arXiv subjects

Olivier Coquand

Publications and source records attributed to Olivier Coquand.

9 recordsLinked to original sources

arXiv2026

Large scale behavior in the Kuramoto-Sivashinsky equation: The Schwinger-Dyson route

The present paper is a study of the large scale properties of the Kuramoto-Sivashinsky equation. By using a Schwinger-Dyson framework, we aim to provide a proof that the only solutions that can sustain a stable scaling in the infrared limit have a negative effective viscosity (in the Kuramoto-Sivashinsky sense) with a minimal set of hypotheses, thereby showing that this constitutes a general property of the wave equation that does not depend on a specific set of truncations of a renormalisation group flow, or limitations of a given numerical scheme for example.

cond-mat.stat-mech↗

arXiv2026

Direct Boltzmann inversion method from particle configurations at arbitrary state points

We introduce a direct Boltzmann inversion method to infer the interaction potential in particle systems using as input particle configurations generated at an arbitrary state point of the system. Unlike iterative Boltzmann inversion, the proposed method does not require performing a new Monte Carlo simulation at each step of the iteration process. It relies instead on enforcing consistency between two independent estimates of the pair correlation function, respectively obtained from interparticle distances and from pairwise forces. As a result, the approach is computationally inexpensive and straightforward to implement. Because it relies on the sole expression of interparticle forces, our method naturally applies to any state point, including when the density is large and alternative methods may fail. Here we present the basic principles of the method and benchmark its performance on a diverse set of test potentials studied using computer simulations. Practical aspects and detailed implementation of the method are also discussed. Owing to its simplicity and generality, the method should be broadly applicable, from the construction of coarse-grained interaction potentials to the inference of effective interactions in non-equilibrium systems.

cond-mat.stat-mech↗

arXiv2026

Getting rid of the ghosts: a toy-model of membrane melting

The theory of thermal fluctuations in crystalline membranes is put under scrutiny. In particular, the two critical regimes of the renormalisation group diagram, which are often left out of the discussion because of their instability in one direction, are examined in details. After studying the proper Goldstone mode counting around each of them, the properties of the fluctuations dominating the large scale spectrum are analysed. This shows that the fixed point P2 is a good candidate to describe the melting of a crystalline membrane. The properties of the melted membrane are then compared to the known properties of fluid membranes. As a byproduct of this analysis, we show that the generation of a fluid membrane by melting a bidimensional crystal allows to formulate its correlation functions without being plagued by the ghosts that inevitably show up in the usual Canham-Helfrich action relying on the Monge parametrisation.

cond-mat.soft↗

arXiv2025

Granular rheology: a tale of three time scales

We adapt statistical models of the physics of complex fluids to study the rheology of granular liquids. This allows us to provide laws of granular rheology based on first principles, which compare well with previously established phenomenological laws. In particular, the very successful law of mu(I) rheology can be understood within our model as the lowest order non trivial Pade approximant of the macroscopic laws of rheology. Our model's ability to describe granular physics outside of the Bagnold scaling regime allows for a natural extension to the rheology of granular suspensions.

cond-mat.soft↗

arXiv2025

The power spectrum of compressible turbulence: A story in symmetries

This article presents a theoretical study of the scaling properties of the kinetic energy spectrum in compressible turbulence. From the fundamental symmetries and linear transformations of the microscopic action, we derive exact relations between the correlation functions and their generators. These relations put strong constraints on the possible scaling relations in the system as a function of scale. One of the main results of this study is that the action can be split between an incompressible part, which is the same as in the usual stochastic Navier-Stokes theory whatever the value of the Mach number, and a longitudinal part, whose behavior is to be compared to the three dimensional Burgers equation, which presents a much richer phase diagram as its usually discussed one dimensionalcounterpart.

physics.flu-dyn↗

arXiv2021

Rheology of granular liquids in extensional flows: Beyond the $μ(\mathcal{I})$-law

The Granular Integration Through Transients (GITT) formalism gives a theoretical description of the rheology of moderately dense granular flows and suspensions. In this work, we extend the GITT equations beyond the case of simple shear flows studied before. Applying this to the particular example of extensional flows, we show that the predicted behavior is somewhat different from that of the more frequently studied simple shear case, as illustrated by the possibility of non monotonous evolution of the effective friction coefficient $μ$ with the inertial number $\mathcal{I}$. By the reduction of the GITT equations to simple toy-models, we provide a generalization of the $μ(\mathcal{I})$-law true for any type of flow deformation. Our analysis also includes a study of the Trouton ratio, which is shown to behave quite similarly to that of dense colloidal suspensions.

cond-mat.soft↗

arXiv2020

Integration through transients approach to the $μ(\mathcal{I})$ rheology

This work generalises the granular integration through transients formalism introduced by Kranz et al. [Phys. Rev. Lett. 121, 148002 (2018)] to the determination of the pressure. We focus on the Bagnold regime, and provide theoretical support to the empirical $μ(\mathcal{I})$ rheology laws, that have been successfully applied in many granular flow problems. In particular, we confirm that the interparticle friction is irrelevant in the regime where the $μ(\mathcal{I})$ laws apply.

cond-mat.soft↗

arXiv2014

Refractive properties of graphene in a medium-strong external magnetic field

1-loop quantum corrections are shown to induce large effects on the refraction index $n$ inside a graphene strip in the presence of an external magnetic field $B$ orthogonal to it. To this purpose, we use the tools of Quantum Field Theory to calculate the photon propagator at 1-loop inside graphene in position space, which leads to an effective vacuum polarization in a brane-like theory of photons interacting with massless electrons at locations confined inside the thin strip (its longitudinal spread is considered to be infinite). The effects factorize into quantum ones, controlled by the value of $B$ and that of the electromagnetic coupling $α$, and a "transmittance function" $U$ in which the geometry of the sample and the resulting confinement of electrons play the major roles. We consider photons inside the visible spectrum and magnetic fields in the range 1-20\; Teslas. At $B=0$, quantum effects depend very weakly on $α$ and $n$ is essentially controlled by $U$; we recover, then, an opacity for visible light of the same order of magnitude $πα_{vac}$ as measured experimentally.

hep-ph↗

arXiv2014

From Quantum Field Theory to Nano-Optics : Refractive Properties of Graphene in a Medium-Strong Magnetic field

1-loop quantum corrections are shown to induce large effects on the refraction index n inside a graphene strip in the presence of an external magnetic field B orthogonal to it. To this purpose, we use the tools of Quantum Field Theory to calculate the photon propagator at 1-loop inside graphene in position space, which leads to an effective vacuum polarization in a brane-like theory of photons interacting with massless electrons at locations confined inside the thin strip (its longitudinal spread is considered to be infinite). The effects factorize into quantum ones, controlled by the value of B and that of the electromagnetic coupling alpha, and a "transmittance function" U in which the geometry of the sample and the resulting confinement of electrons play the major roles. We consider photons inside the visible spectrum and magnetic fields in the range 1-20 Teslas. At B=0, quantum effects depend very weakly on alpha and n is essentially controlled by U; we recover, then, an opacity for visible light of the same order of magnitude pi * alpha_{vac} as measured experimentally.

hep-ph↗