Proximity Effects between Conventional and Unconventional Superconductors
In this article I 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 etlects 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.
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