Superconductor-normal Metal-superconductor Junctions Research Articles

Statistics of avalanches in the self-organized criticality state of a Josephson junction

Magnetic flux avalanches in Josephson junctions that include superconductor-insulator-superconductor (SIS) tunnel junctions and are magnetized at temperatures lower than approximately 5 K have been studied in detail. Avalanches are of stochastic character and appear when the magnetic field penetration depth λ into a junction becomes equal to the length a of the Josephson junction with a decrease in the temperature. The statistical properties of such avalanches are presented. The size distribution of the avalanches is a power law with a negative noninteger exponent about unity, indicating the self-organized criticality state. The self-organized criticality state is not observed in Josephson junctions with a superconductor-normal metal-superconductor (SNS) junction.

Theory of Microwave-Assisted Supercurrent in Diffusive SNS Junctions

The observation of very large microwave-enhanced critical currents in superconductor-normal-metal-superconductor (SNS) junctions at temperatures well below the critical temperature of the electrodes has remained without a satisfactory theoretical explanation for more than three decades. Here we present a theory of the supercurrent in diffusive SNS junctions under microwave irradiation based on the quasiclassical Green's function formalism. We show that the enhancement of the critical current is due to the energy redistribution of the quasiparticles in the normal wire induced by the electromagnetic field. The theory provides predictions across a wide range of temperatures, frequencies, and radiation powers, both for the critical current and the current-phase relationship.

Utilization of a cryo-prober system for operation of a pulse-driven josephson junction array

We demonstrated the operation of pulse-driven Josephson junction arrays (JJAs) for AC voltage standard using a wideband cryo-prober system with a 4-K Gifford-MacMahon (GM) cooler. This unique system was originally developed for high-speed network switch applications of rapid-single-flux-quantum (RSFQ) circuits and enables wideband data transmission at bit rates of higher than 10 Gbps between room-temperature and cryogenic environments. JJA chips were fabricated using NbN-based superconductor-normal metal-superconductor (SNS) junctions. A 5-mm chip was mounted on a 16-mm chip carrier using flip-chip bonding technology for probe contact. To obtain bipolar output voltages, we tried two types of testing based on the AC coupling technique proposed by the National Institute of Standards and Technology (NIST). A pulse pattern generator (PPG) with a large memory of 134 Mbit was used for covering a wide frequency range of output signals. As a result, we succeeded in bipolar operation of the JJA, generating waveforms at frequencies from 60 Hz to several tens of kilo hertz. The maximum rms voltage obtained for a single array was 12.7 mV. The observed spurious level was lower than -93 dBc at 16 kHz.

Resonant Andreev transmission in two-dimensional array of SNS junctions

We present an experimental study of transport properties of a large two-dimensional array of superconductor–normal-metal–superconductor (SNS) junctions comprised of the nanopatterned superconducting film, ensuring that NS interfaces of our SNS junctions are highly transparent. We find the anomalously high charge transmission at certain applied voltages commensurate with the magnitude of the gap in superconducting islands. This indicates the nonlocal nature of the charge transfer in multiply connected SNS systems. We propose the mechanism of the correlated transmission of Cooper pairs in large arrays of SNS junctions based on the combined action of the proximity effect and the simultaneous Andreev conversion processes at many NS-interfaces.

Influence of interface transmissivity and inelastic scattering on the electronic entropy and specific heat of diffusive superconductor-normal metal-superconductor Josephson junctions

We study theoretically the electronic entropy and specific heat in diffusive superconductor-normal metal-superconductor Josephson junctions. In particular, we consider the influence of nonidealities occurring in an actual experiment, such as the presence of barriers at the normal (N) metal-superconductor interfaces, the spin-flip, and inelastic scattering in the N-metal region and quasiparticle subgap states in the superconductors. We find that spin-flip and inelastic scattering do not have, for typical parameters values, a large effect. On the contrary, the presence of barriers suppresses the superconducting correlations in the N region, with the consequence that the entropy and the specific heat get reduced eventually to those in the absence of superconductivity for opaque interfaces. Finally we suggest an experiment and check that it is possible, under realistic conditions, to measure the dependence of electronic specific heat on the phase difference between the superconductors.

Pairing Symmetry Conversion by Spin-Active Interfaces in Magnetic Normal-Metal–Superconductor Junctions

We study the proximity-induced superconducting correlations in a normal metal connected to a superconductor when the interface between them is spin active and the normal metal is ballistic or diffusive. Remarkably, for any interface spin polarization there is a critical interface resistance, above which the conventional even-frequency proximity component vanishes completely at the chemical potential, while the odd-frequency component remains finite. We propose a way to unambiguously observe the odd-frequency component.

