JPS5896786A - Thin film rf squid - Google Patents

Abstract

PURPOSE:To obtain a SQUID having low noise by a method wherein two holes are provided on the thin film rf SQUID which constitutes a superconductive ring, a weak junction part is provided in the middle part of a narrow groove which connects said holes, and a ground plane having a protrusion corresponding to the weak junction part is laminated on the SQUID in the middle part of said groove, through the intermediary of an insulating flat plate. CONSTITUTION:Two holes 1a and 1b are provided on the thin film SQUID consisting of thin film type superconductive material, these holes are connected by narrow grooves 1c anc 1c', and a weak junction part 1d consisting of Josephson junction is provided in the middle of the grooves. Also, holes 3a and 3b, same as the above holes, are provided on the ground plane 3 of superconductive material facing the SQUID 1, these two holes are connected by the narrow grooves 3c and 3c', and an island-like protrusion 3d is provided on the above junction part corresponding to the part 1d. Subsequently, the SQUID 1 and the plane 3 are laminated in one body through the intermediary of an insulating mateial 2. Thus, the critical current of the ring is increased, thereby enabling to adjust the critical current easily.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention includes one weak bond within a superconductor ring;
The present invention relates to a thin film RF SQUID that performs switching by a predetermined magnetic flux given the weak coupling.

In such a thin film RF SQUID, a two-hole SQUID pattern is usually formed, which has two holes and a weak connection part provided in the middle of a narrow groove connecting these holes. In addition, in order to obtain a Josephson device with excellent characteristics using the above-mentioned coupling portion (so-called bridge-type Josephson junction), the device size must be approximately the same as the coherence length of the superconductor used, and the device size must be satisfied. Therefore, microfabrication techniques such as photolithography techniques have been used for a long time.

However, in the conventional example described above, the width WK of the joint portion
In comparison, the length l of the connecting portion corresponding to the width of the narrow groove becomes longer, resulting in the following disadvantages. In other words, the self-inductance L of the above-mentioned coupling part increases, and when viewed as an equivalent circuit, it becomes as if the above-mentioned inductance L was added in series to the Josephson junction corresponding to the above-mentioned coupling part, and as a result, the current of the Josephson junction and There was a drawback that the phase relationship deviated from a sine function.

The present invention has been made in view of the above circumstances, and its object is to reduce the self-inductance of the superconductor ring and the self-inductance of the Josephson junction, thereby providing a thin film RF SQUID that exhibits excellent characteristics. Our goal is to provide the following.

Hereinafter, the present invention will be explained in detail using figures. 1st
The figure is an exploded perspective view of an embodiment of the present invention. In the figure, 1 is a thin film SQUID made of a thin film-like superconducting material, 2 is, for example, a flat insulator, and 3 is a ground plane made of a superconducting material. The thin film SQUID 1 also has first and second holes 1a.
, lb are provided and these holes 1a, lb
A weak connecting portion 1d called a Josephson junction is provided in the middle of the narrow grooves 1c and l'c that connect the two. Furthermore,
Ground plane 3 (then the first and second holes 1
The third and fourth holes 3 correspond to a and lb, respectively.
a, 3b are provided, and these holes 3a,
A narrow groove 3c connecting 3b.

In the middle of 3'c, a convex portion 3d, for example, in the shape of an island, is provided so as to correspond to the connecting portion 1d. Further, FIG. 2 is a sectional view of a main part showing the usage state of the embodiment of the present invention. In the figure, the same symbols as in FIG. 1 are used with the same meanings, and redundant explanation will be omitted here.

A land plane 3, an insulator 2, and a thin film 1 are fixed in a layered manner. Further, FIGS. 3 and 4 are respectively a plan view of an embodiment of the present invention and an enlarged plan view of a part of the factor, and the same symbols as in No. 11 are used with the same meanings, and repeated explanations here will be omitted. .

Further, in FIGS. 3 and 4, the portion formed by the broken line indicates the ground plane 5. As shown in FIG.倚、4th
The figure is an enlarged view of a portion corresponding to the main part 5 of FIG. 3.

The operation of the embodiment of the present invention will be described below.

