JP2921697B2 - Superconducting storage element - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to a storage element using a superconductor.

[Prior art and its problems]

Conventionally, as a storage element using a superconductor, an element using a Josephson junction has been proposed. This has already been prototyped using a metallic superconductor. However, the metal-based superconductor has a very low critical temperature, and requires helium which is expensive and unevenly distributed in terms of resources for cooling.
On the other hand, in recent years, superconductors exhibiting superconductivity even at a high temperature equal to or higher than the temperature of liquid nitrogen have been found in oxides. It is a so-called oxide high-temperature superconductor. It is considered that a storage element can be operated by cooling with liquid nitrogen if a storage element is made by using it as in the related art. However, these oxide high-temperature superconductors have an extremely short coherent length and strong chemical reactivity, so that no artificial Josephson junction has been produced.
The Josephson effect reported to date is due to naturally occurring crystal grain boundaries, which are difficult to control artificially, making fabrication of the device impossible. Therefore, development of an element (including a storage element) that does not use a Josephson junction at all has been required.

As described above, an object of the present invention is to provide a device that does not include any difficult-to-manufacture device such as a Josephson device.

[Means for solving the problem]

For this purpose, a superconductor-insulator-normal conductor junction called a quasiparticle tunnel junction or SIN junction is used instead of a Josephson junction. This is because the Josephson junction is a superconductor-insulator-superconductor junction, so even after all, high-temperature processing is necessary to form the superconductor again after forming the insulator layer. On the other hand, once the superconductor layer is formed, the subsequent insulator layer and normal conductor layer can be manufactured at room temperature with a simple device (for example, a vacuum deposition device or a sputtering film deposition device) with good controllability. It is. In addition, the speed of this junction is by no means slower than that of the Josephson junction.

The storage element according to the present invention comprises an oxide superconductor layer, a thin film of an insulator and a metal as shown in FIG. 1 (d). In some cases, the insulator film between the oxide superconductor and the metal layer is It may not be provided. This element may not be provided with an insulator layer. This element is essentially a superconductor-insulator-normal conductor (SIN) with or without an insulating layer.
(Abbreviated as). In this example, there are two SIN junctions, but three or four junctions may be connected in series as needed.

Next, the operation principle of this element will be described. FIG. 3 shows a conceptual diagram of voltage-current characteristics at a finite temperature below the critical temperature of the superconductor of this element. Here, Δ is the energy gap of the superconductor (the unit is electron volt ev), and e is the elementary charge 1.
6 × 10 ^-19 c. When there is one SIN junction, the magnitude of the voltage when the current rises is Δ / e. In this case, since there are two SIN junctions, it is twice as large as 2Δ / e.
If the energy gap becomes smaller for some reason, the voltage-current characteristic at that time becomes as shown by the broken line in FIG.

Now, when the energy gap is Δ, V
Suppose that a bias voltage V of <2Δ / e is applied. The energy level at this time is as shown in FIG. Second
The figure shows the energy levels of the present invention, where (22) is a superconductor, (21) is a normal conductor, Δ is an energy gap, and V is a bias voltage.

The superconductor (22) has a gap in its energy level by 2Δ around its Fermi surface (23). Arrows indicate tunnel current. A tunnel current is observed due to a change in the energy gap due to light irradiation. Here, it is assumed that light having an energy of 2Δ / e or more is irradiated. The superconducting carrier pair absorbs this light and becomes two normal carriers or quasiparticles, which are excited above the gap. At the same time, the energy gap of the superconductor is narrowed by the non-equilibrium superconductivity effect. The energy gap Δ at this time is expressed as follows: Δ = Δ (0) −2n _qp / N (0) (1) Where n _qp is the amount of quasiparticles, Δ (0)
Is the superconducting energy gap at absolute zero, N
(0) is the state density of the Fermi surface in the normal state of the superconductor.

When the energy gap becomes equal to or less than V, a current flows through the device due to a tunnel effect (FIG. 2B). However, carriers that flow from the left normal conductor to the superconductor behave as quasiparticles. If the current is sufficiently small, the change in Δ is also small, as is apparent from equation (1).
It returns to the original superconducting state when the light irradiation ends, but if a large amount of current is flowing, the light is shut off and Δ does not return to the original state. Flow (FIG. 2 (c)). That is, it can be regarded as a memory state. Referring to FIG. 3, the bias voltage V
Is at the point A when the state is just applied. When this is irradiated with light, it moves to point B, and remains at that position even when the light is gone. The movement from the point A to the point B is determined by the speed of change of the energy gap. However, since the time required for this process is considered to be 1 to 10 ps, the operation can be performed at an extremely high speed.

