arXiv subjects

Michele Giusfredi

Publications and source records attributed to Michele Giusfredi.

4 recordsLinked to original sources

arXiv2026

Generalized diffusion theory for radiative transfer in fully anisotropic scattering media

A generalized anisotropic-diffusion framework is developed for transport problem in media described by a tensorial scattering coefficient and a scalar Henyey--Greenstein asymmetry factor. In this regime the classical similarity relation between scattering and transport parameters fails, and each principal diffusion coefficient depends on all components of the microscopic scattering rate. Explicit expressions are derived for the direction-averaged mean free path, the diagonal elements of the diffusion tensor, and boundary condition lengths via rapidly convergent spherical-harmonics expansions, along with open-source implementations. The resulting predictions are validated against anisotropic Monte Carlo simulations, showing excellent agreement across broad ranges of structural anisotropy and phase-function asymmetry factors. The theory provides a compact, general route connecting microscopic anisotropic scattering to macroscopic diffusion coefficients and boundary conditions in bounded geometries.

physics.optics↗

arXiv2025

Mean-field theory of the DNLS equation at positive and negative absolute temperatures

The Discrete Non Linear Schr\"odinger (DNLS) model, due to the existence of two conserved quantities, displays an equilibrium transition between a homogeneous phase at positive absolute temperature and a localized phase at negative absolute temperature. Here, we provide a mean-field theory of DNLS through a suitable approximation of the grandcanonical partition function which makes it factorizable and can be used to describe the equilibrium state at positive temperatures as well as the metastable state at negative temperatures. By comparing our mean-field results with numerically exact ones, we show that this approximation is good-to-excellent in the whole grandcanonical phase diagram. Explicit approximate expressions for equilibrium observables are provided in the high-temperature limit. Our theory represents a clear advancement over the model that neglects the interaction between sites.

cond-mat.stat-mech↗

arXiv2025

Infinite-temperature thermostats by energy localization in a nonequilibrium setup

Some lattice models having two conservation laws may display an equilibrium phase transition from a homogeneous (positive temperature - PT) to a condensed (negative temperature) phase, where a finite fraction of the energy is localized in a few sites. We study one such stochastic model in an out-of-equilibrium setup, where the ends of the lattice chain are attached to two PT baths. We show that localized peaks may spontaneously emerge, acting as infinite-temperature heat baths. The number $N_b$ of peaks is expected to grow in time $t$ as $N_b \sim \sqrt{\ln t}$, as a consequence of an effective freezing of the dynamics. Asymptotically, the chain spontaneously subdivides into three intervals: the two external ones lying inside the PT region; the middle one characterized by peaks superposed to a background lying along the infinite-temperature line. In the thermodynamic limit, the Onsager formalism allows determining the shape of the whole profile.

cond-mat.stat-mech↗

arXiv2024

Localization in boundary-driven lattice models

Several systems display an equilibrium condensation transition, where a finite fraction of a conserved quantity is spatially localized. The presence of two conservation laws may induce the emergence of such transition in an out-of-equilibrium setup, where boundaries are attached to different and subcritical heat baths. We study this phenomenon in a class of stochastic lattice models, where the local energy is a general convex function of the local mass, mass and energy being both globally conserved in the isolated system. We obtain exact results for the nonequilibrium steady state (spatial profiles, mass and energy currents, Onsager coefficients) and we highlight important differences between equilibrium and out-of-equilibrium condensation.

cond-mat.stat-mech↗