JP5144143B2 - Superconducting material and manufacturing method thereof - Google Patents

Description

  The present invention relates to a superconducting material based on nanotechnology, and relates to a superconducting material for producing and using a novel superconducting nanocomposite material and a method for producing the same.

  Since the discovery of the superconducting phenomenon in 1911, the search for materials that exhibit superconductivity has been investigated for a large number of material systems. Niobium-based superconducting materials are still being actively searched even at present when superconducting magnets and SQUID devices are put into practical use, and expectations for new superconducting materials are high (Patent Document 1, 2, 3, 4, see Non-Patent Documents 1, 2, 4, 5, 6, 7, and 8).

Japanese Patent Laid-Open No. 10-053494 Japanese Patent Application Laid-Open No. 8-306973 Japanese Patent No. 2701732 Japanese Patent No. 3248210 JP 2003-096555 A JP 2002-162374 A J. Mannhart, et al., "Influence of Effect Fields on Pinning in YBa2Cu3O7-σ Films", Phys. Rev. Lett., Vol. 67, No. 15, pp. 2099-2101, 1991. H. Takayanagi, et al., "Superconductivity Proximity Effect in the Native Inversion Layer on InAs", Phys. Rev. Lett., Vol.54, No.22, pp.2449-2452,1985. S. Hirono, et al., "Superhard conductive carbon nanocrystallite films", Appl. Phys. Lett., Vol. 80, No. 3, pp. 425-427, 2002. Y. Takano, et al., "Superconductivity in diamond thin films well above liquid helium temperature", Appl. Phys. Lett., Vol. 85, No. 4, pp.2851-2853, 2004. M. Kocial, et al., "Superconductivity in Ropes of Single-Walled Carbon Nanotubes", Phys. Rev. Lett., Vol.86, No.11, pp.2416-2419, 2001. I. Takesue, et al., "Superconductivity in Entirely End-Bonded Multiwalled Carbon Nanotubes", Phys. Rev. Lett., PRL 96, pp. 057001-1-057001-4, 2006. T.E.Weller, et al., "Superconductivity in the intercalatedgraphite compounds C6Yb and C6Ca", Nature Physics, Vol.1, pp.39-41, 2004. K. Tanigaki, et al., "Superconductivity in sodium- and lithium-containing alkai-metal fullerides", Vol.356, .pp.419-421,1992.

  By the way, as you can see from the examples of copper oxide high-temperature superconductors discovered in the 1980s and magnesium diboride discovered in the 2000s, it was a material system that was not expected to have superconductivity before the discovery. This accidental discovery is the key to the development of new superconducting materials. Such a situation is due to strong constraints on the appearance of the superconducting phenomenon. In general, a high electron concentration and a large electron-lattice interaction are required, and there is a limit to a material system in which a superconducting state appears.

  In most cases of superconducting research and development, there is a material exhibiting superconductivity first, and a desired performance is obtained by precise composition control and structure control of this material. Materials that do not become superconducting at the bulk level are generally not considered. However, many material systems have already been studied so far, and it is difficult to find a new material system that is in a superconducting state at a bulk level.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a novel material system that is in a superconducting state at a bulk level.

  The superconducting material according to the present invention includes a substrate and a plurality of superconducting regions that are disposed in the substrate and that exhibit superconducting characteristics under a predetermined condition made of the same element as the substrate. It is designed to be separated by a distance indicating a conductive proximity effect.

  The superconducting material according to the invention includes a substrate and a plurality of superconducting regions formed by partially modifying the substrate and exhibiting superconducting characteristics under predetermined conditions disposed in the substrate. The matching superconducting regions are separated by a distance exhibiting a superconducting proximity effect.

  In the superconducting material, for example, the substrate has a lower carrier concentration than the superconducting region.

In the superconducting material, the substrate is composed of carbon atoms of A Amorphous state, the superconducting region, Ru der those composed of carbon binding by sp ² hybrid orbital. Incidentally, disposed within the substrate, it may be to include a plurality of regions made up of carbon binding by sp ³ hybrid orbital.

