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Abstract:
In this article we review recent theoretical work on the possibility of using tunneling and proximity
effect experiments to study unconventional superconductors. The basic idea is simple: as shown in
Fig. 1 one places a thin layer of some well understood conventional superconductor in good metallic
contact with the superconductor one wishes to study, and then measures (e.g. by tunneling at the outer
edge the conventional layer) how the superconducting properties of the conventional material are altered
by its proximity to the unconventional material. To study this question theoretically one has to solve
the gap equation for the inhomogeneous system. Because the symmetry and the physical origin of the pairing
interaction may be different for conventional than for unconventional superconductors, it is possible that
the proximity effect is different between two conventional superconductors than it is between a conventional
and an unconventional superconductor. Our understanding of the nature and observability of these differences
is still preliminary. A useful theoretical technique for calculating such proximity effects has only recently
been proposed, and only a few calculations in simple model systems have been done. The results from this
preliminary work are not encouraging: proximity effects involving unconventional superconductivity seem to
differ only in subtle ways from those involving only conventional superconductivity; further, proximity
effect experiments do not seem to provide a useful method of distinguishing unconventional singlet
(e.g. d-wave) from triplet superconductivity.
Paper: [PDF]
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