JPH01313977A - Josephson junction element - Google Patents

Abstract

PURPOSE:To reduce magnetic noise and thermal noise by installing a slit on at least on end-portion of a superconducting electrode having a junction length longer than or equal to twice the Josephson penetration depth. CONSTITUTION:Superconducting electrodes are so stacked that barrier material 3 composed of insulator, semiconductor and, mainly, conductor of metal and the like is sandwiched, between end-portions in the extending direction. On the end-portions of each of the superconducting electrodes 1a, 2a, of the part where the barrier material 3 is sandwiched, a slit 1a and a slit 2a of a specified length are formed, from the central part of the end-portion in the extending direction toward the extending direction. The width dimension from both ends (p), (q) in the width direction of the superconducting electrodes 1, 2 to both ends (m), (n) of the width direction of the slits 1a, 2a is desirable to be longer than or equal to twice the Josephson penetration depth lambdaj. Thereby adverse influence due to magnetic noise and thermal noise can be decreased.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a Josephson junction element having a junction length that is more than twice the Josephson penetration depth.

[Conventional technology]

FIG. 3 is a schematic diagram showing a conventional Josephson junction element, and FIG. 4 (A) is a cross-sectional view taken along the line IV-IV in FIG. It shows. Both superconducting electrodes 11, 12 are stacked and joined with their ends sandwiched between them with a barrier material 13 made of an insulator, semiconductor, or conductor such as metal.

Superconducting electrode 1 in such a Josephson junction device
The current flowing through the barrier material 1.1 and the barrier material 13 due to the tunnel effect is as shown in FIG. 4(b).

The graph shown in Figure 4 (b) shows the junction length shown in Figure 4 (a) on the horizontal axis, specifically the position of each part in the width direction of the joint, and the vertical axis the current density at the joint. As is clear from this graph, the current density is large within a certain length λj (referred to as the Josephson penetration depth) from both ends of the joint widthwise toward the center, and is extremely small in other parts. It has become localized. Therefore, the current flowing through the junction becomes saturated when the junction length is about 2λj, and at most about 3λj.

The Josephson penetration depth λj is determined by the following formula (1)% formula % λ : Magnetic field penetration depth t : Thickness of barrier material jo : Current density flowing through the junction Figure 5 shows the superconductivity of the Josephson junction element Electrode 1
A control line 14 is placed in a direction perpendicular to the extending direction of the superconducting electrodes 11 and 12, facing the junction of the superconducting electrodes 11 and 12, and the Josephson A case is shown in which the junction element is configured as a so-called magnetic field controlled Josephson switching gate that performs a switching operation.

FIG. 6 is an equivalent circuit diagram thereof, and L, Lt indicate the inductance of the control line 14 and the junction, respectively. The maximum current flowing through the junction of the Josephson junction element due to the magnetic field formed by the control current 1c flowing through the control line 14 is as shown in FIG.

In Figure 7, the horizontal axis is IC/Φ. , and the maximum current flowing through the junction is plotted on the vertical axis.

The switching operation is performed as follows.

First, a DC current of 1 g (point A), which is slightly smaller than the maximum current I0, is passed through the superconducting electrodes 11 and 12, and in this state, an input signal current ic is passed through the control line 14 to apply a magnetic field to the junction. In addition, the maximum current flowing through the junction is reduced by the action of this magnetic field (point B). As a result, the operating point becomes above the dark value curve, the junction changes from a zero voltage state to a finite voltage state, and a switching operation is performed.

By the way, the condition for such a switching gate to operate stably is that when an input signal current Ic (or input magnetism, Ic) is applied to the control line 14, an output current flows through the superconducting electrodes 11, 12 and the barrier 13. The current rg is Ic
In other words, the current gain is greater than or equal to 1. The conditions for this current gain are for each control line 14,
Assuming that the inductance of the junction is #L, =L, and that the junction is in a good coupling state, the magnetic flux quantum Φ.

Although the maximum change in output current 1g when applying is about 10, the magnetic flux quantum Φ. The current to the control line 14 for applying Ice is given as shown in equation (2) below.

■.

LIc,=Φ. Therefore, equation (2) can be rewritten as equation (3) below.

Ll, ≧Φ. ...(3) Equation (3) is 1. =j, a” (however, a2: joint area)
Based on the relational expression and equation (1), it is expressed as the following equation (4).

a≧F Tλj ~2.5 λj = (4) As is clear from the equation (4), in order to obtain a current gain of 1 or more, the minimum dimension of the junction length must be 2 to 3λj.

