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Revue de Physique Appliquée

Physical processes in proximity microbridges

James E. Mercereau

Published Jan 1, 1974• 5 citations
Publisher

Abstract

Weakly superconducting circuits have been developed - based on the proximity effect in hard superconductors - which show Josephson-like phenomena over a wide range of electrical and superconducting parameters. Characteristics of these circuits will be discussed and a simple physical model will be presented to account for a modification of the Josephson phenomena at high current density in these devices. Response of the devices at high frequency will be emphasized with special emphasis on detection processes and the Dayem-Wyatt effect.

DOI
10.1051/rphysap:019740090104700

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•Jan 1, 2013•2 citations
Weakly superconducting circuits have been developed - based on the proximity effect in hard superconductors - which show Josephson-like phenomena over a wide range of electrical and superconducting parameters. Characteristics of these circuits will be discussed and a simple physical model will be presented to account for a modification of the Josephson phenomena at high current density in these devices. Response of the devices at high frequency will be emphasized with special emphasis on detection processes and the Dayem-Wyatt effect.
Techniques have been developed with which it is possible to fabricate superconducting thin-film structures with a wide variety of electrical and superconducting parameters which show Josephson-like phenomena. These bridges-based on the proximity effect-are made in layered thin-film ``substrates'' using a large variety of superconducting and nonsuperconducting materials. In this paper we detail the fabrication techniques and the electrical and superconducting characteristics for these proximity effect bridges including a simple low-frequency (<10 GHz) equivalent circuit based on analysis and experiment which closely describes the quantum phase coherence operation of the bridges in their voltage sustaining state. Extension of these fabrication techniques to entire thin-film electrical circuits including resistors, capacitors, and inductors with these bridges and leading to an entire superconducting microcircuit technology is discussed.
Experimental investigations on the proximity effect bridge (a Josephson device) at zero voltage and at finite voltages in the µV range are reported. The phase-super current relation at zero voltage was measured using an asymmetric superconducting quantum interferormeter circuit. The data are in agreement with the Josephson supercurrent-phase relation I_S=I_C sinδ with deviation less than 5% of the critical current I_c. The supercurrent density in the measured bridges reached as high as 50-100 µA/µm^2. Using microcircuitry techniques, proximity effect bridges were strongly coupled to superconducting microstrip resonators. Selfinduced steps in the I-V characteristics of bridges coupled to resonators were observed in the GHz region at voltages (frequencies) corresponding to the expected modes of the resonators. Two types of steps were seen depending on whether the resonator impedance on resonance was much higher or much smaller than the bridge resistance. A simple two fluid model of the bridge-resonator circuit was developed and the size and shape of self-induced steps were calculated for a generalized Josephson oscillator relation I_S = I_c(l-q + q sin∫ 2e/hV dt) where q = 1 corresponds to the original Josephson relation and q = 1/2 represents the phase slip regime. At low critical currents (I_c < 10 µA) and low voltages (V < 3µV) the size and shape of experimentally observed self-induced steps agree with the q = 1 model. At higher voltages and/or critical currents the step size increasingly deviates from the q = 1 model towards q = 1/2. These observations are interpreted to indicate a progressive reduction of the amplitude of the oscillating Josephson supercurrent in proximity effect bridges from I_c towards I_c /2 as the critical current and/or voltage are increased.
Static electric fields have a negligible influence on the electric and transport properties of a metal because of the screening effect. This belief was extended to conventional metallic superconductors. However, recent experiments have shown that the superconductor properties can be controlled and manipulated by the application of strong electrostatic fields. Here, the authors review the experimental results obtained in the realization of field-effect metallic superconducting devices exploiting this phenomenon. The authors start by presenting the pioneering results on superconducting Bardeen–Cooper–Schrieffer wires and nanoconstriction Josephson junctions (Dayem bridges) made of different materials, such as titanium, aluminum, and vanadium. Then, the authors show the mastering of the Josephson supercurrent in superconductor-normal metal-superconductor proximity transistors, suggesting that the presence of induced superconducting correlations is enough to see this unconventional field-effect. Later, the authors present the control of the interference pattern in a superconducting quantum interference device, indicating the coupling of the electric field with the superconducting phase. The authors conclude this review by discussing some devices that may represent a breakthrough in superconducting quantum and classical computation.
Although the two superconducting phase transitions manifest in the same manner, just one aspect of which is the abrupt disappearance of DC electrical resistance shown in Figure 1.1, scientists speculated they are probably a result of unlike physical processes.This claim is nowadays well accepted in the scientific community, there being much evidence about the different nature of the superconducting phase of high-T c and low-T c materials (even in the absence of an exact microscopic model for the former).Indeed, a few key characteristics differentiate the two groups of superconductors, the most prominent being the already mentioned unconventional order parameter symmetry.It is a feature superficially similar to anisotropy in crystals and results in a number of interesting effects distinctive to high-T c superconductors.The aspects of the unconventional order parameter symmetry relevant to superconducting circuits, which is the main focus of this thesis, are presented in Chapter 2.Chapter 4 describes the improvements to the hybrid YBCO-Nb fabrication process and discusses the results of the implemented changes that, together with the knowledge gained about particular procedures, show a positive trend in process robustness.Also presented are electromagnetic simulation techniques modelling electrical parameters of the superconducting thin films as well as comparison with experimentally obtained values.Subsequently, the same Chapter briefly addresses the experimental difficulties with magnetic shielding, where measurements were Superconducting (Josephson) Networks
The recent interest in developing highly controllable quantum bits (qubits) based on superconducting circuits makes it necessary to get a deeper understanding of the physics of small Josephson junctions. Such devices have to be wisely engineered and well isolated from the noisy environment to observe non-classical physics. In this thesis, a real-time noise analysis was implemented by an autocorrelation calculation to identify the type of environmental effects affecting a superconducting circuit. This was used during switching current measurements of small Josephson junctions to track the effects of unwanted signals and identify their frequency components. The temperature dependence of the switching current distribution was used to further characterise the small Josephson junctions. Furthermore the fabrication of smal1 Josephson junctions is further developed by analysing the reliability and reproducibility of them. A systematic approach is shown to solve typical fabrication problems for example identifying and reducing mechanical stress between resist layers. As Josephson junctions show quantum mechanical effects on a macroscopic scale they are used in a wide range of applications especially in superconducting circuits.