JPH0774401A - In-situ type josephson junction structure - Google Patents

Abstract

PURPOSE:To realize in-site type Josephson junction by crystal synthesis without depending on a high thin film technique and an ultrafine technique by realizing energization direction of c-axis direction inside a crystal in an oxide superconductor having laminar crystal structure vertical to c-axis. CONSTITUTION:It is demonstrated by following two points that unit cell of a single crystal Bi2Sr2CaCu2O8 formed from liquid phase reaction forms respective Josephson junctions. At first, since it is a good single crystal crystallographically, the content of an impurity layer does not exceed 0.1%. If an impurity layer provides characteristic as a Josephson junction, fine steps which appear in current/voltage characteristic are at most several. Therefore, a number of steps which appear in current/voltage characteristic are not due to inclusion of an impurity layer. Secondly, if a hopping of current/voltage characteristic is due to inclusion of an impurity layer, fine step does not appear at a minimum unit of 0.2 to 0.3mV.

Description

Detailed Description of the Invention

BACKGROUND OF THE INVENTION This invention relates to in-situ (I
n Situ) type Josephson junction structure.
More specifically, the present invention is useful for devices operating in a wide range of frequencies from the microwave region to the infrared region, and is expected to contribute significantly to the development of microelectronics.
tu) type Josephson junction structure.

2. Description of the Related Art Conventionally, the Josephson junction structure has been attracting attention as one that opens up a new world of electronics, and various attempts have been made to achieve it. In order to form this Josephson junction, it is necessary to provide a barrier of the same length as the superconducting coherence length between the superconductors, so in the past, it was necessary to make full use of extremely advanced thin film forming technology and ultrafine processing technology. Its production has been tried. However, unlike the case of the conventional metal or alloy system which has been the object of the Josephson junction, in the case of the oxide high temperature superconductor, which is expected to lead to new electronics innovation in recent years, The superconducting coherence length is extremely short, which corresponds to the interatomic distance. For this reason, control technology at the atomic layer level is required for forming the Josephson junction, and this is extremely difficult in reality, so it is the current situation that good-quality junction formation has not yet been achieved. is there. Therefore, it has been desired to realize a new technical means capable of forming a new Josephson junction while taking advantage of the characteristics of the oxide-based high temperature superconductor.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, overcomes the limitations of the prior art, and provides a Josephson junction structure with an oxide high temperature superconductor. It is a thing. That is, the present invention is an oxide-based superconductor having a layered crystal structure perpendicular to the c-axis, wherein the in-crystal c-axis direction is the current-carrying direction. Provide structure.

