We carry out an extensive experimental and theoretical study of the Josephson effect in S-N-S junctions made of a diffusive normal metal ͑N͒ embedded between two superconducting electrodes ͑S͒. Our experiments are performed on Nb-Cu-Nb junctions with highly transparent interfaces. We give the predictions of the quasiclassical theory in various regimes on a precise and quantitative level. We describe the crossover between the short-and the long-junction regimes and provide the temperature dependence of the critical current using dimensionless units eR N The Josephson effect is well known to exist in weak links connecting two superconducting electrodes S, e.g., a tunnel barrier I, a short constriction C or a normal metal N ͑S-I-S, S-C-S, and S-N-S junctions͒. This effect manifests itself in a nondissipative dc current flowing through the Josephson junction at zero voltage. At weak coupling, e.g., in the S-I-S case, the Josephson current can be expressed as I s ϭI c sin , where is the phase difference between the two superconducting condensates and the maximum supercurrent I c is called the critical current.The Josephson effect in S-N-S junctions has been studied in a variety of configurations. The early experiments of Clarke 1 and Shepherd 2 were performed in Pb-Cu-Pb sandwiches. In these experiments and in the pioneering calculations by de Gennes, 3 it was already realized that the presence of a supercurrent in such structures is due to the proximity effect. This can be understood as the generation of superconducting correlations in a normal metal connected to a superconductor, mediated by phase-coherent Andreev reflections at the S-N interfaces. The critical current I c is limited by the ''bottleneck'' in the center of the N-layer, where the pair amplitude is exponentially small: I c ϰe ϪL/L T . Here, L T ϭͱបD/2k B T is the characteristic thermal length in the diffusive limit and L is the length of the junction. These calculations, as well as those by Fink, 4 analyzed the temperature dependence of I c within the Ginzburg-Landau theory in the vicinity of the superconducting critical temperature T c . Later, the critical current of diffusive S-N-S microbridges 5,6 was successfully described by Likharev 7 with the aid of the quasiclassical Usadel equations. 8 In this work, the emphasis was put on the high-temperature regime where the superconducting order parameter is smaller than the thermal energy ⌬Ӷk B T. A more general study of the Josephson effect in diffusive S-N-S junctions was made in Ref. 9.More recently, experimental data on long Josephson junctions 10 showed a surprising temperature dependence, which turned out to be in a strong disagreement with the early theory by de Gennes. These data have been discussed by some of us 11 within the quasiclassical approach, which we will also use in the present work. Fink 12 attempted to analyze the data 10 by means of an extrapolation of the GinzburgLandau theory to low temperatures.The proximity effect in mesoscopic hybrid structures consisting of normal and superc...

We investigate hysteresis in the transport properties of superconductor -normal-metal -superconductor (S-N-S) junctions at low temperatures by measuring directly the electron temperature in the normal metal. Our results demonstrate unambiguously that the hysteresis results from an increase of the normal-metal electron temperature once the junction switches to the resistive state. In our geometry, the electron temperature increase is governed by the thermal resistance of the superconducting electrodes of the junction.

We present an experimental study of the transport properties of a ferromagnetic metallic wire (Co) in metallic contact with a superconductor (Al). As the temperature is decreased below the Al superconducting transition, the Co resistance exhibits a significant dependence on both temperature and voltage. The differential resistance data show that the decay length for the proximity effect is much larger than we would simply expect from the exchange field of the ferromagnet. 74.50.+r, 74.80.Fp, 85.30St Superconducting proximity effect consists in inducing superconductive properties in a non-superconducting metal. Although this effect has been studied for a long time 1 , it has gained some renewed interest due to recent experiments performed on samples of mesoscopic size. In such samples, the electron phase-breaking length L ϕ is larger than the sample length L. One can thus probe experimentally the characteristic energy scale of the proximity effect ǫ c =hD/L 2 , which is the Thouless energy related to the sample length. This has led for instance to the observation of large magnetoresistance oscillations in normal metal (N) loops in contact with a superconducting (S) island 2-4 . These oscillations provide a direct evidence for the long-range (up to L ϕ ) nature of the proximity effect. Another recent and striking result is the reentrant behaviour. The excess conductance induced by proximity effect is maximum at a temperature or a bias voltage equivalent to the sample Thouless energy 5 , but the normal state conductance reappears at lower energy.Most experiments were performed in noble metals or semiconductor 2D electron gas, where electron interactions are negligible. In a free electron model, the zerotemperature, zero-bias resistance of a mesoscopic metallic wire is predicted to recover the normal state value 6-8 . In the presence of interactions, theoretical studies 7,9 predict a severe modification of the transport properties. Attractive (respectively repulsive) electron-electron interactions are believed to result in a resistance lower (respectively higher) than the normal-state one 7 . This could provide a probe for interactions in normal metals like Au, Ag, etc 10 . In this communication, we present an experimental study of the superconducting proximity effect in a ferromagnetic metal (F). Magnetic metals are in the strong interaction limit. Exchange interactions between electrons in a ferromagnet usually lead to efficient Cooperpair breaking in F-S structures. However, it is worthwhile re-examining the actual proximity effect in a small ferromagnetic wire 11 . Some experiments 12,13 suggested long-range coherence effects, but without any clear conclusion.