Theory at sea 2026

4 Jun 2026, 13:30 → 5 Jun 2026, 18:15 Europe/Brussels

Vayamundo Oostende

On Thursday and Friday June 4-5, 2026 theoretical and mathematical physicists working in Flanders meet again in an informal joint workshop at Vayamundo Oostende.

The goal: To enjoy knowing the various theory groups at the Flemish universities, and to hear about current research themes and plans. We hope that it fosters collaborations for organizing education (Master- and Ph.D.level) and research, to benefit future careers and developments in theoretical and mathematical physics in Flanders.

 

        

 

 

Thursday 4 June

13:30 → 14:00 Arrival

Registration and welcome coffee.

14:00 → 15:03 Gong show - Day 1

14:00 Electron interactions in a rotating reference frame

Speaker: Arthur Bril (Ghent University)

14:07 Crossing Symmetry violation in Chern-Simons theories

Matter-coupled Chern–Simons theories in 2+1 dimensions give rise to anyonic statistics and unusual scattering phenomena. Recent large-N studies have suggested that their S-matrices exhibit modified crossing symmetry and additional forward-scattering contributions. We present the first direct perturbative study of these effects in non-Abelian Chern–Simons theories coupled to fundamental fermions, focusing on the large-k, fixed-N regime. While forward-scattering modifications are expected to emerge at one loop, unitarity arguments already reveal signatures of these effects at tree level. We establish a framework for systematic one-loop calculations that will provide a direct perturbative test of the proposed modifications.

Speaker: Enrico Di Salvo (VUB)

14:14 Probing the full system through a single interaction - A general definition of Stochastic Impedance in non-equilibrium stochastic networks

Stochastic Impedance can be used to extract information from an entire
stochastic network by considering the response of the probability current between two sites to a periodic driving. However, the theory of stochastic impedance is formulated for periodically driven equilibrium systems. That is, in absence of the driving, the system obeys detailed balance and is as such in equilibrium. The present work aims to extend this framework to driven non-equilibrium systems. This would allow the formalism to not only be applied to the already wide extent of biological and chemical processes that are perturbed from equilibrium, but also to transport and search processes, which are of great interest in condensed matter physics and computer science.

Speaker: Branko Meeus (UHasselt)

14:21 Finite temperature simulations of 2-dimensional quantum systems using tensor networks

Speaker: Sander De Meyer (Ghent University)

14:28 Holographic Correlators for Non-Conformal Maximally Supersymmetric Yang-Mills Theory

Maximally supersymmetric Yang-Mills theory in dimension d≠4 is perhaps the simplest non-conformal quantum field theory with a holographic gravity dual. Its large-N, strong-coupling BPS dynamics is captured by supergravity fluctuations around the D-brane solutions of string theory. After consistent truncation, and in the appropriate conformal frame, these solutions reduce to an AdS metric plus an active scalar, the dilaton, which breaks the scale invariance of AdS spacetime. In this talk, I will report recent progress in the computation of holographic correlators in this setup. I will argue that propagators in this type of background can be constructed by dimensionally reducing AdS propagators and explain how the 3pt-function of scalar operators in the dual strongly-coupled QFT follows from this simple fact.
Based on arXiv:2503.18770 and work in progress with N. Bobev, P. Bomans, F. F. Gautason and H. Paul.

Speaker: Guillermo Mera Alvarez (KU Leuven)

14:35 Electron-Hole bilayer superfluidity

Electron-hole bilayers heterostructures have emerged as a fascinating platform for studying unique electronic and optical phenomena in condensed matter physics. By precisely manipulating the spatial separation between electrons and holes and the particle density in two adjacent layers, researchers can access a rich spectrum of interaction-driven effects and exotic many-body states. I will introduce the concept of bilayer heterostructures and highlighting how it is possible to explore novel quantum phases such as excitonic superfluidity, BCS-BEC crossover, Wigner crystal formation, supersolidity.
However, despite extensive experimental efforts, conclusive evidence of exciton superfluidity remains elusive due to challenges in traditional probing methods, partly because of the neutral nature of excitons.
In this regard, we investigate collective modes, Josephson effect and Berezinskii-Kosterlitz-Thouless transition to identify alternative fingerprints of exciton superfluidity.

