  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 A$_g$ 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 towards 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 (DFT) calculations reveal that
the photoinduced Immm lattice instability arises from electron excitation from
the Se 4$p$- and Sn 5$s$-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.
