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|Title:||Magnetic field resistivity of superconducting bismuth oxides|
|Authors:||Krause, Walter Thomas|
|Abstract:||<p>The superconducting transition in the magnetic resistance of members of the family of bismuth based high temperature superconductors was investigated. Measurements were performed in magnetic fields up to 1.7 T. Small current densities ranging from 0.03 A/cm² to 3.0 A/cm² were applied. The resistivity of Bi₂Sr₂CaCu₂Ox single crystals was analyzed in terms of the thermally activated flux flow expression, ρ =ρ₀U/T exp(-U/T) where T is the temperature. It was found that the activation energy was given by Uα(Hsinθ)^(⁻α) where α≈1/3 and that the prefactor had the form, ρ₀Hsinθ, where H was the applied field and θ the angle of the field with respect to the CuO₂ planes. Results demonstrated that dissipation could be accounted for by the motion of two-dimensional vortices whose density is given by the field, Hsinθ, projected on the CuO₂ planes. Measurements of the resistivity and current dependent resistivity were performed with two Sn-doped and two Sb-doped polycrystalline Bi(1.7)Pb(0.3)Sr₂Ca₂Cu₃O(y) samples. Features in the temperature derivative of the resistivity curves were associated with the presence of a superconducting transition between superconducting grains, coupled by weak links with a distribution of critical currents and critical temperatures, and the superconducting transition within grains. The transition between grains was more strongly suppressed in temperature with the application of a magnetic field in samples with weaker coupling between grains. The presence of a transition in a magnetic field due to weak links between grains was verified at 77 K by the observation of a current dependent resistivity in a magnetic field. Measurements of a Bi₂Sr₂CaCu₂Ox diffusion grown thick film ring were done. The transverse voltage, the voltage at the centre of a 120 μm thick branch with respect to the centre of a 76 μm thick branch, was measured. A higher critical temperature from the presence of more texturing in the 76 μm branch as determined by separate resistivity, x-ray and scanning electron microscopy measurements was consistent with the measurement of a crossover from a negative to positive transverse voltage as the temperature of the sample went through its superconducting transition.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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