JPH05294620A - Superconductor - Google Patents

Abstract

(57) [Summary] [Objective] To provide a superconductor having a high critical current density and various superconducting transition temperatures. [Structure] (La _1-α Sr _α ) ₂ CuO _{4 + δ} (where 0
<Α ≦ 0.25) and at least 1 selected from the following group
A superconductor having a layered structure with a seed material. _{(Bi 1-β Pb β)} 2 Sr 2 Ca n Cu n + 1 O 6 + 2n + δ Tl 2 Ba 2 Ca m Cu m + 1 O 6 + 2m + δ MBa 2 Cu 3 O 7 + δ (Sr 1-η Ca _η ) _1-λ CuO _{2 + δ}

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nuclear fusion reactor, a magnetohydrodynamic generator, an accelerator, a rotating electric machine (motor, generator, etc.),
Suitable for magnetic coils, magnetic separators, magnetic levitation trains, nuclear magnetic resonance measuring devices, magnetic propulsion vessels, electron beam exposure devices, various experimental devices, etc., as well as power transmission lines, electrical energy storage, transformers, rectifiers, Suitable for applications where power loss is a problem, such as phase shifters, as well as Josephson devices and SQUs.
The present invention relates to a superconductor suitable as an element such as an ID element and a superconducting transistor, and further as a functional material such as an infrared detection material and a magnetic shielding material.

[0002]

2. Description of the Related Art Conventionally, a series of copper complex oxide superconductors as described below are known.

That is, "Physical Review Letters"
, 67, 1362-1365 (1991).
Describes that substances having the same crystal structure but different constituent elements are alternately stacked. This is YB
a superconductor of a ₂ Cu ₃ O _{7 + δ} and PrBa ₂ Cu ₃ O _{7 + δ} , YBa ₂ Cu ₃ O which is a superconductor between layers of non-superconductor PrBa ₂ Cu ₃ O _{7 + δ 7 + δ} layers are located.

In addition, "Japanese Journal of Applied Ph
ysics ", Volume 30, L1381-1383 (19
91) describes a superconductor formed by stacking two kinds of substances having different crystal structures. This is Bi ₂ Sr
₂ CuO _{6 + δ} and Bi ₂ Sr ₂ CaCu ₂ O _{8 + δ} are stacked, which is also a non-superconductor Bi ₂ Sr
Bi ₂ Sr ₂ C which is a superconductor between the layers of ₂ CuO _{6 + δ}
A layer of aCu ₂ O _{8 + δ} is located.

Furthermore, "Applied Physics Letters",
Volume 58, pp. 1443-1445 (1991),
It is a so-called T-structure superconductor (La _1-x Sr _x ) ₂
CuO _{4 + δ} and S, which is a so-called T'structure non-superconductor
A superconductor formed by stacking m ₂ CuO _{4 + δ} is described.

It is known that the superconducting property of the copper complex oxide superconductor depends on the amount of charge carriers. However, the above-mentioned conventional superconductors are both superconducting layers and non-superconducting layers. The superconducting property is controlled by changing the thickness of the body layer. Since the superconducting current does not flow in the non-superconductor layer, the superconducting transition temperature can be changed, but the critical current density is low.

[0007]

The object of the present invention is to solve the above-mentioned problems of a series of conventional copper complex oxide superconductors, to have a high critical current density, and to have various superconducting transition temperatures. To provide a superconductor to obtain.

[0008]

In order to achieve the above object, the present invention provides (La _1-α Sr _α ) ₂ CuO _{4 + δ} (where 0 <α ≦ 0.25) and the following group: Provided is a superconductor having a layered structure with at least one selected substance.

[0009] _{(Bi 1-β Pb β)} 2 Sr 2 Ca n Cu n + 1 O 6 + 2n + δ Tl 2 Ba 2 Ca m Cu m + 1 O 6 + 2m + δ MBa 2 Cu 3 O 7 + δ (Sr _1-η Ca _η ) _1-λ CuO _{2 + δ} where 0 ≦ β ≦ 0.3 0 ≦ γ ≦ 0.5 0 ≦ η ≦ 0.95 0 ≦ λ ≦ 0.2 −0.5 ≦ δ ≦ 0.5 m: 1, 2 or 3 n: 2 or 3 M: Y, La, Nd, Sm, Eu, Gd, Dy, Ho or Er In the superconductor of the present invention, each layer is a copper complex oxide superconductor. The superconducting transition temperature (Tc) can be changed by the amount of charge carriers (carriers) doped in the material forming each layer, particularly the amount of charge carriers (carrier concentration) per CuO ₂ surface. it can.

