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Current noise measurements allow to enlighten the transport mechanisms in mesoscopic samples in a complementary way to conductance measurements. The noise gives directly access to the charge of the current carriers and is modified by interactions. This is particularly instructive in the context of hybrid superconductor-normal metal structures where charge pairs generated by Andreev reflection compete with quasiparticles in the current transport. We present investigations of current noise in various hybrid geometries at temperatures down to 50mK using a SQUID as fluctuations detector.
The first two types of structures involve one superconductor in contact with a diffusive normal metal reservoir. To obtain a nearly perfect contact between a superconductor and the normal metal (high transmittive contact), we used niobium as a superconductor and copper as normal metal. Then, the current transport is mediated by charge pairs induced from the superconductor (proximity effect) and the noise is doubled S=2/3x2eI, compared to the case where the two electrodes are normal S=1/3x2eI (where I is the mean bias current and e the electron charge). The 1/3 reduction from the full Poisson noise (S=2eI), observed e.g. in a tunnel junction between two normal reservoirs, is due to the diffusive character of the normal metal. The noise doubling is observed as long as the voltage drop at the junction is smaller than the superconducting gap. Above the gap the shot noise has the same behavior as in the normal case.
In the second type of structures under study, where a superconductor (here TiN) is in contact with a strongly disordered metal (heavily doped silicon), the contact is degraded because of the Schottky barrier that forms at the interface (low transmittive junctions). The noise in such junctions is also doubled, but is twice the full Poisson noise S=2x2eI since the noise is mainly generated at the interface barrier. This confirms that the enhancement of the conductance observed at low bias voltage is due to coherent backscattering of quasiparticles towards the interface by the strong disorder in the silicon (reflectionless tunneling). When this excess of subgap conductance vanishes at finite voltage, the noise slope crosses over to the Poisson value 2e indicating a large quasiparticle contribution to the current.
The third type of structures investigated is a long diffusive S/N/S junction made of aluminum and copper. The noise is enhanced compared to the N/N/N-case due to the confinement of the electron gas between the two superconducting reservoirs and the current transport involve Incoherent Multiple Andreev Reflections. Inelastic processes are important in our samples because the lengths of the junctions (4, 10, and 60 μm) are of the same order of magnitude as the inelastic scattering length. We analyze the results quantitatively with recent semi-classical theory taking into account electron-electron interaction and heat transfer through the SN interfaces in the context of S/N/S junctions. For the longer junctions, we also considered electron-phonon-interaction as a possible cooling mechanism. Finally we show that the energy dependence of the re-entrance of the resistance, observed at low voltage, is essentially due to the increasing effective temperature of the quasiparticles in the normal metal.
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