Critical Current Of Junction Research Articles

Electromagnetic properties andHe+irradiation effects onYBa2Cu3O7−xgrain-boundary Josephson junctions

We have analyzed the electromagnetic properties of small and long YBa2Cu3O7-x grain-boundary Josephson junctions. The Swihart velocity increases with junction width (rv) while the ratio of the relative dielectric constant to the barrier thickness (epsilon/t) decreases. We found that the product wX epsilon/t is approximately constant. These results have been explained in the framework of the filamentary model, where the barrier can be regarded as a disordered dielectric medium with a high density of superconducting filaments. Experiments demonstrate that a controllable variation of these parameters can be achieved by helium irradiation at 80 keV. We give examples of an enhancement of weak-link properties of junctions for doses in the range of 10(13) cm(-2) Raising the dose we can sweep the modification of the weak-link properties from an increase of the junction critical current of about 10% to a severe degradation of the coupling energy of the barrier, although the superconducting properties of the electrodes always worsen gradually.

Behaviour of HTS Josephson vortex flow transistors

We have studied both theoretically and experimentally the low-frequency behaviour of discrete HTS Josephson vortex flow transistors. The devices were fabricated from YBa 2Cu 3O 7 thin films grown on 24° SrTiO 3 bicrystal substrates. They consist of an asymmetric array of 10 or 20 Josephson junctions, with a control current fed through an independent line that is inductively coupled to the array. We have measured current gains up to 40 at 77 K and substantially higher at lower temperatures. We discuss the results of simulations of the device behaviour and consider the effects of junction critical current, number of junctions and array loop inductance. We compare the measured device behaviour at 77 K with simulations which include the effects of thermal noise.

Design of a scanning Josephson junction microscope for submicron-resolution magnetic imaging

We describe a magnetic field scanning instrument designed to extend the spatial resolution of scanning superconducting quantum interference device microscopy into the submicron regime. This instrument, the scanning Josephson junction microscope, scans a single Josephson junction across the surface of a sample, detecting the local magnetic field by the modulation of the junction critical current. By using a submicron junction and a scanning tunneling microscope feedback system to maintain close proximity to the surface, magnetic field sensitivity of 10 μG with a spatial resolution of 0.3 μm should be attainable, opening up new opportunities for imaging vortex configurations and core structure in superconductors and magnetic domains in magnetic materials.

Single flux quantum comparators for HTS AD converters

We have investigated different comparators for Flash and Σ–Δ high temperature superconducting ADCs designed using three-layer HTS tri-crystal junction integrated circuit technology with spread of the junction critical currents less than 10% and features size 0.6 μm. Using the theoretical estimations for the bit error rate in RSFQ circuit under the restrictions of the given technology, we have estimated working temperature as T=62 K. At this temperature, the circuits were optimized in order to achieve maximum performance of the related ADCs in terms of input bandwidth, resolution and operating margins. For design purposes, a novel method was developed for inductance calculation with 3D magnetic field distribution in multilayer superconducting technology and extraction of the inductance matrix of the equivalent circuit.

Low frequency voltage noise in high temperature superconductor Josephson junctions

The origin of 1/f voltage noise in different types of Josephson junctions fabricated from the high temperature superconductors (HTS) have been traced back to the trapping and release of charge carriers in trapping centers in an insulating barrier giving rise to correlated fluctuations of the junction critical current Ic and normal state resistance Rn. For the normalized fluctuations SI and SR a linear scaling with Rn has been observed which suggests an almost constant density of trapping centers for all investigated HTS Josephson junctions. Using this linear scaling we have made an approximate calculation of the density of the trapping centers.

