SNS Josephson Junctions Research Articles

New Barriers for Fast-Switching SNS Josephson Junctions

We report recent results on the development of two different kinds of SNS Josephson junctions which can be used as quantum D/A converters. We have fabricated these devices using as a barrier either a thick 50-100 aluminum layer or a not-stabilized tantalum oxide layer. For both technologies we have determined the dependence of the main electrical properties of the junctions on the fabrication process. Measurements of the rf properties for the Nb/Al/Nb junctions and of the magnetical properties for the Nb/TaO x/ Nb junctions are also reported.

Growth Mechanisms and Surface Morphology of Ybco Based High-Tc Thin Films and Heterostructures for sns Junctions

AbstractWe have controlled the nanoscale growth mechanism and surface morphology of YBa2Cu3O7 (YBCO) based high-Tc thin films and heterostructures, using miscut SrTiO3 substrates. On exact (001) SrTiO3 substrates, the YBCO films grow in a screw dislocation growth mode. The barrier layers (La6.4Sr1.6Cu8O20 and PrBa2Cu3O7) grown on top of such a YBCO film also show spiral growth features, indicating pseudomorphic growth. On miscut substrates (with miscut angle ≥ 4° toward [010]) the YBCO films grow by step-flow. However, the La6.4Sr1. 6Cu8O20 layers grown on such YBCO bottom electrodes, show a high degree of step bunching with rough surface. In contrast, the PrBa2Cu3O7 layers show clear step-terrace surface morphology similar to the underlying YBCO bottom electrode, suggesting the existence of periodic nanoscale steps at the S-N interface. These heterostructures can be used for the fabrication of SNS Josephson junctions to take advantage of the proximity effect coupling at the nanoscale steps at the interface.

Intrinsic Josephson effect in La2−xSrxCuO4 mesa junctions with niobium counter electrode

We reveal the intrinsic Josephson effect in a La2−xSrxCuO4 (LSCO) high-Tc superconductor. Nb counterelectrodes are deposited on top of mesas, which are formed on surfaces either parallel or perpendicular to the ab plane of LSCO single crystals. Nb/LSCO interfaces behave as SNS Josephson junctions. For the junctions parallel to the ab plane, we observe many branches in the current–voltage characteristics due to intrinsic Josephson effects. The voltage gaps between the branches are typically 0.1 mV, which is much smaller than the superconducting energy gap 2Δ≅11 mV expected for LSCO from Bardeen–Cooper–Schrieffer theory.

HTS SNS Josephson junctions: interfaces and mechanisms

YBa 2Cu 3−xCo xO 7−δ goes through a metal-insulator transition near x = 0.4. When these doped YBCO materials are used as barriers in SNS junctions, a clean interface can be formed due to their excellent match of lattice constants and thermal expansion coefficients. By using three doping levels, x = 0.12, 0.21 and 0.42, we have estimated the effects of the dopant from the temperature and thickness dependences of I c and R n. In the framework of conventional de Gennes proximity effect theory, we have observed a crossover from the clean limit to the dirty limit as the doping level approaches the metal-insulator transition. In the case of x = 0.42, which no longer superconducts on its own, we had to invoke a pair-breaking mechanism by a cutoff scattering rate in order to explain the slower increase of the critical currents at low temperatures than the standard proximity effect would predict. Several ideas on the possible origins of such a cutoff scattering rate will be discussed. By adding Pr dopants in the Co-doped barrier, we were able to deduce the elastic scattering effect of Pr dopants, which results in smaller diffusion constants. The interfaces YBCO forms with other barriers such as (CaSr)RuO 3 and (LaSr)CoO 3 have been investigated in detail. The defect states on the surface of YBCO seem responsible for the observed zero bias conductance anomaly.

Superconducting properties in bulk (Bi1.6Pb0.4)(Sr1.8Ba0.2)Ca2Cu3Oy system

U(H,T) dependencies for activation energies obtained from Arrhenius plots were well explained by vortex cutting mechanism in an entangled vortex phase. JcJ(T) dependencies near Tc from χ″(T) agree with TAPS in the case of SNS-Josephson junctions.