The superconducting material manufacturing method according to the present invention includes a step in which a substrate is formed on a substrate, and the substrate is irradiated with a particle beam to locally modulate the structure and use the same element as the substrate. A plurality of superconducting regions exhibiting superconducting characteristics under a predetermined condition, wherein the superconducting regions adjacent to each other are formed in the substrate at a distance apart from each other to exhibit a superconducting proximity effect , and the substrate comprises: consist of carbon in the amorphous state, the superconducting region is obtained by the so that configure carbon binding by sp ² hybrid orbital.

  As described above, according to the present invention, it is provided with a plurality of superconducting regions arranged in a substrate and having superconducting characteristics under a predetermined condition made of the same element as the substrate, and adjacent superconducting regions are superconducting. Since the distance indicating the proximity effect is separated, an excellent effect of providing a new material system that is in a superconducting state at the bulk level can be obtained.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

First, an embodiment of the present invention will be described. FIG. 1 is a sectional view schematically showing a configuration example of a superconducting thin film 101 made of a superconducting material in an embodiment of the present invention. The superconducting thin film 101 includes a substrate 111 made of amorphous carbon, and a plurality of fine nanographites (super ² ) composed of a portion of sp ² hybrid orbitals locally formed in the substrate 111 (sp ² bond). Conductive region) 112 and a plurality of fine nanodiamonds 113 composed of a portion of sp ³ hybrid orbitals locally formed in substrate 111 (sp ³ bonding). Adjacent nanographite 112 is formed at a distance that exhibits a superconducting proximity effect.

The sp ² -bonded carbon does not exhibit superconductivity in the bulk state, but has a diamond state portion (nanodiamond 113) rich in sp ³ bonds that have excellent mechanical strength. Is in a state where a large distortion is applied. In the superconducting material in the present embodiment, the internal stress derived from the high mechanical strength of the nanodiamond 113 causes the nanographite 112 to be strained to exhibit superconductivity.

  This superconducting material forms a part where local structural modulation is made on carbon, which is one of the material systems that have not been confirmed to be superconducting in the past. Is formed. As is well known, when two superconducting regions are close to each other, a superconducting current flows in a non-superconducting region between the two superconducting regions (superconducting proximity effect; (See Patent Documents 1, 2, 4, and Non-Patent Document 2). Therefore, according to the superconducting material in the present embodiment, adjacent nanographites 112 are connected to each other by the superconducting proximity effect, and the superconductor is formed as a whole. Thus, according to this Embodiment, the novel material which will be in a superconducting state in a bulk level is obtained.

  Next, a method for manufacturing the superconducting thin film 101 will be briefly described with reference to FIG. First, as shown in FIG. 2A, a substrate 102 made of single crystal silicon is prepared. Next, as shown in FIG. 2B, silicon oxide is formed on the surface of the substrate 102 by a well-known thermal oxidation method. An insulating layer 103 made of is formed. Next, using a well-known ECR (Electron Cyclotron Resonance) sputtering apparatus, sputtering is performed by argon ECR plasma using high-purity graphite as a target, and carbon is deposited on the insulating layer 103. As a result, as shown in FIG. 2C, a state in which the superconducting thin film 101 is formed on the insulating layer 103 is obtained.

  In sputtering in the formation of the superconducting thin film 101 described above, an ECR ion flow of argon gas ionized using an ECR ion source is irradiated onto the insulating layer 103 (substrate 102) and deposited on the insulating layer 103. The carbon film is irradiated with low energy and high density ions. By this ion irradiation, the carbon film deposited on the insulating layer 103 is partially modified, and the nanographite 112 and the nanodiamond 113 are formed in the substrate 111. These states are confirmed by observation with a transmission electron microscope. Further, by applying RF (bias) to the substrate 102 to accurately control and adjust the ion irradiation energy, the state of local (partial) modification of the formed carbon film can be adjusted. Is also possible. Such film formation control makes it possible to manufacture a higher-performance superconducting material, and for example, it is considered possible to increase the temperature at which the superconducting state (electric resistance is 0) can be increased.

  The superconducting thin film 101 formed as described above is formed into a predetermined pattern by patterning using a known photolithography technique and etching technique, and the current-voltage characteristics are measured by four-terminal measurement using the obtained pattern. As a result of the investigation, it was confirmed that the electric resistance became 0 at a temperature of 4K. Moreover, it was calculated from the measured current-voltage characteristic that the superconducting transition temperature (Tc) was 25K.

