EP0941482A2

EP0941482A2 - Device for coupling an rf-squid to a super conducting tank circuit

Info

Publication number: EP0941482A2
Application number: EP97951807A
Authority: EP
Inventors: Jürgen Schubert; Yi Zhang; Willi Zander; Marko Banzet; Huai-Ren Yi
Original assignee: Forschungszentrum Juelich GmbH
Current assignee: Forschungszentrum Juelich GmbH
Priority date: 1996-11-28
Filing date: 1997-11-26
Publication date: 1999-09-15
Also published as: AU5548098A; US6300760B1; AU738360B2; WO1998023969A3; JP2001504589A; WO1998023969A2

Abstract

Device for coupling an rf-SQUID to a super conducting tank circuit and a base plate, in which the tank circuit and the rf-SQUID form a coplanar structure, and the tank circuit has a slit. The base plate (10) is configured as an outer loop (10a) which is coplanar to the rf-SQUID (2) and to the tank circuit (1), and has a slit (11). The tank circuit (1) encircles an inner loop (1a), in which the slit (4) is embodied. The orientation of the slits (4; 11) of the inner loop (1a) and the outer loop (10a) to one another determines the resonance frequency fr.

Description

Arrangement for coupling an rf-SQUID to a superconducting tank circuit

The present invention relates to an arrangement for coupling an rf-SQUID to a superconducting tank resonant circuit and to a base plate, in which the tank resonant circuit and the rf-SQUID form a coplanar structure and the tank resonant circuit has a slot.

Various proposals have so far been followed to couple rf-SQUID magnetometers to superconducting tank resonant circuits.

One possibility is to use a λ resonator to which an rf-SQUID is galvanically coupled and which also functions as a flow pickup loop. Such a SQUID magnetometer can have a tank frequency of 3 GHz.

However, the use of a λ resonator is problematic since it only shows a low quality of a few 100. In view of the qualities of a few 1000 already achieved with the λ / 2 resonators, this represents a very small size. In addition, the fact that a parameter that is difficult to calculate, namely the high-frequency current distribution, must also be taken into account due to the galvanic coupling , to considerable problems. The high-frequency current distribution is a variable that is not easy to calculate or experimentally control. The SQUID layout is therefore difficult to optimize.

Another possibility is to produce planar LC resonant circuits from YBaCuO thin films with high frequency and high quality. These LC resonant circuits are operated in a flip-chip arrangement with the rf-SQUID in a washer-SQUID structure. The parasitic Capacities between the LC resonant circuit and the rf-SQUID reduce the quality of the LC resonant circuit and make the current distribution in the combined LC resonant circuit / washer-SQUID structure complicated.

In the still unpublished application 196 11 900.6, the applicant has described the arrangement mentioned at the beginning. This solves the problem of parasitic capacitances. However, there is still the problem that, due to the fact that the coplanarly arranged rf-SQUID and tank resonant circuit cannot be arranged in one plane with the base plate, and the base plate also represents a possible source of noise that requires the use of an rf-SQUID -Magnetometers can restrict.

The object of the present invention is therefore to create an arrangement which eliminates the above problems when coupling an rf-SQUID magnetometer to a superconducting resonant circuit.

The object is achieved according to claim 1 in that the base plate is designed as an outer loop coplanar with the rf-SQUID and the tank resonant circuit and has a slot, and in that the tank resonant circuit comprises an inner loop in which the slot is formed and the orientation of the Slits of the inner loop and the outer loop to each other determines the resonance frequency f _r .

The arrangement according to the invention relates to the possibility of an advantageous, optimal coupling of an rf-SQUID to a tank resonant circuit and a base plate which does not have the disadvantages mentioned at the outset. With the fully integrated arrangement of rf-SQUID, tank resonant circuit and base plate and the formation or orientation of the slots in the rf-SQUID and in the base plate according to claim 1, the tank frequency can be adjusted in a simple manner depending on the geometry and thus offers a significant advantage, for example: B. when building a multi-channel SQUID Systems for medical applications. In addition, noise caused by the base plate can be suppressed.

