Photoinduced Lattice Instability in SnSe Yijing Huang, 1, 2, 3 Shan Yang, 4 Samuel Teitelbaum, 1, 3 Gilberto De la Pe˜ na, 1, 3 Takahiro Sato, 5 Matthieu Chollet, 5 Diling Zhu, 5 Jennifer L. Niedziela, 6, 7 Dipanshu Bansal, 6 Andrew F. May, 7 Aaron M. Lindenberg, 1, 3, 8 Olivier Delaire, 4, 9, 10 David A. Reis, 1, 2, 3, 11 and Mariano Trigo 1, 3 1 Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA 2 Department of Applied Physics, Stanford University, Stanford, California 94305, USA 3 PULSE Institute of Ultrafast Energy Science, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA 4 Department of Mechanical Engineering and Materials Science, Duke University, Durham, North Carolina 27708, USA 5 Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA 6 Department of Mechanical Engineering and Materials Science, Duke University, Durham, North Carolina 27708, USA 7 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 8 Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA 9 Department of Physics, Duke University, Durham, North Carolina 27708, USA 10 Department of Chemistry, Duke University, Durham, North Carolina 27708, USA 11 Department of Photon Science, Stanford University, Stanford, California 94305, USA We report femtosecond optical pump and x-ray diffraction probe experiments on SnSe. We find that under photoexcitation, SnSe has an instability towards an orthorhombically-distorted rocksalt structure that is not present in the equilibrium phase diagram. The new lattice instability is accom- panied by a drastic softening of the lowest frequency Ag phonon which is usually associated with the thermodynamic Pnma-Cmcm transition. However, our reconstruction of the transient atomic displacements shows that instead of moving towards the Cmcm structure, the material moves to- wards a more symmetric orthorhombic distortion of the rock-salt structure belonging to the Immm space group. The experimental results combined with density functional theory (DFT) simulations show that photoexcitation can act as a state-selective perturbation of the electronic distribution, in this case by promoting electrons from Se 4p Sn 5s derived bands from deep below the Fermi level. The subsequent potential energy landscape modified by such electronic excitation can reveal minima with metastable phases that are distinct from those accessible in equilibrium. These results may have implications for optical control of the thermoelectric, ferroelectric and topological properties of the monochalcogenides and related materials. Ultrafast photoexcitation can alter the delicate ener- getic balance between nearly-degenerate material phases and the energy barriers separating them, potentially pro- ducing structures with novel properties not accessible in thermal equilibrium [1]. Photoexcitation can induce a large, macroscopic atomic motion through displacive ex- citation of coherent phonons (DECP) [2–7]. This is often seen in materials that exhibit second-order phase transi- tions that are characterized by soft-mode behavior. Ul- trafast electron and x-ray scattering have been used to study phonon dynamics related to phase transitions and their relation to DECP [8–20]. In particular, x-ray free electron laser (FEL) sources provide extremely bright and ultra-short pulses that can probe the atomic posi- tions on a sub-phonon timescale [21–26]. Here we present ultrafast x-ray diffraction results using an x-ray FEL on photoexcited SnSe. We observe coherent oscillations as- sociated with A g phonons of the room temperature struc- ture. From the measured amplitude of these modes, we reconstruct the transient atomic displacements and to- gether with ab-initio simulations demonstrate a new lat- tice instability towards a different structure than those accessible in thermal equilibrium. SnSe and related rocksalt-like IV-VI compounds host a number of lattice instabilities associated with their nearly cubic resonant bonding network. Differences in ionicity and spin-orbit coupling control the orbital hybridizations and lead to a diverse range of structure phases [27–29]. The stability of these phases is sensitive to external pa- rameters including temperature, pressure [30], as well as stoichiometry [31], stemming from the large polariz- ability that has its origin in unsaturated resonant bond- ing [32] and electron phonon interactions [23]. The large polarizability gives rise to a high lattice anharmonicity that leads to exceptional thermoelectric performance [33– 40], and structural instabilities that lead to phase change behavior [31, 41], and ferroelectricity in these materials’ 2D form [42–45], or antiferroelectricity in their bulk form. In this regard, light-matter interactions can be used to manipulate materials, including, for example, disordering the antiferroelectric alignment of in-plane dipole field in the bulk, which could give rise to negative electrocaloric responses [46, 47], or screening the internal ferroelectric polarization with electron-hole excitation, thus driving towards a higher symmetry phase through photostrictive responses [48]. SnSe sits close to the boundary between orthorhombic and cubic phases [28, 29]. The rocksalt phase of SnSe has been realized at room temperature, but only as a metastable phase on epitaxial films [49, 50]. The rocksalt arXiv:2106.07863v1 [cond-mat.mtrl-sci] 15 Jun 2021