JPH0525190B2 - - 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 from superconducting thin films. The temperature _T of a perfect superconducting thin film with critical temperature T C is from T>T _C to T
<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<Tc, but will be trapped. The remaining 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 the superconducting thin films produced by conventional methods are perfect, and
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 in which magnetic flux quanta are trapped. In the figure, 10 is a ground thin film, 6 is an interferometer loop, 7 is a Josephson junction,
8 is vortex (trapped magnetic flux quantum),
9 indicates lines of magnetic force. The diameter of vortex 8 is approx.
50nm, the cross section of the junction through which the magnetic field penetrates is approximately 300nm x
5000nm, the diameter of the interferometer is about 10000nm, and the thickness of all films is about 300nm. When the trapped magnetic flux is coupled to Josephson junction 7 as shown in FIG. bring about changes in characteristics. 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 in this type of magnetic shield is about 10 μG, which generates about 5000 magnetic flux quanta in a Josephson computer, which consists of several Josephson integrated circuit chips with a total chip area of 10 cm x 10 cm. This can cause the computer to become trapped and malfunction. In reality, a computer with the size described above has even more magnetic flux quanta (tens to hundreds of thousands) due to the current induced by the 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) Means provided by the present invention to solve the above-mentioned problems is a Josephson integrated circuit comprising a ground thin film that is a superconducting thin film, the ground thin film being a first a plurality of thin films comprising a first thin film having a superconducting critical temperature and a second thin film having a second superconducting critical temperature lower than the first superconducting critical temperature, and supplying a vortex driving current to the first thin film; terminals are provided, and the second thin film is provided with the vortex drive that flows across the first thin film between the terminals when the ambient temperature is intermediate between the first and second superconducting critical temperatures. It is characterized in that it is arranged in such a way that the current path is narrowed or the current path between the terminal and the ground terminal is blocked.

(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. 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 for keeping the grounded thin film as described above in a superconducting state and at the same time 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. There is a method in which a current is injected, or a method in which the environmental temperature is set to the superconducting critical temperature T of the material of the superconducting thin film other than the ground thin film, T _CJ < T < T _CG . In this state, when a current with current density J is passed through the ground thin film, 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 membrane in a direction perpendicular to the ground membrane 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 grounding thin film by the current flowing through the superconducting thin film never exceeds the critical magnetic field H _C of the material composing the grounding 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 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, the ground thin film is made of two different materials having different superconducting critical temperatures, and this operates on the following principle. When the environmental temperature of the circuit is set between the two critical temperatures, the first thin film among the ground thin films maintains a superconducting state, but the second thin film forming the other part remains superconducting.
The thin film becomes a normal conductor. The second thin film in the normal conducting state becomes a barrier for the flow of superconducting current in the process of passing a superconducting current through the first thin film and driving a vortex therein. In other words, when a vortex drive current is passed through a grounded thin film, a normal conduction transition occurs, and by devising the shape of the second thin film, the flow of this current can be controlled. . In this invention, the second thin film narrows the superconducting current path in the first thin film and increases the current density J, thereby making it easier for the Lorentz force F to become larger than the pinning force F _p . .
Note that under normal circuit operating conditions in which both types of ground thin films have undergone superconducting transition, the second thin film does not pose any problem in signal transmission.

