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

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

Figure 1

(a)–(d) Reflectivity spectra of YbS measured under different conditions. The diagram illustrates multiple reflections of the IR ray. See the text for details.

Figure 2

(a) Reflectivity $R_d(\omega )$ of YbS measured in DAC at pressures up to 20 GPa. (b) Drude-Lorentz fitting to the raw $R_d(\omega )$ data in (a).

Figure 3

(a) $\sigma (\omega )$ of YbS at 0.3–20 GPa, and that obtained with KK analysis of $R(\omega )$ measured in vacuum. The spectra are offset for clarity. A logarithmic scale is used below 0.1 eV. (b),(c) Examples of fitting for 15 GPa data.

Figure 4

(a) Pressure dependence of the gap peak, bare plasma frequency (${\omega }_p$), MIR and NIR peaks. Previous data from Ref. 8 are also plotted. The dotted lines and the gray area are guide to the eye. (b) Pressure dependence of the effective electron density ($N^{*}$) per Yb. (c) Schematic illustrations for the electronic structures in YbS.