JPH0525191B2 - - 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 and its Problems) 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 consists of Josephson junctions, interferometer loops, wiring, etc.

The phenomenon of magnetic flux traps 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 T C is from T>T _C to T
By lowering T C to <T _C , 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 extent by impurities, lattice defects, etc., the magnetic field will not be completely exhausted from within the superconducting film in the state of T < T _C and will become trapped. The magnetic flux remains within the thin film. 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. Actual Josephson integrated circuits are 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. 4 shows the above state.

FIG. 4 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, 6
is an interferometer loop, 7 is a Josephson junction, 8 is a vortex (trapped magnetic flux quantum), and 9 is a magnetic field line. The diameter of the vortex 8 is approximately 50 nm, the cross section of the junction through which the magnetic field passes 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. If the trapped magnetic flux is coupled to Josephson junction 7 as shown in Figure 4, then the junction 7
The Josephson current becomes smaller, and its coupling with the interferometer loop 6 causes a change in 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 10μG;
This traps approximately 5,000 magnetic flux quanta in the Josephson computer, which consists of multiple Josephson integrated circuit chips with a total chip area of 10 cm, 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.

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 for supplying a vortex drive current to a ground thin film, and a plurality of terminals for supplying a vortex drive current to a ground thin film. A conducting wire film is provided for controlling the current flow path, and the conducting wire film is disposed on the upper surface or the lower surface of the ground thin film with an insulating film interposed therebetween, and the ground thin film is in a superconducting state. In some cases, the conductor film has a current capacity that allows the control current to flow in a magnitude sufficient to transfer the ground thin film in the vicinity of the conductor film to a normal conduction state, and the normal conduction state transition control current is applied to the conductor film. When flowing, the conductor film is arranged in such a way that it narrows the path of the vortex driving current flowing through the ground thin film between the terminals or blocks the current path between the terminal and the ground terminal. Features.

(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 vortices 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.
Keeping the grounded thin film mentioned above in a superconducting state and at the same time keeping other superconducting thin films (Josefson circuit thin film)
For example, a method of injecting a current higher than the superconducting critical current of the thin film into a superconducting thin film other than the grounded thin film, or a method of making the material of the superconducting thin film other than the grounded thin film into a normal conduction state, or
There is a method of selecting a material whose T _CJ is lower than the superconducting critical temperature T _CG of the grounding thin film material and setting the environmental temperature at a temperature T such that T _CJ < T < T _CG . 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. There are two directions of the trapped magnetic flux, upper and lower, and these are driven in opposite directions. The vortex then concentrates on the edges of the ground membrane and stops there. What should be noted in this process is that the magnetic field H induced on the surface of the ground thin film by the current flowing through the superconducting thin film never exceeds the critical magnetic field H _C of the material composing the ground thin film (if the material is In the case of conduction, this means that the lower critical magnetic field H _C1 ) must not be exceeded. Above the critical magnetic field, a new vortex is created in the ground film. Therefore, there is an upper limit to the current density J.
J _Max exists. On the other hand, if various lattice defects exist in the ground thin film, a pinning force acts on the vortices captured by the lattice defects. The pinning force is the force that prevents the vortices from escaping from the lattice defect that serves as the pinning center. Pinning force F _P
The size of the thin film is determined by the properties of the thin film, and in general, the closer the film is to a single crystal and the lower the impurity concentration, the weaker the pinning force. When moving a vortex with Lorentz force F, F > F _P must be satisfied. After going through the above process, 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. This results in a Josephson integrated circuit that is free from the effects of magnetic flux trapped in the grounded film. 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 extremely weak. It is also very unlikely that the magnetic flux will be trapped in the Josephson circuit thin film itself.

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 brought into a normally conducting 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 normally conducting 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,
Narrow the path of the vortex drive current flowing through the ground thin film between the vortex drive current supply terminals, or connect the vortex drive current supply terminal and the ground terminal (provided on the ground thin film, and connect the ground thin film to the ground plane of the chip mounting board). A conductor film is placed in such a way as to block the current path between the terminals used for the 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.

