Takekazu Ishida

Complex Susceptibility of High-Tc Superconductor ErBa2Cu3O6+x

The magnetic response of ErBa2Cu3O6+x has been studied in terms of complex susceptibility χ=χ-iχ. In the superconducting transition, χ reflects the Meissner effect and χ forms a single peak. Careful measurements revealed that the onset temperature of χ is slightly higher than that of χ and there exist two superconducting phases. χ and χ of the lower phase are very sensitive to the field amplitude. The upper phase may be interpreted as a bulk superconductivity while the lower phase corresponds to a weak coupling between bulk superconducting inclusions.

Anomalous Tc alteration of quenched Bi2Sr2CaCu2Ox

The full superconducting transition of a single-phase low-Tc specimen was investigated as a function of a quenching temperature Tq. The midpoint Tc varied from 68.8 K to 95.9 K. A surprising feature was that the Tc had a steep minimum at Tc=300°C. The oxygen stoichiometry did not seem to be a dominant factor for the determination of Tc. We speculate that the (BiO)2 layer was responsible for controlling the hole concentration of the CuO2 plane.

Superconducting Transition Temperature of the Quenched Bi0.96Pb0.24SrCaCu1.6Ox: 110 K Phase

The superconducting transition of a 110 K Bi compound has been examined as a function of quenching temperature Tq. We found that Tc is insensitive to Tq below 650°C. This is in marked contrast with the drastic Tc variation of low-Tc phase reported previously. But the influence of quenching on normal-state conduction resembles that in the case of the low-Tc phase. This situation is difficult to fathom. We propose the possibility that Tc is governed by a central layer of three CuO2 stackings.

London penetration depth of YBa2Cu3O7−δ

We have measured the ac magnetic susceptibility of powdered YBa2Cu3O7-δ as a function of temperature. The temperature dependence of the London penetration depth estimated from the susceptibility data is quite different from that expected by the BCS theory. The result is well reproduced by the empirical formula λ(T)=λ(0)(1-T/Tc)-1/2, suggesting the existence of a gapless region on the Fermi surface in the high-Tc oxide superconductor.

Systematic characterization of upper critical fields for MgB2 thin films by means of the two-band superconducting theory

We present experimental results for the upper critical fields Hc2 of various MgB2 thin films prepared by molecular beam epitaxy, multiple-targets sputtering, and co-evaporation deposition. Experimental data for the Hc2(T) are successfully analyzed by applying the Gurevich theory of dirty two-band superconductivity in the case of Dπ/Dσ>1, where Dπ and Dσ are the intraband electron diffusivities for the π and σ bands, respectively. We find that the parameters obtained from the analysis are strongly correlated to the superconducting transition temperature Tc of the films. We also discuss the anomalous narrowing of the transition width at intermediate temperatures confirmed by the magnetoresistance measurements.

Resistivity Zero of Quenched Bi2Sr2CaCu2Ox at Above 90 K

It has been verified that Bi2Sr2CaCu2Ox does not remain as an 80-K superconductor upon quenching. The resistivity zero of the low-Tc Bi compound exceeds 90 K. We emphasize that Bi2Sr2CaCu2Ox is equivalent to YBa2Cu3O7-δ as far as Tc is concerned. The characteristic combination of two CuO2 layers may be responsible for the specification of Tc.

Bulk nature of quench-enhanced superconductivity in Bi2Sr2CaCu2Ox

The lower Tc of the Bi-Sr-Ca-Cu-O (BSCCO) system can be raised by more than 15 K by quenching. This characteristic has been confirmed for a stoichiometric 2212 compound. From the ac susceptibility measurement of the powdered specimens, we revealed that the observed Tc enhancement is not filamentary but has a bulk origin. X-ray diffraction study indicated that our specimens are almost single phase and the crystalline cage is not altered drastically by quenching.

Superconducting Transitions of BiSrCaCu2Ox Quenched from High Temperatures

The BiSrCaCu2Ox specimens have been quenched from various elevated temperatures (200°C ~860°C) to liquid nitrogen. The X-ray diffraction study revealed that the crystalline structure of specimens is essentially tetragonal. We found that the lower Tc as well as the higher Tc vary remarkably but differently as a function of a quenching temperature Tq. We present evidence that the opposite Tc variation observed between 400°C and 700°C cannot be attributed to a microscopic interaction between two phases but might be inherent.

Complex Susceptibility of the Oxide Superconductor BiSrCaCu2Ox

The magnetic response of a sintered BiSrCaCu2Ox pellet has been studied in terms of the ac susceptibility χ=χ-i χ. Both the resistive and inductive transitions occur in two steps (at 110 K and at 82 K). From the field-dependent susceptibility, we revealed that both bulk and weakly coupled superconductive components contribute to the χ profile of the lower transition. This behavior is similar to that of other oxide superconductors.

Anisotropic Diamagnetism of 110 K Bi-Pb-Sr-Ca-Cu-O

The 110 K Bi compounds have been prepared by means of a Pb-doping technique. The 110 K phase was obtained in an almost single-phase form. The ac susceptibility of the pellet can be interpreted in terms of bulk and coupling contributions. The powdered specimen was molded in a Stycast 1266 under a 8.9 T magnetic field to align the sense of the c axis. We found that the ac susceptibility was very anisotropic with respect to the c axis.