Thermal diffusivity and its lower bound in orthorhombic SnSe

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 ($D>{v}_s²{\ensuremath{\tau}}_P$, where ${v}_s$ is the sound velocity and ${\ensuremath{\tau}}_P=\ensuremath{\hbar}/{k}_BT$) 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 ${\mathrm{v}}_s$, ${\ensuremath{\tau}}_P$ and the `melting' velocity derived from the melting temperature.