Conduction-band electronic states of YbInCu4 studied by photoemission and soft x-ray absorption spectroscopies

We have studied conduction-band (CB) electronic states of a typical valence-transition compound YbInCu4 by means of temperature-dependent hard x-ray photoemission spectroscopy (HX-PES) of the Cu 2p3/2 and In 3d5/2 core states taken at hν = 5.95 keV, soft x-ray absorption spectroscopy (XAS) of the Cu 2p3/2 core absorption region around hν ∼ 935 eV, and soft x-ray photoemission spectroscopy (SX-PES) of the valence band at the Cu 2p3/2 absorption edge of hν = 933.0 eV. With decreasing temperature below the valence transition at TV = 42 K, we have found that (1) the Cu 2p3/2 and In 3d5/2 peaks in the HX-PES spectra exhibit the energy shift toward the lower binding-energy side by ∼40 and ∼30 meV, respectively, (2) an energy position of the Cu 2p3/2 main absorption peak in the XAS spectrum is shifted toward higher photon-energy side by ∼100 meV, with an appearance of a shoulder structure below the Cu 2p3/2 main absorption peak, and (3) an intensity of the Cu L3VV Auger spectrum is abruptly enhanced. These experimental results suggest that the Fermi level of the CB-derived density of states is shifted toward the lower binding-energy side. We have described the valence transition in YbInCu4 in terms of the charge transfer from the CB to Yb 4f states.