Mechanistic Transition from Phonon Propagation to Thermal Hopping in Two-Dimensional Solids

TL;DR

This study investigates how thermal transport in two-dimensional solids transitions from phonon propagation to hopping mechanisms as disorder increases, identifying a critical disorder level where the dominant heat transfer process changes.

Contribution

The paper provides a detailed analysis of the transition in heat transport mechanisms in 2D solids, combining atomistic simulations with theoretical analysis to identify the critical disorder level and key indicators.

Findings

Crossover from phonon to hopping transport at disorder level ~0.3

Thermal conductivity shows a turnover in temperature dependence at the transition

Participation ratio and heat flux localization serve as transition indicators

Abstract

Thermal transport in solids changes its nature from phonon propagation that suffers from perturbative scattering to thermally activated hops between localized vibrational modes as the level of disorder increases. Models have been proposed to understand these two distinct extremes that predict opposite temperature dependence of the thermal conductivity, but not for the transition or the intermediate regime. Here we explore thermal transport in two-dimensional crystalline and amorphous silica with varying levels of disorder, {\alpha}, by performing atomistic simulations as well as analysis based on the kinetic and Allen-Feldman theories. We demonstrate the crossover between the crystalline and amorphous regimes at {\alpha} ~ 0.3, which can be identified by a turnover of the temperature dependence in thermal conductivity, and explained by the dominance of thermal hopping processes. The determination of this critical disorder level is also validated by the analysis of the participation ratio of localized vibrational modes, and the spatial localization of heat flux. These factors can serve as key indicators in characterizing the transition in heat transport mechanisms.