Vortex penetration and hysteretic behaviour of narrow planar Josephson junctions in a magnetic field

We have studied narrow YBa 2Cu 3O 7 (YBCO) step edge junctions (width w=0.8–18 μm) in applied magnetic fields up to B a∼35 mT. At B a=0, their I– V characteristics show good agreement with other YBCO grain boundary junctions. We use the magnetic field dependence of the junction critical current, I c( B a), to study the penetration of vortices. At 77 K, the junctions exhibit jumps in I c( B a) due to the creation, annihilation and motion of a small number of vortices close to the junction. The fields at which the jumps occur and the number of jumps measured for a particular B a show a dependence on w. These data suggest that field enhancement affects the penetration of vortices and support a field enhancement factor which is proportional to w. The data also reveal information about the hysteretic trapping of vortices near the junction. The entire I c( B a) pattern demonstrates a shift along the B a axis depending on the size and direction of the maximum applied field, B max. The magnitude of this hysteretic behaviour increases both with w and with B max until showing signs of saturation for B max greater than ∼15 mT.

Flux trapping and magnetic field dependence of critical currents in narrow YBCO step edge junctions

We have studied narrow (0.8-18 /spl mu/m wide) YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// step edge junctions in moderate applied magnetic fields as high as B/sub a//spl sim/25 mT. The magnetic field dependence of the junction critical current, I/sub c/, resembles a Fraunhofer diffraction pattern for all the junctions studied, although the field values of the pattern minima are modified due to the planar geometry. At 77 K these narrow junctions exhibit abrupt discontinuities in the I/sub c/(B/sub a/) pattern due to jumps in I/sub c/. These jumps are most likely due to sudden changes in the position of an individual vortex in close proximity to the junction. The I/sub c/(B/sub a/) patterns show hysteretic behaviour for applied fields B/sub a/>1-2 mT at 77 K, and the magnitude of this hysteretic behaviour appears to plateau for B/sub a/>/spl sim/12 mT, depending on junction width.

Determination of the Parameters of High- T C RF SQUID from Its Small Signal Voltage-Frequency Characteristics

The present study is the extension of a recent theory of high-TC RF SQUIDs, operating in the presence of large thermal fluctuations. First, the explicit analytical expressions for the amplitudes of higher harmonics of the current circulating in a SQUID loop which are necessary for the determination of the junction critical current IC and the inductive coupling coefficient of the SQUID loop to its tank circuit, k2 , are found. Second, in order to find the junction resistance R with acceptable accuracy, the solution of Fokker–Plank equation to first order in parameter q = ωL/R, where ω is a bias frequency, L is a loop inductance, is obtained. These results allow one to derive the simple expressions for the determination of the junction critical current IC, its normal resistance R, and the inductive coupling coefficient of the SQUID loop to its tank circuit, k2 , from the experimental measured small signal voltage-frequency characteristics.

Influence of ramp shape and morphology on the properties of YBa 2Cu 3O 7− δ-ramp-type junctions

The correlation between the shape and the morphology of ramps prepared by ion-beam etching of YBa 2Cu 3O 7− δ thin films, and the properties of ramp-type junctions were investigated in detail. We examined the influence of different fabrication parameters on the YBa 2Cu 3O 7− δ ramps by atomic force microscopy. Ramp-type junctions were fabricated using PrBa 2Cu 2.9Ga 0.1O 7− δ as barrier material. We observed a strong influence of the shape of the ramp on the homogeneity and, thus, on the transport properties of the junctions. Furthermore, we observed that the roughness of the ramps is strongly influenced by the voltage of the ion-beam during etching. Best results are achieved when an additional wet cleaning step by bromine solution in ethanol is introduced prior to the deposition of the barrier and the top electrode. As a result from our optimization, the on-chip spread of the junction critical current was reduced to 11%.

Proton damage on Nb-based Josephson junctions

The effects of proton irradiation on Nb-based Josephson junctions have been investigated. The fabrication technology employed is essentially the same of Josephson digital circuit designs, the main difference being a much lower Josephson critical current density. We have analysed in detail the magnetic field dependence of the junction critical current, and the quasiparticle tunnelling current, in order to observe possible occurrence of permanent changes produced by the ionizing particles. No evidence of radiation induced damage on the properties of the junctions has been found. The profile of the energy lost by protons within the Nb-targets and the dose is calculated by means of Monte Carlo simulations. A schematic description of the vacuum scattering chamber used for proton irradiation is also reported.