By the way, for example, (La _1-α Sr _α ) ₂
In CuO _{4 + δ} , when La is replaced with Sr and carriers are doped, it is an insulator at first, but eventually becomes a superconductor, and Tc also rises. And α = 0.075
Tc shows a maximum value of 36K, but when α becomes larger than this, Tc decreases and eventually becomes a non-superconductor. It is known that some other substances show almost the same tendency.

As described above, in order to maximize Tc, it is necessary to dope a proper amount of carriers. As a method therefor, a method of substituting a part of the constituent elements and a heat treatment of the obtained superconductor are further performed. There is a way to do it.

The former method for substituting a part of the constituent elements is described in "Zeitschrift fur Physik B-condensed Matter".
, Vol. 83, pp. 7-17 (1991), by substituting the elements of the blocking layer and mediating layer with elements of different valences.
This is a method of doping carriers on the O ₂ surface. However, usually, in the copper composite oxide superconductor, the substitution element is
It forms a solid solution only in a narrow range, or does not form a solid solution at all. For example, (La _1-α Sr _α ) ₂ CuO _{4 + δ} is 0 <α ≦ 0.
In the range of 25, La can be replaced with Sr. That is, 0.
It is not possible to form a solid solution with more than 25 Sr.
_{(Bi 1-β Pb β)} 2 Sr 2 Ca n Cu
_{The same} applies to _{n + 1} O _{6+ 2n + δ} and (Sr _1-η Ca _η ) _1-λ CuO _{2 + δ} , where β, η, and λ are 0 ≦ β ≦ 0.
Only in the range of 3, 0 ≦ η ≦ 0.95 and 0 ≦ λ ≦ 0.2, a single phase cannot be obtained. Oxygen is the only anion in these crystals and has a large effect on the carrier concentration. However, in order for the copper complex oxide superconductor to form its crystal structure, the oxygen content is -0. It must be in the range of 5 ≦ δ ≦ 0.5.

On the other hand, in the method by heat treatment, the superconductor obtained by sintering is heated in an oxidizing atmosphere such as air or oxygen or under a high oxygen partial pressure to take in oxygen depleted during sintering. This is a method for increasing δ.
This method is relatively simple and can be made into a single phase, but is often used when the superconducting properties are not so good. For example,
In YBa ₂ Cu ₃ O _{7 + δ} , oxygen deficiency often occurs in the Cu-O chain, and Tc is 40 to 6 when δ is about -0.4.
Although it is as low as 0K, when it is heat-treated in an oxidizing atmosphere at 300 to 600 ° C. and oxygen is injected until δ becomes about −0.05, it rises to about 90K.

The superconductor of the present invention is formed by stacking copper composite oxide superconductors having different crystal structures. (La _1-α Sr _α ) ₂ CuO _{4 + δ} is the above-mentioned _two types. It is a substance that is easily doped with carriers depending on the method, and can be doped more than the carrier amount that maximizes Tc. On the other hand, the substance selected from the above-mentioned specific group is a substance that is difficult to dope the carrier even if the two methods are used, or the trapped carrier easily escapes.

Therefore, these (La _1-α Sr _α ) ₂ Cu
The total carrier concentration is adjusted by stacking O _{4 + δ} and a substance selected from a specific group and forming a superlattice,
Control c. Here, (La _1-α Sr _α ) ₂ CuO
_{4 + δ} is larger than that of the substance selected from the specific group, and is considered to supply the carrier to the substance selected from the specific group. Considering such an action, in the superconductor of the present invention, (La _1-α S
Since the carrier must exude from r _α ) ₂ CuO _{4 + δ} into a substance selected from a specific group, it is preferable that the layer of each substance is thin, and especially the layer of the substance selected from the specific group is It is preferably thinner than 20 unit cells. Further, (La _1-α Sr _α ) ₂ CuO _{4 + δ} , the repeating number of layers of a substance selected from a specific group, if the number of repetitions is too large, the crystallinity decreases. Is preferred. More preferably, it is 200 or less.

