Hirokazu Kugai

Preparation of Ag-Sheathed (Tl, Bi)–Ca–Sr–Cu–O Superconducting Wire

In the Tl-Ca-Ba-Cu-O system, Tl-O single layered and double layered structures with general formulas Tl1Can-1Ba2CunO2n+3(n=1~6) and Tl2Can-1Ba2 CunO2n+4 (n=1 ~4) have been obtained. We fabricated Ag-sheathed Tl-Ca-Ba- Cu-O superconducting wires using powders of Pb-added Tl1Ca 3Ba2Cu4+O11 and of Tl2Ca2Ba2Cu3O10 phases whose critical temperatures are the highest among the respective structures. The production process of the wires was cold- working and heat treatment. The measurement of transport critical current was carried out in the range of magnetic fields from zero to 2.5T. The critical current density Jc at 77K for the wire of (Tl,Pb)1Ca 3Ba2Cu4O11 is 10800A/cm2, and that value for Tl2Ca2Ba2Cu3O10 wire is 5500A/cm2. There is a tendency for the wire specimens with homogeneous structure of superconducting phases to have high Jc values. The Jc of the (Tl,Pb)1Ca3 Ba2Cu4O11 wire decreases rapidly by 0.1T, while the decrease of Jc above 1T is not so remarkable. The magnetic field dependency of Jc of the Tl2Ca2Ba2Cu3O10 wire shows a two-step decrease in Jc versus B curves; the first step is from zero to 0.1T, and the second step is beyond 1T. The direction of the magnetic field against the plane of the wire does not considerably influence the Jc values of either of the wires, because the anisotropic property of the wire is small.

Resistive transition in TlBiSrCaCuO thin films in magnetic fields

Abstract The resistive transition for a c -axis oriented (Tl, Bi)Sr 2 Ca 2 Cu 3 O χ thin film with a critical temperature of 106 K was investigated in magnetic fields up to 9 T. The magnetic field-induced resistive transition shows a considerable anisotropic broadening behavior; a much broader transition in the field perpendicular to the ab -plane than in a parallel field. This behavior is strongly influenced by the magnetic field component perpendicular to the ab -plane. The resistivity in magnetic fields approaches zero exponentially versus temperature. The activation energy U 0 (0) deduced from the slope of the Arrhenius curve is expressed by the power law of U 0 (0)∝ B -1 . The resistive transition in magnetic fields for (Tl, Bi)Sr 2 Ca 2 Cu 3 O χ is attributed to thermally activated flux creep.

Preparation of Alkaline-Metal-Doped C60 Superconducting Film and Its Stabilization in Air with Protective Coatings

Preparation conditions of C60 film were studied by varying substrates, substrate temperature and the rate of evaporation. Crystalline C60 thin films were prepared on GaAs(100) at the substrate temperature of 100° C in order to obtain superconductors by alkaline metal doping. The Rb/Cs-doped film showed a superconducting transition with T c( onset)=24±3 K and T c( zero)=8 K. The composition of the superconducting film was measured by micro-Auger electron spectroscopy. Air stability of Rb3C60 superconducting film with protective coatings applied was studied. A Si protective film maintained superconductivity for 220 min in air. In addition a Si/Al protective coatings maintained superconductivity for 8 h.

YBCO Thin film Flux Transformer with Multiturn Input Coil

We have fabricated a YBa2Cu3O7-x (YBCO) thin film magnetometer involving a flux transformer with a multiturn input coil. The flux transformer was fabricated on a 20x20 mm2 SrTi03 (STO) substrate. A STO film was used as an intermediate insulating layer. The pickup coil was a single square loop and the input coil was 20-turn square spirals. The magnetometer was constructed by coupling the flux transformer to a superconducting quantum interference device (SQUID) with step-edge Josephson junctions in a flip-chip configuration. At 77 K the magnetic field resolution of the magnetometer with the effective area of 3.8 mm2 was 25 fT Hz-1/2 at frequencies above 200Hz and 250 fT Hz-1/2 at 1 Hz.

