Double-barrier Josephson Junctions Research Articles

Resonant tunneling in a double-barrier Josephson junction

Resonant tunneling in a double-barrier Josephson junction

Current–voltage characteristics of asymmetric double-barrier Josephson junctions

We develop a theory for the current–voltage characteristics of diffusive superconductor-normal metal–superconductor Josephson junctions with resistive interfaces and the distance between the electrodes smaller than the superconducting coherence length. The theory allows for a quantitative analytical and numerical analysis in the whole range of the interface transparencies and asymmetry. We focus on the regime of large interface resistance compared to the resistance of the normal region, when the electron–hole dephasing in the normal region is significant and the finite length of the junction plays a role. In the limit of strong asymmetry we find pronounced current structures at the combination subharmonics of Δ+Δg, where Δg is the proximity minigap in the normal region, in addition to the subharmonics of the energy gap 2Δ in the electrodes. In the limit of rather transparent interfaces, our theory recovers a known formula for the current in a short mesoscopic connector – a convolution of the current through a single-channel point contact with the transparency distribution for an asymmetric double-barrier potential.

Current-phase relation of double-barrier Josephson junctions with a two-gap superconductor as intermediate electrode

The current-phase (I-ϕ) relation of a double-barrier Josephson junction with a two-gap superconductor as intermediate electrode is derived by means of a simplified version of Ohta’s model. As in conventional double-barrier Josephson junctions, a marked skewness in the I-ϕ curves is present. Moreover, as in heterotic Josephson devices, a reduction of the maximum Josephson current is predicted. An appropriate experiment to verify the rich behavior of this type of Josephson device is suggested.

Feynman's and Ohta's models of a Josephson junction

The Josephson equations are derived by means of the weakly coupled two-level quantum system model given by Feynman. Adopting a simplified version of Ohta's model, starting from Feynman's model, the strict voltage–frequency Josephson relation is derived. The contribution of Ohta's approach to the comprehension of the additional term given by the classical external system is recognized. Extension of the model to double-barrier Josephson junctions and multilayered systems is finally briefly commented on.

Skewness in the current-phase relation of double-barrier Josephson junctions

The current-phase relations of a double-barrier Josephson junction with a thin intermediate electrode show deviations from the usual sinusoidal dependence of a single Josephson junction. The skewness of these curves can be analytically found for small values of the coupling between the two outer electrodes of the double-barrier Josephson junction.

Quantum behavior of a superconducting loop interrupted by a double-barrier Josephson junction

The Hamiltonian of a superconducting thin loop interrupted by a double-barrier Josephson junction is derived. The parameter ε, which takes account of the difference in the Josephson coupling of the two single junctions in the SISIS structure, is seen to allow variation of the energy gap ΔE01 between the ground state and the first excited flux number state of the system. An additional parameter γ, describing the coupling between the outer electrodes of the double-barrier Josephson junction, is introduced. Persistent currents in the system are studied in terms of the usual parameter α, representing the ratio between the Josephson energy and the magnetic energy, and of the externally applied flux.

Magnetic states of a rf superconducting quantum interference device containing a double-barrier Josephson junction

The magnetic properties of a rf superconducting quantum interference device containing one overdamped double-barrier junction are studied. The effective nonsinusoidal expression for the current-phase relation with an additional half harmonic term is used for the double-barrier junction. Devices with inhomogeneous double-barrier junctions show a characteristic feature: for fixed and not too high superconducting loop inductance values, transition from irreversible to reversible magnetic behavior can be induced by increasing the difference in the Josephson coupling of the two junctions.