The layer of a substance selected from a specific group may contain two or more types of substances selected from the group. That is, in the present invention, (La _1-α S
It suffices that layers of r _α ) ₂ CuO _{4 + δ} and layers of at least one substance selected from a specific group are alternately formed.

The superconductor of the present invention has a tape shape, a wire shape,
It can be used in various shapes such as a fiber shape and a sheet shape. Further, it can be used by being formed on a reinforcing wire made of carbon fiber, ceramics, or metal such as gold or silver. Further, it can be used by filling it with a hollow material for reinforcement such as a silver sheath. Furthermore, it can be used as a superconducting wire having a multi-core wire structure using copper or the like as a matrix. In addition, Si, MgO, LaGaO ₃ , LaAlO
It can be formed as a thin film on a substrate made of ₃ , NdGaO ₃ , NdAlO ₃ , SrTiO ₃ or the like, and can be used as various elements or wiring for LSI.

The superconductor of the present invention can be produced by various methods, but laser ablation method, molecular beam epitaxy method, electron beam evaporation method, various sputtering methods, etc., which can control each layer in atomic order. The film is preferably formed by the physical vapor deposition method of. For example, in the case of the laser ablation method, the following is performed.

That is, first, powders of oxides or carbonates of La, Sr, Cu are mixed and calcined in air or an oxidizing atmosphere at 800 to 1050 ° C. for 1 to 12 hours to form the calcined powder into pellets. The molded body at 800 to 1150 ° C.
Sintered for 12 hours to obtain (La _1-α Sr _α ) ₂ CuO
_Get a target of _{4 + δ} (target 1). Since the film composition and the target composition may deviate during film formation, the powders are weighed and mixed so that the film composition becomes a desired composition. Further, La may be insufficient depending on the film forming conditions, and it may be preferable to increase the La amount by about several percent.

On the other hand, similarly, a target (target 2) of a substance selected from a specific group is obtained. However, the calcination temperature is 650 to 1000 ° C, and the sintering temperature is 700 to 1100.
℃. In this case as well, since the film composition and the target composition may deviate during film formation, the respective powders are weighed and mixed so that the film composition becomes a desired composition.

Next, the targets 1 and 2 are set in the chamber, the substrate is set at a position facing these targets and separated by ⁵ to 150 mm, and the partial pressure in the chamber is set to 1 × 10 ^-5 〜. The substrate is heated to 400 to 900 ° C. while the atmosphere is set to 5 × 10 ³ Pa in an oxidizing atmosphere.
Then, the targets 1 and 2 are alternately irradiated with an excimer laser to deposit constituent materials of each target on the substrate.

In the above, if the distance between the target and the substrate is less than 5 mm, the substrate becomes a hindrance and the irradiation angle of the excimer laser to the target is obliged to be small, so that the ablation of the target is less likely to occur. In addition, the film thickness is likely to be uneven. When the distance between the target and the substrate is longer than 150 mm, the deposition rate on the substrate becomes low, which is not practical.

To create an oxidizing atmosphere in the chamber,
Oxygen, ozone, NO ₂ , or N ₂ O is used, or ozone or active oxygen is generated by irradiating an oxygen atmosphere with ultraviolet rays or the like.

The partial pressure of the oxidizing atmosphere in the chamber is 1 ×
When it is lower than 10 ^-5 Pa, the oxygen uptake amount of the copper complex oxide deposited on the substrate is small, and when it is higher than 1 × 10 ³ Pa, impurities are easily mixed in the copper complex oxide. The morphology of the obtained film will be significantly reduced.

When the temperature of the substrate is lower than 400 ° C., crystallization of the target material deposited on the substrate is less likely to occur, while when it is higher than 900 ° C., re-evaporation of the deposited substance occurs and the film is formed. The composition deviates from the intended composition.

As the excimer laser, ArF or Kr is used.
F, XeCl or the like can be used. The pulse frequency depends on the type of target and the desired ablation excitation species, but if it is too high, the rearrangement of the atoms deposited on the substrate may be incomplete and the crystallinity may decrease, so about 1 to 80 Hz. Is preferred.

The target is switched when the film reaches a desired thickness while monitoring the film thickness with a film pressure gauge or monitoring the film forming time from the relationship between the film forming time obtained by a standard sample and the film thickness. Reflection high-speed electron diffraction (R
The intensity of the diffraction grating points may be monitored on the image obtained by HEED), and the vibration pattern may be switched when the number of unit gratings reaches a desired value. If the laser irradiation is stopped for several seconds to several minutes when the target is switched, the rearrangement of the atoms deposited on the substrate may be more complete and the film quality may be improved.

