JPS62176179A - Josephson integrated circuit - Google Patents

Abstract

PURPOSE:To increase the upper limit of a vortex driving current flowing in a grounding thin film and to remove vortexes from the central part of the grounding thin film, by providing a superconductive thin film comprising a material different from the grounding thin film in a region, in which a circuit is provided at least one part of the grounding thin film in a region, where a Josephson circuit is not provided. CONSTITUTION:In a grounding thin film 1 in a region 11 wherein Josephson circuits are distributed, it is ideal to have a pinning forces 10 as follows: a weak pinning force in the Josephson circuit so that vortexes can be removed as easily as possible; and a large pinning force at the peripheral part of the grounding thin film 1 so that the newly formed vortexes are not moved. In order to realize these factors, the central part of the grounding thin film and its peripheral part are constituted with different superconductive metal materials. In general, when a current density is increased too much, a magnetic field higher than a superconductive critical magnetic field is generated at the peripheral part of a superconductive thin film, which is in parallel with a current in the thin film. Therefore an ideal vortex driving current J becomes JC>J> JF. When the grounding thin film having such a structure is used, the vortex can be normally removed from the central part of the grounding thin film even at a current density of JPE>J>JC>JP or JPE>J>HP>JC (a).

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to Josephson integrated circuits having means for removing magnetic flux trapped within a ground film.

(Prior Art) A Josephson integrated circuit is formed by forming a superconducting thin film on a substrate, and the superconducting thin film consists of a grounded thin film and a Josephson circuit thin film provided on the grounded thin film. This Josephson circuit thin film forms Josephson circuits such as Josephson junctions and interferometer loops.

The phenomenon of magnetic flux trapping in superconducting thin films has long been a problem that hinders the normal operation of Josephson integrated circuits made of superconducting thin films. Temperature T of a perfect superconducting thin film with critical temperature Tc
When T falls from T>Tc to T<Tc, the weak magnetic field that initially penetrated the superconducting thin film is completely eliminated from the superconducting thin film by the Meissner effect. However, if the superconductivity of this superconducting thin film is impaired to some extent by impurities, lattice defects, etc., the magnetic field will not be completely exhausted from within the superconducting film in the state of T<Tc, and the trapped magnetic flux will remain. remains in the thin film as When the magnetic field is sufficiently weak, the trapped magnetic flux is a quantized magnetic flux called a vortex (magnetic flux quantum Φ. = 2 × 10−
7G/cm2) None of the superconducting thin films produced by conventional methods are perfect;
Transactions on Magnetics (IEE)
E Transactions on Magneti
It is known that a trapping phenomenon of magnetic flux quanta occurs as described in Vol. MAG-19° No. 3.1983. Actual Josephson circuits are built on a grounded thin film to reduce magnetic coupling between each gate and between lines. However, if flux quanta are trapped within this grounded film (i.e., a vortex exists), and the trapped flux is coupled to the interferometer loop or to the Josephson junction itself, then the Josephson integration The circuit will malfunction. FIG. 2 shows the above state.

FIG. 2 is a schematic cross-sectional view of a superconducting thin film that traps magnetic flux quanta. In the figure, 1 is a ground thin film, 2 is an interferometer loop, 3 is a Josephson junction, 4 is a vortex (trapped magnetic flux quantum), and 5 is a magnetic field line. vortex 4
The diameter of the junction is approximately 50 nm, and the cross section of the junction through which the magnetic field penetrates is approximately 300 nm.
nmX5000nm, the diameter of the interferometer is approximately 110000n,
All film thicknesses are approximately 300 nm. If the trapped magnetic flux is coupled to the Josephson junction 3 as shown in Figure 2, the Josephson current in that junction 3 becomes small, and if it is coupled to the interferometer loop 2, the interferometer Changes the control characteristics of the gate. In either case, depending on the degree of magnetic coupling, the magnetic coupling may induce malfunction of the gate. Josephson integrated circuits typically operate in very low magnetic fields within a magnetic shield.

The magnetic field within this type of magnetic shield is about 101 IG, which traps about 5000 magnetic flux quanta in a Josephson computer consisting of several Josephson integrated circuit chips with a total chip area of 10 cm x 10 cm, for example. This may cause the computer to malfunction. In reality, a computer of the above size would trap even more (tens to hundreds of thousands) of magnetic flux quanta due to the current induced by thermoelectromotive force when cooling the Josephson computer. Normal operation under such an environment is almost impossible.