(Example) A first example of the present invention is shown in FIG. 1 in Fig. 1 is a thin film (corresponding to the above-mentioned first thin film) that forms the main part of the ground thin film of the Josephson integrated circuit chip;
Reference numeral 2 designates a vortex drive current barrier having a superconducting critical temperature lower than that of the thin film 1 (corresponding to the aforementioned second thin film), 3 a vortex drive current supply terminal, and 4 a power source. FIG. 1 is a plan view of the Josephson integrated circuit chip of this embodiment, in which a thin film 1 and a partition wall 2 form a ground thin film. The Josefson gate is not shown in this figure, but is made on top of the thin film 1. The superconducting critical temperature of thin film 1 is T _CG ,
If the superconducting critical temperature of partition wall 2 is T _CG ′, then T _CG >
T _CG ′. Therefore, the environmental temperature T of the circuit is T _CG ＞
When T>T _CG ', the partition wall 2 becomes a normal conduction state,
The other large part of the grounded membrane (film 1) becomes superconducting. In such a state, as mentioned in the section on the effect, if the Josephson circuit thin film is brought into a normal conduction state and then a direct current is injected into the ground thin film from the vortex drive current supply terminal 3, the vortex will be caused by the Lorentz force. and move within the ground thin film. As mentioned before, the vortex cannot be moved by the pinning force unless the condition F=J×Φ ₀ > _FP is satisfied. The magnitude of the pinning force is determined by the quality of the thin film, but in the worst case J = 1
Unless the current density is ×10 ⁶ A/cm ² , the above condition J×Φ ₀ >F _P cannot be satisfied. For example, if this current density is simply passed through a ground thin film with a width of 5 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, 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 is divided by the double comb-shaped partition wall 2 in the normal conduction state, and the current injected from the current supply terminal 3 is as indicated by the arrow in FIG. It flows in a meandering manner through the ground thin film as if it were flowing. Comparing the case where the current flows in this manner at a constant current density with 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 2. As described above, according to this embodiment, it is possible to move the vortex using a small amount of vortex drive current and to concentrate it around the ground thin film side and the vortex drive current partition wall. In this embodiment, the number of combs is two on each side, but this is not the essence of this embodiment; the greater the number, the smaller the vortex driving current is required. Furthermore, although this embodiment has been explained using a vortex driven current barrier wall having a double comb-like structure as an example, this double comb-like structure is not the essence of this embodiment. Even if a drive current partition is placed and a vortex drive current is passed in a spiral pattern, the vortex on the entire chip grounding film can be moved with a small current. The essence of this embodiment is to increase the current density J by dividing the ground thin film with partition walls having a critical temperature lower than its own critical temperature, and to move the vortex with as small a vortex-driving side current as possible. It is. After applying the vortex drive current, the environmental temperature T is changed from T _CG > T >T' _CG to T _CG >
When T′ _CG >T, the initialization of the embodiment is completed.

A second embodiment of the invention is shown in FIG. In FIG. 2, 1 to 4 are the same as 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 on the chip and the ground plane on the card (chip mounting board). Grounding terminal 5 in Figure 2
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 on the chip, a large amount of the current will also flow through the ground terminal to the superconducting ground plane on the card. It flows away. In other words, in order to cause the required current density J to flow through the ground thin film on the chip in order to drive the vortex, an extra current must be passed through 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. critical temperature as in this example
By separating the ground terminal 5 and the vortex drive current supply terminal 3 by the vortex drive current partition wall 2 having T' _CG , it is possible to prevent the current from wetting onto the card as described above. In other words, as in the first embodiment, the environmental temperature T is set to T _CG > T >T' _CG ,
When a vortex drive current is applied, the current
As can be seen from the figure, the flow is limited to the ground thin film on the chip. The essence of this embodiment is to electrically separate the ground terminal and the vortex drive current supply terminal by a vortex drive current partition wall whose critical temperature is lower than that of other ground thin film parts, and the method of separation is as follows: The number of each terminal is not limited to the configuration shown in FIG. 2. For example, the ground terminals may be framed one by one by the vortex drive current barrier, or the ground terminals may be formed directly on the vortex drive current barrier. 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. 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 ground thin films having two types of superconducting critical temperatures shown in the first, second, and third embodiments are formed using different materials in two steps by means such as vacuum evaporation. Alternatively, it can be made by irradiating with ion beams, electron beams, alpha rays, beta rays, etc. to give locally different critical temperatures.

(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. 1...Thin film that forms the main part of the ground thin film on the chip,
2... Vortex drive current partition, 3... Vortex drive current supply terminal, 4... Power supply, 5... Ground terminal,
6... Interferometer loop, 7... Josephson junction, 8...
Vortex (trapped magnetic flux), 9... lines of magnetic force, 10... ground thin film.

Claims (1)

[Claims] 1. A Josephson integrated circuit comprising a grounded thin film that is a superconducting thin film, wherein the grounded thin film has a first thin film at a first superconducting critical temperature and a second superconducting critical temperature lower than the first superconducting critical temperature. a second thin film, the first thin film is provided with a plurality of terminals for supplying a vortex drive current, and the second thin film has an environmental temperature that is equal to or less than the first and second superconducting critical temperatures. is arranged in such a way that the path of the vortex driving current flowing through the first thin film between the terminals is narrowed or the current path between the terminal and the ground terminal is blocked when the current is in the middle of the terminal. Josephson integrated circuit.