JPH0455354B2 - - Google Patents

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 in 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 ground thin film and a Josephson circuit thin film provided on the ground 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. The temperature T of a perfect superconducting thin film with critical temperature Tc is from T>Tc to T
By lowering the magnetic field to <Tc, the weak magnetic field that initially penetrated the superconducting thin film is completely removed from the superconducting thin film by the Meissner effect. However, if the superconductivity of this superconducting thin film is impaired by impurities, lattice defects, etc., then T<
In the Tc state, the magnetic field is not completely exhausted from within the superconducting film and remains within the thin film as a trapped magnetic flux. When the magnetic field is sufficiently weak, the trapped magnetic flux is a quantized magnetic flux called vortex (magnetic flux quantum Φ ₀ = 2 × ^10-7 G/cm ² ). None of these are complete, and the IE Transactions on Magnetics (IEEE
Transactions on Magnetics) Vol.MAG−19,
No. 3, 1983, it is known that a magnetic flux quantum trap phenomenon occurs. The actual Josephson circuit is built on a grounded thin film to reduce magnetic coupling between each gate and between lines. However, if flux quanta are trapped in this grounded film (i.e., a vortex exists), and the trapped flux is coupled to the interferometer loop or the Josephson junction itself, then the Josephson integrated circuit Causes malfunction.
FIG. 2 shows the above state.

FIG. 2 is a schematic cross-sectional view of a superconducting thin film in which magnetic flux quanta are trapped. 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. The diameter of vortex 4 is approximately 50 nm, the cross section of the junction through which the magnetic field penetrates is approximately 300 nm x 5000 nm, the diameter of the interferometer is approximately 10000 nm, and the thickness of all films is approximately 300 nm.
It is. As shown in Figure 2, the trapped magnetic flux is
If it is coupled to Josephson junction 3, the Josephson current in that junction 3 will be small, and if it is coupled to interferometer loop 2, it will change the control characteristics of the interferometer gate. In either case, depending on the degree of magnetic coupling, the magnetic coupling may induce malfunction of the gate. Normally, Josephson integrated circuits operate in very low magnetic fields within a magnetic shield. The magnetic field within this type of magnetic shield is about 10 μG;
This traps about 5,000 magnetic flux quanta in the Josephson computer, which consists of multiple Josephson integrated circuit chips with a total chip area of 10cm x 10cm, causing the computer to malfunction. In reality, a computer with the above size can generate even more magnetic flux quanta (tens to hundreds of thousands) due to the current induced by thermoelectromotive force when cooling the Josephson computer. It is assumed that this will trap the data, and normal operation of the calculation under such an environment is almost impossible.

Conventionally, the following methods were used to initialize the circuit so as to avoid the influence of the magnetic flux trapped in the ground thin film. In that 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. This allows the magnetic flux vortices trapped in the ground thin film to move through the ground thin film in a superconducting state without being affected by the Josephson circuit thin film. As a method of keeping the grounded thin film as described above in a superconducting state and simultaneously making other superconducting thin films (Josefson circuit thin films) in a normal conducting state, for example, a superconducting thin film other than the grounded thin film is charged with a superconducting critical current higher than the superconducting critical current of that thin film. A method _of injecting a current _of
There is a method of selecting a material with a lower temperature than T _CJ < T < T _CG and setting the environmental temperature to T. In this state, when a current with a current density J (vortex drive current) is passed through the ground thin film, the Lorentz force F = J
×Φ ₀ acts on the vortex (Φ ₀ is a flux quantum and F, J, Φ ₀ are vector quantities). 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 _P exists in a superconducting thin film that prevents the vortex from escaping from its initial position, called the pinning center. The magnitude of this pinning force F _P depends 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>F _P must be satisfied. In other words, the vortex cannot be driven unless a current with a certain current density J _P =F _P /Φ ₀ or higher 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>J _P 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, 7 is the current flowing through the ground 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 in 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 H _C
(If it is a type 2 superconducting conductor, H _CI ) or more magnetic field is generated, and a new vortex is generated in the thin film. The critical current in this state is J _C =
Assuming KH _C (K is a constant), the ideal vortex drive current J becomes J _C > J > J _F. In reality, the current density in a superconducting thin film is high at the edges of the film and low at the center, but this does not affect the discussion of this patent, so all current densities are assumed to be uniform J.

