Thermal conductivity decomposition in two-dimensional materials: Application to graphene

TL;DR

This paper introduces a method to decompose heat transport in two-dimensional materials into in-plane and out-of-plane components, revealing the significant role of flexural phonons in graphene's thermal conductivity.

Contribution

It presents a novel decomposition approach for microscopic heat current in atomistic simulations, enabling direct calculation of thermal conductivity components in 2D materials.

Findings

Flexural phonons contribute about two thirds of graphene's thermal conductivity.

Flexural component causes logarithmic divergence under tensile strain.

Decomposition method improves understanding of heat transport mechanisms.

Abstract

Two-dimensional materials have unusual phonon structures due to the presence of flexural (out-of-plane) modes. Although molecular dynamics simulations have been extensively used to study heat transport in such materials, conventional formalisms treat the phonon dynamics isotropically. Here, we decompose the microscopic heat current in atomistic simulations into in-plane and out-of-plane components, corresponding to the in-plane and out-of-plane phonon dynamics, respectively. This decomposition allows for direct computation of the corresponding thermal conductivity components in two-dimensional materials. We apply this decomposition to study heat transport in suspended graphene, using both equilibrium and non-equilibrium molecular dynamics simulations. We show that the flexural component is responsible for about two thirds of the total thermal conductivity in unstrained graphene, and the acoustic flexural component is responsible for the logarithmic divergence of the conductivity when a sufficiently large tensile strain is applied.