Y-Ba-Cu-O/Nb Josephson tunnel junctions

The authors fabricated Y-Ba-Cu-O/Au/AlO_x/Nb and Y-Ba-Cu-O/AlO_x/Nb Josephson tunnel junctions using electron-beam evaporation of Al and Nb films and natural oxidation. Sintered Y-Ba-Cu-O was used as the base electrode. Superconducting Josephson current and hysteresis of the current-voltage characteristics, which are typical features of Josephson tunnel junctions, have been observed at 4.2 K. RF-induced voltage steps at a voltage greater than 0.4 mV have been clearly observed, and RF-induced subharmonic steps have also appeared. The superconducting Josephson current was modulated by the magnetic field     

Josephson tunnel junctions with refractory electrodes

Fabrication of high quality Josephson tunnel junctions with refractory electrodes is made difficult by the electrode's short coherence lengths and the sensitivity of the their superconducting properties to tunnel barrier processing. Several innovative procedures have been developed to address this problem. In this review, we describe these tunnel barrier processes and the junction characteristics that were obtained using them. In particular, attention is focused on the junction's subgap conductance and the reproducibility of the maximum Josephson current because of their importance as device design parameters for integrated circuits.     

Properties of niobium nitride-based Josephson tunnel junctions

High T_c (16.8 K) niobium nitride thin films have been prepared by rf diode sputtering in N₂/Ar atmosphere and subsequent high temperature annealing. Josephson tunnel junctions have been made by thermal oxidation of the films. The geometry is defined by high resolution photolithography. The Josephson junctions have been characterized by magnetic field diffraction measurements and other techniques and are shown to be particularly suitable for applications especially in superconducting microwave integrated devices.     

Quantum dynamics of the charge in Josephson tunnel junctions

It is shown that (quasi) charge of a Josephson tunnel junction can be considered as a macroscopic quantum degree of freedom. In the adiabatic limit we derive the effective action of the tunnel junction in the quasicharge representation and calculate the quantum decay rate of the charge states.     

Annular Josephson tunnel junctions in an external magnetic field

We have investigated the static and dynamic configurations of the phase inside an annular Josephson tunnel junction in the presence of an externally applied magnetic field. We report here a detailed study of the dependence on the magnetic field of the critical current and of the Fiske singularities. The behaviour is investigated perturbatively and numerically.     

Properties of niobium-based Josephson tunnel junctions with a lead-indium alloy top electrode

A fabrication process for niobium-based Josephson superconductive tunnel junctions with a Pb/In alloy top electrode which showed very good mechanical and electrical properties and high oxide uniformity is described. The dependence of the junction parameters on the In-percentage in the alloy is also reported, and the results, in connection with data on the barrier composition, obtained via Auger electron-spectroscopy analysis, are discussed.     

Plasma frequency of the tunnel Josephson junctions with anharmonic current-phase relationship

The effect of alternating current on the plasma frequency of a tunnel Josephson junction with anharmonic current-phase relationship under the conditions of simultaneous passage of direct (dc) and alternating (ac) current has been studied. It is established that the presence of the ac component decreases, whereas the anharmonicity increases the plasma frequency of the tunnel Josephson junction.     

Low specific capacitance by ion-beam oxidation of Niobium-based Josephson tunnel junctions

Josephson tunnel junctions were fabricated on niobium by ion-beam oxidation where an oxide thickness independent of the processing time can be achieved. Low specific capacitance of about 5 µF/ cm²at a maximum current density of 1 kA/cm²have been measured via junction and interferometer resonances. A model for the structure of those low-capacitance barriers is proposed.     

Theoretical investigation of Josephson tunnel junctions with spatially inhomogeneous superconducting electrodes

The microscopic theory of the Josephson effect in tunnel structures with electrodes having spatially inhomogeneous superconducting properties is formulated. Two mechanisms of inhomogeneity are considered. The first is associated with the presence of a thin transition normal layer located near the tunnel barrier, which is relevant for junctions based on refractory superconductors. The second case is the trapping of Abrikosov vortices by junction electrodes. The tunnel current components are calculated numerically in the whole temperature range 0<T<T _c and magnetic field range 0<H<H _c2. It is shown that the tunnel current is extremely sensitive to the type of smearing of the singularities of the classical tunnel theory ateV=2. The results allow experimental determination of the characteristics of real tunnel junctions.     

Spatial distribution of the maximum Josephson current in superconducting tunnel junctions

The distribution of the maximum Josephson current in two-dimensional tunnel junctions is investigated both theoretically and experimentally. Measurements of the Josephson current density are performed using low-temperature scanning electron microscopy. Josephson current densities are calculated by means of a numerical iteration procedure, which is presented in detail. These theoretical calculations are found to be essential for the correct interpretation of the measured Josephson current densities. Combining these experimental and theoretical methods, we investigate trapped transverse vortices. Further, the strength of the elementary pinning centers acting upon the trapped flux quanta is measured by recording the shift of the vortex position as a function of an applied Lorentz force. Ringlike vortices are also observed, and the influence of different current feed configurations on the Josephson current distribution is investigated.     

