POLARIZED COPPER K-EDGE X-RAY-ABSORPTION SPECTRA IN YBA2CU3O7-Y

The polarized copper K-edge x-ray-absorption near-edge spectra (XANES) in the high-Tc superconductor YBa2Cu3O7 and related semiconductor oxide YBa2Cu3O6 were calculated. The initial states and the final bound states were obtained from the self-consistent-field discrete-variational X method (SCF DV-X). Continuum states were found by the multiple-scattering method using the muffin-tin truncation of the SCF DV-X 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 YBa2Cu3O7, 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 YBa2Cu3O7 and absent in the semiconducting phase. © 1990 The American Physical Society.