Searcharxiv⌕ Search

arXiv subjects

Hugues Chaté

Publications and source records attributed to Hugues Chaté.

At least 19 recordsLinked to original sources
arXiv2026

Coexistence phase of the two-dimensional Vicsek model

The Vicsek model continues to be central in statistical physics studies of active matter, with major recent surprises such as the partial metastability of the famous Toner-Tu flocking phase. Here we report on the diversity of band patterns observed in its coexistence phase --some of them never observed before--, but we also argue that most disappear in the infinite-size limit, leaving nevertheless a different picture than anticipated previously. We finally show that these rather regular, globally ordered band configurations can be destroyed by a localized perturbation: the coexistence phase is also at least partially metastable, and thus must be, asymptotically, a disordered phase made of a variety of chaotically interacting traveling bands.

cond-mat.stat-mech↗
arXiv2026

Phase Diagram of the Two-Dimensional Vicsek Model

Motivated by gaps in our current knowledge of the Vicsek model, we present a major update of its phase diagram in two space dimensions, summarizing comprehensive numerical simulations at length- and timescales never explored before. Our main conclusions are that multiple phase separation scenarios are at play, not just the simple microphase separation put forward in [Phys. Rev. Lett. 114, 068101 (2015)], and that the coexistence phase is subdivided in two. We also map out the region of metastability of the homogeneous Toner-Tu liquid phase.

cond-mat.stat-mech↗
arXiv2026

From particles to continuum theories in effective models of self-propelled rods

Models of colliding self-propelled rods are abstractions of many real systems such as gliding bacteria and biofilaments displaced by molecular motors. They have been studied mostly in two space dimensions and shown to display a wealth of different collective regimes with many parameters involved in the ensuing phase diagrams. Continuum-level theories faithful to this phenomenology are largely lacking since coarse-graining models of explicit rods is notoriously difficult. Here we study collections of particles in which the rods geometry is taken into account effectively, by implementing the nematic alignment of their velocities and two types of repulsion. These Vicsek-style models are simple enough to be starting points for systematic coarse-graining. We thus derive hydrodynamic equations that we show to have solutions qualitatively faithful to their particle-level origin.

cond-mat.stat-mech↗
arXiv2026

Long-Range Order in Coupled $D$-dimensional Kuramoto Oscillators

We show that the long-range order (LRO) strikingly emerges in systems of locally coupled $D$-dimensional vector Kuramoto oscillators on low-dimensional lattices ($d=1,2$), but only for odd $D$. This parity-dependent effect is traced to two-oscillator dynamics, where odd-$D$ units synchronize for any coupling, while even-$D$ pairs require a finite threshold. This fundamental difference selectively seeds collective order in large-scale systems, a phenomenon demonstrated by our numerical simulations. A renormalization group analysis reveals a RG flow to a weak-coupling fixed point for $d \le 2$. In this limit, odd-$D$ systems effectively map to a ferromagnetic model, developing an ordered ``hemisphere" phase, whereas even-$D$ systems remain disordered. Our findings further reveal orientational LRO emerges in both $d=1$ and $d=2$, but frequency LRO requires $d=2$. We contrast these results with the established behavior of models possessing continuous symmetry, highlighting how quenched disorder provides a fundamentally new route to order.

cond-mat.stat-mech↗
arXiv2026

XY Model with Persistent Noise

We consider a 2D XY model subjected to time-correlated noise, a model of direct relevance to active crystals, which were shown recently to be able to support very large deformations without melting in the presence of persistent fluctuations. We find that our persistent XY model can remain quasi-ordered in spite of correlations decaying much faster than allowed in equilibrium. We then investigate theoretically and numerically the order-disorder transition and conclude that it remains of the Berezinskii-Kosterlitz-Thouless type, but with scaling exponents that vary with the persistence time of the noise.

cond-mat.stat-mech↗
arXiv2026

How Continuous Symmetry Stabilizes the Ordered Phase of Polar Flocks

We study the stability of the ordered phase of compressible polar flocks against the nucleation of counter-propagating droplets, using a combination of analytical theory, microscopic and hydrodynamic simulations. For discrete-symmetry flocks, such droplets are known to always grow and propagate, making the ordered phase metastable. We explain how, on the contrary, continuous symmetry can stabilize the ordered phase at small enough noise by destabilizing the leading edge of growing droplets. Flocking models with continuous symmetries thus have a lower critical dimension than their discrete-symmetry counterparts, in contrast to equilibrium physics.

