JPH01137684A - Superconducting switching element - Google Patents

Abstract

PURPOSE:To reduce a leak current as well as to facilitate the control of magnetic field by a method wherein a third superconducting material is formed astride first and second interregion insulated superconducting materials, and a photoconductive material is formed thereon through the intermediary of a superconducting material of a thickness that enables tunnelling. CONSTITUTION:The first and the second superconducting materials 2 and 5, consisting of an Ln-Ae-Cu oxide thin film, are connected by the third superconducting material 4 consisting of an Ln-Ae-Cu oxide film, astriding the superconducting materials 2 and 5 through the intermediary of a thin interlayer insulating film 3 consisting of a zirconia thin film. When said element is cooled down to the critical temperature or below, the three superconducting materials are brought into the state wherein they are separated by a thin interlayer insulating film 3. If the interlayer insulating film 3 is formed so as to have a thickness that enables tunnelling, or below, a tunnel current can be made to flow. The Ln in the formula indicates that it contains at least one of La, Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, and the Ae indicates that it contains at least one of Ba and Sr.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a superconducting switching element that switches using light.

BACKGROUND OF THE INVENTION Josephson devices have been known as switching devices using superconductors. The Josephson element is
As shown in FIG. 4, it has a structure in which two superconductors are separated by a thin insulating film.

In FIG. 4, 21 is a superconductor such as Nb, 22 is an insulator such as niobium oxide, and 23 is a substrate. At cryogenic temperatures, Nb, 21 is in a superconducting state. At this time, the current (I)-voltage (V) characteristics of the Josephson element are as shown in FIG. 5 due to the tunneling effect of the superconductor on both ends of the insulator. Therefore, it can be used as a two-terminal switching element. Switching is accomplished by varying the current or by applying a magnetic field to the junction. As a method of applying a magnetic field, a magnetic field is generated by placing a conductive wire near it and passing a current through it.
Some devices perform switching by controlling the maximum current value that maintains a zero voltage state.

Problems to be Solved by the Invention However, conventional Josephson devices have problems such as large leakage current and difficulty in controlling the threshold value when performing switching.

The present invention has been made in view of these points, and an object of the present invention is to provide a light-based superconducting switching element with low leakage current and whose threshold value can be easily controlled.

Means for Solving the Problems In order to solve the above problems, the present invention straddles the first and second superconductors which are insulated between regions, and is capable of tunneling with the first and second superconductors. It consists of a third superconductor and a photoconductor formed thereon via a thick superconductor.

As described above, the present invention utilizes the change in the superconducting energy gap of the third superconductor formed through the thin glabella insulating film due to photoelectron injection, and has low leakage current and easy switching control. It's easy.

EXAMPLE Hereinafter, an example of the superconducting switching element of the present invention will be described with reference to the drawings.

A YI Ba 2 Cu oxide thin film of about 2 μm is formed on a strontium titanate substrate using a sputtering method, and after making it superconducting by heat treatment, one of the Y, Ba 2 Cu oxide thin films is formed using a photolithography technique. A thin film of about 80 zirconium oxides is formed on the whole by sputtering, and on top of this a thin film of Y I B a 2 Cu B f11 of about Lpm is removed. A thin film was formed.

After that, heat treatment was performed to make the thin film superconducting. Furthermore, about 5000 CdS photoconductive thin films were formed thereon by vacuum deposition. Its structure is shown in FIG.

In FIG. 1, 1 is a strontium titanate substrate, 2
is the first superconductor formed thereon, Y, Ba2
Cua oxide thin film, 3 is zirconium oxide film, 5
4 is the second superconductor, YIB a 2 Cu 2 oxide thin film, and 4 is the third superconductor, YI Ba 2 C.
It is a ua oxide thin film. That is, it has a structure in which the first and second superconductors are straddled and connected by the third superconductor via a thin interlayer insulating film. 6 is a CdS photoconductive film formed on the third superconductor.

Electrodes 7, 8.9 are formed on each superconductor, and an appropriate bias voltage can be applied to each superconductor.

Y, Ba2Cu3 oxide is a type 2 superconductor that becomes a superconductor at about 90%. A type 2 superconductor is a superconductor in which a magnetic field can penetrate into the superconductor while maintaining a superconducting state. Therefore, when an element having such a structure is cooled to a temperature below the critical temperature, for example, below 77, the three superconductors are separated by a thin glabellar insulating film. If the thickness of the glabellar insulating film is set below the thickness that allows tunneling, a tunneling current will flow. Therefore, the same [-V characteristics as a normal Josephson element can be obtained between the first and second superconductors and the third superconductor. This situation is shown in FIG. 2A. When a current exceeding the maximum current required to maintain zero voltage is passed through this junction, zero voltage is no longer maintained and a voltage is generated across the junction. This state is shown in FIG. 2B. In this state, the photoconductor is irradiated with light. Electrons generated within the photoconductor are thereby injected into the third superconductor immediately below. The energy bandgap of a superconductor changes and becomes smaller when a large amount of electrons are injected from the outside.

Therefore, the tunnel current starts to flow from a lower bias voltage and changes to the r-v characteristic shown in FIG. 2C. Therefore, if a constant voltage is applied to the junction, the current flowing through the circuit will increase to a value corresponding to D, and if a resistor is used as the load, the voltage across it will rise rapidly. If a constant current is flowing, the voltage will correspond to E. Switching can be performed in any case.

L n , B a 2 Cu a oxide (Ln is Nd, Sm, Eu, Gd, Tb, Dy, Ho.

