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IEEE Transactions on Magnetics

Properties of superconducting weak links prepared by ion implantation and by electron beam lithography

E. P. Harris, Robert B. Laibowitz

Published Jan 1, 1977• 20 citations
Publisher

Abstract

In this paper, we present our latest results on the properties of superconducting weak links prepared in Mo films by ion-implantation and in Nb films by electron-beam lithography. In the case of the ion-implanted weak links, which structurally resemble proximity-effect bridges, we find that the static properties are quite well described by a recent theory of Likharev and Yakobson. We further find that the rf response of these weak links is in excellent agreement with quasistatic resistively-shunted-junction model calculations based on a sinusoidal current-phase relation at those temperatures for which such a current-phase relation is predicted by the static theory of Likharev and Yakob-son. For the e-beam fabricated Nb bridges, which have the Dayem bridge geometry, our sample preparation techniques have allowed us to produce bridges and two-junction interferometers in which the link dimensions are as small as about 2000 Angstroms. We find that Josephson effects are observable in these bridges even when the bridge dimensions exceed the temperature-dependent coherence length by about an order of magnitude, but are still less than the effective thin-film penetration depth.

DOI
10.1109/tmag.1977.1059315

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In this paper, we present our latest results on the properties of superconducting weak links prepared in Mo films by ion-implantation and in Nb films by electron-beam lithography. In the case of the ion-implanted weak links, which structurally resemble proximity-effect bridges, we find that the static properties are quite well described by a recent theory of Likharev and Yakobson. We further find that the rf response of these weak links is in excellent agreement with quasistatic resistively-shunted-junction model calculations based on a sinusoidal current-phase relation at those temperatures for which such a current-phase relation is predicted by the static theory of Likharev and Yakob-son. For the e-beam fabricated Nb bridges, which have the Dayem bridge geometry, our sample preparation techniques have allowed us to produce bridges and two-junction interferometers in which the link dimensions are as small as about 2000 Angstroms. We find that Josephson effects are observable in these bridges even when the bridge dimensions exceed the temperature-dependent coherence length by about an order of magnitude, but are still less than the effective thin-film penetration depth.
Amplification, down-conversion and detection of the microwaves in thin-film Josephson variable thickness bridges of submicron dimensions have been investigated experimentally. The results for indium and tin bridges are discussed taking into account the influence of the nonequilibrium electrons in a weak link region. The same experiments were carried out with SNS variable thickness Nb-Al-Nb bridges. They showed that the properties of SNS bridges agreed better with the theoretical predictions of the resistively shunted junction model in a wide temperature range. Moreover the parameters of the Nb-Al-Nb bridges have possessed better stability and reliability.
We report here numerical calculations on the properties of superconducting weak links as mixers with external local oscillators (LO). Our calculations are based on the resistively shunted-junction model. We assume that the LO and the signal are ideal current sources of amplitudes ILO and IS and of frequencies ωLO and ωs, respectively. We calculate the IF voltage across the weak link as a function of the bias current, the LO current amplitude ILO, and the normalized signal frequency Ωs = ħωs/(2 e Ic R), where Ic is the critical current and R is the shunt resistance. If ωIF/ωLO and Is/Ic are small, we find a linear IF response. Using this method we find the frequency dependence of the IF response, estimate the conversion efficiency for the matched case in both the absence and the presence of noise, and calculate the required LO power. We have fabricated thin-film constrictions by using electron lithography. We have made constrictions of indium films with thicknesses of ~ 1 200 A with widths as narrow as 2 000 A and lengths as short as 1 000 A and have studied their I-V characteristics in the presence of RF radiation and measured their X-band mixing properties. We have measured an NEP of 10-13 W/Hz1/2 and a dynamic range of 23 dB (3 dB deviation from linearity) and estimate that, with optimum matching, these would become 3 × 10-18 W/Hz1/2 and 74 dB, respectively.
An experimental study of small superconducting microbridges in order to examine parameter conditions (i.e., junction resistance and junction inductance) necessary for these junctions to exhibit the Josephson effect is described. Nb variable thickness junctions with very narrow width (<50 nm) and short length (<100 nm) were fabricated using electron beam lithography. The junctions which showed the Josephson effect were thinned by ion beam etching, and consequent changes of junction characteristics were measured. The experimental results indicate that the Josephson effect disappears at some critical resistance. These parameter limits of the Josephson junction are compared with theory.
The requirements of quantum metrology and nanoscience are driving the need for single-particle detection capability across a wide variety of physics areas, including quantum information processing, single-photon detection, nanoelectromechanical systems, nanomagnetism and spintronics. The single particles to be detected include atoms, molecules, photons, spins and even phonons, in the future. Nanoscale superconducting quantum interference devices (nanoSQUIDs) represent a new manifestation of an old but exciting superconducting technology which addresses some of these requirements. In this paper we describe a straightforward approach to fabricating Nb microbridge weak links using combined optical lithography and focused ion beams which may be used to fabricate nanoSQUIDs. The devices show non-hysteretic current?voltage characteristics and demonstrate very low noise, even at operating temperatures above 4.2?K. To improve our understanding of the superconducting properties of the Josephson microbridge/nanobridge junctions which have proved very successful in realizing low noise nanoSQUIDs we have carried out a combination of investigations. These include cryogenic resistance versus temperature (R(T)) and current?voltage characteristic measurements, atomic force microscope scans, controlled gallium (Ga) ion milling and implantation, and ion beam track modelling of these Josephson devices and Nb thin films.
While recent theoretical and experimental evidence have indicated that many commonly used superconducting weak links might have distinctly nonsinusoiual current-phase relations, precise measurement of these relations has been difficult to realize. Now a high resolution technique has been developed for the measurement of current-phase relations of weak links. This technique was used to determine the current-phase relations of various weak link structures including point contacts, thin film microbridges and proximity effect bridges. (GRA)
A new single flux quantum device is proposed, and, in conjunction with it, fabrication process of Josephson weak links using electron beam lithography and ion-etching is reported. Two types of niobium weak links, microbridges and thick-thin-thick type bridges, have been fabricated. They showed dc supercurrent, but microwave-induced steps were observed for thick-thin-thick type bridge only. The proposed logic device is constructed with a magnetically coupled rf-SQUID cells. These cells behave in the similar manner as Domino-toppling, and basic logic operations, AND, OR and NOT, are demonstrated by numerical calculations. Switching delay of about 10ps is expected to be practical.