In addition, the fully integrated arrangement enables a simple estimation of the coupling between the rf-SQUID and the tank circuit.

According to claim 2, it is advantageous that an adjustment of the slots to each other causes a change in the resonance frequency f__ 300 <Mhz.

Another advantage is that a defined superconducting short circuit is installed between the rf-SQUID and the tank circuit. The resonance frequency of the resonance circuit increases with decreasing dimension. Above a cut-off frequency of 1 GHz, the SQUID electronics required become very complex and expensive. The defined superconducting short circuit significantly reduces the resonance frequency, so that there is a very simple possibility of obtaining discrete frequency ranges within a span of 600 MHz by simple geometry changes, and still resonance frequencies of up to 500 MHz with very small dimensions. These discrete frequencies are a necessary prerequisite for the implementation of a multi-channel HTSL SQUID system.

Further advantages of the present invention are obtained from the features of subclaims 4 to 12.

According to claim 13, it is advantageous that a flux transformer is integrated in the arrangement in order to further increase the magnetic field sensitivity of an rf-SQUID.

Another advantage according to claim 14 is that the flux transformer comprises a coupling coil which is short-circuited. This results in a decoupling of two forms of current that are ex- act. The two forms of current differ in their high and low frequencies. The decoupling eliminates the parasitic contributions of the high-frequency current that occur at the crossovers of the flux transformer. In the decoupled state, only low-frequency current flows through the crossings.

According to claim 15, it is particularly advantageous if the short circuit occurs at a specific position of the capacitor.

Another advantage according to claim 16 is that the field direction of the insert loop is opposite to the field direction of the coupling coil. This amplifies the SQUID signal in this geometry.

Embodiments of the present invention are described below with reference to the drawings. Show it:

La shows a schematic view of a first geometry of a SQUID magnetometer with a base plate designed as a coplanar loop according to the first embodiment of the present invention;

1b shows a schematic view of a second geometry of a SQUID magnetometer with a base plate designed as a coplanar loop according to the first embodiment of the present invention;

1c shows a schematic view of a third geometry of a SQUID magnetometer with a base plate designed as a coplanar loop according to the first embodiment of the present invention;

FIG. 2 shows a diagram of a test measurement with a geometry according to FIG. 3a shows a schematic view of a SQUID magnetometer without a built-in short circuit;

3b shows a schematic view of a SQUID magnetometer according to a second embodiment of the present invention with a built-in short circuit and with a first geometry;

3c shows a schematic view of a SQUID magnetometer according to a second embodiment of the present invention with built-in short circuit and with a second geometry;

3d shows a schematic view of a SQUID magnetometer according to a second embodiment with a built-in short circuit and with a third geometry;

4a and 4b show a basic illustration of an rf-SQUID with a planar tank resonant circuit and a base plate;

5 shows a schematic top view of an rf-SQUID and a tank resonant circuit without a coplanar base plate;

6 shows a schematic top view of a single-layer flux transformer with a base plate designed as a coplanar loop according to the present invention;

Fig. 7 is a schematic view of a single-layer flux transformer with a base plate designed as a coplanar loop and a multi-layer flux transformer according to the present

Invention; 8 shows a schematic view of a single-layer flux transformer with a base plate designed as a coplanar loop and a multi-layer flux transformer with short circuit according to the present invention;

Fig. 9 is a schematic plan view of a gradiometer SQUID or two-hole SQUID.

4a and 4b each show a tank resonant circuit 1 and an rf-SQUID magnetometer 2 with planar resonant circuits and λ / 2 or λ resonators which are coupled to a base plate 10 made of metal or superconductor material. As a result, the tank resonant circuit 1 with integrated SQUID 2 and the base plate 10 cannot be arranged in one plane. In addition, the base plate 10 represents a possible source of noise, which can restrict the use of an rf-SQUID magnetometer 2. FIG. 5 shows a tank resonant circuit 1 with a rf-SQUID magnetometer 2 arranged coplanarly without a base plate 10.

