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The Proximity Effect in Mesoscopic Normal/Superconducting StructuresThe proximity effect between normal and superconducting metals was well studied in the 1960's, culminating in the review articles by deGennes describing the experimental phenomena in terms of a Ginzburg-Landau theory. In the 1990's several groups applied the techniques of mesoscopic physics to fabricate and study submicron hybrid samples consisting of both normal and superconducting metals in contact. They soon discovered several phenomena that could neither be understood from the old Ginzburg-Landau theory, nor from the mesoscopic quantum transport theories of the 1980's. While most groups focused on transport properties, the Quantronics Group at Saclay, France measured the equilibrium properties of the proximity effect. During my sabbatical working with that group, we measured the single-particle density of states (DOS) in a normal metal wire very close to a good contact with a superconductor. Because the wire was highly disordered, we could analyze the data using the Usadel equation from non-equilibrium superconductivity theory (also called the "quasiclassical" theory). The data and theory show that the DOS in the normal wire displays a dip near the Fermi energy, whose width in energy dE depends on the distance x from the N/S contact as dE » hD/x2, where D is the electron diffusion constant. The depth of the of the feature decreases as x approaches the phase-breaking length. Several experimental groups have studied the transport properties of NS structures. Two papers that represent a subset of that work are H. Courtois et al., J. Low Temp. Phys. 116, 187 (1999), and V.T. Petrashov et al., Phys. Rev. B 58, 15088 (1998). |
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