K. Nakao

Influence of Bending and Torsion Strains on Critical Currents in YBCO Coated Conductors

A YBCO coated conductor is expected to be a next generation high performance superconductor. To design superconducting machines using YBCO conductors, evaluation of not only superconducting properties but also mechanical properties is very important. Hence, we applied bending and torsion to the YBCO conductor and measured degradation of the conductor against the strain. To compare with the YBCOs data, a Bi2223 conductor was also tested under the same experimental condition. From the bending test, degradation of the Bi conductor was faster than that of the YBCO conductor and the critical current gradually decreased with increasing bending strain. The YBCO conductor kept better performance than the Bi conductor, and degradation suddenly occurred. And when the superconducting layer in the YBCO conductor was subjected to compressive strain in the bending test, the critical current hardly decreased at approximately one percent of the bending strain.

Characteristics of Compressive Strain and Superconducting Property in YBCO Coated Conductor

We have applied compressive stress perpendicular to YBCO coated tapes, and measured the degradation of critical currents. When the tape was pushed from the silver surface of the tape, the performance of the tape hardly decreases. On the other hand, compressing from the hastelloy face, degradation of the tape occurred. We observed the damage of the tapes by a magneto-optical imaging technique. The observed results were consistent with the degradation data. From those results, we think that it is better not to compress strongly from the hastelloy side.

Role of oxygen on the flux pinning and vortex phase transitions in NdBa2Cu3O7−δ crystals

Abstract The origin of flux pinning is studied in NdBa 2 Cu 3 O 7− δ single crystals grown by the flux method with varying O 2 partial pressure during the growth and using different oxygen reduction schedules. The critical current density, J c is remarkably enhanced when O 2 defects are introduced in both crystals grown at low (0.03%) and intermediate (0.1%) O 2 -partial pressure. However, for excessive O 2 vacancies, both T c and J c are decreased. Our results demonstrate that oxygen vacancy clusters along with the substitutional defects and twin boundaries play the major role in flux pinning. Our results also demonstrate that the first-order melting transition observed in a clean system can transform into a glassy state with increasing defect density.

Magnetic and magneto-transport properties of NdBaCuO single crystals with varying Nd/Ba substitution and oxygen content

Abstract The effects of Nd/Ba substitution and oxygen contents on the peak effect of J c and transport properties in the normal states of Nd 1+ y Ba 2− y Cu 3 O 7− x single crystals are studied. We find that the Nd/Ba substitution plays a more important role for the peak effect than the oxygen vacancies in nearly optimum-doped crystals. In heavily underdoped crystals with y ∼0, we observe a peculiar charge localization in the out-of-plane and in-plane conductivity followed by superconducting transition at low temperature. Magnetization studies reveal that this behavior can be explained by the existence of spin-glass-like antiferromagnetic ordering above T c in the oxygen deficient region of x =0.4–0.6.

Crossover from the vortex-lattice melting to the continuous transition in twinned NdBa2Cu3Oy single crystals

Abstract We report on the observation of the first-order melting transition of the flux line lattice in a twinned NdBa 2 Cu 3 O y single crystal by transport measurements. The magnetization J c does not exhibit peak effect in crystals grown up to 0.05% oxygen partial pressure. The observed melting transition is a strong function of O 2 partial pressure during growth. The first-order melting transition becomes continuous and peak in critical current density first emerges with oxygen partial pressure≥0.07%. Our results demonstrate the existence of a first-order melting transition in twinned RE-123 system containing rare-earth magnetic ions. We have shown that twin planes do not suppress the melting transition in the absence of defects due to Nd–Ba substitution.

Tunneling spectra of Bi-based high-Tc thin film of which the c-axis is tilted about 45° to the substrate

Abstract High T c Bi-based superconductors are easily cleaved between the Bi-O layers. Therefore, most tunneling spectra so far were measured through the Bi-O layer. It is also interesting to study spectra for tunneling parallel to the a-b plane. We studied the tunneling spectra of Bi-S-Ca-Cu-O superconducting thin film of which the c -axis tilted about 45° to the substrate. We observed a gap structure around ± 25mV. We did not observe a double-gap structure as was reported in the case of YBa 2 Cu 3 O 7 . We also observed that the humps at the edge of the gap structure were reduced in an applied magnetic field. We suggest that a low T c superconducting layer exists between the insulating layer and superconductor and this low T c superconducting layer becomes a normal metal in an applied magnetic field. Quasi-particles in tunneling scattered in the normal metal layer.

Scanning tunneling microscopy studies of NdBa2Cu3O7-δ single crystals

Abstract We have performed scanning tunneling microscope/spectroscopy measurements on NdBa2Cu3O7−δ (Nd123) single crystals. Two characteristic structures, i.e., square lattice and one-dimensional atomic structures are observed on the naturally terminated layers. For a cleaved surface, nm size bright and dark regions consisting of a 4×4 A2 square lattice are observed and the contrast is explained by the inhomogeneous distribution of oxygen vacancies on the surfaces.

Tunneling spectra for junctions parallel and perpendicular to the ab plane of Y1Ba2Cu3O7

Abstract We studied tunneling spectra of junctions fabricated parallel and perpendicular to the CuO plane of nearly single crystalline Y1Ba2Cu3O7 prepared by MPMG(Melt-Powder-Melt-Growth) method. We also studied effect of a magnetic field on the tunneling spectrum.