The constriction model for SNS Josephson junctions

The approach developed by Blonder, Tinkham and Klapwijk (BTK) for calculation of the quasiparticle (qp) current in NcS contacts is generalized to the case of ScNS and SNcNS contacts with disordered NS electrodes. Amplitudes of Andreev and normal reflections at the ballistic constriction are calculated. The relation between the qp current and energy spectrum of NS proximity sandwich is found for arbitrary transparencies of the constriction and NS interfaces. The appearance of new series of subharmonic peaks associated with the two-gap structure in the density of states of NS electrodes is demonstrated. The model is also applied to calculation of the critical and the excess currents in ScNS and SNcNS for different interface parameters. The relevance of the model to transport mechanisms in step-edge HTS SNS contacts is discussed.

Fabrication and characterization of high temperature superconductor Josephson junctions with a novel device design

A unique normal-metal (N) layer construction was used to fabricate high temperature superconducting (S) YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// SNS Josephson junctions. The normal-metal included a gradient Pr-doped Y/sub 1-x/Pr/sub x/Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// layer which was composed of a light doping (x=0.1) next to both YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// electrodes, a slightly higher doping (x=0.3) towards the center, and a doping concentration of x=0.5 in the middle of the N-layer. A gradient of the doping profile of the N-layer instead of an abrupt one provides good thermal, structural, and chemical compatibility between adjacent regions. The multilayer configuration of the gradient Pr-doped N-layers on YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// showed good growth structure as confirmed by X-ray diffraction and Rutherford backscattering channeling tests. The SNS junctions fabricated in such a way showed resistively shunted junction current vs voltage characteristics above 77 K. Microwave induced Shapiro steps above 77 K and voltage modulation of dc SQUIDs at 77 K were both demonstrated with this technology. >

Epitaxial Thin Films and Heterostrcutures of Copper-Oxide-Based Isotropic Metallic Oxides for Device Applications

AbstractWe have grown epitaxial thin films of a new family of copper-oxide-based isotropic metallic oxides such as La6.4Sr1.6Cu8O20, La5BaCu6O13 and La6BaYCu8O20in situ by 900° off-axis sputtering. These metallic oxides are pseudo-cubic perovskites with essentially isotropic properties, which could be ideal normal metals for SNS junctions in superconducting devices. We have also grown epitaxial SNS superconducting heterostructures (c-axis YBa2Cu3O7 / La6.4Sr1.6Cu8O20 / c-axis YBa2Cu3O7) with a copper-oxide-based isotropic metallic oxide (La6.4Sr1.6Cu8O20) normal metal barrier. X-ray diffraction and cross-sectional transmission electron microscopy reveal these heterostructures to have high crystalline quality and clean interfaces. This material will facilitate fabrication of ideal SNS Josephson junctions with low boundary resistance due to its excellent chemical compatibility and lattice match with cuprate superconductors and will be useful for determining the source of interface resistance in such heterostructures.

High Temperature Superconducting Step-Edge SNS Josephson Junctions on Silicon Substrates

AbstractWe have fabricated and tested YBCO step-edge SNS Josephson junctions on silicon substrates. The silicon step edges were patterned photolithographically and reactively ion etched using an SF6 plasma. The structures were fabricated through sequential angled pulsed laser deposition of yttria stabilized zirconia, YBCO, and gold layers, followed by photolithographic patterning and ion milling. The completed devices showed resistively shunted junction (RSJ)-like current voltage characteristics and microwave induced Shapiro steps. Critical currents as large as 84 PA and resistances of order 0.5 Ω were obtained. Measurable critical currents were observed up to 76 K. We report on the fabrication and properties of these junctions.

Interface and proximity effects in SNS Josephson junctions based on YBCO with epitaxial barriers

We report the properties of SNS Josephson junctions based on YBCO in an edge geometry. Pseudo-cubic perovskite materials such as (CaSr)RuO 3 and (LaSr)CoO 3, as well as the layered perovskite materials such as (LaSr) 2CuO 4, Co-doped YBCO, and Ca-doped YBCO, have been used as epitaxial barriers. We have observed interface resistances on the order of 10 -8 to 10 -7 Ωcm 2 in the case of the cubic perovskite barriers, and interface resistances smaller than 10 -10 Ωcm 2 in the case of the layered perovskite barrier materials. We found that the junctions with smaller interface resistance values behave close to predictions of proximity effect theory.

