WO1996009654A1

WO1996009654A1 - A method and device for improving the performance of thin-film josephson devices in magnetic fields

Info

Publication number: WO1996009654A1
Application number: PCT/AU1995/000617
Authority: WO
Inventors: Graeme John Sloggett
Original assignee: Commonwealth Scientific And Industrial Research Organisation; The Broken Hill Proprietary Company Limited
Priority date: 1994-09-21
Filing date: 1995-09-20
Publication date: 1996-03-28
Also published as: AUPM832994A0

Abstract

A shielded superconducting device (11) includes a thin-film superconducting device, such as a Josephson device (12), and an adjacent superconducting shield (13). The device is arranged to operate as superconducting device when operated at a temperature below the critical temperature of the superconducting material chosen for the device. By virtue of the adjacent location of the Josephson device and the shield element, the Josephson device is partially shielded or totally shielded from an external magnetic field (14) applied to the superconducting device, due to the Meissner effect whereby magnetic flux is wholly or partly excluded from penetrating through the shield.

Description

A METHOD AND DEVICE FOR IMPROVING THE PERFORMANCE OF THIN- FILM JOSEPHSON DEVICES IN MAGNETIC FIELDS Introduction

The present invention relates generally to an active superconducting device and more particularly to improving the performance of thin-film Josephson devices in magnetic fields. Background of the Invention

In active superconducting devices which incorporate Josephson Junctions, it is often the case that the magnetic field in the region of the device is sufficiently strong that it impinges upon the ability of the device to operate in the desired manner.

It is a characteristic of such devices that their operation will suffer if the field to which they are subjected, is greater than a critical level and in the extreme case the device in question may even cease to function in the desired manner. Summary of the Invention The present invention consists in a device arranged to operate as a superconducting device when operated at a temperature below a critical temperature, including a superconducting thin-film device and a superconducting shield, wherein the superconducting thin-film device is located adjacent to, but separate from the superconducting shield such that the thin-film device is partially or totally shielded from a magnetic field in the region of the device.

In one preferred form of the invention the superconducting thin-film device is a Josephson junction device made of material selected from high or low critical temperature superconducting materials, and including one or more junctions selected from grain boundary, bicrystal, biepitaxial, step-edge, multilayer and other junction types.

In a first embodiment of this invention, the shield element is formed as a piece of bulk superconducting material and is separated from the thin-film Josephson device which is formed on a substrate.

In a second embodiment of the invention the shield is made from thin-film superconducting material formed on the same substrate as the thin-film Josephson device and separated from it by a thin insulating layer.

In a third embodiment of the invention, the shield is made from thin-film superconducting material grown on a separate substrate to the Josephson device and the two substrates are placed adjacent to one another, such that the shield element and the Josephson device are on opposite sides of an intervening insulating layer or gap.

Embodiments of this invention are particularly useful in ameliorating the present limitation(s) which apply to active superconducting devices like Josephson Junctions and SQUIDS when operating in magnetic fields, by reducing the magnetic field strength impinging on the active device, thereby enhancing the performance of the device when used in magnetic fields. Brief Description of the Drawings The features and advantages of the present invention will become apparent from the following description of the preferred embodiments thereof, by way of example only, with reference to the accompanying drawings:

Figure 1 is a schematic sectional view of one embodiment of the present invention indicating the magnetic field around the shield and thin-film Josephson device;

Figure 2a is a schematic top view of an thin-film Josephson device; Figure 2b is a cross-sectional side view of figure 2a;

Figure 3a is a section view wherein the Josephson device is placed on a substrate and the shield is a bulk superconductor;

Figure 3b is a section view wherein the Josephson device is placed on the same substrate as the shield;

Figure 3c is a section view, wherein the Josephson device and shield element are each placed on a separate substrate;

Figure 4a is a graph of the junction critical current as a function of the applied magnetic field for an unshielded step edge junction; and

Figure 4b is a graph of the junction critical current as a function of the applied magnetic field for a shielded step edge junction. Detailed description of the Preferred Embodiments

With reference to Figure 1, the device 11 comprising a thin-film Josephson device 12 and a shield element 13 is arranged to operate as a superconducting device 11 when operated at a temperature below the critical temperature of the superconducting material chosen for the device. The Josephson device 12 is arranged to be located near the shield element 13, such that the Josephson device 12 is partially shielded or totally shielded from an external magnetic field 14 applied to the superconducting device. The shielding arises from the well-known Meissner effect whereby magnetic flux is wholly or partly excluded from penetrating through the shield element 13. The Josephson device 12 and shield element 13 are separated and the plane of the largest surface of the thin-film Josephson device 12 (hereinafter referred to as the Josephson device plane) is substantially parallel to the largest surface of the shield element 13 (hereinafter referred to as the shield element plane) . The device 11 is also capable of operating under conditions where the Josephson device plane and the shield element plane are not parallel.

