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 the IV−VI class 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-frequency Ag phonon. This mode has previously been identified as the soft mode in the thermally driven phase transition to a Cmcm structure. 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 with Immm symmetry. The Immm structure of SnSe is an orthorhombic distortion of the rocksalt structure and does not occur in equilibrium. Density functional theory calculations reveal that the photoinduced Immm lattice instability arises from electron excitation from the Se 4p- and Sn 5s-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.8 MoreReceived 15 June 2021Revised 10 November 2021Accepted 20 December 2021DOI:https://doi.org/10.1103/PhysRevX.12.011029Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectron-phonon couplingOptical phononsTechniquesUltrafast pump-probe spectroscopyX-ray diffractionCondensed Matter, Materials & Applied Physics
Highlights
There is growing interest in using ultrafast light pulses to drive functional materials into nonequilibrium states with novel properties
We report a novel nonthermal lattice instability induced by ultrafast above-gap excitation in SnSe, a representative of the IV − VI class of semiconductors that provides a rich platform for tuning material functionality with ultrafast pulses due to their multiple lattice instabilities
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 with Immm symmetry
Summary
The Ag displacement connecting Pnma to the new structure must be consistent with the experimental observation ΔxSe > 0, ΔzSe < 0, ΔxSn > 0, and ΔzSn > 0 Based on these criteria, we identify Immm as the space group associated with the photoexcited lattice instability (see Appendix A). We show that ultrafast NIR photoexcitation of SnSe favors a structural instability toward Immm, an orthorhombically distorted rocksalt structure, rather than toward the thermodynamic Cmcm phase Though both Cmcm − Pnma and the Immm − Pnma instabilities can be thought of as symmetry lowering due to a Peierls-like mechanism, they are related to different electron orbitals. This could be exploited to direct a particular structural distortion to desirable outcomes with particular functionality beyond those accessible in thermal equilibrium
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