SITE-SPECIFIC X-RAY-ABSORPTION SPECTROSCOPY OF Y1-XCAXBA2CU3O7-Y - OVERDOPING AND ROLE OF APICAL OXYGEN FOR HIGH-TEMPERATURE SUPERCONDUCTIVITY

Using polarization-dependent O 1s and Cu 2p near-edge x-ray absorption spectroscopy on detwinned Y1-xCaxBa2Cu3O7-y single crystals, the unoc cupied electronic structure of the CuO2 planes and CuO3 chains has bee n studied. Unlike the conventional picture where superconductivity is related predominantly to the number of planar holes, we find clear evi dence for a prominent role of the apical sites. For both under- and ov erdoped samples the hole states in the planes and chains rue identifie d and hole counts for the relevant orbitals near the Fermi level are d erived. Oxygen-deficient as well as oxygen-rich samples show that the holes introduced by replacing Y3+ with Ca2+ appear solely in the plane s.