High-T/sub c/ superconductor oversampled delta modulator for analog-to-digital converters

Based on a single-level YBCO thin film topology and seven bicrystal grain-boundary junctions, a high temperature superconductor delta modulator for analog-to-digital converters has been designed, fabricated, and tested. A single bit quantizer based on a quantum flux parametron (QFP) comparator is used for the modulator. A large superconducting inductor is used both for the signal input and for the feedback. A small resistor is chosen for connecting the comparator to the read-out section. The oversampling provides large input signal bandwidth with high dynamic range. Circuit simulations prove the concept of the design. The effective LSB and MSB of the modulator are discussed in terms of input inductance, junction critical current, and QFP noise current. The density of latched output pulses is proportional to the change of input amplitude. Au wires were employed to make crossover of bias lines. Although this modulator did not function as expected for low-speed measurements, it shows the proper latching behaviour of the output section at 4.2 K and 35 K.

Fabrication of superconductor-normal metal-superconductor Josephson junctions with integrated high-T/sub c/ groundplanes

We describe the fabrication of superconductor-normal metal-superconductor YBa/sub 2/Cu/sub 3/O/sub x/ (YBCO) edge junctions with a cobalt-doped YBCO barrier. The process includes an integrated high-T/sub c/ groundplane with a low /spl epsiv//sub r/ interlevel dielectric layer of Sr/sub 2/AlNbO/sub 6/ (SAN). We have used groundplanes of both sputtered YBCO and YBCO and NdBa/sub 2/Cu/sub 3/O/sub x/ (NBCO) deposited by thermal coevaporation. Junction to groundplane isolations of >3.5/spl times/10/sup 8/ /spl Omega//spl middot/cm have been measured with a groundplane T/sub c/ as high as 90 K. The proper functioning of the groundplane was verified by measuring the inductance of identical SQUIDs on and off the groundplane. The SQUID inductance on the groundplane was as low as 1.2 pH//spl square/ at 70 K. The spreads in the junction critical currents are comparable for devices on and off the groundplane Groundplane and junction uniformity issues such as film morphology, and edge and barrier formation will be discussed.

Effect of intense proton irradiation on properties of Josephson devices

We have experimentally investigated the effects of intense proton beam irradiation (up to 10/sup 15/ p/cm/sup 2/) on Josephson junctions and junction arrays. The devices we have studied were realized using state of the art full-Nb technology, employing same materials and thicknesses of common Josephson digital circuit designs. We have analysed in detail the magnetic field dependence of the junction critical current, and the quasiparticle tunneling current, in order to observe possible occurrence of permanent changes produced by the ionizing particles. No evidence of radiation induced damage on the properties of the junctions has been found.

Laser trimming of Y-Ba-Cu-O step-edge Josephson junctions

We report our studies on the laser-induced permanent changes in electrical transport of YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) step-edge Josephson junctions. The junctions were fabricated from 200-nm-thick YBCO films deposited by laser ablation onto 300-nm-high steps etched in LaAlO/sub 3/ substrates. Laser annealing was performed in-situ by illuminating the junctions at 50 K with a focused Ar-ion laser beam of the intensity up to 10/sup 5/ W/cm/sup 2/. Under optimal irradiation, we observed up to 50% increase of the junction critical current. The modified critical currents were stable and did not change with time or during the helium-to-room-temperature thermal cycling. The annealed junctions exhibited RSJ-like current-voltage characteristics and an enhancement of the I/sub c/R/sub n/ product of up to 50%. The I/sub c/(B) characteristics of the illuminated junctions showed up to 10% shift of the first minima, indicating changes in the junction-barrier area. The I/sub c/(B) simulations, based on the Yanson model of large junction-barrier fluctuations, demonstrated that laser treatment improved barrier homogeneity. We believe that upon laser irradiation, photo-assisted thermal oxygen reordering takes place in the vicinity of the junction barrier, consequently leading to grain boundaries with increased oxygen content and improved junction dc characteristics. An illumination power exceeding 10/sup 5/ W/cm/sup 2/ resulted in oxygen out-diffusion from the junction area and loss of the Josephson-like properties.

