Dismiss
InnovationQ will be updated on Sunday, Oct. 22, from 10am ET - noon. You may experience brief service interruptions during that time.
Browse Prior Art Database

Well-Cooled Edge Structure for Superconducting Junction Devices

IP.com Disclosure Number: IPCOM000060852D
Original Publication Date: 1986-May-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 3 page(s) / 46K

Publishing Venue

IBM

Related People

Gallagher, WJ: AUTHOR [+3]

Abstract

A superconducting edge-junction structure is provided with a unique trilayer base electrode. The conductive trilayer structure consists of a sandwich of three metal films. The center film is relatively thin, and the tunnel barrier formed on this film is thinner than that formed on the outer films. The structure is shown in Fig. 1. An example of materials for the structure is Nb for the outer two films and NbN for the middle film. An edge junction is constructed using this base electrode. The flow of tunneling current into or out of the three film sandwich will be completely dominated by the thin tunnel barrier to the thin middle layer and the current to the outer layers will be negligible due to the exponential dependence of current on oxide thickness.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 39% of the total text.

Page 1 of 3

Well-Cooled Edge Structure for Superconducting Junction Devices

A superconducting edge-junction structure is provided with a unique trilayer base electrode. The conductive trilayer structure consists of a sandwich of three metal films. The center film is relatively thin, and the tunnel barrier formed on this film is thinner than that formed on the outer films. The structure is shown in Fig. 1. An example of materials for the structure is Nb for the outer two films and NbN for the middle film. An edge junction is constructed using this base electrode. The flow of tunneling current into or out of the three film sandwich will be completely dominated by the thin tunnel barrier to the thin middle layer and the current to the outer layers will be negligible due to the exponential dependence of current on oxide thickness. However, the current leaving the junction region can fan out into the outer two films, as can the flux of diffusing non-equilibrium quasiparticles and phonons.

The trilayer edge-junction structure is thus much better "cooled" than a conventional edge junction. The advantages of the structure are illustrated in the following comparison of the removal of non-equilibrium species from the junction region in planar junctions, in standard edge junctions, and in this trilayer edge- junction structure. In comparison with planar structures, in addition to allowing simple fabrication of small junctions, the standard edge structure has other advantages over planar structures including: allowing high junction current densities without exceeding the thin film superconductor critical current densities, and providing for efficient removal of non-equilibrium quasiparticles and phonons from the region of the junction. The first point is illustrated in Figs. 2 and 3. Current is represented by the arrows, which illustrate crowding on entering the base electrode of a planar device, as shown in Fig. 2, in which the current density in the thin films can be considerably higher than that of the junction, so that film critical current densities can be exceeded even though the lower junction current density is not. This situation is avoided in an edge junction, as shown in Fig. 3, in which film current densities are less than or equal to junction current densities. In general, critical current densities are higher in films than in junctions. Thus, edge structures eliminate one major barrier to the use of high current density junctions. When a junction supports a voltage drop in excess of the average energy gap of the two superconductors, pair-breaking occurs and non- equilibrium quasiparticles that are essentially unpaired electrons, are created at a rate proportional to the current density in the junction. When the local quasiparticle density exceeds a critical value, a transition out of the superconducting state occurs. Thus, in a high current density junction, the superconducting electrodes can be driven normal even though the critical curren...