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Title:Heat capacity measurements of yttrium barium(2)copper(3)oxygen(7-delta) near T(c): Fluctuation effects in a bulk superconductor
Author(s):Inderhees, Sue Ellen
Doctoral Committee Chair(s):Salamon, Myron B.
Department / Program:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Physics, Condensed Matter
Abstract:High-resolution ac heat capacity measurements of YBa$\sb2$Cu$\sb3$O$\sb{\rm 7-\delta}$ near the critical temperature are reported and discussed. The samples are high-quality single crystals, several of which display untwinned regions making up at least 50% of the sample mass. The heat capacity data show a cusp-like anomaly instead of the discontinuous step observed in "classic" superconductors. This divergence is indicative of the presence of fluctuation contributions and is due to the exceptionally short Ginzburg-Landau coherence length of this material. Because there is considerable uncertainty in the temperature of cross-over from mean-field to critical behavior, the zero-field heat capacity data are analyzed both as a lowest-order correction to mean-field BCS theory (the Gaussian approximation) and in terms of true critical behavior. The analysis shows that the data are best described within the Gaussian approximation as three-dimensional fluctuations of a two-component order parameter, but the results do not rule out a more complicated order parameter or a logarithmic (critical) divergence. The underlying BCS step indicates strong-coupling behavior.
Application of a magnetic field broadens and suppresses the heat capacity transition, with no apparent shift in the onset of superconductivity, and with the effects more pronounced for fields applied parallel to the c-axis than for fields in the ab-plane. We find this behavior cannot be explained in terms mean-field theory. Rather, the anisotropic broadening and suppression may be understood in the context of critical finite-size scaling, since the length scale of the fluctuations is essentially restricted to values less than the lowest-order Landau radius in directions perpendicular to the applied field.
Issue Date:1990
Rights Information:Copyright 1990 Inderhees, Sue Ellen
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9114278
OCLC Identifier:(UMI)AAI9114278

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