Conventionally, the following methods have been used to initialize the circuit so as to avoid the influence of the magnetic flux trapped in the ground thin film. In this method, first, the superconducting thin film other than the ground thin film, that is, the Josephson circuit thin film, is brought into a normal conducting state. By doing this, the magnetic flux vortex trapped in the ground thin film can move through the ground thin film in a superconducting state without being affected by the Josephson circuit thin film. As a method for keeping the grounded thin film as described above in a superconducting state and at the same time making other superconducting thin films (Josephson circuit thin films) in a normal conducting state, for example, a superconducting thin film other than the grounded thin film is charged with the superconducting critical current of that thin film. The method of injecting the above current, or the superconducting critical temperature TCJ of the material of the superconducting thin film other than the grounding thin film, is the superconducting critical temperature T of the material of the grounding thin film. . Choose a material with a lower value than T < T < T. 0
There is a method of setting the environmental temperature J degrees to the temperature T. In this state, when a current with a current density J (vortex driving current) is passed through the ground thin film, a Lorentz force F=JXΦ0 acts on the vortex (Φ. is a magnetic flux quantum, and F, J, and Φ. are is a vector quantity). This Lorentz force drives the vortex within the ground thin film in a direction perpendicular to the vortex drive current.

It is known that a pinning force F exists in a superconducting thin film that prevents the vortex from escaping from its initial position, called the pinning center. The pinning force F and its magnitude depend on the material, film quality, film thickness, etc. of the thin film. Therefore, in order to drive the vortex in the thin film by the Lorentz force F, the condition F>FP must be satisfied. In other words, a certain current density JP=FP/Φ. The vortex cannot be driven unless more current is passed through the thin film.

FIG. 3 is a schematic cross-sectional view of the superconducting ground thin film when a current density J of J>JP is applied to the superconducting ground thin film. In Figure 3, 1 is the ground thin film, 4 is the vortex, 6 is the anti-vortex, and 7
is the current flowing through the grounded thin film, and 8 is the Lorentz force. The anti-vortex has magnetic flux trapped in the opposite direction of the vortex. As can be seen from FIG. 3, in such a case the vortex or anti-vortex is removed from the center of the ground membrane.

(Problem to be solved by the invention) However, if the current density is increased too much, the superconducting critical magnetic field H6 (H61 in the case of a type 2 superconducting conductor) or more will be generated at the side of the superconducting thin film parallel to the current in the thin film. A magnetic field is generated and a new vortex is generated within the thin film. The critical current in this state is J. :KHo (K is a constant), the ideal vortex drive current J is J>J>J. In reality, the current density in a superconducting thin film is high at the edges of the film;
Although the current density is lower in the center, it is not discussed in this patent, so the current density is assumed to be uniform J.

Needless to say, the smaller the pinning force F of the thin film is, the more advantageous it is.
Some have extreme conditions.

Even if it is not extreme as mentioned above, if the pinning force is large, J, and J. The difference between JP and J is reduced, and JP<J<J. The operating range of J that satisfies this condition becomes narrower.

(Means for Solving the Problems) Means provided by the present invention to solve the above-mentioned problems is a Josephson integrated circuit in which a Josephson circuit is provided in a certain area on a ground thin film, Terminals to which a current is supplied to apply a Lorentz force to the vortex of the ground thin film to move the vortex are provided on opposing edges of the ground thin film, and terminals of the ground thin film in areas where the Josephson circuit is not provided are provided. The Josephson integrated circuit is characterized in that a superconducting thin film made of a different material from the ground thin film in the region where the Josephson circuit is provided is provided in at least a portion thereof.

(Function) In order to widen the narrow operating region of the vortex driving current density J, in the present invention, a region made of a material with a particularly strong pinning force is provided on the side of the superconducting thin film parallel to the current in the thin film. make a profit. If the pinning force on this side is FPg, the current density J is J even if it is a little. Even if it is above, JPE
” FPE'Φ. If this is the case, the vortex that entered from the edge of the thin film will be fixed to the edge of the thin film due to the large pinning force FPE on the edge, and will not be able to move toward the center of the thin film due to the Lorentz force. .

