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

**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.

**Misaki Ozawa (Univ. Grenoble Alpes)**

### Renormalization Group Approach for Machine Learning Hamiltonian

Reconstructing, or generating, high dimensional probability distributions starting from data is a central problem in machine learning and data sciences. We will present a method —The Wavelet Conditional Renormalization Group —that combines ideas from physics (renormalization group theory) and computer science (wavelets, Monte-Carlo sampling, etc.). The Wavelet Conditional Renormalization Group allows reconstructing in a very efficient way classes of high dimensional distributions and the associated Hamiltonians hierarchically from large to small length scales. We will present the method and then show its applications to data from statistical physics and cosmology.

**Ramgopal Agrawal (LPTHE)**

### Critical dynamics of the \(\pm J\) Ising model

The \(\pm J\) Ising model is a simple frustrated spin model, where the exchange couplings independently take the discrete value \(-J\) with probability \(p\) and \(+J\) with probability \(1-p\). Here, we investigate the nonequilibrium critical behavior of the bi-dimensional \(\pm J\) Ising model, after a quench from different initial conditions to a critical point T_c(p) on the paramagnetic-ferromagnetic (PF) transition line, especially, above, below and at the multicritical Nishimori point (NP). The dynamical critical exponent \(z_c\) seems to exhibit non-universal behavior for quenches above and below the NP, which is identified as a pre-asymptotic feature due to the repulsive fixed point at the NP. Whereas, for a quench directly to the NP, the dynamics reaches the asymptotic regime with \(z_c \simeq 6.02(6)\). We also consider the geometrical spin clusters (of like spin signs) during the critical dynamics. Each universality class on the PF line is uniquely characterized by the stochastic Loewner evolution (SLE) with corresponding parameter \(\kappa\).