# Heat perturbations spreading in the Fermi-Pasta-Ulam-$\beta$ system with next-nearest-neighbor coupling: Competition between phonon dispersion and nonlinearity

## TL;DR

This study investigates how phonon dispersion and nonlinearity influence heat transport in a 1D Fermi-Pasta-Ulam-$eta$ lattice with next-nearest-neighbor interactions, revealing complex scaling behaviors and a transition in transport regimes.

## Contribution

It demonstrates the interplay between phonon dispersion and nonlinearity in heat transport, uncovering multiscaling and transition phenomena in a modified FPU-$eta$ model.

## Key findings

- Ballistic heat transport at low anharmonicity
- Superdiffusive heat transport at high anharmonicity
- Nonmonotonic delocalization in intermediate nonlinearity

## Abstract

We employ the heat perturbations correlation function to study thermal transport in the one-dimensional (1D) Fermi-Pasta-Ulam-$\beta$ lattice with both nearest-neighbor and next-nearest-neighbor couplings. We find that such a system bears a peculiar phonon dispersion relation, and thus there exists a competition between phonon dispersion and nonlinearity that can strongly affect the heat correlation function' shape and scaling property. Specifically, for small and large anharmoncity, the scaling laws are ballistic and superdiffusive types, respectively, which are in good agreement with the recent theoretical predictions; whereas in the intermediate range of the nonlinearity, we observe an unusual multiscaling property characterized by a \emph{nonmonotonic} delocalization process of the central peak of the heat correlation function. To understand this multiscaling laws, we also examine the momentum perturbations correlation function and find a transition process with the same turning point of the anharmonicity as that shown in the heat correlation function. This suggests coupling between the momentum transport and the heat transport, in agreement with the theoretical arguments of mode cascade theory.