JPH085713B2 - Y-Ba-Cu-O-based oxide photoconductive material and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to the optical properties of a composition outside the composition range showing the superconductivity of an oxide superconductor and a composition leading to the composition, particularly high-speed pulsed photoconductivity. We discovered a substance that exhibits a photoconductivity phenomenon that is deeply related to superconductivity, which is unpredictable in common sense. In its manufacturing method, the composition ratio x, y (for example, y = 0, x = 0 → 2), or by controlling z by constant cooling of x and y (for example, x = 2, y = 3) or by gradually cooling, it is close to x = 0 to 1 or rapidly cooled. It is a semiconductor with photoconductivity, it is close to x = 2, and it is a superconductor when it is cooled slowly. As an industrial application field, it can be expected to be applied to superconducting optoelectronics. Is.

(Prior Art) Until now, superconducting materials have mainly used metals and their alloys. Also, in recent oxide high temperature superconductors (for example, Y-Ba-Cu-O-based oxide superconductors), a large amount of additional elements (Ba, Sr) and the like are used for the purpose of raising the critical temperature. Therefore, measurement of their optical properties near the visible has been largely limited to light reflection or scattering experiments, reflecting these metallic properties.

(Problems to be solved by the invention) As shown by this, light does not enter a superconductor by only reflecting it, so superconductivity and optical properties are not reflected or scattered at academic conferences and international conferences both in Japan and abroad. It is considered to be an almost unrelated field.

However, if a material with photoconductivity corresponding to superconductivity can be manufactured, it is possible to combine these materials, for example, devices such as superconducting phototransistors, and the “superconducting” based on the currently pursued Josephson device. It becomes possible to fabricate a device having both the characteristics of the "conduction computer" and the "optical computer" proposed in optoelectronics, that is, a "superconducting optical computer".

(Means for Solving the Problems) An object of the present invention is to address the occurrence of superconductivity at a temperature below the critical temperature at which the superconducting phenomenon of a Y-Ba-Cu-O-based oxide superconducting material occurs. Y-Ba-Cu- that produces photoconductivity
An object is to provide an O-based oxide photoconductive material.

Y-Ba-Cu-O based oxide photoconductive substance according to the invention have the general formula _{Y 3-x -Ba x -Cu y} -O z , where, 0 ≦ x <1,3 ≦ y ≦ 6,6 It has a composition of ≦ z ≦ 12, and is an insulator or a semiconductor in a dark place at a temperature substantially corresponding to the transition temperature of the Y-Ba-Cu-O-based oxide superconducting substance to the superconducting state, It is characterized by producing photoconductivity by irradiation.

Furthermore, the manufacturing method of the Y-Ba-Cu-O based oxide photoconductive substance according to the invention have the general formula _{Y 3-x -Ba x -Cu y} -O z , where, 0 ≦ x <1,3 ≦ The starting material having a composition of y ≦ 6,6 ≦ 6 ≦ 12 is mixed with the solid-state reaction at a temperature of 650 to 1050 ° C. for 1 to 10
Heat for an hour, then slowly cool, press-shape, then 570-1
Secondary sintering is performed at 050 ° C., and ultra-quick cooling is performed at a cooling temperature of 2000 to 900 ° C./sec, or slow cooling is performed at a cooling rate of 150 to 200 ° C./H to obtain a Y-Ba-Cu-O system. It is characterized in that a substance which is an insulator or a semiconductor in a dark place and which exhibits photoconductivity by light irradiation is obtained at a temperature substantially lower than a transition temperature of an oxide superconducting substance to a superconducting state.

The reason why the substance of the present invention is limited to the composition represented by the general formula is as follows.
If the composition range is about 650 to 105
Heat at 0 ℃ for 1-10 hours, gradually cool, press-mold, and further sinter at 570-1050 ℃, super-cool at 2000-900 ℃ / sec, or 150-200 ℃ This is because a substance having photoconductivity in parallel with superconductivity can be obtained by slowly cooling with / H, and the composition of the numerical value of (x, y, z) corresponding to this general formula is x = 0 or x From ≦ 1, as x = 0 approaches, it becomes insulating and shows semiconductor-like properties, and exhibits photoconductivity for light of a specific wavelength, whereas from x = 1 toward x = 2, it changes to superconductivity. I found out that I would do it. However, x = 2, y = 3
In the case of z = 6.75 to 6.97, the well-known superconductivity is exhibited and the photoconductivity cannot be observed. Therefore, this range is excluded from the composition range of the present invention. Among the substances obtained by the present invention, an example of the substance showing an insulating property is x = 0,
y = 3, z = 7.5 to 12, and the substance exhibiting superconductivity is, for example, x = 2, y = 3, z = 6.5 to 6.97, and as shown in the examples, superconductivity is obtained even in the region of the insulating substance. The present invention is completed by systematically clarifying that it exhibits photoconductivity having temperature dependence and excitation light wavelength dependence as if it has latent properties.

