Regensburg 2004 – scientific programme

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TT: Tiefe Temperaturen

TT 8: Postersitzung I: Supraleitung

TT 8.34: Poster

Monday, March 8, 2004, 14:30–19:00, Poster A

Evidence for a percolation-driven transition to coherent surface superconductivity — •Lars von Sawilski, Sara Casalbuoni, and Jürgen Kötzler — Institut für Angewandte Physik und Zentrum für Mikrostrukturforschung, Universität Hamburg, Jungiusstrasse 11, D-20355 Hamburg, Germany

Above the upper critical field H_c2 we have investigated the field dependences of the surface conductance, G′−iG″, and the critical current J_c of different treated Niobium cylinders: as grown, chemically and electrolytically polished. The low frequency limits of G′(H) and G″(H) and of J_c(H) display power-law singularities, defining a transition to coherent surface superconductivity at H_c3^c>H_c2. The critical exponents as well as the dynamical scaling of G′−iG″ studied between 0.1 Hz and 1 MHz are consistent with predictions for a two-dimensional percolation transition. Relating H_c3^c to the conventional onset field for surface superconductivity H_c3, we find H_c3^c/H_c3=0.81. Surprisingly, this ratio turns out to be independent of significant variations of H_c3 due to differently treated surfaces. This points to a new universal relation between the nucleation and the onset of long-range coherent surface superconductivity.