XANES Study of Superconducting Lu0.7Ca0.3Ba2Cu3O7-δ Annealed in Argon, Air and Oxygen

Abstract

Cu K-XANES mesurements have been made on argon, air and oxygen annealed superconducting Lu 0.7 Ca 0.3 Ba 2 Cu 3 O 7-δ compounds. The Cu 1+ peak which is absent for the oxygen annealed sample, appears on air annealing and becomes prominent on argon annealing. For the argon annealed sample, the white line, assigned to Cu 2+, shifts towards the lower energy side and its width increases compared to that of the oxygen and the air annealed samples. From the ratio of areas of Cu 1+ and Cu 2+ deconvoluted peaks and the oxygen content ( O 7-δ), it has been possible to estimate the effective charges (n) on Cu ions separately for the Cu(1) chain and the Cu(2) plane sites. The value of n>2 on Cu of the Cu(2) sheets is found essential for superconductivity. In the light of the observed Tc values, the results are interpreted in terms of under-, near optimum- and over-doping of holes in the CuO 2 planes respectively for the argon, air and oxygen annealing conditions. The analysis of the results provides convincing evidence for hole doping of the CuO 2 planes directly by the divalent Ca in LuBa 2 Cu 3 O 7-δ.