Subharmonic Energy Gap Structure Research Articles

Andreev reflections and magnetotransport in 2D Josephson junctions

In this paper, the subharmonic energy gap structures (SGS) and induced superconductivity, due to multiple Andreev reflections, in indium gallium arsenide (In0.75Ga0.25As) two-dimensional electron gas (2DEG) are discussed at different temperatures and magnetic fields. Strong suppression of both SGS and induced gap are observed as a function of temperature and magnetic field. The differential conductance of the Josephson junctions (JJs) as a function of external in-plane magnetic fields shows an asymmetric response to positive and negative magnetic field sweeps with a conductance maximum at close to zero. Our approach to quantum transport studies of the ballistic 2D JJs may open a new road towards scalable and integrated quantum processing and help pave the way for the development of topological circuits for the realization of the next generation of quantum processors.

Supercurrent and Multiple Andreev Reflections in InSb Nanosheet SNS Junctions

In this study, the realization of mesoscopic Josephson junctions based on free‐standing InSb nanosheet grown by molecular‐beam epitaxy is reported. Below the critical temperature of superconducting aluminium electrodes (≈1.1 K), the high transparency of the contacts gives rise to proximity‐induced superconductivity. A dissipationless supercurrent which can be modulated by a gate voltage acting on the electron density in the nanosheet flows through the superconducting weak links. At finite bias voltage, subharmonic energy‐gap structures (SGS) originating from multiple Andreev reflections (MARs) are observed, indicating a highly transparent InSb nanosheet–superconductor interface. At last, a superconducting hybrid device with niobium electrodes is shown, suitable for further higher temperature and magnetic field transport measurements.

Proximity induced superconductivity in indium gallium arsenide quantum wells

We report on the experimental observation of the proximity induced superconductivity in an indium gallium arsenide (In0.75Ga0.25As) quantum well. The Josephson junction was fabricated by several photo-lithographic processes on an InGaAs heterojunction and Niobium (Nb) was used as superconducting electrodes. Owing to the Andreev reflections and Andreev bound states at the Nb-In0.75Ga0.25As quantum well-Nb interfaces, the subharmonic energy gap structures (SGS) are observed at the differential conductance (dI/dV) versus voltage (V) plots when the applied source-drain bias voltages satisfy the expression VSD = 2Δ/ne. The dI/dV as a function of applied magnetic field B shows a maximum at zero B which decreases by increasing B. When decreasing B to below ±0.4 T, a hysteresis and shift of the conductance maxima close to B = 0 T are observed. Our results help to pave the way to the development of integrated coherent quantum circuitry.

Anomalous behavior near T c and synchronization of Andreev reflection in two-dimensional arrays of SNS junctions

We have investigated low-temperature transport properties of two-dimensional arrays of superconductor--normal-metal--superconductor (SNS) junctions. It has been found that in two-dimensional arrays of SNS junctions (i) a change in the energy spectrum within an interval of the order of the Thouless energy is observed even when the thermal broadening far exceeds the Thouless energy for a single SNS junction; (ii) the manifestation of the subharmonic energy gap structure (SGS) with high harmonic numbers is possible even if the energy relaxation length is smaller than that required for the realization of a multiple Andreev reflection in a single SNS junction. These results point to the synchronization of a great number of SNS junctions. A mechanism of the SGS origin in two-dimensional arrays of SNS junctions, involving the processes of conventional and crossed Andreev reflection, is proposed.

Diffusive single and multiply connectedSNS systems with high-transparentinterfaces

We present low-temperature transport measurementson single and multiply connected SNS systems fabricated on thebasis of superconducting polycrystalline PtSi film. Zero biasanomaly (ZBA) and subharmonic energy gap structure (SGS)originating from the proximity effects and the multiple Andreevreflections were observed and carefully studied. It was foundthat the ZBA is not described by any theories developed forSNS junctions with high-transparent NS interface. The value ofexcess conductance and the width of ZBA for single SNSjunction far exceed values predicted by up-to-date theories.The NS interfaces of our SNS junctions are really high-transparent, for superconducting and normal metal parts aremade of the same material. On the other hand, as we have todeal with long diffusive SNS junctions one might expect thatimpurities could provide the conditions for incoherent multipleAndreev reflections.In comparison with single SNS junctions we observe significant narrowing of the ZBA in multiply connected SNS systems:one-dimensional (1D) and two-dimensional arrays (2D) of SNSjunctions. In 2D arrays an appreciable SGS appears, with up to n = 16(eV = ±2Δ/n) and some numbers being lost. One of themost interesting results obtained on 1D arrays consists in theappearance of symmetrical dips on the dependences dV/dI - Vat dc voltage corresponding to some multiples of Δ/e. Presentedresults show that coherent phenomena governed by Andreevreflection are not self-averaging and are maintained over amacroscopic scale.