The oxide-based high-temperature superconductor usually has a layered crystal structure perpendicular to the c-axis, and the flow of the superconducting current is limited to a specific layer (for example, Cu-O plane). An insulating layer or a normal conducting layer is interposed between the two. For example,
B in the case of Bi-Sr-Ca-O-based oxide superconductor
The i-O plane plays this role. Strong anisotropy,
In Bi ₂ Sr ₂ CaCu ₂ O _{8 and the} like, which have a short coherence length, experimentally observed semiconductor-like or insulator-like characteristics different from the ordinary metal-like conduction mechanism as the conductivity characteristics in the c-axis direction. It is expected that the conduction in the c-axis direction in the superconducting state is weakly coupled, unlike the conduction in the ab-direction. However, up to now, it has been extremely difficult to obtain a good quality single crystal, and therefore, an experimental example demonstrating this experimentally has not been published. The present invention was made by paying attention to the original property of the crystal inherent in such a high-temperature superconductor, and using the intrinsic weak bond existing in the c-axis direction, The principle structure for forming a high-quality Josephson junction, which has not yet succeeded in the current highly developed thin film growth technology, has been established. That is, the inventor of the present invention pays attention to the layer structure itself of the high-temperature superconductor, and the layer structure itself is a Josephson junction (essential junction, Intrinsic Junction), or the layer at the time of crystal growth. We confirmed that the structural changes that might be introduced during the construction functioned as the Josephson junction, and provided this as a new Josephson junction structure. The feature of this structure itself is that it is introduced at the time of synthesizing superconductors (In Situ Junction), as compared with the conventional artificial construction, so it is extremely easy and of high quality. A Josephson device with constant characteristics can be obtained. The structure is characterized by being controllable during synthesis.
In fact, as the oxide-based superconductor in the present invention, L
a-Sr-Cu-O system, La-Ba-Cu-O system, Y-
Ba-Cu-O system, Ln (lanthanide) -Ba-Cu-
O-based, Bi (Pb) -Sr-Ca-Cu-O-based, Tl-
Ba (Sr) -Ca-Cu-O system, Hg-Ba-Ca-
Various materials such as Cu-O type are used. The Josephson junction structure in these superconductors may be a single crystal or a polycrystalline body having crystallinity similar to that of a single crystal, such as a single crystal produced by a melting method or a vapor phase method. Provided is the Josephson junction of the invention. Looking at the coherence length of superconductivity in a typical high-temperature superconductor, the coherence length in the c-axis direction is particularly short in the Bi system and Tl- system, which have strong anisotropy, and it depends on the method. It is about 1Å or less. Even in the La-system and the Y (Ln) -system, which have relatively small anisotropy, it is about several Å to 10Å. From the crystal structure, these superconducting C
The distance between uO ₂ planes is 15Å (2212 system) in Bi-system, 1
The values of 8Å (2223-system) and Tl-system are almost the same because of the similarity in crystal structure. La- with relatively small anisotropy
The system and the Y (Ln) -system also have about 11Å. Furthermore, the electric resistance in the c-axis direction in the normal state is Bi with strong anisotropy.
The − system and the Tl − system show a tendency to increase as the temperature decreases, and are so-called semiconductor-like. In this regard, La-
In the system and Y (Ln)-system, the situation is slightly different, and the c-axis conduction morphology is sensitively dependent on the hole concentration of the system. Generally, in the region where the hole concentration is low, it is more anisotropic and the conduction is more semiconducting. Things are known. As for the anisotropy in the superconducting state, the anisotropy in the normal state can be directly taken from many experimental results. The most definitive experimental evidence as a Josephson junction is that anomalies peculiar to the Josephson junction are observed especially in the c-axis current-voltage characteristics of anisotropic superconductors. This Josephson characteristic is confirmed by precisely measuring the temperature dependence of the current-voltage characteristic in the c-axis direction using a Bi-type high-quality single crystal that is the most anisotropic high-temperature superconductor. Since the weak coupling property in the voltage-current characteristic depends on the cross-sectional area in the direction of current flow, the Josephson junction characteristic does not depend only on the essential characteristic in the c-axis direction, but its cross-sectional area, that is, the normal characteristic. Resistance value R of weak junction
It is also defined by n. In the case of Bi-system, the IcRn value (Ic is the Josephson critical current value) as a value defining the weak junction is about 0.1 to 1 mV for each layer. This means that even in a system where the anisotropy is small and the weak junction in the c-axis direction is not so weak, the Josephson weak junction can be formed by limiting the cross-sectional area in the current direction. In the structure of the Josephson junction of the present invention, the Josephson junction is formed during the synthesis of the material, and it is not necessary to use the advanced and cumbersome vapor deposition technique as in the past, and the formation is extremely easy. is there. Also, a uniform and high quality joint can be obtained. Hereinafter, the present invention will be described in more detail with reference to examples.

