OXIDE SUPERCONDUCTOR

Abstract: An oxide superconductor having a superconductive layer interposed between two blocking layers compositionally different from each other. A superconductive layer is provided in any of the following structures: a single layer of one Cu-O₂ surface (single layer system); a double layer system containing two Cu-O₂ surfaces with a mediating layer interposed between said two surfaces; and a triple layer system containing three Cu-O₂ surfaces with a mediating layer interposed between respective Cu-O₂ surfaces. Because of difference in composition between the blocking layers, seventy seven (77) kinds, far more than before, of oxide and application thereof to a wider field is possible.

Description: TECHNICAL FIELD The present invention relates to oxide superconductors adapted for use as magnet coil materials in nuclear fusion furnaces, electromagnetic fluid generators, accelerators, rotary electrical apparatuses (electric motors, generators, etc.), magnetic separators, magnetic floating trains, magnetic floating automobiles, magnetic floating elevators, nuclear magnetic resonance computed tomographs, magnetically-propelled ships, electron beam exposure devices, single crystal manufacturing apparatuses, various laboratory devices, etc., suited for applications which involve the problem of power loss in transmission lines, electrical energy storing devices, transformers, rectifiers, etc., further adapted for use in various elements, such as Josephson devices, SQUID, superconducting transistors, superconducting microwave transmission circuits, etc., and furthermore, adapted for use as various function materials, such as infrared detection materials, magnetic shielding materials, etc. BACKGROUND ART Conventionally about 20 kinds of copper compound oxide superconductors with different crystal structures have been found. Any of these copper compound oxide superconductors commonly includes, in its crystal structure, Cu-O₂ sheets through which superconducting current flows, and has a stratified structure composed of repeated units each including the Cu-O₂ sheet and another layer situated at a predetermined distance therefrom. In the case of the crystal structure of La₂CuO₄, for example, repeated units each including an La₂O₂ layer and a Cu-O₂ sheet are arranged in layers in the direction of a c-axis (direction perpendicular to the Cu-O₂ sheet), as shown in Fig. 1. From a different standpoint, one Cu-O₂ sheet is sandwiched between each two La₂O₂ layers. Having one Cu-O₂ sheet between the aforesaid layers, this structure will be referred to as one-layer system, hereinafter. In the case of this crystal structure, there is a slip for a 1/2 unit cell in each La₂O₂ layer with respect to the directions of a- and b-axes (directions within the plane of the Cu-O₂ sheet). Crystallographically, therefore, two La₂O₂ layers and two Cu-O₂ sheets constitute a repeated unit. One-layer system oxide superconductors of this type include, for example, Bi₂Sr₂CuO₆, Tl₂Ba₂CuO₆, Nd₂CuO₄, and Pb₂SrLaCu₂O₆+δ , which is described in "Physica C," vol. 166, 1990, pp. 502 to 512. In the case of the crystal structure of Y₁Ba₂Cu₃O₇, moreover, repeated units each including a BaO-CuO-BaO layer, a first Cu-O₂ sheet, a Y layer, and a second Cu-O₂ sheet are arranged in layers in the direction of the c-axis, as shown in Fig. 2. From a different standpoint, therefore, a unit composed of the first Cu-O₂ sheet, Y layer, and second Cu-O₂ sheet is sandwiched between each two BaO-CuO-BaO layers. Having two Cu-O₂ sheets in the aforesaid unit, this structure will be referred to as two-layer system, hereinafter. In the case of this crystal structure, in contrast with the case of the aforementioned La₂CuO₄, there is no slip with respect to the directions of the a- and b-axes, so that the aforesaid repeated units are not different from crystallographic versions. Two-layer system oxide superconductors of this type include, for example, Bi₂Sr₂Ca₁Cu₂O₈, Tl₂Ba₂CaCu₂O₈ described in "Nature", vol. 332, March 31, 1988, pp. 420 to 422, YBa₂Cu₄O₈ described in "Nature", vol. 334, July 14, 1988, pp. 141 to 143, Ba₂YCu₃O 7-x described in "Japanese Journal of Applied Physics", vol. 26, No. 5, May, 1987, pp. L649 to L651. La 2-x Ca 1 + x Cu₂O 6-x/2 + δ described in "Materials Chemistry", vol. 7, 1982, pp. 413 to 427, and Pb₂Sr₂(R, Ca)₁Cu₃O 8 + y (R: rare earth element) described in "Physica C", vol. 157, 1989, pp. 124 to 130. In the case of the crystal structure of Bi₂Sr₂Ca₂Cu₃O₁₀, furthermore, repeated units each including a SrO-Bi₂O₂-SrO layer, a first Cu-O₂ sheet, a first Ca layer, a second Cu-O₂ sheet, a second Ca layer, and a third Cu-O₂ sheet are arranged in layers in the direction of the c-axis, as shown in Fig. 3. From a different standpoint, therefore, a unit composed of the first Cu-O₂ sheet, first Ca layer, second Cu-O₂ sheet, second Ca layer, and third Cu-O₂ sheet is sandwiched between each two SrO-Bi₂O₂-SrO layers. Having three Cu-O₂ sheets in the aforesaid unit, this structure will be referred to as three-layer system, hereinafter. Also in the case of this crystal structure, as in the case of the aforementioned La₂CuO₄, there is a slip for a 1/2 unit cell in each SrO-Bi₂O₂-SrO layer with respect to the directions of the a- and b-axes. Crystallographically, therefore, the aforesaid two repeated units constitute a repeated unit. Three-layer system oxide superconductors of this type include, for example, a Pb-Bi-Sr-Ca-Cu-O-based superconductor described in "Japanese Journal of Applied Physics", vol. 28, No. 5, May, 1989, pp. L787 to L790, a Tl-Pb-Sr-Ca-Cu-O-based superconductor described in "Science", vol. 242, October, 1988, pp. 249 to 252, and Tl₁Ca n-1 Ba₂Cu n O 2n + 3 (n = 1, 2, 3) described in "Physical Review Letters", vol. 61, No. 6, August, 1988, pp. 750 to 753. It is known that the more the Cu-O₂ sheets in each unit, in the aforementioned one-, two-, and three-layer system superconductors, the higher the superconducting transition temperature (Tc) of the materials is. The inventors hereof analyzed the correlations between the Tc of the copper oxide superconductors described above and the interlayer distances between the Cu-O₂ sheets in their crystal structures, and presented the results of this analysis in "Physica C", vol. 167, 1990, pp. 515 to 519. Thereupon, the inventors hereof found the fact that the shorter the interlayer distances between the Cu-O₂ sheets of the crystal structures, the higher the value of Tc is. In order to assign reasons for this phenomenon, the inventors hereof proposed that other layers interposed between the Cu-O₂ sheets through which superconducting current flows should be classified into blocking layers and mediating layers as they are studied. In this case, the blocking layer, in the crystal structure, may be regarded as a layer for cutting off the interaction between the Cu-O₂ sheets, through which the superconducting current flows, with the distance between the Cu-O₂ sheets not shorter than 6 angstroms, and further supplying carriers to the Cu-O₂ sheets to give the Cu-O₂ sheets a function for a superconducting current flow. The mediating layer, which is interposed between the Cu-O₂ sheets having a -2-valent electric charge as a whole, may be regarded as a layer for neutralizing the electric charge, thereby enabling the whole crystal structure to be formed, and causing an interaction between the Cu-O₂ sheets with the distance between them not longer than 4 angstroms. From this point of view, the one-layer system oxide superconductor shown in Fig. 1 can be considered to be constructed so that one Cu-O₂ sheet serving as a superconducting layer is sandwiched between each two identical La₂O₂ blocking layers. In the case of the two-layer system oxide superconductor shown in Fig. 2, on the other hand, two blocking layers are formed individually of identical BaO-CuO-BaO layers, and the unit composed of the first Cu-O₂ sheet, Y layer as a mediating layer, and second Cu-O₂ sheet, sandwiched between the blocking layers, functions as a superconducting layer. In the case of the three-layer system oxide superconductor shown in Fig. 3, the unit composed of the first Cu-O₂ sheet, first Ca layer as a first mediating layer, second Cu-O₂ sheet, second Ca layer as a second mediating layer, and third Cu-O₂ sheet is sandwiched between each two identical SrO-Bi₂O₂-SrO-(blocking) layers, and this unit functions as a superconducting layer. Thus, according to the concept proposed by the inventors hereof, the crystal structure of the conventionally proposed copper oxide superconductors can be considered to be designed so that a superconducting layer is sandwiched between each two identical blocking layers, and it can be concluded that this superconducting layer is composed of one Cu-O₂ sheet, or of two (for a two-layer system) or three (for a three-layer system) Cu-O₂ sheets with a mediating layer between them. If the conventionally known copper oxide superconductors are viewed at this angle, the blocking layers may be classified into the following 8 types: La₂O₂-type, BaO-CuO-BaO-type, BaO-CuO-CuO-BaO-type, SrO-Bi₂O₂-SrO-type, BaO-Tl₂O₂-BaO-type or BaO-TlO-BaO-type, SrO-PbO-Cu-PbO-SrO-type, SrO-(Pb, Cu)O-SrO-type or SrO-(Pb, Sr)O-SrO-type, and Ln₂O₂-type (Ln is selected from Nd, Sm, Eu, and Gd). Further, the mediating layer may be formed of Ca, Sr, Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, or Lu. The materials of the one-layer system shown in Fig. 1 have blocking layers of only one kind, except those ones which include Ln₂O₂-type blocking layers. In either of the two- and three-layer systems shown in Figs. 2 and 3, moreover, the blocking layers in the crystal structures are of only one kind. Accordingly, the conventional copper oxide superconductors are limited to about 20 kinds, so that fields of their industrial application are restricted. DISCLOSURE OF THE INVENTION An object of the present invention is to provide copper oxide superconductors, in which about 20 kinds of conventionally found copper oxide superconductors can be increased to about 77 kinds, so that the materials can be applied to a wider variety of fields. Another object of the present invention is to provide a copper oxide superconductors having Tc higher than that of conventional copper oxide superconductors. According to the present invention, there is first provided an oxide superconductor comprising repeated units each including a first blocking layer having a composition selected from the following group, a first Cu-O₂ sheet, a second blocking layer having a composition selected from the following group and different from the composition of the first blocking layer, and a second Cu-O₂ sheet, arranged in layers in the order named, the group including: La₂O₂, BaO-CuO-BaO, BaO-CuO-CuO-BaO, SrO-Bi₂O₂-SrO, BaO-Tl₂O₂-BaO, BaO-TlO-BaO, SrO-PbO-Cu-PbO-SrO, SrO-(Pb, Cu)O-SrO, and SrO-(Pb, Sr)O-SrO. This superconductor will hereinafter be referred to as one-layer system oxide superconductor. According to the present invention, moreover, there is provided an oxide superconductor comprising repeated units each including a first blocking layer having a composition selected from the following group a, a first Cu-O₂ sheet, a first mediating layer composed of elements selected from the following group b , a second Cu-O₂ sheet, a second blocking layer having a composition selected from the following group a and different from the composition of the first blocking layer, a third Cu-O₂ sheet, a second mediating layer composed of elements selected from the following group b , and a fourth Cu-O₂ sheet, arranged in layers in the order named, the group a including: La₂O₂, BaO-CuO-BaO, BaO-CuO-CuO-BaO, SrO-Bi₂O₂-SrO, BaO-Tl₂O₂-BaO, BaO-TlO-BaO, SrO-PbO-Cu-PbO-SrO, SrO-(Pb, Cu)O-SrO, SrO-(Pb, Sr)O-SrO, and Ln₂O₂ (where Ln is selected from Nd, Sm, Eu and Gd), the group b including: Ca, Sr, Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu. This superconductor will hereinafter be referred to as two-layer system oxide superconductor. According to the present invention, furthermore, there is provided an oxide superconductor comprising repeated units each including a first blocking layer having a composition selected from the following group a , a first Cu-O₂ sheet. a first mediating layer composed of elements selected from the following group b , a second Cu-O₂ sheet, a second mediating layer composed of elements selected from the following group b , a third Cu-O₂ sheet, a second blocking layer having a composition selected from the following group a and different from the composition of the first blocking layer, a fourth Cu-O₂ sheet, a third mediating layer composed of elements selected from the following group b , a fifth Cu-O₂ sheet, a fourth mediating layer composed of elements selected from the following group b , and a sixth Cu-O₂ sheet, arranged in layers in the order named, the group a including: La₂O₂, BaO-CuO-BaO, BaO-CuO-CuO-BaO, SrO-Bi₂O₂-SrO, BaO-Tl₂O₂-BaO, BaO-TlO-BaO, SrO-PbO-Cu-PbO-SrO, SrO-(Pb, Cu)O-SrO, SrO-(Pb, Sr)O-SrO, and Ln₂O₂ (where Ln is selected from Nd, Sm, Eu and Gd), the group b including: Ca, Sr, Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu. This superconductor will hereinafter be referred to as three-layer system oxide superconductor. In the oxide superconductor according to the present invention, Tc thereof can be increased if the interlayer distance between the Cu-O₂ sheets in the crystal structure is made shorter. The present invention is based on an inference that the interlayer distance between the Cu-O₂ sheets may possibly be restricted by the state of the blocking layers adjacent to the sheets. As mentioned before, the blocking layers serve to supply the carriers to the Cu-O₂ sheets, as well as to cut off the interaction between the Cu-O₂ sheets. The each known copper oxide superconductor has blocking layers of the same composition. It can be inferred, however, that even materials which include different kind of blocking layer compositions can become superconductor as long as they can fulfill the aforementioned functions. But it can be guessed that the presence of