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

Epitaxial trilayer and superlattice structures grown by pulsed laser a blation have been used to study the superconducting-to-normal transiti on of ultrathin (one and two c-axis unit cells) YBa2Cu3O7-x layers. Th e normalized flux-flow resistances for several epitaxial structures co ntaining two-cell-thick YBa2Cu3O7-x films collapse onto the ''universa l'' curve of the Ginzburg-Landau Coulomb gas (GLCG) model. Analysis of normalized resistance data for a series of superlattices containing o ne-cell-thick YBa2Cu3O7-x layers also is consistent with the behavior expected for quasi-two-dimensional layers in a highly anisotropic, lay ered three-dimensional superconductor. Current-voltage measurements fo r one of the trilayer structures also are consistent with the normaliz ed resistance data, and with the GLCG model. Scanning tunneling micros copy, transmission electron microscopy, and electrical transport studi es show that growth-related steps in ultrathin YBa2Cu3O7-x layers affe ct electrical continuity over macroscopic distances, acting as weak li nks. However, the perturbation of the superconducting order parameter can be minimized by utilizing hole-doped buffer and cap layers, on bot h sides of the YBa2Cu3O7-x layer, in trilayers and superlattices. Thes e results demonstrate the usefulness of epitaxial trilayer and superla ttice structures as tools for systematic, fundamental studies of high- temperature superconductivity.