Rayleigh waves, surface disorder, and phonon localization in nanostructures

TL;DR

This paper presents a novel FDTD-based method to compute thermal conductivity in disordered nanostructures, revealing how surface disorder and Rayleigh waves significantly influence phonon localization and heat transport.

Contribution

The study introduces a scalable FDTD technique combined with Green-Kubo for analyzing phonon behavior in large, surface-disordered nanostructures, highlighting the impact of Rayleigh waves and surface disorder.

Findings

Free edges lower thermal conductivity compared to fixed edges.

Surface disorder localizes energy at disordered edges, reducing heat transport.

Rayleigh waves and surface modes significantly affect phonon localization.

Abstract

We introduce a technique to calculate thermal conductivity in disordered nanostructures: a finite-difference time-domain (FDTD) solution of the elastic wave equation combined with the Green-Kubo formula. The technique captures phonon wave behavior and scales well to nanostructures that are too large or too surface disordered to simulate with many other techniques. We investigate the role of Rayleigh waves and surface disorder on thermal transport by studying graphenelike nanoribbons with free edges (allowing Rayleigh waves) and fixed edges (prohibiting Rayleigh waves). We find that free edges result in a significantly lower thermal conductivity than fixed ones. Free edges both introduce Rayleigh waves and cause all low-frequency modes (bulk and surface) to become more localized. Increasing surface disorder on free edges draws energy away from the center of the ribbon and toward the disordered edges, where it gets trapped in localized surface modes. These effects are not seen in ribbons with fixed boundary conditions and illustrate the importance of phonon surface modes in nanostructures.