JPH0546111B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Technical Field of the Invention The present invention relates to superconducting circuits, and in particular to nonlinear inductance circuits that have the required current phase characteristics and can be used over a wide current range.

(b) Prior art and problems The Josephson effect in a superconducting state is applied to logic circuits and memory devices as a Josephson element, and has various application fields such as magnetometers, voltage standards, electromagnetic wave detection, and thermometers. In these application fields, a loop structure consisting of one or two Josephson junctions and a superconducting inductance is one of the basic structures.

Figure 1 shows an example of this type of loop structure, with 1
is a loop made of a superconductor such as a lead (Pb)-based alloy, which serves as the lower electrode, and 2 is the upper electrode, between which a Josephson junction 3 and a contact junction 4 with a sufficiently larger junction area are formed. has been done.

An equivalent circuit of the loop structure shown in FIG. 1 can be expressed as shown in FIG. 2a. In the figure, J represents the Josephson junction 3, and L represents the inductance of the loop. The signal current flowing into and out of this loop structure is I, the current passing through J is I _J , and the current passing through L is I _L.
If the magnetic flux passing through the loop is Φ, the critical current value of Josephson junction J is Io, the phase difference between the superconductors at both ends of the tunnel oxide film is φ, and the magnetic flux quantum is Φo (=h/2e=2.07 ×10 ^-15 Wb), I _J = Io sinφ (1) Φ = Φo/2π×φ (2) From the relationship, I _L = Φo/2πL×φ (3).

If the inductance L is set to L = Φo / 2πIo (4), then I _L = Io × φ (5) from the above formula (3), which can be expressed as I = I _J + I _L = Io (sinφ + φ) (6) I can do it.

The relationship between the signal current I and the phase difference φ according to the above equation (6) is shown in FIG. 2b with I/Io as the horizontal axis. In the figure, curve a shows I _J /Io according to equation (2), straight line b shows I _L /Io according to equation (5), and curve c shows I/Io according to equation (6).

The differential inductance of this loop structure is expressed as
From (2), dΦ/dI = Φo/2π×dφ/dI (7) In proportion to the differential slope of curve c, the period is 2π of the phase difference φ of the loop, and the low inductance region A and the high inductance region B has a nonlinear response including

When trying to apply the nonlinearity of this loop structure in the application field as mentioned above, not only is the inductance L a periodic function as mentioned above, but also because region B is narrower than region A, high inductance is extremely low. Being limited to a narrow current range makes its application extremely difficult, and it is necessary to obtain a wider range of low and high inductance regions.

(c) Object of the Invention An object of the present invention is to provide a superconducting circuit having required current phase characteristics that can utilize the nonlinearity of inductance over a wide current range.

(d) Structure of the Invention The object of the present invention is to provide a superconducting circuit having two or more n Josephson junctions, two or more n superconducting inductances, and a pair of terminations connected to an external circuit. One end of n Josephson junctions is connected to one terminal end and the kth (1≦k≦n-1)
The other end of the th Josephson junction and the other end of the k+1th josefson junction are connected via the k+1th superconducting inductance, and the other end of the nth joosefson junction is connected to one other end of the ladder. This is accomplished by a superconducting circuit forming a shaped circuit and having a shunt comprising a first superconducting inductance between the other end of the first Josephson junction and said one termination.

(e) Embodiments of the Invention The present invention will be specifically explained below using embodiments with reference to the drawings.

FIG. 3a is an equivalent circuit of a loop structure having the same configuration as the loop structure described above, and FIG. 3b is a diagram showing the relationship between the current ratio I/Io and the phase difference φ. In this cited example, a subscript 1 is added to the same reference numerals as in the above example.

In this cited example, the inductance L ₁ is set to L ₁ =Φo/√2πIo ₁ (8), and as a result, the current I L ₁ shown on the straight line b ₁ increases from the above example. In this case, a hysteresis phenomenon appears in the signal current I ₁ as shown by the curve c ₁ .