the different kind of blocking layers causes some crystal strain in the crystal structure obtained. First, diagrams of these three kinds of oxide superconductors are shown in Figs. 4, 5 and 6. In the one-layer system oxide superconductor of Fig. 4, a first blocking layer B11 having a composition selected from the aforesaid group, a first Cu-O₂ sheet C11, a second blocking layer B12 having a composition different from that of the first blocking layer, and a second Cu-O₂ sheet C12 are arranged in layers In the c-axis direction, in the order named, thus constituting a repeated unit U1, and the Cu-O₂ sheet C11 (C12) sandwiched between these two blocking layers B11 and B12 forms a superconducting layer. Further, the Cu-O₂ sheet situated between these two blocking layers B11 and B12 is a single layer, and minimum units L11 and L12, which are necessary for superconductivity, constitute base units. In the two-layer system oxide superconducting material of Fig. 5, a first blocking layer B21 having a composition selected from the aforesaid group a , a first Cu-O₂ sheet C21, a first mediating layer M21 composed of elements selected from the aforesaid group b , a second Cu-O₂ sheet C22, a second blocking layer B22 having a composition different from that of the first blocking layer, a third Cu-O₂ sheet C23, a second mediating layer M22 composed of elements selected from the aforesaid group b , and a fourth Cu-O₂ sheet C24 are arranged in layers in the c-axis direction, in the order named, thus constituting a repeated unit U2, and a unit S21, which includes the first Cu-O₂ sheet C21, mediating layer M21, and second Cu-O₂ sheet C22, and a unit S22, which includes the third Cu-O₂ sheet C23, second mediating layer M22, and fourth Cu-O₂ sheet C24, form superconducting layers. In this case, L21 and L22 constitute base units. In the three-layer system oxide superconductor of Fig. 6, a first blocking layer B31 having a composition selected from the aforesaid group a , a first Cu-O₂ sheet C31, a first mediating layer M31 composed of elements selected from the aforesaid group b , a second Cu-O₂ sheet C32, a second mediating layer M32 composed of elements selected from the aforesaid group b , a third Cu-O₂ sheet C33, a second blocking layer B32 having a composition different from that of the first blocking layer, a fourth Cu-O₂ sheet C34, a third mediating layer M33 composed of elements selected from the aforesaid group b , a fifth Cu-O₂ sheet C35, a fourth mediating layer M34 composed of elements selected from the aforesaid group b , and sixth Cu-0₂ sheet C36 are arranged in layers in the c-axis direction, in the order named, thus constituting a repeated unit U3, and a unit S31, which includes the first Cu-O₂ sheet C31, first mediating layer M31, second Cu-O₂ sheet C32, second mediating layer M32, and third Cu-O₂ sheet C33, and a unit S32, which includes the fourth Cu-O₂ sheet C34, third mediating layer M33, fifth Cu-O₂ sheet C35, fourth mediating layer M34, and sixth Cu-O₂ sheet C36, form superconducting layers. In this case, L31 and L32 constitute base units. In any of the above-described cases of the one-, two-, and three-layer systems, one repeated unit directly constitutes a crystallographic repeated unit if neither of the first and second blocking layers is slipped with respect to a direction or directions (a-axis direction and/or b-axis direction) perpendicular to the c-axis. If either of the first and second blocking layers is slipped in the a-axis direction and/or b-axis direction, however, two repeated units constitute one crystallographic repeated unit. If both of the first and second blocking layers are slipped, and if the first and second blocking layers are slipped in the same manner, one repeated unit directly constitutes a crystallographic repeated unit. If the first and second blocking layers are slipped in different ways, however, two repeated units constitute one crystallographic repeated unit. In the case of the oxide superconductors found so far, each of which includes only one kind of blocking layers, that is, in the case where the first and second blocking layers are identical with each other, the aforementioned base unit and repeated unit are identical. In each of these oxide superconductors, the composition which constitutes each blocking layer is not always a stoichiometric composition on account of excess or defect of oxygen. In the case of SrO-Bi₂O₂-SrO, for example, excessive oxygen is often contained in practice, and the composition of this blocking layer is Bi₂Sr₂O₄ + δ. The same may be said of BaO-Tl₂O₂-BaO or BaO-TlO-BaO, and the respective compositions of these blocking layers are Tl₂Ba₂O₄ + δ and TlBa₂O₃ + δ . Also in the case of SrO-PbO-Cu-PbO-SrO, oxygen is often excessively introduced in practice, and the composition of this blocking layer is Pb₂Sr₂CuO₄ + δ . In the cases of other compositions, that is, BaO-CuO-BaO or BaO-CuO-CuO-BaO, La₂O₂, SrO-(Pb, Cu)-O-SrO, SrO-(Pb, Sr)O-SrO, and Ln₂O₂, on the other hand, oxygen is often in defect, and the respective compositions of these blocking layers are Ba₂CuO₃₋δ , Ba₂Cu₂O₄₋δ , La₂O₂₋δ , (Pb, Cu)Sr₂O₃₋δ , (Pb, Sr)-Sr₂O₃₋δ , and Ln₂O₂-δ . In any of the aforesaid compositions, the length of its crystal cell is equal or approximate to the Cu-O bond length of the Cu-O₂ sheets. In forming the whole crystal structure by the method mentioned later, therefore, the blocking layers of these compositions fit the Cu-O₂ sheets, thus stabilizing the crystal structure. In forming each superconductor with the blocking layers of these compositions, different compositions are selected for the first and second blocking layers mentioned before. This is because it is estimated, although without any obvious reasons, that if the superconducting layer is sandwiched between the different kind of blocking layers, a strain is caused in the superconducting layer, so that the Tc is higher than in the case where the superconductor is sandwiched between same kind of blocking layers. If the composition used for each blocking layer of the two- and three-layer systems is Ln₂O₂, moreover, only one of the aforesaid elements may be used as Ln, or two or more may be suitably selected for use. Likewise, one or two or more elements may be selected from the group b . In the oxide superconductors of the present invention, whether the one-, two-, or three-layer type, superconducting charge carriers are holes. As in the case of the known copper oxide superconductors, therefore, superconductors are obtained when the hole density of the Cu-O₂ sheet is not lower than 0.01 or not higher than 0.5 per Cu atom. In order to adjust the carrier concentration to the aforesaid ranges, the oxygen content in the composition used may be somewhat deviated from its stoichiometric value, or some of cations which constitutes the composition may be substituted by other cations, in forming the blocking layers by the method mentioned later. In oxide superconductors one of whose blocking layers consist of SrO-PbO-Cu-PbO-SrO, in this case, the valency of Cu in the blocking layer, which