Epitaxial Thin Films And Heterostrcutures of Various Isotropic Metallic Oxides for Device Applications

AbstractWe have grown epitaxial thin films of various isotropic metallic oxides such as Sr1-xCaxRuO3 and La8-xSrxCu8O2Oin situ by 90° off-axis sputtering. These metallic oxides are pseudo-cubic perovskites with essentially isotropic properties, which could be ideal normal metals for SNS junctions in superconducting devices and for electrodes in ferroelectric devices. We have fabricated epitaxial ferroelectric heterostructures [SrRuO3/Pb(Zr0. 52 Ti0.4 8) O3 /SrRuO3] employing isotropic metallic oxide (SrRuO3) electrodes on substrates of (100) SrTiO3 and (100) Si with an yttria stabilized zirconia buffer layer. They exhibit superior fatigue characteristics over those made with metal electrodes, showing little degradation over 10 cycles, with a large remnant polarization (27 μC/cm2 ). We have also grown epitaxial superconducting heterostructures (YBa2Cu3O7 / La8-xSrxCu8O2O / YBa2Cu3O7 ) with a copper-oxide-based isotropic metallic oxide (La8-xSrxCu8O20) normal metal barrier. X-ray diffraction and cross-sectional transmission electron microscopy reveal these heterostructures to have high crystalline quality and clean interfaces. This material will facilitate fabrication of ideal SNS Josephson junctions with low boundary resistance due to its excellent chemical compatibility and lattice match with cuprate superconductors and will be useful for determining the source of interface resistance in such heterostructures.

Optimizing SNS Josephson junctions for high critical current density in BiSrCaCuO-metal composites

The use of superconducting BiSrCaCuO (BiSCO) wires or tapes in electronic devices implies the optimization of the transport properties in ohmic contacts between the superconductor and the “normal” metal in the circuit. This paper presents results of theoretical calculations, based on modifications of tunneling theory in SNS, in both the “pure” and “dirty” limits. The aim is to sort the relevant parameters for the maximum current density. For a specific superconductor and a “normal” metal, the critical current density has an optimum value, depending on the “normal” metal resistivity in the “dirty” limit, and on the ratio of Fermi velocities in the “pure” limit.

YBa2Cu3O7−δ-based, edge-geometry SNS Josephson junctions with low-resistivity PrBa2Cu3O7−δ barriers

Abstract We investigated the properties of all-high- T c edge-geometry Josephson weak links using superconducting YBa 2 Cu 3 O 7−δ (YBCO) electrodes and PrBa 2 Cu 3 O 7−δ (PBCO) normal-metal layers. The fabrication of the weak links involved first ion-milling the base YBCO film using a MgO mask to form the edge and then depositing PBCO and YBCO layers in situ by pulsed-laser deposition. The optimum PBCO films had resistivities of 2 to 4 mΩ cm at room temperature and ≈30 mΩ cm at 4.2 K. Current-versus-voltage ( I–V ) characteristics were qualitatively consistent with the resistively-shunted-junction (RSJ) model. Strong Shapiro steps were present in the I–V characteristics of junctions under microwave irradiation and the critical current of the devices modulated with a Fraunhofer-like dependence as a function of magnetic field. The device parameters exhibited good scaling with area and PBCO thickness. The critical-current density varied exponentially with PBCO thickness, yielding a normal-metal coherence length of 9 nm for the PBCO.

High-T C superconductor-normal metal-superconductor Josephson microbridges with high-resistance normal metal links

We have developed an in situ process for fabricating high transition temperature superconductor-normal metal-superconductor microbridges using a step edge to define the normal metal length. Critical current-normal resistance products over 1 mV have been measured at low temperature in devices with high-resistivity Ag-Au alloy bridges. Results on samples with Ag bridges are compared with the alloy data as an initial test of recent theories of SNS Josephson junctions. Josephson effects have been demonstrated in these devices at temperatures higher than 80 K. Clearly defined rf steps have been observed, with power dependence qualitatively similar to theoretical predictions.

