Phonon anomalies at the valence transition of SmS: An inelastic x-ray-scattering study under pressure

The phonon dispersion curve of SmS under pressure was studied by inelastic x-ray scattering around the pressure-induced valence transition. Between 0 and 0.9 GPa, a significant softening of the longitudinal acoustic modes propagating along the [111] direction was observed spanning a wide q region from (2π/3a,2π/3a,2π/3a) up to the zone boundary. The largest softening occurs at the zone boundary. At higher pressures up to 7.6 GPa, these phonon modes become much less pressure sensitive, whereas the low q modes display a significant hardening. This phonon spectrum indicates favorable conditions for the emergence of pressure-induced superconductivity in SmS.