Extrapolitan procedures in high- Tc gap-function experiments

Recent experiments have employed the phase sensitive nature of the Josephson effect to determine the wave-vector dependent phase of the gap function in high-temperature superconductors. A parallel combination of Josephson junctions, commonly referred to as a dc SQUID configuration, has been used in many experiments to probe the gap function symmetry via transport measurements. Asymmetries in the junction properties have required various extrapolation procedures to extract symmetry information from measured field-dependent transport measurements. Here we analyze the finite-temperature properties of this system in the experimentally relevant limit of large self-inductance, where the asymmetry is due to a difference in junction critical currents. We derive a scaling relation between systems of different self-inductance, leading to a new extrapolition procedure to determine the gap function symmetry.

Magnetic flux quanta in annular Josephson junctions in a barrier-parallel de magnetic field

Flux quanta (fluxons) trapped in an annular Josephson junction reside in a spatially periodic pinning potential when a uniform external magnetic field is applied parallel to the plane of the junction's tunnel barrier. We investigate the fluxon behavior by measuring the current versus voltage curves, the magnetic dependence of the junction critical current, and by means of spatially resolved measurements imaging the pinned fluxons. We derive the fluxon potential energy including fluxon-fluxon interaction. Fair agreement of this calculation with our experimental results is obtained.

The phase-dependent dissipation effect in a d.c. SQUID

The magnetic field modulation of the voltage output of a d.c. SQUID, calculated numerically including the phase-dependent dissipation (PDD) term with a negative prefactor ε in Josephson junctions, exhibits maxima (minima) at integer (half-integer) values of the magnetic filling factor f in a larger bias current beyond I ∗ , the values of which depends on the inductance parameter β L and the prefactor ε. Asymmetry in both junction critical currents and inductances between the two branches of a d.c. SQUID loop does not change the essential feature of the magnetic-field modulation of the resistance due to the PDD effect.

Synchronization and phase locking in two-dimensional arrays of Josephson junctions.

We report numerical studies of phase locking in two-dimensional (2D) arrays of Josephson junctions. In the conventional 2D arrays biased with dc current in the horizontal (X) direction, the phase-locked solution is only possible in arrays with no disorder. In the presence of disorder in the junction critical currents, we see vertical (Y) columns of phase locked clusters form in the array with no intercolumn locking. As an extension, we introduced a geometry called XY-biased array, where the array is biased in both X and Y directions. In XY-biased array we found a dynamical state, where all junction voltages are strictly dc and the only oscillating components are the supercurrents in each junction. This dynamical state is stable against experimentally achievable disorder in the junction critical currents. \textcopyright{} 1996 The American Physical Society.

Laser-induced modification of the critical current in Y-Ba-Cu-O step-edge Josephson junctions

Transport properties of YBa2Cu3O7−x (YBCO) step-edge weak links were measured in the absence and presence of the magnetic fieldB, both before and after the cw-laser annealing. Depending on the laser power density, either an increase (up to 75%) or decrease (up to 100%) of the junction critical currentIc was observed. TheIc(B) characteristics of the laser-treated junctions exhibited about 10% variations of theIc(B) periodicity, indicating permanent changes in the junction barrier and/or adjacent YBCO electrodes.

Effect of disorder on synchronization in prototype two-dimensional Josephson arrays.

We study the effects of quenched disorder on the dynamics of two-dimensional arrays of overdamped Josephson junctions. Disorder in both the junction critical currents and resistances is considered. Analytical results for small arrays are used to identify a physical mechanism which promotes frequency locking across each row of the array, and to show that no such locking mechanism exists between rows. The intrarow locking mechanism is surprisingly strong, so that a row can tolerate large amounts of disorder before frequency locking is destroyed.