ISSN:
1573-7357
Source:
Springer Online Journal Archives 1860-2000
Topics:
Physics
Notes:
Abstract Thermodynamic aspects of the proximity effect have been studied via calorimetric measurements on bulk samples composed of alternating super-conducting (S) and normal (N) layers. Specifically, the thermal properties of lamellar Pb/Sn alloys are reported for samples with periodicities λ in the range 0.6≲λ≲6 µm and for temperatures between 1.5 and 8 K. It is shown that if the thickness of the lead-rich (S) domains is greater than about ten coherence lengths, the proximity effect can be described in terms of a surface free energy whose high-temperature behavior is compatible with the cubic Ginzburg-Landau equation. Values of a parameter related to the extrapolation length are derived from the surface free energies, and are shown to lead to transition temperatures which are in good agreement with the observed values in this regime. At the opposite extreme, where the S-domain thickness is comparable to or less than the coherence length, the results show that the thermal properties are virtually indistinguishable from those expected for a homogeneous superconductor with a very low gap ratio Δ0/kT c. The condition for such “pseudohomogeneous” behavior is estimated to be1/2D S≲2ξ(T), whereD S is the thickness of the lead-rich lamellae and ξ(T) is a temperature-dependent coherence length. The transition temperatures of these alloys agree very well with the theory of Moormann for the large periodicities, and moderately well for the small periodicities if the ratio(NV) S/(NV) N of the theory is regarded as an adjustable parameter. However, if this is taken as the ratio of coupling strengths obtained from tunneling experiments—as suggested by the theory of Silvert for the proximity effect in strong coupling materials—then the agreement is only fair for small lamellar periods. The specific heat jump at the transition temperature is found to be in reasonable accord with calculated values based on the Fulde and Moormann theory, provided that one defines the “jump” suitably and that it is regarded as a function ofT c rather than of lamellar thickness. When the same calculations are extended to the region well belowT c, they give results which agree remarkably well with the data in the neighborhood of the specific heat “jump” associated with the tin-rich lamellae. The effects of lamellar stability, irregularities, strains, and concentration gradients are also discussed.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00655281
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