Enhancement of the transport properties in a-axis oriented YBa2Cu3O7-δ thin films by calcium doping

High critical current densities (J_c) are required for applications, but grain boundaries suppress the attainable J_c values. Because of the presence of 90° grain boundaries and because of poor structural and electrical properties, a-axis thin films are of great interest to investigate the role played by Y_0.8Ca_0.2Ba₂Cu₃O_7-δ (YCaBCO) cap layers in the transport properties. The transport critical current densities of the films under various magnetic fields and the angular dependence of J_c at fixed temperature were measured before and after irradiation. A thin YCaBCO cap layer deposited on top of a-axis oriented YBa₂Cu₃O_7-δ (YBCO) films, is found to improve the electrical transport properties. Ca-doping decreases the resistivity and strongly increases J_c of a-axis oriented YBCO films. Furthermore, strong pinning by 90° grain boundaries was observed.