Cu K-edge polarized x-ray-absorption near-edge structure of ${\mathrm{Bi}}_2$${\mathrm{CaSr}}_2$${\mathrm{Cu}}_2$${\mathrm{O}}_8$

The linearly polarized Cu K-edge x-ray-absorption near-edge structure (XANES) of ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{8+\mathrm{\delta }}$ (Bi 2:2:1:2) has been measured, and the spectra are interpreted by the multiple-scattering approach in real space. The polarized spectra can be predicted in terms of a one-electron dipole (Δl=+1) transition Cu 1s→εp, probing the unoccupied p-like (l=1) density of states projected on the Cu site with orbital angular momentum ${\mathit{m}}_{\mathit{l}}$=0 in the E∥z spectra, and the ${\mathit{m}}_{\mathit{l}}$=1 in the E⊥c spectra. Therefore we show that the electronic structure of the high-energy conduction bands, beyond the Cu 3d band, are well described in terms of the one-electron approximation. Final-state effects induced by the core hole have been studied and calculated. A satellite 7 eV above the main K-XANES peak in both polarizations is shown to be a multielectron shake-up excitation, and it is enhanced going into the insulating phase. Structural information on the copper site in ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{8+\mathrm{\delta }}$ and in ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{YCu}}_2$ ${\mathrm{O}}_{8+\mathrm{\delta }}$ has been extracted from XANES spectra, providing evidence for the decrease of the Cu-O distance in the metallic phase.