makes no change, is + 1, so that the hole density per Cu atom is calculated taking valency of Cu as + 1. In this calculation, moreover, all the respective valency of La, Bi, Tl, Ln (Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu) are all regarded as + 3, all the respective valency of Ba, Sr and Ca as + 2, and the valency of O as -2, Pb can take valency of + 2 and + 4. In this case, therefore, the valency of Pb is regarded as + 3, the average of + 2 and + 4. In forming blocking layers of SrO-PbO-Cu-PbO-SrO, however, the valency of Pb is regarded as + 2. The holes are supplied mainly by changing the heat treatment conditions under various oxygen partial pressures to control the oxygen content in the crystal structure, during the manufacture of an oxide superconductor, or by substitution of some of the elements in the blocking layers. In the case of the heat treatment under the various oxygen partial pressures, for example, the oxygen partial pressure is varied within the range of 0.001 to 1,000 atm. When the oxygen partial pressure is lower than 1 atm., the superconductor is annealed for from 1 to 100 hours at a temperature ranging from about 300°C to less than the melting point of the material, and is then rapidly cooled to the temperature of liquid nitrogen. By doing this, an oxygen deficiency is caused to reduce the oxygen content, whereby the hole concentration is lowered. When the oxygen partial pressure is higher than 1 atm., on the other hand, the superconductor is annealed for from 1 to 100 hours at a temperature ranging from about 300°C to less than the melting point of the material. Then, the oxygen deficiency is eliminated by slowly cooling the superconductor in a furnace, or excessive oxygen is introduced into the furnace to increase the oxygen content, whereby the hole concentration is increased. Moreover, the hole concentration can be increased by increase of oxygen content by mixing ozone with oxygen in an oxygen atmosphere during the heat treatment. Since this hole concentration control method can be carried out as a separate process even after the intended crystal structure is manufactured, it is particularly suited for the case where the holes are just enough in number so that no crystal can be obtained unless the electric charge on the whole crystal is balanced, or the case where the range of the partial pressure of oxygen in the atmosphere must be limited in order to grow the crystal. The following is a description of a method for controlling the hole concentration by the substitution of some of the elements in the blocking layers. In the case of La₂O₂, for example, holes can be supplied by substituting + 2-valent Ba, Sr or Ca or + 1-valent Na for some of + 3-valent La. In the case of BaO-CuO-BaO or BaO-CuO-CuO-BaO, the hole concentration can be controlled by substituting a + 1-valent alkaline metal, such as Na, for Ba to supply holes, or by substituting Sr or Pb for some of Ba to change the atom packing state of the crystal structure, thereby changing the facility of oxygen introduction. Also, the hole concentration can be controlled by substituting Al or Ga for Cu. In the case of SrO-Bi₂O₂-SrO, the hole concentration can be controlled by substituting + 2-valent Pb for some of + 3-valent Bi to supply holes, or by substituting + 3-valent La for some of + 2-valent Sr to reduce holes. In the case of BaO-Tl₂O₂-BaO or BaO-TlO-BaO, the hole concentration can be controlled by substituting + 2-valent Pb for some of + 3-valent Tl to supply holes, or by substituting + 3-valent La for some of + 2-valent Ba to change the atom packing state of the crystal structure, thereby changing the facility of oxygen introduction. In the case of SrO-PbO-Cu-PbO-SrO, SrO-(Pb, Cu)O-SrO, or SrO-(Pb, Sr)O-SrO, the hole concentration can be controlled by substituting + 3-valent La for some of + 2-valent Sr to reduce holes, or by substituting Ba or Ca for some of Sr to change the atom packing state of the crystal structure, thereby changing the facility of oxygen introduction. In the case of Ln₂O₂, the carrier concentration can be controlled by substituting a + 2-valent alkaline earth metal, such as Ca or Mg, for some of + 3-valent Ln to supply holes, or by substituting + 4-valent Ce or Th for some of + 3-valent Ln to reduce holes. In the cases of two- and three-layer system oxide superconductors, moreover, the hole concentration can be controlled by changing the combination ratios between + 2-valent Ca or Sr and + 3-valent Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu in selecting the elements for the mediating layers from the aforesaid group b . The oxide superconductors of the present invention will now be enumerated with reference to models of the repeating units (U1, U2 and U3) shown in Figs. 4 to 6. These models are combinations of the blocking layers, Cu-O₂ sheets, and mediating layers. The compositions of the blocking layers are expressed in the same manner as the composition group for the one-layer system and the group a for the two- and three-layer systems. Actually, however, the oxygen content may be somewhat deviated from its stoichiometric value, or some cations may be replaced, in order to adjust the carrier concentration, as mentioned before. For the two- and three-layer system oxide superconductors, moreover, the mediating layers composed of the elements selected from the group b are designated by M's. Further, the one-, two-, and three-layer system oxide superconductors are designated individually by serial numbers, e.g., "1-16," "2-10," and "3-5." There are 21 kinds of one-layer system oxide superconductors as follows: 1-1: BaO-CuO-BaO/Cu-O₂/La₂O₂/Cu-O₂, 1-2: Bao-CuO-CuO-BaO/Cu-O₂/La₂O₂/Cu-O₂, 1-3: BaO-CuO-CuO-BaO/Cu-O₂/BaO-CuO-BaO/Cu-O₂, 1-4: SrO-Bi₂O₂-SrO/Cu-O₂/La₂O₂/Cu-O₂, 1-5: SrO-Bi₂O₂-SrO/Cu-O₂/BaO-CuO-BaO/Cu-O₂, 1-6: SrO-Bi₂O₂-SrO/Cu-O₂/BaO-CuO-CuO-BaO/Cu-O₂, 1-7: (BaO-Tl₂O₂-BaO) or (BaO-TlO-BaO)/Cu-O₂/La₂O₂/Cu-O₂, 1-8: (BaO-Tl₂O₂-BaO) or (BaO-TlO-BaO)/Cu-O₂/BaO-CuO-BaO/Cu-O₂, 1-9: (BaO-Tl₂O₂-BaO) or (BaO-TlO-BaO)/Cu-O₂/BaO-CuO-CuO-BaO/Cu-O₂, 1-10: (BaO-Tl₂O₂-BaO) or (BaO-TlO-BaO)/Cu-O₂/SrO-Bi₂O₂-SrO/Cu-O₂, 1-11: SrO-PbO-Cu-PbO-SrO/Cu-O₂/La₂O₂/Cu-O₂, 1-12: SrO-PbO-Cu-PbO-SrO/Cu-O₂/BaO-CuO-BaO/Cu-O₂, 1-13: SrO-PbO-Cu-PbO-SrO/Cu-O₂/BaO-CuO-CuO-BaO/Cu-O₂, 1-14: SrO-PbO-Cu-PbO-SrO/Cu-O₂/SrO-Bi₂O₂-SrO/Cu-O₂, 1-15: SrO-PbO-Cu-PbO-SrO/Cu-O₂/(BaO-Tl₂O₂-BaO) or (BaO-TlO-BaO)/Cu-O₂, 1-16: (SrO-(Pb, Cu)O-SrO) or SrO-(Pb, Sr)O-SrO/Cu-O₂/La₂O₂/Cu-O₂, 1-17: (SrO-(Pb, Cu)O-SrO) or SrO-(Pb, Sr)O-SrO/Cu-O₂/BaO-CuO-BaO/Cu-O₂, 1-18: (SrO-(Pb, Cu)O-SrO) or SrO-(Pb, Sr)O-SrO/Cu-O₂/BaO-CuO-CuO-BaO/Cu-O₂, 1-19: (SrO-(Pb, Cu)O-SrO) or SrO-(Pb, Sr)O-SrO/Cu-O₂/SrO-Bi₂O₂-SrO/Cu-O₂, 1-20: (SrO-(Pb, Cu)O-SrO) or SrO-(Pb, Sr)O-SrO/Cu-O₂/(BaO-Tl₂O₂-BaO) or (BaO-TlO-BaO)/Cu-O₂, 1-21: (SrO-(Pb, Cu)O-SrO) or SrO-(Pb, Sr)O-SrO/Cu-O₂/SrO-PbO-Cu-PbO-SrO/Cu-O₂. There are 28 kinds of two-layer system oxide superconductors as follows: 2-1: BaO-CuO-BaO/Cu-O₂/M/Cu-O₂/La₂O₂/Cu-O₂/M/Cu-O₂, 2-2: BaO-CuO-CuO-BaO/Cu-O₂/M/Cu-O₂/La₂O₂/Cu-O₂/M/Cu-O₂, 2-3: BaO-CuO-CuO-BaO/Cu-O₂/M/Cu-O₂/BaO-CuO-BaO/Cu-O₂M/Cu-O₂, 