JPS61287180A - Josephson integrated circuit - Google Patents

Abstract

PURPOSE:To eliminate an effect by magnetic flux trapped by a grounding thin-film by arranging conductor films in a shape that paths for vortex driving currents flowing through the grounding thin-film among terminals are narrowed or current paths among the terminals and a grounding terminal are interrupted when transition control currents under a normal conductive state flow through the conductor films. CONSTITUTION:Currents IN are made to flow through vortex driving current control lines 2, grounding thin-film 1 sections magnetically coupled with the control lines 2 are brought to a normal conductive state, and other grounding thin-film 1 sections are brought to a superconductive state. When a Josephson circuit except the grounding thin-films 1 is brought to the normal conductive state under the state and DC currents are injected into the grounding thin-films 1 from a vortex driving-current supply terminal 3, vortexes are moved in the grounding thin-films 1 by Lorentz's force. The grounding thin-films 1 are divided by double pectinate partition walls under the normal conductive state manufactured by the control lines 2 on the injection of driving currents, and currents injected from the terminal 3 move in a zigzag direction and flow in the grounding thin-films 1. Accordingly, vortexes are shifted by a small quantity of driving currents, and can be concentrated to the peripheral sections of the grounding thin-films and the peripheries of the control lines 2, thus avoiding an effect by magnetic flux trapped by the grounding thin-films.

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 and Problems Therewith) 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 consists of Josephson junctions, interferometer loops, wiring, etc.

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
is T〉τ0 to T. By lowering the magnetic field to 100%, the 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 to some degree by impurities, lattice defects, etc., the magnetic field will not be completely exhausted even within the superconducting film in the Tc state, and the trapped magnetic flux will be remains in the thin film as When the magnetic field is sufficiently weak, the trapped magnetic flux is a fully quantized magnetic flux called a vortex (magnetic flux quantum Φm −2X 10− 'G/
an') None of the superconducting thin films produced by conventional methods are perfect, and the
Transactions on Magnetics
) Vol, ! l! AG (9, No. 3, 19
It is known that the trapping phenomenon of magnetic flux quanta occurs as described in 83. Actual Josephson integrated circuits are built on a grounded thin film to reduce magnetic coupling between each gate and between lines. However, if a flux quantum is trapped within this ground thin extension (i.e., a vortex exists), and the trapped flux is coupled to the interferometer loop or to the Josephson junction itself, then the Josephson FIG. 4 shows the above-mentioned situation in which the integrated circuit malfunctions.

Figure 4 is a schematic cross-sectional view of a superconducting thin film that traps magnetic flux quanta. ), 9 indicates magnetic field lines, vortex 8
The diameter of is approximately 50nff1, and the cross section of the junction through which the magnetic field penetrates is approximately 3
00no+85000nm, the diameter of the interferometer is approximately 10000nm
No+, the film thickness is all about 300 nm.

41st! ! ! When the magnetic flux trapped like l is coupled to the Josephson junction 7, the Josephson current in that junction 7 becomes small, and when it is coupled to the interferometer loop 6, the interferometer gate resulting in a change in the control characteristics, in any case depending on the degree of magnetic coupling that magnetic coupling may induce gate malfunctions. Normally the operation of Josephson integrated circuits is very low in magnetic shielding. It is carried out in a magnetic field. The magnetic field within this type of magnetic shield is approximately 10 μG;
There are approximately 500 Josephson integrated circuit chips in a Josephson computer with a total chip area of
This traps zero magnetic flux quanta and causes the combination unit to malfunction. In reality, due to the current induced by thermoelectromotive force when cooling the Josesoter Combi-21, a computer with the size mentioned above will require even more (
It is assumed that magnetic flux quanta (from several layers to tens of blades) will be trapped, and normal operation in such an environment is almost impossible.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a Josephson integrated circuit in which the effects of magnetic flux trapped in a grounded thin film can be avoided.

