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Register a new userRe-entrance of the metallic conductance in a mesoscopic proximity superconductor
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P. Charlat
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We present an experimental study of the diffusive transport in a normal metal near a superconducting interface, showing the re-entrance of the metallic conductance at very low temperature. This new mesoscopic regime comes in when the thermal coherence length of the electron pairs exceeds the sample size. This re-entrance is suppressed by a bias voltage given by the Thouless energy and can be strongly enhanced by an Aharonov Bohm flux. Experimental results are well described by the linearized quasiclassical theory.
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In a mesoscopic metal in proximity with a superconductor, the electronic conductance is enhanced in a very energy-sensitive way. In this paper, we discuss the spectral conductance of a proximity superconductor from both the theoretical and experimental point of view. The dependence of the spectral conductance on the phase-breaking length, gap of the superconductor and interface transparency is theoretically investigated. We present experimental data on the re-entrance of the normal-state conductance at very low temperature and bias voltage. A complete description of the experimental data needs taking into account heating of the reservoirs by the bias current. In addition, we show that the energy sensitivity of the proximity effect enables one to access the energy distribution of the conduction electrons inside a mesoscopic sample.
We study heat transport in hybrid normal metal - superconductor - normal metal (NSN) structures. We find the thermal conductance of a short superconducting wire to be strongly enhanced beyond the BCS value due to inverse proximity effect. The measurements agree with a model based on the quasiclassical theory of superconductivity in the diffusive limit. We determine a crossover temperature below which quasiparticle heat conduction dominates over the electron-phonon relaxation.
We review transport measurements in a normal metal (N) in contact with one or two superconducting (S) islands. From the experiment, we distinguish the Josephson coupling, the mesoscopic fluctuations and the proximity effect. In a loop-shaped N conductor, we observe large h/2e-periodic magnetoresistance oscillations that decay with temperature T with a 1/T power-law. This behaviour is the signature of the long-range coherence of the low-energy electron pairs induced by the Andreev reflection at the S interface. At temperature and voltage below the Thouless energy $hbar D / L^2$, we observe the re-entrance of the metallic resistance. Experimental results agree with the linearized quasiclassical theory.
We investigated the local electronic density of states in superconductor-normal metal (Nb-Au) bilayers using a very low temperature (60 mK) STM. High resolution tunneling spectra measured on the normal metal (Au) surface show a clear proximity effect with an energy gap of reduced amplitude compared to the bulk superconductor (Nb) gap. Within this mini-gap, the density of states does not reach zero and shows clear sub-gap features. We show that the experimental spectra cannot be described with the well-established Usadel equations from the quasi-classical theory.
The superconducting proximity effect has played an important role in recent work searching for Majorana modes in thin semiconductor devices. Using transport measurements to quantify the changes in the semiconductor caused by the proximity effect provides a measure of dynamical processes such as screening and scattering. However, in a two terminal measurement the resistance due to the interface conductance is in series with resistance of transport in the semiconductor. Both of these change, and it is impossible to separate them without more information. We have devised a new three terminal device that provides two resistance measurements that are sufficient to extract both the junction conductance and the two dimensional sheet resistance under the superconducting contact. We have compared junctions between Nb and InAs and Nb and 30% InGaAs all grown before being removed from the ultra high vacuum molecular beam epitaxy growth system. The most transparent junctions are to InAs, where the transmission coefficient per Landauer mode is greater than 0.6. Contacts made with ex-situ deposition are substantially more opaque. We find that for the most transparent junctions, the largest fractional change as the temperature is lowered is to the resistance of the semiconductor.
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