Conventionally known Y ₃ -Cu ₆ -O _z (where, z = 6 to 12)
And most of the oxide-based compounds such as _{Y 3-x -Ba x -Cu 3} -O y is a semiconductor in the dark otherwise irradiated with light in a ground state (ground state) i.e. a low temperature. Therefore, elementary excitation can be created by giving an appropriate amount of momentum and an appropriate amount of energy to the ground state of these substances. For ordinary superconductors, elementary excitation beyond the energy gap destroys the superconducting ground state in BCS theory. However, there is a possibility that an insulating semiconductor can create a coherent state of elementary excitation like bipolaron and excitons even in a thermally non-equilibrium state. In parallel with the research on superconductors with high critical temperature (Tc), unlike the trend of research, it is a completely new superficial in the field of basic physics and applied physics from a new perspective, that is, from the viewpoint of elementary excitation concept. The present invention is completed by finding a substance having a composition that does not become a conductor and having a photoconductive property corresponding to superconductivity in a composition close to that of a superconductor.

(Example) As an example of such a _{material, Y 3-x -Ba x -Cu} y -
This is O _z , and this research was conducted to clarify the whole face diagram. Here, the present inventor has conducted research to clarify not only the superconducting phase of the above substances but also the semiconductor phase or the insulating phase.

A large number of samples of Y-Cu-O system and Y-Ba-Cu-O system were used for Y ₂
O _3, BaCO ₃ and conventionally powder CuO known methods namely 650
A method in which primary calcination is performed by solid-state reaction at ~ 1050 ° C, and after press-molding using the product, secondary sintering is performed at 570 ~ 1050 ° C, annealing at 500 ~ 600 ° C, and slow cooling. Manufactured in. The composition of the starting material was studied in detail, and although it was in contact with the area as a simple superconducting material, it could be controlled once in the area without them. In addition, z was controlled by paying attention to the speed of the cooling method.

The samples thus obtained are sample numbers S22, S21, S20.
Samples such as.

Note samples Y ₃ -Cu ₃ -O _7.5 is # S61 in the composition, 1
Secondary firing (1000 to 1050 ° C x 2H) Slow cooling, pressure shaping, secondary firing (1000 to 1050 ° C x 2H) Cooling, annealing (600 ° C x 2H)
Was slowly cooled.

x, is in the range of some portion of the value of y and _{z Y 3 -Cu 3 -O z,} Y
₃ -Cu ₆ -O _z and _{Y 3-x -Ba x -Cu y} -O samples _z many (x,
Since it becomes highly insulating in the y, z) region, or becomes at least a semiconductor at low temperatures, we encountered some severe difficulties with ordinary resistance measurement techniques. Especially in combination with non-ohmic contact power supply and space charge storage and low signal / noise ratio due to low carrier density in high impedance materials, it became a serious problem.

In this experiment, two types of techniques were adopted for resistance measurement and conductivity measurement. First, for the insulating samples S22 and S61 (ρ10 ⁸ Ω · cm), the fast pulse technique with a blocking electrode was adopted [see FIG. 1 (A)]. Specimen 13H with dye laser light having a pulse width of 3 ns
Excited in synchronism with a pulsed electric field up to E≈5 kV / cm with a repetition rate of z and an interval of 10 ms [see FIG. 1 (b)], and the magnitude of the photoconductive response was measured to overcome the above-mentioned difficulties. did it. Secondly, for samples with moderate conductivity (ρ10 ^-2 to 10 ^-1 Ωcm) at 300K, such as S20 and S21,
The resistance was measured by adopting a normal four-terminal method without using light.

Sample S20 is a mixture of Y ₂ O ₃ 0.40 g, BaCO ₃ 1.40 g, and CuO 0.85 g, and is fired.Sample S61 is Y ₂ O ₃ 2.25 g, CuO1.591.
It was obtained by mixing g and firing so as to obtain Y ₃ Cu ₃ O _z . Sample S21 is a mixture of Y ₂ O ₃ 0.40g, BaCO ₃ 0.35g, and CuO 0.425g,
_It was fired so that the formula of ₂ Ba ₁ Cu ₃ O _z was obtained. Sample S22 is Y ₂ O ₃ 0.
72 g and CuO (1.00 g) were mixed and fired to obtain Y ₃ Cu ₆ O _z .
z represents the amount of oxygen, z changes depending on the firing method, and the product obtained accordingly varies.