(Means for Solving the Problems) In order to solve the above-mentioned problems, the Josephson integrated circuit provided by the present invention includes a plurality of terminals that supply a vortex drive current to a ground thin film, and a plurality of terminals that supply a vortex drive current to a ground thin film, and A conducting wire film is provided for passing a current to control the flow path of the drive current, and the conducting wire film is disposed on the upper surface or lower surface of the ground thin film with an insulating film interposed therebetween, and the ground thin film is in a superconducting state. The conductor film has a current capacity that allows the control current to flow in a magnitude sufficient to transition the ground thin film in the vicinity of the conductor film to a normal conduction state at a certain time, and the normal conduction state transition control current flows through the conductor film. When the vortex drive current is flowing through the ground thin film between the terminals, the conductive wire film is arranged in such a way as to narrow the path of the vortex drive current flowing through the ground thin film between the terminals or to block the current path between the terminal and the ground terminal. It is characterized by

(effect) Next, the operation of the present invention will be described.

In the present invention, in order to initialize so as to avoid the influence of the magnetic flux trapped in the ground thin film, first the superconducting thin film other than the ground thin film, that is, the Josephson circuit thin film, is brought into a normal conduction 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 di5-Sefson circuit thin film. As a method for keeping the ground thin film as described above in a superconducting state and at the same time making other superconducting thin films (di5-Sefson circuit thin films) in a normal conducting state, for example, a superconducting thin film other than the ground thin film is made to have its superconducting criticality. A method of injecting a current higher than the current, or a material whose superconducting critical temperature 'fcJ is lower than the superconducting critical temperature XCa of the material of the grounding thin film is selected as the material of the superconducting thin film other than the grounding thin film, τc, <'r<
There is a method of setting the environmental temperature to a temperature T called τca. In this state, a current (with current density J) in the ground thin film (
When a vortex drive current) is applied, Lorentz force F-JX
Φ acts on the vortex (Φ is a magnetic flux quantum and F, J, Φ are vector quantities), and this Lorentz force causes the vortex to move in the ground thin film in a direction perpendicular to the vortex driving current. is driven towards. There are two directions of the trapped magnetic flux, upper and lower, and these are driven in opposite directions. Therefore, the vortex concentrates on the edges of the ground thin film and stops there. What should be noted in this process is that the magnetic field induced on the surface of the ground thin film by the current flowing through the superconducting thin film never exceeds the critical magnetic field He of the material composing the ground thin film (if the material is a type 2 superconductor, This means that the critical magnetic field 11c,) must not be exceeded. If the above-mentioned critical magnetic field is exceeded, a new vortex will be generated in the ground thin film, so there is an upper limit JMayo for the current density J. On the other hand, when various lattice defects exist in the ground thin film, a binding force acts on the vortices captured by the lattice defects.The binding force is the force that prevents the vortices from escaping from the lattice defect, which becomes the binding center. It is. The magnitude of the bottle-holding force F is determined by the properties of the thin film, and in general, films that are close to single crystals and have a low impurity concentration have a weak bottle-holding force.If the vortex is moved by the Lorentz force F, F > F
After going through the process described above, the Josephson circuit thin film, which had been set to a normal conductive state, is returned to a superconducting state by controlling current, temperature, etc., and then it becomes trapped in the grounded thin film. A Josephson integrated circuit without the influence of magnetic flux is obtained. In addition, when the Josephson circuit thin film transitions to a superconducting state in the final process, the external magnetic field is already concentrated around the ground thin film as explained above, so the magnetic field near the Josephson circuit thin film is Trapping of the magnetic flux into the very weak Josephson circuit thin film itself is also very unlikely.

In the Josephson integrated circuit of the present invention, a conductor film through which a current for controlling the vortex drive current in the ground thin film passes is disposed on the upper or lower surface of the ground thin film via an insulating film.

By passing a current through this conductive wire film, a large magnetic field can be created near the conductive wire film, and this magnetic field can destroy the superconducting state of the ground thin film that is locally coupled to the conductive wire film.

