Determination of nonthermal bonding origin of a novel photoexcited lattice instability in SnSe

TL;DR

This study uses time-resolved X-ray scattering to reveal that photoexcitation induces a nonthermal lattice instability in SnSe, primarily affecting interlayer bonds rather than intralayer resonant bonds, offering insights into non-equilibrium material behavior.

Contribution

It provides the first detailed analysis of the nonthermal bonding origin of photoinduced lattice instability in SnSe using time-resolved lattice dynamics measurements.

Findings

Photoexcitation causes a lattice instability in SnSe.

The instability is mainly due to changes in fourth-nearest neighbor interlayer bonds.

Intralayer resonant bonds are relatively unaffected.

Abstract

Interatomic forces that bind materials are largely determined by an often complex interplay between the electronic band-structure and the atomic arrangements to form its equilibrium structure and dynamics. As these forces also determine the phonon dispersion, lattice dynamics measurements are often crucial tools for understanding how materials transform between different structures. This is the case for the mono-chalcogenides which feature a number of lattice instabilities associated with their network of resonant bonds and a large tunability in their functional properties. SnSe hosts a novel lattice instability upon above-bandgap photoexcitation that is distinct from the distortions associated with its high temperature phase transition, demonstrating that photoexcitation can alter the interatomic forces significantly different than thermal excitation. Here we report decisive time-resolved X-ray scattering-based measurements of the nonequlibrium lattice dynamics in SnSe. By fitting interatomic force models to the excited-state dispersion, we determine this instability as being primarily due to changes in the fourth-nearest neighbor bonds that connect bilayers, with relatively little change to the intralayer resonant bonds. In addition to providing critical insight into the nonthermal bonding origin of the instability in SnSe, such measurements will be crucial for understanding and controlling materials properties under non-equilibrium conditions.