Effect of characteristic size on the collective phonon transport in crystalline GeTe

TL;DR

This study investigates how the size of crystalline GeTe influences phonon thermal transport, revealing a collective hydrodynamic regime between ballistic and diffusive limits, with implications for low-thermal conductivity materials.

Contribution

It introduces a detailed analysis of size and temperature effects on phonon hydrodynamics in GeTe using first-principles methods and identifies key length scales and scaling relations.

Findings

Identification of a collective phonon transport regime in GeTe.

Extraction of nonlocal and heat wave propagation lengths.

Scaling relations linking thermal conductivity and size.

Abstract

We study the effect of characteristic size variation on the phonon thermal transport in crystalline GeTe for a wide range of temperatures using the first-principles density-functional method coupled with the kinetic collective model approach. The characteristic size dependence of phonon thermal transport reveals an intriguing collective phonon transport regime, located in between the ballistic and the diffusive transport regimes. Therefore, systematic investigations have been carried out to describe the signatures of phonon hydrodynamics via the competitive effects between grain size and temperature. A characteristic nonlocal length associated with phonon hydrodynamics and a heat wave propagation length has been extracted. The connections between phonon hydrodynamics and these length scales are discussed in terms of the Knudsen number. Further, the scaling relation of thermal conductivity as a function of characteristic size in the intermediate size range emerges as a crucial indicator of the strength of the hydrodynamic behavior. A ratio concerning normal and resistive scattering rates has been employed to understand these different scaling relations, which seems to control the strength and prominent visibility of the collective phonon transport in GeTe. This systematic investigation emphasizes the importance of the competitive effects between temperature and characteristic size on phonon hydrodynamics in GeTe, which can lead to a better understanding of the generic behavior and consequences of the phonon hydrodynamics and its controlling parameters in low-thermal conductivity materials.