Photoinduced anisotropic lattice dynamic response and domain formation in thermoelectric SnSe

TL;DR

This study combines ultrafast electron diffraction, simulations, and calculations to directly observe anharmonic lattice distortions in SnSe, revealing domain formation mechanisms that influence thermoelectric efficiency.

Contribution

It introduces an innovative method to directly identify anharmonic lattice distortions and domain formation in thermoelectric materials using ultrafast electron diffraction.

Findings

Soft, anharmonic lattice distortions are linked to interlayer shear strain.

Photoexcitation induces domain formation in SnSe.

The approach aids in understanding ultralow thermal conductivity mechanisms.

Abstract

Identifying and understanding the mechanisms behind strong phonon-phonon scattering in condensed matter systems is critical to maximizing the efficiency of thermoelectric devices. To date, the leading method to address this has been to meticulously survey the full phonon dispersion of the material in order to isolate modes with anomalously large linewidth and temperature-dependence. Here we combine quantitative MeV ultrafast electron diffraction (UED) analysis with Monte Carlo based dynamic diffraction simulation and first-principles calculations to directly unveil the soft, anharmonic lattice distortions of model thermoelectric material SnSe. A small single-crystal sample is photoexcited with ultrafast optical pulses and the soft, anharmonic lattice distortions are isolated using MeV-UED as those associated with long relaxation time and large displacements. We reveal that these modes have interlayer shear strain character, induced mainly by c-axis atomic displacements, resulting in domain formation in the transient state. These findings provide an innovative approach to identify mechanisms for ultralow and anisotropic thermal conductivity and a promising route to optimizing thermoelectric devices.