Stanford University / PULSE Institute

Lead telluride (PbTe) is a widely-used thermoelectric material in the high-temperature regime (400-800 K), largely as a result of its naturally low thermal conductivity. Recent INS measurements link this inherent property of PbTe to observed anomalous behavior of the transverse optical (TO) phonon branch in collected phonon dispersion spectra. It is speculated that this branch may experience strong anharmonic coupling with heat-carrying acoustic phonons, dampening their activity. We investigate the atomic mechanisms behind this purported coupling by measuring photoexcited phonon dispersion spectra of PbTe at LCLS using the novel pump-probe technique Fourier-transform inelastic x-ray scattering (FT-IXS). Here, we impulsively disturb the system out of equilibrium by pumping carriers above the band gap and then track the resulting relaxation dynamics involving carrier-phonon and phonon-phonon interactions with ultrafast (femtosecond) time- and very fine (< 0.1 rlu) momentum-resolution. The results reveal extensive carrier effects that influence the trajectory of various phonon branches.