JPH085714B2 - La-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 photoconduction phenomenon closely related to superconductivity that is unpredictable in common sense, and the former ultra-quenched one depends on whether it is super-quenched or gradually cooled in its manufacturing method. The semiconductor with photoconductivity, the latter slow-cooled one, is a superconductor that also contains photoconductivity, and it can be expected to be applied to "superconducting optoelectronics" as an industrial application field. It is what is done.

(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 up to the visible region 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 simply by being reflected or scattered, and therefore the optical properties of superconductivity are not reflected or scattered at academic conferences and international conferences both at home and abroad. It is considered to be an almost unrelated field except for.

However, if a material having photoconductivity corresponding to superconductivity can be manufactured, for example, devices such as superconducting phototransistors, and “superconducting computers” and optotypes based on the currently pursued Josephson devices will be developed. It is possible to fabricate a device having the characteristics of an "optical computer" proposed in electronics, that is, a "superconducting optical computer".

(Means for Solving the Problems) An object of the present invention is to provide a photoconductive layer that responds to the occurrence of superconductivity at a temperature equal to or lower than a critical temperature at which a superconducting phenomenon of a La-Cu-O-based oxide superconducting substance occurs. An object is to provide a La-Cu-O-based oxide photoconductive material that produces conductivity.

The La—Cu—O-based oxide photoconductive material according to the present invention has the general formula La ₂ —Cu ₁ —O _z where z = 3.84 to 4.02, and La—Cu—O
It is characterized in that it is an insulator or a semiconductor in the dark at a temperature substantially corresponding to the transition temperature of a superconducting oxide-based material to a superconducting state or less, and that it exhibits photoconductivity by light irradiation.

Furthermore, the manufacturing method of the La-Cu-O based oxide photoconductive substance according to the invention has the general formula La ₂ -Cu ₁ -O _z where, z = the solid phase reaction of the starting material of the composition from 3.84 to 4.02 Is heated at a temperature of 800 to 1050 ℃ for 5 to 10 hours, then slowly cooled, press-molded, secondarily sintered at 600 to 1200 ℃, and then rapidly cooled at a cooling temperature of 2000 to 900 ℃ / sec. , 150 ~
By gradually cooling at a cooling rate of 200 ° C / H, La-Cu-O
It is characterized in that a substance which is an insulator or a semiconductor in the dark and which exhibits photoconductivity by light irradiation is obtained at a temperature substantially lower than a transition temperature of a superconducting oxide-based 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 set to about 800 to 105
After heating at 0 ℃ for 5-10 hours, press-molding, secondary sintering at 600-1200 ℃, and then ultra-quenching at 2000-900 ℃ / sec, or 150
When gradually cooled at ~ 200 ° C / H, as shown in the examples, a material having photoconductivity having temperature dependence and excitation wavelength light dependence such as latent superconductivity even in an insulating material region, This is because a substance that generates photoconductivity corresponding to the transition of a superconducting substance to the superconducting state can be obtained, and the present invention has been completed by systematically clarifying these phenomena.

Conventionally known La-Cu-O and Y-Cu-O or Y-
Most of oxide-based compounds such as Ba-Cu-O are usually insulators or semiconductors in a ground state, that is, in a dark place where light is not radiated at 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 superconductors, it is generally said that elementary excitation over the energy gap destroys the superconducting ground state in BCS theory. However,
Insulating semiconductors may be able to create elementary coherent states such as 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 that research, from a new perspective, that is, from the perspective of elementary excitation concept The present invention has been completed by discovering a substance that has a composition that does not become a conductor and that is photoconductive at a temperature below the superconducting transition temperature in a composition that leads to a superconductor.

One example of such a substance is La ₂ —Cu ₁ —O _z , and this study was conducted to clarify the face diagram, especially the change due to z, that is, the effect of oxygen defects. 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 insulator phase. A large number of La ₂ —Cu ₁ —O _z samples were prepared from La ₂ O ₃ and CuO powders. Although the composition of the starting material was studied in detail, here, the oxygen content z
It has been clarified that can be controlled. Sample P2 is La ₂ O
₃ 1.70 g and CuO 0.469 g were mixed and fired so that the formula of La ₂ Cu ₁ O _z was obtained. Sample S30 was prepared by mixing 3.26 g of La ₂ O ₃ and 0.80 g of CuO and firing it so as to become La ₂ Cu ₁ O _z . Z represents the amount of oxygen, and z changes depending on the firing method. The physical properties of the obtained products are different.

