Pressure Tuning of an Ionic Insulator into a Heavy Electron Metal: An Infrared Study of YbS

Abstract

Optical conductivity [$σ(ω)$] of YbS has been measured under pressure up to 20 GPa. Below 8 GPa, $σ(ω)$ is low since YbS is an insulator with an energy gap between fully occupied 4$f$ state and unoccupied conduction ($c$) band. Above 8 GPa, however, $σ(ω)$ increases dramatically, developing a Drude component due to heavy carriers and characteristic infrared peaks. It is shown that increasing pressure has caused an energy overlap and hybridization between the $c$ band and 4$f$ state, thus driving the initially ionic and insulating YbS into a correlated metal with heavy carriers.