In the embodiment of the present invention having the above configuration, the hole la,
The superconductor ring consisting of 'lb and transverse grooves 1c and l'c has a magnetic flux Φ in which the magnetic flux Φ penetrating the ring is determined by only physical constants due to quantum mechanical effects. is fixed to an integer multiple of . When a magnetic field is applied from the outside in this state, a blocking current flows through the ring and the magnetic flux Φ inside the ring is always kept constant, but the connecting portion 1d is a Josephson junction with a critical current value Ic. Therefore, when the above-mentioned magnetic field increases and becomes (5) I=Ic, the above-mentioned coupling part cannot withstand the above-mentioned coupling part and magnetic flux enters into the above-mentioned ring. Also, due to the intrusion of the magnetic flux, the above-mentioned shielding current (2) decreases to (III).

Eventually, the ring will again hold a magnetic flux that is an integer multiple of the magnetic flux quantum. In this way, while the magnetic field applied from the outside increases at a constant rate, the magnetic flux inside the ring is equal to the flux quantum Φ. The magnetic flux inside the ring increases in a stepwise manner as a unit, and conversely, when the magnetic field decreases, the magnetic flux inside the ring also decreases in a stepwise manner. It shows the periodic response of a triangular wave with one period. By the way, the self-inductance L of the ring and the magnetic flux quantum Φ. , and the critical current Ic0 is given as below, and the equation (2
) and the above equation (1) lead to the following equation (3).

LIc”＞Φ.
(S(4) (↓) SctLL(3) However, L: Self-inductance, IC: Critical current (i)
.

SJ, characteristic noise spectral density Φ O: magnetic flux quantum WRF: rf angular frequency KB = Boltzmann constant T: feed temperature In the embodiment of the present invention, as described above, the coupling portion 1d is convex through the insulator 2. Since the ring is attached to the portion 3d, the self-inductance of the ring is smaller than that in the case where the thin film SQUID 1 is used alone. Therefore, from the above equation (1), if L→small, re→large, and K becomes such that the critical current IC increases by the amount that the self-inductance L decreases. Also, according to the above formula (3), if L → small, s(:i
) → As the self-inductance L decreases, the noise also decreases.

According to the embodiment of the present invention as described in detail above, since the critical current Ia is increased, there is an advantage that the critical current Ic can be easily adjusted.

Furthermore, since the noise is reduced as described above, there is also the advantage that the SIN ratio is significantly improved. Furthermore, in the conventional example, if the length t of the coupling part is long, what kind of circuit is formed, such as an inductance (Lo) inserted in series with the coupling part (x), as shown in the circuit diagram of FIG. Although the I-V characteristics deviated greatly from the ideal Josephson junction, according to the embodiment of the present invention, the self-inductance Lo is reduced by the superconducting ground plane as described above, so that the I-V characteristics deviated greatly from the ideal Josephson junction.
It also has the advantage that the V characteristic approaches that of an ideal Josephson junction.

[Brief explanation of drawings]

FIG. 1 is an exploded perspective view of an embodiment of the present invention, FIG. 2 is a cross-sectional view of essential parts showing the usage state of the embodiment of the present invention, and FIGS. 3 and 4 are a plan view and essential parts of the embodiment of the present invention, respectively. The enlarged plan view, FIG. 5, is an equivalent circuit diagram of a conventional example. DESCRIPTION OF SYMBOLS 1...Thin film SQUID, 2...Insulator, 3...Ground brain, 4...Substrate, 5...Main part, la,
lb, 3a, 3b...hole, lc, l'c, 3
c, 3'c...Horizontal groove, 1d...Joining part, 3d
...Protrusion.

Claims (1)

[Claims] A thin film RF SQUID using a bridge-type Jita Zefson element includes a thin film SQUID made of a thin film-like superconducting material having two holes and a weak bonding part provided in the middle of a narrow groove connecting these holes; A ground plane made of a superconducting material having two corresponding holes and a convex part provided in the middle of a narrow groove connecting these holes so as to correspond to the coupling part, and sandwiched between the ground plane and the thin film SQUID. A thin 1 [RF SQUID] which is equipped with a plate-shaped insulator attached to the RF SQUID.