  Layered tin selenide (SnSe) has recently emerged as a high-performance
thermoelectric material with the current record for the figure of merit (ZT)
observed in the high-temperature Cmcm phase. So far, access of the Cmcm phase
has been mainly obtained via thermal equilibrium methods based on sample
heating or application of external pressure, thus restricting the current
understanding only to ground-state conditions. Here, we investigate the
ultrafast carrier and phononic dynamics in SnSe. Our results demonstrate that
optical excitations can transiently switch the point-group symmetry of the
crystal from Pnma to Cmcm at room temperature in a few hundreds of femtoseconds
with an ultralow threshold for the excitation carrier density. This
non-equilibrium Cmcm phase is found to be driven by the displacive excitation
of coherent Ag phonons and, given the absence of low-energy thermal phonons,
exists in SnSe with the status of 'cold lattice with hot carriers'. Our
findings provide important insight for understanding non-equilibrium
thermoelectric properties of SnSe.
