Discovery and Characterization of a Novel Lattice Instability in SnSe

We use ultrafast X-ray scattering to study SnSe, a resonantly bonded material. Resonantly bonded materials have various functional properties directly associated with the

structures. They host a number of structural phases that are sensitive to external parameters (e.g., temperature, pressure, and chemical doping) and are expected to exhibit tunability by

the light field. The large polarizability in resonantly bonded materials means pronounced coupling between phonons and electronic states, which yields large responses of the X-ray

probe.

We show that using a combination of ultrafast optical and X-ray lasers, we can understand materials on the natural time and length scales of their chemical bonding, which is not achievable with purely optical probes. The knowledge of the microscopic interactions in the non-equilibrium states will ultimately help us explore possible new functionalities in the non-equilibrium phases.

In particular, we use time-resolved X-ray diffraction to obtain amplitude as well as the phase of atomic motion, which allows us to reconstruct the lattice structure of SnSe. The structural distortions and the related new phase are unexpected, and cannot be correctly concluded from a purely optical (e.g., Raman scattering) measurement. We also use time-resolved X-ray diffuse scattering to access the excitedstate dispersion of SnSe, which elucidates how photoexcitation alters the strength of specific bonds leading to this the novel lattice instability observed in diffraction.