Cluster-configuration-interaction analysis of Cu 2pand valence-band photoemission measurements onBi2Sr2CaCu2O8andBi2Sr2CuO6superconductors

Abstract

The electronic structure of ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ and ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CuO}}_6$ high-${\mathit{T}}_{\mathit{c}}$ superconductors is investigated by Cu 2p x-ray photoemission spectroscopy and valence-band resonant photoemission at the Cu ${\mathit{L}}_3$ edge. The data suggest strong correlation effects for the Cu 3d electron states due to a large on-site Coulomb interaction. By means of a simplified cluster-configuration-interaction model we derive the model Hamiltonian parameters which quantify the Cu d-d interaction energy, the O to Cu charge-transfer energy, and the degree of Cu d–O 2p hybridization. The solutions for the two compounds differ mainly in the charge-transfer energy, which in both cases is lower than the energy required for a Cu d-d charge fluctuation. An energy gap of about 1 eV for both compounds is found. This identifies them as charge-transfer insulators, in analogy with what was already found for CuO and other copper oxide based high-${\mathit{T}}_{\mathit{c}}$ superconductors. The density of states at the Fermi level is assigned to doping induced states in the semiconductor gap.