High T(c) Superconductors: in Situ Thin Film Synthesis and Magnetic Vortex Motion-Related Dissipation Studies.

To achieve superior superconducting properties, high T_{rm c} superconductors of YBa_2Cu_3O _7 are synthesized into high quality epitaxial thin films via magnetron sputter deposition technologies. The growth process makes use of a single stoichiometric ceramic target and requires no composition adjustments, yielding reproducible epitaxial films on a variety of substrates. The critical current density of such films reach values as high as 6 times 10^7 A/cm^2 at 4.2 K. Novel properties of the normal state conductivity as well as new defect structures are revealed. To further understand the limiting factors on critical current densities and dissipation processes in these high T_{ rm c} superconductors, systematic studies of the magnetic vortex state are carried out with both electrical transport and magnetometry measurements. It is found that ample inhomogeneity-related distribution of pinning interactions and critical current density can cause significant dissipation. Such dissipation is shown to be dramatically reduced when the superconductor operates slightly below the critical current density. This will help directing future efforts in eliminating undesired dissipation in these superconductors.