JPS61196587A - Direct-current superconducting quantum interference device - Google Patents

Abstract

PURPOSE:To enable a critical current value of a junction to be controlled as required during the operation of a device, by providing a control means for equalizing the critical current values of two tunnel-type Josephson junctions. CONSTITUTION:A direct-current superconducting quantum interference device (DC-SQUID) comprises, in addition to components identical with those of a conventional device, a main coil 1, a modulation coil 7 provided on the main coil 1 with electrical isolation therefrom, an insulation layer 2, a Josephson junction 3, an insulation layer 8 covering an upper electrode 4 and a shunt resistance 5, and control lines 9 provided over the tunnel-type Josephson junctions 3. Control current IC1 and IC2 is supplied to right-hand and left-hand control lines 9 so that the critical current values 1o of the right-hand and left- hand tunnel-type Josephson junctions 3 are controlled by means of magnetic fields produced by the control current IC1 and IC2. In this manner, the right-hand and left-hand critical current values Io can be equalized.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a DC-driven superconducting quantum interference device (hereinafter referred to as DC-3QUID) used, for example, to detect a weak magnetic field, and in particular to a DC-3QUID during its operation. This study concerns the control of the critical current value of the Son junction.

[Conventional technology]

Figure 6 shows, for example, Applied Physics Letters (
Applied Physics Letters,
VoL43. P, 694-696, 1983) is a plan view showing the conventional (7) DC-3QUID, in which 1 is a main coil, 2 is an insulating layer covering a part of the main coil l, and 3 is an insulating layer that covers a part of the main coil l. This is a tunnel junction type Josephson junction formed on the main coil 1, and its junction area is determined by the size of the window formed in the insulating layer 2. 4
5 is an upper electrode connected to the main coil 1 through the two tunnel junction type Josephson junctions 3, 5 is a shunt resistor that short-circuits the main coil 1 and the upper electrode 4 through a window formed in the insulating layer 2, and 6 is a main This is an input coil formed on the coil 1 and electrically insulated therefrom. Note that insulating layers other than the insulating layer 2 are omitted in FIG.

These coils and the like are formed by stacking them on, for example, a Si substrate, and the stacking order is, from the substrate side, the shunt resistor 5 - the main coil 1 - the insulating layer 2 - the upper electrode 4 - the human powered coil 6. be. Also, main coil 1. Upper electrode 4. The input coil 6 is made of a superconducting material such as niobium or a lead alloy, the insulating layer 2 is made of SiO, and the shunt resistor 5 is made of an alloy of copper and gold.

Next, the operation will be explained.

Current-voltage characteristics (hereinafter referred to as 1-V characteristics) of DC-3QUID when the magnetic flux φ in the superconducting loop formed by the main coil 1 and the upper electrode 4 is φ=nφ0 and φ−(n+1/2)φ0. ) are shown by curves A and B in Figure 7 (al), and the IV characteristics continuously change between the curves A and B as the magnetic flux φ changes. A bias current 1b larger than twice the critical current value 1o per junction type Josephson junction 3 is caused to flow from the main coil 1 toward the upper electrode 4, and the DC-3QUID is kept in a voltage state. In this case, the change in magnetic flux generated within the main coil 1 due to the signal current Is flowing through the input coil 6 will be extracted as a change in voltage.

In this case, the relationship between the magnetic flux in the superconducting loop composed of the main coil l and the upper electrode 4 and the output voltage is shown in Fig. 7(b).
As shown in , the magnetic flux quantum φO (=2.07x) is
This is possible when the hysteresis parameter βC shown in equation (1) is smaller than 1. Therefore, conventionally, in order to set βcal, a shunt resistor 5 that is sufficiently smaller than the normal conduction resistance of the tunnel junction type Josephson junction 3 has been provided.

φ0 Here, C is the junction capacitance of the Josephson junction 3, and R is the normal conduction resistance value when the shunt resistor 5 and the Josephson junction 3 are connected in parallel.

In the DC-3QUID that operates as described above, its sensitivity is expressed by the magnetic flux-voltage conversion coefficient α
Determined by

φ0 Here, ΔV is the amplitude of the output voltage of DC-3QUID, and the value of this ΔV is as shown in equation (3) when the critical currents 1o of the left and right junctions are the same, and the two When the critical current value 1o varies, it becomes smaller than the value given by equation (3).

Δ■U Io-R (3) Therefore, in order to achieve high sensitivity, it is necessary to make the critical current values 1o of the left and right junctions equal. Furthermore, DC-5QU
According to the Journal of Low Temperature Physics, the energy resolution of ID is minimized when equation (4) is satisfied.
ow Temperature Physics, Vo
L29. P., 301-331° 1977).

