Polarized copper K-edge x-ray-absorption spectra in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$

The polarized copper K-edge x-ray-absorption near-edge spectra (XANES) in the high-${\mathit{T}}_{\mathit{c}}$ superconductor ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ and related semiconductor oxide ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_6$ were calculated. The initial states and the final bound states were obtained from the self-consistent-field discrete-variational Xα method (SCF DV-Xα). Continuum states were found by the multiple-scattering method using the muffin-tin truncation of the SCF DV-Xα potentials. XANES for each of the two Cu sites in these compounds were identified by using the calculated difference in Cu 1s ionization potentials. Features in XANES were interpreted with the help of the projected density of states and photoelectron trapping times. A quantitative comparison with the measured spectra leads us to assign shakedown features with relaxation energy of 8.3 eV and intensities of 20% and 12% of the total transition in the a-axis- and b-axis-polarized Cu K-edge XANES of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$, respectively. This shakedown feature apparently involves ligand→3d transitions in the core-hole excited state. It is found that this relaxation process is present in the metallic ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ and absent in the semiconducting phase.