Superconductive junction

Details Superconductive junction Superconductive junction

1996-09-23

1998-10-13

Whitehead, Jr., Carl W.

Active solid-state devices (e.g., transistors, solid-state diode

Thin active physical layer which is

Tunneling through region of reduced conductivity

257 34, 505234, 505238, H01L 2906, B32B 0000

Patent

active

058215565

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to superconductive junctions and more particularly to superconductive junctions fabricated from a plurality of superconducting layers.
2. Discussion of Prior Art
Superconducting Josephson junctions fabricated from metal films are well known in the prior art and are reviewed by K. K. Likharev in Review of Modern Physics (1979), Volume 51, page 101 et seq.. Josephson junctions in metal superconductors conventionally comprise a tri-layer structure of superconductor, insulator, and superconductor, with the insulator forming a tunnel barrier. Microbridges comprising a link of constrained dimensions between two regions of a single layer of superconducting material are also known to produce junctions with Josephson-like properties. If a microbridge is of the order of the superconducting coherence length wide, an Anderson Dayem microbridge results. If a microbridge is of the order of the London penetration depth in size, the microbridge is known as a Likharev microbridge. The Josephson-like properties of a Likharev microbridge arise as a result of vortex flow rather than Josephson tunnelling. Josephson junctions incorporating tunnel barriers are distinguished from microbridges. Likharev microbridges are not true Josephson junctions but they do have many of the properties of Josephson junctions. The differences between Anderson Dayem microbridges in which the dimensions of the microbridge normal to a direction of current flow through the microbridge are comparable with the superconducting coherence length and Likharev microbridges in which the dimensions are comparable with the superconducting penetration depth are discused by Aslamazov and Larkin in Soviet Physics JETP (1975), Volume 41, Number 2, pages 381-386 and by others elsewhere.
U.S. Pat. No. 4,539,741 describes a Josephson junction element having a first layer of superconductor and a second layer of superconductor overlaying the first layer with an insulating layer disposed between the two superconducting layers. The two superconducting layers are electrically connected by a weak link which may be, inter alia, any kind of superconducting material, copper, or other normal metal. The weak link has a length equal to the thickness of the insulating layer in a range from a few hundred to a few thousand .ANG.. The superconducting layers are made of superconducting metallic elements or their alloys. The width, length, and thickness of the weak link are such that an Anderson Dayem microbridge results with a sin .phi. current phase characteristic.
Oxide superconductors having critical temperatures higher than those of metals or intermetallic compounds were discovered in 1986. The term critical temperature in the field of superconductivity means the temperature at which a material loses all electrical resistivity. The fabrication of reliable reproducible Josephson junctions in thin films of oxide superconductors is less straightforward than the fabrication of junctions in metal superconductors. The difficulties arise from the conditions required to deposit the thin films, the superconducting parameters of the oxide superconductors, and their physical and chemical sensitivity to processing techniques.
A variety of techniques for fabricating Josephson junctions in oxide superconductors have been described in the literature. Grain boundary junctions may be formed by patterning tracks into a superconducting film deposited over a grain boundary of a substrate, as described by D. Dimos et at. in Physical Review Letters (1988), Volume 61, page 2476 et seq. Grain boundary junctions may also be fabricated by depositing a superconducting film over a step in a substrate. Grain boundary junctions do not have sufficient reproducibility for use other than in the most simple electrical circuits. Attempts have been made to fabricate tri-layer junctions incorporating an insulating tunnel barrier layer though these have not been successful. The tunnel barrier must be very thin, of the order 20 .ANG.,

REFERENCES:
patent: 4224630 (1980-09-01), Kroger
patent: 4263603 (1981-04-01), Jillie, Jr.
patent: 4366494 (1982-12-01), Ohta
patent: 4539741 (1985-09-01), Ohta

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