2-4: SrO-Bi₂O₂-SrO/Cu-O₂/M/Cu-O₂/La₂O₂/Cu-O₂/M/Cu-O₂, 2-5: SrO-Bi₂O₂-SrO/Cu-O₂/M/Cu-O₂/BaO-CuO-BaO/Cu-O₂/M/Cu-O₂, 2-6: SrO-Bi₂O₂-SrO/Cu-O₂/M/Cu-O₂/BaO-CuO-CuO-BaO/Cu-O₂/M/Cu-O₂, 2-7: (BaO-Tl₂O₂-BaO) or (BaO-TlO-BaO)/Cu-O₂/M/Cu-O₂/La₂O₂/Cu-O₂/M/Cu-O₂, 2-8: (BaO-Tl₂O₂-BaO) or (BaO-TlO-BaO)/Cu-O₂/M/Cu-o₂/BaO-CuO-BaO/Cu-O₂/M/Cu-O₂, 2-9: (BaO-Tl₂O₂-BaO) or (BaO-TlO-BaO)/Cu-O₂/M/Cu-O₂/BaO-CuO-CuO-BaO/Cu-O₂/M/Cu-O₂, 2-10: (BaO-Tl₂O₂-BaO) or (BaO-TlO-BaO)/Cu-O₂/M/Cu-O₂/SrO-Bi₂O₂-SrO/Cu-O₂/M/Cu-O₂, 2-11: SrO-PbO-Cu-PbO-SrO/Cu-O₂/M/Cu-O₂/La₂O₂/Cu-O₂/M/Cu-O₂, 2-12: SrO-PbO-Cu-PbO-SrO/Cu-O₂/M/Cu-O₂/BaO-CuO-BaO/Cu-O₂/M/Cu-O₂, 2-13: SrO-PbO-Cu-PbO-SrO/Cu-O₂/M/Cu-O₂/BaO-CuO-CuO-BaO/Cu-O₂/M/Cu-O₂, 2-14: SrO-PbO-Cu-PbO-SrO/Cu-O₂/M/Cu-O₂/SrO-Bi₂O₂-SrO/Cu-O₂/M/Cu-O₂, 2-15: SrO-PbO-Cu-PbO-SrO/Cu-O₂/M/Cu-O₂/(BaO-Tl₂O₂-BaO) or (BaO-TlO-BaO)/Cu-O₂/M/Cu-O₂, 2-16: (SrO-(Pb, Cu)O-SrO) or SrO-(Pb, Sr)O-SrO/Cu-O₂/M/Cu-O₂/La₂O₂/Cu-O₂/M/Cu-O₂, 2-17: (SrO-(Pb, Cu)O-SrO) or SrO-(Pb, Sr)O-SrO/Cu-O₂/M/Cu-O₂/BaO-CuO-BaO/Cu-O₂/M/Cu-O₂, 2-18: (SrO-(Pb, Cu)O-SrO) or SrO-(Pb, Sr)O-SrO/Cu-O₂/M/Cu-O₂/BaO-CuO-CuO-BaO/Cu-O₂/M/Cu-O₂, 2-19: (SrO-(Pb, Cu)O-SrO) or SrO-(Pb, Sr)O-SrO/Cu-O₂/M/Cu-O₂/SrO-Bi₂O₂-SrO/Cu-O₂/M/Cu-O₂, 2-20: (SrO-(Pb, Cu)O-SrO) or SrO-(Pb, Sr)O-SrO/Cu-O₂/M/Cu-O₂/(BaO-Tl₂O₂-BaO) or (BaO-TlO-BaO)/Cu-O₂/M/Cu-O₂, 2-21: (SrO-(Pb, Cu)O-SrO) or SrO-(Pb, Sr)O-SrO/Cu-O₂/M/Cu-O₂/SrO-PbO-Cu-PbO-SrO/Cu-O₂/M/Cu-O₂, 2-22: Ln₂O₂/Cu-O₂/M/Cu-O₂/La₂O₂/Cu-O₂/M/Cu-O₂, 2-23: Ln₂O₂/Cu-O₂/M/Cu-O₂/BaO-CuO-BaO/Cu-O₂/M/Cu-O₂, 2-24: Ln₂O₂/Cu-O₂/M/Cu-O₂/BaO-CuO-CuO-BaO/Cu-O₂/M/Cu-O₂, 2-25: Ln₂O₂/Cu-O₂/M/Cu-O₂/SrO-Bi₂O₂-SrO/Cu-O₂/M/Cu-O₂, 2-26: Ln₂O₂/Cu-O₂/M/Cu-O₂/(BaO-Tl₂O₂-BaO) or (BaO-TlO-BaO)/Cu-O₂/M/Cu-O₂, 2-27: Ln₂O₂/Cu-O₂/M/Cu-O₂/SrO-PbO-Cu-PbO-SrO/Cu-O₂/M/Cu-O₂, 2-28: Ln₂O₂/Cu-O₂/M/Cu-O₂/(SrO-(Pb, Cu)O-SrO) or SrO-(Pb, Sr)O-SrO/Cu-O₂/M/Cu-O₂. There are 28 kinds of three-layer system oxide superconductors as follows: 3-1: BaO-CuO-BaO/Cu-O₂/M/Cu-O₂/M/Cu-O₂/La₂O₂/Cu-O₂/M/Cu-O₂/M/Cu-O₂, 3-2: BaO-CuO-CuO-Ba/Cu-O₂/M/Cu-O₂/M/Cu-O₂/La₂O₂/Cu-O₂/M/Cu-O₂/M/Cu-O₂, 3-3: BaO-CuO-CuO-BaO/Cu-O₂/M/Cu-O₂/M/Cu-O₂/BaO-CuO-BaO/Cu-O₂/M/Cu-O₂/M/Cu-O₂, 3-4: SrO-Bi₂O₂-SrO/Cu-O₂/M/Cu-O₂/M/Cu-O₂/La₂O₂/Cu-O₂/M/Cu-O₂/M/Cu-O₂, 3-5: SrO-Bi₂O₂-SrO/Cu-O₂/M/Cu-O₂/M/Cu-O₂/BaO-CuO-BaO/Cu-O₂/M/Cu-O₂/M/Cu-O₂, 3-6: SrO-Bi₂O₂-SrO/Cu-O₂/M/Cu-O₂/M/Cu-O₂/BaO-CuO-CuO-BaO/Cu-O₂/M/Cu-O₂/M/Cu-O₂, 3-7: (BaO-Tl₂O₂-BaO) or (BaO-TlO-BaO)/Cu-O₂/M/Cu-O₂/M/Cu-O₂/La₂O₂/Cu-O₂/M/Cu-O₂/M/Cu-O₂, 3-8: (BaO-Tl₂O₂-BaO) or (BaO-TlO-BaO)/Cu-O₂/M/Cu-O₂/M/Cu-O₂/BaO-CuO-BaO/Cu-O₂/M/Cu-O₂/M/Cu-O₂, 3-9: (BaO-Tl₂O₂-BaO) or (BaO-TlO-BaO)/Cu-O₂/M/Cu-O₂/M/Cu-O₂/BaO-CuO-CuO-BaO/Cu-O₂/M/Cu-O₂/M/Cu-O₂, 3-10: (BaO-Tl₂O₂-BaO) or (BaO-TlO-BaO)/Cu-O₂/M/Cu-O₂/M/Cu-O₂/SrO-Bi₂O₂-SrO/Cu-O₂/M/Cu-O₂/M/Cu-O₂, 3-11: SrO-PbO-Cu-PbO-SrO/Cu-O₂/M/Cu-O₂/M/Cu-O₂/La₂O₂/Cu-O₂/M/Cu-O₂/M/Cu-O₂, 3-12: SrO-PbO-Cu-PbO-SrO/Cu-O₂/M/Cu-O₂/M/Cu-O₂/BaO-CuO-BaO/Cu-O₂/M/Cu-O₂/M/Cu-O₂, 3-13: SrO-PbO-Cu-PbO-SrO/Cu-O₂/M/Cu-O₂/M/Cu-O₂/BaO-CuO-CuO-BaO/Cu-O₂/M/Cu-O₂/M/Cu-O₂, 3-14: SrO-PbO-Cu-PbO-SrO/Cu-O₂/M/Cu-O₂/M/Cu-O₂/SrO-Bi₂O₂-SrO/Cu-O₂/M/Cu-O₂/M/Cu-O₂, 3-15: SrO-PbO-Cu-PbO-SrO/Cu-O₂/M/Cu-O₂/M/Cu-O₂/(BaO-Tl₂O₂-BaO) or (BaO-TlO-BaO)/Cu-O₂/M/Cu-O₂/M/Cu-O₂, 3-16: (SrO-(Pb, Cu)O-SrO) or SrO-(Pb, Sr)O-SrO/Cu-O₂/M/Cu-O₂/M/Cu-O₂/La₂O₂/Cu-O₂/M/Cu-O₂/M/Cu-O₂, 3-17: (SrO-(Pb, Cu)O-SrO) or SrO-(Pb, Sr)O-SrO/Cu-O₂/M/Cu-O₂/M/Cu-O₂/BaO-CuO-BaO/Cu-O₂/M/Cu-O₂/M/Cu-O₂, 3-18: (SrO-(Pb, Cu)O-SrO) or SrO-(Pb, Sr)O-SrO/Cu-O₂/M/Cu-O₂/M/Cu-O₂/BaO-CuO-CuO-BaO/Cu-O₂/M/Cu-O₂/M/Cu-O₂, 3-19: (SrO-(Pb, Cu)O-SrO) or SrO-(Pb, Sr)O-SrO/Cu-O₂/M/Cu-O₂/M/Cu-O₂/SrO-Bi₂O₂-SrO/Cu-O₂/M/Cu-O₂/M/Cu-O₂, 3-20: (SrO-(Pb, Cu)O-SrO) or SrO-(Pb, Sr)O-SrO/Cu-O₂/M/Cu-O₂/M/Cu-O₂/(BaO-Tl₂O₂-BaO) or (BaO-TlO-BaO)/Cu-O₂/M/Cu-O₂/M/Cu-O₂, 3-21: (SrO-(Pb, Cu)O-SrO) or SrO-(Pb, Sr)O-SrO/Cu-O₂/M/Cu-O₂/M/Cu-O₂/SrO-PbO-Cu-PbO-SrO/Cu-O₂/M/Cu-O₂/M/Cu-O₂, 3-22: Ln₂O₂/Cu-O₂/M/Cu-O₂/M/Cu-O₂/La₂O₂/Cu-O₂/M/Cu-O₂/M/Cu-O₂, 3-23: Ln₂/O₂/Cu-O₂/M/Cu-O₂/M/Cu-O₂/BaO-CuO-BaO/Cu-O₂/M/Cu-O₂/M/Cu-O₂, 3-24: Ln₂O₂/Cu-O₂/M/Cu-O₂/M/Cu-O₂/BaO-CuO-CuO-BaO/Cu-O₂/M/Cu-O₂/M/Cu-O₂, 3-25: Ln₂O₂/Cu-O₂/M/Cu-O₂/M/Cu-O₂/SrO-Bi₂O₂-SrO/Cu-O₂/M/Cu-O₂/M/Cu-O₂, 3-26: Ln₂O₂/Cu-O₂/M/Cu-O₂/M/Cu-O₂/(BaO-Tl₂O₂-BaO) or (BaO-TlO-BaO)/Cu-O₂/M/Cu-O₂/M/Cu-O₂, 3-27.' Ln₂O₂/Cu-O₂/M/Cu-O₂/M/Cu-O₂/SrO-PbO-Cu-PbO-SrO/Cu-O₂/M/Cu-O₂/M/Cu-O₂, 3-28: Ln₂O₂/Cu-O₂/M/Cu-O₂/M/Cu-O₂/(SrO-(Pb, Cu)O-SrO) or SrO-(Pb, Sr)O-SrO/Cu-O₂/M/Cu-O₂/M/Cu-O₂. The oxide superconductor of the present invention can be prepared by various methods. For example, the superconductor can be prepared by the well-known powder mixing method. Preferably, it may be prepared by various deposition methods capable of deposition control of elements on atomic order, such as the electron beam deposition method, laser deposition method, etc., or various sputtering methods, such as the magnetron sputtering method. Also, it may be prepared by the chemical vapor deposition method using halides, organic metals, etc., atomization method using nitrates, organic acids, or application method using alcoxides and the like. If the total pressure of the atmosphere during the manufacture is 1 atm. or less, as in the cases of the electron beam deposition, laser deposition, magnetron sputtering, chemical vapor deposition method, etc., the oxygen deficiency in the superconductor can be covered by as aforesaid by mixing ozone with the atmosphere. Among these manufacturing methods, the laser deposition method in the oxygen atmosphere is particularly preferable, since layers of intended compositions can be formed by deposition under fine control according to this method. This laser deposition method is described in, for example, "Japanese Journal of Applied Physics", vol. 27, No. 7, July, 1988, pp. L1293 to L1296, or "Appl. Phys. Lett.", vol. 54, No. 18, May, 1989, pp. 1802 to 1804, and "Appl. Phys. Lett.". vol. 57, No. 2, July 9, 1990, pp. 198 to 200. In general, the oxide superconductors of the present invention are prepared by the laser deposition method as follows. First, an intended crystal structure is designed, and the respective compositions of the first and second blocking layers are selected. Then, sintered bodies having the respective compositions of the individual blocking layers are prepared. Also, a sintered body of Cu or CuO₂(sic) and the elements for the mediating layers are prepared for the superconducting layers. In the case of the one-layer system oxide superconductor, it is necessary only that sintered bodies for the individual blocking layers and a sintered body of Cu or CuO₂(sic) are prepared. In the cases of the two-and three-layer system oxide superconductors, moreover, sintered bodies containing CuO₂(sic) and the elements for the mediating layers in a desired ratio may be prepared for the superconducting materials. These individual sintered bodies are separately located in a chamber to be used as targets, and a single crystal substrate of, e.g., MgO (100), heated up to a desired temperature, are opposed to the targets. In this state, the chamber is loaded with an oxygen atmosphere having a desired oxygen partial pressure, and a pulsed excimer laser with a desired