Monday, August 20, 2018

Growth and characterization of Bi2Sr2Ca2Cu3O10 and (Bi,Pb)2Sr2Ca2Cu3O10−δ single crystals

Large and high-quality single crystals of both Pb-free and Pb-doped high-temperature superconducting compounds (Bi1−xPbx)2Sr2Ca2Cu3O10−y (x = 0 and 0.3) were grown by means of a newly developed 'vapour-assisted travelling solvent floating zone' technique (VA-TSFZ). This modified zone-melting technique was performed in an image furnace and allowed for the first time the growth of large Pb-doped crystals, by compensating for the Pb losses that occur at high temperature. Crystals up to 3 × 2 × 0.1 mm3 were successfully grown. Post-annealing under high pressure of O2 (up to 10 MPa at T = 500 °C) was applied to enhance Tc and improve the homogeneity of the crystals. Structural characterization was performed by single-crystal x-ray diffraction (XRD). Structure refinement confirmed a commensurate modulated superlattice in the Pb-free crystals. The space group is orthorhombic, A2aa, with cell parameters a = 21.829(4) Å, b = 5.4222(9) Å and c = 37.19(1) Å. Superconducting studies were carried out by ac and dc magnetic measurements. Very sharp superconducting transitions were obtained in both kinds of crystals (ΔTc ≤ 1 K). In optimally doped Pb-free crystals, critical temperatures up to 111 K were measured. Magnetic critical current densities of 2 × 105 A cm−2 were measured at T = 30 K and μ0H = 0 T. A weak second peak in the magnetization loops was observed in the temperature range 40–50 K, above which the critical current density was found to rapidly decrease as a function of T and H. A comparison between the pinning properties in Pb-free and Pb-doped crystals is reported and discussed.


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