Speaker: Filippo Pascucci (U Antwerpen)

14:42 Fermi Surface Coupled to Spin Glass Critical Point

Speaker: Jagannath Sutradhar (Ghent University)

14:49 Wormholes from Gauging Nonlocal Symmetries

The wormhole contribution to the gravitational path integral may be interpreted as smooth remnant of correlations among the erratic large-N behaviors of dual CFTs. In this work, we investigate this idea in (2+1)-dimensional gravity. We show that one-sided boundary gravitons are intrinsically incomplete in the sense that the associated observable algebra has a nontrivial center regardless of choices of boundary conditions. Based on asymptotic symmetries, we bootstrap a general Poisson bracket to construct completions of the boundary gravitons. In the simplest completion, the commutant of the boundary graviton observable algebra is given by an observable algebra of monodromy data which we interpret as an effective description of one-sided black holes. We show that, to describe Lorentzian multi-boundary wormholes, only the monodromy data with a positivity restriction is needed. The positivity restriction results in emergent erratic large-N behaviors for some observables. We filter out the erratic observables by restricting to a subspace on which they act trivially. The monodromy observables generate nonlocal symmetries lack of corresponding local currents. We show that gauging the nonlocal symmetries is equivalent to filtering out the erratic observables. For one CFT, gauging the nonlocal symmetries at the quantum level removes all black hole states. Filtering the partition function of CFTs leads to an apparent ensemble averaging. For two CFTs, a Hilbert subspace describing wormholes survives after gauging global part of the nonlocal symmetries. The filtered partition function of the two CFTs is an ensemble average over quantum gates entangling the monodromy degrees of freedom the two CFTs. The correlation between the erratic observables of the two CFTs is preserved, which contributes to the filtered partition function as a wormhole term.

Speaker: Qi-Feng Wu (Ghent University)

14:56 Moving on a curved surface: metric perturbations

Speaker: Forouh Maleki (KU Leuven)

15:05 → 15:45 Coffeebreak

check-in and photo session on the beach

15:45 → 16:45 Sorting by Resetting

A novel paradigm for sorting is introduced, based upon resetting. Using simple examples, we demonstrate that sorting is achieved by resetting the velocity component(s) or the orientation of the particles, rather than their positions. The objects to be sorted are microparticles, modeled as suspended and spatially extended Brownian particles. This sorting-by-resetting scheme illustrates that stochastic resetting can create non-equilibrium conditions which enable tasks forbidden at thermodynamic equilibrium.

Speaker: Prof. Bart Cleuren (UHasselt)

16:45 → 17:30 15 min talks - Day 1

16:45 Nonthermal Fixed Points: towards hydrodynamics far from equilibrium

Far-from-equilibrium quantum many-body systems can exhibit universal behavior long before reaching thermal equilibrium. A prominent example is provided by Nonthermal Fixed Points, attractive dynamical regimes characterized by self-similar evolution that have been predicted in contexts ranging from early-universe cosmology to heavy-ion collisions and observed in ultracold-atom experiments.
Our current understanding of Nonthermal Fixed Points is largely restricted to spatially homogeneous systems. Recently, it has been suggested that spatial inhomogeneities around these states may lead to the emergence of fluid-like dynamics. In this talk, I will show how Nonthermal Fixed Points in expanding systems can be understood as ideal hydrodynamic realizations of their non-expanding counterparts. Focusing on Bjorken flow, a longitudinally expanding system relevant for heavy-ion collisions, I will further demonstrate how the expansion-induced inhomogeneity gives rise to second-order hydrodynamic behavior. These results establish a direct connection between Nonthermal Fixed Points and far-from-equilibrium hydrodynamics.

Speaker: Matisse De Lescluze (Ghent University)

17:00 First principles theory of nonlinear long-range electron-phonon interaction

Electron-phonon interactions are often written using the approximation of linear interaction, where one only keeps the process where one electron interacts with one phonon. This is usually sufficient to quantitatively describe material properties. However, this is no longer true in anharmonic materials with significant electron-phonon interaction, such as quantum paraelectrics and halide perovskites. Currently, the only available models for nonlinear electron-phonon interaction are model Hamiltonians, written in terms of phenomenological parameters. Here, we provide a microscopic semi-analytical expression for the long-range dipole part of the 1-electron-2-phonon matrix element, which can be interfaced with first principles techniques. We show that unlike for the long-range 1-electron-1-phonon interaction, the continuum approximation is not sufficient and that the entire phonon dispersion must be considered. We calculate an expression for the quasiparticle energies and show that they can be written in terms of a 1-electron-2-phonon spectral function. To demonstrate the method in practice, we calculate the 1-electron-2-phonon spectral function for LiF and CsPbI3 from first principles, and we show that the nonlinear interaction contributes significantly to the electron mobility of CsPbI3. The framework presented here bridges the gap between model Hamiltonians and first-principles calculations for the 1-electron-2-phonon interaction.