1a to 1c schematically show an arrangement according to the invention in a first embodiment, in which the base plate 10 is designed as a coplanar arrangement in the form of an outer loop 10a. As a result, the tank resonant circuit 1, the SQUID 2 and the base plate 10 can be arranged in one plane.

FIGS. 1 a, 1 b and 1 c each show a different geometry of a SQUID magnetometer 2, which is present as a rf resonant circuit with a coplanar arrangement of an inner loop 1 a of the tank resonant circuit 1 and the outer loop 10 a of the base plate 10. A further external superconducting loop 10a is added to the SQUID geometry in FIGS. 4a, 4b. This loop 10a with a slot 11 represents the coplanar resonant circuit. The area A in FIGS. 1 a, 1 b, 1 c can be designed as a washer, multiloop or current injection SQUID structure his. It is also possible to use this surface as a flux concentrator or flux transformer in order to combine the coplanar resonant circuit with a washer SQUID magnetometer in flip-chip geometry.

A great advantage of these coplanar resonant circuits 10a is the geometry-dependent resonance frequency of the resonant circuit. The only difference in the geometries in Figures la to lc lies in the orientation of the slot 11 in the outer loop to the slot 4 in the inner loop la. In Fig. La both slots 4, 11 are aligned one above the other, the difference in orientation is 0 degrees or 360 degrees. The resonance frequency is f _E. = 850 MHz. If you change the orientation to 180 degrees (Fig. Lc), the resonance frequency decreases to f ₁ _ = 550 MHz. At 90 degrees (Fig. Lb), the resonance frequency f _I _ = 650 MHz. This results in a very simple possibility of reaching discrete frequency ranges within a range of 300 MHz by simply changing the geometry. These discrete frequencies are a necessary prerequisite for the implementation of a multi-channel HTSL squid system.

The simple change in the geometry of the coplanar resonant circuit 10a results in a frequency change in the tank frequency of the rf-SQUID 2. This is a significant advantage over the prior art, especially with regard to the construction of a multi-channel SQUID system for medical applications, for example.

2 shows test measurements with this layout which, at a resonant frequency of the tank resonant circuit of 850 MHz, have a quality factor of approximately 5000 and a white noise of

1.85 * 10 ^{~ 5} Φ _O / VHZ.

Figures 3b to 3d show the geometry of a coplanar resonant circuit according to a second embodiment of the present invention with a defined superconducting short circuit 5. In Fig. 3a, the resonant circle shown without this short circuit 5. The frequency in FIG. 3a is f ₀ = 1.1 GHz with a quality of Q> 5000. Defined superconducting short circuits 5 are installed in FIGS. 3b to 3d. These short circuits 5 are attached at 180 degrees (FIG. 3b), 90 degrees (FIG. 3c) and 360 degrees (FIG. 3d). The changes in the resonance frequency in FIGS. 3b to 3d with the short circuits 5 compared to the resonance frequency without a short circuit are considerable. In FIG. 3b the resonance frequency is f ₁ = 920 MHz, in FIG. 3c the resonance frequency is f ₂ = 803 MHz and in FIG. 3d the resonance frequency is f ₃ = 620 MHz. The quality is still better than Q> 5000 for all manufactured resonant circuits. Since the quality determines the coupling between the SQUID and the resonant circuit, the SQUID function is not affected by the improvement presented here.

This results in the very simple possibility of reaching discrete frequency ranges within a span of 600 MHz by simply changing the geometry. Even with substrate dimensions of 10 ^β 10 mm ² , resonance frequencies of up to 500 MHz can be achieved by the invention.

6 schematically shows a further embodiment of an arrangement according to the invention, in which the base plate 10 is designed as a coplanar arrangement in the form of an outer loop 10a and the tank resonant circuit 1 and the base plate 10 are arranged in one plane. A flux transformer with an insert loop 3.1 is arranged within the tank circuit 1. A gradiometer SQUID 2 or two-hole SQUID (FIG. 9) is used in such a way that the Josephson contact is positioned at the point of contact of the two washer surfaces. This SQUID 2 is applied to the flux transformer in a flip-chip arrangement.