cond-mat.soft↗
arXiv2025

Inescapable anisotropy of non-reciprocal XY models

We investigate non-reciprocal XY (NRXY) models defined on two-dimensional lattices in which the coupling strength of a spin with its neighbors varies with their position in the frame defined by the current spin orientation. As expected from the seminal work of Dadhichi et al., Phys. Rev. E 101, 052601 (2020), we first show that non-reciprocity is akin to a self-propulsion: we derive a mean-field continuous theory identical to that of constant-density flocks. Like the latter, NRXY models exhibit a long-range ordered phase that is metastable to the nucleation of topological defects, and their asymptotic state is a dynamic foam of asters. We then show that in the metastable ordered phase, the lattice always induces anisotropy on large scales, pinning the direction of order and imposing a finite correlation length. Crucially, we demonstrate that this anisotropy is inescapable since, even when not explicitly present in the model, it is generated by fluctuations. In short, the ordered phase of lattice NRXY models is that of active clock models.

cond-mat.stat-mech↗
arXiv2025

Fluctuation-Induced First Order Transition to Collective Motion

The nature of the transition to collective motion in assemblies of aligning self-propelled particles remains a long-standing matter of debate. In this article, we focus on dry active matter and show that weak fluctuations suffice to generically turn second-order mean-field transitions into a `discontinuous' coexistence scenario. Our theory shows how fluctuations induce a density-dependence of the polar-field mass, even when this effect is absent at mean-field level. In turn, this dependency on density triggers a feedback loop between ordering and advection that ultimately leads to an inhomogeneous transition to collective motion and the emergence of inhomogeneous travelling bands. Importantly, we show that such a fluctuation-induced first order transition is present in both metric models, in which particles align with neighbors within a finite distance, and in `topological' ones, in which alignment is based on more complex constructions of neighbor sets. We compute analytically the noise-induced renormalization of the polar-field mass using stochastic calculus, which we further back up by a one-loop field-theoretical analysis. Finally, we confirm our analytical predictions by numerical simulations of fluctuating hydrodynamics as well as of topological particle models with either k-nearest neighbors or Voronoi alignment.

cond-mat.soft↗
arXiv2024

Condensation and Synchronization in Aligning Chiral Active Matter

We show that spontaneous density segregation in dense systems of aligning circle swimmers is a condensation phenomenon at odds with the phase separation scenarios usually observed in two-dimensional active matter. The condensates, which take the form of vortices or rotating polar packets, can absorb a finite fraction of the particles in the system, and keep a finite or slowly growing size as their mass increases. Our results are obtained both at particle and continuous levels. We consider both ferromagnetic and nematic alignment, and both identical and disordered chiralities. Condensation implies synchronization, even though our systems are in 2D and bear strictly local interactions. We propose a phenomenological theory based on observed mechanisms that accounts qualitatively for our results.

cond-mat.soft↗
arXiv2024

Flocking in One Dimension: Asters and Reversals

We study the one-dimensional active Ising model in which aligning particles undergo diffusion biased by the signs of their spins. The phase diagram obtained varying the density of particles, their hopping rate and the temperature controlling the alignment shows a homogeneous disordered phase but no homogeneous ordered one, as well as two phases with localized dense structures. In the flocking phase, large ordered aggregates move ballistically and stochastically reverse their direction of motion. In what we termed the "aster" phase, dense immobile aggregates of opposite magnetization face each other, exchanging particles, without any net motion of the aggregates. Using a combination of numerical simulations and mean-field theory, we study the evolution of the shapes of the flocks, the statistics of their reversal times, and their coarsening dynamics. Solving exactly for the zero-temperature dynamics of an aster allows us to understand their coarsening, which shows extremal dynamics, while mean-field equations account for their shape.

cond-mat.stat-mech↗
arXiv2024

Dynamic Scaling of Two-Dimensional Polar Flocks

We propose a hydrodynamic description of the homogeneous ordered phase of polar flocks. Starting from symmetry principles, we construct the appropriate equation for the dynamics of the Goldstone mode associated with the broken rotational symmetry. We then focus on the two-dimensional case considering both "Malthusian flocks" for which the density field is a fast variable that does not enter the hydrodynamic description and "Vicsek flocks" for which it does. In both cases, we argue in favor of scaling relations that allow to compute exactly the scaling exponents, which are found in excellent agreement with previous simulations of the Vicsek model and with the numerical integration of our hydrodynamic equations.

