On the strength of electron correlations in high-T c superconducting copper oxides

Abstract

We analyze the strength of electronic correlations in the half-filled antibonding Cu−O orbitals of high-Tc superconducting copper oxides by considering a Cu12O 17 n− cluster withn=8 or 10, respectively. The correlated ground state is calculated by the method of the local approach (LA) in a version which allows the treatment of stronger correlated electrons. As mean-field basis a semiempirical Hamiltonian of the ZDO (zero differential overlap) type has been adopted. It is found that the correlations are particularly strong in the Cu 3dx2−y2 orbitals. The nonintegral orbital occupation allows for valence fluctuations between Cu+ and Cu2+ in spite of the remarkable correlations. According to the present model excess holes are located at the oxygen sites. The theoretical findings are compared with the results of spectroscopic investigations. The present electronic-structure analysis allows for a straighforward rationalization of previous experimental measurements. The strong connection between the importance of electronic correlations and the symmetry properties of the electronic wave function is emphasized.