I. Kawano

Possibility of superconductivity with highT c in La-Sr-Nb-O system

We prepared La-Sr-Nb-O system samples both in bulk form and as a film, with sharp resistive transitions at temperatures of 228–323 K, depending on the annealing conditions. The sharp resistive transitions might be attributable to superconductivity. Magnetic ordering was observed over a narrow temperature range around ∼40 K for some of the samples.

La-Sr-Nb-O film as a candidate for superconductor withT ce up to 255 K

A possible high-Tccsuperconductivity in a La-Sr-Nb-O film is reported. Onset of the resistive transition is at a temperature (Tco) of 315 K, with a zero-resistance state at a temperature (Tce) of 255 K. Diamagnetization of about 5% that of the pure Nb superconductor and a noticeable current dependence ofTcoandTceare observed. The yield probability of this film is ∼50%. The characteristics were observed even 17 days after preparation. However, a thermal cycle (cooling and warming) of more than ten cycles gradually deteriorates the characteristics.

EFFECTS OF A PARTIAL SUBSTITUTION OF Cu ELEMENTS BY Nb ELEMENTS IN YBaCuO SYSTEM

Samples of the YBaCuNbO system with different molecular ratios of YBa2NbOy to YBa2Cu3O7−d, were prepared in air by the solid-state reaction method. The X-ray powder diffraction patterns showed that the sample was composed of two phases, one corresponding to the YBa2Cu3O7−d phase and the other to the YBa2NbOy phase with a cubic lattice parameter of 8.425 A to 8.436 A depending on the Nb content. The superconducting zero resistivity temperature, Tc0, of the YBaCuNbO system increased with the increase of the molecular ratios, from 91.2 K up to a maximum temperature of 92.8 K, and then, by a further increase in the molecular ratio, the Tc0 was drastically reduced with a gradient of −1.94 K/%x.

In situ quenching process and properties of Bi-Sr-Ca-Cu-O superconducting thin films by magnetron sputtering

As-grown crystallized superconducting thin films of Bi-Sr-Ca-Cu-O have been prepared on MgO (100) at 700 degrees C by DC magnetron sputtering using a single target. The as-grown process, followed by quenching, yielded c-axis oriented superconducting thin films with smooth surfaces. These films displayed the three-layer phase with a c-axis lattice constant of about 37 AA. Although the superconducting properties of the films were very sensitive to the oxygen pressure during the quenching process, reproducible results were obtained. The best film showed zero resistivity at 81 K and the critical current density increased from 8*104 A cm-2 at 77 K to 3*106 A cm-2 at 4.2 K.

Observation of large diamagnetism in La-Sr-Nb-O films up to room temperature

Large diamagnetic transitions along with sharp resistive transitions were observed in the La-Sr-Nb-O system near room temperature (∼290 K). The amplitude of the diamagnetism reaches 35% of that of a pure Nb sheet. In addition, a behavior similar to weak magnetic spin ordering was observed for some samples at a temperature of about 290 K, over a temperature range of 30 K. The diamagnetism reappears above this temperature and continues up toT ∼320 K. It is not clear what composition ratio of La-Sr-Nb-O is responsible for this large diamagnetism and the high critical temperature. The yield probability of these samples is around 50%. The characteristics of the samples having not passed through many thermal cycles remain stable for about 1 month.

Sputter-Deposited Films of Perovskite Compound and Nb–Si–O System

One of the authors of this manuscript has reported a resistance anomaly of very thin Nb–Si films. The anomaly is marked by a sudden drop in the resistance by a factor of ~103 at a temperature (TD) ~180K. This behavior bears many resemblance to that of a superconductor. It became apparent to us that the presence of oxygen during sputtering remarkably improved the stable yield of the films with this anomaly and the highest TD obtained is ~250K. These might be attributable to that in the ceramic superconductor.

Superconductivity of SrxCuO2-δ Film in YBCO/SrxCuO2-δ/YBCO System

The zero resistance behavior of a SrxCuO2-δ film (x=0.05 ~ 1.0) sandwiched between YBa2Cu3O7-z film was examined. The highest transition temperatures of the sandwiched-type-SrxCuO2-δ, which was deposited optimally at x=0.1 and with a thickness ≈2200 A, was Tc-end = 93.8 K and Tc-onset = 99.0 K. In the region of x=0.05 ~ 1.0, the scattering order of Tc tended to decrease with increase of x, and the resistivity in a normal state had a minimum value at x=0.1. XRD exhibited a phase transformation of a infinite layer type (at x=1.0)-to-NaCl type like C/2c (at x=0.1). It was determined by XPS that Sr-3d5/2,3/2 core lines were the same value for both concentrations of x=1.0 and 0.1 in SrxCuO2-δ.

Transport properties of SrxCuO2-σ film in YBCO/SrxCuO2-σ/YBCO system

Abstract The zero resistance behavior of a SrxCuO2-σ film sandwiched with YBa2Cu3O7−z films has been examined. The transition temperatures of the sandwiched-type-SrxCuO2-δ, which was deposited optimally at x=0.1 and with thickness ≈2200A, was Tc-end=93.8K and Tc-on=99.0K.

Fabrication and superconductivity of YBCO/CuO/YBCO films

The AC susceptibility of a CuO film sandwiched with YBa2Cu3O7−δ (YBCO) films has been examined. The trilayer films are fabricated on a SrTiO3 substrate by using in-situ DC magnetron sputtering. The superconducting transition of AC susceptibility of the CuO sandwiched with YBCO reached the onset temperature of 94.0K followed by the end of transition at 92.5K. The critical temperature for the zero resistivity of YBCO electrodes in this trilayer sample was 88.1K. The thin CuO layer with the thickness of ∼300Å was speculated to have defects induced with diffusion of Ba from YBCO. The chemical shifts of ∼1eV was observed in the binding energy of Ba3d5/2 from that of both YBCO and BaO by X-ray photoemission spectroscopy.

TRANSPORT PROPERTIES OF CuO IN YBCO/CuO/YBCO JUNCTION

The zero resistance behavior of a CuO film sandwiched with YBa2Cu3O7–δ films has been examined. The CuO is of tenorite C2/c structure which is a deformed NaCl-type lattice structure without being separated by an alkaline earth, and it forms a bulk in which copper is surrounded by nearly a square of Oxygen. The transition temperature Tce (R=0) in the fabricated sandwiched-type-CuO optimally increases over that of YBa2Cu3O7–δ film-electrodes.