Approximate analysis for stationary current flow in two-dimensional Josephson tunnel junctions

The stationary current distribution in Josephson tunnel junctions is discussed within the framework of an approximate linear analysis. It is shown that such an approach can provide very useful information also on the behavior of two-dimensional junctions. Self-limiting effects and the magnetic field dependence of the critical current for various geometries are discussed in some detail. The special case of a square junction is analyzed extensively.     

Thermal noise generated by quantum phase relaxations in Josephson tunnel junctions

An analysis of thermal noise generated by energy gap relaxations in a small superconductive tunnel diode is presented. Thermodynamic aspects are discussed which make the Werthamer functional accessible for the application of a generalized fluctuation dissipation theorem. The current fluctuation related to equilibrium states is evaluated without restrictions concerning the nonlinearity in the phase difference.Supported by Grant of the Federal Department of Commerce (West Germany).     

Spin-filter Josephson junctions

Josephson junctions with ferromagnetic barriers have been intensively investigated in recent years. Of particular interest has been the realization of so called π-junctions with a built-in phase difference, and induced triplet pairing. Such experiments have so far been limited to systems containing metallic ferromagnets. Although junctions incorporating a ferromagnetic insulator (I(F)) have been predicted to show a range of unique properties including π-shifts with intrinsically low dissipation and an unconventional temperature dependence of the critical current I(c), difficulties with the few known I(F) materials have prevented experimental tests. Here we report supercurrents through magnetic GdN barriers and show that the field and temperature dependence of I(c)is strongly modified by the I(F). In particular we show that the strong suppression of Cooper pair tunnelling by the spin filtering of the I(F) barrier can be modified by magnetic inhomogeneity in the barrier.     

The effect of alternating current on the plasma frequency of the tunnel Josephson junctions

The effect of alternating current on the plasma frequency of the tunnel Josephson junctions simultaneously carrying both direct (dc) and alternating (ac) currents has been studied. The presence of the ac component leads to a decrease in the plasma frequency. An analytical expression describing the plasma frequency as a function of the ac current amplitude is proposed. The obtained results agree with the recent experimental data on the properties of tunnel Josephson junctions.     

Measurement of the spatial distribution of the maximum Josephson current in superconducting tunnel junctions

The spatial distribution of the maximum Josephson current densityJ _max is imaged in quasi-one-dimensional in-line tunnel junctions by scanning the junction surface with an electron beam and by recording the beam-induced change ofI _max as a function of the coordinates of the beam focus. In addition to the modulation of the Josephson current density for the different vortex states, the nonlocal effect resulting from the beam-induced change of the phase-difference function is observed. The results are in good agreement with the theoretical predictions. The electron beam scanning technique also serves well for imaging inhomogeneities in the barrier or in the electrodes of the junction.     

Observation of subgap structures in high-quality Nb/Al-AlOx/Nb Josephson tunnel junctions

We have performedI-V measurements on high-quality Nb/Al-AlO_x/Nb Josephson junctions. All curves exhibit subgap structures. Some junctions exhibit in theI-V curves the (Δ₁–Δ₂)/e singularity, due to the presence of niobium films with different gaps. In these devices we observed sharply separated structures corresponding to the series 2Δ₁/ne and 2Δ₂/ne, with n≤3 corresponding to the tunneling of two and three electrons. The respective amplitudes of the two structures atn=2 allow one to conclude that the phenomenon is a multiparticle tunneling process.     

Analysis of magnetic field patterns of single Josephson tunnel junctions with large idle regions

We have experimentally investigated the effect of a combined lateral and longitudinal idle region on the magnetic field dependence of the critical current of Nb/Al−AlO_x/Nb Josephson junctions. All the samples were high quality devices with a good uniformity and reproducibility of the electrical parameters. An excellent agreement has been found with calculations of the magnetic field patterns made using a simple model of junction with idle region. A comparison with the magnetic field response of the same devices after a proper trimming of the idle region has also been done.     

The effect of alternating current on the thermal activation threshold of the tunnel Josephson junctions

The process of thermal activation in the tunnel Josephson junctions simultaneously carrying both constant (dc) and alternating (ac) currents has been studied. The presence of the ac component leads to a decrease in the potential barrier for a metastable state of the Josephson junction. The thermal activation threshold is expressed as a function of the ac current amplitude. The results agree with the data of recent experiments on the statistics of switching from a superconducting to resistive state in the Josephson tunneling junctions.     

Temperature dependence of the maximum Josephson current in V-V x O y -Pb tunnel junctions

Measurements of the temperature dependence of the maximum dc Josephson current in V-V _x O _y -Pb tunnel structures have been performed. Junctions exhibiting a vanadium critical temperature close to the bulk value and a uniform supercurrent density have been considered. In spite of the high quality of the samples, the results still differ markedly from the BCS calculation of Ambegaokar and Baratoff.This work is partially supported by the Consiglio Nazionale delle Ricerche under the Progetto Finalizzato Superconduttivitá.