CALCULATION OF ABSORPTION-SPECTRA OF YBA2 CU-3 O-7-DELTA BY MEANS OF X-ALPHA METHOD

In order to clarify the absorption spectra of YBa2Cu3O7-delta (YBCO) i n both the insulating state (delta=1) and superconducting state (delta =0), the electronic states are calculated by cluster approximation by means of the X alpha method. The absorption peak observed in the mid-I R region in the superconducting state of YBCO is assigned to the trans ition from the O2 2p(z) orbital to the hybrid orbital of O1 2p(z) and Cu II 3d(z2) in a hole-doped (CuO5)(7-) pyramid cluster, and the peak at 4.1 eV observed in the insulating state, to the transition from the O1 2p(z) orbital to the Cu I 4p(x) one in a (CuO2)(3-) chain cluster. The overall spectra are reproduced well using the transition energies and the oscillator strengths calculated by the X alpha method.