Preparation of high T/sub c/ and J/sub c/ films of Ba/sub 2/YCu/sub 3/O/sub 7-δ/ by laser evaporation and observation of superconductivity in a 27A phase

The remarkable discovery of high T/sub c/ superconducting ceramics has been followed by intense efforts to realize their potential in the form of devices, films and wires. A number of techniques have been shown to be effective for growing thin films of these superconductors including electron beam/thermal co-evaporation, RF and DC sputtering, laser evaporation, and spin-on/pyrolysis. Of all of these techniques, the e-beam/thermal co-evaporation methods have been shown to give the best electrical characteristics in Ba/sub 2/YCu/sub 3/O/sub 7/ films, particularly with respect to the critical currents which have reached --10/sup 6/ A/cm/sup 2/ at 77K. Despite the major advantage of the laser evaporation method for orders of magnitude scale-up, it has not been useful for growing films with state-of-the-art electrical characteristics. In this report, the authors show that this problem is not inherent to this technique and that the desired electrical properties can be achieved by laser evaporation of a composite target onto a suitable substrate. This is achieved by using a target where the barium is incorporated in the form of BaF/sub 2/ and annealing the deposited material in moist oxygen to allow for formation of the superconducting Ba/sub 2/YCu/sub 3/O/sub 7/ phase. In addition, another superconducting phase of Ba-Y-Cu-O has been grown as thin films. This phase has been previously seen and ascribed to a variation in the stacking sequence of the normal orthorhombic Ba/sub 2/YCuO/sub 7/ phase which contains a c-axis elongated to 27.3A. This is confirmed and additional information is found concerning its structure. Several films have been grown which contain this phase nearly exclusively which allows for positive characterization of it as a superconductor with T/sub c/(R=O) of --79K, and an onset T/sub c/ of --82K