Synchrotron Terahertz Spectroscopy of Solids under Extreme Conditions

We report on the end station for terahertz spectroscopy at high pressures and low temperatures at the UVSOR Facility of the Institute for Molecular Science, and present one set of experimental results for the pressure-dependent valence transition of samarium sulfide (SmS) at the temperatures of 6 and 300 K. The end station covers the photon energy range of 5 meV to 1.2 eV with pressures of up to 6 GPa and temperatures down to 3.7 K. Using the end station, the valence transition, namely, the black golden phase transition, of SmS was clearly observed at 0.65 GPa. From Drude and Lorentz fitting of the pressure-dependent reflectivity spectra, the carrier density at the critical pressure was found to be about 10(-2)/SmS at both 6 and 300 K. The carrier density appear to play an important role in the valence transition.