We report on a photoemission study of Ta2NiSe5 that has a quasi-one-dimensional structure and an insulating ground state. Ni 2p core-level spectra show that the Ni 3d subshell is partially occupied and the Ni 3d states are heavily hybridized with the Se 4p states. In angle-resolved photoemission spectra, the valence-band top is found to be extremely flat, indicating that the ground state can be viewed as an excitonic insulator state between the Ni 3d-Se 4p hole and the Ta 5d electron. We argue that the high atomic polarizability of Se plays an important role to stabilize the excitonic state.

We report temperature-dependent angle-resolved photoemission spectroscopy measurement of Ta2NiSe5 which shows a semiconductor-semiconductor structural phase transition at around 330 K. Characteristically, flat band at the top of the valence band is observed, which is ascribed to the excitonic insulator effect. The top valence band shifts to higher binding energy and its bandwidth increases as the temperature decreases. As the system exceeds the transition temperature, the flat feature of the valence band weakens though the exciton fluctuations remain finite

We have studied the electronic structure of the diluted magnetic semiconductor Ga 1−x Mn x N ͑x = 0.0, 0.02, and 0.042͒ grown on Sn-doped n-type GaN using photoemission and soft x-ray absorption spectroscopy. Mn L-edge x-ray absorption have indicated that the Mn ions are in the tetrahedral crystal field and that their valence is divalent. Upon Mn doping into GaN, new states were found to form within the band gap of GaN, and the Fermi level was shifted downward. Satellite structures in the Mn 2p core level and the Mn 3d partial density of states were analyzed using configuration-interaction calculation on a MnN 4 cluster model. The deduced electronic structure parameters reveal that the p-d exchange coupling in Ga 1−x Mn x N is stronger than that in Ga 1−x Mn x As.

A persistent photoinduced metal-to-insulator transition has been confirmed in a manganite thin film, Pr_(0.55)(Ca_(0.75)Sr_(0.25))_(0.45)MnO3, near a multicritical point by monitoring with transport measurements and x-ray photoemission spectroscopy. Together with the previously reported reverse effect, the photoinduced insulator-to-metal transition, it is found that the relative stability of the metallic and insulating phases interchanges around 80 K in the middle of a very wide hysteresis loop, which is a manifestation of the large potential barrier due to the long-range elastic energy. It is shown that photons are much more effective in overcoming the barrier via the electronically excited intermediate states than via the heat mode.

We have studied the electronic structure of the d-electron heavy-fermion system CaCu 3 Ru 4 O 12 using x-ray photoemission spectroscopy and a cluster model calculation. The Ru 3d core level spectrum shows a doublepeak structure as commonly observed in metallic Ru oxides. In CaCu 3 Ru 4 O 12 , the well-screened peak has dominating intensity, indicating that the Ru 4d electrons in CaCu 3 Ru 4 O 12 are highly itinerant. On the other hand, the Cu 2p 3/2 core level peak is accompanied by a satellite and shows that the valence state of Cu is close to 3d 9 ͑Cu 2+ ͒ with localized character. In addition, the main Cu 2p 3/2 peak shows an asymmetric line shape due to the screening effect, suggesting the hybridization effect between the Cu 3d and Ru 4d orbitals. The present results show that, among the d-electron heavy-fermion materials, the electronic structure of CaCu 3 Ru 4 O 12 best resembles that of the f-electron Kondo system.