MULTI-CHANNEL HIGH Tc SQUID SYSTEM FOR MCG

We have developed a multi-channel high Tc SQUID system for magnetocardiography. The system consists of a 16ch SQUID in a liquid nitrogen dewar, a cylindrical permalloy magnetic shield including a movable bed, an adjustable gantry and electronics with a personal computer. The SQUIDs whose diameter is 17mm were arranged in 4 by 4 matrix with spacing of 20mm. The field resolution of the SQUID is less than 1pT/√Hz @10Hz. The liquid nitrogen dewar is compact with the diameter of 190mm. The magnetic shield is compact with the diameter of 0.85m and the length of 1.6m. It has open ends for a patient to feel relaxed. The development of a compact and low cost system is aimed for use in hospitals and bioengineering laboratories. We obtained a 16ch magnetocardiogram of the 6cm by 6cm area above the human heart and constructed time-varying magnetocardiac isofield contour maps. System noise level was about 25pT in real time mode and less than 5pT by averaging of 11 times.

High Tc SQUID Gradiometer

We have fabricated a YBa2Cu3O7-x (YBCO) single layer SQUID gradiometer with step-edge Josephson junctions. We have measured the noise of the gradiometer with and without a magnetic shield. Each case was measured both in the zero field cool (ZFC) and in the field cool (FC). The noise spectra with the magnetic shield in the ZFC and in the FC are almost the same, which means that there is no flux trapping or that the flux trapping does not contribute to the noise increase. We have also measured the magnetic field of a watch with the gradiometer without the magnetic shield. The signal to noise ratio of the gradiometer is about twenty times larger than that of a commercial fluxgate at a distance of 60 mm.

High Tc SQUID with a Single Layer Flux Transformer on Sapphire

We have fabricated a HoBa2Cu3O7-y SQUID magnetometer with a single layer flux transformer on a sapphire substrate. It consists of a small SQUID chip (2mm by 2mm) on a SrTi03 substrate and a large single layer flux transformer (17mm by 17mm) on a sapphire substrate. The transformer is mounted on the SQUID by a flip-chip method. We use a sapphire substrate and a large area HoBa2Cu3O7-y excimer laser deposition technique to reduce the cost. Effective area of the bare SQUID is 0.04mm2, and that of the SQUID with the flux transformer is 0.16mm2. The magnetic field resolution is 130fTHz-1/2 at 10Hz.

High-Tc Superconducting Crossing Lines and Contact

YBa2Cu307-x (YBCO) crossing lines with a SrTi03 insulating layer and a YBCO-YBCO contact sample with a via hole were prepared. The critical current densities Jc might be enough for superconducting device applications at 77K. The microstructure of the upper YBCO layer over the edge of the lower YBCO line was observed by TEM. Although step-like structures were observed at the upper side of the SrTi03 layer, the a-b plane of the upper YBCO layer almost grew parallel to (100) plane of the MgO substrate. Neither large grain boundaries nor large defects were in the upper YBCO layer.

Alkaline Metals Doping into C60 Thin Films

C60 thin films were prepared on GaAs(100) and quartz substrates by two different vapour deposition methods. Rb and Cs were found to be doped in crystalline C60 thin films by the resistance measurement during the course of the doping experiment. The Rb/Cs-doped film showed a superconducting transition at Tc(onset)=24±3 K and Tc(zero)=8 K. The composition of the superconducting film was investigated by µ-Auger Electron Spectroscopy (µ -AES). This is the first confirmation of superconductivity in a Cs/Rb-doped Cgo thin film.

Preparation of C60 Thin Films and Cs-Rb Doping

Abstract The effect of preparation conditions onto the crystallinity of C60 thin films were studied by using two different vapor deposition methods. Rb and Cs were found to be doped in C60 thin films by the resistance measurement during the course of the doping experiment. The Rb/Cs-doped film showed a superconducting transition at Tc(onset)=24±3K and Tc(zero)=8K.