intensity is applied to the targets. At this time, the laser is applied in the order of the target for the first blocking layer, target for a superconducting layer, target for the second blocking layer, target for another superconducting layer. As a result, a film including repeated units each composed of the first blocking layer, superconducting layer, second blocking layer, and superconducting layer is formed on the surface of the single crystal substrate. Thereafter, the obtained film is subjected to heat treatment under the desired oxygen partial pressure, as mentioned before, so that carrier concentration is adjusted, whereby the superconductor is obtained. In order to form this crystal structure, the repeated units should be electrically neutral. This is because if the electric charge of the whole repeated units is not neutral, the bulk body having crystal structure cannot grow. If each layer is formed on the basis of a designed crystal structure, the repeated units obtained are not always be electrically neutral, and have a positive or negative electric charge as a whole. This can be estimated beforehand from the electric charges of the aforesaid constituent elements. Table 1 shows the results of calculation of the respective electric charges of repeated units (Case 1) of a one-layer system oxide superconductor and a two- or three-layer system oxide superconductor whose mediating layers are formed of Ca, repeated units (Case 2) of a two-layer system oxide superconductor whose mediating layers are formed of Y, and repeated units (Case 3) of a three-layer system oxide superconductor whose mediating layers are formed of Y. No. Case 1 Case 2 Case 3 1 -1 +1 +3 2 -2 0 +2 3 -3 -1 +1 4 0 +2 +4 5 -1 +1 +3 6 -2 0 +2 7 0 or -1 +2 or -1 +4 or +3 8 -1 or -2 +1 or 0 +3 or +2 9 -2 or -3 0 or -1 +2 or +1 10 0 or -1 +2 or +1 +4 or +3 11 -1 +1 +3 12 -2 0 +2 13 -3 -1 +1 14 -1 +1 +3 15 -1 or -2 +1 or 0 +3 or +2 16 -2 0 +2 17 -3 -1 +1 18 -4 -2 0 19 -2 0 +2 20 -2 or -3 0 or -1 +2 or +1 21 -3 -1 +1 Based on this table, therefore, partial substitution of the constituent elements of the mediating layers or other elements in the composition constituting each blocking layer can be adopted, and the target composition can be selected so that the electric charge of the whole repeated units is 0. In preparing a repeated unit No. 7 of Case 3, for example, the electric charge of the whole repeated unit can be decreased to 0 by substitution + 3-valent Y for + 2-valent Ca which form meditating layer. Whether or not the material thus obtained is superconductive can be analyzed by cooling this material to a very low temperature and measuring its electrical resistance or magnetic susceptibility. Even at room temperature, however, it can be analyzed by the method mentioned below. The following is a description of this method. The optical properties of obtained samples are measured under predetermined conditions by means of a spectroellipsometry, whereupon a real part ε₁ and an imaginary part ε₂ of a complex dielectric function given by ε * = ε 1 + ε 2 i (i: imaginary unit) are simultaneously obtained. The respective values of the real and imaginary parts ε₁ and ε₂ are plotted with respect to the measurement wave-number, and whether or not the value of the real part ε₁ passes zero on the resulting curve is read. If the value of the real part ε₁ for a material passes zero, then the material is superconducting. Then, a reciprocal function a* (= 1/ε*) of the aforesaid complex dielectric function ε* is calculated. In this case, this reciprocal function is also a complex function, which is given by a* = a 1 + a 2 i (i: imaginary). The respective values of real and imaginary parts a₁ and a₂ of the reciprocal function a* are plotted with respect to the aforesaid measurement wave-number. A curve representing the imaginary part a₂ for a superconductive material has a clear peak at almost the same wave-number which gives ε₁ = 0 in the aforesaid complex dielectric function. For a material which is not superconducting, no such peak appears, or if any, the peak is shifted to the low-wave-number side, and is broad. Thus, if the curve representing the real part ε₁ of the aforesaid complex dielectric function passes the zero value, and if the curve representing the imaginary part a₂ of the reciprocal function of the complex dielectric function has its peak value at almost the same wave-number for ε₁ = 0, then the material is judged to be superconducting. For example, an oxide superconductor A, which has a composition expressed as Bi₂Sr₂CaCu₂O y and Tc = 85 K, and a non-superconducting material B, which has a composition expressed as Bi₂Sr₂CuO z and is not superconducting unless Tc = 0 K is obtained, were prepared, and the respective complex dielectric functions of these materials were measured under the following conditions by using a phase difference measuring apparatus (NPDM-1000 from Nippon Kogaku K.K.). Spectroscope: M-70. Light source: Halogen lamp, Sensor: Si and Ge, Polarizer and analyzer: Gran-Thomson Number of revolutions of analyzer: 2, Angle of incidence: 80° , Measurement wave-number: 7,000 to 25,000 cm⁻¹. Fig. 7 shows the relationships between the real and imaginary parts ε₁ and ε₂ of the complex dielectric function ε* of the material A and the measurement wave-number, and Fig. 8 shows the relationships between the real and imaginary parts a₁ and a₂ of the reciprocal function a* of the complex dielectric function ε* of the material A and the measurement wave-number. In this drawing, open circles and closed circles represent the real and imaginary parts, respectively. As seen from Fig. 7, the curve representing the real part ε₁ passes the zero value when the wave-number is nearly 8,100 cm⁻¹. As seen from Fig. 8, moreover, the curve representing the imaginary part a₂ has its peak value when the wave-number is nearly 8,100 cm⁻¹. On the other hand, Fig. 9 shows the relationships between the real and imaginary parts ε₁ and ε₂ of the complex dielectric function ε* of the material B and the measurement wave-number, and Fig. 10 shows the relationships between the real and imaginary parts a₁ and a₂ of the reciprocal function a* of the complex dielectric function ε* of the material B and the measurement wave-number. In this drawing, open circles and closed circles represent the real and imaginary parts, respectively. In the case of the material B, as seen from Fig. 9, the real part ε₁ of the complex dielectric function takes the zero value when the wave-number is nearly 14,500 cm⁻¹. As seen from Fig. 10, however, the curve representing the imaginary part a₂ has no such distinct peak as Fig. 8 shows in the vicinity of 14,500 cm⁻¹. As seen from Fig. 10, moreover, the imaginary part a₂ has a gentle peak in the vicinity of 8,000 cm⁻¹. In Fig. 9, however, ε₁ does not take the zero value in the vicinity of 8,000 cm⁻¹. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a conceptual diagram of a conventional La₂CuO₄ superconductor; Fig. 2 is a conceptual diagram of a conventional Y₁Ba₂Cu₃O₇ superconductor; Fig. 3 is a conceptual diagram of a conventional Bi₂Sr₂Ca₂Cu₃O₁₀ superconductor; Fig. 4 is a conceptual diagram of a one-layer system oxide superconductor according to the present invention; Fig. 5 is a conceptual diagram of a two-layer system oxide superconductor according to the present invention; Fig. 6 is a conceptual diagram of a three-layer system oxide superconductor according to the present invention; Fig. 7 is a graph showing the relationships between real and imaginary parts ε1 and ε2 of the complex dielectric function of the oxide superconductor and the measurement wave-number; Fig. 8 is a graph showing the relationships between real and imaginary parts a1 and a2 of the reciprocal function of the complex dielectric function of the oxide superconductor and measurement wave-number; Fig. 9 is a graph showing the relationships between the real and imaginary parts ε1 and ε2 of the complex dielectric function of the non-superconducting oxide material and the measurement wave-number; Fig. 10 is a graph showing the relationships between real and imaginary parts a1 and a2 of the reciprocal function of the complex dielectric function of the non-superconducting oxide material and the measurement wave-number; Fig. 11 is a model diagram of a crystal structure (Model 1-4) of Embodiment 3 as a one-layer system oxide superconductor according to the present invention; Fig. 12 is an X-ray diffraction pattern diagram of the oxide superconductor of Embodiment 3; Fig. 13 is a model diagram of a crystal structure (Model 1-15) of Embodiment 8 as a one-layer system oxide superconductor according to the present invention; Fig. 14 is an X-ray diffraction pattern diagram of the oxide superconductor of Embodiment 8; Fig. 15 is a model diagram of a crystal structure (Model 1-16) of Embodiment 9 as a one-layer system oxide superconductor according to the present invention; Fig. 16 is an X-ray diffraction pattern diagram of the oxide superconductor of Embodiment 9; Fig. 17 is a model diagram of a crystal structure (Model 1-17) of Embodiment 10 as a one-layer system oxide superconductor according to the present invention; Fig. 18 is an X-ray diffraction pattern diagram of the oxide superconductor of Embodiment 10; Fig. 19 is a model diagram of a crystal structure (Model 2-6) of Embodiment 15 as a two-layer system oxide superconductor according to the present invention; Fig. 20 is an X-ray diffraction pattern diagram of the oxide superconductor of Embodiment 15; Fig. 21 is a model diagram of a crystal structure (Model 2-8) of Embodiment 17 as a two-layer system oxide superconductor according to the present invention; Fig. 22 is an X-ray diffraction pattern diagram of the oxide superconductor of Embodiment 17; Fig. 23 is a model diagram of a crystal structure (Model 2-11) of Embodiment 18 as a two-layer system oxide superconductor according to the present invention; Fig. 24 is an X-ray diffraction pattern diagram of the oxide superconductor of Embodiment 18; Fig. 25 is a model diagram of a crystal structure (Model 3-5) of Embodiment 27 as a three-layer system oxide superconductor according to the present invention; Fig. 26 is an X-ray diffraction pattern diagram of the oxide superconductor of Embodiment 27; Fig. 27 is a model diagram of a crystal structure (Model 3-10) of Embodiment 30 as a three-layer system oxide superconductor according to the present invention; Fig. 28 is an X-ray diffraction pattern diagram of the oxide superconductor of Embodiment 30; Fig. 29 is a model diagram of a crystal structure (Model 3-20) of Embodiment 32 as a three-layer system oxide superconductor according to the present invention; and Fig. 30 is an X-ray diffraction pattern diagram of the oxide superconductor of Embodiment 32. EMBODIMENTS OF THE INVENTION [Embodiment 1] Powders of PbO and CuO were weighed so that the molar ratio Pb:Cu was 2:1, and these powders were mixed together. The resulting powder mixture was pressed into pellets at 500 kg/cm², and the resulting pellet was then sintered for 5 hours in air at 800°C. Composition: A sintered pellet of Pb₂CuO x was obtained. This will be referred to as sintered body I. Powders of SrCO₃, La₂(CO₃)₃ · nH₂O, and CuO were weighed so that the molar ratio Sr:La:Cu was 0.75:0.25:1, and these powders were mixed together. The resulting powder mixture was pressed into pellets at 500 kg/cm², and the resulting pellet was then sintered for 5 hours in air at 900°C. Composition: A sintered pellet of Sr 0.75 La 0.25 CuO x was obtained. This will be referred to as sintered body II. Powders of CaCO₃, Y₂O₃, and CuO were weighed so that the molar ratio Ca:Y:Cu was 0.8:0.2:1, and these powders were mixed together. The resulting powder mixture was pressed into pellets at 500 kg/cm², and the resulting pellet was then sintered for 5 hours in air at 900°C. Composition: A sintered pellet of Ca 0.8 Y 0.2 CuO x was obtained. This will be referred to as sintered body III. Powders of La₂(CO₃)₃ · nH₂O, BaCO₃, PbO, and CuO were weighed so that the molar ratio La:Ba:Pb:Cu was 3:0.75:0.5:2, and these powders were mixed together. The resulting powder mixture was pressed into pellets at 500 kg/cm², and the resulting pellet was then sintered for 5 hours in air at 900°C. Composition: A sintered pellet of La₃Ba 0.75 Pb 0.25 Cu₂O x was obtained. This will be referred to as sintered body IV. Then, these sintered bodies I, II, III and IV were set individually on target holder which was located in a vacuum chamber, an MgO (100) single crystal substrate was set in a position opposite to these sintered bodies, and this substrate was heated to 600°C. O₂ gas and N₂O gas were introduced into the vacuum chamber, and an excimer laser was applied to the respective targets of the individual sintered bodies with the degree of vacuum in the chamber kept at 0.067 Pa (5 × 10⁻⁴ Torr) (oxygen partial pressure: 0.034 Pa (2.5 × 10⁻⁴ Torr)). The intensity of the laser used was 150 mJ/pulse. The laser was applied to the respective targets of the sintered bodies in a manner such that the sintered bodies I, II, III, IV, III and II were irradiated for each cycle in the order named, and this cycle was repeatedly executed. A film was formed on the MgO substrate. After the film formation, the substrate was cooled at the rate of 20°C/min by introducing oxygen gas containing 8% by volume of ozone into the chamber at the flow rate of 25 ml/min. The general composition of the film obtained was (Pb 0.5 Cu 0.5 ) (Sr 0.75 La 0.25 )₂(La 0.75 Ba 0.19 Pb 0.06 )₂ - (Ca 0.8 Y 0.2 )₂Cu₅O 13.2 . When an X-ray diffraction spectrum was observed, the crystal structure of the film proved to be identical with that of the aforementioned Model No. 2-16. Subsequently, real and imaginary parts ε1 and ε2 of the complex dielectric function of this film were measured at room temperature by the aforesaid spectroellipsometry. Thereupon, the value ε1 proved to be zero when the measurement wave-number was about 9,500 cm⁻¹, and also, a peak was observed in a curve of an imaginary part a2 of a reciprocal function when the wave-number was about 9,500 cm⁻¹. Thus, it was confirmed that this