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

Abstract

The role of a charge buffer layer in the superconductivity of high-T_c materials is best studied by cationic substitutions. In this work, the chain copper in YBCO single crystals is substituted by Co³⁺ ion and consequent effect on superconducting transition temperature (T_c) studied. The T_c is measured using non-resonant Microwave Absorption technique, which is a highly sensitive and contactless method. It is seen that T_c of as-grown crystals is considerably enhanced by cobalt doping in low concentration regime. In contrast, higher T_c is achieved in undoped crystals only after extended oxygen anneal. When dopant concentration increases beyond an optimal value, T_c decreases and the system does not show superconductivity when cobalt content is high (x>0.5 in YBa₂Cu_3-xCo_xO₇±δ). This behaviour consequent to cobalt substitution is discussed with reference to the apical oxygen model. Optimal cobalt doping can be thought of as an alternative to extended oxygen anneal in as-grown crystals of YBCO.