Photoinduced lattice instability in SnSe*

We demonstrate via time-resolved x-ray diffraction that SnSe has a photo-induced lattice instability associated with an orthorhombic distortion of the rock-salt structure. This lattice instability is distinct from the one associated with the high-temperature Cmcm phase. Our findings show that photoexcitation can transiently induce lattice instability inaccessible on the equilibrium phase diagram. This lattice instability is signaled by the drastic softening behavior of the soft phonon while its displacement direction is away from the thermally existent higher symmetry phase. The resonant bonding theory that explains the lattice instability of many rock-salt structure type semiconductor, and predicts the Pnma-Cmcm lattice instability in SnSe, may not be relevant under photoexcitation. The nonthermal distribution of photoexcited carriers in this semiconductor do not impact the lattice like merely raising electron temperatures. The results point to the need for investigating electron phonon coupling in the nonequilibrium region, as well as implications for manipulating matter with light.