Superconductive photoconductive-substance of the Y-Ba-Cu-O system and a method for producing the same

The disclosed substance has a general chemical formula of Y.sub.3-x -Ba.sub.x -Cu.sub.y -O.sub.z, x being 0 to 2, y being 3 to 6, and z being 6 to 12 (but excluding compositions with x=2, y=3 and z=6.75-6.97). At a temperature below 90-95 K, the disclosed substance shows superconductive photoconductivity or even both superconductivity, either real or potential, and photoconductivity in response to incident exciting light in a wavelength range of 420 to 640 nm. The substance is produced by heating a mixture of starting material therefor at 750.degree.-1,050.degree. C. for 1-10 hours so as to cause solid phase reaction, cooling gradually, shaping under pressure, sintering at 670.degree.-1,050.degree. C., and cooling either quickly at a rate of 2,000.degree.-900.degree. C./sec or slowly at a rate of 150.degree.-200.degree. C./hour.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to superconductive photoconductive-substance of the 
Y-Ba-Cu-O system defined to be photoconductive substance correlative with 
superconductivity whose composition is outside but includes even areas 
continuously close to that of regular oxide superconductors. Experiments 
on the optical properties, especially on the photoconduction in response 
to high-speed pulses, e.g. a pulsed light out of a dye laser, of substance 
with a chemical formula of Y.sub.3-x -Ba.sub.x -Cu.sub.y -O.sub.z revealed 
an unexpected correlation between the superconductivity and the 
photoconductivity of the substance. 
The invention also relates to a method for producing substance with a 
chemical formula of Y.sub.3-x -Ba.sub.x -Cu.sub.y -O.sub.z, in which 
method either x and y of the chemical formula are controlled for instance 
by varying x from 0 to 2 while keeping y=3, or z of the chemical formula 
is controlled by quick cooling or slow cooling while keeping x and y 
constant such as x=2 and y=3. When the substance has a low value of x near 
x=0 or is cooled quickly, the substance becomes a photoconductive 
insulator or semiconductor. On the other hand, when the substance has a 
value of x near x=1 and is cooled slowly, the substance becomes a 
photoconductive superconductor. 
The substance in the invention is expected to be useful in developing new 
industrial field of "Superconductive Opto-Electronics". 
2. Related Art Statement 
There has been no publications on such a system of substance which has 
superconductive photoconductivity or both superconductivity and inherent 
photoconductivity. 
Conventional superconductors are metals or alloys in the main. Recently, 
much attention has been paid to high-temperature oxide superconductors, 
such as superconductors of the Y-Ba-Cu-O group, and considerable amounts 
of additives such as barium (Ba) and strontium (Sr) are used to raise the 
superconductive critical temperature (T.sub.c). Studies and measurements 
on the optical properties of the superconductors at and in the proximity 
of visible wavelengths have been limited to the study of reflection and 
scattering of light therefrom due to a part of the metallic properties of 
such substance. 
It has been believed that light is simply reflected from or scattered by 
the surface of a superconductor and is not allowed to enter therein. Study 
of optical properties, except the phenomena of reflection and scattering, 
has been treated as a completely different field from that of 
superconductivity in academic institutions, domestic and abroad, and in 
international conferences. 
On the other hand, if any substance having superconductive photoconductive 
capability or even both superconductive capability and photoconductive 
capability at the boundary compositions is produced, a number of new 
electronic and optoelectronic devices may be developed; for instance, a 
superconductive phototransistor, a "superconductive optical computer" with 
a combined characteristics of the "superconductive computer" based on the 
currently studied Josephson devices and the "optical computer" proposed in 
optoelectronics, "superconductive optical fiber", and the like. 
SUMMARY OF THE INVENTION 
Therefore, an object of the present invention is to provide superconductive 
photoconductive-substance which reveals superconductive photoconductivity 
or both superconductivity and photoconductivity at a temperature below its 
critical temperature for superconductivity. 
The superconductive photoconductive-substance according to the invention 
has a general chemical formula of Y.sub.3-x -Ba.sub.x -Cu.sub.y -O.sub.z, 
x being 0 to 2, y being 3 to 6, and z being 6 to 12 (but excluding 
compositions with x=2, y=3 and z=6.75-6.97). The substance of the 
invention shows superconductive photoconductivity or even both 
superconductivity and photoconductivity at a temperature below 90-95K, the 
photoconductivity being valid in a wavelength range of 420 to 640 nm. The 
superconductivity of the substance of the invention may be either 
potential or real. 
