Sub-amorphous thermal conductivity in amorphous heterogeneous nanocomposites

TL;DR

This study demonstrates that heterogeneous amorphous nanocomposites can achieve thermal conductivities lower than their individual components by localizing vibrational modes, revealing new limits for heat transport in disordered solids.

Contribution

It introduces a novel approach to significantly reduce thermal conductivity in amorphous nanocomposites through vibrational mode localization, expanding understanding of heat transport limits.

Findings

Amorphous SiGe nanocomposites have lower thermal conductivity than pure amorphous Si or Ge.

Vibrational modes above Ge cutoff are localized in the Si host, reducing heat transport.

Localization of vibrational modes is a key mechanism for lowering thermal conductivity.

Abstract

Pure amorphous solids are traditionally considered to set the lower bound of thermal conductivity due to their disordered atomic structure that impedes vibrational energy transport. However, the lower limits for thermal conductivity in heterogeneous amorphous solids and the physical mechanisms underlying these limits remain unclear. Here, we use equilibrium molecular dynamics to show that an amorphous SiGe nanocomposite can possess thermal conductivity substantially lower than those of the amorphous Si and Ge constituents. Normal mode analysis indicates that the presence of the Ge inclusion localizes vibrational modes with frequency above the Ge cutoff in the Si host, drastically reducing their ability to transport heat. This observation suggests a general route to achieve exceptionally low thermal conductivity in fully dense solids by restricting the vibrational density of states available for transport in heterogeneous amorphous nanocomposites.