In this way, a thin film is formed on the substrate,
When the total film thickness reaches a desired value, the laser irradiation is stopped, and the film is cooled to 300 to 650 ° C at a rate of 1 to 100 ° C / min. When the temperature reaches 300 to 650 ° C., the partial pressure of the oxidizing atmosphere in the chamber is set to 1 × 10 ⁻³ to 1 × 10 ⁵ Pa.
And hold for 1-60 minutes. At this time, the substrate temperature is 3
Lower than 00 ℃ or partial pressure of oxidizing atmosphere is 1 × 10 ^-3
When it is lower than Pa, the oxygen uptake amount of the copper composite oxide decreases. On the other hand, if the temperature is higher than 650 ° C., the oxygen taken in may be released, and the amount of oxygen taken up also decreases. When the partial pressure of the oxidizing atmosphere is higher than 1 × 10 ⁵ Pa, impurities are likely to be mixed in the deposited copper composite oxide.

In order to make oxygen uptake more complete, the obtained film is annealed at 300 to 1000 ° C. for 0.1 to 20 hours under an oxygen partial pressure of 0.1 to 1 atm, 300-110 under oxygen partial pressure of 1-400 atmospheres
It is also preferable to perform HIP (Hot Isostatic Press) treatment at 0 ° C. for 1 to 20 hours.

In the above, one layer is formed for each layer.
Although one target was used, multiple targets can also be used. For example, (Bi _1-β Pb _β ) ₂
Pellets formed by mixing and sintering Bi and Pb oxide powders instead of the target 2 in the formation of the Sr ₂ CaCu ₂ O _{8 + δ} layer, and Cu oxide powders and Sr carbonate powders. It is possible to use a pellet obtained by mixing and sintering and a pellet obtained by mixing and sintering a carbonate powder of Ca and an oxide powder of Cu.

[0031]

【Example】

Example 1 La ₂ O ₃ , SrCO ₃ , and CuO powders were mixed with La: S.
Weigh so that r: Cu is 1.85: 0.3: 1,
The target 1 was obtained by mixing, calcining in air at 900 ° C. for 12 hours, crushing, shaping into pellets, sintering in air at 1000 ° C. for 24 hours, and slow cooling.

Further, Bi ₂ O ₃ powder was molded into pellets, sintered in air at 750 ° C. for 5 hours, and gradually cooled to obtain a target 2.

Further, SrCO ₃ and CuO powders are added to S
The r: Cu was weighed so as to be 2: 1, mixed, calcined in air at 800 ° C. for 5 hours, crushed, formed into pellets, and baked in air at 900 ° C. for 5 hours. After binding, it was gradually cooled to obtain a target 3.

Further, each powder of CaCO ₃ and CuO is weighed so that Ca: Cu becomes 1: 1, mixed, calcined in air at 700 ° C. for 5 hours, pulverized and molded into pellets. Then, it was sintered in air at 800 ° C. for 5 hours and gradually cooled to obtain a target 4. Next, targets 1, 2,
3 and 4 are set in the chamber, SrTi having a (100) plane orientation is located at a position facing these targets and being 25 mm away from each target.
An O ₃ substrate is placed and an oxidizing atmosphere (N ₂ O
The partial pressure of (gas) was adjusted to 0.01 Pa, and the substrate was heated to 600 ° C.

Next, ArF having a wavelength of 193 nm, an oscillation frequency of 8 Hz and a pulse energy density of 1 J / cm ² was applied to each target.
An excimer laser is used to monitor the thickness of the substance deposited on the substrate by RHEED, and the target is set to a target 1 so that the thickness of the layer formed by the target 1 and the layers formed by the targets 2, 3 and 4 are each one unit lattice.
Irradiation was sequentially switched to 3, 4, 3, 2, 3, 4, 3, 2, ....

Thus, a thin film having a thickness of about 50 nm was obtained on the substrate. When this thin film was analyzed by X-ray diffraction or the like, it was found that (La _0.85 S
r _0.15 ) ₂ CuO _{4 + δ} layer and Bi ₂ Sr ₂ Ca ₂ Cu ₃
It was a stack of 10 alternating layers of O _{10+ δ} .

