Fluctuations, relaxation and proximity effect in superconducting circuits

Abstract

Mesoscopic physics investigates structures smaller than the everyday macroscopic scale but larger than the scale of individual atoms, with properties that can often only be explained in terms of the laws of quantum mechanics. A typical mesoscopic electrical component is a tunnel junction, formed by a thin insulating oxide layer separating two metallic electrodes. In this thesis, various mesoscopic circuits containing sub-micron tunnel junctions between normal (N) and superconducting (S) metals are studied experimentally at sub-kelvin temperatures. An emphasis is placed on the influence of electrical fluctuations on the systems, as well as the strong connection between electrical and thermal transport in them. We first demonstrate that a Josephson tunnel junction between two S electrodes functions as an on-chip detector of current fluctuations in a wide band of frequencies, potentially useful for studying charge transport in various mesoscopic systems. The lifetime of the zero-voltage state in a current-biased junction is very sensitive to the fluctuations in the bias current. We are able to observe the non-Gaussian nature of the shot noise generated by electrons tunneling across another nearby tunnel junction coupled to the detector junction. Several of the experiments in this thesis probe how the transport close to a transparent interface between a superconducting and a normal conducting electrode is modified by the phenomenon of superconducting proximity effect. We present direct measurements of electron overheating in a normal metal weak link between two superconductors, explaining the routinely observed hysteretic current-voltage characteristic. Electronic temperature is probed locally by contacting the N island to an additional S electrode via an oxide barrier (I), thereby forming an NIS tunnel junction. Connecting the S electrodes of such a proximity SNS weak link into a closed loop, we further demonstrate use of the structure as a sensitive magnetometer with low dissipation. We probe also the electronic thermal conduction of short S wires between two N terminals. Due to the inverse proximity effect, the thermal conductance is found to be strongly enhanced beyond the value for a bulk superconductor. We consider theoretically the prospects for rectifying thermal fluctuations by an NIS junction in a suitable electromagnetic environment, thereby realizing a Brownian refrigerator. Finally, we report the observation of increased cooling power in a voltage-biased NIS junction in small applied magnetic fields. This is attributed to enhanced relaxation of the hot electrons injected into the S lead of the junction.