# Effect of discrete breathers on macroscopic properties of the Fermi-Pasta-Ulam chain

## TL;DR

This study investigates how discrete breathers influence the macroscopic physical properties of the Fermi-Pasta-Ulam chain, revealing their effects on specific heat, thermal expansion, and Young's modulus during instability and thermalization processes.

## Contribution

It provides new insights into the impact of discrete breathers on macroscopic properties of nonlinear chains with symmetric and asymmetric potentials.

## Key findings

- DBs reduce the specific heat across parameters.
- DBs increase thermal expansion in asymmetric potentials.
- Young's modulus varies with potential asymmetry in the presence of DBs.

## Abstract

The effect of discrete breathers (DBs) on macroscopic properties of the Fermi-Pasta-Ulam chain with symmetric and asymmetric potentials is investigated. The total to kinetic energy ratio (related to specific heat), stress (related to thermal expansion), and Young's modulus are monitored during the development of modulational instability of the zone boundary mode. The instability results in the formation of chaotic DBs followed by the transition to thermal equilibrium when DBs disappear due to energy radiation in the form of small-amplitude phonons. It is found that DBs reduce the specific heat for all the considered chain parameters. They increase the thermal expansion when the potential is asymmetric and, as expected, thermal expansion is not observed in the case of symmetric potential. The Young's modulus in the presence of DBs is smaller than in thermal equilibrium for the symmetric potential and for the potential with a small asymmetry, but it is larger than in thermal equilibrium for the potential with greater asymmetry. Our results can be useful for setting experiments on the identification of DBs in crystals by measuring their macroscopic properties.