cond-mat.stat-mech↗
arXiv2024

Effect of Persistent Noise on the XY Model and Two-Dimensional Crystals

Two-dimensional (2D) crystals made of active particles were shown recently to be able to experience extremely large spontaneous deformations without melting. The root of this phenomenon was argued to lie in the time-persistence of the orientation of the intrinsic axes of particles. Here, we pursue this idea and consider passive systems subjected to time-persistent external perturbations. We first study a 2D XY model subjected to time-correlated noise and find that it can remain quasi-ordered in spite of correlations decaying much faster than allowed in equilibrium. We then study a simple model of a passive 2D crystal immersed in a bath of active particles, and show that it can sustain large deformations without melting.

cond-mat.soft↗
arXiv2023

Extreme Spontaneous Deformations of Active Crystals

We demonstrate that two-dimensional crystals made of active particles can experience extremely large spontaneous deformations without melting. Using particles mostly interacting via pairwise repulsive forces, we show that such active crystals maintain long-range bond order and algebraically-decaying positional order, but with an exponent $η$ not limited by the $\tfrac{1}{3}$ bound given by the (equilibrium) KTHNY theory. We rationalize our findings using linear elastic theory and show the existence of two well-defined effective temperatures quantifying respectively large-scale deformations and bond-order fluctuations. The root of these phenomena lies in the sole time-persistence of the intrinsic axes of particles, and they should thus be observed in many different situations.

cond-mat.soft↗
arXiv2023

Metastability of Discrete-Symmetry Flocks

We study the stability of the ordered phase of flocking models with a scalar order parameter. Using both the active Ising model and a hydrodynamic description, we show that droplets of particles moving in the direction opposite to that of the ordered phase nucleate and grow. We characterize analytically this self-similar growth and demonstrate that droplets spread ballistically in all directions. Our results imply that, in the thermodynamic limit, discrete-symmetry flocks -- and, by extension, continuous-symmetry flocks with rotational anisotropy -- are metastable in all dimensions.

cond-mat.soft↗
arXiv2023

Global method for gender profile estimation from distribution of first names

As social issues related to gender bias attract closer scrutiny, accurate tools to determine the gender profile of large groups become essential. When explicit data is unavailable, gender is often inferred from names. Current methods follow a strategy whereby individuals of the group, one by one, are assigned a gender label or probability based on gender-name correlations observed in the population at large. We show that this strategy is logically inconsistent and has practical shortcomings, the most notable of which is the systematic underestimation of gender bias. We introduce a global inference strategy that estimates gender composition according to the context of the full list of names. The tool suffers from no intrinsic methodological effects, is robust against errors, easily implemented, and computationally light.

stat.AP↗
arXiv2022

Finite-size scaling and double-crossover critical behavior in two-dimensional incompressible polar active fluids

We study the order-disorder transition in two-dimensional incompressible systems of motile particles with alignment interactions through extensive numerical simulations of the incompressible Toner-Tu (ITT) field theory and a detailed finite-size scaling (FSS) analysis. The transition looks continuous in the explored parameter space, but the effective susceptibility exponent $γ/ν$ and the dynamic exponent $z$ exhibit a strong, non-monotonic variation on the system size in the form of double crossovers. At small sizes, mean-field exponents are observed for the homogeneous $k=0$ mode whereas spatial fluctuations follow Gaussian statistics. A first crossover marks the departure from this regime to one where the system behaves like the equilibrium XY model with long-ranged dipolar interaction and vortex excitations. At larger sizes, scaling deviates from the dipolar XY behavior and a second crossover is observed, to presumably the asymptotic ITT universality class. At this crossover to genuinely off-equilibrium behavior, advection comes in to expedite transport of fluctuations, suppress large-scale fluctuations and help stabilize long-range order. We obtain estimates and bounds of the universal Binder cumulant and exponents of the ITT class. We propose a reduced hydrodynamic theory, previously overlooked, that quantitatively describes the first scaling regime. By providing a relatively comprehensive numerical picture and a novel analytical description, our results help elucidate finite-size effects in critical active matter systems, which have been argued to be relevant for understanding scale-free behavior in real flocks or swarms.

cond-mat.stat-mech↗