Entropy production and collective excitations of crystals out of equilibrium: the concept of entropons

TL;DR

This paper introduces and analyzes entropons, a new type of collective excitation in out-of-equilibrium crystals, which dominate spectral entropy production and extend the concept beyond self-propelled particle systems.

Contribution

The paper generalizes the concept of entropons to a broader class of active crystals, providing theoretical derivation and explicit examples of their existence.

Findings

Entropons coexist with phonons in out-of-equilibrium crystals.

Entropons dominate over phonons far from equilibrium.

Existence of entropons confirmed in various active crystal models.

Abstract

We study the collective vibrational excitations of crystals under out-of-equilibrium steady conditions that give rise to entropy production. Their excitation spectrum comprises equilibrium-like phonons of thermal origin and additional collective excitations called entropons because each of them represents a mode of spectral entropy production. Entropons coexist with phonons and dominate over them when the system is far from equilibrium while they are negligible in near-equilibrium regimes. The concept of entropons has been recently introduced and verified in a special case of crystals formed by self-propelled particles. Here, we show that entropons exist in a broader class of active cyrstals that are intrinsically out of equilibrium and characterized by the lack of detailed balance. After a general derivation, several explicit examples are discussed, including crystals consisting of particles with alignment interactions and frictional contact forces.