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We report the pressure-dependent optical reflectivity spectra of a stronglycorrelated insulator, samarium monosulfide (SmS), in the far- andmiddle-infrared regions to investigate the origin of the pressure-induced phasetransition from the black phase to the golden phase. The energy gap becomesnarrow with increasing pressure in the black phase. A valence transition fromSm2+ in the black phase to mainly Sm3+ in the golden phase accompanied byspectral change from insulator to metal were observed at the transitionpressure of 0.65 GPa. The black-to-golden phase transition occurs when theenergy gap size of black SmS becomes the same as the binding energy of theexciton at the indirect energy gap before the gap closes. This result indicatesthat the valence transition originates from an excitonic instability.

Excitonic Instability in the Transition from the Black Phase to the Golden Phase of SmS under Pressure Investigated by Infrared Spectroscopy