JPH01137683A - Superconducting switching element - Google Patents

Abstract

PURPOSE:To reduce a leak current as well as to facilitate the control of a magnetic field by a method wherein a thin superconducting material is provided astride two interregion insulated superconducting materials through the intermediary of an insulating films which can be tunneled. CONSTITUTION:The first and the second substrate side superconducting 2 and 5, consisting of an Lu-Ae-Cu oxide thin film subjected to dielectric isolation, are connected by the third superconducting material 4 consisting of the Ln-Ae-Cu oxide thin film, which is thinner than the superconducting materials 2 and 5, through the intermediary of a thin interlayer insulating film 3 consisting of a zirconia thin film. The critical magnetic field of the third superconducting material 4 is lower than that of the first and the second superconducting materials. Said element is cooled down to the critical temperature or less, the three superconducting materials are formed into the state wherein they are separated by a thin interlayer insulating film. When the interlayer insulating film 3 is formed in the thickness which can be tunneled, a tunnel current can be made to flow. The Ln in the formula contains at least one of Y, La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, and Ae contains at least one of Ba, Sr and Ca.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a superconducting switching element that switches using a magnetic field.

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. The current (1)-voltage (V) characteristics of the Josephson element at this time 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 using a magnetic field.

The present invention has been made in view of the above, and an object of the present invention is to provide a superconducting switching element which has a small leakage current and whose magnetic field threshold can be easily controlled.

Means for Solving the Problems In order to solve the above-mentioned problems, the present invention provides a tunnel between the first and second superconductors in a manner that straddles the first and second superconductors which are insulated between regions. The superconductor has a structure in which a third superconductor thinner than the first and second superconductors is provided through an insulating film having a thickness as high as possible.

As described above, the present invention uses a phenomenon in which a magnetic field causes the superconducting-normal conductive transition of the third superconductor formed through the thin glabella insulating film, so leakage current is small and the switching magnetic field can be controlled. is easy.

EXAMPLE An example of the present invention will be described below with reference to the drawings.

A Y I Ba 2 Cu a oxide thin film of about 2 μm was formed on a strontium titanate substrate using a sputtering method, and after making it superconducting by heat treatment, one of the Y+BatCLIs oxide thin films was formed using a photolithography technique. A portion was removed by etching to isolate it into two regions, and a zirconium oxide Fjl film of about 80 layers was formed on the entire surface by sputtering, and a Y+BazCu3 oxide thin film of about 2000 layers was further formed on top of that by sputtering. . After that, heat treatment was performed to make the thin film superconducting.

Its structure is shown in Fig. 1. In Fig. 1, 1 is a strontium titanate substrate, 2.5 is a first
and the second Y, Ba2 Cua oxide thin film, 3 is the zirconium oxide thin film, and 4 is the third superconductor Y+Ba
The film thicknesses of the gCu3 oxide thin film and the third superconductor thin film are thinner than the film thickness of the first superconductor thin film. That is, the first. It has a structure in which it straddles the second substrate-side superconductor and is connected via a thin glabellar insulating film to a third superconductor that is thinner than the second substrate-side superconductor.

Electrodes 6, 7.8 are formed on each superconductor, and an appropriate bias voltage can be applied to each superconductor.

Y, Ba2 Cua oxide becomes a superconductor at about 90 volts and is a type 2 superconductor with a bulk critical magnetic field of 80 Tesla (T) or more. A type 2 superconductor is a superconductor in which a magnetic field can penetrate into the superconductor while maintaining a superconducting state.

However, the critical magnetic field of a superconductor becomes smaller as the film becomes thinner and its thickness becomes extremely thin (Y1Ba2
This tendency is remarkable in the Cu8 oxide superconductor, and the critical magnetic field is extremely sensitive to the film thickness. Therefore, the critical magnetic field of the third superconductor having the structure of this example is such that the thickness of the third superconductor is
．． Since the thickness of the second superconductor is thinner than that of the second superconductor, the first superconductor is made of the same material. Its critical magnetic field is lower than that of the second superconductor.

If an element having such a structure is cooled to a temperature below the critical temperature, in this case, below 77, the three superconductors will be 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 IV characteristics as a normal Josephson element can be obtained between the first and second superconductors and the third superconductor. This situation is shown in Figure 2A. When a current higher than the maximum current that maintains zero voltage is passed through this junction,
Zero voltage is no longer maintained and a voltage develops across the junction.

This state is shown in FIG. 2B. A magnetic field is applied to this junction.

When a magnetic field equal to or higher than the critical magnetic field of the first superconductor is applied, the m Y I B a 2 Cu s oxide rapidly transitions to the normal conducting state. In the normal conducting state, the energy gap is much larger than in the superconducting state. Become. Therefore, electron tunneling from the second superconductor to the first superconductor is no longer possible, and current no longer flows.

This changes the electron tunneling state, and since the resistance value in the normally conducting state is very high, the current is also limited by this resistance. The IV characteristics at that time are shown in FIG. 2C. Therefore, if a constant voltage is applied to the junction, the current flowing through the circuit will decrease to a value corresponding to D, and if a resistor is used as the load, the voltage across it will drop rapidly. When a constant current is flowing, a voltage corresponding to the current B is generated across the junction on straight line C. Switching can be performed in any case.

