EXPERIMENTAL INVESTIGATIONS OF SUPERCONDUCTIVITY IN QUASI-2-DIMENSIONAL EPITAXIAL COPPER-OXIDE SUPERLATTICES AND TRILAYERS

Epitaxial trilayer and superlattice structures grown by pulsed laser ablation have been used to study the superconducting-to-normal transition of ultrathin (one and two c-axis unit cells) YBa2Cu3O7-x layers. The normalized flux-flow resistances for several epitaxial structures containing two-cell-thick YBa2Cu3O7-x films collapse onto the ''universal'' curve of the Ginzburg-Landau Coulomb gas (GLCG) model. Analysis of normalized resistance data for a series of superlattices containing one-cell-thick YBa2Cu3O7-x layers also is consistent with the behavior expected for quasi-two-dimensional layers in a highly anisotropic, layered three-dimensional superconductor. Current-voltage measurements for one of the trilayer structures also are consistent with the normalized resistance data, and with the GLCG model. Scanning tunneling microscopy, transmission electron microscopy, and electrical transport studies show that growth-related steps in ultrathin YBa2Cu3O7-x layers affect electrical continuity over macroscopic distances, acting as weak links. However, the perturbation of the superconducting order parameter can be minimized by utilizing hole-doped buffer and cap layers, on both sides of the YBa2Cu3O7-x layer, in trilayers and superlattices. These results demonstrate the usefulness of epitaxial trilayer and superlattice structures as tools for systematic, fundamental studies of high-temperature superconductivity.