Formation and properties of novel artificially layered cuprate superconductors using pulsed-laser deposition

Pulsed-laser deposition and epitaxial stabilization have been effectively used to engineer artificially-layered thin- film materials. Novel cuprate compounds have been synthesized using the constraint of epitaxy to stabilize (Ca,Sr)CuO₂/(Ba,Ca,Sr)CuO₂ superconducting superlattices in the infinite layer structure. Superlattice chemical modulation can be observed from the x-ray diffraction patterns for structures with SrCuO ₂ and (Ca,Sr)CuO₂ layers as thin as a single unit cell. X-ray diffraction intensity oscillations, due to the infinite thickness of the film, indicate that (Ca,Sr)CuO₂ films grown by pulsed-laser deposition are extremely flat with a thickness variation of only approximately 20 angstrom over a length scale of several thousand angstroms. This enables the unit-cell control of (Ca,Sr)CuO₂ film growth in an oxygen pressure regime in which in situ surface analysis using electron diffraction is not possible. With the incorporation of BaCuO₂ layers, superlattice structures have been synthesized which superconduct at temperatures as high as 70 K. Dc transport measurements indicate that (Ca,Sr)CuO₂/BaCuO ₂ superlattices are two dimensional superconductors with the superconducting transition primarily associated with the BaCuO₂ layers. Superconductivity is observed only for structures with BaCuO₂ layers at least two unit cells thick with T_c decreasing as the (Ca,Sr)CuO₂ layer thickness increases. Normalized resistance in the superconducting region collapse to the Ginzburg-Landau Coulomb gas universal resistance curve consistent with the two-dimensional vortex fluctuation model.