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Modification of Tunnel Junction Resistance in Josephson Devices

IP.com Disclosure Number: IPCOM000079975D
Original Publication Date: 1973-Oct-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 3 page(s) / 38K

Publishing Venue

IBM

Related People

Matiso, J: AUTHOR

Abstract

These Josephson devices allow independent adjustment of single particle tunnel resistance below the gap voltage, thereby permitting proper memory cell operation. Specifically, these structures permit adjustment of single particle tunneling resistance in Josephson tunneling devices, where the adjustment is independent of current density which, in turn, is primarily controlled by the tunnel barrier properties (mainly the thickness). This is achieved by changing the number of electronic states in the superconducting energy gap of the electrodes, using proximity effects in dissimilar metal film layers.

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Modification of Tunnel Junction Resistance in Josephson Devices

These Josephson devices allow independent adjustment of single particle tunnel resistance below the gap voltage, thereby permitting proper memory cell operation. Specifically, these structures permit adjustment of single particle tunneling resistance in Josephson tunneling devices, where the adjustment is independent of current density which, in turn, is primarily controlled by the tunnel barrier properties (mainly the thickness). This is achieved by changing the number of electronic states in the superconducting energy gap of the electrodes, using proximity effects in dissimilar metal film layers.

A memory cell using Josephson tunneling devices is basically an RLC circuit. For proper memory operation sufficient damping must be provided to critically damp the circuit. The most important resistance in this case is the resistance R(j) below the gap 2 delta.

In good tunnel junctions, this resistance is determined by the operating temperature and the density of states of the superconducting electrodes, as well as by the overall current density level of the junction (which is in turn determined by R(nn), the resistance of the junction in the normal state). Since the operating temperature is fixed, one way to design a circuit is to choose the dimensions and current density appropriate to provide critical damping. Another way is to provide external resistors. However, each of these techniques has its disadvantages. With the first technique, design freedom is restricted rather severely, while the second technique does not always work well, particularly with nonlinear junctions. Additionally, the use of external resistors tends to use considerable area.

The proposed technique allows alteration of the ratio R(j)/R(nn) independently of Josephson current density J(1). Additionally, it is compatible with fabrication technology and does not reduce the current density. This technique modifies the density of states of the electrodes (either both or one) by the proximity effect. Therefore, the current voltage characteristic of an ideal Josephson junction (shown in Fig. 1A) can be altered to the smeared characteristic shown in Fig. 1B.

Two structures for achieving this are shown in Figs. 2 and 3. These structures utilize a Josephson junction comprised of superconducting electrodes and an additional metal layer which alters the density of states in an electrode. For instance, Fig. 2 shows a metal layer M2 located beneath the base electrode M1. A tunnel barrier separates the base electrode and the counter electrode M3. The base electrode M1 is a superconductor while the metal M2 is conveniently a normal metal or a weak superconductor. These metal layers can be used in any combination of superconductors or metals, and their thicknesses can be varied to provide different degrees of...