Heat capacity measurements of yttrium barium(2)copper(3)oxygen(7-delta) near T(c): Fluctuation effects in a bulk superconductor
Inderhees, Sue Ellen
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https://hdl.handle.net/2142/22402
Description
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
Issue Date
1990
Doctoral Committee Chair(s)
Salamon, Myron B.
Department of Study
Physics
Discipline
Physics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Condensed Matter
Language
eng
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.
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