Contrasting pressure evolutions of $f$ electron hybridized states in CeRhIn${}_5$ and YbNi${}_3$Ga${}_9$: an optical conductivity study

Abstract

Optical conductivity [$\sigma (\omega )$] of CeRhIn${}_5$ and YbNi${}_3$Ga${}_9$ have been measured at external pressures to 10 GPa and at low temperatures to 6 K. Regarding CeRhIn${}_5$, at ambient pressure the main feature in $\sigma (\omega )$ is a Drude peak due to free carriers. With increasing pressure, however, a characteristic mid-infrared (mIR) peak rapidly develops in $\sigma (\omega )$, and its peak energy and width increase with pressure. These features are consistent with an increased conduction ($c$)-$f$ electron hybridization at high pressure, and show that the pressure has tuned the electronic state of CeRhIn${}_5$ from very weakly to strongly hybridized ones. As for YbNi${}_3$Ga${}_9$, in contrast, a marked mIR peak is observed already at ambient pressure, indicating a strong $c$-$f$ hybridization. At high pressures, however, the mIR peak shifts to lower energy and becomes diminished, and seems merged with the Drude component at 10 GPa. Namely, CeRhIn${}_5$ and YbNi${}_3$Ga${}_9$ exhibit some opposite tendencies in the pressure evolutions of $\sigma (\omega )$ and electronic structures. These results are discussed in terms of the pressure evolutions of $c$-$f$ hybridized electronic states in Ce and Yb compounds, in particular in terms of the electron-hole symmetry often considered between Ce and Yb compounds.

20 pages, 6 figures, accepted to Physical Review B