Abstract
High temperature superconducting bi-epitaxial grainboundary junctions have been fabricated in our laboratory. The first part of this thesis is to develop the techniques for making HTS homo-bi-epitaxial grainboundary junctions. We have studied the in-plane orientations of epitaxial growth of YBCO on YSZ substrates under different deposition conditions and found improved processes for growing 0-degree and 45-degree films. In addition to the two kinds of homogeneous films, we can also produce films of mixed orientations with different ratio. The critical current densities of films with different in-plane orientations, pure and mixed, are measured as a function of temperature and applied perpendicular magnetic fields. The critical current densities of films with both pure orientations are in the order of 107 A/cm2 at 5K, while those of the mixed orientations are at least three orders of magnitude smaller. In addition, the temperature dependences of critical current are quite different for these two groups of films. Aided with the knowledge of producing films of pure orientations, we have successfully fabricated reproducible bi-epitaxial grainboundary junctions. The critical current densities of asymmetric 45-degree grainboundary junctions are measured and analyzed. The I-V curves show substantial deviations from that of RSJ model with a current density of the same order of those of mixed films. The reason for deviation from RSJ model is probably due to the existence of facets along the grain boundary. Work is under progress to control faceting by producing zigzag boundary line with e-beam lithography.The second part of this thesis is a theoretical study of epitaxial growth of BaZrO3 on YSZ, which is believed to be the main reason for producing YBCO films with 0-degree orientation. We have used the ab-initio calculations to calculate the total energies of several different models of BaZrO3 / YSZ stacking. It is interesting to note that there is a volume expansion of 0-degree BZO in contrast to nearly no volume change for the 45-degree BZO on YSZ. After the atoms are relaxed, the atomic movements of the latter are much greater than the former. We believe that is the reason for favoring 0-degree orientation. However, due to the complexity of the real stack YBCO/BaZrO3/YSZ, we defer such more demanding calculations to future work.