Toshiya Matsubara

Evidence of Critical Oxygen Concentration at y=6.7∼6.8 for 90 K Superconductivity in Ba2YCu3Oy

A study of precise oxygen concentration dependence of the superconducting volume fraction in Ba2YCu3Oy (6.22≤y≤6.96) revealed the phase diagram of this superconducting system. Three characteristic phases are found based on the oxygen concentration: (1) a nonsuperconducting tetragonal phase for 6.2<y<6.4, (2) a low-Tc (Tc= 55~70 K) orthorhombic phase for 6.4≤y<6.8 and (3) a high-Tc (Tc=90 K) orthorhombic phase for 6.8≤y<7.0. Evidence for a distinct critical concentration at y=6.7~6.8, necessary to sustain the bulk high-Tc (90 K) superconductivity, is presented.

Valence Study of Orthorhombic and Tetragonal Ba2YCu3Oy: The Role of Oxygen Vacancies in High-Tc Superconductivity

The valency of Cu ions in the orthorhombic and tetragonal phases of high-Tc superconductor Ba2YCu3Oy (6.22<y<6.96) has been studied by near-edge X-ray absorption spectroscopy. The results indicate that the Cu valency at Cu1 sites on the z=1/2 plane is strongly correlated with the high-Tc (90 K) superconductivity, while the Cu valency of Cu2 sites on the basal planes affects the low-Tc (50-60 K) superconductivity of the orthorhombic phase.

Possibility of Superconductivity at 65°C in Sr-Ba-Y-Cu-O System

Transition to a zero-resistance state at 65°C (338 K) was observd in a Sr-Ba-Y-Cu-O system for the first time. A nonlinear I-V characteristic curve was observed at that temperature. The resistivity changed from 5×10-4 Ωcm to below a measurable limit of 1×10-8 Ωcm between 71 and 65°C. The zero-resistance state reminiscent of superconductivity was continued for ten days at room temperature in air. A magnetization measurement indicated a very small volume fraction of the room temperature superconducting state, if any. X-ray diffractogram showed almost a single phase of oxygen-deficient perovskite with a trace of unidentified phase. Photoemission measurement showed the coexistence of multivalent Cu ions and a relatively low density of state at Fermi level. The superconductivity can be interpreted by the previously proposed d-hole mechanism.

Local Structure in Orthorhombie and Tetragonal Ba2YCu3O7-y: The Role of Oxygen Vacancies for High Tc Superconductivity

Cu valency and local structure in the orthorhombic and tetragonal phases of high Tc superconductor Ba2YCu3O7-y (0.03<y<0.64) have been studied by the near-edge and extended X-ray absorption fine structure (EXAFS). Oxygen vacancies on the z = 1/2 plane modify both Cu valency and local structure. The near-edge spectra show that Cu ions at Cu1 sites in tetragonal Ba2YCu3O7-y are monovalent. From the temperature dependence of the Cu-O distance and its mean relative displacement, it is suggested that the breathing mode Cu-O stretching vibration is slightly suppressed in the tetragonal phase, which may degrade superconducting properties.

Preparation of Y-Ba-Cu-O Superconductors with High Critical Current Density by Unidirectional Melt Solidification

Y-Ba-Cu-O superconductors with high Jc have been prepared by a unidirectional melt solidification method. Uniformly oriented plate-shaped YBa2Cu3O7-y crystals containing small particles of Y2BaCuO5 have been grown successfully using the material with the nominal composition of Y1+xBa2-xCu3-2xOz (x0.4) as a starting material. The Jc values of the unidirectionally solidified Y-Ba-Cu-O samples, measured at 77 K using a four-probe method with continuous direct current, were 4600 and 12000 A/cm2 in 1 and 0.25 T, respectively. These values nearly coincided with those estimated from magnetization hysteresis loops using Beans critical state model.

Addition of New Pinning Center to Unidirectionally Melt Solidified Y-Ba-Cu-O Superconductor

Y-Ba-Cu-O bulk superconductor including small particles of BaSnO3 was synthesized by the unidirectional melt solidification method. Finely dispersed Y2BaCuO5 precipitates and BaSnO3 particles were observed in YBa2Cu3Oy matrix. The critical current density at 77 K in 8 kOe was 1.8×104 A/cm2, which was about four times higher than that of the BaSnO3-free specimen. Fine particles of BaSnO3 (0.1–1 µm) can act as effective flux pinning centers in the specimen and result in an increase of the critical current density.

Strong correlation between Tc and lattice parameters in Ba2YCu3Oy

Superconducting properties and lattice parameters were systematically studied for Ba 2 YCu 3 O y ( 6 T c value and lattice parameter have a strong correlation in a stepwise change with the oxygen content. The lattice parameters of c and (b-a) change stepwisely at y=6.7 and 6.35 like Cu valency. The bond length of Cu-0 along the pyramidal height strongly correlates to the Tc value. The distortion of the pyramidal basal plane also has a relation to the Tc value.

Properties of YBa2Cu3Oy Films on BaSnO3 Layers

We have investigated the potentiality of BaSnO3 as the buffer layer between a superconducting Y-Ba-Cu-O layer and a substrate. At first, the reactivities of some barium double oxides with the Y-Ba-Cu-O system were examined at the partial melt region (1050°C), and BaSnO3 was found to be the most stable. It was then confirmed that BaSnO3 formed a thin and dense layer on a (100)MgO single crystal by a CVD method. Superconducting films of YBa2Cu3Oy with critical current densities of up to 3 x 105A/cm2 in zero magnetic field at 77K were prepared on the BaSnO3 layers.

Fabrication and Properties of Long (Y, Ho)-Ba-Cu-O Rods Prepared by Unidirectional Solidification Technique

Unidirectional solidification was used to enhance the grain alignment and critical current density (Jc) of (Y,Ho)-Ba-Cu-O samples. The composition of the samples used in this study was YBa2Cu3Oy: Ho2BaCuO5: BaSnO3: Ag with a molar ratio of 3: 2: 1: 0.1, respectively. Solidifying parameters involving the maximum temperature of furnace and growing rate, were varied from 1000 to 1080°C and 1 to 7mm/h, maintaining a temperature gradient of approximately 180°C/cm at 980°C. Optimizing these conditions produced highly c-axes aligned, textured samples. Samples with cross sectional area of 0.055cm2 and length of 77mm had Jc values, greater than 9.1×103A/cm2 (IC>500A,77K,0T).

Anisotropic magnetic-field dependence of the critical current density and microstructure for YBaCuO thin films

Abstract Anisotropic magnetic-field dependence of the critical current density (J c ) of polycrystalline YBa 2 Cu 3 O 7−δ thin films deposited on (100)MgO. (100)SrTiO 3 and a-axis oriented BaSnO 3 was studied. For the films on MgO 2 . J c changes from J c (B//substrate surface)≈1×10 5 A/cm 2 to J c (B⊥ substrate surface)≈4×10 4 A/cm 2 at 77.3K and 0.6T. On the other hand, for the films on SrTiO 3 , J c is almost constant at about J c (B//)≅J c (B⊥)≈3×10 5 A/cm 2 under the same conditions. Transmission electron microscopy shows that the films on MgO consist of c-axis oriented small YBa 2 Cu 3 O 7−δ grains (∼ 1 μm in size), while the films on SrTiO 3 consist of a mixture of a-axis and c-axis oriented small YBa 2 Cu 3 O 7−δ grains (∼0.5μm in size). These facts suggest that the dependence of J c on the direction of the applied magnetic field strongly reflect the microstructure of the thin films.