L n 1B a 2 Cu a oxide (Ln is La, Y, Nd, Sm, Eu, Gd, Tb, Dy.

At least one of Ho, Er, Tm, Yb, and Lu) superconductors are all second type superconductors with a critical temperature of about 90°C. In addition, L a +, as-A eo, +5-Cu, and oxides (Ae is at least one of Ba, Sr, and Ca) are all type 2 superconductors with a critical temperature of about 30. . Furthermore, when the film is made thinner, the critical magnetic field becomes smaller as the film thickness becomes thinner. Therefore, in the configuration of the example, it is clear that by using the above oxidized superconductor, a switching operation similar to that of the example can be obtained.

In Lrz Ba2Cu3 oxide, Ba is replaced by Sr, C
Even if substituted with a, a superconductor having characteristics similar to that of a is obtained. Therefore, as a superconductor, Ba can be used as Sr.
It is clear that similar effects can be obtained even when substituted with or Ca.

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. A superconductor has an energy gap in its superconducting state, which can be expressed by a valence band, a forbidden band, and a conduction band, just like normal semiconductors. 3 is an insulating film between the eyebrows. Now, the valence band of superconductor 2 is above the bottom of the conduction band of superconductor 4, and the valence band of superconductor 4 is above the bottom of the conduction band of superconductor 4.
Suppose that the bias is placed above the bottom of the conductive band. Then, tunneling from the valence band of the superconductor 2 to the conduction band of the superconductor 4 becomes possible, and a large amount of electrons are injected from the superconductor 2 to the superconductor 4. Electrons injected into the superconductor 4 are directly tunnel-injected into the conductive band of the superconductor 5 unless energy is lost within the superconductor 4. Even if they lose energy and fall into the valence band of the superconductor 4, tunneling from the valence band of the superconductor 4 to the conduction band of the superconductor 5 is possible, and in any case, a large amount of electrons will fall into the valence band of the superconductor 5. is injected into. Therefore, by applying a magnetic field in such a bias state and causing the superconductor 4 to transition from the superconducting state to the normal conducting state, switching of the current can be performed. If the potential of superconductor 4 is made a little higher so that the bottom of the conduction band of superconductor 4 is above the valence band of superconductor 2, no current will flow even when there is no magnetic field. As shown in this example, by adjusting the bias potential of each superconductor, the degree of freedom in controllability by current and magnetic field is increased.

Super 1! By changing the potential of the conductor 4, a type of amplification is also possible by biasing the conductor 4 to a state where switching can be controlled.

As a method for controlling the magnetic field of the element of this example, a conductor is provided near the element, and a magnetic field is generated or extinguished around the conductor by passing or cutting current through the conductor, thereby performing switching control. can be done easily.

Effects of the Invention As described above, according to the method of the present invention, since tunnel junctions between superconductors having different critical magnetic fields are used, the magnetic field can be easily controlled. In addition, when off, the two superconductors are separated by a normal conductor sandwiched between two insulating films, which is much more effective than a conventional Josephson element, which is separated by a single interlayer insulating film. leakage current is low.

The effect of the present invention is that the first and second two regions are insulated from each other.
The first and second M4
This is obtained from a structure in which a third superconductor, which is thinner than the first and second superconductors, is provided via an insulating film having a thickness that allows tunneling with the conductor. There are many possible materials that can form this structure, but
It is applicable to both.

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 may be any value within the range where the tunnel effect occurs, and the thickness of each superconductor thin film may also be within the range in which it can be formed. is optional. However, if the film becomes too thick, the critical magnetic field becomes the same as the bulk value and becomes independent of film thickness. The critical thickness is 10 μm.

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

In this example, zirconium oxide was used as the glabellar insulating film, but it is clear that the material 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. .

As described above, the present invention straddles the two superconductors, the first and second superconductors, which are insulated between regions. The present invention provides a switching element having a structure in which a third superconductor is provided which is thinner than the first and second superconductors, has a small leakage current, and can easily control a magnetic field.

[Brief explanation of the drawing]

Fig. 1 is a cross-sectional view showing an example 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 the energy band diagram of the device of the present invention. FIG. 4 is a cross-sectional view showing an example of the structure of a conventional Josephson element, and FIG. 5 is a graph showing the IV characteristics of the conventional Josephson element. 1... Strontium titanate substrate, 2,4°5... Y I B a 2 Cu 3 oxide W4 superconductor, 3... Interlayer insulating film, 6. 7.8
······electrode. Name of agent: Patent attorney Toshio Nakao
8

Claims (3)

[Claims] (1) Straddling the first and second superconductors that are insulated between regions, the first and second superconductors are 2
A superconducting switching element having a structure including a third superconductor having a thickness thinner than that of the superconductor. (2) As a superconductor, Ln (Ln is Y, La
, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er,
at least one of Tm, Yb, Lu) - Ae (however, A
The superconducting switching element according to claim 1, wherein e is at least one of Ba, Sr, and Ca)-Cu oxide. (3) A superconducting switching element according to claim (1), characterized in that each superconductor is provided with a terminal for applying a bias voltage.