Photo-Induced Ultrafast Symmetry Switch in SnSe

TL;DR

This study demonstrates that ultrafast optical excitation can transiently switch the symmetry of SnSe from Pnma to Cmcm at room temperature within hundreds of femtoseconds, revealing new non-equilibrium phases relevant for thermoelectric applications.

Contribution

It shows that optical excitation can induce a non-equilibrium Cmcm phase in SnSe at room temperature, expanding understanding of its dynamic symmetry properties.

Findings

Ultrafast optical pulses switch SnSe symmetry from Pnma to Cmcm.

The phase transition occurs within a few hundred femtoseconds.

The non-equilibrium phase is driven by coherent Ag phonons.

Abstract

Layered tin selenide (SnSe) has recently emerged as a high-performance thermoelectric material with the current record for the figure of merit (ZT) observed in the high-temperature Cmcm phase. So far, access of the Cmcm phase has been mainly obtained via thermal equilibrium methods based on sample heating or application of external pressure, thus restricting the current understanding only to ground-state conditions. Here, we investigate the ultrafast carrier and phononic dynamics in SnSe. Our results demonstrate that optical excitations can transiently switch the point-group symmetry of the crystal from Pnma to Cmcm at room temperature in a few hundreds of femtoseconds with an ultralow threshold for the excitation carrier density. This non-equilibrium Cmcm phase is found to be driven by the displacive excitation of coherent Ag phonons and, given the absence of low-energy thermal phonons, exists in SnSe with the status of 'cold lattice with hot carriers'. Our findings provide important insight for understanding non-equilibrium thermoelectric properties of SnSe.