Speaker: Matthew Houtput (U Antwerpen)

17:15 Inducing activity by chemo-mechanical coupling

In biophysical systems, oriented locomotion is not imposed directly but emerges from the underlying chemistry. Molecules consume fuel, reactions proceed out of equilibrium, and somehow this chemical driving turns into persistent mechanical motion. The present work [1] addresses how to generate sustained mechanical activity from coupling to chemically driven stochastic dynamics, while maintaining a transparent structure of action, semi-reciprocal coupling, and local detailed balance. We study which characteristic features get transferred, and what new phenomena appear. To be specific, as illustrated in Fig. 1, we consider a heavy Newtonian probe (point particle) moving on a circle, coupled to a collection of fast independent but driven jump processes. Under time-scale separation, we treat the probe as the analogue of a Brownian particle bombarded by the nonequilibrium medium and derive the induced Langevin dynamics with explicit expressions for the streaming term, friction coefficient, and noise variance. These parameters are computed exactly in a weak coupling expansion. The induced friction is a sum of two terms: one entropic, proportional to the noise variance as in the Einstein relation for a thermal equilibrium bath, and a frenetic contribution that can take both signs. The frenetic part wins over a regime of parameters, making the total linear friction negative, and hence creating a linear instability. Detailed simulations confirm the initial growth driven by this anti-damping and exhibit a rich steady-state behavior (e.g., bimodal velocity distribution, U−shaped position distribution with peaks away from the minima of the potential), akin to active particles. Based on these numerical explorations, we conjecture which corrections to the induced Langevin equation are needed beyond time-scale separation to recover the numerical results.

Publications
[1] A. Beyen, F. Casini and C. Maes, A Rayleigh criterion for mechanical instability: inducing
activity by chemo–mechanical coupling (in preparation).

Speaker: Aaron Beyen (KU Leuven)

17:30 → 18:30 Localisation effects in many-body physics

Localisation is a phenomenon in Hamiltonian mechanics due to absence of effective resonances.
I will discuss several examples, mostly in a many-body context.
Most prominently: anomalous spreading of wave packets in non-linear oscilator systems, and absence of thermalisation and transport in interacting quantum chains.

Speaker: Prof. Wojciech De Roeck (KU Leuven)

19:00 → 21:00 Dinner

Friday 5 June

07:30 → 09:00 Breakfast

09:00 → 10:00 Developments in lower-dimensional quantum gravity

Speaker: Prof. Thomas Mertens (Ghent University)

10:00 → 11:00 Gong show - Day 2 (morning)

10:00 TBD

Speaker: Katharine Hyatt (Ghent University)

10:07 Integrability from Discrete Holomorphicity

Speaker: Anton Martin (Ghent University)

10:14 How much SYK is enough for Quantum Chaos?

We propose a two-body variant of bosonic SYK-type models that represents a minimal setting in which chaotic dynamics emerge, a phenomenon we refer to as quadratic quantum chaos. In contrast to integrable quadratic fermionic systems, the corresponding hard-core bosonic models exhibit genuine many-body chaos. We diagnose this behaviour using spectral statistics and multiple probes of operator growth. Owing to their simplicity and bosonic nature, these minimal models provide promising and resource-efficient platforms for investigating quantum chaos and information scrambling on near-term quantum devices.

Speaker: Pratik Nandy (VUB)

10:21 Superconductivity from emergent dipolar interactions in a fractionalized Fermi liquid

Speaker: Mina-Lou Schleith (Ghent University)

10:28 Fragmentation temperature of 1D and 3D quantum droplets in a BEC mixture

In a mixture of two Bose-Einstein condensates, the interactions can be tuned such that self bound objects called quantum droplets appear. Whereas the ground states of such quantum droplets at finite temperature have been studied for three- and one-dimensional configurations, the possible fragmentation of these droplets has so far not been considered in these studies. In this work we show that droplets can lower their free energy by splitting or fragmenting in a combination of multiple smaller droplets and/or a gas. Three-dimensional droplets will split when the interspecies interaction strength is considerably stronger than the intraspecies interaction strength, and the number of atoms is of the same order as the minimum number of atoms necessary to form a droplet. One-dimensional droplets will fragment as long as the intraspecies and interspecies interactions strength do not vary too much in strength and the density is not to big compared with the scattering length. If the temperature rises, 1D droplets will split by expelling atoms, forming a gas of predominantly free atoms and pairs of atoms. These pairs remain present in the system up to considerably high temperatures compared to the transition temperature. Our results provide important insights on the stability of these droplets.

Speaker: Jeroen Van Loock (U Antwerpen)

10:35 Extracting average properties of random spin chains with translationally invariant tensor networks

Disordered quantum spin chains are interesting systems as they can behave very differently from their clean counterparts. However simulating these systems pose signifciant challenges. First translation symmetry is broken when one considers a specific disorder realization, Secondly if one wants to study averaged observables it is necessary to average over many different disorder realization. In this talk I will present a tensor network method that addresses both issues by constructing a single, translationally-invariant Matrix Product Operator (MPO) which encodes the mixture of Gibbs states of the system in the thermodynamic limit. As a benchmark of the method we study the Infinite Randomness Fixed Point of the random transverse field Ising model.