Lp ・IoΣ1/2φ0 ... (4)
Here, t and p are the self-inductances of the superconducting loop composed of the main coil l and the upper electrode 4.

[Problem that the invention seeks to solve]

Since the conventional DC-3QUID is configured as described above, the device must be formed so that the values of the critical currents 10 of the two Josephson junctions are equal and also satisfy the above formula (4). It is difficult to manufacture, and furthermore, it may change over time after the device is fabricated.

The value of .beta.C increases and the value of .beta.C exceeds 1, which causes problems such as hysteresis occurring on the I--characteristics and failure to operate normally.

This invention was made in order to solve the above-mentioned problems, and during operation, the critical current values Io of the two Josephson junctions can be made equal on the left and right sides, and Ll)"toΣ1/
2φ0. DC-3QUID that can be controlled to satisfy βc<l.

The purpose is to obtain.

[Means for solving problems]

The DC-3QUID according to the present invention is provided with a control means for generating a magnetic field to equalize the critical current values of two tunnel junction type Josephson junctions.

[Effect]

In the DC-3QUID of this invention, the magnetic field generated by the current flowing through the control means enters the tunnel junction type Josephson junction and changes the current distribution inside the junction, thereby changing the critical current value IO of the two junctions. control so that they are equal.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a plan view showing a first embodiment of the invention, FIG. 2 is an enlarged plan view of the portion surrounded by a dashed line in FIG. 1, and FIG.
The figure is a sectional view taken along line mm in FIG. 2.

In the figure, 1 to 6 are exactly the same as the above conventional device,
7 is a modulation coil provided on the main coil 1 and electrically insulated therefrom, and 8 is the main coil 1.

Insulating layer 2. Josephson junction 3. An insulating layer 9 covering the upper electrode 4 and the shunt resistor 5 is a control line provided above the tunnel junction type Josephson junction 3 via an insulating layer 8. Note that in FIGS. 1 and 2, the insulating layer 2. Insulating layers other than insulating layer 8 are omitted.

The main coil 1 and the like are formed by laminating them on a substrate made of, for example, Si or 5i02, and the order of lamination is from the substrate side, for example, shunt resistor 5 - main coil l - insulating layer 2 -
Josephson junction 3 - upper electrode 4 - insulating layer 8 - human power coil 6, modulation coil 7° control line 9. In addition, main coil 1j upper electrode 4° input coil 6. The modulation coil 7 and control line 9 are made of a superconducting material such as niobium or a lead alloy, the insulating layer 2.8 is made of SiO, for example, and the shunt resistor 5 is made of a resistor such as AuIn2.

Next, the effects will be explained.

In the DC-3QUID configured as above,
Control currents I cl and I c2 are passed through the left and right control lines 9, and the control currents Ic1. Due to the magnetic field generated by Ic2, the critical current value of the left and right tunnel junction Josephson junctions 3! Controls 0.

FIG. 4 is a diagram for explaining the control of the critical current value Io of the tunnel junction type Josephson junction 3 by the control line 9. In the figure, WH is the line width of the control line 9, and d is the tunnel junction type Josephson junction. The film thickness of the barrier layer of No. 3, λ1. λ2 is the London magnetic field penetration depth of the superconductor constituting the main coil 1 and the upper electrode 4, respectively, and 1 is the distance between the control line 9 and the barrier layer.

For example, when the angle between the upper electrode 4 and the control line 9 is a right angle as shown in FIGS. 2 and 4, the magnetic field generated by the control current C1 becomes direction only, and its magnitude Hx is /<<WH
In this case, equation (5) is obtained.

WH Here, for example, l is 9000 (person) and WH is 20μm
If so, it can be said that the condition l<<WH is fully satisfied. At this time, the current density distribution inside the tunnel junction type Josephson junction 3 changes due to Hx, and the critical current value Io is calculated by the formula (
6), +7).

...(7) Here, μO (=4πxlOH/m) is the magnetic permeability of vacuum, w
y is the width of the tunnel junction type Josephson junction 3 in the X direction,
io is the critical current value when no external magnetic field is applied. Therefore, by changing the control current 1c1,
The critical current 1o can be controlled within the range of equation (8).

0≦io≦io...(
8) The effect of the control current 1c2 is also exactly the same as in the case of Icl.

In this way, in the device of this embodiment, even if the critical current value 1o of the left and right tunnel junction type Josephson junctions 3 varies during device fabrication, by flowing the control current 1 (1, IC2) during operation, the left and right critical current values can be adjusted. The current values IO can be made uniform.Furthermore, the critical current Io can be controlled so as to satisfy the optimum drive condition Lplo=1/2φO, which minimizes the energy resolution of the DC-3QUID.