(Example) A first example of the present invention is shown in FIG. In FIG. 1, 1 is a grounding thin film in the Josephson integrated circuit chip of this embodiment, 2 is a vortex drive current control line (corresponding to the above-mentioned conducting wire film), 3 is a vortex drive current supply terminal, and 4 is a power supply. FIG. 1 is a plan view of a Josephson integrated circuit chip. Although the Josephson circuit thin film is not shown in this figure, it is fabricated on top of the thin film 1. By passing a current I _N through the vortex drive current control line 2, the part of the ground thin film 1 that is magnetically coupled to the control line 2 is made to be in a normal conduction state, and the other part of the ground thin film 1 is kept in a superconducting state. I can do it. In such a state, as described in the operation section, if the Josephson circuit other than the ground thin film 1 is brought into the 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 is the Lorentz force F
It moves within the ground thin film 1 by. As mentioned before, the vortex cannot be moved by the pinning force F _P unless the condition F=J×Φ ₀ >F _P is satisfied. The magnitude of the pinning force is determined by the quality of the thin film, but in the worst case J = 1 × 10 ⁶ A/cm ²
Unless the current density is such that the above J×Φ ₀ >
The condition F _P cannot be satisfied. For example, if this current density is simply passed through a grounded thin film with a width of 50 nm and a thickness of 300 nm, the total current will be a large current of 15 A.
Flowing such a large current into a superconducting system increases heat generation in the normal conducting parts of the system.
Destruction of the superconducting state may also occur. 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 is divided by the double comb-shaped normally conducting partition wall created by the control line 2, and the current injected from the current supply terminal 3 is 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 at a constant current density as shown by the arrow in Figure 1 and the case where there is no partition wall in the normal conduction state, the current value in the former case is only a fraction of that in the latter case. Taking FIG. 1 as an example, the vortex drive current can be reduced to one-fifth of that without the partition wall. As described above, according to this embodiment,
A small amount of vortex drive current can move the vortex and concentrate it around the ground thin film edge and the vortex drive current 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 less vortex drive current is required. Furthermore, although this embodiment has been explained using a vortex drive current control line having a double comb-like structure as an example, this double comb-like structure is not the essence of this embodiment. Even if a vortex drive current control line is arranged and the vortex drive current is passed in a spiral pattern, the vortex on the entire trap grounding thin film can be moved with a small current. The essence of this embodiment is to increase the drive current J by dividing the ground thin film with a control line that is magnetically coupled to the ground thin film, and to move the vortex with as small a vortex drive current as possible. After the vortex drive current is applied, the control current I _N is made zero to complete the initialization of the embodiment.

A second embodiment of the invention is shown in FIG. In FIG. 2, 1 to 4 are the same as those in FIG. 1, and 5 is a ground terminal. In an actual Josephson integrated circuit, as shown in FIG. 2, a plurality of ground terminals 5 are used to electrically connect the ground thin film 1 on the chip to the ground plane on the card (chip mounting board). In FIG. 2, the ground terminal 5 is grounded to the ground plane on the card.
Ideally, this ground terminal 5 should be superconducting. If such a superconducting ground terminal exists, when a current is supplied from the vortex drive current supply terminal 3 to the ground thin film 1 on the chip, the current will pass through the ground terminal to the superconducting ground plane on the card. Also flows in large quantities. In other words, in order to cause the required current density J to flow through the ground thin film 1 on the pin in order to drive the vortex, an extra current must be applied to the card. As explained in the first embodiment, it is undesirable to cause an extra current to flow through a circuit system that utilizes a superconducting state. 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 prevent the current from leaking onto the card as described above. I can do it. In other words, the first
As in the embodiment shown in FIG. 2, when a current is applied to the control line 2 to bring the ground thin film in the vicinity into a normal conduction state, and then a vortex drive current is applied, the current flows into the ground thin film 1 on the chip, as can be seen from FIG. Limited flow. The essence 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 shown in FIG. It is not limited to such a configuration.

A third embodiment of the invention is shown in FIG. Each component in FIG. 3 is the same as that in FIGS. 1 and 2. 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 the first, second and third embodiments are modified so that the vortex drive current control line 2 is connected to the ground thin film 1.
By embedding it in the lower part of the superconductor, a uniform superconducting ground thin film is obtained in which the control line 2 is not exposed on the surface, and the Josephson circuit thin film can be easily placed on top of the superconducting ground thin film.

(Effects of the Invention) According to the present invention, magnetic flux trapped in the ground thin film of an unmounted Josephson integrated circuit chip or a Josephson integrated circuit chip mounted on a card can be driven by a small vortex drive current. Therefore, according to the present invention, a Josephson integrated circuit can be provided in which the effects of magnetic flux trapped in the 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 source, 5...Ground terminal, 6...Interferometer loop, 7...Josephson junction, 8...Vortex (trapped) (magnetic flux), 9... Lines of magnetic force.

Claims (1)

[Claims] 1 A plurality of terminals for supplying a vortex drive current to a 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 arranged on the upper surface of the ground thin film or The control current is disposed on the lower surface with an insulating film interposed therebetween, and has 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 conducting wire film has a current capacity that allows the current to flow, and the normal conduction state transition control current is flowing through the conducting wire film, the path of the vortex drive current flowing through the ground thin film between the terminals is narrowed or A Josephson integrated circuit characterized in that the conductor film is arranged in such a way that it blocks a current path between it and a ground terminal.