Unusual valence state and metal-insulator transition in BaV10 O15 probed by hard x-ray photoemission spectroscopy

We have studied the electronic structure of BaV10O15 across the metal-insulator transition with V trimerization by means of hard-x-ray photoemission spectroscopy (HAXPES) and mean-field calculations. The V 2p HAXPES indicates V2.5+-V3+ charge fluctuation in the metallic phase, and V2+-V3+ charge order in the insulating phase. The V2.5+-V3+ charge fluctuation is consistent with the mean-field solution where a V 3d a1g electron is shared by two V sites with face-sharing VO6 octahedra. The valence-band HAXPES of the metallic phase exhibits pseudogap opening at the Fermi level associated with the charge fluctuation, and a band gap ∼200 meV is established in the insulating phase due to the switching of charge correlation.