Another object of the present invention is to provide a method for 
producing the above-mentioned superconductive photoconductive-substance 
which reveals superconductive photoconductivity or even both 
superconductivity and photoconductivity at a temperature below its 
critical temperature for superconductivity. 
With a method according to the invention for producing the superconductive 
photoconductive-substance with a general chemical formula of Y.sub.3-x 
-Ba.sub.x -Cu.sub.y -O.sub.z, x being 0 to 2, y being 3 to 6, and z being 
6 to 12 (but excluding compositions with x=2, y=3 and z=6.75-6.97) a 
mixture of starting materials for a composition of the above-mentioned 
chemical formula is heated at 750.degree.-1,050.degree. C. for 1-10 hours 
so as to cause solid phase reaction in the mixture. The heated mixture is 
cooled gradually after that the cooled mixture is shaped under pressure. 
The shaped mixture is sintered at 670.degree.-1,050.degree. C., and then 
cooled either quickly at a rate of 2,000.degree.-900.degree. C./sec or 
slowly at a rate of 150.degree.-200.degree. C./hour. The substance thus 
produced reveals superconductive photoconductivity or both 
superconductivity and photoconductivity. 
The reason for limiting the composition of the substance of the invention 
to the above-mentioned general chemical formula is in that the substance 
of such composition reveals superconductive photoconductivity or even both 
superconductivity and photoconductivity provided that it is treated by the 
method of the invention; namely, the method comprising steps of heating at 
750.degree.-1,050.degree. C. for 1-10 hours so as to cause solid phase 
reaction in a starting material mixture, cooling gradually the heated 
mixture, shaping the cooled mixture under pressure, sintering the shaped 
mixture at 670.degree.-1,050.degree. C., and cooling the sintered 
substance either quickly at a rate of 2,000.degree.-900.degree. C./sec or 
slowly at a rate of 150.degree.-200.degree. C./hour. 
The inventors have found that, in the substance with a general chemical 
formula of Y.sub.3-x -Ba.sub.x -Cu.sub.y -O.sub.z, if x is zero (x=0) or 
close to zero, the substance becomes insulating and shows semiconductive 
properties as well as photoconductive properties for certain wavelengths 
of light. On the other hand, if x departs from 0 and increases toward 2, 
the superconductive properties of the substance becomes more apparent 
while the photoconductive properties of the substance is maintained. It is 
noted that when the composition of a substance is such that x=2, y=3 and z 
is 6.75 to 6.97 in the chemical formula of Y.sub.3-x -Ba.sub.x -Cu.sub.y 
-O.sub.z, the substance with such composition shows only superconductive 
properties, so that such composition is excluded from the scope of the 
composition of the invention. 
Of the substance according to the invention, substance of insulating type 
has for instance x=0, y=3-6 and z of 7.5 to 12 in the above chemical 
formula, while substance of superconductive type has for instance x=2, y=3 
and z of 6.5 to 6.97 in the above chemical formula. It is an important 
finding of the inventor that even the above substance of insulating type 
has such photoconductivity which depends both on temperature and 
wavelength of exciting light, and the manner of its dependence on 
temperature and wavelength suggests a potential correlation with 
superconductivity in the insulating type substance. The present invention 
is based on such finding.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Most of conventional oxide compounds such as Y.sub.3 -Cu.sub.6 -O.sub.z (z 
being 6 to 12) and Y.sub.3-x -Ba.sub.x -Cu.sub.3 -O.sub.z-y are normally 
semiconductors at the ground state, e.g., at low temperatures and in the 
dark. An elementary excitation can be created by giving the many-body 
ground state an appreciable amount of energy with relevant magnitude of 
momentum. Usually, for superconductors, these excitations beyond the 
energy gap destroy the superconductive ground state in the BCS theory. 
There is a possibility, however, to create a coherent state of elementary 
excitations above the ground state of insulating semiconductors such as 
bipolarons and excitons even in a thermally non equilibrium state. We have 
found a new substance as an outcome of studies in fundamental physics and 
applied physics from the standpoint of the elementary excitation concept, 
in a sense parallel to, but rather orthogonal to the trend of studies of 
high-T.sub.c (critical temperature) superconductors. Namely, our finding 
relates to substance whose composition does not result in perfect 
superconductor, but the substance has a composition close to that of 
superconductor and reveals superconductive photoconductivity or even both 
superconductivity and photoconductivity. The present invention has been 
completed based on that finding. 
