JP2978738B2 - Electrostatic Josephson interferometer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-speed and high-sensitivity device using a superconductor and a normal conductor, and more particularly to an electrostatic Josephson interference device utilizing the property of an electron wave.

[0002]

2. Description of the Related Art In a conventional device using a superconducting ring, a magnetic field is mainly applied to the center of the ring to control the current and conductance of the ring. When a magnetic field is applied to the center of the superconducting ring, the phase of the wave function of electrons passing through the upper and lower branches is shifted by θ and −θ, respectively, so that the Josephson current of the superconducting ring is periodically shifted as a magnetic field function. Vibrates.

[0003] There have been remarkable microfabrication and crystal growth techniques for the last 10 years, and in recent years, it has become possible to produce a phase-coherent sample at a low temperature even for a normal metal. In such a situation, the electrons passing through the sample move ballistically like electrons moving in a superconductor or in a vacuum.

[0004] An experiment for controlling the conductance by applying a magnetic field or an electrostatic potential to such a fine ring has been described by S. K. Data et. al. , Applied
Physics Letters (Applied Physics Letters) 48, P487 (1986) and P.S. Vegar et. al. , PhysicalRe
view B (physical review bee) 40, P
3491 (1989). Because such devices are normal metal, the conductance must be at a temperature of a few Kelvin to obtain a high peak valley ratio (1 and 0 in logic circuits) as a function of magnetic field and electrostatic potential. When the temperature is high, the peak valley ratio decreases in proportion to 1 / T. In addition, there is a disadvantage that the sample size must be processed to about several thousand angstroms.

[0005]

With superconductivity having a high _Tc , operation at a temperature at least 100 K higher than that of a normal metal becomes possible. A device that controls a current by a magnetic field is known as a SQUID, but an electrostatic voltage is more preferable than a magnetic field in device application. A superconductor generally has a much higher electron density than a semiconductor and requires a strong voltage to provide a phase difference between the upper and lower branches. In general, the resistance is almost zero in the superconducting state.
, It is almost impossible to sufficiently control the current by the voltage.

[0006]

The electrostatic Josephson interference device of the present invention is connected between two terminals by a first superconducting branch and a second superconducting branch having the same electric characteristics. A superconducting ring provided with a normal conductor junction in the middle of each of the first superconducting branch and the second superconducting branch, and applying a static voltage to at least one of the normal conductors of the superconducting branch, A difference in electron density is generated between the conduction branch and the second superconducting branch to control current and conductance. Also, a plurality of normal conductor junctions are provided in the middle of each of the first superconducting branch and the second superconducting branch. Also, the invention is characterized in that electrostatic Josephson interference elements are connected in series. Further, a normal conductor is inserted between the terminal and the superconducting ring, and the normal conductor has a resonant tunnel barrier structure.

[0007]

According to the present invention, a superconductor-normal conductor-superconductor SNS sandwich structure is used to apply an electrostatic voltage to one of the normal regions of the Josephson current above and below the superconducting ring, thereby forming the upper and lower branches. A gradient occurs in the wave number (carrier density) of the passing electrons, and the current oscillates as a function of the static voltage.

[0008]

Embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a schematic diagram of a first embodiment of the present invention. FIG. 1A is a schematic diagram of the first embodiment.
In the figure, 1 is a source electrode, 6a is a drain electrode,
2, 6 are branch points of the superconducting ring, 3, 4 are normal conductors, and 5 is a gate electrode.

Electrons emitted from the source electrode 1 are distributed at equal probability by two branches of the superconductor 2. The split electrons further pass through the normal conductors 3 and 4, respectively, and merge again at the superconductor 6.

[0011] Here, the electron wave number that has passed through the normal conductor 4 by applying a static voltage V _G to the gate electrode 5 with one side of the Josephson junction as shown in FIG. 1 (a),

[0012]

(Equation 1)

From

[0014]

(Equation 2)

Changes to On the other hand, the wave number of the electron passing through the normal conductor 3 is

[0016]

(Equation 3)

## EQU1 ## Thus, the probability distribution of electrons difference in electron density above and below the branch has reached the superconductor 6 occurs is modulated by an electrostatic voltage V _G applied to the gate electrode. That is, the modulation of the current with the static voltage (magnetic field 0) becomes possible.

If there is no magnetic field and no electrostatic potential,
The Josephson current J _tot of the system is

[0019]

(Equation 4)

Δ ₀ is the phase shift between the superconductors. K
Is a proportional constant, and a constant J _tot is obtained when δ ₀ ≠ nπ.

[0021] If If applying the gate voltage V _G to the gate electrode 5 in the normal conductor 4,

[0022]

(Equation 5)

Next, J _tot becomes as shown in FIG.
Amplitude as a function of φ

[0024]

(Equation 6)

Vibrates. By applying a gate voltage at the gate electrode 5 of several mV, modulation of several periods becomes possible.

FIG. 2 shows a second embodiment of the present invention. FIG.
(A) shows a schematic diagram of the second embodiment. In FIG.
Is a source electrode, 14 is a drain electrode, 13 is a superconductor,
Reference numerals 9, 10, and 11 are normal conductors, and 12 is a gate electrode. As shown in FIG. 2 (a), normal conductors 9, 10, 11 opposing each other above and below the superconducting ring are arranged to form a Josephson junction. By making the gate electrode 12 one of the upper and lower ones, the oscillation period of the Josephson current is increased, and the element can be operated at a lower voltage. FIG. 2B shows a case where three junctions are formed, and the voltage required to change the vibration by one cycle is 1 /.
It becomes 3. Therefore, in the case of N upper and lower parts, the voltage is 1
/ N.

