Cu K-edge polarized x-ray-absorption near-edge structure of Bi2CaSr2Cu2O8.

Abstract

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