The role of reduced dimensionality in the new perovskite superconductors

Given the strong anisotropy of the copper perovskite superconductors, the question arises as to whether the superconductivity in these materials is intrinsically two-dimensional, and if not, whether the superconductivity is enhanced or suppressed by the structural two-dimensionality.; My first approach to this question involved making films of similar, but isotropic, material thin enough to be effectively two-dimensional. Films of doped strontium titanate, a low temperature superconductor, were made by magnetron sputtering and pulsed laser deposition. Rutherford Backscattering and X-ray diffraction show these films to be highly epitaxial with tantalum dopant atoms located predominantly on titanium sites as expected. Transport measurements reveal the importance of defect structure and impurities on the electrical properties of these doped semiconductors.; Direct studies of the cuprate superconductors were later initiated. Mutual inductance and noise power spectrum measurements were used to investigate the nature of the superconducting transition in ultrathin films of {dollar}rm Bisb2Srsb2Casb1Cusb2Osb9{dollar} (2212) and {dollar}rm YBasb2Cusb3Osb7{dollar} (YBCO). Samples of different thicknesses and with different proximity layers were investigated.; The effective penetration depth of 2212 films 31 to 62 A thick, with {dollar}rm Bisb2Srsb2CuOsb7{dollar} proximity layers, is approximately 1 {dollar}mu{dollar}m. The apparent superconducting onset temperature is suppressed below the mean-field transition temperature and is frequency-dependent. The effective penetration depth of YBCO films thinner than 90 A and with PrBa{dollar}sb2{dollar}Cu{dollar}sb3{dollar}O{dollar}sb7{dollar} proximity layers depends on YBCO thickness, ranging from 0.6 to 0.2 {dollar}mu{dollar}m for films 24 to 90 A thick. The observed superconducting transitions are similar to those of 2212 films. YBCO samples with {dollar}rm Prsb{lcub}.5{rcub}Ysb{lcub}.5{rcub}Basb2Cusb3Osb7{dollar} proximity layers have shorter penetration depths and higher transition temperatures than comparable samples with PrBa{dollar}sb2{dollar}Cu{dollar}sb3{dollar}O{dollar}sb7{dollar} proximity layers.; Ultrathin films of both materials show strong evidence for the existence of vortex-antivortex excitations, however, the observed superfluid response differs from Kosterlitz-Thouless predictions. The vortex diffusion follows an Arrhenius law with zero-temperature activation energies proportional to the films' inverse inductance as expected for vortex core pinning: E{dollar}sb{lcub}rm a{rcub}{dollar}(0) {dollar}approx{dollar} 4 {dollar}Phisb0sp2{dollar}L{dollar}sp{lcub}-1{rcub}{dollar} for 2212 and 0.4 {dollar}Phisb0sp2{dollar}L{dollar}sp{lcub}-1{rcub}{dollar} for YBCO. We explain our results in terms of a "vortex pinning" model, in which the superconducting transition is actually a kinetic crossover dominated by vortex-pin rather than vortex-antivortex interactions.