ELECTRON-ENERGY-LOSS STUDIES OF CORE EDGES IN (TL0.5PB0.5)(CA1-XYX)SR2CU2O7-DELTA

Superconductivity is observed in the (Tl0.5Pb0.5)(Ca(1-x)Y(x))Sr2Cu2O7-delta system in the composition range 0 less-than-or-equal-to x < 0.6, with a maximum T(c) of 108 K at x approximately 0.2. For x greater-than-or-equal-to 0.6, the material becomes semiconducting. We have investigated the electronic structure of these materials using transmission electron-energy-loss spectroscopy on individual crystallites, over the entire composition range, 0 less-than-or-equal-to x less-than-or-equal-to 1. In the semiconducting end of the composition range, the material already shows a pre-edge feature in the O 1s absorption edge, which we have attributed to an empty density of states in the conduction band containing O 2p character. This feature therefore should not be used as a direct measure of the hole concentration in the material. We found that the holes associated with superconductivity do give rise to a secondary pre-edge feature, about 1.5 eV lower in energy. The emergence of this feature also correlates with that of another secondary feature in the copper L3 edge, suggesting a strong covalency between oxygen and copper in the CuO2 plane. Evidence is also presented for the existence of holes responsible for normal conductivity but not for superconductivity.