Optical imaging of the transport properties of S-Sm-S junctions

We study the optical effects on superconductor-normal metal superconductor (S-Sm-S) junctions composed of two-dimensional electron gas (2DEG) in a GaAs/AlGaAs heterostructure and NbN superconducting electrodes. When the whole junction area was illuminated at λ = 800 nm, we observe a reduction in the normal resistance due to an increase in the sheet carrier density of the 2DEG, and the enhancement of the Andreev reflection probability. To reveal its origin, we performed scanning photo-voltage measurement by employing an optical microscope. The obtained image plots show maxima and minima of the photo-voltage change along the S-Sm interfaces. Those structures are considered to reflect the modulation of the barrier height at S-Sm interface and the increase in the scattering by photo-generated carriers. It is demonstrated that the scanning photo-voltage measurement is one of the most powerful tools as a local probe of the transport properties of S-Sm-S junctions.

Supercurrent transferring throughc-axis cuprate Josephson junctions with thick normal-metal bridge

With a simple but exactly solvable model, we investigate the supercurrent transferring through thec-axis cuprate superconductor–normal metal–superconductor junctions with theclean normal metal much thicker than its coherence length. It is shown thatthe supercurrent as a function of thickness of the normal metal decreasesmuch slower than the exponential decaying expected by the proximity effect.The present result may account for the giant proximity effect observed in thec-axis cuprate SNS junctions.

The Vortex State and Josephson Critical Current of a Diffusive SNS Junction

We study theoretically the electronic and transport properties of a diffusive superconductor-normal metal-superconductor (SNS) junction in the presence of a perpendicular magnetic field. We show that the field dependence of the critical current crosses over from the well-known Fraunhofer pattern in wide junctions to a monotonic decay when the width of the normal wire is smaller than the magnetic length \(\xi_{\mathrm{H}}=\sqrt{\varPhi_{0}/H}\) , where H is the magnetic field and Φ0 the flux quantum. We demonstrate that this behavior is intimately related to the appearance of a linear array of vortices in the middle of the normal wire, the properties of which are very similar to those in the mixed state of a type II superconductor. This novel vortex structure is also manifested in a strong modulation of the local density of states along the transversal dimension, which can be measured with existing experimental techniques.

Influence of photon-assisted tunneling on heat flow in a normal metal–superconductor tunnel junction

We have investigated theoretically the influence of an ac drive on heat transport in a hybrid normal metal-superconductor tunnel junction in the photon-assisted tunneling regime. We find that the useful heat flux out from the normal metal is always reduced as compared to its magnitude under the static and quasistatic drive conditions. Our results are useful to predict the operative conditions of ac-driven superconducting electron refrigerators.

Performance of a Cryocooled Nb DC Programmable Voltage Standard at 4 K

We report on the performance of a 1-V dc programmable Josephson voltage standard (PJVS) operating on a commercially available Gifford-McMahon cryocooler at 4 K. The NIST-fabricated niobium superconductor-normal metal-superconductor junction chip has been shown to operate successfully as integrated with this cryopackage, generating 1.229 V at the maximum operating frequency of 19.364 GHz with Shapiro steps at constant voltage that are flat to within less than 100 nV. Because of the large operating margin (greater than 1 mA), these steps were highly stable and immune from noise. Differences in behavior of the PJVS in liquid helium and on the cryocooler are discussed. This demonstration paves the way towards automated warming and cooling of the Josephson device and a fully automated voltage standard system.

Magnetic Interference Patterns and Vortices in Diffusive SNS Junctions

We study theoretically the electronic and transport properties of a diffusive superconductor-normal metal-superconductor junction in the presence of a perpendicular magnetic field. We show that the field dependence of the critical current crosses over from the well-known Fraunhofer pattern in wide junctions to a monotonic decay when the width of the normal wire is smaller than the magnetic length xi(H)=square root Phi(0)/H, where H is the magnetic field and Phi(0) the flux quantum. We demonstrate that this behavior is a direct consequence of the magnetic vortex structure appearing in the normal region and predict how this structure is manifested in the local density of states.

Josephson current in a superconductor-ferromagnet-superconductor junction with in-plane ferromagnetic domains

We study a diffusive superconductor-ferromagnet-superconductor (SFS) junction with in-plane ferromagnetic domains. Close to the superconducting transition temperature, we describe the proximity effect in the junction with the linearized Usadel equations [Phys. Rev. Lett. 25, 507 (1970)]. We find that properties of such a junction depend on the size of the domains relative to the magnetic coherence length. In the case of large domains, the junction exhibits transitions to the pi state, similar to a single-domain SFS junction. In the case of small domains, the magnetization effectively averages out, and the junction is always in the zero state, similar to a superconductor-normal metal-superconductor junction. In both those regimes, the influence of domain walls may be approximately described as an effective spin-flip scattering. We also study the inhomogeneous distribution of the local current density in the junction. Close to the 0-pi transitions, the directions of the critical current may be opposite in the vicinity of the domain wall and in the middle of the domains.

Divergence at Low Bias and Down-Mixing of the Current Noise in a Diffusive Superconductor–Normal-Metal–Superconductor Junction

We present current noise measurements in a long diffusive superconductor-normal-metal-superconductor junction in the low voltage regime, in which transport can be partially described in terms of coherent multiple Andreev reflections. We show that, when decreasing voltage, the current noise exhibits a strong divergence together with a broad peak. We ascribe this peak to the mixing between the ac-Josephson current and the noise of the junction itself. We show that the junction noise corresponds to the thermal noise of a nonlinear resistor 4k B T/R with R=V/I(V) and no adjustable parameters.

