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

Arrival

Registration and welcome coffee.

Gong show - Day 1

1 TBD

Speaker: Arthur Bril (Ghent University)

2 TBD

Speaker: Enrico Di Salvo (VUB)

3 TBD

Speaker: Branko Meeus (UHasselt)

4 TBD

Speaker: Sander De Meyer (Ghent University)

5 TBD

Speaker: Guillermo Mera Alvarez (KU Leuven)

6 TBD

Speaker: Filippo Pascucci (U Antwerpen)

7 TBD

Speaker: Jagannath Sutradhar (Ghent University)

8 TBD

Speaker: Qi-Feng Wu (Ghent University)

15:00 Coffeebreak

check-in and photo session on the beach

9 TBD

Speaker: Prof. Bart Cleuren (UHasselt)

15 min talks - Day 1

10 TBD

Speaker: Matisse De Lescluze (Ghent University)

11 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)

12 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 Dinner

Friday 5 June

07:30 Breakfast

13 TBD

Speaker: Prof. Thomas Mertens (Ghent University)

Gong show - Day 2 (morning)

14 TBD

Speaker: Katharine Hyatt (Ghent University)

15 TBD

Speaker: Anton Martin (Ghent University)

16 TBD

Speaker: Pratik Nandy (VUB)

17 TBD

Speaker: Mina-Lou Schleith (Ghent University)

18 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)

19 TBD

Speaker: Kevin Vervoort (Ghent University)

20 TBD

Speaker: Maxime Grandjean (VUB)

21 TBD

Speaker: Vic Vander Linden (Ghent University)

11:00 Coffeebreak

22 TBD

Speaker: Dr Andrew Rolph (VUB)

12:30 Lunch

23 Physics-Inspired Modeling of Socio-Economic Systems

Speaker: Prof. Jan Ryckebusch (Ghent University)

15 minute talks - Day 2

24 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)

25 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)