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

TL;DR

This study demonstrates how applying pressure transforms YbS from an insulator into a heavy electron metal by inducing band overlap and hybridization, as revealed through infrared optical conductivity measurements.

Contribution

It provides the first infrared evidence of pressure-induced insulator-to-metal transition and heavy fermion behavior in YbS.

Findings

Optical conductivity increases dramatically above 8 GPa.

Development of a Drude component indicating heavy carriers.

Pressure causes band overlap and hybridization between c band and 4f state.

Abstract

Optical conductivity [$\sigma(\omega)$] of YbS has been measured under pressure up to 20 GPa. Below 8 GPa, $\sigma(\omega)$ 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, $\sigma(\omega)$ 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.