Electronic structure of YbNiX3 (X=Si, Ge) studied by hard X-ray photoemission spectroscopy

Electronic structure of the Kondo lattices YbNiX3 (X=Si, Ge) has been investigated by means of hard x-ray photoemission spectroscopy (HAXPES) with h nu = 5.95 keV. From the Yb 3d HAXPES spectra, the Yb valence in YbNiSi3 is estimated to be similar to 2.92, which is almost temperature-independent. On the other hand, the valence in YbNiGe3 is estimated to be 2.48 at 300 K, showing significant valence fluctuation, and gradually decreases to 2.41 at 20 K on cooling. The Ni 2p(3/2) and Yb3+ 4f peaks exhibit opposite energy shifts amounting to similar to 0.6 eV between YbNiSi3 and YbNiGe3. We propose a simple model for the electronic structure of YbNiX3 based on the HAXPES results.