Impact of the ground-state $4f$ symmetry for anisotropic $cf$-hybridization in the heavy fermion superconductor CeNi${}_2$Ge${}_2$

We report the ground-state symmetry of the Ce $4f$ states in the heavy fermion superconductor CeNi${}_2$Ge${}_2$, yielding anisotropic $cf$-hybridization between the Ce $4f$ states and conducting electrons. By analyzing linear dichroism in soft x-ray absorption and core-level hard x-ray photoemission spectra, the $4f$ symmetry is determined as $\mathrm{\Sigma }$-type ${\mathrm{\Gamma }}_7$, promoting predominant hybridization with the conducting electrons originating from the Ge site. The band structures probed by the soft x-ray angle-resolved photoemission indicates that the Ge $4p$ components contribute to the band renormalization through the anisotropic hybridization effects, suggesting that the control of the electronic structures of Ge orbital gives an impact to achieve the exotic phenomena in CeNi${}_2$Ge${}_2$.