Thermal diffusivity and its lower bound in orthorhombic SnSe

TL;DR

This study investigates the thermal diffusivity of orthorhombic SnSe, revealing it remains above the Planckian limit across temperatures and its anisotropy is governed by sound velocity, contributing to understanding thermoelectric materials.

Contribution

It provides the first detailed measurement of anisotropic thermal diffusivity in SnSe and compares it with related materials to establish a universal diffusivity bound.

Findings

Thermal diffusivity in SnSe exceeds the Planckian limit.

Anisotropy in diffusivity is determined by sound velocity.

Diffusivity in IV-VI compounds relates to melting temperature and sound velocity.

Abstract

The orthorhombic monochalcogenide SnSe has attracted much attention in recent years as a promising high-temperature thermoelectric material. We present a study of its thermal conductivity and specific heat of SnSe between 2~K and 300~K and quantify its anisotropic thermal diffusivity, $D$. For both crystallographic orientations, thermal diffusivity remains above the recently identified Planckian limit ($D > v_s^2 \tau_P$, where $v_s$ is the sound velocity and $\tau_P= \hbar/k_BT$) and its anisotropy in $D$ is set by the anisotropy of $v_s$. Comparison with cubic members of the IV-VI family leads to a consistent picture, where the diffusivity in all members of the family is set by the product of v$_s$, $\tau_P$ and the 'melting' velocity derived from the melting temperature.