University of Birmingham > Talks@bham > Theoretical Physics Seminars > Supercritical Fields and Flux Structures in Mesoscopic Type I Superconductors

Supercritical Fields and Flux Structures in Mesoscopic Type I Superconductors

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  • UserProf Simon Bending, U of Bath
  • ClockThursday 26 April 2012, 13:45-15:00
  • HouseTheory Library.

If you have a question about this talk, please contact Dr Dimitri M Gangardt.

The properties of type I superconducting materials were extensively studied in the 1960s and 1970s with a view to understanding surface superconductivity and superheating effects, as well as the role played by shape-dependent demagnetisation factors. In bulk samples the latter lead to the formation of an intermediate state of coexisting superconducting and normal domains which limit the local field inside the sample while the maximum sustainable external field remains Hc. Surprisingly, however, the properties of mesoscopic type-I superconductors, comparable in size to the superconducting coherence depth, remain largely unexplored. Such structures can be made too small to support an intermediate state when their properties are dominated by surface and geometrical effects. We have used electrochemical deposition to fabricate highly facetted micron-sized type I Sn and Pb single crystals with well-defined shapes. Hall micromagnetometry measurements and Ginzburg-Landau (G-L) simulations for strongly type I -Sn rods reveal distinctly size- and temperature-dependent superheating (Hsh) and supercooling (Hsc) fields, that depart dramatically from the classical thermodynamic critical field, Hc. As the temperature increases (or samples become smaller), Hsh/Hc and Hsc/Hc meet and both diverge as Tc is approached, resulting ultimately in a reversible, second-order superconducting-to-normal phase transition at fields many times larger than Hc. This behaviour will be contrasted with that of mesoscopic Pb structures in which an intermediate state is always observed, and strong quantum confinement leads to chains of single flux quanta in nanowire samples. A further level of control can be achieved by fabricating hybrid structures combining materials with different functionalities, e.g., a superconductor and a ferromagnet (S-F) or two different superconductors (S1-S2). S-F (Pb-Ni and Sn-Ni) core-shell structures exhibit superconductivity at applied fields well in excess of the bulk Hc(T) due to the compensation of stray fields, as well as a form of superconducting ‘noise’ linked to the Barkhausen effect in the ferromagnetic shell. In addition we demonstrate that modification of the boundary condition at the Pb/Sn interface of S1-S2 (Sn-Pb) core-shell structures leads to enhanced superconductivity in the Sn core which appears to remain superconducting well above its bulk critical temperature.

This talk is part of the Theoretical Physics Seminars series.

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