Hiromi Mukaida

X-Ray Diffraction Study of a High-Tc Bi-Ca-Sr-Cu-O Superconductor Having Incommensurate Superstructure

The incommensurate superstructure of a Bi-Ca-Sr-Cu-O single crystal has been investigated by the X-ray precession method. The specimen crystal showed superconductivity at 79 K. It was found that the crystal has orthorhombic symmetry and the space group obtained from reflections of the subcell is D2h20-Bbmb or C2v13-Bb2b, with the cell dimensions of a=5.42, b=5.42 and c=30.91(2) ?. The first- and second-order satellites were observed systematically along the b* direction with intervals of (1/4.7)|b*| and indicate a modulated structure.

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.

Resistive Transition in Magnetic Fields for a Single Crystal of a Tl-Ba-Ca-Cu-O Superconductor

Resistivity-versus-temperature curves are measured for the single crystal of a Tl-Ba-Ca-Cu-O superconductor in applied magnetic fields up to 12T. The resistive superconducting transition broadens extremely in the field perpendicular to the c-plane. The anisotropic factor of Hc2 is ~4. The resistivity shows an exponential temperature dependence in the lower resistivity region, suggesting the occurrence of a thermally activated flux motion.

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.

Temperature Dependence of Forward Current-Voltage Characteristics of Ba 1-x K x BiO 3 /Nb-Doped SrTiO 3 Schottky Junctions

The temperature dependence of ideality factor n on Ba1-xKxBiO3/Nb-doped SrTiO3 Schottky junctions was investigated using an interfacial layer model. We conclude that the increment of n as temperature decreases is due to the temperature dependence of the permittivity of Nb-doped SrTiO3.

Scanning Tunneling Microscopy of the Cleaved Surface of Bi2Sr2CaCu2O8+δ

We have obtained scanning tunneling microscopic (STM) images with atomic resolution of the cleaved surface of Bi 2 Sr 2 CaCu 2 O 8+δ single crystals. The sample was cleaved at a Bi-O plane and observed with an STM at ultra high vacuum conditions at room temperature. The images clearly show the superstructure along b axis and depressed atomic rows running along a axis called missing atom rows. In contrast to previous reports, from our STM images the topographic structure of the superstructure is a rectangle wave like structure rather than sinusoidal one, and the periodicity of the superstructure is 5 fundamental units (5 dots in the STM images).

Structural Characterization of Bi2Sr2CaCu2O8/Bi2Sr2CuO6 Superlattices Prepared by MBE Methods

X-ray diffraction studies for Bi2Sr2CaCu2O8(BiSrCaCuO)/Bi2Sr2CuO6(BiSrCuO) superlattices have been performed. Intensity analysis based on a step model explains the observed main peaks and satellite peaks rather well, and the analysis confirms that the superlattices are well ordered on a half-unit-cel1 by half-unit-cell basis with (001)BiSrCaCuO/(001)BiSrCuO-texture. In addition, the observed peak-broadening and intensity-reduction of high-order reflections are analyzed by an extended step model which assumes a random distribution of superlattice period (Λ). As a result, the variation of Λ is estimated to be within a small width (ΔΛ/Λ = 1.4%) of a gaussian distribution.

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.

A Study of Y211/123 Layered Films

Anisotropic-thermal-expansion measurements of Y2BaCuO5 were carried out for the first time. The thermal-expansion coefficients (β a, β b and β c (R.T. - 450 °C) are 11.8 × 10−6, 10.9 × 10-6 and 9.8 × 10−6 K−1, respectively. It was found that there are little differences in thermal expansions between a (001) oriented YBa2Cu3O7 - δ layer and a (230) oriented Y2BaCuO5 layer below about 450 °C. Moreover, lattice mismatches between these layers are 1.1 – 3.4 %, which are relatively small.