The static magnetic susceptibility or the magnitude of magnetization itself was measured in a weak magnetic field up to H≈500 Oe using the microwave SQUID method [see FIGS. 2 (A) and (b)] operating at 9 GHz. This system was normally operated in a mode locked in the Q pattern [see FIG. 2 (C)]. The bandwidth of this system extends from DC to 200 KHz, and the twist rate is about 10 ⁴ φ ₀ / sec.
Set to.

The photoconductive spectral response was studied by selecting the wavelength λ from the dye laser with a resolution Δλ ≈ 1 nm, with reference to the appropriate standardization procedure for the incident power from the laser and the spectral sensitivity of the pyroelectric detector. It was recognized that the possibility that the sample would be heated by photoexcitation was small and negligible. Photoexcited carrier density is 10 ⁶
It is of the order of 10 ⁸ / cm ³ . All optical signals were typically detected in synchronous mode using a Bockker integrator.

Experimental Results Sample S20 except (x = 2), and all _{Y 3-x -Ba x -Cu 3} -O z, in the Y ₃ -Cu ₆ -O _z (sample S20), the above-described transient A clear signal was observed indicating the photoconductivity using the pulse technique.

First, the dependence of photoconductivity Q (λ, T, E, H) on E is 77K.
It was found that the linearity was almost linear up to E≈kV / cm. No significant magnitude of lateral and longitudinal magnetoresistance in Q (λ, T, E, H) was observed up to H≈15 kOe at 77K.

FIG. 3 is _{(a) Y 3 -Cu 6 -O} z ( where z ≒ 10.5) (sample S
_{22), (b) Y 3} -Cu 3 -O z ( where z ≒ 7.5) (Sample S6
_{1), (c) Y 2} -Ba 1 -Cu 3 -O z ( where z ≒ 6.6) (Sample S2
1) pulsed pumping optical response in the range of wavelength λ ≈ 460 to 640 nm,
A representative spectrum of Q (λ, T) is shown.

Second, the Y ₃ -Cu ₆ -O _z (where z ≒ 10.5) (sample S22) and
Y ₃ -Cu ₃ -O _z (where z ≒ 7.5) of the magnetization (Sample S61) size M (T, H), it was confirmed to be positive sign paramagnetic clearly the 4.2K least (See FIG. 4). The most prominent feature is that the magnitude of magnetization M (of Y ₁ —Ba ₂ —Cu ₃ —O _z (Sample S20), especially Y ₂ —Ba ₁ —Cu ₃ —O _z (z≈6.6) (Sample S21) It was confirmed that (T, H) clearly showed the diamagnetism peculiar to the superconducting state. The magnitude of magnetization of sample S21 is about 1/30 of the magnitude of magnetization of sample S20.

Thirdly, the wavelength region of photoconductive response Q (λ, T) λ≈500 to 5
As illustrated in FIGS. 5 (a) and 5 (b), the temperature dependence at 70 nm is examined for the sample S22 which is insulating at a temperature below the critical temperature and the sample S21 which exhibits superconductivity. It was Surprisingly, the dark resistance ρ is too large for sample S22 and despite the tremendous drop in dark resistance ρ (T) that only sample S21 can show in FIG. 5 (c). There are striking similarities between the features of the photocurrent response Q (λ, T) of It is clearly recognized that the temperature decreases as "photoconductivity" begins to appear at around 100K and reaches a maximum value at an absolute temperature of 70-80K, and the slightly flat part continues to an absolute temperature of about 10K. Below that, the photoconductive response Q (λ, T) rapidly decreases in the superconducting sample S21.

Note finally superconductivity of the sample _{Y 2 -Ba 1 -Cu 3 -O z} ( sample S2
The dark resistance ρ (T) of 1) and Y ₁ —Ba ₂ —Cu ₃ —O _z (Sample S20) is shown in FIG. 5 (c) as a function of temperature. What we notice immediately here is that this sample exhibits superconductivity at the critical temperature Tc ≈ 50 to 90K or less. However, the photoconductive response Q (λ, T) was not observed for the sample S20.