Needless to say, the smaller the pinning force F _P of a thin film is, the more advantageous it is, but depending on the thin film, extreme conditions such as J _P >J _C may apply. Even if it is not extreme as described above, when the pinning force is large, the difference between J _P and J _C narrows, and the operating range of J that satisfies the condition J _P < J < J _C 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 thin film to move the vortex are provided on opposite edges of the ground thin film, and at least a portion of the ground thin film in an area where the Josephson circuit is not 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.

(Function) In order to widen the narrow operating region of the vortex drive 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. Assuming that the pinning force at this side is F _PE , even if the current density J is slightly higher than J _C , if J _PE = F _PE / Φ ₀ or more, the vortex that has entered from the thin film side will be absorbed by the large side It is fixed to the thin film edge by the pinning force F _PE ,
It cannot move towards the center of the thin film due to 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 force. It is a part of the ground thin film made of a material that has strength, and 1
0 is the pinning force. The anti-vortex has magnetic flux trapped in the opposite direction of the vortex.

Figure 4 shows that the current density increases when J>J _C >J _P or J>J _P
This is the vortex distribution when >J _C.
This is because vortexes and anti-vortexes constantly invade the membrane from the edges of the ground thin film, are driven to the center of the membrane by the Lorentz force, and collide with each other almost at the center of the membrane and disappear. It is in a state of flux. In Figure 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 Figure 4, the pinning force on the side of the membrane F _PE = Φ ₀ J _PE is J _PE ＞J＞J _P ＞J _C or
If J _PE > J > J _C > J _P , the state in which the vortex constantly flows can be avoided, and a state in which there is no vortex at the center of the thin film is shown.

(Example) An example of the present invention is shown in FIG. 1a and FIG. 1b. FIG. 1a is a top view of this embodiment, and FIG. 1b is a sectional view thereof. In both figures, 1 is a ground thin film, and 9 is a part of the ground thin film made of a superconducting material with a strong pinning force, such as a material for superconducting magnets. 11 is an area where Josephson circuits are distributed, 12 is a power supply, 13 is a current supply line, and 14 is a vortex drive current supply terminal. The grounding thin film in the distributed area 11 has a weak pinning force of the Josephson circuit to make it as easy to remove the vortex as possible, and a large pinning force is applied to the edges of the grounding thin film 1 to prevent the newly formed vortex from moving. Having power 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.

Using a grounding thin film with such a structure,
J _PE > J > J _C > J _P or J _PE > as shown in Figure 5
Even at current densities such as J > J _P > J _C , 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 the vortex can be removed even from the center of the ground thin film, which has a film quality that has a relatively strong pinning force against the Lorentz force.

[Brief explanation of the drawing]

FIG. 1a is a schematic top view of an embodiment of the present invention;
FIG. 1b is a cross-sectional view of the superconducting integrated circuit, and FIG. 2 is a schematic cross-sectional view showing the superconducting thin film of the Josephson integrated circuit in a state where magnetic flux quanta are trapped in the grounded thin film.
Figure 3 is a schematic cross-sectional view showing the state in which the vortex is removed from the center of the membrane by a vortex drive current that generates a Lorentz force greater than the pinning force, and Figure 4 shows the state in which the vortex is constantly flowing. Schematic sectional view. FIG. 5 is a schematic sectional view of a state where a portion with strong pinning force is arranged on the side of the ground thin film. 1...Ground thin film, 2...Interferometer loop, 3...
Josefson junction, 4... Vortex, 5...
Magnetic field lines, 6... anti-vortex, 7... current flowing through the ground thin film, 8... Lorentz force, 9... portion of the ground thin film made of material with large pinning force, 10... pinning force, 11 ...Area where Josephson circuits are distributed, 12...Power supply, 13...
...Current supply line, 14...Vortex drive current supply terminal.

Claims (1)

[Claims] 1. 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 located opposite the ground thin film. A superconducting thin film, which is provided on the edge and is made of a different material from the ground thin film in an area where the Josephson circuit is provided, is provided on at least a portion of the ground thin film in an area where the Josephson circuit is not provided. integrated circuit.