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 a fully occupied 4f state and an 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 4f state, thus driving the initially ionic and insulating YbS into a correlated metal with heavy carriers.
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