Photoinduced dynamics of nematic order parameter in FeSe

Formation of electronic nematicity is a common thread of unconventional superconductors. We use ultrafast electron diffraction to probe the lattice interactions with electronic degrees of freedom in superconducting FeSe and find a significant lattice response to local nematicity. We observe that a perturbation by a laser pulse leads to a surprising enhancement of the high-symmetry crystalline order as a result of suppression of low-symmetry local lattice distortions, which are signatures of nematic fluctuations. The distortions are present at temperatures both below and above the nematic phase transition, as corroborated by our x-ray pair distribution function analysis and transmission electron microscopy measurements. Nonequilibrium lattice behavior of FeSe reveals two distinct time scales of nematic response to photoexcitation, 130(20) and 40(10) ps, corresponding to diffusive and percolative dynamics of nematic fluctuations respectively.