O KVV AUGER EMISSION VERSUS RESONANT PHOTOEMISSION AT THE O K EDGE OFHIGH-T-C SUPERCONDUCTORS

Photoelectron spectroscopy results on single crystals of the supercond uctors Bi2Sr2CaCu2O8,Bi2Sr2CuO6, Ba0.6K0.4BiO3 and the semiconductor B a0.9K0.1BiO3 are reported for the photon energy region around the O K absorption threshold. The development of the O-KVV Auger structure has been carefully monitored as a function of photon energy. A non-monoto nic behavior displaying a feature at a constant binding energy of abou t 14 eV was found for Bi2Sr2CaCu2O8 and Bi2Sr2CuO6 in a narrow photon energy region of 1 eV at the main edge of the O K absorption spectrum around 530 eV. The corresponding enhancement, connected with the autoi onization of O 2 p states, is absent in Ba1-xKxBiO3 in contrast to Bi2 Sr2CaCu2O8 and Bi2Sr2CuO6. The resonant enhancement is more pronounced for Bi2Sr2CuO6 as compared to Bi2Sr2CaCu2O8, which can be explained b y a lower charge carrier concentration in the former case, leading to a more localized nature of intermediate O 2 p states. The model parame ters Cu d-d and O p-p Coulomb interactions and the charge transfer ene rgy Delta are estimated from the experiments. (C) 1998 Elsevier Scienc e B.V. All rights reserved.