[Example] The first feature of the Josephson device is as follows.
It means that the device is weakly superconducting.
This weak coupling can be directly measured by the current-voltage curve of the junction, and the critical current value Ic and the electrical resistance value Rn in the normal state can be measured.
And IcRn = constant relationship holds. And
In the case of the in-situ Josephson junction, the electrical resistance value in the normal state is defined by the specific resistivity value ρ _{c in} the c-axis direction and the shape of the sample (cross-sectional area S and thickness t). Here, since Rn is proportional to the thickness t of the sample and inversely proportional to the cross-sectional area S, the cross-sectional area is small and the Rn value can be large in the thick sample. Since the critical current value of the sample is proportional to the cross-sectional area, IcRn = ρ _c J _C t
The IcRn value is proportional to the material-specific parameters ρ _c and J _c (critical current density) and the thickness t. Since the parameter peculiar to a substance is an amount that is determined if the substance is determined, the IcRn value is ultimately determined by the thickness of the sample. In practice, it is preferable that the current value is low for the convenience of measurement, and therefore the cross-sectional area must be small. Therefore, a good quality single crystal Bi ₂ Sr ₂ CaCu ₂ O ₈ grown from the liquid phase reaction was used as a sample, and the cross-sectional area of this sample was set to about 0.1 mm × 0.1 mm.
The thickness was 0.03 to 0.05 mm. Rn-100-
It is 300Ω. The direct current two-terminal method as shown in FIG. 1 was adopted for the measurement of the characteristics. FIG. 2 schematically shows an example of current-voltage characteristics. It can be seen that at a temperature of 67.5 k, the voltage suddenly rises near 11 mA when the current is increased, and reaches the electrical resistance in the normal state.
This means that the superconducting weak bond is broken at this point. When the current value is gradually decreased from this state, it does not coincide with the rising time as shown in FIG. 2, and the normal state continues to the low current side, and eventually becomes zero at around 6 mA. Indicates that superconductivity has been restored. Such a characteristic hysteresis phenomenon is a characteristic of the Josephson junction that is capacitively coupled. Although not shown in the figure, it is also a characteristic of the Josephson junction that a negative current value appears as a negative voltage in contrast to the case of a positive current. Such current-voltage characteristics appear in the entire temperature range from just below the critical temperature to the low temperature side. FIG. 3 shows the temperature dependence of the critical current density J _c . This curve almost agrees with the Ambegaokar-Baratoff formula for J _c of Josephson junctions,
The critical current density on the low temperature side is about J _c (0) = 240 ± 30
A / cm ² . Further, the critical current density J _c ^{ab in the ab-} plane of the crystal has already been known from many experimental values, and J _c
^ab (0) to 10 ^{5 to} 10 ⁶ A / cm ² , the magnitude of the anisotropy γ is γ = J _c ^ab (0) / J _c = 400−.
The measured value is 4000, and the other measured results, for example, the anisotropy of the electric resistance and the anisotropy value obtained from the measured result of the magnetic torque, are consistent with the measured result. As shown in FIG. 1, for example 4.2K,
Although it appears remarkably on the low temperature side, a tail appears in the current-voltage characteristic when the current value approaches the critical current value (the curve of the rising edge of the current-voltage characteristic becomes dull). FIG. 4 is an enlarged view of this portion. The skirt of this curve is made up of many small voltage jumps, and its minimum step width is about 0.2-0.3 mV when directly read. Since the total voltage is about 3V, the number of jumps is 1 × 10 ⁴ −1.5 × 10 ⁴ . If the thickness of the sample is t = 30 μm, the number of in-situ Josephson junctions is 2 × (30 μm / 30Å) = 2 × 10 ⁴ , and the number of voltage jumps and the number of in-situ Josephson junctions are almost the same. . This means that single crystal Bi ₂
It has been proved that each unit cell of the crystal of Sr ₂ CaCu ₂ O ₈ forms one Josephson junction. In addition, since the crimp type two-terminal method as shown in FIG. 1 is used as the cause of the tail of the current-voltage characteristic, about 10% of the upper and lower surface layer side of the sample is exposed to pressure and other factors. It can be considered that, due to a mechanical factor, a bond different from the internal bond state was formed (the bond strength further deteriorated due to damage). Since the base of this current-voltage characteristic appears large or small when the sample is changed and the measurement condition is changed, there are some that show a very sharp transition depending on the condition, so the nonuniformity of the Josephson junction of the sample Is considered to be a reflected result. Incidentally, from the above, the i _c r _n value as the basic unit of the in-situ Josephson junction can be estimated as about 0.3 mV. That is, the voltage jump is composed of a large number of small jumps when expanded, and the product of the number of jumps n and the jump voltage v is approximately equal to the voltage output V in the normal state (V to nv). This indicates that the voltage output of the voltage-current characteristic is that each superconducting layer forms a Josephson junction and is connected in series. It should be noted that the current-voltage characteristics as described above do not reflect the impurities contained in the inside of the sample, and the Bi ₂ Sr ₂ CaCu ₂ O ₈
It is clear from the following that it is due to the inherent properties of the sample. That is, first of all,
The single crystal of Bi ₂ Sr ₂ CaCu ₂ O ₈ used for the sample is
This is because it is a crystallographically high quality single crystal containing very few impurities. As can be seen immediately from FIG. 5 showing this crystal structure, the crystal has a layered structure of blocks of each element, and CuO ₂ --SrO--
(BiO-BiO) -SrO-CuO 2 -CaO-Cu
It has a repeating structure of O ₂ . In this structure, superconductivity is CuO ₂ -CaO-CuO ₂ layer CuO
_It has been reported that the _two surfaces are responsible, and the other layer between these two layers of CuO ₂ , especially the bilayer of (BiO—BiO) is a layer with weak superconductivity. The impurities emphasized here do not simply refer to the impurity elements substituted with Bi, Sr, Ca, Cu, and O, but mean layer defects (Intergrowth) that enter each block layer. In particular,
What is known as an impurity layer is that the CuO ₂ —Ca layer is missing from the structure of FIG. 5, Bi ₂ Sr ₂ CuO ₆ , and C.
uO ₂ Bi -Ca layer is inserted ₂ Sr ₂ Ca ₂ Cu ₃
It is O ₁₀ . According to a precise measurement using a detailed high resolution electron microscope, the single crystal Bi ₂ Sr ₂ C used in this example was
The content of these impurity layers in aCu ₂ O ₈ is 0.1%
It is the following. That is, about 1 in a sample with a thickness of 30 μm.
Mixing of 0 impurity layer is expected, but by carefully selecting the sample, it is possible to take out a sample free from mixing of different phases. Such a sample is used in this example.
However, if such an impurity layer is mixed in the sample used in this example to give the characteristics as a Josephson junction, it is possible to obtain the minute voltage as shown in the current-voltage characteristics of FIG. Only about 10 steps are expected, so it is clear that the large number of steps as seen in FIG. 4 is not due to the incorporation of such an impurity layer. Secondly, although related to the above point, if the jump of the current-voltage characteristic is caused by the mixing of the impurity layer, the minute step as seen in FIG. It is unlikely that it will appear in the form of a minimum unit of about 0.3 mV. Naturally, an irregular number of irregular jump values appear. The number of in situ Josephson junctions included in the sample,
The fact that the number of junctions expected from the jumping of the basic unit of the current-voltage characteristic is almost the same with good reproducibility indicates that the periodic structure in the unit envelope of the crystal structure derived from the basic structure is the basic unit of the Josephson junction. It is a direct proof of this. From the above two points, it can be concluded that Bi ₂ Sr ₂ CaCu ₂ O ₈ has an intrinsic Josephson junction formed in its crystal. Therefore, in an oxide superconductor having a layered crystal structure perpendicular to the c-axis direction, an in-situ Josephson junction structure is realized by the means of the present invention in which the in-crystal c-axis direction is the conducting direction. become. Of course, the present invention is not limited to the above examples and can be variously applied to various crystals.