- For example, λl = 1370 (people), λ2 = 7
00 (person), d=80 (person), Wy=5μm, WH
-5μm, Ic required to set φX=φ0
The value of 1 is about 16 (mA), and in this case, the value of 0 can be reduced to zero.

The driving method of the entire DC-3QUID is the same as the conventional one, and generally consists of a main coil 1 and an upper electrode 4 in order to linearize the input/output relationship of the DC-3QUID. The magnetic flux in the superconducting loop is n・φ0 or (n+1
/2) A feedback current is passed through the modulation coil 7 to drive it so that it is fixed at φ0. This driving method is described in, for example, the Journal of Low Temperature Physics.
w Temperature Physics, ν0L
25, P, 99-'144.1976).

In the above embodiment, the case where the angle θ between the upper electrode 4 and the control line 9 was 90° was explained, but if O≦θ, 90°
Even if the critical current is 1 due to the control current 1cl and Ic2
o can be controlled.

FIG. 5 shows a second embodiment of the present invention. In this embodiment, a control line 9 is provided in the same manner as in the first embodiment, and an insulating layer 10 is further provided under the main coil l. A superconducting ground plane 11 is provided under the insulating layer 10, and the superconducting ground plane 11 and the control line 9 constitute a control means. In this embodiment, the angle θ between the upper electrode 4 and the control line 9 is 90°, and insulating layers other than the insulating layer IO are not shown in FIG.

In this example, the control current 1c1 and the image current -Ic1 of Icl induced in the superconducting ground plane ll
generates a magnetic field in the X direction. Further, an image current -IJ of the current Ij flowing through the tunnel junction type Josephson junction 3 generates a magnetic field in the X direction. Since these X-direction and X-direction magnetic fields invade the Josephson junction 3, the critical current 1o
are as shown in equations (9) to (11), and the left and right critical current values can be controlled by ICI and Ic2, as in the case without the superconducting ground plane 11. In addition, Wx
is the width of the Josephson junction 3 in the X direction.

...(9) ...QOI ...(11) In this way, in this embodiment, the superconducting ground plane 11
Due to the Meissner effect, the magnetic flux generated by the control line 9 is confined between the control line 9 and the superconducting ground plane 11, improving the magnetic coupling efficiency between the control line 9 and the Josephson junction 3, and thus, Control of the critical current value IO becomes easier.

In the embodiment shown in FIG. 5, the case where the angle θ between the upper electrode 4 and the control line 9 was 90° was explained.
Even if 0≦θ<90°, the control current 1c1. The critical current value IO can be controlled by Ic2.

〔Effect of the invention〕

As described above, according to the present invention, since the control means for equalizing the critical current values of the two tunnel junction type Josephson junctions is provided, the critical current value of the junction can be freely controlled during operation. This has the effect of suppressing variations in critical current caused by the manufacturing process or changes over time, and also allows the DC-3QUID to be driven in a state with the lowest energy resolution.

[Brief explanation of drawings]

FIG. 1 shows a DC-3QU according to a first embodiment of the present invention.
A plan view showing ID, FIG. 2 is an enlarged plan view of the dashed-dotted line portion in FIG. 1, and FIG.
4 is a diagram for explaining the control of the critical current value 1o of the tunnel junction type Josephson junction 3 by the control line 9,
The figure is a sectional view showing a second embodiment of the present invention, FIG. 6 is a plan view showing a conventional DC-3QUID, and FIG. 7(a).
(b) is the general I- of DC-3QU ID, respectively.
FIG. 3 is a characteristic diagram showing V characteristics and magnetic response characteristics. In the figure, l is the main coil, 3 is a tunnel junction type Josephson junction, 4 is an upper electrode, 8 is an insulating layer, 9 is a control line,
IO is an insulating layer, and 11 is a superconducting ground plane. Note that the same reference numerals in the figures indicate the same or equivalent parts.

Claims (3)

[Claims] (1) A superelectric ring composed of a main coil and an upper electrode, two tunnel junction type Josephson junctions formed between the main coil and the upper electrode, and the two tunnel junction type Josephson junctions. 1. A direct current driven superconducting quantum interference device, comprising: control means for generating a magnetic field to equalize the critical current values of . (2) A patent characterized in that the control means is provided with an insulating layer on the upper surface of the upper electrode, and a control line magnetically coupled to the Josephson junction on the upper surface of the insulating layer. A direct current driven superconducting quantum interference device according to claim 1. (3) The control means includes an insulating layer provided on the upper surface of the upper electrode, a control line magnetically coupled to the Josephson junction on the upper surface of the insulating layer, and an insulating layer provided on the lower surface of the main coil. , a superconducting ground plane is provided on the lower surface of the insulating layer.
DC-driven superconducting quantum interference device as described in 2.