JPH0627794B2 - DC driven superconducting quantum interference device - Google Patents

Description

BACKGROUND OF THE INVENTION [Field of the Industrial] magnetic sensor of the present invention is for example a high sensitivity, ammeter, displacement meter, or DC-driven superconducting quantum interference device for applications such as high-frequency signal amplifier (DC S uperconducting Q uantum I n
terference D evice, hereinafter referred to as DC SQUID. ) Is related to.

[Conventional technology]

FIG. 6 is a schematic structural diagram showing a conventional DC SQUID, FIG. 7,
FIG. 8 is a sectional view taken along lines VII-VII and VIII-VIII in FIG. 6, respectively. Applied Physics Letters, Vol. 43, 694 (1983), or Vol. 40, 736 (1982) [Ap.
plied Physics Letters, Vol.43, 694 (1983), or V
ol.40,736 (1982)].

In the figure, 1 is a substrate, 2 is a main coil, 3 is a counter electrode,
Numerals 4 and 5 are Josephson elements formed one by one at the connection between the main coil 2 and the counter electrode 3, 6 is an insulating layer for determining the area of the Josephson elements 4 and 5, and 7 and 8 are the Josephson elements 4 and 5. 5 are resistors connected in parallel to each other between the main coil 2 and the counter electrode 3. The main coil 2 and the counter electrode 3 form one superconducting ring on the substrate 1, and 1 to 8 are the basic constituent elements of the DC SQUID body.

Reference numeral 9 is an insulating film that covers the main coil 2, and 10 is a spiral input coil that is located on the insulating film 9 and that is electrically insulated from the main coil 2 and is magnetically coupled. The input coil 10
One wire that is pulled out from the center of the spiral formed by to the outer circumference is insulated from the main coil 2 and the input coil 10 itself by another layer of insulating film (not shown).

Reference numeral 11 is a modulation coil located on the insulating film 9 similarly to the input coil 2. Further, in FIG. 6, the insulating layer 6 and the insulating film 9 are omitted.

Here, the main coil 2, the counter electrode 3, the input coil 10, and the modulation coil 11 are made of a superconducting metal such as Nb or Pb, and the insulating layer 6 and the insulating film 9 are made of, for example, SiO, SiO _2, or Nb ₂ O _5. Etc., and the resistors 7 and 8 are made of normal conductive metal such as Mo and Au.
Materials such as Si, SiO ₂ and sapphire are used for the substrate 1, respectively.

Although the number of turns of the input coil is abbreviated to 3 turns in the figure, it is actually about 50 turns, and the line width is 5 turns.
It is about μm. Further, the film thickness of the superconducting metal thin film used for the main coil 2 and the input coil 10 is required to be several times the magnetic field penetration length of the material used. For example, when Nb is used, 3000Å
The degree.

12 is an output terminal connected to the main coil 2, 16 is an output terminal connected to the counter electrode 3, 13 and 17 are modulation coils 1
1 input terminal, 14 is one input terminal of the input coil 10,
Reference numeral 15 is the other input terminal of the input coil 10.

Next, the operation will be described. First, the output terminals 12 and 16
If a DC bias current Ib is flown in between and the critical current values of the Josephson devices 4 and 5 are Io, the magnitude of Ib is set to around Ib2Io. At this time, input terminal 1
4, 15 or input terminals 13, 17 to input coil 1
0 or a signal current flows through the modulation coil 11, the main coil 2
Signals are transmitted as magnetic flux to the superconducting ring composed of the counter electrode 3 and the counter electrode 3. Input coil 10 and modulation coil 11
Is the sum of the signal magnetic flux transmitted from φ _EX , output terminals 12, 1
If the output voltage generated across 6 is V _out , V for φ _EX
The change in _out (φ-V characteristic) is as shown by the solid line in Fig. 9, and V
_out fluctuates in the cycle of magnetic flux quantum φ _o (= 2.07 × 10 ^-15 wb). That is, the DC SQUID serves to convert the signal current in the input coil 10 and the modulation coil 11 into an output voltage.

Here, when a voltage is generated between the output terminals 12 and 16, the Josephson elements 4 and 5 operate as an alternating current source that oscillates at a frequency _J = V _out / φ _o , and this high frequency current flows in the superconducting ring. . When V _out = 40 uV As an example, the frequency of the high frequency current is about 19GH _z.

