Enhancement of the transport properties in a-axis oriented YBa/sub 2/Cu/sub 3/O/sub 7-(delta)/ thin films by calcium doping

High critical current densities (J/sub c/) are required for applications, but grain boundaries suppress the attainable J/sub c/ values. Because of the presence of 90(degree) grain boundaries and because of poor structural and electrical properties, aaxis thin films are of great interest to investigate the role played by Y/sub 0.8/Ca/sub 0.2/Ba/sub 2/Cu/sub 3/O/sub 7(delta)/ (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/sub c/ at fixed temperature were measured before and after irradiation. A thin YCaBCO cap layer deposited on top of a-axis oriented YBa/sub 2/Cu/sub 3/O/sub 7-(delta)/ (YBCO) films, is found to improve the electrical transport properties. Ca-doping decreases the resistivity and strongly increases J/sub c/ of aaxis oriented YBCO films. Furthermore, strong pinning by 90(degree) grain boundaries was observed.