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Oxford University Press eBooks

Barriers in Josephson Junctions: An Overview

Martin P. Weides

Published Jul 6, 2017• 2 citations
Publisher

Abstract

This article considers Josephson junction barriers, focusing on barriers made from insulators, metals, semiconductors, magnets, and nanowires. The main characteristic of Josephson junctions is the local reduction or even suppression of the critical current in the barrier. These barriers affect the static and dynamics properties of Josephson junctions, including coupling strength, ground state, phase damping, and tunability of the critical current. The article first provides an overview of the fundamental physics of Josephson junctions, with particular emphasis on the Josephson effect, before describing the properties of two coupled superconductors. It then discusses tunnel barriers, metallic barriers, semiconducting barriers, and magnetic barriers.

DOI
10.1093/oxfordhb/9780198738169.013.15

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Improved high frequency coupling to Josephson tunneling junctions must involve impedance transformation techniques or the fabrication of junctions with more appropriate impedance values. Two kinds of calculations about efficiency of junction coupling with a cavity, one with an antenna and another with an inductive loop show clearly the importance of the term Ic r0/Γ where Γ is the capacitance, Ic the critical current and r0 a dimension related to the size of the junction. Several ways may be proposed to increase this « figure of merit ». It appears that, when considering only tunneling phenomena, there is an advantage to decrease the barrier potential height. A number of people have employed semiconductors as barriers because it is possible to use either a naturally narrow forbidden band, or to use the metal-semiconductor interface properties, to get small barrier heights. This paper is devoted to a review of experimental works on junctions using low barrier materials. Attempts have been made with various deposited materials such as Te, PbTe, CdS, CdSe, Ge, InSb, etc., ... But at the present time these trials have not yet been clearly successful. Many problems are related to the use of these materials, some of the more important being : presence of surface states, variation of doping with time, diffusion near the interface, pinholes and amorphous-polycrystalline behaviour. Moreover, these various phenomena can lead to several possible mechanisms for explaining conduction in superconductor-semiconductor-superconductor sandwiches. Some of these mechanisms are not compatible with good high frequency coupling.