As described above, the element of the present invention is an element having two or more junctions of a superconductor layer and a metal layer (SN junction) or junctions of a superconductor layer, an insulating layer and a metal layer (SIN junction) in series. At a temperature below the conduction critical temperature, nΔ / e (where Δ is the energy gap of the superconductor (unit: electron volt ev), e is the elementary charge 1.6 × 10 ^-19 c,
(n is the number of junctions) Write and store data by applying a light to the superconductor layer while applying a voltage of less than or equal to the number of junctions, thereby causing a tunnel current to flow and continuing to flow even after the light is cut off. It is an element that can be used.

The superconductor used in the present invention is a superconductor composed of yttrium (or another lanthanoid element) -barium-copper-oxygen, or mainly bismuth-strontium-calcium, which is known as a low carrier concentration superconductor. A superconductor composed of copper-oxygen, or a superconductor composed mainly of thallium-barium-calcium-copper-oxygen, or a superconductor composed of lanthanum-alkaline earth metal-copper-oxygen, or barium-potassium-bismuth A superconductor composed of oxygen or a superconductor composed of barium-bismuth-lead-oxygen can be used.

 Hereinafter, the present invention will be described in more detail with reference to Examples.

Embodiment FIG. 1 shows a manufacturing process of a superconducting memory element of the present invention.

Oxide superconductor Bi ₂ Sr ₂ CaCu ₂ O ₈ thin film (2) (thickness is about
0.1 μm) was formed on a magnesium oxide single crystal (100) plane substrate (3) by a usual sputtering method, and an aluminum oxide thin film (1) having a thickness of about 10 nm was further deposited thereon (FIG. 1 (a) )). The superconductor film (2) and the aluminum oxide film (1) were extremely flat films, and no grain boundary was observed. X-ray analysis revealed that the C-axis of this superconductor film was perpendicular to the substrate. In addition, the susceptibility measurement revealed that superconductivity was exhibited below 90K.

The insulating layer (1) and the oxide superconductor thin film (2) were etched by ordinary photolithography to form a square having a side of 5 μm (FIG. 1 (b)).
Further, as shown in FIG. 1 (c), a thin film (5) of aluminum oxide having a thickness of about 0.1 μm was formed thereon by a vacuum deposition method using a mask (4). Finally, a gold thin film (6) (thickness of about 0.1
μm) to form a SIN junction. This gold thin film (6)
Serves also as an electrode of the element. The spacing between two SIN junctions is about 2μ
m.

The characteristics of the device manufactured in this manner were examined. The device was cooled with liquid nitrogen, and 10 mv was applied as a bias voltage. In this state, Nd: YAG laser light (wavelength 1.06 μm) having a pulse width of about 10 ns was irradiated. It was found that the element was brought into a memory state by irradiation with light of 0.1 mJ / cm ² or more. At this time, the magnitude of the current flowing through the element was about 5 μA.

〔The invention's effect〕

According to the present invention, a conventionally known superconductor (particularly, a low carrier concentration superconductor) and an oxide high-temperature superconductor having a critical temperature exceeding a critical temperature exceeding a liquid nitrogen temperature recently discovered have been obtained. It can be applied to the storage element of the present invention. The storage element of the present invention requires only a very short time for writing and has a simple structure and manufacturing method because there is no Josephson junction. The storage element of the present invention can be widely used as a storage element of a computer using a superconductor element or a computer using a semiconductor.

According to the present invention, a memory element using an oxide superconductor excellent in mass productivity and reproducibility can be obtained. Since this storage element is a superconductor element that operates sufficiently even at the temperature of liquid nitrogen, the operating cost can be lower than that of a conventional superconductor element using a metal-based superconductor. Also, since no Josephson junction is used, the structure is simple and the fabrication is easy.

According to the present invention, after the superconductor layer is manufactured, the insulator layer or the normal conductor layer can be manufactured with a simple device at room temperature, which facilitates the manufacturing.

As is clear from the above, the present invention is an industrially useful invention.

[Brief description of the drawings]

FIG. 1 is a diagram showing a method for manufacturing a memory device of the present invention. FIG. 2 is an energy level diagram of the memory device of the present invention. FIG. 3 is a diagram showing current-voltage characteristics of the memory device of the present invention. when not irradiated, when the broken line is irradiated with light, and thereafter) 1 ...... insulating layer (aluminum oxide) 2 ...... superconductor layer _{(Bi 2 Sr 2 CaCu 2 O} 8) 3 ...... substrate (magnesium oxide) 4 Mask 5 Insulating film (aluminum oxide) 6 Normal conductor film (gold)

Claims (1)

(57) [Claims] 1. A superconducting memory element having a superconductor layer-metal layer junction or a superconductor layer-insulating layer-metal layer junction, wherein said superconducting memory element is written or stored in said superconducting memory element. A superconducting storage element characterized in that a body layer is irradiated with light.