FIG. 3 is a diagram showing a third embodiment of the present invention. FIG. 3A is a schematic diagram of the third embodiment. In the figure, 15 is a source electrode, 17, 19, and 21 are Josephson junctions, 16, 18, 20, and 22 are normal conductor lines, 23, 24, and 25 are gate electrodes, and 26 is a drain electrode. In the third embodiment, as in the first embodiment, a normal conductor is provided on a superconductor, and a gate electrode is provided on one of the normal conductors. Such a superconducting ring is connected to each superconducting ring and the source electrode 15 and the drain electrode 2 by the normal conductor wires 16, 18, 20 and 22.
Connected to 6. In the third embodiment, three superconducting rings are arranged in series as shown in the figure.

The operation of the third embodiment will be described. The electrons emitted from the source electrode 15 pass through the respective Josephson junctions and normal conductor wires and reach the drain electrode 26. The conductors of the entire system are modulated by modulating the gate voltages at the gate electrodes 23, 24, and 25. Normal conductor wires 16, 18, 20, 22 have the same length L
Then, when kL = nπ (k is the wave number of an electron), a Fabry-Perot type pulse-like resonance peak is generated in the system conductor as shown in FIG. 3B. The peak width at this time becomes narrower by increasing the number of connected superconducting rings,
The distinction between peaks and valleys (1.0 of logic circuit) becomes clear. In this embodiment, a normal conductor is used as a line connecting the superconducting rings, but a superconducting wire may be used.

FIG. 4 is a diagram showing a fourth embodiment of the present invention. FIG. 4A is a schematic diagram of the fourth embodiment. 1 in the figure
5 is a source electrode, 27 and 32 are resonance tunnel barrier structures,
28 and 31 are superconductors, 29 and 30 are normal conductors, 3
Reference numeral 3 denotes a drain electrode. The fourth embodiment is the first embodiment except that a normal conductor resonance tunnel barrier structure is arranged between the branch portion of the superconductor 28 and the source electrode and between the junction of the superconductor 31 and the drain electrode. The configuration is similar to that of the example.

Electrons emitted from the source electrode selectively pass through the resonant tunneling barrier structure to reduce the state of complete transmission (transmission probability of the entire system is 1). Therefore, the conductance can be changed more rapidly than in the first embodiment having no barrier at both ends, and the switching ratio can be increased.

FIG. 4 by modulating here V _G
As shown in (b), the state of the conductance [1, 0] can be controlled as a function of the gate voltage. In addition,
Adjustment of the switching ratio is obtained by increasing the thickness of the resonant tunnel barrier structure on both sides.

In the above embodiments, the superconductivity does not depend on the type of metal, ceramic, organic material or material, but it is preferable that the superconducting transition temperature is high. In addition, a semiconductor such as GaAs is used as a normal conductor. It can also be used. The resonant tunnel barrier structure is, for example, GaAs / AlGa
It can be formed of an As semiconductor.

[0033]

According to the present invention, a Josephson interferometer having a large on / off ratio (peak valley ratio) of a current ratio can be directly and electrically controlled without a magnetic field at a higher temperature than conventionally.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a first embodiment of the present invention and a diagram showing current modulation characteristics of the first embodiment.

FIG. 2 is a schematic diagram showing a second embodiment having a composite junction according to the present invention, and a diagram showing current modulation characteristics of the second embodiment.

FIGS. 3A and 3B are a schematic diagram showing a third embodiment of the present invention connected in series, and a diagram showing current modulation characteristics of the third embodiment; FIGS.

FIG. 4 is a schematic diagram showing a fourth embodiment of the present invention having a resonant tunnel barrier structure at both terminals, and a diagram showing current modulation characteristics of the fourth embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Source electrode 2 Branch point of superconducting ring 3 Normal conductor 4 Normal conductor 5 Gate electrode 6 Branch point of superconducting ring 7 Source electrode 8 Superconductor 9 Normal conductor 10 Normal conductor 11 Normal conductor 12 Gate electrode 13 Superconductor 14 Drain electrode 15 Source electrode 16 Normal conductor wire 17 Josephson junction 18 Normal conductor wire 19 Josephson junction 20 Normal conductor wire 21 Josephson junction 22 Normal conductor wire 23 Gate electrode 24 Gate electrode 25 Gate electrode 26 Drain electrode 27 Resonant tunnel barrier structure 28 Superconductor 29 Normal conductor 30 Normal conductor 31 Superconductor 32 Resonant tunnel barrier layer 33 Drain electrode

Claims (4)

(57) [Claims] Between 1. A drain, a superconducting branch <br/> equal have first superconductive branch and the second electric characteristics are connected together, the said first superconducting branch first A normal conductor junction is provided in the middle of each of the two superconducting branches, and electrons emitted from the source terminal are equally distributed by the two branches of the superconductor, and the two separated electrons further pass through the normal conductor. A superconducting ring that passes through and rejoins the superconductor, and applies an electrostatic voltage to at least one of the normal conductors of the superconducting branch, so that the first superconducting branch and the second superconducting branch are applied. An electrostatic Josephson interference device, wherein a difference in electron density is generated between the two to control current and conductance. 2. The electrostatic Josephson interference device according to claim 1, wherein a plurality of normal conductor junctions are provided in each of the first superconducting branch and the second superconducting branch. 3. The electrostatic Josephson interference device according to claim 1, wherein said electrostatic Josephson interference devices are connected in series. 4. The electrostatic Josephson interference device according to claim 1, wherein a normal conductor is inserted between the terminal and the superconducting ring, and the normal conductor has a resonance tunnel barrier structure.