Fabrication of monolithic lateral SNS junction structure for Bi-oxide systems

A novel monolithic lateral SNS structure of Bi-oxide systems has been developed in which the compositions of films were locally controlled in the lateral direction. Lateral SNS (BiSrCaCuO/BiSrCuO/BiSrCaCuO) structures which were made from a Ca-F/BSCO layer on MgO substrates have been developed. The BiSrCaCuO area, with a thickness of 150 nm, is composed of a 2212 phase with high crystallinity of the c-axis preferred orientation, and shows zero resistance at 85 K. The BiSrCuO indicates sufficiently low resistivity to be used as a normal barrier for SNS Josephson junctions. BSCO formation using Ca-diffusion into the surfaces of 2201 single crystals was successfully carried out.

Fabrication and properties of YBCO/Ag/YBCO planar SNS junctions

We fabricated planar YBCO Ag/YBCO junctions having lateral dimensions of 1mm × 1mm and a nominal thickness of 300Å. The junctions were prepared in situ using the laser ablation technique and contact masks to define the geometry. The critical current of the junctions in a magnetic field and in a microwave radiation shows response characteristic of an SNS Josephson Junction. In addition, the junctions show a novel nonlinear effect which to the best of our knowledge was not observed with conventional superconductors.

Fractional giant Shapiro steps in 2D Josephson arrays

Radio-frequency signals were applied to 1000 by 1000 arrays of SNS Josephson junctions and giant Shapiro steps were observed at voltages 1000 times that expected for a single junction, Vn= nNhv/2e, where N=1000 is the number of junctions in the direction of applied current. For certain perpendicular magnetic fields the field-induced vortices are strongly commensurate with the array and are arranged in superlattices with qxq unit cells. In the presence of these strongly commensurate fields novel fractional giant steps were observed at voltages, Vn =n(N/q)hv/2e, directly related to q. The locking of the motion of the vortex superlattice with the applied rf current is used to explain these novel steps. The possibility of subharmonic fractional steps and preliminary measurements on disordered arrays is discussed.

Fabrication and properties of YBCO/Ag/YBCO Planar SNS junctions

We fabricated planar YBCO Ag/YBCO junctions having lateral dimensions of 1mm × 1mm and a nominal thickness of 300Å. The junctions were prepared in situ using the laser ablation technique and contact masks to define the geometry. The critical current of the junctions in a magnetic field and in a microwave radiation shows response characteristic of an SNS Josephson Junction. In addition, the junctions show a novel nonlinear effect which to the best of our knowledge was not observed with conventional superconductors.

Observation of integer and fractional giant Shapiro steps in arrays of SNS Josephson junctions

We observe giant Shapiro steps in the I-V curves of NxM arrays of SNS Josephson junctions; these steps occur at N times the single junction voltage. Both integer and fractional giant steps are observed as a function of temperature, external magnetic field, and rf current. The fractional steps occur at multiples of 1/q times the giant Shapiro step voltage and are seen at well defined values of the applied field, p/q flux quantum per placquette. We also observe a fractional step at 1/2 integer voltages, even in zero applied field. These fractional steps decrease in size if random site disorder is introduced to the array.

Airy pattern, weak-link modeling of critical currents in high-Tc superconductors

We have measured the transport critical current density at very low magnetic fields in samples of superconducting bulk sintered Y 1 Ba 2 Cu 3 O x , Y 1 Ba 2 Cu 4 O x obtained from several sources. The results are analyzed at low fields (≤10 mT) with a statistical model which assumes that the current is limited by Josephson weak links (SNS or SIS Josephson junctions or microbridges) whose locations are to be determined. Each weak link is assumed to be described by an Airy current-field pattern rather than a Fraunhofer pattern. The former has a better theoretical foundation and is in better agreement with the data, varying as H -3 2 upon averaging. The fitting procedure yields the average cross sectional area of the weak links. By assuming the link thickness to be twice the London penetration depth at 77 K, we find that the average linear dimensions of the links are in all cases comparable to the grain dimensions. The quantitative analysis also confirms the percolation concept, in which a subset of weakest links controls the transport current.