Ryohkan Yuasa

Superconductivity in BiSrCaCuO Superlattices: Two-Dimensional Properties of CuO Planes

We report resistive measurements on epitaxial superlattices consisting of alternating Bi2Sr2CaCu2O8 (2212) and Bi2Sr2CuO6 (2201) layers grown by atomic-layer-by-layer molecular beam epitaxy (MBE) using oxygen radical beams. The influence of interplane coupling between Cu-O planes was studied by varying the superlattice sequence of different c-axis half-unit cells. A single-half-unit-cell-thick 2212 layer was found to be superconducting in a 2201 matrix, independent of the conductivity and the thickness of neighboring 2201 layers. The fact that all of our superlattices showed superconductivity essentially equal to that of a 2212 single-phase film suggests that this cuprate is a two-dimensional superconductor due to its Cu-O double planes.

Magnetron Sputtering of Bi-Ca-Sr-Cu-O Thin Films with Superconductivity above 80 K

Superconducting Bi-Ca-Sr-Cu-O thin films on (100) MgO were prepared by rf magnetron sputtering from a sintered Bi1Ca1Sr1Cu2Ox target. Post-annealing after sputtering resulted in superconducting films with a resistance anomaly around 110 K and zero resistance at 80 K. X-ray diffraction patterns indicated a preferential orientation, where the c axis of a fourfold stacked perovskite, with a lattice constant c=15.38 A, is perpendicular to the substrate plane.

Sputtered Tl-Ca-Ba-Cu-O Thin Films with Zero Resistivity at 98 K

Superconducting Tl-Ca-Ba-Cu-O thin films were prepared by rf magnetron sputtering from a sintered Tl3Ca2Ba2Cu3Ox target. After post-annealing, films grown on (100) MgO substrates exhibited the best superconducting properties with an onset temperature as high as 110 K and zero resistivity at 98 K. No resistive tails were observed at measuring current densities of 0.5–5 A/cm2. X-ray diffraction patterns indicated that the major phase of the films is Tl2Ca1Ba2Cu2Ox.

Magnetic Field Effects in the High-Tc Superconductor Y-Ba-Cu-O

The single-phase compound YBa2Cu3O7 prepared in pure oxygen atmosphere exhibited an extremely high upper critical field Hc2 (0 K) above 110 T (midpoint) as well as high Tc above 92 K (end point). Magnetic field dependence of resistance R has been measured as a function of temperature T. Shoulders appeared in the R-T transition curves of (Y1-xBax)CuO3-y(x=0.6) prepared in air, suggesting the coexistence of two kind of phases, i.e., a high-Hc2 phase and a low-Hc2 phase. Hc2(77 K) defined by a zero-resistance state was at most ~0.05 T in the low-Hc2 phase.

Long-Range Josephson Coupling in BiSrCaCuO-Based Coplanar Junctions

We present measurements of the characteristics of superconducting weak links prepared in epitaxial bilayer structures of Bi2Sr2CuO6 (BSCO)/Bi2Sr2CaCu2O8 (BSCCO) by focused ion beam lithography. The bilayer films with each layer thickness of 150 nm have been grown in situ by atomic-layer- by-layer molecular beam epitaxy and then patterned by implanting Si+ ions to destroy the superconductivity of the BSCCO top layer. We find S-N-S-like I-V characteristics for junctions in which the separation between the two superconducting BSCCO banks is about 300–400 nm. Shapiro steps in the nonhysteretic I-V curves appear in response to microwave radiation. We further find that the critical currents I c for all the weak links fit well with the form given by the Ginzburg-Landau theory, (1-T/T c’)3/2, where T c’ is estimated to be 26–33 K. We conclude that our junctions are not proximity-effect-type but microbridge-type S-S’-S weak links. This characteristics can be interpreted as the formation of filamentary superconducting paths in BSCO.

BiSrCaCuO/BiSrCuO Superlattices: A Prove of Cu-O Plane Superconductivity

The parallel and perpendicular superconducting transport properties of epitaxial superlattices that consist of alternating Bi2Sr2CaCu2O8 (BSCCO) and Bi2Sr2CuO6 (BSCO) layers are reported. The interlayer coupling of Cu-O planes within a cell and in adjacent cells is studied by varying the superlattice sequence of different c-axis half-unit cells. All of our superlattices show superconductivity in the plane of the layer, including superlattices containing isolated Cu-O double planes of BSCCO in a semiconducting BSCO matrix. The overall shapes of superconducting transitions are found to be quite similar independent of the separation of the BSCCO layer. In the perpendicular direction, the transition width increases with the number of BSCO half-unit cells, but a residual resistivity appears for five half-unit cells. We conclude that superconductivity is essentially independent of the BSCCO and BSCO half-unit-cell sequence, and proximity-effect coupling along the c-axis direction cannot occur through the BSCO layer more than two unit cells (49 A) thick.

Magnetic Field Effects in High-Tc Superconductors

Superconducting and structural properties of high-Tc superconductors RE-Ba-Cu-O (RE = Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm and Yb) were investigated. Each compound with nominal composition, RE1Ba2Cu3O7-δ, displayed superconductivity around 90 K. Magnetic field effects on resistance suggested that each compound was almost single phase except La-Ba-Cu-O. An X-ray study showed that the 90K-range superconducting phase is an orthorhombic perovskite common to all systems. Compounds of tetravalent rare earth elements (Ce, Pr, Tb) were found nonconducting under the same preparation conditions.