Next, as shown in FIG. 4a, the
If L ₂ having the same inductance as L ₁ is connected, the current I ₁ ' is represented by a curve d from the curve c ₁ and the straight line b ₁ in FIG. 4b.

Furthermore, as shown in FIG. 5a, a second Josephson junction _J2 is connected in parallel to the circuit shown in FIG. 4a.
However, the critical current of the second Josephson junction J ₂
Let Io ₂ be 1/2 of Io ₁ .

The relationship between the current I ₂ and the phase difference φ ₂ in this case is represented by a curve f in FIG. 5b from the curve d and the curve e representing IJ ₁ . In this embodiment, the low inductance region A and the high inductance region B can be obtained with approximately the same current width for the current I ₂ . Furthermore, since the current phase relationship is a periodic function of 4π, inductance changes can be used over a wide range for the phase.

By further connecting inductances and Josephson junctions in multiple stages as shown in FIG. 6, and selecting each constant, it is possible to realize the required nonlinear inductance characteristics with sufficient accuracy.

The nonlinear inductance circuit according to the present invention described above can be applied, for example, as follows.

Figure 7a shows an example of a circuit that controls signal current transmission by applying the present invention, in which C is a nonlinear inductance circuit according to the present invention, LT is a transformer, and Lo is the primary side of the transformer LT. Indicates inductance. An offset current Ioff is input from the input terminal 11 of the circuit shown in the figure, superimposed on the signal current Ic.

By giving the nonlinear inductance circuit C a current phase characteristic as shown in the curve of FIG. 7b, and by selecting a binary value of the offset current Ioff passing through the circuit C, the operating point of the signal current Ic can be set to the low inductance region A or the high inductance region. The signal current through the primary of the transformer LT by setting within B, and therefore the transformer
The signal current transmitted to the secondary side of the LT is controlled.

Furthermore, FIG. 8 shows an example of a circuit that discriminates whether the signal current is larger or smaller than the threshold value. The nonlinear inductance circuit C according to the present invention is set to shift from a low inductance region to a high inductance region at the threshold value, and is connected in parallel to a control gate circuit in which the inductance of Josephson junction Jo is Lo. When the signal current is smaller than the threshold value, the current on the control gate circuit side is suppressed, improving the stability of the discrimination operation, and making it possible to prevent malfunctions due to noise, for example.

(f) Effects of the Invention As explained above, according to the present invention, a nonlinear inductance in which the low inductance region and the high inductance region are set with a sufficient current width with the required current phase characteristics is realized. It becomes possible to apply it to many superconducting circuits.

[Brief explanation of the drawing]

Figure 1 is a plan view showing an example of a loop structure consisting of Josephson junction and inductance, Figure 2a
is its equivalent circuit;
Figures a and b are diagrams showing an equivalent circuit in which an inductance is connected in series with a loop structure and its current phase characteristics, Figure 5 a is an equivalent circuit of an embodiment of the present invention, and b
6 is a diagram showing a generalized equivalent circuit of an embodiment of the present invention, FIG. 7 a is an equivalent circuit diagram showing an application example of the present invention, and b is an explanation of its operation. 8 are equivalent circuit diagrams showing other application examples. In the figure, 1 is a loop serving as a lower electrode, 2 is an upper electrode, 3 is a Josephson junction, 4 is a contact junction, J, Jo, J ₁ to Jn are Josephson junctions,
L, Lo, _L1 to Ln represent inductance.

Claims (1)

[Claims] 1 A superconducting circuit having 2 or more n Josephson junctions, 2 or more n superconducting inductances, and a pair of terminals connected to an external circuit, the n
One end of the Josephson junction is connected to one termination, and the other end of the k-th (1≦k≦n-1) Josephson junction and the other end of the k+1-th Josephson junction are connected to the k+1-th superconducting inductance. and the other end of the nth Josephson junction is connected to one other termination, forming a ladder circuit, and the first junction is connected between the other end of the first Josephson junction and said one termination A superconducting circuit comprising a shunt including a second superconducting inductance.