Mode Localization and Suppressed Heat Transport in Amorphous Alloys

TL;DR

This study explores how mass disorder affects heat transport in amorphous silicon-germanium alloys, revealing mode localization as a key factor in suppressing thermal conductivity and providing insights for designing materials with lower heat conduction.

Contribution

It demonstrates the impact of mass disorder on thermal conductivity in amorphous alloys using molecular dynamics and lattice dynamics, highlighting mode localization effects.

Findings

Thermal conductivity exhibits a smooth U-shape with Ge concentration.

Localization of low-frequency modes causes initial conductivity drop.

Broad minimum thermal conductivity occurs between 25% and 75% Ge concentration.

Abstract

Glasses usually represent the lower limit for the thermal conductivity of solids, but a fundamental understanding of lattice heat transport in amorphous materials can provide design rules to beat such a limit. Here we investigate the role of mass disorder in glasses by studying amorphous silicon-germanium alloy (a-Si$_{1-x}$Ge$_x$) over the full range of atomic concentration from $x=0$ to $x=1$, using molecular dynamics and the quasi-harmonic Green-Kubo lattice dynamics formalism. We find that the thermal conductivity of a-Si$_{1-x}$Ge$_x$ as a function of $x$ exhibits a smoother U-shape than in crystalline mass-disordered alloys. The main contribution to the initial drop of thermal conductivity at low Ge concentration stems from the localization of otherwise extended modes that make up the lowest 8\% of the population by frequency. Contributions from intermediate frequency modes are decreased more gradually with increasing Ge to reach a broad minimum thermal conductivity between concentrations of Ge from $x=0.25$ to $0.75$.