Subharmonic energy-gap structures and 1/ f noise properties in Nb/Al+AlO x/Nb tunnel junctions

We have investigated subharmonic energy-gap structures (SGSs) and 1/ f noise spectra in Nb/Al+AlO x /Nb tunnel junctions as a function of bias voltage. The clear SGSs were observed in d V/d I vs. V curves at 4.2 K. The quasiparticle characteristic was explained by the Octavio, Tinkham, Blonder and Klapwijk theory based on multiple-Andreev reflection model. To estimate the 1/ f noise parameter α/ N c in the Nb/Al+AlO x /Nb junctions, we used Hooge's empirical theory. The α/ N c of these junctions biased above superconducting energy-gap did not depend on bias voltage, while below the gap voltage it was larger than that above the gap voltage. Moreover, some peak structures related to the SGSs were observed in α/ N c vs. V characteristics biased below the gap voltage. This enhanced 1/ f noise might be caused by MAR current.

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

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.

Mesoscopic SNS junctions on the basis of superconducting PtSi films

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 and 6 μm and the same widths W=0.3 μm are studied. The zero bias resistance dip is observed for all junctions whereas the subharmonic energy gap structure (SGS) originating from multiple Andreev reflections occurs only in the SNS junctions with short wires.

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

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 $\mu$m and L=6$\mu$m and the same widths W=0.3$\mu$m 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.

Photon-assisted Andreev transport and sub-gap structures

We report new measurements of microwave-induced perturbations of the sub-harmonic energy gap structures in the current–voltage characteristics of superconductor–semiconductor-superconductor junctions. Around the sub-gap bias voltages associated with the enhanced quasi-particle transfer mediated by multiple Andreev reflection processes we observe microwave induced satellites, shifted in voltage by multiples of hf/ en, where hf is the photon energy and n is the number of quasi-particle traversals as determined by the Andreev processes. The observed behavior is the analogue of the so-called photon-assisted tunneling but here associated with the multiple Andreev reflections.

Superconductor-semiconductor-superconductor planar junctions of aluminium on δ-doped gallium-arsenide

We have fabricated and characterized planar superconductor-semiconductor-superconductor (S-Sm-S) junctions with a high quality (i.e. low barrier) interface between an n/sup ++/ modulation doped conduction layer in MBE grown GaAs and in situ deposited Al electrodes. The Schottky barrier at the S-Sm interface was compensated by inserting several Si /spl delta/-doped layers above the conduction layer and close to the surface of the GaAs heterostructure. Below 1.2 K, the transition temperature of Al, the dc I-V curves of such S-Sm-S junctions with a wide and short GaAs channel exhibited the classic features of S-N-S junctions including subharmonic energy gap structure (SGS) and excess current (EC) due to Andreev reflections at the interfaces.

Evidence for a proximity-induced energy gap in Nb/InAs/Nb junctions

We investigate the subharmonic energy gap structure (SGS) in coplanar Nb/p-type InAs(2DEG)/Nb junctions. The junctions are produced by employing a technique which enables us to achieve distances between the superconducting electrodes considerably below 100 nm. In the coplanar geometry of our junctions, a smearing of the SGS is expected when a voltage drop along the Nb/InAs interfaces is present. This behavior is indeed observed in junctions with interfaces of minor quality. In junctions with high-quality interfaces, the SGS is well developed despite the coplanar geometry. We interpret the absence of smearing, together with a temperature-dependent shift of the SGS, in terms of a recent theory by Aminov et al. Within this theory, the influence of the proximity effect on the InAs inversion layer under the Nb electrodes is taken into account, leading to the opening of a gap in the density of states. Because of the narrow inversion layer and the high interface transparency, the gap is comparable in magnitude to the superconducting energy gap \ensuremath{\Delta} of the Nb. For our junction, we find from the shift of the SGS a gap of 0.81\ensuremath{\Delta} at temperature T=0 induced in the inversion layer.

Decay Lengths for Diffusive Transport Activated by Andreev Reflections in Al/n-GaAs/Al Superconductor-Semiconductor-Superconductor Junctions

In a highly doped GaAs semiconductor with superconducting contacts of Al, clear conductance peaks are observed at zero voltage bias and at $V\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ifmmode\pm\else\textpm\fi{}2\ensuremath{\Delta}/e,$ $\ifmmode\pm\else\textpm\fi{}\ensuremath{\Delta}/e$. The subharmonic energy gap structure originates from Andreev scattering with diffusive, but energy conserving, transport in the GaAs. The zero bias excess conductance is due to phase-coherent transport. Both effects are suppressed when the distance between the superconducting electrodes exceeds the inelastic diffusion length in the GaAs normal channel.