For example, prepare according to the composition ratio of the charged material, stir well, pulverize, and then perform primary calcination at 800 to 1050 ° C for 4 hours to perform solid phase reaction, and after slowly cooling, add pellets using the product. It is made by pressure shaping. Furthermore, these are 1000 ~ 4
Secondary sintering was performed for 5 hours, one sample P2 was ultra-quenched to liquid nitrogen temperature (absolute temperature 77K), and the other sample S30 was 80%.
After annealing at 0 ° C. for 3 hours and 600 ° C. for 2 hours, it is gradually cooled to room temperature over 4 hours.

Experimental method The phase diagram of the La ₂ -Cu ₁ -O _z system begins to pursue some details,
It is still hard to say that it is perfect, but it is still in the process of being investigated. Especially important is the control of oxygen defects. Despite the hard work of many scientists, it will probably take some time to complete. Here we have been concerned not only with the superconducting phase but also with the semiconductor phase. A lot of samples la-CuO _Z lines were prepared from La ₂ O _3, CuO powders. The composition of the charged material has been investigated in detail in the process of slow cooling and rapid cooling and can be controlled to some extent.

The La ₂ -Cu ₁ -O _z system sample has extremely high insulating property at a certain value of z, or is semiconductive at least at low temperature. Two types of technology were adopted. First of all, an insulating sample (ρ10 ⁸ Ω · cm) and sample P2 at an absolute temperature of 4.2K.
It has been found that the high-speed pulse technique (see FIG. 1 (A)) in which a blocking electrode is arranged overcomes some of the difficult measurement problems previously noted. At this time, for example, the electric field pulse E is maintained at a constant value up to E≈3 kV / cm for 10 msec, and this is repeated at a repetition frequency of 13 Hz. The excitation light pulse has a width of 3 nsec and is synchronized with an appropriate time within the application time of the electric field pulse [see FIG. 1 (B)]. Next, for samples with moderate conductivity (ρ10 ^-2 to 10 ¹ Ω · cm), such as sample S30, the resistance measurement was not performed in this case as a matter of course. The terminal method was adopted.

Static magnetic susceptibility or magnitude of magnetization M (T, H) is H ≈ 500 Oe
Microwave SQUID in the 9GHz band in a weak magnetic field up to. The characteristics of this measuring system are as described above [see FIGS. 2 (A), (B) and (C)].

The sample was photoexcited using a pulsed dye laser for the measurement of photoconductivity. The spectral response was also carefully examined with the same caution. The density of the excited optical carriers is on the order of 10 ⁶ -10 ⁸ / cm ³ . All optical signals were usually detected in synchronous mode using a boxcar integrator.

Experimental Results Samples of La ₂ —Cu ₁ —O _z like sample S30 appear black and the resistance is usually in the order of ρ 10 ⁻¹ Ω · cm. Nevertheless, according to the place where we have observed, La ₂ -Cu ₁ -O _z sample S30
Applying the transient pulse technique described above to both (superconductivity) and sample P2 (semiconductor) confirmed that a signal exhibiting photoconductivity appears reliably at an absolute temperature of 30 K or less. Was given.

First of all, the dependence of the photoconductivity Q (λ, T, E, H) on the electric field E is almost linear up to E≈3 kV / cm at T≈4.2K. On the other hand, with respect to the magnetic field H, at a temperature of 4.2 K in absolute temperature, neither a lateral effect nor a longitudinal effect has been observed in the magnetoresistance having a size capable of detecting a magnetic field H≈15 kOe. Figure 3 (b) shows La
_2- Cu ₁ -O _z sample P2, Fig. 3 (c) shows La ₂ -Cu ₁ -O _z sample S30
Photoconductive response Q in the wavelength region of λ ≈ 420 to 640 nm
It is a typical spectrum of (λ, T). By the way, FIG. 3 (a) shows the data of optical absorption of Cu ₂ O reported by Grossman, which is a standard established in this field.

Next, the value of the magnitude M (T, H) of the magnetization of the superconducting La ₂ —Cu ₁ —O _z sample S30 is at least │χ│ <3 at an absolute temperature of 4.2K.
It is observed as very small as × 10 ^-8 . A similar phenomenon was reported by Kang et al. In the La-Cu-O _z system. But they have observed critical currents and fields even in such samples. That is, it suggests superconductivity.

Next, as shown in FIGS. 4 (a) and 4 (b), the temperature dependence of Q (λ, T) in the wavelength region λ≈500 to 570 nm is shown in both the semiconducting sample P2 and the superconducting sample S30. I investigated about. What is surprising here is that despite the tremendous difference in dark resistivity ρ (T) between sample S30 and sample P2, as shown in FIG. It was observed that there are certainly significant similarities between the general features of conductive Q (λ, T). As anyone has to admit clearly, “photoconductive response Q (λ, T)” appears at an absolute temperature of 30 to 40K as the temperature decreases in both semiconducting and superconducting samples, Absolute temperature after increasing to 5
It decreases a little below K.

