85 research outputs found

    Multiple Andreev reflections in diffusive SNS structures

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    We report new measurements on sup-gap energy structure originating from multiple Andreev reflections in mesoscopic SNS junctions. The junctions were fabricated in a planar geometry with high transparency superconducting contacts of Al deposited on highly diffusive and surface d-doped n++-GaAs. For samples with a normal GaAs region of active length 0.3um the Josephson effect with a maximal supercurrent Ic=3mA at T=237mK was observed. The sub-gap structure was observed as a series of local minima in the differential resistance at dc bias voltages V=2D/ne with n=1,2,4 i.e. only the even sub-gap positions. While at V=2D/e (n=1) only one dip is observed, the n=2, and the n=4 sub-gap structures each consists of two separate dips in the differential resistance. The mutual spacing of these two dips is independent of temperature, and the mutual spacing of the n=4 dips is half of the spacing of the n=2 dips. The voltage bias positions of the sub-gap differential resistance minima coincide with the maxima in the oscillation amplitude when a magnetic field is applied in an interferometer configuration, where one of the superconducting electrodes has been replaced by a flux sensitive open loop.Comment: 20 pages, 7 figure

    Observation of supercurrent enhancement in SNS junctions by non-equilibrium injection into supercurrent carrying bound Andreev states

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    We report for the first time enhancement of the supercurrent by means of injection in a mesoscopic three terminal planar SNSNS device made of Al on GaAs. When a current is injected from one of the superconducting Al electrodes at an injection bias V=Δ(T)/eV=\Delta(T)/e, the DC Josephson current between the other two superconducting electrodes has a maximum, giving evidence for an enhancement due to a non-equilibrium injection into bound Andreev states of the underlying semiconductor. The effect persists to temperatures where the equilibrium supercurrent has vanished.Comment: 7 pages + 3 figures. Resubmitted to Phys. Rev. Lett. Contents change

    Subgap anomaly and above-energy-gap structure in chains of diffusive SNS junctions

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    We present the results of low-temperature transport measurements on chains of superconductor--normal-constriction--superconductor (SNS) junctions fabricated on the basis of superconducting PtSi film. A comparative study of the properties of the chains, consisting of 3 and 20 SNS junctions in series, and single SNS junctions reveals essential distinctions in the behavior of the current-voltage characteristics of the systems: (i) the gradual decrease of the effective suppression voltage for the excess conductivity observed at zero bias as the quantity of the SNS junctions increases, (ii) a rich fine structure on the dependences dV/dI-V at dc bias voltages higher than the superconducting gap and corresponding to some multiples of 2\Delta/e. A model to explain this above-energy-gap structure based on energy relaxation of electron via Cooper-pair-breaking in superconducting island connecting normal metal electrods is proposed.Comment: RevTex, 5 pages, 4 figure

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

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    Low temperature transport measurements on superconducting film - normal metal wire - superconducting film (SNS) junctions fabricated on the basis of 6 nm thick superconducting polycrystalline PtSi films are reported. The structures with the normal metal wires of two different lengths L=1.5 μ\mum and L=6μ\mum and the same widths W=0.3μ\mum are studied. Zero bias resistance dip related to pair current proximity effect is observed for all junctions whereas the subharmonic energy gap structure originating from phase coherent multiple Andreev reflections have occurs only in the SNS junctions with short wires.Comment: ReVTex, 4 pages, 4 eps figures include

    DC current through a superconducting two-barrier system

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    We analyze the influence of the structure within a SNS junction on the multiple Andreev resonances in the subgap I-V characteristics. Coherent interference processes and incoherent propagation in the normal region are considered. The detailed geometry of the normal region where the voltage drops in superconducting contacts can lead to observable effects in the conductance at low voltages.Comment: 11 pages, including 7 postscript file

    Proximity Effect and Multiple Andreev Reflections in Chaotic Josephson junctions

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    We study the dc-current transport in a voltage biased superconductor-chaotic dot-superconductor junction with an induced proximity effect(PE) in the dot. It is found that for a Thouless energy EThE_{Th} of the dot smaller than the superconducting energy gap Δ\Delta, the PE is manifested as peaks in the differential conductance at voltages of order EThE_{Th} away from the even subharmonic gap structures eV2(Δ±ETh)/2neV \approx 2(\Delta\pm E_{Th})/2n. These peaks are insensitive to temperatures kTΔkT \ll \Delta but are suppressed by a weak magnetic field. The current for suppressed PE is independent of EThE_{Th} and magnetic field and is shown to be given by the Octavio-Tinkham-Blonder-Klapwijk theory.Comment: 4 pages, 3 figure

    Direct demonstration of circulating currents in a controllable π\pi-SQUID generated by a 0 to π\pi transition of the weak links

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    A controllable π\pi-SQUID is a DC SQUID with two controllable π\pi-junctions as weak links. A controllable π\pi-junction consists of a superconducting - normal metal - superconducting Josephson junction with two additional contacts to the normal region of the junction. By applying a voltage VcV_c over these contacts it is possible to control the sate of the junction, i.e. a conventional (0) state or a π\pi-state, depending on the magnitude of VcV_c. We demonstrate experimentally that, by putting one junction into a π\pi-state, a screening current is generated around the SQUID loop at integer external flux. To be able to do this, we have fabricated controllable π\pi-junctions, based on Cu-Nb or Ag-Nb, in a new geometry. We show that at 1.4 K only the Nb-Ag device shows the transition to a π\pi-state as a function of VcV_c consistent with theoretical predictions. In a controllable π\pi SQUID based on Nb-Ag we observe, a part from a screening current at integer external flux, a phase shift of π\pi of the VSQUIDBV_{SQUID}-B oscillations under suitable current bias, depending on the magnitude of VcV_c.Comment: 11 pages, 12 figures, subm. to Phys. Rev.

    Two-dimensional array of diffusive SNS junctions with high-transparent interfaces

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    We report the first comparative study of the properties of two-dimensional arrays and single superconducting film - normal wire - superconducting film (SNS) junctions. The NS interfaces of our SNS junctions are really high transparent, for superconducting and normal metal parts are made from the same material (superconducting polycrystalline PtSi film). We have found that the two-dimensional arrays reveal some novel features: (i) the significant narrowing of the zero bias anomaly (ZBA) in comparison with single SNS junctions, (ii) the appearance of subharmonic energy gap structure (SGS), with up to n=16 (eV=\pm 2\Delta/n), with some numbers being lost, (iii) the transition from 2D logarithmic weak localization behavior to metallic one. Our experiments show that coherent phenomena governed by the Andreev reflection are not only maintained over the macroscopic scale but manifest novel pronounced effects as well. The behavior of the ZBA and SGS in 2D array of SNS junctions strongly suggests that the development of a novel theoretical approach is needed which would self-consistently take into account the distribution of the currents, the potentials, and the superconducting order parameter.Comment: RevTex, 5 pages, 5 figure
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