UC Davis

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-Si1-xGex) over the full range of atomic concentration from x=0 to x=1, using molecular dynamics and the quasiharmonic Green-Kubo lattice dynamics formalism. We find that the thermal conductivity of a-Si1-xGex 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. Modal analysis unravels the correlations among localization, line broadening, and the contribution to thermal transport of modes within different frequency ranges.