film is a two-layer system oxide superconductor as it is called according to the present invention. [Embodiments 2 to 35] Sintered bodies of various compositions shown in Tables 2-(1) and 2-(2) were manufactured by the powder mixing method in the same manner as in the case of Embodiment 1. Then, films were formed on substrates shown in Tables 3-(1) and 3-(2) by the laser deposition method using these sintered bodies as targets, and these films were heat-treated under the oxygen partial pressure in the same manner as in Embodiment 1. During the film formation, an excimer laser was applied to the targets by repeating the cycles shown in Tables 3-(1) and 3-(2). Tables 3-(1) and 3-(2) show the respective general compositions of the films obtained, and also show models of crystal structures obtained X-ray diffraction patterns. A spectroellipsometry measurement was made on each of these films in the same manner as in Embodiment 1. Thereupon, the curve of the real part ε₁ of the complex dielectric function ε* passed the zero value, and a peak was observed in the curve of the imaginary part a2 of its reciprocal function at the wave-number which gave ε₁ = 0 in the aforesaid complex dielectric function. Thus, it was confirmed that the films are superconducting materials. Among these 34 kinds of superconducting films, Embodiments 2 to 11 were one-layer system oxide superconductors, Embodiments 12 to 24 were two-layer system oxide superconductors, and Embodiments 25 to 35 were three-layer system oxide superconductors. The following is a description of model figures of the respective typical crystal structures of the one-, two-, and three-layer systems, among these individual superconductors, and X-ray diffraction pattern corresponding individually to the structures. First. Figs. 11 to 18 show details of the one-layer systems, in which Figs. 11 and 12 are a model diagram and an X-ray diffraction pattern, respectively, of Embodiment 3 (Model 1-4), Figs. 13 and 14 are a model diagram and an X-ray diffraction pattern, respectively, of Embodiment 8 (Model 1-15), Figs. 15 and 16 are a model diagram and an X-ray diffraction pattern, respectively, of Embodiment 9 (Model 1-16), and Figs. 17 and 18 are a model diagram and an X-ray diffraction pattern, respectively, of Embodiment 10 (Model 1-17). Figs. 19 to 24 show details of the two-layer systems, in which Figs. 19 and 20 are a model diagram and an X-ray diffraction pattern, respectively, of Embodiment 15 (Model 2-6), Figs. 21 and 22 are a model diagram and an X-ray diffraction pattern, respectively, of Embodiment 17 (Model 2-8), and Figs. 23 and 24 are a model diagram and an X-ray diffraction pattern, respectively, of Embodiment 18 (Model 2-11). Further, Figs. 25 to 30 show details of the three-layer systems, in which Figs. 25 and 26 are a model diagram and an X-ray diffraction pattern, respectively, of Embodiment 27 (Model 3-5), Figs. 27 and 28 are a model diagram and an X-ray diffraction pattern, respectively, of Embodiment 30 (Model 3-10), and Figs. 29 and 30 are a model diagram and an X-ray diffraction pattern, respectively, of Embodiment 32 (Model 3-20). Practically, also in the individual crystal structures shown in these model figures, the oxygen content may be somewhat deviated from its stoichiometric value, due to carrier concentration adjustment, or some cations may be substituted, as mentioned before. Although only 35 kinds of oxide superconductors of the present invention, among 77 kinds in total, have been given as the embodiments, the remaining kinds can be also manufactured by the same method as these embodiments. Thus, according to the present invention, about 20 kinds of conventionally found copper oxide superconductors can be increased to about 77 kinds by using a combination of two blocking layers of different compositions and one CuO₂ sheet, a combination of two blocking layers of different compositions, one mediating layer, and two Cu-O₂ sheets, or a combination of two blocking layers of different compositions, two mediating layers, and three Cu-O₂ sheets, as has been also described in connection with the embodiments, so that the materials can be applied to a wider variety of fields.

Claims: An oxide superconductor comprising repeated units each including a first blocking layer having a composition selected from the following group, a first Cu-O₂ sheet, a second blocking layer having a composition selected from the following group and different from the composition of said first blocking layer, and a second Cu-O₂ sheet, arranged in layers in the order named, said group including: La₂O₂, BaO-CuO-BaO, BaO-CuO-CuO-BaO, SrO-Bi₂O₂-SrO, BaO-Tl₂O₂-BaO, BaO-TlO-BaO, SrO-PbO-Cu-PbO-SrO, SrO-(Pb, Cu)O-SrO, and SrO-(Pb, Sr)O-SrO. An oxide superconductor comprising repeated units each including a first blocking layer having a composition selected from the following group a , a first Cu-O₂ sheet, a first mediating layer composed of elements selected from the following group b , a second Cu-O₂ sheet, a second blocking layer having a composition selected from the following group a and different from the composition of said first blocking layer, a third Cu-O₂ sheet, a second mediating layer composed of elements selected from the following group b , and a fourth Cu-O₂ sheet, arranged in layers in the order named, said group a including: La₂O₂, BaO-CuO-BaO, BaO-CuO-CuO-BaO, SrO-Bi₂O₂-SrO, BaO-Tl₂O₂-BaO, BaO-TlO-BaO, SrO-PbO-Cu-PbO-SrO, SrO-(Pb, Cu)O-SrO, SrO-(Pb, Sr)O-SrO, and Ln₂O₂ (where Ln is selected from Nd, Sm, Eu and Gd), said group b including: Ca, Sr, Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu. An oxide superconducting material comprising repeated units each including a first blocking layer having a composition selected from the following group a , a first Cu-O₂ sheet, a first mediating layer composed of elements selected from the following group b , a second Cu-O₂ sheet, a second mediating layer composed of elements selected from the following group b , a third Cu-O₂ sheet, a second blocking layer having a composition selected from the following group a and different from the composition of said first blocking layer, a fourth Cu-O₂ sheet, a third mediating layer composed of elements selected from the following group b , a fifth Cu-O₂ sheet, a fourth mediating layer composed of elements selected from the following group b , and a sixth Cu-O₂ sheet, arranged in layers in the order named, said group a including: La₂O₂, BaO-CuO-BaO, BaO-CuO-CuO-BaO, SrO-Bi₂O₂-SrO, BaO-Tl₂O₂-BaO, BaO-TlO-BaO, SrO-PbO-Cu-PbO-SrO, SrO-(Pb, Cu)O-SrO, SrO-(Pb, Sr)O-SrO, and Ln₂O₂ (where Ln is selected from Nd, Sm, Eu and Gd), said group b including: Ca, Sr, Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu.

Inventors: NOBUMASA HITOSHI, SHIMIZU KAZUHARU, NOBUMASA, HITOSHI, SHIMIZU, KAZUHARU, NOBUMASA, HITOSHI, 2-9, CHUO 4-CHOME, SHIMIZU, KAZUHARU, 12-26, SONOYAMA 2-CHOME
Applicants: TORAY INDUSTRIES, TORAY INDUSTRIES, INC.
Application year: 1991
Main classification: C01G3
IPC codes: C01G15, C30B29, C01G1, H01L39, H01B12, C01G21, C01G3, C01G29