At least one of Er, Tm, Yb, and Lu) superconductors all have a critical temperature of about 90°C. Further, La+, ss Aeo, +s Cu1 oxides (where Ae is at least one of Ba, 'Sr, C)a) all become superconductors with a critical temperature of about 30. Therefore, in the configuration of the example, by using the L n 1B a 2 Cu a oxide superconductor or the La+, as Aeo, +s Cu+ oxide superconductor as the superconductor, the same switching operation as in the example can be obtained. it is obvious.

Even if Ba is replaced with S in Lrz Ba2Cu3 oxide, a superconductor with similar characteristics can be obtained.Therefore, even if a superconductor in which Ba is replaced with Sr is used, similar results can be obtained. It is clear that the effects can be obtained.

An electrode is formed on each superconductor, and a bias voltage can be applied to each superconductor.

This makes it extremely easy to control the switching current value or voltage value.

FIG. 3 shows that in the device of this example, a bias voltage is applied to each superconductor electrode so that superconductor 5 is at the highest potential, superconductor 2 is at the lowest potential, and superconductor 4 is at an intermediate potential. It is an energy band diagram when adding. In the superconducting state, the four superconductors have an energy gear, which can be expressed as a valence band, a forbidden band, and a conductive band, just like in normal semiconductors. 3 is an interlayer insulating film. Now, the valence band of superconductor 2 is above the bottom of the shock zone of superconductor 4, and the valence band of super TL conductor 4 is above the bottom of the conduction band of superconductor 5. Suppose we bias it as follows. Then, tunneling from the valence band of superconductor 2 to the conduction band of superconductor 4 becomes possible, and a large number of electrons are transferred to superconductor 2.
is injected into the superconductor 4 from If the electrons injected into the superconductor 4 do not lose energy within the superconductor 4,
It is directly tunnel-injected into the conductive band of the superconductor 5. Even if it loses energy and falls into the valence band of the super-'QL' 4 body 4, it is possible to tunnel from the valence band of the superconductor 4 to the conduction band of the superconductor 5, and in any case, a large amount of electrons is injected into the superconductor 6. Therefore, when the superconductor 4 is biased so that the bottom of the conductive band is slightly above the valence band of the superconductor 2, almost no tunnel current flows. .

When the photoconductive film is irradiated with light in such a biased state, photoelectrons generated in the photoconductive film enter the superconductor 4. When a large amount of electrons are injected into the superconductor 4 from the outside, its energy band gap becomes smaller, and the bottom of the conduction band of the superconductor 4 comes to be below the valence band of the superconductor 2.

Then, tunnel current begins to flow rapidly, and current switching can be performed. By adjusting the bias potential of each superconductor as shown in this example, the degree of freedom in controllability using current and light is increased. superconductor 4
A type of amplification is also possible if the voltage is biased to a state where switching can be controlled by changing the potential.

Effects of the Invention As described above, according to the method of the present invention, the change in energy band gap due to the injection of photoelectrons into a superconductor by light is utilized, and switching control is easy. In addition, when off, the two superconductors are separated by a normal conductor sandwiched between two insulating films, which is better than a conventional Josephson element that is separated by a single interlayer insulating film. , much less leakage current.

The effects of the present invention are such that a third superconducting layer straddles the first and second superconductors which are insulated between regions, and a superconductor having a thickness that allows tunneling with the first and second superconductors. It is obtained from a structure consisting of a photoconductor and a photoconductor formed on it, and there are many combinations that can form this structure other than this example, but any of them can be applied. .

In addition, in this example, a specific value was used as the thickness of the thin film, but the thickness of the glabella insulating film can be arbitrary within the range where the tunnel effect occurs, and the thickness of each superconductor thin film and photoconductive film can also be set to that value. It is arbitrary within the range that can be formed.

In this example, a strontium titanate single crystal was used as the substrate, but it is obvious that the substrate is not limited to this, and any substrate on which a superconducting thin film can be formed may be used.

In this example, zirconium oxide film was used as the glabellar insulating film, but it is clear that it is not limited to this, as long as it does not react with the superconducting film during film formation or during heat treatment after formation. be.

In this example, a CdS thin film was used as the photoconductive film, but it is not limited to this as long as it generates electrons when irradiated with light.

As described above, the present invention straddles the first and second superconductors that are insulated between regions, and the first and second superconductors are connected via a superconductor having a thickness that allows tunneling with the first and second superconductors. The present invention has a structure consisting of a superconductor (No. 3) and a photoconductor formed thereon, and provides a switching element with low leakage current and easy magnetic field control.

[Brief explanation of the drawing]

Fig. 1 is a cross-sectional view of one embodiment of the structure of the present invention, Fig. 2 is a graph showing the IV characteristics of the device of the present invention, and Fig. 3 is an example of an energy band diagram of the device of the present embodiment. An explanatory diagram, FIG. 4 is a cross-sectional view showing an example of the structure of a conventional Josephson element,
FIG. 5 is a graph showing the [-V characteristics of a conventional Josephson element. 1... Strontium titanate substrate, 2...
... Y 1B a 2 Cu a oxide thin film superconductor, 3 ... Interlayer insulating film, 4 ... Y, Ba
2Cu3 oxide thin film superconductor, 5...Y113
a 2 Cu trioxide thin film superconductor, 6...
・Photoconductor, 7, 8.9... Electrode. Name of agent: Patent attorney Toshio Nakao Hakaichi Meijyo
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Claims (3)

[Claims] (1) Straddling the first and second superconductors that are insulated between regions, the third superconductor and its A superconducting switching element consisting of a photoconductor formed thereon. (2) As a superconductor, Ln (Ln is La,
Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er
, Tm, Yb, and Lu)-Ae (where Ae is at least one of Ba and Sr)-Cu oxide according to claim (1). switching element. (3) A superconducting switching element according to claim (1), characterized in that each superconductor is provided with a terminal for applying a bias voltage.