Influence of cobalt doping on superconducting transition in as-grown YBCO single crystals

The role of a charge buffer layer in the superconductivity of high-T-c mate rials is best studied by cationic substitutions. In this work, the chain co pper in YBCO single crystals is substituted by Co3+ ion and consequent effe ct on superconducting transition temperature (T-c) studied. The T-c is meas ured using non-resonant Microwave Absorption technique, which is a highly s ensitive and contactless method. It is seen that T-c of as-grown crystals i s considerably enhanced by cobalt doping in low concentration regime. In co ntrast, higher T-c is achieved in undoped crystals only after extended oxyg en anneal. When dopant concentration increases beyond an optimal value, T-c decreases and the system does not show superconductivity when cobalt conte nt is high (x > 0.5 in YBa2Cu3-xCOxO7+/-delta). This behaviour consequent t o cobalt substitution is discussed with reference to the apical oxygen mode l. Optimal cobalt doping can be thought of as an alternative to extended ox ygen anneal in as-grown crystals of YBCO. (C) 1999 Elsevier Science Ltd. Al l rights reserved.