FIGS. 4 and 5 illustrate each of the above situations. Each of these figures is a cross-sectional view of a superconducting ground thin film, where 1 is the ground thin film, 4 is the vortex, 6 is the anti-vortex, 7 is the current flowing through the ground thin film, 8 is the Lorentz force, and 9 is the large pinning. 10 is the pinning force. The anti-vortex has magnetic flux trapped in the opposite direction of the vortex.

Figure 4 shows that as the current density increases, J>J>J or J>J,>
This is the vortex distribution in the case of P Jo. This is due to the constant flow of vortices and anti-vortexes that constantly invade the membrane from the edges of the ground thin membrane, are driven to various parts of the membrane by the Lorentz force, and collide with each other in the center of the membrane and disappear. It is a state of In Fig. 5, a part 9 made of a material with strong pinning force is placed on the side of the ground thin film, and even at the current density as shown in Fig. 4, the pinning force F, E = ΦOJPE is J, E>J>J
,>Jo or JPE>J>Jo>J, the state in which the vortex constantly flows can be avoided, and a state in which no vortex exists at the center of the thin film is shown.

(Example) An example of the present invention is shown in FIG. 1(a) and FIG. 1(b).

FIG. 1(a) is a top view of this embodiment, and FIG. 1(b) is a sectional view thereof. In both figures, 1 is a grounding thin film, and 9 is a part of the grounding thin film made of a superconducting material with a strong pinning force, such as a material for superconducting magnets. 11 is the area where the Josephson circuit is distributed, 12 is the power supply, 13 is the current supply line, 14
is the vortex drive current supply terminal. Distribution area 1
The grounding thin film 1 has a weak pinning force of the Josephson circuit so that the vortex can be removed as easily as possible, and a large pinning force on the sides of the grounding thin film 1 so that the newly formed vortex does not move. is ideal. In order to meet these requirements, the present invention is characterized in that the center and side parts of the ground thin film are made of different superconducting metal materials.

If a ground thin film with such a structure is used, J as shown in FIG. > J > Jo > J, or J,
Even at current densities such as F, > J > JP > Jo, the vortex can be successfully removed from the center of the ground thin film.

(Effects of the Invention) As explained above, by using the present invention, it is possible to increase the upper limit of the vortex drive current that is passed through the ground thin film in order to remove the vortex in the Josephson circuit. Therefore, the operating margin of the vortex drive current becomes larger, and vortex can be removed even from the central part of the ground thin film, which has a film quality that has a relatively strong pinning force against the Lorentz force.

[BRIEF DESCRIPTION OF THE DRAWINGS] FIG. 1(a) is a schematic top view of an embodiment of the present invention.
Figure (b) is a step view, Figure 2 is a schematic cross-sectional view showing the superconducting thin film of a Josephson integrated circuit in which magnetic flux quanta are trapped in the grounded thin film, and Figure 3 is a pin-up diagram. A schematic cross-sectional view showing the state in which the vortex is removed from the zero part of the film by the vortex drive current that generates the Lorentz force on the retaining force. Figure 4 is a schematic cross-sectional view showing the state in which the vortex is constantly flowing. , FIG. 5 is a schematic cross-sectional view of a state in which a portion with strong pinning force is arranged on the side of the ground thin film. 1 Ground thin film 2 Interferometer loop 3 Josephson junction 4 Portex 5 Magnetic field lines 6 Anti-vortex 7 Current flowing through the ground thin film 80 - Lenz force 9 Part of ground thin film made of material with large pinning force 10 Pinning force 11 Joseph Area where the son circuit is distributed 12 Power supply 13 Current supply line 14 Vortex drive current supply terminal 1 bottle) @ξjitei 5 pictures

Claims (1)

[Claims] In a Josephson integrated circuit in which a Josephson circuit is provided in a certain area on a ground thin film, a terminal to which a current is supplied that applies a Lorentz force to the vortex of the ground thin film to move the vortex is connected to the ground thin film relative to the ground thin film. A superconducting thin film is provided on the edge of the ground where the Josephson circuit is not provided, and at least a portion of the ground thin film in the area where the Josephson circuit is not provided is provided with a superconducting thin film made of a different material from the ground thin film in the area where the Josephson circuit is provided. Josephson integrated circuit.