As described in detail above, according to the present invention, an in-situ Josephson junction is formed by crystal synthesis without depending on the advanced thin film technology or the ultrafine technology. Intrinsic weak junctions associated with crystals
Junction) can be regarded as a series connection of individual Josephson junctions in a unit package.In addition, by controlling the current in the vertical Josephson connection in the middle layer, a multifunctional ultra-fast switching device, a micro It is considered to have a wide range of applications such as wave mixers, and will have a great impact on high-speed, high-density computers, high-sensitivity microwave communication, etc. in the near future, and its social effect is extremely large.

[Brief description of drawings]

FIG. 1 is a perspective view illustrating a sample and a two-terminal clamp measuring device.

FIG. 2 is a schematic diagram of current-voltage characteristics as an example.

FIG. 3 is a correlation diagram showing the temperature dependence of the critical current value J _c .

FIG. 4 is an enlarged view of an experiment chart of the skirt field for the current-voltage characteristic as an example.

FIG. 5 is a crystal structure diagram of Bi ₂ Sr ₂ CaCu ₂ O ₈ .

Claims (4)

[Claims] 1. An in-situ Josephson junction structure, characterized in that, in an oxide-based superconductor having a layered crystal structure perpendicular to the c-axis, the in-crystal c-axis direction is the conducting direction. 2. The Josephson junction structure according to claim 1, wherein the crystal is a single crystal or a polycrystalline body having crystallinity close to that of a single crystal. 3. The Josephson junction structure according to claim 1, wherein the crystal is a single crystal produced by a melting method. 4. The Josephson junction structure according to claim 1, wherein the crystal is a single crystal prepared by a vapor phase method.