MoRe-based and MgB2 -based tunnel junctions and their characteristics

Perspective Josephson Mo-Re alloy-oxide-Pb, MgB2- oxide – Mo-Re alloy and Mo-Re alloy-normal metal-oxide- normal metal-Mo-Re alloy junctions have been fabricated and investigated. Thin (~50-100 nm) MoRe superconducting films are deposited on Al2O3 substrates by using a dc magnetron sputtering of MoRe target. Thin (~50-100 nm) MgB2 superconducting films are deposited on Al2O3 substrates by using e-beam evaporation of boron and thermal coevaporation of magnesium. To investigate a transparency spread for the fabricated junctions barriers the computer simulation of the measured quasiparticle I–V curves have been done in framework of the model of multiple Andreev reflections in double-barrier junction interfaces. It's demonstrated the investigated junctions can be described as highly asymmetric double-barrier Josephson junctions with great difference between the two barrier transparencies. Results of computer simulation of quasiparticles I–V curves of junctions are presented and discussed. The IC(T) characteristics, measured for Josephson heterostructures with different thickness of metal layer s and exposure dose E, essentially deviate from an Ambegaokar- Baratoff (A&B) IC(T) behavior and Kulik-Omelianchuck (K&O) curves, because of proximity effect caused by the comparatively high value of s (up to 100 nm).

Theoretical study of ac Josephson effect in a double barrier Josephson junction

We study an ac Josephson effect in a SC/I/X/I/SC junction, where SC is an s-wave superconductor, I is an insulator, and X is either a normal metal (NM) or a ferromagnetic metal (FM). Using a tunneling Hamiltonian and Green’s function method, we obtain two types of superconducting phase dependent current. One is the Josephson current and the other is the quasiparticle-pair-interference current (QPIC). In a SC/I/NM/I/SC junction, the Josephson critical current ( I c 1 ) and the amplitude of QPIC ( I c2 ) are functions of the thickness of NM and the applied voltage ( V ) . I c1 shows a logarithmic divergence called Riedel peak at the gap voltage, V = 2 Δ / e , where Δ is the superconducting gap, and I c2 shows a discontinuous jump at the gap voltage. These behaviors are similar to these of SC/I/SC junctions. On the other hand, in a SC/I/FM/I/SC junction, I c1 and I c2 exhibit the strong dependence on the thickness of FM and changes their signs by crossing the 0–π transition point due to the magnetic exchange splitting between the up- and down-spin bands in FM. It is predicted that the Riedel peak in I c1 disappears at the 0–π transition and the 0–π transition occurs in I c2 similarly to I c1 .

Two-qubit gate based on a multiterminal double-barrier Josephson junction

Two-qubit gate based on a multiterminal double-barrier Josephson junction

A Particle Detector Based on a Double Barrier Josephson Junction

We analyze the performance of a particle detector based on a double-barrier Nb/Al/AlOx/Al/AlOx/Al/Nb junction where the Al trapping layer is embedded between two insulating AlO/sub x/ barriers. Within this layer, a very sharp Andreev bound state (ABS) level E/sub 0/ is formed. Because of a very long recombination time, the charge amplification factor is expected to be 10-100 times larger in the double-barrier detector compared to ordinary single-barrier devices. The double barrier detector also has much higher idle-state subgap resistance at low temperatures. Because the middle Al quasiparticle trap layer is separated from the adjacent layers by two insulating tunneling barriers, the proximity effect is minimized. This provides much better alignment between the maximum of nonequilibrium quasiparticle distribution function and the maximum in the electron density of states. All the factors listed result in much higher sensitivity and the energy resolution of the double barrier Nb-based STJ as compared to their single-barrier analogs.

Optimization of the Double-Barrier Josephson Junction Switching Dynamics

The switching dynamics of a double-barrier Josephson junction is analyzed as a function of the microscopic properties of its electrodes. In particular, it is found that the nonstationary behavior of the Josephson phase difference is very sensitive to dissipation mechanisms acting inside the intrinsic shunt. The leading factor that determines the dissipation is the local electron density of states N(E) inside the electrodes. The roles of junction geometry, electrode purity, and interface quality are discussed and how they affect the details of N(E), hence the resulting phase dynamics. The microscopic analyses allow optimization of the performance of double-barrier Josephson junction-based rapid-single-flux-quantum circuits in two ways: 1) decreasing the switching time of Josephson elements and 2) reducing the excess wiring. Such an analysis is facilitated with the aid of a lumped circuit representation which generalizes the nonlinear resistive-shunted-junction model.