Speaker: Kevin Vervoort (Ghent University)

10:42 Metastable strings at PTA: classical stability analysis

Metastable strings can arise because of a two-step symmetry breaking chain of the type $SU(2)\to U(1)\to 1$. These strings can decay via quantum tunnelling through the nucleation of monopole anti-monopole pairs, and have recently been considered as candidates for explaining the gravitational-wave background observed by pulsar timing arrays. In our work, we study the classical stability of such metastable strings in a concrete model realizing this symmetry breaking pattern. We identify regions of parameter space in which the string solutions are classically stable or unstable. Our results show that classical instabilities can impact the parameter space relevant for explaining the PTA signal.

Speaker: Maxime Grandjean (VUB)

10:49 Noise Thresholds for Topological Codes

Speaker: Vic Vander Linden (Ghent University)

11:00 → 11:30 Coffeebreak

11:30 → 12:30 Energy, how low can it go?

Energy plays a ubiquitous role in physics. In GR, energy conditions forbid exotic spacetimes such as traversable wormholes, and play a central role in singularity theorems and black hole thermodynamics. Yet all quantum field theories violate all pointwise energy conditions: states with arbitrarily negative local energy density always exist. In this talk, I will discuss what survives of energy inequalities once matter is quantised. I will start by reviewing the classical energy conditions and their role in gravity. I will then introduce several quantum energy inequalities—including the averaged null energy condition, the quantum null energy condition, and smeared null energy bounds—which place nontrivial constraints on negative energy despite the failure of classical energy conditions. Finally, I will discuss how these quantum energy inequalities can be used to recover gravitational results that were previously derived from classical pointwise energy conditions, focusing on modern quantum singularity theorems. Along the way, I will describe recent progress on higher-dimensional bounds on smeared null energy and their connections to causality, entropy, and quantum information.

Speaker: Dr Andrew Rolph (VUB)

12:30 → 14:00 Lunch

14:00 → 15:00 Physics-Inspired Modeling of Socio-Economic Systems

Speaker: Prof. Jan Ryckebusch (Ghent University)

15:00 → 15:30 15 minute talks - Day 2

15:00 Feynman variational principle and other analytic approaches to polarons in tight-binding conduction bands

We develop and compare analytical approaches for the polaron problem in finite-width, non-parabolic conduction bands [1, 2]. We revisit analytical methods originally formulated for continuum polarons, including canonical transformations and improved self-consistent Wigner–Brillouin approximations, and generalize them to lattice systems. In a finite-bandwidth lattice, these approaches exhibit qualitative features absent in the continuum case, such as a nontrivial connection between weak- and strong-coupling limits. An improved Wigner–Brillouin scheme yields a momentum-dependent polaron self-energy free of resonances and consistent with perturbation theory at zero momentum.

Our main result is an extension of the Feynman variational method to tight-binding lattices [1], where the effective-mass approximation breaks down. We show that, for lattice polarons, the modified Feynman variational method yields ground-state energies that are at least as accurate as those obtained from the well-recognized momentum-average approximation [3] and, in many cases, even closer to numerically exact results.

The methods are applied to the Holstein model and benchmarked against numerically exact calculations, including Diagrammatic Monte Carlo, exact diagonalization, and density-matrix renormalization-group results, and are further extended to polarons with Rashba spin–orbit coupling.

References
[1] S. N. Klimin, J. Tempere, M. Houtput, I. Zappacosta, S. Ragni, T. Hahn, L. Celiberti, C. Franchini and A. S. Mishchenko, arXiv:2603.09609 (2026).
[2] S. N. Klimin, J. Tempere, M. Houtput, S. Ragni, T. Hahn, C. Franchini and A. S. Mishchenko, Phys. Rev. B 110, 075107 (2024).
[3] G. L. Goodvin, M. Berciu, and G. A. Sawatzky, Phys. Rev. B 74, 245104 (2006).

Speaker: Serghei Klimin (U Antwerpen)

15:15 Bremsstrahlung function in superconformal gauge theories

The radiation emitted by an accelerated charged particle is one of the most basic observables in gauge theories and is governed by the Bremsstrahlung function. While this quantity can be computed exactly in Maxwell theory, its generalization to non-Abelian gauge theories is far more intricate. In superconformal field theories, however, the enhanced symmetry structure provides powerful tools to make the problem accessible. I will discuss recent developments in deriving exact results for the Bremsstrahlung function at arbitrary coupling using localization in the planar limit of different superconformal gauge theories. I will then focus on the strong-coupling regime of these models and analyze this quantity using analytical methods.

Speaker: Paolo Vallarino (VUB)