Multiple Andreev-reflection in semiconductor-coupled superconducting weak-links

We present investigations of the carrier transport through a highly doped semiconducting(Sm) n-InSb-film sandwiched between two superconducting(S) electrodes. We measured the I(V)- and dV dI ( V) -characteristics in dependence on the temperature and the influence of an external rf-field. These characteristics show a subharmonic energy gap structure(SGS) and an excess- or a deficit-current, indicating the occurrence of multiple Andreev-reflection(AR). The theoretical background of AR is given in terms of the OTBK-model. We proposed and calculated some modifications of this theory.

Andreev-reflection in highly doped Pb/n?InSb/Pb junctions

We studied the carrier transport through n-InSb-films sandwiched between two superconducting lead electrodes. These n-InSb-films were about 30 nm thick, polycrystalline and very highly doped (1021 cm−3). TheV(I)- and dV/dI (V)-characteristics indicated all the properties of Andreev-reflection (subharmonic energygap structures (SGS) and excess current) according to the theory of Octavio et al. (OTBK-model) [1, 2]. Our measurements show the temperature dependence of these structures and also the influence of magnetic fields up to 80 mT and give a further reference to the importance of Andreev-reflection in semiconductor-coupled superconducting junctions (SSmS junctions).

Transport properties in an InAs-inserted-channel In0.52Al0.48As/In0.53Ga0.47As heterostructure coupled superconducting junction.

Supercurrent is observed in a superconductor-semiconductor-superconductor (S-Sm-S) junction using a two-dimensional-electron-gas system in an InAs-inserted-channel ${\mathrm{In}}_{0.52}$${\mathrm{Al}}_{0.48}$As/${\mathrm{In}}_{0.53}$${\mathrm{Ga}}_{0.47}$As heterostructure. The temperature dependence of the supercurrent cannot be explained by a simple exponential expression, but is here explained in terms of the clean-limit theory, taking account of the higher-order effect of the coherence length. The differential resistance of the junction shows clear subharmonic energy-gap structures due to multiple Andreev reflection since the phase coherence is maintained during the Andreev-reflection process. The Cooper pair transport explained by the clean-limit theory is attributed to the fact that the mean free path is longer than the coherence length and the length between the superconducting electrodes.

Nonstationary properties and subharmonic energy-gap structure of resistivity state in superconducting films

The authors have studied a subharmonic energy-gap structure in the dependence dV/ dI(V) in thin tin films with phase slip centers, and its relationship to non-Josephson generation. Some singularities have been revealed that are due to an additional conduction channel for quasi-particles with the energies of the order of magnitude of the gap. The influence external high frequency radiation on Δ and the generation effect have been investigated.

Subharmonic energy-gap structure and heating effects in superconducting niobium point contacts.

We present experimental data of the temperature-dependent subharmonic energy-gap structure (SGS) in the current-voltage ({ital I}-{ital V}) curves of superconducting niobium point contacts. The observed SGS is modified by heating effects. We construct a model of the quasiparticle conductance of metallic superconducting weak links that includes the heating effects self-consistently. Our model is combined with that of Octavio, Blonder, Klapwijk, and Tinkham (Phys. Rev. B 27, 6739 (1983)), which is based on the idea of multiple Andreev scattering in the contact. The shape and the temperature variation of the calculated SGS is found to be in good agreement with the experimental curves for contacts with resistance larger than 5 {Omega}.

Subharmonic energy-gap structure in superconducting weak links.

We present corrected calculations of the subharmonic energy-gap structure using the model of Octavio, Tinkham, Blonder, and Klapwijk, which includes the effect of normal scattering in the weak link. We show that while the overall predictions of this model do not change qualitatively, the details of the predicted curves are different and in better agreement with experiment. We also present calculation of the current-voltage characteristics and of the excess currents for T = 0, as the normal scattering parameter Z is varied. We also show how the calculation can be shortened using symmetry arguments and discuss how this model improves the understanding of the connection between weak links and tunnel junctions.

Andreev Reflections and Current Voltage Characteristics of SNS-Junctions

The Andreev reflected, accelerated electron-hole wave packet solutions of the time-dependent Bogoliubov-de Gennes equations for a three-dimensional, phase-coherent superconductor-normal metal-superconductor junction in a uniform electric field are used in order to calculate the total time-averaged current density of the system at any temperature below Tc. Scattering from impurities and phonons is taken into account in the relaxation time approximation. The theory yields the excess current experimentally observed in weak links at “high” voltages eV≫Δ (Δ=pair potential). At low voltages eV≪Δ the current voltage characteristic corresponds to the so-called “foot” of variable-thickness microconstrictions. In the intermediate voltage range eV≃Δ subharmonic energy gap structures are obtained.