That is, the ground thin film near the conducting wire membrane can be placed in a normal conduction state, and when a vortex drive current is passed through the ground thin film at that time, the current flows avoiding the portion of the ground thin film that is in the normal conduction state. In other words, by carefully arranging the conductor film, the flow of the vortex drive current can be freely controlled. Therefore, in the present invention, the path of the vortex drive current flowing through the ground thin film between the vortex drive current supply terminals is narrowed, or the vortex drive current supply terminal and the ground terminal (provided on the ground thin film, and the ground thin film is connected to the chip mounting board). A conductive film is placed in such a way as to block the current path between the terminal (used to connect to the ground plane). By arranging the conductive wire film in this manner, it is possible to flow a control current through the conductive wire film, increase the density of the vortex drive current, and thereby increase the Lorentz force acting on the vortex.

(Embodiment) A first embodiment of the present invention is shown in FIG. 1. In FIG. (equivalent to the membrane), 3 is the vortex drive current supply terminal, 4 is 1t
It is R. Figure $1 is a top view of a Josephson integrated circuit chip. Although the Josephson circuit membrane is not shown in this figure, it is fabricated on top of membrane 1. By passing a current through the vortex drive current control 1112, the part of the ground thin film 1 that is magnetically coupled to the control line 2 can be brought into a normal conduction state, and the other parts of the ground thin film 1 can be kept in a superconducting state. . In such a state, as described in the operation section above, if the Josephson circuits other than the ground thin film 1 are brought into normal conduction state, and then a DC current is injected into the ground thin film 1 from the vortex drive current supply terminal 3, The vortex moves within the ground thin film 1 by the Lorentz force F. As mentioned before, unless the condition F-JXΦ,〉F is satisfied, the vortex cannot be moved by the bottle-holding force F. The magnitude of the bottle-holding force is determined by the quality of the thin film, but the worst case is In this case, the above condition J×Φ, >F cannot be satisfied unless the current density is J-IXIO°A / If it is passed through a thin film, the total current is 15
It becomes a large current of A. Flowing such a large current into a superconducting system increases heat generation in the normal conducting parts of the system, and may even destroy the superconducting state. Another major problem is the inflow of heat into the system due to large diameter electric wires used to introduce such a large current into the system from the outside. According to this embodiment, when a vortex drive current is injected, the ground thin film 1 and the control line 2 are divided by the double comb-shaped normal-conducting partition walls, and the current completely injected from the current supply terminal 3 is divided by the control line 2. 1, it flows in a meandering manner through the ground thin film 1, as indicated by the arrows in FIG. Comparing the case where the current flows as shown by the arrow in Figure 1 at a constant current density and the case where there is no partition wall in the normal conduction state, the current value in the former case is only a few times lower than that in the latter case, Figure 1. For example, the vortex drive current can be reduced to one-fifth of that without the partition wall. According to this embodiment, the vortex is moved by a small amount of vortex drive current, and this is transferred to the ground thin film side and The vortex drive current can be concentrated around the control line 2.

Although the number of combs is two on each side in this embodiment, this is not the essence of this embodiment, and the greater the number, the smaller the vortex driving current is required. Furthermore, although this embodiment has been explained using a vortex drive current control line with a double comb-like structure as an example, this double comb-like structure is not the essence of this embodiment. Even if the vortex drive current control line is arranged and the vortex drive current is passed in a spiral pattern, the vortex on the entire chip ground thin film can be moved with a small current. The current density J is increased by dividing it with a control line that connects to the vortex, and the vortex is moved with as small a vortex drive current as possible. After the vortex drive current is applied, the control current A is made zero to complete the initialization of the embodiment.

