Hisashi Kawabe

Strong flux pinning centers in Y‐Ba‐Cu‐O films prepared by chemical vapor deposition

The Y‐Ba‐Cu‐O film prepared by chemical vapor deposition showed Tc=91.5 K and Bc2 (77.3 K)=60 T defined by zero resistance. The Jc values measured at 77.3 K were 7.8×105 A/cm2 at B=0 and 1.0×105 A/cm2 at 16 T, magnetic fields perpendicular to the c axis. Small disk‐shaped precipitates possibly regarded as strong flux pinning centers in the Y‐Ba‐Cu‐O films were observed using transmission electron microscopy. The evaluation of Jc related to the concentration and configuration of the precipitates was in reasonable agreement with the measured Jc values.

Resistive transition in YBa2Cu3Oz and Bi2(Sr,Ca)3Cu2Oz CVD films under high magnetic field

Abstract Electrical resistance of YBa 2 Cu 3 O z and Bi 2 (Sr,Ca) 3 Cu 2 O z CVD films in the mixed state was investigated in magnetic field up to 25 T. The sheet resistance in the broad transition is expressed by R □ = R 0 exp(- U 0 / k B T ) on the low temperature side, and it is independent of the configuration of current with respect to the magnetic field. The activation energy U 0 is larger in YBa 2 Cu 3 O z films. The U 0 value becomes smaller proportional to B -α with increasing field. Moreover, the resistance follows a power law of B γ at constant temperature. The value of exponent γ is larger than unity and increases with decreasing temperature. These behaviors are discussed based on the flux creep models by Tinkham and by Dew-Hughes.

Normal and superconducting properties of Pb2Sr2Y1−xCaxCu3O8+δ: II. Dependence on the oxygen concentration

The dependence on the oxygen concentration of normal and superconducting properties of ceramic samples of Pb2Sr2Y1−xCaxCu3O8+δ has been investigated in the composition range of 0⩽x⩽0.5 and 0⩽δ<2. The superconductivity in each sample with different x is suppressed with increasing δ, which is in good correspondence to the increase in resistivity and Hall coefficient with increasing δ, but is contradictory to the speculation by Ohta and Maekawa [Phys. Rev. B41 (1990) 6524]. It is found from chemical analysis that holes introduced by the extra oxygen first enter the CuOδ layer and then the PbO layer rather than the pyramidal CuO2 layer. With increasing δ, a structural phase transition from the orthorhombic phase to the tetragonal phase is observed at δ∼1 in every sample with different x. The dependence on the oxygen concentration of the magnetic susceptibility is also reported.

Critical Current Criterion in High-Tc Superconducting Films

Critical current densities at 77.3 K obtained by a voltage criterion of 2 µV/cm were 5.6×105 A/cm2 at 0 T and 3.1×104 A/cm2 at 27 T for the best Y1Ba2Cu3O7-δ film prepared by chemical vapor deposition. Current-voltage curves were fitted by a power law of the form V∝In with small n-values of 4–14. Since its n-value is 7 at 27 T, the Jc corresponds to 2.8×104 A/cm2 in the 1 µV/cm criterion and also to 2.0×104 A/cm2 in the 0.1 µV/cm one. In the critical current measurement for superconducting oxides, the n-value together with the criterion should always be described.

Transport critical current and morphology for YBaCuO films prepared by chemical vapor deposition

Abstract Y 1 Ba 2 Cu 3 O 7−δ films with various morphologies were prepared by a chemical vapor deposition. Electron probe micro-analysis (EPMA) exhibited many precipitates with the higher copper content in the matrix of CVD films. For the best film, T c = 92 K, B c2 (77K) = 62 T and J c (77K, 27T) = 3.1×10 4 A/cm 2 were obtained by resistive measurements.

Correlation between magnetic relaxation and Jc of YBa2Cu3O7 films

The magnetic relaxation arising from thermally activated flux motion has been measured for the high-J c YBa 2 Cu 3 O 7 films prepared by chemical vapor deposition. The distribution of activation energies is discussed on the basis of a flux creep model proposed by Hagen and Griessen. The distrubution is related to the high-J c property of the YBa 2 Cu 3 O 7 films.

Upper Critical Field and Critical Current Density in Ceramic Samples of Pb2Sr2Y0.5Ca0.5Cu3O8+δ

The electrical resistance of ceramic samples Pb2Sr2Y0.5Ca0.5Cu3O8+δ with zero resistance at 75 K has been measured in magnetic fields up to 21 T. The superconducting transition curve becomes as broad in the field longitudinal to the current direction as in the transverse field. The value of -dHc2/dT is estimated as 1.7–1.8 T/K. The resistive tail in the lower portion of the transition curve is well expressed as ρ=ρ0 exp (-U0/kBT). The critical current density Jc shows hysteresis with respect to the magnetic field sweep, and is as low as 0.1 A cm-2 at 4.2 K in magnetic fields between 1 T and 13 T.

Angular dependence of critical current density of YBa2Cu3O7- delta films in magnetic fields

The anisotropic behaviour of the transport critical current density, Jc, of YBa2Cu3O7- delta films prepared by CVD has been investigated in high magnetic fields up to 27 T. The strong angular dependence of Jc is discussed based on the intrinsic pinning model.

Electric resistance in the mixed state of the high Tc oxide films

Abstract The thermally activated behavior in the resistive transition and the relaxation phenomenon of the dc magnetization of Y Ba Cu O films have been investigated in magnetic fields up to 25 T. The behavior of the electrical resistance is not consistent with the simple flux creep model based on the Lorentz force. Moreover, the activation energy estimated from the resistance measurements is about one order larger than that from the magnetic relaxation.

Superconducting properties of Pb2Sr2Y1−xCaxCu3O8+δ

Superconducting properties of ceramic samples Pb2Sr2Y1−xCaxCu3O8+δ have been investigated in the wide composition-range of 0≦x≦0.8 and 0≦0