In Fig. 7 is within the tank circuit 1

Flow transformer provided that a multi-layer single-head pel coil 3.2 includes. The multi-layer coupling coil 3.2 also has a crossover 6.

Due to the fact that a high-frequency and a low-frequency current form are present in the present arrangement and the high-frequency current has a parasitic effect at the crossovers 6 of the flux transformer and the quality of the resonant circuit is reduced at a tank frequency of approximately 900 MHz, a position A (FIG. 8) provided on a capacitor 7 short circuits that decouple the two current forms, so that only low-frequency currents flow through the crossings 6 in the decoupled state. The short circuits are normally conductive high-frequency metal short circuits. The second superconducting loop 10a is used for high-frequency coupling between SQUID 2 and the flux transformer. The high-frequency current acts here and ensures the coupling between SQUID 2 and tank resonant circuit 1. Because the field direction of the insert loop 3.1 and the multi-layer coil 3.2 are exactly opposite, the SQUID signal is amplified in this geometry.

Claims

claims

1. Arrangement for coupling an rf-SQUID to a superconducting tank resonant circuit and to a base plate, in which the tank resonant circuit and the rf-SQUID form a coplanar structure and the tank resonant circuit has a slot, characterized in that the base plate acts as an outer loop ( 10a) is coplanar with the rf-SQUID (2) and the tank circuit (1) and has a slot (11), and that the tank circuit (1) comprises an inner loop (la) in which the slot (4) is formed and the orientation of the slots (4; 11) of the inner loop (la) and the outer loop (10a) to each other determines the resonance frequency f _^ .

2. Arrangement according to claim 1, characterized in that an adjustment of the slots (4, 11) causes a change in the resonance frequency f__ of less than or equal to 300 MHz.

3. Arrangement according to claim 1 or 2, characterized in that a defined superconducting short circuit (5) is installed between the rf-SQUID (2) and the tank resonant circuit (1).

4. Arrangement according to claim 3, characterized in that the defined superconducting short circuit (5) between the inner loop (la) and the outer loop (10a) is installed.

5. Arrangement according to claim 3 or 4, characterized in that the orientation of the short circuit (5) to the slot (4) of the inner loop (la) determines the resonance frequency f _r .

6. Arrangement according to one of claims 3 to 5, characterized. that the installation of the defined superconducting short circuit (5) causes a change in the resonance frequency f._ of less than or equal to 600 MHz.

7. Arrangement according to one of the preceding claims, characterized in that the rf-SQUID (2) has a SQUID with washer-SQUID structure.

8. Arrangement according to one of claims 1 to 6, characterized in that the rf-SQUID (2) has a SQUID with a multiloop SQUID structure.

9. Arrangement according to one of claims 1 to 6, characterized in that the rf-SQUID (2) has a SQUID with current injection SQUID structure.

10. Arrangement according to one of claims 1 to 6, characterized in that the rf-SQUID (2) has a SQUID with single-layer or multi-layer transformers with several turns.

11. Arrangement according to one of claims 1 to 6, characterized in that the rf-SQUID (2) has a SQUID with a double coil gradiometer.

12. The arrangement according to claim 9, characterized in that the double coil gradiometer is formed with two series-connected opposing coils.

13. Arrangement according to one of claims 1 to 6, characterized in that a flux transformer is formed within the tank circuit (1), which the coplanar outer loop (2), has an insert loop (3.1) and a capacitor (7).

14. Arrangement according to claim 13, characterized in that a second coupling coil (3.2) with a crossover (6) is provided which is short-circuited.

15. The arrangement according to claim 14, characterized in that the short circuit occurs at a position A of the capacitor (7) in order to decouple a high-frequency current.

16. Arrangement according to one of claims 13 to 15, characterized in that the field direction of the insert loop (3.1) runs opposite to the field direction of the coupling coil (3.2).

17. Arrangement according to one of the preceding claims, characterized gekennzeichneet that the tank resonant circuit (1) has dimensions of less than 10 x 10 mm ² .