[Problem to be solved by the invention]

By the way, as mentioned above, <when a conventional Josephson junction element is used as a magnetically controlled switching gate, the junction length needs to be at least 2 to 3 times the Josephson penetration depth λj,
Moreover, the current flowing through the junction is saturated when the junction length is about two to three times the Josephson penetration depth, and the current I0 flowing through the junction shown in FIG. 7 cannot be increased.

However, with such a level of junction current, there is a problem in that, for example, when thermal noise or magnetic noise occurs, even a slight change has a large effect, making it impossible to obtain stable operation.

The present invention was developed in view of the above circumstances, and its purpose is to provide a stable switching operation even when the current flowing through the junction is subject to slight fluctuations due to magnetic noise or thermal noise. A Josephson junction device is provided.

[Means to solve the problem]

In the Josephson junction element according to the present invention, a slit is provided at the junction of at least one superconducting electrode.

[Effect]

According to the present invention, the current flowing through the junction can be localized in a region ranging from both ends of the superconducting electrode junction and from both ends of the slit in the width direction, respectively, within the range of the Josephson penetration depth λj.

[Examples] The present invention will be specifically described below based on drawings showing examples thereof.

FIG. 1 is a schematic diagram of a Josephson junction device according to the present invention (hereinafter referred to as the device of the present invention), and FIG. 2 is an enlarged cross-sectional view taken along the line ■-■ in FIG. The electrode 3 indicates a barrier material. The double-sided conductive electrodes 1.2 are stacked with a barrier material 3 made of an insulator, semiconductor, or mainly metal conductor sandwiched between their ends in the extending direction, and the part where the barrier material 3 is sandwiched is At the ends of each of the superconducting electrodes 1.degree.2, slits la and 2a each having a predetermined length are formed in the extending direction from the center of the end face in the extending direction. Both ends P of the superconducting electrodes 1 and 2 in the width direction.

The width dimension from q to both ends m and n of the slit la and 2a in the width direction is a Josephson penetration depth of 13 or more, preferably 2
It is desirable to set it to λj or more.

The width and length of the slits a and 2a are not particularly limited, and, for example, the length is usually the same as that of the superconducting electrode 1. 2
less than or equal to the length of the overlapping ends of the Further, in the above embodiment, each superconducting electrode 1, 2 has slits la, 2a.
Although a configuration in which one slit is provided has been described, the slit may be formed in at least one of the superconducting electrodes, and it goes without saying that a plurality of slits may be provided in accordance with the bond length.

The current flowing through the junction in the device of the present invention is as shown in FIG. In Figure 3, the horizontal axis shows the position of each part in the width direction of the joint, and the vertical axis shows the current density.As is clear from this graph, the slits la,
Regions where high current density can be obtained are formed on the left and right sides of 2a over a width more than twice the Josephson penetration depth λj, in other words, over 4λj or more, and the superconducting electrodes 1, 2 and barrier 3 are The maximum value I0 of the flowing current can be significantly increased, and the influence of magnetic noise and thermal noise can be relatively reduced.

〔Effect of the invention〕

As described above, in the device of the present invention, the Josephson penetration depth λ
Since a slit is provided at at least one end of the superconducting electrode, which has a junction length of more than twice j, the current flowing through the junction is extremely large, and when used as a switching element, for example, the junction may be damaged by magnetic noise and thermal noise. The present invention has excellent effects, such as being able to operate stably even if the current flowing through it changes slightly.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram of the device of the present invention, Fig. 2 (a) is an enlarged sectional view taken along the line ■-■ in Fig. 1, and Fig. 2 (b) is the current density distribution at various parts in the junction length direction of the device of the invention. Figure 3 is a schematic diagram of the conventional element, Figure 4 (a) is an enlarged sectional view taken along the line IV-IV in Figure 3, and Figure 4 (b) is the current density distribution at various parts in the junction length direction of the conventional element. Figure 5 is a schematic diagram of the conventional element configured as a magnetic field control type switching element, Figure 6 is an equivalent circuit diagram of the conventional element shown in Figure 5, and Figure 7 is the conventional element shown in Figure 5. FIG. 1.2...Superconducting electrode 3...Barrier material la,
2a...Slit In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] 1. In a Josephson junction element formed by sandwiching a barrier material between two superconducting electrodes, the junction length of which is at least twice the Josephson penetration depth, at least one of the superconducting electrodes is A Josephson junction element characterized by having a slit in the junction part.