By the way, in the above, an example was shown in which a nano-graphite was induced to be a superconducting material by internal stress derived from the high mechanical strength of nano-diamond. It is possible to realize a superconducting state by controlling to. For example, as shown in FIG. 3, a plurality of fine nano-particles composed of a substrate 301 made of amorphous carbon and a bond (sp ² bond) part formed by sp ² hybrid orbitals locally formed in the substrate 301. What is necessary is just to set it as the structure provided with the graphite 302. FIG. Adjacent nanographites 302 are formed at a distance apart that exhibits a superconducting proximity effect.

  By forming the nanographite 302 by controlling the dimensions (particle diameter) and the state of the edges (corners, edges) precisely, the nanographite 302 can be in a state in which a large strain is applied. If superconducting properties are imparted by applying a large strain to the nanographite 302 as described above, a superconducting material having a plurality of fine regions exhibiting superconducting properties can be obtained as described above.

  By the way, it has been reported that a superconducting phenomenon is observed in carbon materials such as graphite and fullerene into which impurity atoms are introduced. From these facts, the superconducting material in the above-described embodiment can be expected to improve the superconducting characteristics by introducing similar impurities. For example, superconducting properties can be improved by introducing less than 10% of alkali metals and alkaline earth metals such as potassium and calcium.

  In the above-described embodiment, the nanographite 112 has a sufficient carrier concentration even if no impurity is introduced, and thus can be a superconducting region. However, the nanographite 112 induces carriers in the nanodiamond 113 portion. It cannot be a superconducting region. However, by introducing impurities such as boron, sufficient carriers can be induced even in the nano diamond 113 portion, and this region can also function as a superconducting region. In this case, in addition to the nanographite 112, the nanodiamond 113 portion may be formed in a state of being spaced apart by a distance indicating the superconducting proximity effect.

  As a method for producing the carbon film, it is known that a sputtering method is suitable, and high results can be obtained by using high-purity graphite as a target and a high-purity inert gas as a sputtering gas. Even if other targets and sputtering gas are used, there is no problem as long as the same effect can be obtained.

Carbon-based thin films prepared by methods other than sputtering using a carbon target, such as adamantane (C ₁₀ H ₁₆ ) thin films mainly composed of sp ³ bonds, or similar compounds, or phenanthrenes mainly composed of sp ² bonds Polycyclic aromatics such as (C ₁₄ H ₁₀ ) and pentacene (C ₂₂ H ₁₄ ) may be deposited on the substrate by a sublimation method or spin coating. However, if hydrogen is contained in the carbon film, a film having good electrical characteristics cannot be obtained. Therefore, in order to make this a superconductor, at least a part to be used as a superconducting region is completely removed. It is necessary to do quality.

  In addition, HOPG (Highly Oriented Pyrolytic Graphite) is used as a carbon-based material that does not contain hydrogen, and a minute region that has been partially modified by irradiation with particle beams such as ion irradiation. By forming it stochastically, a superconducting material similar to that described above may be used. However, since only the surface layer of the material is modified by ordinary modification means, it is necessary to use it in consideration of this.

  As described above, according to the present invention, a material system that has not been confirmed to have superconductivity has been subjected to local structural modulation and partially provided with superconductivity. Is to be obtained. Heretofore, since the superconducting properties of a material having a uniform composition and structure are considered to be the best, a homogeneous material system has been the object in many superconducting material searches. In the material search process, a material system that can obtain a homogeneous sample by a normal and relatively simple method, and in particular a material system that does not provide superconducting properties to be observed, is almost searched. Not.

  However, the inventors have speculated that if nanostructure control technology is applied, superconducting properties will be developed even in material systems that do not exhibit superconducting properties, and various studies have been conducted. It was discovered that this is possible, leading to the recall of the present invention. The inventors discovered that a superconducting state is manifested in bulk carbon, which is a material that has not been reported so far, by local structural control.