Originally, in the DC SQUID φ-V characteristic, the maximum and minimum of V _out appear every 1 / 2φ _o , and should be as shown by the dotted line in FIG. 9. However, as described above, the input coil 10 is
In the DC SQUID stacked on the superconducting ring composed of the main coil 2 and the counter electrode 3, since the high frequency current flowing in the superconducting ring causes resonance between the superconducting ring and the input coil 10, φ− The V characteristic is distorted as shown by the solid line in FIG.
The maximum and minimum appear twice during the period φ _o .
Therefore, although the input / output relationship of the DC SQUID is non-linear as described above, in order to maintain the linearity between the input and output over a wide input magnetic flux signal range, a drive circuit as shown in FIG. 10 is used. .

FIG. 10 shows an example of the most widely used DC SQUID drive circuit, which is a low temperature physical journal (Journal of Low Tem).
perature Physics), Vol, 25, 99 (1976). In the figure, 20 is a coil of the LC resonance circuit, 21 is the capacitance of the LC resonance circuit, 22 is a preamplifier, 2
3 is a phase detector (lock-in amplifier), 24 is a feedback resistor, 25 is an oscillator, and 26 is a pick-up coil.
The part within the dotted line is the portion corresponding to FIG.

In this drive circuit, from the oscillator 25, for example, φ _m = 1 / 2φ
A modulation magnetic flux corresponding to _o (-p) is applied, the output voltage of the DC SQUID is phase-detected, and the output of the phase detector 23 is fed back to the feedback resistor 24.
Through to DC SQUID again. That is, DC SQUID
The operating point is fixed at the maximum or minimum position of the φ-V characteristic shown in FIG. 9, and the output voltage proportional to the signal current flowing through the input coil 10 is obtained as the output voltage of the phase detector 23.
Here, the pickup coil 26 is connected to the input terminals 14 and 15.
If it is connected, it will operate as a magnetometer, and if a signal current source is connected, it will operate as an ammeter. Since this drive circuit operates by fixing the operating point of the DC SQUID at a certain magnetic flux position, it is called a Flux Locked Loop circuit.

[Problems to be solved by the invention]

Since the conventional DC SQUID is constructed as described above, for example, when the operating point is fixed at the positions shown by 18 and 19 in the solid line in FIG. 9, the loop gain of the drive circuit shown in FIG. On the right side of the operating points 18 and 19, it becomes lower than on the left side, and the operating point shifts to the right side when a large signal is input. That is, the tracking speed (slew rat)
There is a problem that e) is lower than that of the DC SQUID having the original φ-V characteristic. Also, the modulation magnetic flux φ _m is 1 / 2φ
Even if it is smaller than _o (pp), φ-V shown by the dotted line in Fig. 9
Compared to DC SQUID, which has its own characteristics, the operating point is easier to move,
There was also the problem that the tracking speed would be low.

The present invention has been made to solve the above problems, have less distortion phi-V characteristic having a period of phi _o, as shown by a dotted line in FIG. 9, and the Furatsukusu -The purpose is to obtain a DC SQUID with a high following speed when driven by a lock loop circuit.

[Means for solving problems]

The DC SQUID according to the present invention is one in which an input capacitance having a capacitance reactance that is sufficiently small with respect to the oscillation frequency of the Josephson element is connected between two input terminals of an input coil.

[Work]

The input capacitance according to the present invention is connected in parallel with the inductance of the input coil, and damps the resonance occurring between the superconducting ring and the input coil due to the high frequency current flowing in the superconducting ring.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. First
In the figure, 1 to 17 are exactly the same as the conventional device. 28 is a first electrode connected to one terminal 14 of the input coil 10, 29 is a dielectric formed on the first electrode 28, 30 is connected to the other terminal 15 of the input coil 10,
The second electrode is stacked on the dielectric 29 (see FIG. 2). Here, the first electrode 28 is formed of, for example, a superconducting material such as Nb, and a part of the first electrode 28 is anodized into Nb ₂ O ₅ to form the dielectric 29. Similarly, the second
The electrode 30 is also made of a superconducting material such as Nb or Pb.

Since the input capacitance is formed between the first electrode 28 and the second electrode 30, when viewed from the terminals 14 and 15, it is connected in parallel to the inductance of the input coil 10. Become. As an example, Nb ₂ O ₅ (relative permittivity ε _r = 29) is used as the dielectric 29, and the film thickness is 200Å,
When the area is 0.5 mm × 0.5 mm = 2.5 × 10 ⁻⁷ m ² , the input capacitance size Ci is about 3.2 μF. This input capacitance Ci
Is to have a sufficiently small capacitive reactance with respect to the oscillation frequency _J of the Josephson element, that is, Is set to satisfy.

