Seiichi Tokunaga

SiO2 passivation film effects on YBCO junctions

Abstract We investigate SiO 2 passivation effects on YBCO junctions fabricated by the focused ion beam (FIB) apparatus. The 600 nm thick SiO 2 film is deposited by a sputtering method with the Ar+O 2 plasma. T c of the junction increases a little during the SiO 2 film deposition. Two conditions of junction degradation are performed. Electrical properties of 600 nm SiO 2 coated junction are not changed at all by 10 min humidity test at 85°C and 85% relative humidity. T c of the 600 nm SiO 2 coated junction decreases a little after 200°C and 2 h annealing. It indicates that the SiO 2 film on the junction is a good passivation film for humidity and annealing.

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.

Step Angle Dependence of Tl2212 Josephson Junction Properties on MgO (100) Substrates

We have investigated the step-angle dependence of Tl2Ba2CaCu2Ox Josephson junction properties. The junctions were formed on MgO (100) substrates. A high-angle (nearly 83°) step was obtained by using an Au/Nb mask. The junction formed on this step showed the same electrical properties as an intrinsic Josephson junction. It had a very high IcRn compared with a junction formed on a low-angle (approximately 28°) step. This junction is suitable for use as a microwave detector.

Fabrication of FIB Josephson Junctions with High Microwave Impedance

We tried to apply junctions patterned using the focused ion beam (FIB) apparatus to the series-connected junction arrays. The number of lines damaged by FIB were increased from 1 to 6. Rn of a 6 lines junction is nearly 4 times larger than that of a 1 line junction. It means that we succeeded in the fabrication of series-connected junctions with the FIB junctions.

HTS Millimeter Wave Mixer Antenna-II

We report on high temperature superconducting (HTS) millimeter wave mixer-antenna-IC performances, such as frequency conversion and fundamental / harmonic mixings in the wide frequency range from 10GHz up to more than 100 GHz. The antenna was a log-periodic microstrip and was linearly polarized and had a cross-polarization level of -10 dB at 100 GHz. The mixer was weak links in series with high lc x Rn product using such fabrications as a focused-ion-beam (FIB) to damage MgO substrates before Y-Ba-Cu-0 film deposition. S/N of IF=1 GHz at RF=100GHz was more than 45 dB at 26 K and more than 10 dB at 77 K, respectively.

Comparisons between NdBaCuO and YBaCuO-Based Resonators

We present comparisons between the NdBaCuO and YBaCuO-based micro-stripline resonators to estimate application of the NdBaCuO thin films to the microwave devices. The NdBaCuO thin films fabricated by the RF magnetron sputtering method exhibit flatter surface than the YBaCuO thin films fabricated by the laser ablation method. At the low microwave power, the higher unloaded Q factors (Qo) of the NdBaCuO-based resonators were obtained compared to the YBaCuO-based resonators. However, at the relatively high power, the Qo of the NdBaCuO-based resonators decreased because of the loss at the grain boundaries.

Transport Property of a-Axis Oriented YBa2Cu3O7−δ Thin Films in Magnetic Field

Transport properties of in-plane aligned a-axis oriented YBa2Cu3O7−δ thin films have been investigated in magnetic fields. The in-plane anisotropy of the film was clearly shown by resistivity measurements for the film with different relative directions of the current, the magnetic field, and the crystallographic axis. The energy dissipation in the film was found dependent on the c-axis component of the magnetic fields and nearly free from macroscopic Lorentz force. No sharp drops in the resistance due to grain-boundary pinning and/or surface pinning were observed in angular traces of magnetoresistance.

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.