When the temperature dependence of the electric resistance of this thin film was measured by the 4-terminal method, a sharp decrease in the resistance due to the superconducting transition started at 100 K (Tc
_onset ), the resistance became zero at 80K (Tc _zero ). The critical current density was 3 × 10 ⁵ A / cm ² at 4.2K.

Example 2 As in Example 1, La: Sr: Cu was added to 1.85:
A target 1 of 0.3: 1, a target ₂ of Bi ₂ O ₃ , a target _{3 of} Sr: Cu of 2: 1 and a target 4 of Ca: Cu of 1: 1 were obtained.

Next, the targets 1, 2, 3 and 4 were set in the chamber, and thereafter, in the same manner as in Example 1, but with the layer according to the target 1 and the targets 2, 3, and 4.
So that the thickness of each of the layers is 1 unit lattice, the targets are 1, 3, 4, 3, 2, 3, 4,
The film was formed by sequentially switching to 3, 2, ....

Thus, a thin film having a thickness of about 57 nm was obtained on the substrate. This thin film has a thickness of 1 unit lattice,
(La _0.85 Sr _0.15 ) ₂ CuO _{4 + δ} layer and Bi ₂ Sr ₂
It was a stack of 10 alternating layers of Ca ₃ Cu ₄ O _{12+ δ} . The Tc _onset of this thin film is 11
0K, Tc _zero is 85K, and the critical current density is 4.2.
It was 4 × 10 ⁵ A / cm ² in K.

Example 3 As in Example 1, La: Sr: Cu was added to 1.85:
A target 1 having a ratio of 0.3: 1 was obtained.

Further, each powder of BaCO ₃ , CaCO ₃ and CuO was weighed and mixed so that Ba: Ca: Cu was 2: 1.5: 2.2, mixed and aired at 850 ° C. for 5 hours. It is calcined and crushed, and Tl ₂ O ₃ is added to it: Ta: Ba: Ca:
Cu was added so as to be 3: 2: 1.5: 2.2, mixed, molded into pellets, sintered in air at 850 ° C. for 4 hours, and slowly cooled to obtain target 2. It was

Next, the targets 1 and 2 were set in the chamber and the same as in Example 1 except that the layer thickness of the target 1 was 10 unit lattices and the layer thickness of the target 2 was 4 unit lattices. Then, the target was sequentially switched to the targets 1, 2, ... The substrate was placed 15 mm away from the target. The oxidizing atmosphere in the chamber was oxygen gas containing 8% ozone (partial pressure 1 Pa). Further, the substrate temperature was 630 ° C., and a film thickness meter was used to monitor the thickness.

Thus, a thin film having a thickness of about 260 nm was obtained on the substrate. This thin film has a thickness of 10 unit cells (La
A layer of _0.85 Sr _0.15 ) ₂ CuO _{4 + δ and} a layer of Tl ₂ Ba ₂ CaCu ₂ O _{8 + δ} having a thickness of 4 unit cells were alternately stacked 10 times. Also, the Tc of this thin film
_{The onset} was 105 K, the Tc _zero was 82 K, and the critical current density was 4.2 K and 3 × 10 ⁵ A / cm ² .

Example 4 In the same manner as in Example 1, La: Sr: Cu was added at 1.80:
Target 1 having 0.4: 1 was obtained.

Further, the respective powders of Y ₂ O ₃ , BaCO ₃ and CuO were weighed and mixed so that Y: Ba: Cu was 1.1: 2: 4.2 and mixed, and at 750 ° C. in air. A target 2 was obtained by calcination for 10 hours, crushing, forming into pellets, sintering in air at 820 ° C. for 10 hours, and slow cooling.

Next, the targets 1 and 2 were set in the chamber, and thereafter, in the same manner as in Example 1, but so that the thickness of the layer formed by the target 1 and the layer formed by the target 2 became 3 unit lattices, respectively. The film was formed by sequentially switching the target to targets 1, 2, .... The oxidizing atmosphere in the chamber was oxygen gas containing 8% ozone (partial pressure 1 Pa). The substrate temperature was 650 ° C., the laser oscillation frequency was 10 Hz, and the pulse energy density was 1.5 J / cm ² . A film thickness meter was used to monitor the thickness.