Microscopic nonequilibrium theory of double-barrier Josephson junctions

We study nonequilibrium charge transport in a double-barrier Josephson junction, including nonstationary phenomena, using the time-dependent quasiclassical Keldysh Green's function formalism. We supplement the kinetic equations by appropriate time-dependent boundary conditions and solve the time-dependent problem in a number of regimes. From the solutions, current-voltage characteristics are derived. It is understood why the quasiparticle current can show excess current as well as deficit current and how the subgap conductance behaves as function of junction parameters. A time-dependent nonequilibrium contribution to the distribution function is found to cause a nonzero averaged supercurrent even in the presence of an applied voltage. Energy relaxation due to inelastic scattering in the interlayer has a prominent role in determining the transport properties of double-barrier junctions. Actual inelastic scattering parameters are derived from experiments. It is shown as an application of the microscopic model, how the nature of the intrinsic shunt in double-barrier junctions can be explained in terms of energy relaxation and the opening of Andreev channels.

A qubit device based on manipulations of Andreev bound states in double-barrier Josephson junctions

A qubit system exploiting manipulations of the Andreev bound state (ABS) levels in the SINIS junction by applying appropriate bias voltages and transport currents is suggested. The parameters of the SINIS setup may be chosen in such a way that only two ABS levels are present; this is in agreement with our experimental data obtained using Nb/Al double-barrier junctions. In the qubit Hamiltonian, H , the two ABS levels are presented as the ‘↑’ and ‘↓’ spin states, while the controlling physical parameters (voltage across one of the barriers and the transport current) are mapped to the ‘magnetic fields’ B x n and B z n. The phase decoherence time is estimated.

A phenomenological model for high-frequency dynamics of double-barrier(SINIS) Josephson junctions

Double-barrier Josephson junctions, with outer electrodes of superconductingNb and a central thin layer of non-superconducting Al, are being developed asself-shunted junctions for high-frequency superconducting integrated circuits.This paper presents a simplified phenomenological model of the behaviour ofsuch junctions (often called SINIS, forsuperconductor-insulator-normal-insulator-superconductor) based on theresistively shunted junction model, using Josephson coupling between thelayers and the time-dependent Ginzburg-Landau equation to describe thedynamics of the weak induced energy gap in the Al layer. This analysis showshow non-hysteretic I-V characteristics can be obtained but suggests somelimitations in their high-frequency performance. Predictions for criticalcurrent generally agree with prior experiments. The technological implicationsof this model are discussed.

Double-barrier Josephson junctions: theory and experiment

New theoretical and experimental results on double-barrier SIS'IS Josephson junctions are presented (I is a tunnel barrier, S' is a thin film with critical temperature lower than that of S). The previously developed microscopic model for the stationary case, which describes the critical currents in Nb/Al/Nb junctions, is extended to the non-equilibrium regime of finite voltage. In particular, an intrinsic shunting resistance is estimated from I-V curves. We formulate the requirements for interface barriers in order to realize non-hysteretic SIS'IS junctions with high critical current density and I/sub c/R/sub N/ products. A comparison with single-barrier SIS junctions with high critical current density is carried out.

Coherence effects in double-barrier Josephson junctions

The general solution for ballistic electronic transport through double-barrier Josephson junctions is derived. We show the existence of a regime of phase-coherent transport in which the supercurrent is proportional to the single barrier transparency and the way in which this coherence is destroyed for increasing interlayer thickness. The quasiparticle dc current at arbitrary voltage is determined.

Stationary properties of SINIS double-barrier Josephson junctions

This paper reports on the evaluation of the effective suppression parameter eff in double-barrier Josephson junctions realized in Nb-Al/AlxOy/Al/AlxOy/Al-Nb SINIS technology. This parameter describes the overall resistance of current flow across the interfaces within the whole structure configuration and characterizes its ICRN product attainable. By comparison with theoretical descriptions of tunnelling structures in the dirty-limit approximation, the effective suppression parameter eff has been extracted from the measured temperature dependence of the ICRN product. For SINIS junctions of a series of different wafer productions with various critical currents of the fabricated junctions, the evaluated values of eff range between 80 and 250. With reference to the proximity effect and to the possibility of removing the Al layers at the Nb electrodes, a value of eff has been deduced, which ranges between 50 and 120. This is very close to the assigned value between 10 and 20 ideally suitable for integrated circuit applications, e.g. Josephson voltage standards and rapid single flux quantum circuits.