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

## TL;DR

This study investigates how external pressure influences the $c$-$f$ hybridized electronic states in CeRhIn$_5$ and YbNi$_3$Ga$_9$ using optical conductivity measurements, revealing contrasting pressure-dependent behaviors.

## Contribution

It provides the first comparative optical conductivity analysis of CeRhIn$_5$ and YbNi$_3$Ga$_9$ under pressure, highlighting opposite evolutions of hybridized states in Ce and Yb compounds.

## Key findings

- CeRhIn$_5$ shows increased $c$-$f$ hybridization with pressure.
- YbNi$_3$Ga$_9$ exhibits decreased hybridization at high pressure.
- Opposite pressure responses in electronic structures of Ce and Yb compounds.

## 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.