Thus, a thin film having a thickness of about 120 nm was obtained on the substrate. This thin film has a thickness of 3 unit lattices,
(La _0.8 Sr _0.2 ) ₂ CuO _{4 + δ} layer and YBa ₂ Cu
It was a stack of 16 alternating 16 layers of ₃ O _{7 + δ} . The Tc _onset of this thin film is 84K, Tc _zero
Is 72K, the critical current density is 4.2K, and 1 × 10 ⁵
It was A / cm ² .

Further, this thin film is formed with an oxygen partial pressure of 400 kg / cm.
^When HIP treatment was performed at 500 ° C. for 5 hours under ^{2 and} the temperature dependence of electric resistance was measured again, Tc _onset was 90 K,
Tc _zero is 85K and critical current density is 4.2K and 4
It was × 10 ⁵ A / cm ² . Example 5 In the same manner as in Example 1, La: Sr: Cu was changed to 1.80:
Target 1 having 0.4: 1 was obtained.

Further, each powder of SrCO ₃ , CaCO ₃ and CuO was weighed and mixed so that Sr: Ca: Cu was 0.2: 0.8: 1, mixed, and then in air at 750 ° C. for 10 hours. Calcination, crushing, forming into pellets, 820 in air
The target was obtained by sintering at 10 ° C. for 10 hours and gradually cooling.

Next, the targets 1 and 2 were set in the chamber, and thereafter, in the same manner as in Example 1 except that the thickness of the layer formed by the target 1 and the layer formed by the target 2 were 4 unit lattices, respectively. The target was sequentially switched to targets 1, 2, ..., A thin film was formed. The substrate was placed 25 mm away from the target.
The oxidizing atmosphere in the chamber was oxygen gas containing 8% ozone (partial pressure 1 Pa). Furthermore, the substrate temperature is 65
It was set to 0 ° C. A film thickness meter was used to monitor the thickness.

Thus, a thin film having a thickness of about 210 nm was obtained on the substrate. This thin film has a thickness of 4 unit cells each,
(La _0.8 Sr _0.2 ) ₂ CuO _{4 + δ} layer and Sr _0.2 Ca
Alternating layers of _0.8 O _{2 + δ} were stacked 30 times. Also, the Tc _onset of this thin film is 50K, Tc
_zero is 42K, critical current density is 4.2K and 1x1
It was 0 ⁴ A / cm ² .

[0053]

The superconductor of the present invention is (La _1-α Sr
Since it has a layered structure of _α ) ₂ CuO _{4 + δ} (where 0 <α ≦ 0.25) and at least one substance selected from the following group, as shown in the examples, the critical current A superconductor having a high density and various superconducting transition temperatures can be obtained.

[0054] _{(Bi 1-β Pb β)} 2 Sr 2 Ca n Cu n + 1 O 6 + 2n + δ Tl 2 Ba 2 Ca m Cu m + 1 O 6 + 2m + δ MBa 2 Cu 3 O 7 + δ (Sr _1-η Ca _η ) _1-λ CuO _{2 + δ} where 0 ≦ β ≦ 0.3 0 ≦ γ ≦ 0.5 0 ≦ η ≦ 0.95 0 ≦ λ ≦ 0.2 −0.5 ≦ δ ≦ 0.5 m: 1, 2 or 3 n: 2 or 3 M: Y, La, Nd, Sm, Eu, Gd, Dy, Ho or Er

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI Technical display location H01B 13/00 565 D 8936-5G H01L 39/24 ZAA D 8728-4M

Claims (1)

[Claims] 1. A superconductor having a layered structure of (La _1-α Sr _α ) ₂ CuO _{4 + δ} (where 0 <α ≦ 0.25) and at least one substance selected from the following group: .. _{(Bi 1-β Pb β)} 2 Sr 2 Ca n Cu n + 1 O 6 + 2n + δ Tl 2 Ba 2 Ca m Cu m + 1 O 6 + 2m + δ MBa 2 Cu 3 O 7 + δ (Sr 1-η Ca _η ) _1-λ CuO _{2 + δ} However, 0 ≦ β ≦ 0.3 0 ≦ γ ≦ 0.5 0 ≦ η ≦ 0.95 0 ≦ λ ≦ 0.2 −0.5 ≦ δ ≦ 0.5 m: 1, 2 or 3 n: 2 or 3 M: Y, La, Nd, Sm, Eu, Gd, Dy, Ho or Er