We have studied conduction-band (CB) electronic states of a typical valence-transition compound YbInCu 4 by means of temperature-dependent hard x-ray photoemission spectroscopy (HX-PES) of the Cu 2p 3/2 and In 3d 5/2 core states taken at hν = 5.95 keV, soft x-ray absorption spectroscopy (XAS) of the Cu 2p 3/2 core absorption region around hν ∼ 935 eV, and soft x-ray photoemission spectroscopy (SX-PES) of the valence band at the Cu 2p 3/2 absorption edge of hν = 933.0 eV. With decreasing temperature below the valence transition at T V = 42 K, we have found that (1) the Cu 2p 3/2 and In 3d 5/2 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 2p 3/2 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 2p 3/2 main absorption peak, and (3) an intensity of the Cu L 3 VV 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 YbInCu 4 in terms of the charge transfer from the CB to Yb 4f states.

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Electronic structure of quasi-one-dimensional BaVS 3 with metal-insulator transition at 70 K has been investigated by hard x-ray photoemission spectroscopy (HAXPES). An energy gap of 60 meV across the transition temperature is observed in the HAXPES spectra near the Fermi level. Line widths of the Ba 3d 5/2 and S 1s spectra continuously decrease on cooling. The analyses based on the Debye model give the Debye temperature of Θ D = 250 ± 50 K, which is consistent with Θ D = 200 K derived from the specific heat measurements.

We have studied the V 3d electronic structure of quasi-one-dimensional BaVS 3 through a metalinsulator transition (MIT) at T MI =70 K by means of angle-resolved photoemission spectroscopy (ARPES). Utilizing the polarization dependence of ARPES, we have succeeded in selective observation of the dispersive a 1g and almost flat e π g bands. The a 1g-derived spectral intensity at the Fermi level (E F) abruptly decreases from 150 to 120 K much higher than T MI , most likely due to its onedimensional (1D) instability. Below 120 K, the a 1g spectral intensity at E F gradually decreases to zero at T MI. In contrast, the e π g intensity at E F is reduced almost linearly with temperature from room temperature and becomes zero at T MI. Similar temperature dependence of the two bands between 120 K and T MI suggests that MIT in BaVS 3 is caused by the electron localization in the a 1g and e π g bands, with a driving force of the 1D instability of the a 1g band.