Electronic structures of CeRu\(_2X_2\) (\(X\) = Si, Ge) in the paramagnetic phase studied by soft X-ray ARPES and hard X-ray photoelectron spectroscopy

Abstract

Soft and hard X-ray photoelectron spectroscopy (PES) has been performed for one of the heavy fermion system CeRu\(_2\)Si\(_2\) and a \(4f\)-localized ferromagnet CeRu\(_2\)Ge\(_2\) in the paramagnetic phase. The three-dimensional band structures and Fermi surface (FS) shapes of CeRu\(_2\)Si\(_2\) have been determined by soft X-ray \(h\nu\)-dependent angle resolved photoelectron spectroscopy (ARPES). The differences in the Fermi surface topology and the non-\(4f\) electronic structures between CeRu\(_2\)Si\(_2\) and CeRu\(_2\)Ge\(_2\) are qualitatively explained by the band-structure calculation for both \(4f\) itinerant and localized models, respectively. The Ce valences in CeRu\(_2X_2\) (\(X\) = Si, Ge) at 20 K are quantitatively estimated by the single impurity Anderson model calculation, where the Ce 3d hard X-ray core-level PES and Ce 3d X-ray absorption spectra have shown stronger hybridization and signature for the partial \(4f\) contribution to the conduction electrons in CeRu\(_2\)Si\(_2\).