Density of states and supercurrent in diffusive SNS junctions: Roles of nonideal interfaces and spin-flip scattering

We present a theoretical study of the density of states and supercurrent in diffusive superconductor-normal metal-superconductor (SNS) junctions. In particular, we study the influence on these two equilibrium properties of both an arbitrary transparency of the SN interfaces and the presence of spin-flip scattering in the normal wire. We show that the minigap that is present in the spectrum of the diffusive wire is very sensitive to the interface transmission. More importantly, we show that at arbitrary transparency the minigap replaces the Thouless energy as the relevant energy scale for the proximity effect, determining, for instance, the temperature dependence of the critical current. We also study in detail how the critical current is suppressed by the effect of spin-flip scattering, which can be due either to magnetic impurities or, under certain circumstances, to an external magnetic field. Our analysis based on the quasiclassical theory of diffusive superconductors can be very valuable in establishing quantitative comparisons between experiment and theory.

A Whole-Head SQUID System in a Superconducting Magnetic Shield

Sensitivity of a SQUID system in a superconducting magnetic shield is studied. The SQUID sensors are made of SNS(superconductor/normal metal/superconductor) junctions. Sensitivities of the SQUID system of SNS junctions in a superconducting magnetic shield are more than 100 times better than those of the same SQUID system in a magnetically shielded room of permalloy. The best sensitivity of the SQUID system of SNS junctions is 0.9 fT/(Hz) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1/2</sup> at 100 Hz in a superconducting magnetic shield. We performed a experiment of somatosensory evoked magnetic held of human brains to study the N20 component at 20 ms after stimulation. We estimated locations in the brain of the evoked current dipoles for N20 by a extension of a radar theory named MUSIC(multiple signal classification approach). The locations of the evoked current dipole for N20 estimated by MUSIC is confirmed exactly the same as the locations of the current dipole estimated by the traditional iterative least-squares fitting. This is the best agreement between the MUSIC estimation and the conventional estimation of current dipoles to our knowledge.

Design of SNS Josephson Arrays for High Voltage Applications

The voltage from a single, microwave-biased Josephson junction is a small quantity; thus useful voltages are generated only through series arrays of many thousands of junctions. Arrays of superconductor-normal metal-superconductor junctions have been fabricated and tested with as many as 16,500 junctions per array. The arrays are optimized for the highest voltage operation with the largest operating margins for the current bias. Measurements show that these arrays, driven with 20 GHz microwaves, generate a dc voltage greater than 680 mV per array with a dc bias margin over 1 mA. To increase the microwave uniformity across the array, the transmission line impedance has been tapered. By use of this technique, ac Josephson voltages over 110 mVrms per array have been generated, also with over 1 mA dc bias margin.

Voltage-induced Shapiro steps in a superconducting multiterminal structure

When a superconducting tunnel junction at a finite voltage is irradiated with microwaves, the interplay between the alternating Josephson current and the ac field gives rise to steps in the dc current known as Shapiro steps. In this work we predict that in a mesoscopic structure connected to several superconducting terminals one can induce Shapiro-like steps in the absence of any external radiation simply by tuning the voltages of the leads. To illustrate this effect we make quantitative predictions for a three-terminal structure which comprises a diffusive superconductor-normal metal-superconductor junction and a tunneling probe, a set-up which can be realized experimentally.

Influence of Charge and Energy Imbalances on the Tunneling Current through a Superconductor–Normal Metal Junction

We consider quasiparticle charge and energy imbalances in a thin superconductor weakly coupled with two normal-metal electrodes via tunnel junctions at low temperatures. Charge and energy imbalances, which can be created by injecting quasiparticles at one junction, induce excess tunneling current $I_{\rm ex}$ at the other junction. We numerically obtain $I_{\rm ex}$ as a function of the bias voltage $V_{\rm det}$ across the detection junction. We show that $I_{\rm ex}$ at the zero bias voltage is purely determined by the charge imbalance, while the energy imbalance causes a nontrivial $V_{\rm det}$-dependence of $I_{\rm ex}$. The obtained voltage-current characteristics qualitatively agree with the experimental result by R. Yagi [Phys. Rev. B {\bf 73} (2006) 134507].

Observation of a Possible Metallic State Induced by a Parallel Magnetic Field in Superconducting Au0.7In0.3 Samples With Very Low Normal-state Sheet Resistance

We report the observation of an apparent metallic state induced by a parallel magnetic field in Au0.7In0.3 samples with very low normal-state sheet resistance. These samples can be modeled as a random array of superconductor-normal metal-superconductor (SNS) junctions. For both the thin planar and cylindrical films, the magnetic field was applied parallel to the substrate of the sample and measurement currents. For the mesoscopic rings, however, a perpendicular field was used. Our electrical transport and tunneling measurements suggest that the samples consist of superconducting In-rich islands not linked by Josephson coupling in the metallic state. The physical origin of the metallic state is discussed.