The thin-film Josephson device 12, shown in Figure 2a and 2b, has a pair of thin film superconducting regions 15, 16, joined together by a thin non-superconducting barrier 17 hereinafter referred to as a weak link. The pair of thin film superconducting regions 15, 16 in the region of the weak link 17 and the weak link itself will together be referred to as the junction 18.

With reference to Figure 2 the thin-film Josephson device 12 is arranged to operate as part of an electrical circuit, with predetermined Junction current flowing through the junction 18. This junction current has a critical value which is the threshold current above which the dc voltage across the junction is non-zero. It is known that a magnetic field applied substantially perpendicularly to the thin film Josephson device plane will be focussed and thread the junction reducing the critical current value or modulating said critical current.

Figure 3a shows the first embodiment of the present invention. The Josephson device 22 is grown on a substrate 29 and is separated from a high critical current density shield element 23 by an insulating gap 20. The shield element 23 comprises either a high critical temperature or a low critical temperature bulk piece of superconducting material. The high critical temperature bulk piece of superconducting material can be for example a single crystal or in a melt-textured form. The shield element 23 is of sufficient thickness 25 and the plane 21 is of large enough area with respect to the gap 20 and the Josephson device 22 to provide the required amount of shielding of the magnetic field onto the Josephson device 22.

In a second embodiment of the present invention, the shield element 33 comprises a thin-film superconducting material grown on the same substrate 39 as the Josephson device 32 using multi-layer thin film deposition and standard lithographic methods. The shield element 33 is separated from the Josephson device 32 by a thin insulating layer 30, in a trilayer structure as illustrated by Figure 3b.

Figure 3c shows a third embodiment of the invention. The shield element 43 is grown on a separate substrate 44 to the Josephson device substrate 45 and the two substrates 44, 45 are placed adjacent, such that the shield element plane 41 and Josephson device plane 48 are facing each other on opposite sides of an insulating layer or gap 40 in what is known as a flip chip configuration. The patterning of the shield element may be achieved by either standard lithographic techniques or by dicing the substrate.

Figure 4 shows a set of graphs of the critical current as a function of the applied magnetic field for a high critical temperature yttrium-barium-copper oxide step-edge junction Josephson device. In Figure 4a the critical junction current is shown for a high critical temperature yttrium-barium-copper oxide step-edge junction Josephson device, without the presence of a shield element, as a function of the applied magnetic field. In figure 4b the critical current is shown as a function of an applied magnetic field for a high critical temperature yttrium-barium-copper oxide Josephson device with a high critical temperature yttrium-barium-copper oxide shield element. The critical junction current reaches a first null at an applied magnetic field of at least one order of magnitude higher than the first null in the critical junction current seen in Figure 4a. The one order of magnitude difference in the applied magnetic field between yttrium-barium-copper oxide step-edge junction with shield Figure 4a and the same step-edge junction without a shield element demonstrates that the presence of shield element allows for the successful operation of the step-edge junction at applied magnetic fields to at least one order of magnitudes higher. It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

CLAIMS:-

1. A device arranged to operate as a superconducting device when operated at a temperature below a critical temperature, including a superconducting thin-film device and a superconducting shield, wherein the superconducting thin-film device is located adjacent to, but separate from the superconducting shield such that the thin-film device is partially or totally shielded from a magnetic field in the region of the device.

2. The device of claim 1 wherein the shield is formed as a piece of bulk superconducting material and the thin- film superconducting device is formed on a substrate the thin film superconducting device being separated from the shield.

3. The device of claim 2 wherein the shield is mounted on the substrate such that the substrate provides separation of the shield and the thin-film superconducting device.

4. The device of claim 1 wherein the shield is formed of thin-film superconducting material the shield and thin- film superconducting device being formed on one substrate and separated from one another by a thin insulating layer.

5. The device of claim 4 wherein the substrate forms the thin insulating layer, the shield and the thin-film superconducting device being formed on opposite sides of the substrate.

6. The device of claim 1 wherein the shield is formed of thin-film superconducting material, the shield and thin-film superconducting device being formed on separate substrates, and the two substrates being located adjacent to one another, such that the shield and the thin-film superconducting device are on opposite sides of an intervening insulating layer or gap.

7. The device as claimed in any one of claims 1 - 6 wherein superconducting thin-film device is a Josephson junction device made of material selected from high or low critical temperature superconducting materials, and includes one or more junctions selected from grain boundary, bicrystal, biepitaxial, step-edge, and multilayer junctions.