Optical Conductivity Study of $f$ Electron States in YbCu$_2$Ge$_2$ at High Pressures to 20 GPa

Optical conductivity [$\sigma(\omega)$] of YbCu$_2$Ge$_2$ has been measured at external pressures ($P$) to 20 GPa, to study the $P$ evolution of $f$ electron hybridized states. At $P$=0, $\sigma(\omega)$ shows a marked mid-infrared (mIR) peak at 0.37 eV, which is due to optical excitations from $f^{14}$ (Yb$^{2+}$) state located below the Fermi level. With increasing $P$, the mIR peak shows significant shifts to lower energy, reaching 0.18 eV at $P$=20 GPa. This result indicates that the $f^{14}$ energy level increases toward the Fermi level with $P$. Such a shift of the $f$ electron level with $P$ has been expected from theoretical considerations, but had never been demonstrated by spectroscopic experiment under high $P$. The obtained results are also analyzed in terms of the $P$ evolution of the conduction-$f$ electron hybridization.