We calculate the transport critical-current density in a granular superconductor in magnetic fields below about 5 x 10/sup -3/ T. The field dependence in this region is assumed to be controlled by intragranular or intergranular Josephson junctions. Various model calculations are fitted to transport critical-current data on bulk Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7/..sqrt../sub delta/ ceramic superconductors, whose average grain size somewhat exceeds 10 ..mu..m. The results yield an average junction cross-sectional area (thickness x length) of 4--6 ..mu..m/sup 2/. If the junctions are at the grain boundaries, a London penetration depth of about 150--300 nm is inferred, consistent with other estimates. We conclude that Josephson junctions are limiting the transport critical current in these samples and that they lie at the grain boundaries. The parameters of the fit are not consistent with Josephson junctions at twinning boundaries.

Josephson Junction Model of Critical Current in Granular Y1Ba2Cu3O7-δ

We identify the preparation procedures to obtain polycrystalline pellets of Bi-2212 and of Bi-2223 high-temperature superconductors, which slow only the superconducting properties and the X-ray diffraction pattern of one or the other phase (“single-phase” pellets). These procedures were found by systematic variation of the preparation, with substitution of PbO for Bi2O3 as well as with addition of PbO to the stoichiometric mixture of starting powders. Bi-2223 in single-phase form has a TZR of 107.5 ± 0.5 K, whereas single-phase pellets of “Bi-2212” can have sharp transitions with TZR's between 55–95 K, depending on the oxygen partial pressure of the atmosphere in the final annealing step.

Preparation and characterisation of single-phase Bi-Pb-Sr-Ca-Cu-O high temperature superconductors

Synthesis and characterization of Er-substituted Bi-2223 H-Tc glass–ceramic superconductors

Electrical resistivity measurements have been performed on the Bi 2 Sr 2 Ca 1−x M x Cu 2 O 8+y (M=Eu, Dy, Tm) system with various heat treatments. A gradual change in normal state resistivity ( ϱ n ) and T c has been observed with increasing dopant concentration. The results provide experimental evidence for a transition from a metallic behaviour to a variable range hopping regime at low temperatures. In the insulator regime (highly semiconducting-like) the resistivity behaviour is found to turn from 2D to 3D variable range hopping for x ⩾0.6. The results are discussed in terms of a theoretical model which determines the intrinsic competition between superconductivity and localization in an essentially localized system. T c data are compared with the earlier reported findings.

Superconductivity and resistivity studies in Bi2Sr2Ca1−xMxCu2O8+y (M=Eu, Dy, Tm and 0⩽x⩽0.6)

The work reported in this paper aims at a better understanding of doping effects on high temperature superconducting YBa2Cu3O7?? (YBCO or M-doped YBCO), in the form of large grains, fabricated by the top seeded melt-growth process. The study focused on the performance of YBCO large grains doped with Co, Ga, Ni and Zn substituting for the Cu site. The rate of Tc suppression with dopant concentration varies significantly among different dopants. Zn?doping shows the highest depression rate, followed by Ni, Ga and Co?doping. This we attribute to the fact that Zn and Ni substitute at Cu?(2) sites on CuO2 planes in the lattice, whereas Co and Ga occupy Cu (1) sites along CuO chains and they have different magnetic moments. Trapped field, levitation force and Jc are enhanced due to pinning caused by local suppressed superconducting regions, and the enhancement effects are the highest in the case of Zn doping.

Effects of ionic doping on superconducting properties of melt textured YBa2(Cu1-xMx)3O7-δ (M = Co, Ni, Zn or Ga) large grains

Superconductivity in the low T c Bi-based 2-1-2-2 cuprate has been studied as a function of processing parameters. T c ( R =0) above 91 K has been realized through the optimization of process conditions. XRD of the samples indicate subtle changes in respect of intensity and splitting of certain peaks. The results are compared with those on Bi-based 2-2-2-3 and Y-based 1-2-3 cuprates. The enhanced T c and the reduced δT c of the 2-1-2-2 compound have been suggested to be the consequence of improved atomic ordering and orthorhombicity.

Superconductivity above 90 K in low Tc phase Bi2Ca1Sr2Cu2Ox

Thermoelectric power of Bi2-xPbxCa2Sr2Cu3O10 based on the two-band model.

Superconductivity and possible carrier localization in YBa2Cu3−xZinxO7−y

The hafnium doped Y-Ba-Cu-O system is characterised by resistivity, EPR and low magnetic field microwave absorption signal (LFMAS) studies for doping concentrations varying from 0.5% to 2%. It is found that Hf doping improves the mechanical strength of the material without changing much of its superconducting properties at least up to 2% of doping. From LFMAS studies, it is observed that the superconducting properties of these ceramics are of intergranular nature for fields ⩽ H m (where LFMAS is maximum) and of intragranular character for fields > H m .

V. N. Moorthy

Papers

Superconductivity above 90 K in low Tc phase Bi2Ca1Sr2Cu2Ox

Thermoelectric power of Bi2-xPbxCa2Sr2Cu3O10 based on the two-band model.

Superconductivity and possible carrier localization in YBa2Cu3−xZinxO7−y

EPR and low magnetic field microwave absorption in hafnium doped YBa2Cu3Oy

Superconductivity of high Tc cuprates with 3d metallic dopants