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Physical review. B, Condensed matter•Feb 1, 1994•29 citations
This article considers Josephson junction barriers, focusing on barriers made from insulators, metals, semiconductors, magnets, and nanowires. The main characteristic of Josephson junctions is the local reduction or even suppression of the critical current in the barrier. These barriers affect the static and dynamics properties of Josephson junctions, including coupling strength, ground state, phase damping, and tunability of the critical current. The article first provides an overview of the fundamental physics of Josephson junctions, with particular emphasis on the Josephson effect, before describing the properties of two coupled superconductors. It then discusses tunnel barriers, metallic barriers, semiconducting barriers, and magnetic barriers.
Improved high frequency coupling to Josephson tunneling junctions must involve impedance transformation techniques or the fabrication of junctions with more appropriate impedance values. Two kinds of calculations about efficiency of junction coupling with a cavity, one with an antenna and another with an inductive loop show clearly the importance of the term Ic r0/Γ where Γ is the capacitance, Ic the critical current and r0 a dimension related to the size of the junction. Several ways may be proposed to increase this « figure of merit ». It appears that, when considering only tunneling phenomena, there is an advantage to decrease the barrier potential height. A number of people have employed semiconductors as barriers because it is possible to use either a naturally narrow forbidden band, or to use the metal-semiconductor interface properties, to get small barrier heights. This paper is devoted to a review of experimental works on junctions using low barrier materials. Attempts have been made with various deposited materials such as Te, PbTe, CdS, CdSe, Ge, InSb, etc., ... But at the present time these trials have not yet been clearly successful. Many problems are related to the use of these materials, some of the more important being : presence of surface states, variation of doping with time, diffusion near the interface, pinholes and amorphous-polycrystalline behaviour. Moreover, these various phenomena can lead to several possible mechanisms for explaining conduction in superconductor-semiconductor-superconductor sandwiches. Some of these mechanisms are not compatible with good high frequency coupling.
The Josephson effect, as an example of a macroscopic quantum phenomenon, reveals itself in the three-layer heterostructures formed by two superconductors coupled by a weak link that usually consists of a 1-2 nm-thick insulating barrier.The traditional way of modelling such-systems' dynamics is based on an equivalent circuit that comprises three parallel elements: a pure superconducting element with a certain supercurrent-versus-Josephson phase difference dependence, a resistor R, and a capacitor C. In this short review, we analyse the practical problem of reducing the junction capacitance while maintaining or slightly impairing other characteristics.Some arguments are presented to explain why the capacitance should be suppressed and how it will affect performance of superconducting quantum interference devices (SQUIDs) and digital electronics circuits.As a solution for low-capacitance junctions, we propose a weak link made of an amorphous-silicon interlayer doped with nanoscale metallic drops between the two superconducting Mo-Re-alloy electrodes.
The Josephson effect is a privileged access to the macroscopic quantum nature of superconductors. We review some ideas and experimental techniques on macroscopic quantum decay phenomena occurring in Josephson structures. The attention is mainly addressed to intermediate levels of dissipation which characterize a large majority of low critical current Josephson devices and are therefore an avoidable consequence of nanotechnology applied more and more to Josephson devices. Phase diffusion phenomena take over thermal activation in some temperature ranges also affecting the transition to macroscopic quantum tunneling, enriching the phase diagram mostly defined by the Josephson energy, the temperature and the level of dissipation.
The Josephson junction is a device and constructed by separating two superconductors with an insulating barrier, which is thin enough so that super-electrons or Cooper pairs, composed of two electrons with opposite momenta and spins, formed by the electron-phonon interaction, and described as probability waves, can quantummechanically tunnel through the barrier and cause superposition and interferences. Due to the quantum e↵ect, a Josephson junction can generate and sustain an indefinitely long direct electric current or potential di↵erence, which becomes alternating one when an external voltage is applied. This paper investigates the Josephson pulse including its generation, property, and control. The results obtained from this study indicate that perturbations of the superconductors can generate strong superconducting pulses in the Josephson junction. The electric potential and field in the barrier oscillate when the two superconductors have di↵erent physical properties including the number densities of super-electrons, the phases of superconductors, and their rates of variations. The oscillations can be harmonic, combined, and purely pulse-like. The strength of the pulse increases with the di↵erences in the rates of densities and phases between the two superconductors. The pulse can be stronger in strength and narrower in width when one superconductor has lower density but higher rates of density and phase than another superconductor. Light or electromagnetic waves can perturb the superconductivities of superconductors and hence can be applied to control the pulses of Josephson junctions.
Quantum computing based on superconducting qubits requires the understanding and control of the materials, device architecture, and operation. However, the materials for the central circuit element, the Josephson junction, have mostly been focused on using the AlO x tunnel barrier. Here, we demonstrate Josephson junctions and superconducting qubits employing two-dimensional materials as the tunnel barrier. We batch-fabricate and design the critical Josephson current of these devices via layer-by-layer stacking N layers of MoS 2 on the large scale. Based on such junctions, MoS 2 transmon qubits are engineered and characterized in a bulk superconducting microwave resonator for the first time. Our work allows Josephson junctions to access the diverse material properties of two-dimensional materials that include a wide range of electrical and magnetic properties, which can be used to study the effects of different material properties in superconducting qubits and to engineer novel quantum circuit elements in the future.
The transport properties of a small superconductor--normal-metal--superconducting tunnel junction can be controlled by a gate electrode coupled capacitively to the central island. We evaluate the critical Josephson current Ic{\mathit{I}}_{\mathit{c}}Ic​ through such a system as a function of the gate voltage Vg{\mathit{V}}_{\mathit{g}}Vg​ taking into account parity effects in the superconductors. The dependence Ic{\mathit{I}}_{\mathit{c}}Ic​(Vg{\mathit{V}}_{\mathit{g}}Vg​) shows resonant singularities and has a qualitatively different character for \ensuremath{\Delta}EC{\mathit{E}}_{\mathit{C}}EC​ and \ensuremath{\Delta}>EC{\mathit{E}}_{\mathit{C}}EC​, \ensuremath{\Delta} being the superconducting gap and EC{\mathit{E}}_{\mathit{C}}EC​=e2{\mathit{e}}^{2}e2/2C being the charging energy. Due to the sweeping of the gate voltage, the system can be driven to states which are metastable with respect to a change of the number of electrons on the central island. Since the lifetime of the metastable states can be macroscopically large, one can observe a remarkable bistability of the Josephson current. A weak magnetic field suppresses the interference of the electrons propagating through the central electrode, and, consequently, the magnitude of the critical current.