We have studied electronic properties of BaV10O15 in the intermediate-temperature phase as well as in the high-temperature metallic phase by using hard x-ray photoemission spectroscopy (HAXPES). The V 2p HAXPES show a shift in the high temperature phase between 300 K and 245 K which is similar to the shift of spectral weight near the Fermi edge. The binding energy of the O 1s HAXPES peak does not change except a slight shift of the lower binding energy edge in the opposite direction between 300 K and 180 K across the transition to the intermediate-temperature phase. This behavior is in sharp contrast to the transition to the low-temperature insulating phase where V 2p and O 1s HAXPES show dramatic shifts in the same direction. This indicates that the charge-orbital change in the intermediate-temperature phase is driven by the correlated V 3d electrons and is electronic. The V 2.5+-V 2.5+ bond ordering is related to the metallic contribution and gradually decreases with cooling from 300 K. The valence-band HAXPES show the metallic features of the pseudogap behavior near the Fermi edge in and above the intermediate-temperature phase due to V 2.5+-V 3+ charge fluctuation. The magnitude of the pseudogap increases from 300 K to 180 K in parallel with the gradual breaking of the V 2.5+-V 2.5+ bond and formation of trimers.
We have studied the electronic structure of Ba1−xSrxV13O18(x = 0, 0.2, and 1) at different temperatures across the trimerization and charge order transitions by using hard x-ray photoemission spectroscopy (HAXPES) experiment. The V 2p HAXPES indicates V 2+ /V 3+ charge order and fluctuation in the high-temperature tetramer phase, low-temperature trimer phase, and intermediatetemperature charge order phase in the series of x = 0, 0.2, and 1. In the valence-band HAXPES, although the spectral weight at the Fermi level tends to be suppressed in all the phases due to strong charge order or fluctuation, it increases in the trimer phase at x = 0.2 in agreement with the decrease of resistivity and the increase of itinerant electrons in the trimer phase. Interestingly, in the most conducting x = 1 without the charge order phase, the spectral weight at the Fermi level is strongly suppressed even in the trimer phase, indicating charge fluctuation in the trimer phase is different between x = 0.2 and 1.
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We have studied the electronic structure of BaV 10 O 15 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 V 2.5+ -V 3+ charge fluctuation in the metallic phase, and V 2+ -V 3+ charge order in the insulating phase. The V 2.5+ -V 3+ charge fluctuation is consistent with the mean-field solution where a V 3d a 1g electron is shared by two V sites with face sharing VO 6 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.
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