Next, the operation will be described. DC configured as above
In the SQUID, a DC bias current Ib (2Io) is passed between the output terminals 12 and 16. At this time, when a signal current flows through the input coil 10 and the modulation coil 11, the signal is transmitted as a magnetic flux to the superconducting ring composed of the main coil 2 and the counter electrode 3, and the output terminal 1 is output according to the magnitude of the signal magnetic flux.
The output voltage V _out appears between 2 and 16.

In this way, in the state in which a voltage is generated between the DC SQUID output voltages 12 and 16, the Josephson devices 4 and 5 operate as a high frequency current source as in the conventional device, so that the high frequency current is applied to the main coil 2 and the counter electrode. Although flowing in the superconducting ring composed of 3, the one terminal 14 and the other terminal 15 of the input coil 10 are short-circuited by the input capacitance composed of the first electrode 28, the dielectric 29, and the second electrode 30. Therefore, the resonance between the superconducting ring and the input coil 10 is damped.

Therefore, the distortion in the φ-V characteristic disappears and the φ-V characteristic becomes DC.
It approaches SQUID's original ideal sinusoidal shape, as shown in Fig. 3. Therefore, when this DC SQUID is driven by the flex-lock loop circuit shown in FIG. 10, a larger follow-up speed than before can be obtained, and it is possible to follow a large input with the operating point fixed. .

In the above embodiment, the first electrode 28 and the second electrode 3
0 is newly provided and is connected to one terminal 14 and the other terminal 15 of the input coil 10. As shown in FIGS. 4 and 5, a dielectric 31 is provided on one terminal 14. Alternatively, the other terminal 15 may be formed on the dielectric 31 so as to be overlapped.

Further, in the above embodiment, the DC SQUID is
The case of driving with a loop circuit has been described, but when used as a high frequency amplifier, the operating point is set to a position of φ _EX = (2n + 1) / 4 (n: integer) in FIG. 3 by using the modulation coil 11. Alternatively, the output voltage between the output terminals 12 and 16 can be detected by fixing a high frequency signal current to the input coil 10.
In this case, the distortion of the φ-V characteristic is reduced and the range in which the linearity between the input and the output is maintained is widened, so that the dynamic range is widened.

〔The invention's effect〕

As described above, according to the present invention, since the input capacitor is connected in parallel to the input coil, the distortion of the input / output characteristic (φ-V characteristic) is improved, and the ideal DC SQUID original ideal. A characteristic close to a sine wave is obtained, and
This has the effect of increasing the signal following speed of the entire system when driven by the loop circuit.

[Brief description of drawings]

FIG. 1 is a schematic structural view showing a DC drive type superconducting quantum interference device (DC SQUID) according to an embodiment of the present invention, FIG. 2 is a sectional view taken along line II-II of FIG. 1, and FIG. Is DC S in Fig. 1
Characteristic diagram showing magnetic response characteristics (φ-V characteristics) of QUID, No. 4
FIG. 6 is a schematic structural view showing a DC SQUID according to another embodiment of the present invention, FIG. 5 is a sectional view taken along line VV of FIG. 4, and FIG.
FIG. 7 is a schematic structural view showing a conventional DC SQUID, FIGS. 7 and 8 are sectional views taken along lines VII-VII and VIII-VIII of FIG. 6, respectively, and FIG. 9 is a DC SQUID of FIG. 10 is a characteristic diagram showing the magnetic response characteristic (φ-V characteristic) of FIG. 10, and FIG. 10 is a block diagram of a DC SQUID drive circuit (flat lock loop circuit). 2 is a main coil, 3 is a counter electrode, 4 and 5 are Josephson elements, 10 is an input coil, 14 is one input terminal, 15 is the other input terminal, 12 and 16 are output terminals, 28 is a first electrode, Reference numeral 29 is a dielectric, and 30 is a second electrode. In addition,
In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (2)

[Claims] 1. An input coil having two input terminals for detecting a signal current, a main coil and a counter electrode which form a superconducting ring magnetically coupled to the input coil, and between the main coil and the counter electrode. In a DC drive type superconducting quantum interference device, which is provided at a connection part of the device and includes a Josephson element that converts the signal current into a voltage output, between the two input terminals, the oscillation frequency of the Josephson element is sufficient. A DC drive type superconducting quantum interference device characterized in that an input capacitance having a small capacitive reactance is connected. 2. The connection of the input capacitor, the first electrode is connected to one terminal of the two input terminals,
A dielectric is formed on the first electrode, and a second electrode connected to the other one of the two input terminals is overlaid on the dielectric. 2. A DC drive type superconducting quantum interference device as set forth in claim 1.