### Attention : désormais les séminaires ont lieu tous les lundis à 10h45 en salle 523 du LPTMC - Tour 12-13

**Benoît Mahault (MPI Göttingen)**

### Emergent organization and pattern formation induced by motility regulation

The regulation of motility in response to environmental cues provides a platform for controlling self-propelled agents through activity landscapes, as well as a means for active systems interacting via quorum-sensing rules to regulate their aggregation dynamics. In this talk, I will present recent works dealing with emergent dynamics induced by motility regulation.

The motility response of microorganisms often involves a complex gene regulatory network, so that it occurs on a characteristic timescale that is not necessarily fast in comparison to other timescales of the dynamics. In the first part of the talk, I will present a minimal model of self-propelled motion that explicitly includes the dynamics of motility regulation, and illustrate how the presence of this additional timescale qualitatively affects the behaviour of self-propelled particles in activity landscapes.

In a second part, I will discuss the collective behaviour of multi-species active particles regulating their motility via quorum-sensing rules. I will in particular show how we identified a mechanism for dynamical pattern formation driven solely by the combined effects of motility and nonreciprocity in the inter-species couplings. Characterizing the phase of chaotic chasing bands emerging this way, we further find that it may be involved in phase-separated configuration, where it leads to anomalously slow coarsening.

References:

B. Mahault, P. Godara and R. Golestanian, Phys. Rev. Res. 5, L022012 (2023).

Y. Duan, J. Agudo-Canalejo, R. Golestanian and B. Mahault, Phys. Rev. Lett. (in press), arXiv:2306.07904.

**Rahul Dandekar (IPhT)**

### Hydrodynamics and Fluctuations in long-ranged 1D systems

Fluctuating hydrodynamic theories have emerged in recent years as a candidate for a generalised formalism for non-equilibrium statistical physics in a wide range of systems. In this talk, I will show how this formalism can be usefully applied to the study of systems with long-ranged interactions, taking the 1D Riesz gas as a concrete example. I will show that the scaling of the tagged particle position in long-ranged interacting single-file systems depends on the tail exponent of the interaction.

**Hadrien Kurkjian (LPT Toulouse)**

### The normal Fermi gas: a Fermi liquid?

Landau's Fermi liquid theory provides an effective description of a low-temperature fermionic system in the form of a dilute gas of quasiparticles confined to the Fermi level. It remarkably applies to systems whose microscopic physics is poorly understood, such as Helium-3, and successfully describes their long wavelength collective modes, in particular the phononic ones. However, due to the difficulty of solving exactly the quasiparticle transport equation in the case of arbitrary interactions, Landau's theory is generally used only in the hydrodynamic or collisionless limiting cases. In the case of a two-component ultracold Fermi gas in its normal phase, the simplicity of contact interactions has allowed us to go further and describe the entire transition from the hydrodynamic to the collisionless regime. In the weakly-interacting limit, our results are in excellent quantitative agreement with density-density response measurements performed by the Yale group, where the resonance corresponding to the first sound emerges from the Lindhard function of the non-interacting gas. In time-of-flight images, which enable tomography of the Fermi liquid, this evolution corresponds to a drastic change in the distribution of quasiparticles on the Fermi sphere. While the density response thus seems to be very well described by Landau's theory, I will show that non-Fermi liquid properties appear in the pairing susceptibility, near the superfluid critical temperature.

**Olivier Gauthé (École Polytechnique Fédérale de Lausanne)**

### Tensor network methods for frustrated magnets at finite temperature

Within strongly correlated systems, frustrated magnetism is the field that study magnetic insulators when different microscopic magnetic interactions favor incompatible orders and no classical spin configuration can fulfill all of them. This realm offers a fertile ground for experimental and fundamental exploration, giving rise to unconventional phenomena such as order by disorder or the enigmatic quantum spin liquid phase. Nevertheless, its study poses challenges due to the presence of competing interactions of comparable magnitude, confounding perturbation theory. On the numerical side, frustration usually prevents the use of quantum Monte Carlo algorithms.

Over the last decades, tensor network methods have emerged as the one of the most powerful numerical approach to tackle the many-body problem in both classical and quantum physics. In this talk, we will review the core principles of tensor network and their applications in condensed matter physics. We will focus on strongly correlated systems in two dimensions and discuss the simulation of frustrated quantum magnets at thermal equilibrium using Projected Entangled Pair States (PEPS).

To illustrate this approach, we will address the spin-1/2 Heisenberg model on the square lattice with nearest-neighbor coupling J1 and next-nearest coupling J2 (J1-J2 model) at finite temperature [1]. We will consider both antiferromagnetic (J1 > 0) and ferromagnetic (J1 < 0) cases. We will expose the first unambiguous and direct evidence of an Ising transition associated with the spontaneous breaking of the C_4v symmetry within the collinear antiferromagnet region of the phase diagram.

[1] O. Gauthé & F. Mila, PRL, 128, 227202 (2022).

**David Martin (University of Chicago)**

### Emergent phenomena in active matter and beyond

Active Matter deals with the study of microscopic agents able to exert self-propulsion forces on their medium. These microscopic agents can model various entities evolving in a large range of scales in Nature; from bacterias and flying birds to man-made self-phoretic colloids. The presence of self-propulsion drives the active agents out of equilibrium and allows for the emergence of landmark phenomena, both at the level of a single agent and at the collective level in ensembles of agents. In this presentation, I will first characterize such nonequilibrium phenomena for a single active particle. I will then move to the characterization of different collective behaviors as a function of the microscopic interactions between the active agents. In particular, I will assess how topological, repulsive and nonreciprocal interactions interplay with the emergence of collective motion.