetic reconnection. It has already been known, in fact, that this two phenomena are strictly linked in space plasmas (Matthaeus and Velli, SSR, 2011). Recently, data analysis of turbulence generated within a high-speed reconnection jet in the terrestrial magnetotail were performed using multipoint in-situ measurements from the Cluster spacecraft (Osman et al., ApJL, 2015). The authors showed that in the outflow region of a reconnection site turbulence is at play forming localized coherent structures by an intermittent cascade. In this work, we use high resolution 3D PIC numerical simulation data to study the features of the turbulence that develops in the outflow regions of magnetic reconnection (Lapenta et al., Nature Phys., 2015). We analyze temporal and spacial spectra of the fluctuations and higher order structure functions and discuss how energy is transmitted from fields to particles. We compare the numerical results with observations.
Speaker: Dr Francesco PUCCI (KU Leuven) -
130
Upgrade of the CMS muon system with triple-GEM detectors
After the upgrades of the Large Hadron Collider (LHC) planned for the second and the third Long Shutdown (LS), the LHC luminosity will reach values up to 2×10^34cm−2s−1 and 5×10^34cm−2 s−1 respectively. Such conditions will deeply affect the performance of the CMS muon system, especially in the very forward region, due to the harsh expected background environment and the reduced magnetic field, together with high pile-up conditions.
The CMS GEM collaboration proposes to upgrade the muon forward region with two additional muon detectors, GE1/1 and GE2/1 stations, to be installed in the second and third LS respectively, in the region 1.6<|η|<2.4 to increase redundancy and enhance the trigger and reconstruction capabilities. These detectors make use of the well established GEM technology which high spatial resolution and high rate capability allows to combine tracking and triggering functions.
In this contribution we will present the status of the GE1/1 developments on both the detector R&D and the physics analysis sides, the impact they will have on the CMS muon spectrometer, and results obtained during test beam campaigns.
Speaker: Mr Thomas LENZI (Université Libre de Bruxelles) -
131
Validation of the response function of the WENDI-2 detector with high-energy quasi-monoenergetic neutron beams for proton therapy centers
In proton therapy, proton beams with energies up to typically 230 MeV are used to treat cancerous tumours very efficiently while sparing surrounding healthy tissues as much as possible. Due to nuclear interactions of the proton beams with matter, mainly inside the cyclotron, the beam line, the treatment nozzle and the patient, secondary neutrons with energies up to 230 MeV are unfortunately produced, as well as photons up to ~10 MeV. In shielding studies for proton therapy facilities, the neutron total ambient dose equivalent H(10) component is often evaluated using the Monte Carlo codes MCNPX or FLUKA. Recent benchmark simulations performed with GEANT4 have shown that this code would also be a suitable tool for the shielding studies of proton therapy centres. The experimental validation of such shielding studies requires the use of a detector with a good sensitivity for neutrons ranging from thermal energies up to 230 MeV, such as for example the extended-range neutron rem meter WENDI-2, developed in the 1990s by R.H. Olsher and nowadays commercialized by Thermo Scientific. Although the response function of the WENDI-2 detector is not ideal, it is rather well-balanced with respect to the ICRP74 fluence-to-H(10) conversion function. However to have an accurate H*(10) measurement, the WENDI-2 detector response function needs to be validated with quasi-monoenergetic neutron beams at high energies. This paper presents the measurements performed with a WENDI-2 detector in high-energy quasi-monoenergetic neutron beams at the Theodor Svedberg Laboratory (Uppsala, Sweden). The results are compared to similar measurements of Olsher et al. as well as to MCNPX and GEANT4 simulations.
Speakers: Mr David NDAYIZEYE (ULB), Mr Gilles DE LENTDECKER (ULB) -
132
Vanadium dioxide thin films prepared on silicon by low temperature MBE growth and ex-situ annealing
Vanadium dioxide (VO2) is a material that shows an insulator to metal transition (IMT) near room temperature. This property can be exploited for applications in field effect devices, electro-optical switches and nonlinear circuit components. We have prepared VO2 thin films on silicon wafers with a native oxide by combining a low temperature MBE growth with an ex-situ annealing at high temperature. We investigated the structural, electrical and optical characteristics of films with thicknesses ranging from 10 to 100 nm. The films grown with our method are polycrystalline with a preferred orientation in the (011) direction of the monoclinic phase. An IMT at around 68 ∘C is observed while the magnitude of the resistance change across the transition increases with thickness. The refractive index at room temperature corresponds with values reported in the literature for thin films. The successful growth of VO2 films on silicon with good electrical and optical properties is an important step towards the integration of VO2 in novel devices. The authors acknowledge financial support from the FP7-ICT-2013-11-61456 SITOGA Project. P.H. acknowledges support from Becas Chile—CONICYT.
