Hiroaki Furukawa

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.

In-situ epitaxial growth of BiSr-Ca-CuO films and superlattices by MBE using an oxygen radical beam

Abstract Epitaxial BiSr-aCuO thin films and superlattices have been grown on MgO (100) and SrTiO 3 (100) substrates by the MBE technique using an oxygen radical beam produced by RF excitation. During the first deposition of Ca atomic layers on MgO, the epitaxial direction was found rotated by 45° between Ca-free and Ca-containing phases. Superlattices of Bi 2 Sr 2 caCu 2 O 8 (2212)/BiSr 2 CuO 6 (2201). in which 2212 layers with a thickness of m c axis half unit cells are separated by nonsuper-conducting 2201 layers of n half unit cells, were grown by this technique. We found that the superlattices exhibit similar superconductivity independent of both the m and n half-unit-cell numbers, indicating the two dimensionality of this superconductor..

Preparation of Bi-Sr-Ca-Cu-O Films by MBE Using an Oxygen Radical Beam Source

Bi-Sr-Ca-Cu-O thin films have been grown in situ on (100) Mg0 substrates using radical beams as a source of reactive oxygen. Oxygen radicals were produced by a RF-excited plasma in a quartz-tube reactor and supplied through fine holes in the beam aperture plate on the substrate. The vacuum pressure was maintained below 4×10−6 Torr during growth. Cu deposited on substrates is selectively oxidized by shadowing the beams. The Bi-based film grown at 750 °C shows a superconducting transition at 75 K without in-situ annealing. In the x-ray diffraction pattern, the periodical peaks are assigned to (00Ω) reflections of Bi2Sr2CaCu2Oy as a main phase.

Comparison of flux pinning in superconducting TlCaBaCuO crystals and thin films

Abstract We report direct observation of the flux pinning behavior of a Tl 1 Ca 2 Ba 2 Cu 3 O y highly oriented thin film, a randomly oriented polycrystalline thin film and a Tl 2 CaBa 2 Cu 2 O y single crystal. From the temperature dependence of the critical current densities J c at zero field, activation energies U 0 are estimated to be 110, 110 and 40 meV, respectively. Strong time-logarithmic magnetic relaxation M ( t , T ) was observed both in single crystals and in thin films. Within a thermally activated flux motion (TAFM) model, the distribution of activation energies ϱ ( U 0 ) is determined from data on the time and temperature dependence of M ( t , T ) for the highly oriented film. The presence of a high-energy tail in the distribution suggests that the TAFM model can not be described by a single activation energy U 0 ; the effective U 0 value increases rapidly with temperature.

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.

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.

Transport Property of Epitaxial Bi2Sr2CaCu2O8/Bi2Sr2CuO6 Superlattices in Magnetic Field

The transport property of epitaxial (Bi2Sr2CaCu2O8)m/(Bi2Sr2CuO6)n superlattices has been investigated in magnetic field. For a superlattice of m=4 and n=3, the angular dependence of resistance was measured below Tc by rotating magnetic fields in the plane perpendicular to the film surface (ab-plane). The curves of the resistance are scaled by an effective field, H (sin2 θ+γ−2cos2 θ)1/2, where θ is the field angle measured from the ab-plane. The anisotropy parameter γ=150−200 yielded by the scaling is almost the same value of Bi2Sr2CaCu2O8 films. This result suggests the behavior of two-dimensional vortex discs is identical between the superlattice and the single phase film in Bi2Sr2CaCu2O8.

Ion Bombardment Enhanced Etching for YBaCuO Superconducting Thin Films

Ion bombardment enhanced etching using Br2 solution, previously applied to BiSrCaCuO films, has also been successfully applied to YBaCuO high-Tc superconducting thin films. It was found that the etch rate of the damaged region which was formed by the ion irradiation with a 200 keV Si++ focused ion beam was enhanced with the Br2 treatment. Bridge structures with a width of 0.3 µm were successfully fabricated with little degradation in Tc and Jc. The 0.3-µm bridge had a Jc value of about 106 A/cm2 at 15 K.

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.

Study on Doping Properties of BSCCO/GaAs Films

We investigated doping properties of Bi2Sr2CaCu2O8/GaAs layered films. Resistivities of the GaAs layers decreased from 10 kΩ · cm to 20 Ω · cm by Si doping. There was no indication that the doping carriers were compensated by deep levels created by oxygen. This suggests that oxygen outdiffusion from the Bi2Sr2CaCu2O8 layer, even though it occurs, has little influence on electrical properties of the Bi2Sr2CaCu2O8/GaAs layered films. To explore the prospect for heteroepitaxy of GaAs on BSCCO, crystal structures of GaAs grown on (001) Bi2Sr2CuO6 single crystals and (100) MgO substrates were investigated. On (100) MgO substrates, the (400) peak which results from the lattice matching was observed.