Finally, superconducting La ₂ -Cu ₁ -O _z samples S30 and semiconducting La ₂
The dark resistivity ρ (T) of both -Cu ₁ -O _z sample P2 is shown as a function of temperature in Figure 4 (c). As anyone will notice immediately, sample S30 becomes superconducting below T≈10-35K, while sample P2 is an insulator.

It is never easy to try to interpret these experimental facts. It can be seen that the heating effect of the sample by the excitation light is sufficiently small when carefully evaluated. Absolute temperature
La ₂ -Cu ₁ -O _z samples S30 in the 300K also sample P2 also are both semiconducting. The "photoconductivity" observed in the arrangement of the blocking electrodes in the superconducting sample S30 is compatible with the "superconductivity". This is probably Fig. 4 (b),
As shown in (c), this is due to the insulating portion in this sample. However, surprisingly, as shown in Fig. 4 (a), even in the semiconducting sample P2, there is a "photoconductivity" that is implicitly correlated as if superconductivity is latent in it. It is the existence of the "appearance of phenomena".

Experimental consideration Although it is widely known, the La ₂ —Cu ₁ —O _z system semiconductor-like samples usually have a deep red or black color. The photoconductive spectral response Q (λ, T) shown in FIGS. 3 (a) and 3 (b) is not layered in the atomic sense inside the La ₂ —Cu ₁ —O _z system sample. Also implies that there is a Cu ₂ O-like region in some sense.

The photoabsorption and photoconductivity of Cu ₂ O itself have been thoroughly elucidated by exciton theory and are well known as typical examples of Mott-Wannier type excitons. The position of the fine structure in Q (λ, T) here is in good agreement with the structure of the basic absorption edge of Cu ₂ O itself. We can recognize some striking fine structures that are probably due to excitons. For example, similar to that of Cu ₂ O, the photoconduction response spectrum of La ₂ −Cu ₁ −O _z λ ≈ 580 nm
A structure that is thought to correspond to the n = 2 state of the yellow excitons of Cu ₂ O is recognized in the vicinity. Therefore, even if we imagine the following, it would be reasonable. La-Cu-O system has Cu ⁺¹ in non-negligible at least finite ratio inside the material
A phase similar to Cu ₂ O exists. And although there are some differences in their crystal structures, the conduction electrons and holes excited by light are certainly in a state of moving around.

It is reported that conduction electrons and holes in a standard type Cu ₂ O crystal form a rather “large polaron” with a coupling constant α ≈ 0.14 to 0.18, as discussed previously. The sign of the optical response by the Denver effect is considered to be conduction electrons in the La-Cu-O system, which has a long lifetime and makes a large contribution to the carriers made of excitation light. However, in both cases, the appearance of “photoconductivity Q (λ, T)” is clearly related to the “appearance of superconductivity” in the insulating material, and even if superconductivity is photoconductivity. It seems to be latent in the back of the phenomenon of sexuality. So the effect of polaron is that it is LO
"Large polaron" based on interaction with phonons
Whether it is a “small polaron” due to the Jahn-Teller effect, or in the region of an intermediate bond based on both, as shown in FIGS. 3 and 4 (a) to (c). It is at least potentially important, as is the "polaron effect due to electronic polarization". They are probably effective as elementary excitations in coherently hybrid form. Here we need to pay special attention to polarons due to electronic polarization, which is also called "exciton polaron".
Looking at the experimental results here, we must admit the close relationship between polarons and excitons.

