Absence of Ni2+/Ni3+charge disproportionation and possible roles of O 2p holes in La3Ni2O7-δ revealed by hard x-ray photoemission spectroscopy

We have investigated the electronic structure of La3Ni2O7-8 (8 approximate to 0.07) by means of hard x-ray photoemission spectroscopy (HAXPES). Although the nominal Ni valence is close to +2.5, the Ni 2p HAXPES spectra show an absence of Ni2+/Ni3+ charge disproportionation. The Ni 2p spectral shape including the main peak and the charge-transfer satellite indicate that oxygen 2p holes are heavily involved in the transport properties. The spectral weight suppression at the Fermi level indicates that the carriers of O 2p character (mixed with Ni 3d) are affected by electronic correlation which would be associated with the density wave transition and the superconductivity controlled by pressure.