Observation of a Novel Lattice Instability in Ultrafast Photoexcited SnSe

Abstract

There is growing interest in using ultrafast light pulses to drive functional materials into nonequilibrium states with novel properties. The conventional wisdom is that above-gap photoexcitation behaves similarly to raising the electronic temperature and lacks the desired selectivity in the final state. Here, we report a novel nonthermal lattice instability induced by ultrafast above-gap excitation in SnSe, a representative of theIV−VIclass of semiconductors that provides a rich platform for tuning material functionality with ultrafast pulses due to their multiple lattice instabilities. The new lattice instability is accompanied by a drastic softening of the lowest-frequencyAgphonon. This mode has previously been identified as the soft mode in the thermally driven phase transition to aCmcmstructure. However, by a quantitative reconstruction of the atomic displacements from time-resolved x-ray diffraction for multiple Bragg peaks and excitation densities, we show that ultrafast photoexcitation with near-infrared (1.55 eV) light induces a distortion toward a different structure withImmmsymmetry. TheImmmstructure of SnSe is an orthorhombic distortion of the rocksalt structure and does not occur in equilibrium. Density functional theory calculations reveal that the photoinducedImmmlattice instability arises from electron excitation from the Se4p- and Sn5s-derived bands deep below the Fermi level that cannot be excited thermally. The results have implications for optical control of the thermoelectric, ferroelectric, and topological properties of the monochalcogenides and related materials. More generally, the results emphasize the need for ultrafast structural probes to reveal distinct atomic-scale dynamics that are otherwise too subtle or invisible in conventional spectroscopies.

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Plain Language Summary

Ultrafast x-ray free-electron lasers enable detailed investigation of the structure and dynamics of materials with unprecedented temporal and spatial resolution. At the same time, there is growing interest in the use of ultrafast optical pulses to generate novel states of matter, with the goal of creating materials’ properties on demand. Here, we use ultrafast x-ray pulses to measure how the atomic lattice of tin selenide (SnSe) responds to ultrafast optical excitation, finding a novel photoinduced instability that drives the material toward a new high-symmetry structure.

SnSe represents a broader class of materials that provides a rich platform for tuning materials’ properties by means of their multiple lattice instabilities. In our experiments, we excite a sample of SnSe with near-infrared laser pulses and use scattered x rays to observe coherent phonon dynamics. The time-resolved x-ray diffraction provides critical information on the atomic motion, including phase, from which we can quantify the atomic-scale structural rearrangements.

From the coherent phonon motion, we find that photoexcitation moves the structure further from the high-temperature phase toward a new structure. We identify this new structure as belonging to a higher symmetry space group that is distinct from any known phase of this material. First-principles calculations support these results and help us identify electronic states that play a decisive role in determining the photoexcited state.

Our results suggest strategies for stabilizing the higher symmetry structure as well as potentially controlling nontrivial topological properties. The results, therefore, have important implications for related materials in and out of equilibrium, as well as the broader goal of controlling materials’ properties with light.