Surface Effects in the Mixed Superconducting State
1965; American Institute of Physics; Volume: 137; Issue: 3A Linguagem: Inglês
10.1103/physrev.137.a818
ISSN1536-6065
Autores Tópico(s)Superconductivity in MgB2 and Alloys
ResumoMeasurements of the critical transport current above ${H}_{c1}$ in a variety of ${\mathrm{Pb}}_{0.95}$ ${\mathrm{Tl}}_{0.05}$ samples reveal that the surface can support a large transport current in the mixed state when the magnetic field vector is aligned with the surface. When a geometry is chosen so that the magnetic field vector can be aligned with a single surface (e.g., a cylinder of triangular cross section), the critical currents of opposite polarities differ by over a factor of 3 when the magnetic field vector is parallel to a surface plane; partial rectification is thus observed. The larger critical current is always in a direction that indicates that magnetic flux crosses out of the surface of a type-II superconductor more easily than into the surface. The "anomalous peak effect" (a maximum in the critical current just below ${H}_{c2}$) is observed in annealed samples and is identified with the strength of the Saint-James-de Gennes surface film below ${H}_{c2}$ because of the manner in which it is affected by a thin surface film of copper. The critical-transport-current results suggest that two separate mechanisms contribute to the surface transport current in the mixed state. One of these mechanisms is identified with the Saint-James-de Gennes surface film below ${H}_{c2}$; the second is tentatively associated with a bulk penetration depth. We measured a mixed-state resistivity and separated the contributions from the surface and the bulk. The bulk mixed-state resistivity is found to be independent of the density of bulk defects.
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