Proximity effects and Andreev reflection in a mesoscopic SNS junction with perfect NS interfaces

Abstract

Low temperature transport measurements on superconducting film-normal wire-superconducting film (SNS) junctions fabricated on the basis of thin superconducting polycrystalline PtSi films are reported. Due to the perfectness of SN boundaries in the junctions, zero bias resistance dip related to pair current proximity effect and subharmonic energy gap structure originating from phase coherent multiple Andreev reflections have been observed and studied. 74.80.Fp, 74.50.+r Research activity in the study of properties of NS and SNS junctions (where N is a normal metal and S is a superconductor) has increased significantly over the past few years, mainly owing to technological advances in fabrication of mesoscopic hybrid systems. Until now, all such systems have been made by a combination of different materials, for example, the superconductor-normal metal pairs Al-Ag [1], Nb-Al and Nb-Ag [2], superconductor-heavily doped semiconductor pairs Nb-n + InGaAs [3], Nb-n + InAs [4], Nb-p + Si [5,6], Al-n + GaAs [7] and so on. One of the important topics of the physics of NS and SNS junctions is the currentvoltage characteristics behavior. Two interesting phenomena can be observed from the dependence of SNS differential resistance versus bias voltage (dV /dI-V ): an anomalous resistance dip at zero bias [2-7], the socalled zero bias anomaly (ZBA); and symmetrical dips at nonzero biases [7,8]. In some cases the existence of tunnel barriers at the NS interfaces does not give the possibility of the definite interpretation of experimental results.In this paper we report the fabrication and study of SNS junctions with perfect SN interfaces. The best way to obtain a perfect junction is to have both superconducting and normal metal parts of an SNS junction made of the same material. It is known that the wires made from some thin superconducting metallic films can have the transition temperature T c less than that of the film itself. We have used this property to fabricate the ideal SNS junctions.We started with fabrication of ultrathin PtSi films. Smooth, continuous and uniform PtSi films with thickness of 6 nm were formed on Si substrates by deposition of a thin Pt layer followed by a rapid thermal annealing step at 450 • C for 60 s to convert the as-deposited Pt into PtSi. The films formed were polycrystalline, with an average grain size of roughly 20 nm.The samples used in the experiments are three Hall bridges with 50 µm in width and 100 µm in length. The main parameters of the films obtained from Hall measure-ments at T > T c = 0.54 K are presented in Table I. The diffusion constant is estimated assuming the simple free electron model. As is seen from Table I, our PtSi films are metal films with small mean free path. It should be noted that they have a hole type conductivity.PtSi wires of length L = 1.5 µm and 6 µm and width W = 0.3 µm were fabricated by means of electron lithography and subsequent plasma etching and placed in one of the Hall bridges. For a schematic picture of the samples, and a scanning electron micrograp...

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