However, it is not easy to interpret these experimental facts easily. Samples S22 in 300K in Y ₃ -Cu ₆ -O _z is already insulator, a sample S21 in Y ₂ -Ba ₁ -Cu ₃ -O _z is a semiconductor.
In the sample S21, the photoconductivity observed by disposing the blocking electrode is compatible with the superconductivity as shown in FIG. 5 (b). This is probably due to an insulator-like portion in the sample S21, as can be seen from the value of the magnitude of magnetization. However, surprisingly, it may be closely related to the superconductivity, which is considered to actually exist behind the insulator sample S22 shown in FIG. 5 (a). There is an unfortunate photoconduction phenomenon.

Although the place is (Experimental Discussion) generally prevalent et al, samples of _{Y 3-x -Ba x -Cu 3} -O z as a sample S21 is the ordinary dark green. It is an insulator at temperatures below the critical temperature. Y ₃ -Cu ₆ -O _z samples, such as samples S22, appear blue-green. Fig. 3 (a)
It is strongly suggested that the spectral response Q (λ, T) of the photoconductivity of (c) is at least for the sample of Y ₃ —Cu ₆ —O _z .
And if not at the stage of tentatively atomic layer even there is more possibility _{Y 3-x -Ba x -Cu 3} -O z, is Cu ₂ O region of similar conditions with Cu ⁺¹ in some sense It means that it exists somewhere. Since the response signal of Q (λ, T) is clearly observed, anyone can insulate Y-Cu-O system or Y-Ba system.
In the inside of the —Cu—O-based sample, it is inevitable that either the conduction electron or the hole becomes movable in some cases. [The sign of the optical signal by the Denver effect shows that holes make a larger contribution as photoexcited carriers. ] The photoabsorption and photoconduction phenomena of Cu ₂ O have been thoroughly elucidated by the exciton theory. We can only recognize Y ₃
-Cu is _{that 6} -O _z and Y _3-x -Ba _x -Cu ₃ spectrum Cu ₂ O similar to 2 and 3 of the microstructure of both in -O _z of Q (lambda) is present. Therefore, it makes sense to assume that there is a non-negligible proportion of Cu ₂ O-like phases somewhere in the Y-Cu-O and Y-Ba-Cu-O systems. Even if there is a slight difference in crystal structure, there is a certain state that the conduction electrons and holes that are photoexcited can move reliably. This is a situation that occurs with a very high probability, considering the degree z of oxygen vacancies and, in particular, the recently claimed ordered state thereof. XPS (X-ray photoelectron spectroscopy), EXAFS (Extended X-ray Absorption Fine)
Struct-ures), XANES (X-ray Absorption Near
Edge Structures) experimental result is divalent Cu ²⁺ ion,
The existence of monovalent Cu ^{1+ in} addition to trivalent Cu ³⁺ ions was implicitly shown. The calculation results for the energy band structure and the evaluation of the density of states (Density of States) show a similar tendency.

Conduction electrons and holes in standard type Cu ₂ O crystals are rather weak coupling constants α ≈ 0.14 to 0.18 in LO polaron in polaron formation (still large compared to ordinary III / V semiconductors) As a result, it will create a "big polaron". In fact, cyclotron resonance due to conduction electrons and holes has been observed at 1.2 to 20K. However, on the other hand, in the Cu ₂ O-like part here, the Y ₃ -Cu ₆ -O _z system, Y _3-x
-Ba _x -Cu ₃ -O _z system sample belongs to the series of ferroelectric materials with large static permittivity, and larger effective permittivity κ and coupling constant strengthened due to the ordered state of oxygen defects. It is considered to have α. Originally, when Mueller started his research together with Betonorz, the oxide has a strong electron-phonon interaction, and is likely to be a strong candidate for polaron formation due to the distortion of the crystal lattice caused by the Jahn-Teller effect. This is because the. The dynamic effect in a polaron, whether it is a "large polaron" combined with LO-phonons, a "small polaron" due to the Jahn-Teller effect, or an intermediate between both, Anyway, they must be as essential as "polarons due to polarization of the electronic part". The polarons due to the phonon part and the electronic part are probably formed as coherently hybridized states as elementary excitations.

