Glass-Like Thermal Conductivity in Nanostructures of a Complex Anisotropic Crystal

TL;DR

This study reveals that nanostructuring complex anisotropic crystals like higher manganese silicide can induce glass-like thermal conductivity due to boundary scattering, confinement effects, and increased incommensurability, offering new pathways for thermal management.

Contribution

It demonstrates the first observation of glass-like thermal conductivity in nanostructured complex anisotropic crystals, highlighting the roles of boundary scattering, confinement, and incommensurability.

Findings

Glass-like thermal conductivity observed in ~20 nm thick nanostructures.

Boundary scattering strongly affects vibrational modes due to anisotropy.

Increased incommensurability suppresses diffusive mode transport.

Abstract

Size effects on vibrational modes in complex crystals remain largely unexplored, despite their importance in a variety of electronic and energy conversion technologies. Enabled by advances in a four-probe thermal transport measurement method, we report the observation of glass-like thermal conductivity in ~20 nm thick single crystalline ribbons of higher manganese silicide, a complex, anisotropic crystal with a ~10 nm scale lattice constant along the incommensurate c axis. The boundary scattering effect is strong for many vibrational modes because of a strong anisotropy in their group velocities or diffusive nature, while confinement effects are pronounced for acoustic modes with long wavelengths along the c axis. Furthermore, the transport of the non-propagating, diffusive modes is suppressed in the nanostructures by the increased incommensurability between the two substructures as a result of the unusual composition of the nanostructure samples. These unique effects point to diverse, new approaches to suppressing the lattice thermal conductivity in complex materials.