Mesoscopic proximity effect in double-barrier superconductor/normal-metal junctions (original) (raw)
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The oxidation state at tunnel junction interfaces
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An experimental and theoretical study is presented of coherent effects in electron transport in the doublebarrier SINIS junctions ͑where S, I, and N denote a superconductor, insulator, and normal metal, respectively͒. The appearance of a steplike subgap structure in the current-voltage characteristics of the Nb/Al/AlO x /Al/AlO x /Al/Nb superconducting junctions at a voltage Vϳ⌬ Nb /e ͑where ⌬ Nb is the superconducting energy gap of Nb͒ is interpreted as a manifestation of a nonequilibrium supercurrent at finite dc bias voltage ͑Finite-Bias Josephson Effect͒. The origin of this effect lies in the energy-band structure associated with a set of macroscopic quantum states characteristic of a SINIS junction. Specifically, the junction can have an energy level near energy ⌬ Nb , which provides an additional channel for dc Josephson current at V ϳ⌬ Nb /e. In addition, sharp features in the conductivity at a voltage near the gap-sum voltage were observed in both SINIS and SINININIS junctions, implying correlated quasiparticle tunneling in multiple-barrier junctions. Our theoretical model provides a good qualitative description of the quasiparticle conductivity, including narrow peaks at finite voltage and a zero-voltage anomaly observed on some samples, and suggests an alternative explanation of a feature interpreted earlier as gap-difference feature associated with the tunneling extraction of quasiparticles from the middle Al layer.
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Eprint Arxiv 1402 6055, 2014
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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New behavior of the conductance of diffusive NS junctions near T_c
We observe a maximum in the conductance of Al͞n-GaAs junctions at temperatures 20 mK lower than the superconducting transition temperature ͑T c ͒. This is the first observation of a peak in the conductance near the superconducting transition in superconducting-normal ͑S͞N͒ junctions. To accommodate this effect we calculate the full temperature dependence of the conductance of these structures, invoking quasiclassical Green's functions in the diffusive limit. In addition to the well-known low-temperature peak at temperatures on the order of the Thouless energy, we find a maximum near T c . This peak has the same origin as the subgap conductance observed in S͞N junctions at low temperatures. PACS numbers: 74.80.Fp, 73.40.Ns The study of transport properties of mesoscopic normal superconducting ͑N͞S͒ structures has revealed a number of novel features [1,2], including long-range, phase-coherent transport, disorder-enhanced subgap conductance , and a nonmonotononic dependence of the conductance on temperature ͑T ͒ and voltage ͑V ͒. Such a nonmonotonic dependence was first predicted in [4] for a short point N͞S contact of length L satisfying L 2 øhD͞D͑0͒, where D is the diffusion coefficient and D͑0͒ is the energy gap of the superconductor at zero temperature. In Ref. it was shown that G, as a function of T or V , increases from G n (G n is the conductance in the normal state) at T 0 and V 0, reaches a maximum at T or eVm of order D͑T ͒, and then decreases to G n as T or V are increased further. In the opposite limit,