Speakers: Mariela MENGHINI (Department of Physics and Astronomy, KU Leuven, Leuven, Belgium), Maryline SOUSA (IBM Research Laboratory, Zurich, Switzerland), Dr Pablo SANCHIS (I. U. I. Centro de Tecnología Nanofotónica, Universitat Politècnica de València, Valencia, Spain), Pia HOMM (Department of Physics and Astronomy, KU Leuven, Leuven, Belgium) -
133
Vector Boson Scattering prospects for High-Luminosity LHC at CMS in the WZ final state
A feasibility study is presented for Vector Boson Scattering measurements in the WZ final state, that can be performed during the high luminosity phase of the LHC. Particular emphasis is given to the expected performances of the detector in different upgrade scenarios, which are compared to each other and to the results attained with the current, aged technology.
Speaker: Deniz POYRAZ (UGent) -
134
Vortices in a rotating Fermi gas within the finite temperature effective field theory
Vortices and vortex arrays in superfluid atomic Fermi gases in the BCS-BEC crossover are investigated within the finite temperature effective field theory (EFT) [1-4] for a macroscopic wave function representing the field of condensed pairs, analogous to the Ginzburg-Landau theory for superconductors. Here, we have established how rotation modifies this effective field theory, by rederiving it starting from the action of Fermi gas in the rotating frame of reference. The rotation vector potential is renormalized within the EFT due to a renormalization of the pair effective mass. The latter one appears to be in agreement with results of the functional renormalization group theory. In the extreme BEC regime, the pair effective mass tends to twice the fermion mass, being in line with the physical picture of a weakly interacting Bose gas of molecular pairs. We use our macroscopic wave function description to study vortices and the critical rotation frequencies to form them. Phase diagrams for vortex states are derived. They are in good agreement with available results of the Bogoliubov - De Gennes theory and with experimental data.
This research was supported by the Flemish Research Foundation (FWO-Vl), project nrs. G.0115.12N, G.0119.12N, G.0122.12N, G.0429.15N, by the Scientific Research Network of the Research Foundation-Flanders, WO.033.09N, and by the Research Fund of the University of Antwerp.
References
S. N. Klimin, J. Tempere G. Lombardi, and J. T. Devreese, Eur. Phys. Journal B 88, 122 (2015); arXiv:1309.1421. S. N. Klimin, J. Tempere, and J. T. Devreese, Physica C 503, 136 (2014). S. N. Klimin, J. Tempere and J. T. Devreese, Phys. Rev. A 90, 053613 (2014). G. Lombardi, W. Van Alphen, S. N. Klimin, and J. Tempere, Phys. Rev. A 93, 013614 (2016).Speaker: Serghei KLIMIN (TQC, Universiteit Antwerpen) -
135
YAGG:Cr$^{3+}$ luminescence : influence of synthesis on luminescent properties
YAGG:Cr3+ luminescence : influence of synthesis on luminescent properties
J.H. Bouman 1, M.Tiberi 2, O.Q. De Clercq 1, K. Korthout1, P.F. Smet 1 and D. Poelman 1
1 Lumilab, Dept. Solid State Sciences, Ghent University, Ghent, Belgium
2 Nanomaterials and Interfaces Group (NIG), Chemistry Dept., University of Milan, Milan, Italy
Synthetic Yttrium Aluminium Garnet (Y$3Al_5O{12},YAG)hasbeenwidelystudiedasahostmaterialforlasingandw−LEDphosphorapplications.Inthiswork,wefocusonCr^{3+}−dopedYAG,whichisanear−infraredemitterandapromisingcandidateforapplicationsinmedicalimaging.ThedevelopedYAG:Cr^{3+}phosphorshowsbrightnear−infraredemission,peakingat690nmandhasahighinternalquantumefficiencyof87^{3+}byGa^{3+}inthehost,resultingintheformationofaY3Al{5-x}GaxO{12}$ (YAGG) host structure, allows for band gap tuning and modification of the luminescent behavior of the phosphor.
We performed characterization of the optical properties of the Cr3+ ion in YAGG via steady-state and time-resolved photoluminescence, cathodoluminescence and thermal quenching experiments. The influence of the preparation method is checked, as we compare the luminescent properties of YAGG:Cr3+ synthesized via solid state synthesis and wet-chemical methods such as coprecipitation. The latter allows for more rigorous control of the grain size and the distribution of Cr-dopants in the host.
Speakers: Mr Jesse BOUMAN (Lumilab, Dept. Solid State Sciences, Ghent University, Ghent, Belgium), Ms Marta TIBERI (Nanomaterials and Interfaces Group (NIG), Chemistry Dept., University of Milan, Milan, Italy), Mr Olivier Q DE CLERCQ (Lumilab, Dept. Solid State Sciences, Ghent University, Ghent, Belgium)
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18:00
Closing Ceremony
Prizes and drinks
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08:15