  Carbon is a material whose superconducting performance is significantly deteriorated when the above-described superconducting region that can be realized locally is increased at a certain rate or more. Therefore, as in the case of carbon, if the superconducting region that can be realized locally is increased at a certain rate, superconducting properties that are particularly noticeable can be obtained in systems where the superconducting performance is significantly degraded, that is, in homogeneous systems. It is considered that the present invention can be applied to materials other than carbon as long as the material is not present. Therefore, the first essential requirement of the present invention is to develop a certain region that develops superconductivity in a homogeneous material. This can be realized, for example, by using a nanostructure control technique.

  Further, in the superconducting material in the present invention, the superconducting property becomes a superconducting state in order to connect the locally developed superconducting regions and operate as a superconductor as a whole. It is considered that the property of leaking out from the region by a certain distance is involved. This is the second essential requirement of the present invention. This property is called a superconducting proximity effect and is a universal phenomenon associated with the superconducting state. For example, in a structure in which two superconducting regions are close to each other, it is known that a superconducting current flows in a non-superconducting region between the two superconducting regions.

  An electronic device that utilizes the fact that a superconducting current flows using the tunnel effect when two superconducting regions are separated from each other by an insulating film is called a Josephson element. As a general device. Also, depending on the material system, a superconducting current may flow even if a gap produced by lithography technology is separated. In such a case, the superconducting current is controlled by controlling the electronic properties of the gap region. It is possible to control and operate as if it were a field effect element.

  Even in a material system that is not in a superconducting state, if there is a region where the superconducting state is locally realized and the region is connected by the superconducting proximity effect, it can be used as a superconductor as a whole. It is done. Special nanostructure control technology is important for this realization.

  A film of a homogeneous material is produced by a technique that can easily obtain a relatively homogeneous material, for example, a sputtering method. In this state, the material may not have superconducting properties. This film is subjected to a treatment for modifying the structure of the film during or after the deposition to partially modify the film so as to have superconducting properties. By modifying the superconducting region so that the proximity effect works effectively, the entire material operates as a superconducting material. At this time, the characteristics of the non-superconducting region around the superconducting region are not particularly limited.

  However, when the above-described degree of modification of the superconducting region progresses and occupies a certain ratio or more, the superconducting characteristics are remarkably deteriorated or lost, and therefore it is necessary to pay sufficient attention to the modification conditions. This state is shown in FIG. When the degree of modification proceeds and the portions 402 exhibiting superconducting properties dispersed in the substrate 401 come into contact with each other, this is the same as the structure of a general substance.

  At this time, if a too complicated material system is used, the effect of local modification may not be sufficient. Therefore, it is preferable that the system is as simple as possible, preferably the main component is composed of a single element. However, superconducting properties may be expressed as a whole by local modification even in complex compound systems.

  Further, the deposition method is not particularly limited, but in order to perform nano-order modification, since a film produced in a thermal equilibrium state is not preferable, plasma CVD, sputtering, It is better to form the film by a thermally non-equilibrium process such as vapor deposition.

  The film modification method may be performed by uniform irradiation with ions having relatively low energy capable of performing local structure modulation. Further, in the case of irradiation with high energy particle beams (ions or electrons), the above-described film modification for making a partially superconducting state may be performed by partial irradiation with non-uniform distribution. However, there is no problem if other methods can perform local modification. Further, it does not matter whether the modification is performed during the deposition or after the deposition, but if it can be performed during the deposition, the process can be simplified.

  Next, let us consider the distance between local parts (superconducting regions) subjected to structural modulation. It is known that the distance to which the proximity effect reaches varies greatly depending on the material system. In so-called high-temperature superconducting materials, it is known that the thickness is 1 nm or less. In order to obtain this state, it is necessary to perform very difficult control. Unfortunately, there is no such local structure control technology at present. Accordingly, it is practically advantageous that the distance between adjacent superconducting regions is 10 nm or more and 1 μm or less. A material system that exhibits superconducting properties as a whole can be configured by the above-described modification method as long as the proximity effect of the proximity effect of the material that has been modified to be in the superconducting state can reach.

Since the proximity effect distance of superconductivity expressed by local structure control can be made larger than that of known superconducting materials, such a modified system can be compared with conventional superconducting materials. High performance characteristics. Of course, it does not deny the advent of a technology for improving the local region approaching to about 1 nm by improving the local reforming technology in the future due to technological progress.
However, if a material system is discovered that exhibits sufficient proximity effect characteristics even with a distance of 1 μm or more, such a system can be locally modified with higher accuracy with the aid of known lithography techniques. It seems that higher quality materials can be obtained by quality.

