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

TL;DR

This study uses infrared spectroscopy to investigate the pressure-induced phase transition in SmS, revealing that excitonic instability drives the black-to-golden phase change through a valence transition.

Contribution

It demonstrates that the black-to-golden phase transition in SmS is driven by excitonic instability, linking valence change to exciton binding energy under pressure.

Findings

Valence transition from Sm2+ to Sm3+ at 0.65 GPa

Energy gap narrows with pressure in black SmS

Transition driven by excitonic instability

Abstract

We report the pressure-dependent optical reflectivity spectra of a strongly correlated insulator, samarium monosulfide (SmS), in the far- and middle-infrared regions to investigate the origin of the pressure-induced phase transition from the black phase to the golden phase. The energy gap becomes narrow with increasing pressure in the black phase. A valence transition from Sm2+ in the black phase to mainly Sm3+ in the golden phase accompanied by spectral change from insulator to metal were observed at the transition pressure of 0.65 GPa. The black-to-golden phase transition occurs when the energy gap size of black SmS becomes the same as the binding energy of the exciton at the indirect energy gap before the gap closes. This result indicates that the valence transition originates from an excitonic instability.