A second embodiment of the present invention is shown in FIG. 2, 1 to 4 in FIG.
is the same as that in FIG. 1, and 5 is a ground terminal. In an actual Josephson integrated circuit, a plurality of grounding terminals 5 are used as shown in Figure 2, and the grounding thin film 1 on the chip and the card (
It is electrically connected to the ground plane on the chip mounting board). In FIG. 2, the ground terminal 5 is grounded to the ground plane on the card.

Ideally, this grounding terminal 5 should be superconducting. If such a superconducting grounding terminal exists, the vortex drive current supply terminal 3 will be connected to the ground 'f# film 1 on the chip.
When a current is supplied to the ground thin film 1 on the chip, a large amount of the current also flows to the superconducting ground plane on the card via the ground terminal. In order to cause the current to flow through the card, an extra current must be passed through the card.The fact that extra current is passed through the circuit system that utilizes the superconducting state was also explained in the first embodiment. 0 As in this embodiment, by passing a current through the vortex drive current control line 2 to separate the ground terminal 5 and the vortex drive current supply terminal 3, it is possible to This can help prevent current leakage.

In other words, as in the first embodiment, when a current is applied to the control I12 to bring the ground thin film in its vicinity into a normal conduction state, and then a vortex drive current is applied, the current flows through the ground thin film 1 on the chip as shown from No. 2150. Flows only inside. In addition, the material of this embodiment is to electrically separate the ground terminal and the vortex drive current supply terminal by the vortex drive current control line, and the method of separation, the number of each terminal, etc. are as shown in Figure 2. It is not something that can be taken as a composition.

A third embodiment of the present invention is shown in FIG.
This embodiment is a combination of the first embodiment and the second embodiment, and has all the features of these two embodiments. In other words, it is possible to move the vortex with a relatively small amount of vortex drive current, and at the same time, it has a structure that prevents this current from leaking to the card side.
The reason why this embodiment is shown individually as shown in FIG. 3 is to show that the first and second embodiments of the present invention can be realized simultaneously and are not mutually exclusive.

Note that if the first, second, and third embodiments are modified and the vortex drive current control line 2 is buried under the ground thin film 1, the control line 2 does not appear on the surface and becomes a uniform superconductor. A ground thin film is obtained, and the Josephson circuit thin film can be easily placed in its place.

(Effects of the Invention) According to the present invention, by a small vortex drive current,
Magnetic flux trapped in the ground thin film of an unpackaged Josephson integrated circuit chip or a mounted Josephson integrated circuit chip on a card can be driven. Therefore,
According to the present invention, a Josephson integrated circuit can be provided in which the influence of magnetic flux trapped in a ground thin film can be avoided.

[Brief explanation of drawings]

1, 2, and 3 are schematic plan views showing the first, second, and third embodiments of the present invention, respectively, and FIG. 4 shows a state in which magnetic flux quanta are trapped in the ground thin film. 1 is a schematic cross-sectional view showing a superconducting thin film of a Josephson integrated circuit in FIG. DESCRIPTION OF SYMBOLS 1... Ground thin film on chip, 2... Vortex drive current control line, 3... Vortex drive current supply terminal, 4... Power supply, 5... Ground terminal, 6... Interferometer loop , 7...Josephson junction, 8...Vortex (trapped magnetic flux), 9...Magnetic field lines.

Claims (1)

[Claims] A plurality of terminals for supplying a vortex drive current to the ground thin film, and a conductor film for passing a current that controls the flow path of the vortex drive current in the ground thin film are provided, and the conductor film is on the top or bottom surface of the ground thin film. is disposed through an insulating film to allow the control current to flow in a magnitude sufficient to transfer the ground thin film in the vicinity of the conducting wire film to a normal conductive state when the ground thin film is in a superconducting state. When the current capacity is in the conductive wire film and the normal conduction state transition control current is flowing through the conductive wire film, the path of the vortex drive current flowing through the ground thin film between the terminals is narrowed or the terminal and ground are connected. A Josephson integrated circuit characterized in that the conductor film is arranged in such a way as to block a current path between the conductor film and the terminal.