X-ray photoelectron investigation of the valence-band electronic structure of superconducting Y-Ba-Cu oxide

Abstract

The valence-band electronic structure of single-phase $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ orthorhombic (superconducting) and tetragonal (nonsuperconducting) materials was studied at room temperature using x-ray photoelectron spectroscopy. The valence-band (VB) data have very clearly resolved spectral features and appear to yield more information than the core-level spectra which are hybrid states of combined $\mathrm{Cu} 2p$, $\mathrm{Cu} 3d$, and $\mathrm{O} 2p$ states. The difference in the density of states (DOS) is striking in that the orthorhombic material has a significant peak at an energy corresponding to bare $\mathrm{Cu} {3d}^8$ holes (${\mathrm{Cu}}^{3+}$ sites), whereas the tetragonal material has a very much smaller contribution at that energy. In the main part of the VB there is a shift of the maximum in the DOS away from $E_F$ for the orthorhombic material with respect to the tetragonal. The DOS near $E_F$ is lower for the former material compared to the latter. When this region is resolved into component peaks the results correspond to $\mathrm{Cu} {3d}^9\mathit{L}$ and $\mathrm{Cu} {3d}^{10}{\mathit{L}}^2$ holes, where $\mathit{L}$ is the ligand or O hole(s), which arise from the hybridization of $\mathrm{Cu} 3d$ and $\mathrm{O} 2p$ states. These are, therefore, not bare $d$ holes but represent holes screened by O-to-Cu charge transfer. The data are in very good agreement with a configuration-interaction calculation for a Cu${\mathrm{O}}_6^{10-}$ cluster by Fujimori et al., both in shape and energies. The increased participation of oxygen in the VB of orthorhombic material is observed.