Ultrafast investigation of photo-excited carrier-lattice dynamics in PbTe using Fourier-transform inelastic X-ray scattering

We report with fine temporal and momentum resolution non-equilibrium electron-phonon and phonon-phonon dynamics in the widely-used thermoelectric material PbTe. The measurements are made possible by both the intense, ultrafast X-ray pulses of the Linac Coherent Light Source instrument and the recently developed Fourier-transform inelastic X-ray scattering (FT-IXS) technique. We demonstrate experimentally the various effects of impulsive optical photo-excitation on the lattice dynamics of the material as carriers are dramatically redistributed in the system. Specifically, coupled two-phonon states are excited throughout various Brillouin zones and possible plasmon-phonon modes are observed very near zone center. Coupled with calculations from density functional theory (DFT), an analysis of these photo-excited modes yields new insight into the origins of the incipient ferroelectricity and zone center anomalies noted in past measurements on PbTe.