These polarons and excitons are all oxygen (2p)
From the hybrid valence state of (3d) of copper and copper, leaving holes in the configuration of (3d) ⁹ and interacting with LO phonons, the optical band-to-band transition to the (4s) conduction band of copper occurs. By (4s) ¹
The conduction electrons of are created. But La-Cu
The -O type polaron can be created even by optical excitation by substituting La or Ba for La. 2p (O) and 3d
Since the holes in the (Cu) hybrid band can be created from the ground state of many-body systems by either intra-zone or inter-zone transitions, the correlation effect between electrons is of course extremely important.
We also have to pay more attention to the dynamic valence fluctuations between Cu ¹⁺ and Cu ²⁺ as well as the dynamic valence fluctuations between Cu ²⁺ and Cu ³⁺ . Therefore, for the superconducting mechanism with high critical temperature, there is sufficient reason to consider the potential role of polaron assemblages, especially those closely related to excitons, regardless of size. These bound polaron and exciton sets here are probably based on bipolarons, polaron exciton sets, and / or most likely dynamic electron-phonon interactions as well as dynamic electron correlations. It seems to be an "exciton-mediated bipolaron". As shown in Fig. 4 (a)
The photoconductive response Q (λ, T) in the La-Cu-O system is La-Ba-Cu-
It reflects the appearance of superconductivity in the O, La-Sr-Cu-O system.
Therefore, we can believe that the elementary excitation study here reveals the nature of the superconducting ground state, despite the enormous carrier density differences. To the best of our knowledge, these are the first clear experimental evidences for polaron and exciton mechanisms to appear in superconductivity with high critical temperatures and few diamagnetisms.

Thus, here we find the expected coincidence of appearance temperatures, i.e. at least superconducting La-C up to an absolute temperature of 4.2-100 K.
In the u-O system and the semiconducting La-Cu-O system, there was almost no diamagnetism (for some reason), but we first discovered a deep correlation between "superconductivity (zero resistance)" and "photoconductivity". Therefore, we reconfirmed the existence of the dynamical mechanism by polarons and excitons, that is, by "exciton-mediated bipolaron" in high temperature superconductivity.

(Effect of Invention) As a result of the above, we can conclude as follows. In the temperature range of absolute temperature 4.2K ~ 100K, the DC four-terminal method and repeated pulse photoconductivity measurement method are applied for conductivity measurement.
As a result of extensively examining the static magnetic susceptibility using microwave SQUID for the first time, diamagnetism is almost nonexistent, but there is a correlation between "superconductivity (zero resistance)" and "photoconductivity". The inventor of the conductive material "La ₂ -Cu ₁ -O _z system (z = 3.84 to 4.02) and the manufacturing method thereof were also invented. This invention was developed in parallel with the theoretical consideration that we have proposed for "high-temperature superconductivity", that is, "dynamic mechanism by polarons and excitons", that is, by "exciton-mediated bipolaron". These new materials will open up a new cutting-edge science and technology field called "superconducting optoelectronics" that directly controls superconductivity with light.

[Brief description of drawings]

Fig. 1 shows the principle of the repeated pulsed photoconductivity measurement method with blocking electrodes, Fig. 2 shows the microwave SQUID device for static magnetic susceptibility measurement, and Fig. 3 shows the superconducting photoconductive material La ₂ -Cu. Characteristic diagram showing wavelength dependence of optical absorption K (λ) and photoconductive response Q (λ, T) in the case of _1- O _z (a), (b), and (c) below, (a) According to GROSMANN Experimental results of light absorption of Cu ₂ O, (b) Photoconductive response Q of semiconductor material P2 (z ≒ 3.88)
A characteristic diagram showing wavelength dependence of (λ, T), (c) Photoconductivity response Q of superconducting sample S30 (z ≈ 3.92)
Fig. 4 is a characteristic diagram showing the wavelength dependence of (λ, T). Fig. 4 shows the following (a) semiconducting sample P2 (z ≈ 3.88) (b) conduction of the superconducting photoconductive material La ₂ -Cu ₁ -O _z. (C) is a characteristic diagram showing the temperature dependence of photoconductive response Q (λ, T) and dark resistance R (T) in the case of sample P2 and sample S30.

Claims (2)

[Claims] 1. A general formula La ₂ —Cu ₁ —O _z where z = 3.84 to 4.02, and corresponds substantially to a transition temperature of a La—Cu—O based oxide superconducting material to a superconducting state. La-Cu-O-based oxide photoconductive substance, which is an insulator or a semiconductor in the dark at a temperature equal to or lower than the above temperature, and exhibits photoconductivity by light irradiation. 2. A starting material having a composition of the general formula La ₂ —Cu ₁ —O _z, where z = 3.84 to 4.02, is heated at a temperature at which the solid-state reaction occurs, at 800 to 1050 ° C. for 5 to 10 hours and then slowly cooled. Then, after pressure shaping and secondary sintering at 600-1200 ℃, 200
Ultra-quick cooling at 0-900 ℃ / sec or 150-200 ℃ / H
By gradually cooling at a cooling rate of 1, the temperature is substantially equal to or lower than the transition temperature of the La-Cu-O-based oxide superconducting substance to the superconducting state, and it is an insulator or a semiconductor in the dark and is irradiated with light. A method for producing a La-Cu-O-based oxide photoconductive substance, which comprises obtaining a substance which exhibits photoconductivity.