JPH0566034B2 - - Google Patents

Description

Detailed Description of the Invention (Field of Industrial Application) This invention relates to a magnetically coupled superconductor equipped with an input coil for magnetically coupling an input signal to a superconducting quantum interference device (hereinafter referred to as SQUID). This relates to conduction quantum interference devices (hereinafter referred to as magnetically coupled SQUIDs).

(Prior Art) Conventionally, as this type of element, the one shown in Fig. 2 is known. A SQUID coil 2 made of a superconducting thin film with a thickness of about 0.3 μm over a wide range, and an input coil 3 made of a superconducting thin film with a thickness of about 0.3 μm over a wide range, through which a signal current flows, are stacked with an insulating layer in between. structure is adopted, which allows approximately 100PH
It is possible to efficiently magnetically couple the SQID coil 2, which has an inductance value of only about 10 nH (nano-Henry), with the input coil 3, which has an inductance value of about 10 nH (nano-Henry). It had the advantage of increasing sensitivity.

(Problem to be solved by the invention) However, the above conventional magnetically coupled SQUID
However, there are serious problems as explained below.

In other words, the frequency of the signal current applied to the SQUID is usually a low-frequency current of 100KHz (kilohertz) or less, but a drive voltage (V) of about 20 μV is applied to conventional magnetically coupled SQUIDs, so 2
From Josephson element 1 shown in the figure, f=V/Φ ₀
An alternating current with a frequency f expressed as (Φ ₀ is a magnetic flux quantum) flows into the SQUID coil 2. In this case, Φ ₀
=2×10 ^-15 Wb (Waver), a high frequency current of approximately f=10 GHz (gigahertz) flows into the SQUID coil 2.

For such high-frequency currents, the stacked structure of the input coil 3 and SQUID coil 2 shown in Fig. 2 operates as a resonator with very low power loss, and therefore the resonance consisting of the input coil 3 and SQUID coil 2 When an electromagnetic wave with a frequency of about 10 GHz passes through the cavity with almost no loss, a standing wave with a frequency of about 10 GHz will be created inside the resonator.

For this reason, the impedance seen from the Josephson element 1 to the SQUID coil 2 side greatly fluctuates from approximately 0Ω to an infinite value around the frequency of the standing wave.

This means that the internal state of the SQUID is at the operating voltage V
This means that the input/output characteristics that should normally be smooth as shown in Figure 3a result in distortion as shown in Figure 3b.
This was causing unstable operation of the SQUID.

(Means for Solving the Problems) In order to solve the above problems, the present invention interposes a normal conductive metal layer between the input coil and the SQUID coil, which is electrically (direct current) insulated from both. This paper proposes a magnetically coupled SQUID.

(Function) As mentioned above, large fluctuations in impedance, which cause unstable operation of conventional SQUIDs,
The input coil and
This was caused by standing waves being created inside the resonator as a result of electromagnetic waves with a frequency of approximately 10 GHz passing through the resonator consisting of the SQUID coil with almost no loss. When a normal metal layer is interposed between the SQUID coil and the electromagnetic wave passing through the resonator, an eddy current is induced in the normal metal layer, and part of the energy of the electromagnetic wave is absorbed by the resistance of the normal metal layer. This is converted into a joule loss. As a result, standing waves are less likely to arise, and impedance fluctuations are kept small.

On the other hand, the frequency of the signal current applied to the SQUID is usually below 100 KHz (kilohertz), and the normal metal layer has no effect on such low-frequency current. That is, the normal metal layer does not interfere with the magnetic coupling between the input signal and the SQUID.

In addition, when the normal conductive metal layer interposed between the input coil and the SQUID coil in this invention becomes thicker, the magnetic coupling efficiency deteriorates, and the thinner input coil tends to peel off, etc. For this reason, it is preferable to reduce the thickness to, for example, 1 μm or less. Examples of normal metal materials that can be easily made into thin films and have a large loss effect against electromagnetic waves of about 10 GHz as mentioned above include gold, silver, copper,
Aluminum etc. can be mentioned. This is due to the following reason.

That is, when the surface of a normal metal is irradiated with electromagnetic waves, the electromagnetic waves penetrate from the surface to δ (skin depth). The following relationship exists between the skin depth δ and the resistivity ρ of the normal conducting metal.

δ=[ρ/(π×μ ₀ ×f)] ^1/2 …(1) However, in the formula, μ ₀ is magnetic permeability, and μ ₀ =4π×10 ^-7 H/m.In other words, for materials with small ρ δ is also small.
On the other hand, the electromagnetic wave loss effect due to the normal metal layer is
The maximum value is reached when the thickness t of the normal conductive layer becomes approximately the same as δ.

Therefore, for the above-mentioned t < 1 μm and f = 10 GHz, the resistivity ρ of the normal conductive metal such that δ < 1 μm
is obtained from the relational expression (1), ρ<4×10 ^-8 Ω・m
However, the operating temperature of SQUID [when Nb is used as material]
For SQUIDs, the resistivity is usually 4.2K], and examples of materials that have the above-mentioned low resistivity values and can be easily made into thin films include gold, silver, copper, and aluminum.

(Examples) Hereinafter, the present invention will be specifically described based on illustrated examples.

FIG. 1 shows an embodiment of this invention.
1 is a Josephson element, 2 is a Josephson element 1
SQUID coil 3 is made of a superconducting thin film with a wide range of thickness of about 0.3 μm and is electrically connected to SQUID coil 2. A wide normal conductive metal layer 4 having a thickness of 1 μm or less is interposed between the metal layer 3 and the metal layer 3 .

An operating voltage of 20 μV is applied to such a magnetically coupled SQUID, and the input coil 3 is
When a signal current of 100KHz or less was applied, the SQUID
Input/output characteristics indicating normal operation were obtained.

(Effects of the Invention) In summary, according to the present invention, the unstable operation of a magnetically coupled SQUID, which was previously impossible, can be improved without interfering with the magnetic coupling between the input signal and the SQUID.

[Brief explanation of the drawing]

Fig. 1 is an exploded perspective view of a magnetically coupled SQUID showing an embodiment of the present invention, Fig. 2 is an exploded perspective view of a conventional magnetically coupled SQUID, and Fig. 3 is a magnetically coupled SQUID.
These are input/output characteristic diagrams of the SQUID; FIG. 3a is an input/output characteristic diagram when normal operation occurs without resonance, and FIG. 3b is an input/output characteristic diagram when resonance occurs. In the figure, 1 is a Josephson element, 2 is a SQUID coil, 3 is an input coil, and 4 is a normal conductive metal layer.

Claims (1)

[Claims] 1. A magnetically coupled superconducting quantum device characterized by interposing a normal conducting metal layer electrically (direct current) insulated between the superconducting quantum interference element coil and the input coil. interference element. 2. The magnetically coupled superconducting quantum interference device according to claim 1, wherein a thin film of gold, silver, copper, or aluminum is used as the normal conducting metal layer.