Abstract
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 SrCuO2 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/BaCuO2 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 Tc 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.