Therefore, it is reasonable enough to imagine that these polarons, whether they are large or small, have a potential role in the superconducting phenomena here, whether they are large or small. it is conceivable that. These coupled polarons and exciton sets are probably due to the effects of dynamic electron-phonon and electron-electron interactions, both in bipolarons, polaron excitons, and / or exciton polarons. . These polarons and excitons are transferred from the mixed valence state of 2p of oxygen and 3d of copper to the conduction band of (4s) ^{1 in} the conduction band of 4s of copper, leaving one hole which can be represented by (3d) ⁹ later. An electron is created by an optical transition accompanied by LO phonon interaction. Polarons can also be created by the optical transitions here, especially in the Y-Cu-O system by substituting Y for Ba. As shown in Fig. 5 (a), the photoconductive response Q (λ, T) in the Y-Cu-O system reflects the appearance of superconductivity in the Y-Ba-Cu-O system. There is. Similar phenomenon has been observed in the La-Cu-O system. Therefore, the study of these elementary excitations here reflects the nature of the superconducting ground state. To the best of our knowledge, this is the first clear experimental evidence for the polaron and exciton mechanism in high critical temperature superconductivity and perfect diamagnetism.

(Effect of Invention) As a result of the above, we can conclude as follows. In the temperature range of 4.2 to 100K, the direct current four-terminal method and the repetitive pulsed photoconductivity measurement method were applied to the conductivity measurement, and the microwave SQUID was used to measure the static magnetic susceptibility. As a result, it was found that there is at least a significant consensus that could not have been predicted until now, that is, superconducting photoconductive materials that have a correlation between superconductivity, perfect diamagnetism, and photoconductivity are at least Y _{3- x} -Ba _x -Cu ₃ -O ₂ system (0 ≦ x <1, y = 3~
6, z = 6 to 12), and invented the manufacturing method. This invention was developed in parallel with the theoretical consideration of "dynamic mechanism by polarons and excitons" for our "high-temperature superconductivity". These new materials are directly exposed to light. It opens a new cutting-edge science and technology field called "superconducting optoelectronics" that controls superconductivity.

[Brief description of drawings]

FIG. 1 (A) is a principle circuit layout diagram of the repeated pulse photoconductivity measurement method in which blocking electrodes are arranged, FIG. 1 (B) is a time-series waveform diagram of measurement, and FIGS. 2 (A) and (B), (C) is a drawing showing the main part of the measuring device by the microwave SQUID for static magnetic susceptibility measurement, and a block diagram of the measuring system, a block diagram of the feedback system, and FIG. 3 is a superconducting photoconductive material Y _3-x -Ba _x- Cu _y- O _z (a)
x = 0, y = 6, z≈10.5, (b) x = 0, y = 3, z≈7.5,
(C) A characteristic diagram showing the wavelength dependence of the photoconductive response Q (λ, T) where x = 1, y = 3, z≈6.6, and FIG. 4 shows Y _3−x −Ba _x −Cu _y −O _z. Fig. 5 shows the waveform of the signal induced in the signal detection coil of the microwave SQUID by the magnitude of the magnetization of each sample in Fig. 5, and Fig. 5 shows the light of the superconducting photoconductive material Y _3-x -Ba _x -Cu _y -O _z . Temperature dependence of conduction response Q (λ, T) (a) x = 0, y = 3, z≈7.
5, (b) x = 1, y = 3, z≈6.6, (c) x = 1, y = 3, z≈6.
6 is a characteristic diagram showing the temperature dependence ρ (T) of the dark resistance of 6 and x = 2, y = 3, z≈6.9.

Claims (2)

[Claims] [Claim ## wherein the general formula _{Y 3-x -Ba x -Cu y} -O z, consists composition of 0 ≦ x <1,3 ≦ y ≦ 6,6 ≦ z ≦ 12, Y-Ba-Cu A Y-characterized by being an insulator or a semiconductor in a dark place at a temperature substantially corresponding to a transition temperature of an O-based oxide superconducting substance to a superconducting state or less, and exhibiting photoconductivity by light irradiation. Ba-Cu-O-based oxide conductive material. 2. Here the general formula _{Y 3-x -Ba x -Cu y} -O z, 0 ≦ x <1,3 ≦ y ≦ 6,6 ≦ 6 ≦ 12 The starting materials of the composition of the solid phase reaction Heat at a temperature of 650 to 1050 ℃ for 1 to 10 hours, then slowly cool, press-shape, and further 570 to 105
Secondary sintering is performed at 0 ° C., and ultra-quick cooling is performed at a cooling temperature of 2000 to 900 ° C./sec, or slow cooling is performed at a cooling rate of 150 to 200 ° C./H. A Y-characterized as a substance which is an insulator or a semiconductor in a dark place and which exhibits photoconductivity upon irradiation with light at a temperature substantially lower than a transition temperature of an oxide superconducting substance to a superconducting state. Ba-Cu-O
-Based oxide photoconductive material manufacturing method.