  By the way, even if a material based on proximity effect coupling of superconductivity by local modification as described above can be realized, if the substrate portion around the fine superconducting region is a good conductor, To distinguish, it is necessary to use advanced microscopy. On the other hand, when the substrate portion is an insulator or a semiconductor with few carriers in the superconducting temperature range, it is possible to distinguish a conventional superconducting material from the temperature dependence of characteristics.

  In a superconducting material in which a superconducting region is partially formed in a substrate of a good conductor, the temperature dependence of the electrical resistance is as shown in FIG. 5, which is indistinguishable from the characteristics of a conventional superconducting material. In this state, in order to confirm the local superconducting state, it is necessary to specify with a high-resolution observation means such as an MFM or a Lorentz microscope.

  On the other hand, in the superconducting material in which the superconducting region is partially formed in the insulating substrate, the temperature dependence of the electrical resistance is as shown in FIG. Further, in a superconducting material in which a superconducting region is partially formed in a semiconductor substrate, the temperature dependence of the electrical resistance is as shown in FIG. When the region surrounding the modified superconducting region is an insulator, it is an insulator as a whole when the temperature is higher than the superconducting transition temperature, and almost no current flows above the superconducting temperature as shown in FIG. Absent. This material exhibits conductivity only when the modified region becomes a superconducting state and the adjacent superconducting regions are connected by the proximity effect.

In addition, when the substrate is a semiconductor with few carriers in the superconducting temperature range, it exhibits semiconducting properties as a whole in a high temperature state, and the resistance is greatly increased because the number of carriers decreases as the temperature decreases. When the temperature further decreases and the modified region becomes a superconducting state, the entire region becomes superconducting due to the proximity effect. In this case, the superconducting thin film 101 in the above-described embodiment has not been found yet, and has been found for the first time by the present invention. A carbon film with a mixture of sp ² and sp ³ bonds has high mechanical strength and is chemically stable, so it is applied to protective coatings and electrodes for electrochemical analysis. (See Patent Documents 5 and 6 and Non-Patent Document 3). However, the inventors have found for the first time that a carbon film in which a sp ² bond portion and a sp ³ bond portion are mixed exhibits superconducting properties as described above.

  Conventional superconductors often have a superconductive state throughout the material. In this case, since the carrier concentration is high, the conduction characteristics cannot be controlled by an electric field. In the superconducting material according to the present invention, it is easily inferred that a good electric field effect is exhibited from an example of a superconducting proximity effect device already reported. This is a characteristic that has not been realized so far as a superconducting material, and is an effect realized for the first time by the nanostructure control technology.

  By the way, as described above, it is the essence of the present invention that a non-equilibrium state is expressed locally by modifying means such as ion irradiation, and the limited region is a superconducting material. It is difficult to enlarge the modified region to about 1 μm, for example. When the reformed region that is in a non-equilibrium state becomes large, relaxation to the equilibrium state is likely to occur, which deteriorates the superconducting characteristics. This is not a desirable situation. In order to prevent such relaxation, it is necessary that the dimensions of the modified region be fine enough to maintain a non-equilibrium state.

  This dimension varies depending on the material system, but is generally 100 nm or less. For example, the minimum grain size obtained by thermal CVD, which is a growth method in a thermal equilibrium state, may be helpful. For example, in many metal materials, the minimum crystal grain size is approximately 100 nm. In many cases, semiconductors such as silicon are several tens of nanometers. As a special example, fullerene and CNT have a crystal size of 1 nm class in a carbon system.

  It is considered that the essence of the present invention is to develop a modified region in a non-equilibrium state with the above dimensions as a guide in a material having a uniform composition. Although it seems that it is possible in principle even with materials composed of multiple elements, in general, phase reforming occurs by local modification, and a structure close to an equilibrium state is formed rather than relaxed. This situation is undesirable. Control is easier when the number of constituent elements is small. From this point of view, if the main constituent element is, for example, one kind, there is no problem even if many kinds of elements are introduced at the impurity level.

