JPS602796B2 - thin film rf squid - Google Patents

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 pongee 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 in order to satisfy the device size, Conventionally, microfabrication techniques such as photolithography techniques have been used. However, in the conventional example, the length of the connecting portion corresponding to the width of the narrow groove is longer than the width W of the connecting portion, resulting in the following disadvantages. .

In other words, the self-inductance of the above-mentioned coupling increases, and when viewed as an equivalent circuit, it becomes as if the above-mentioned inductance L is 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 such circumstances, and its purpose is to provide a thin film that exhibits excellent characteristics by reducing the self-inductance of superconductor IJ and also reducing the self-inductance of Josephson junction. Our goal is to provide rf squid.

Hereinafter, the present invention will be explained in detail using the drawings.

FIG. 1 is an exploded perspective view of an embodiment of the present invention, in which 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 l.
a, lb are provided, and a weak connection portion ld called a Josephson junction is provided in the middle of the thin grooves lc, 1'c connecting these holes la, lb. Furthermore, the ground plane 3 is provided with the first and second holes la and l.
The third and fourth holes 3a, 3b correspond to the holes b, respectively.
In addition, a convex portion 3d, for example, in the shape of an island, is provided in the middle of the thin washers 3c, 3'c connecting these holes 3a, 3b so as to correspond to the above-mentioned connecting portion ld. Further, FIG. 2 is a cross-sectional view showing the state of use of the embodiment of the present invention. In the figure, the same symbols as in FIG. 1 are used with the same meanings, and repeated explanations will be omitted here. 4 is a substrate made of, for example, sapphire, on which a ground plane 3, an insulator 2, and a thin film 1 are fixed in a layered manner. These are a plan view and an enlarged plan view of essential parts of an embodiment of the present invention, respectively. In the figures, the same symbols as those in the first figure are used with the same meaning, and redundant explanation here will be omitted. In Figure 4, the part formed by the broken line is the ground plane 3.
It shows. Incidentally, FIG. 4 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 explained below.

In the embodiment of the present invention having the above configuration, the holes la,
The superconductor ring consisting of lb, Yokomizo c, and 1'c has a magnetic field whose magnetic distance is determined only by 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 current 1 flows through the upper ring, and the magnetic flux of the ring is always kept constant.
Since this is a Josephson junction having c, when the magnetic field increases to 1=lc, the coupling portion cannot withstand the force and the magnetic velocity enters into the ring. Also,
Due to the intrusion of the magnet, the current 1 decreases to 1<1. So, the above ring is again a magnetic flux quantum ■. It will hold a magnetic flux that is an integer multiple of . In this way, while the magnetic field applied from the outside increases at a constant rate, the magnetic flux inside the ring increases stepwise in units of magnetic flux quanta, and conversely, the magnetic field decreases. The magnetic flux within the superconducting ring begins to decrease in a stepwise manner,
As a result, the magnetic flux quantum ■ for the above change in magnetic field. It shows a periodic response of a triangular wave with one period as . by the way,
The self-inductance L of the ring above, and the magnetic flux quantum ■.
, and the above-mentioned critical current lc, a relationship as shown in the following equation [1' is established. Further, the intrinsic noise spectral density Si■ of the rf SQUID is given as shown in the following equation (2), and from the equation (2) and the above equation [1st], the following equation (3) is derived.
(1) L1c ■. (i' (L1c)2 S'Gogo 1.3x ---X (Many Saints)% (2) WRF
IC's.

Si is L (3) However,
L: self-inductance, lc: critical current Si'? : Intrinsic noise spectral density, ■.

: Magnetic flux quantum WRF: M angular frequency KB: Bormann constant T: Absolute temperature In the embodiment of the present invention, as described above, the coupling portion ld is provided with the convex portion 3d via the insulator 2. Therefore, the self-inductance of the ring is smaller than that of the thin film Squidow alone.

Therefore, from the above equation '1), if L → 4, lc → becomes large, and "critical current lc increases by the amount that self-inductance L decreases.
will start to increase. Furthermore, from the above equation {81, if L→small, then Si→small, and the noise also decreases by the amount that the self-inductance L decreases. According to the embodiment of the present invention as described in detail above, since the critical current lc is increased, there is an advantage that the critical current lc can be easily adjusted.

Furthermore, since noise is reduced as described above, there is also the advantage that the S/N ratio is significantly improved. Furthermore, in the subordinate example, if the length of the coupling part is long, an equivalent circuit with an inductance in series with the coupling part x is created as shown in the circuit configuration diagram in Figure 5, and the -V characteristic is ideal. However, 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 t-V characteristic also differs from that of an ideal Josephson junction. It also has the advantage of being closer.

[Brief explanation of the drawing]

The official drawing is an exploded perspective view of the embodiment of the present invention, FIG. 2 is a cross-sectional view of the state of use of the embodiment of the present invention, and FIG. An enlarged plan view v FIG. 5 is an equivalent circuit diagram of a conventional example. Heavy... Thin film SQUID ~ 2...Insulator~ 3...Ground plane, 4...Substrate 5...Main part, military a?lb, 3a, 3b...
Hole "Turtle c, 1'c, 3c Tortoise 3℃...Horizontal groove, ld...Joining part ~ 3d...Convex part. Younger brother' figure younger brother Z box younger brother J face 4th box younger brother's figure

Claims (1)

[Claims] 1. In a thin film RF SQUID using a bridge-type Josephson element, a thin film SQUID made of a thin film-like superconducting material has two holes and a weak connection part provided in the middle of a narrow groove connecting these holes, and a ground plane made of a superconducting material having two corresponding holes and a convex portion provided in the middle of a narrow groove connecting these holes so as to correspond to the connecting portion; and a ground plane that is tightly attached to the thin film SQUID. A thin film RF SQUID comprising a flat plate-shaped insulator.