  When the modification is performed by a known means, local modulation of the composition is likely to occur in the case of a general compound. The modulation of the composition often occurs at the atomic order scale, and is manifested in a form such as a defect or an atomic defect, which is disadvantageous for improving the superconducting properties. On the other hand, in the case of a single material, particularly a transition metal system, it is difficult to develop local structural modulation by modification. On the other hand, in the case of relatively light elements such as boron, carbon, and silicon, local structural modulation is easily expressed.

In particular, in the case of carbon, in the state of a single element, the bonding mode between main atoms includes a bond by sp ² hybrid orbital (sp ² bond) and a bond by sp ³ hybrid orbital (sp ³ bond). Each can be present stably in a certain region. As described above, in the simple element, the two types of bonding modes exist stably only for carbon, and this is an effective characteristic for local modification. The superconducting material in the embodiment described above makes use of this characteristic.

Graphite having sp ² bonds does not become superconducting in the bulk state, but exhibits superconducting properties when a large strain is applied. On the other hand, diamond having sp ³ bonds is excellent in mechanical strength. Combining these properties, embedding nanographite with large strain as a carbon structure with superconducting properties in a carbon matrix containing nanodiamonds with excellent mechanical strength and rich sp ³ bonds, and superconducting proximity effect between nanographites It is possible to create a high-performance superconducting material by disposing them at such a distance that they can be coupled together.

  By the way, it has been reported that superconducting properties are expressed by embedding carbon nanotubes in a metal oxide matrix (see Non-Patent Document 6). Compared with the superconducting property of the carbon nanotube, the superconducting material of the present embodiment described above has a large magnetic pinning effect and a large critical magnetic field. This is also an effect of a carbon matrix, which is an effective point of a superconducting material whose main constituent element is carbon.

It is sectional drawing which shows typically the structural example of the superconducting thin film 101 which consists of a superconducting material in embodiment of this invention. It is process drawing which shows an example of the formation method of the superconducting thin film 101 in embodiment of this invention. It is sectional drawing which shows typically the structural example of the other superconducting material in embodiment of this invention. It is a block diagram for demonstrating the state in which the superconducting region occupies a certain proportion of the degree of modification and the superconducting properties are significantly deteriorated and disappear. It is a characteristic view which shows the temperature dependence of the electrical resistance in the superconductive material which formed the superconductive area | region partially in the substrate of a good conductor. It is a characteristic view which shows the temperature dependence of the electrical resistance in the superconducting material which formed the superconducting region partially in the substrate of the insulator. It is a characteristic view which shows the temperature dependence of the electrical resistance in the superconductive material which formed the superconductive area | region partially in the semiconductor substrate.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 101 ... Superconductive thin film, 102 ... Substrate, 103 ... Insulating layer, 111 ... Substrate, 112 ... Nano graphite, 113 ... Nano diamond

Claims (5)

A substrate,
A plurality of superconducting regions disposed in the substrate and exhibiting superconducting properties under predetermined conditions comprising the same elements as the substrate,
The substrate is composed of amorphous carbon,
The superconducting region is composed of carbon bonded by sp ² hybrid orbitals,
The adjacent superconducting regions are separated by a distance exhibiting a superconducting proximity effect. A substrate,
A plurality of superconducting regions formed by partially modifying the substrate and exhibiting superconducting properties under predetermined conditions disposed within the substrate;
The substrate is composed of amorphous carbon,
The superconducting region is composed of carbon bonded by sp ² hybrid orbitals,
The adjacent superconducting regions are separated by a distance exhibiting a superconducting proximity effect. The superconducting material according to claim 1 or 2,
A superconducting material comprising a plurality of regions arranged in the substrate and composed of carbon bonded by sp ³ hybrid orbitals. In the superconducting material according to any one of claims 1 to 3 ,
The substrate has a carrier concentration lower than that in the superconducting region. A step of forming a substrate on the substrate; and
A plurality of superconducting regions exhibiting superconducting characteristics under a predetermined condition consisting of the same elements as the substrate by locally modulating the structure by irradiating the substrate with particle beams, and adjacent superconducting regions are superconducting proximity effects the by distance apart shown comprising at least a step of a state of being formed on said substrate,
The substrate is composed of amorphous carbon,
It said superconducting region, method for producing a superconducting material, characterized that you consists of carbon atoms of the bond by the sp ² hybrid orbitals.

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