The invention will be described in further detail by referring to 
embodiments. 
EMBODIMENTS 
The composition of the substance found by the inventors can be expressed by 
a chemical formula of Y.sub.3-x -Ba.sub.x -Cu.sub.y -O.sub.z. The inventor 
has tried to seek into details of the complete scheme of a phase diagram 
of Y.sub.3-x -Ba.sub.x -Cu.sub.y O.sub.z. 
A large number of specimens of Y-Cu-O and Y-Ba-Cu-O systems were made from 
the powders of Y.sub.2 O.sub.3, BaCO.sub.3 and CuO by using the method 
already described in numerous references. For instance, primary sintering 
of a mixture of starting materials for a composition of said chemical 
formula was effected at 750.degree.-1,050.degree. C. for 1-10 hours so as 
to cause solid phase reaction in the mixture. After being cooled 
gradually, the sintered mixture was shaped under pressure, and secondary 
sintering was effected at 670.degree.-1,050.degree. C. The specimens were 
then annealed at 600.degree.-700.degree. C. and cooled slowly. The 
composition of the starting material was studied in detail, and the 
composition of the specimen was selected to be close to regular 
superconductors in such a manner that it became more or less controllable 
in a range outside that of the latter. Besides, the value of z in the 
above chemical formula was controlled by carefully regulating the method 
and speed of cooling. 
Specimens No. S20, No. S21 and No. S22 were prepared in the above-mentioned 
manner. 
Specimen No. S61 with a composition of Y.sub.3 -Cu.sub.3 -O.sub.7.5 was 
prepared by primary sintering of a starting mixture of the above 
composition at 1,000.degree.-1,050.degree. C. for 2 hours, slow cooling, 
shaping under pressure, secondary sintering at 1,000.degree.-1,050.degree. 
C. for 2 hours, cooling, annealing at 700.degree. C. for 2 hours, and slow 
cooling. 
Since specimens of the Y.sub.3 -Cu.sub.3 -O.sub.z, Y.sub.3 -Cu.sub.6 
-O.sub.z and Y.sub.3-x -Ba.sub.x -Cu.sub.y -O.sub.z systems within a 
certain part of the values of x, y and z became highly insulating or at 
least semiconducting at low temperatures, the conventional techniques for 
resistivity measurements encountered several serious difficulties, 
especially such as those associated with the non-ohmic contact electrodes, 
the build-up of space charge and with the low signal-to-noise ratio due to 
low carrier density in high-impedance materials. 
In the experiments of the inventors, two types of techniques were adopted 
for resistivity or/and conductivity measurements. First, for insulating 
specimens (.rho..gtoreq.10.sup.8 .OMEGA..multidot.cm) such as No. S22, a 
fast pulse technique with blocking electrodes as shown in FIG. 1A was 
adopted. Referring to FIG. 1B, the difficulties noted above were overcome 
by using a pulsed dye laser of 3 ns width at a repetition rate of 13 Hz in 
the dark synchronized with pulse electric fields up to E.perspectiveto.5 
kV/cm of 10 ms duration for measuring photoresponse, as shown in FIG. 1B. 
Second, for moderately conducting specimens (.rho..gtoreq.(10.sup.-2 
-10.sup.-1) .OMEGA..multidot.cm) at 300K such as SpeCimens No. S20 and No. 
S21, resistivity measurements were performed by adopting the usual 
four-probe method in the dark without using any light. 
The specimen No. S20 was made by mixing 0.40 g of Y.sub.2 O.sub.3, 1.40 g 
of BaCO.sub.3 and 0.85 g of CuO, and sintering the mixture. The specimen 
No. S61 was made by mixing 2.26 g of Y.sub.2 O.sub.3 and 1.59 g of CuO, 
and sintering the mixture so as to produce Y.sub.3 -Cu.sub.3 -O.sub.z. The 
specimen No. S21 was made by mixing 0.40 g of Y.sub.2 O.sub.3, 0.35 g of 
BaCO.sub.3 and 0.425 g of CuO, and sintering the mixture so as to produce 
Y.sub.2 -Ba.sub.1 -Cu.sub.3 O.sub.z. The specimen No. S22 was made by 
mixing 0.72 g of Y.sub.2 O.sub.3 and 1.00 g of CuO, and sintering the 
mixture so as to produce Y.sub.3 -Cu.sub.6 -O.sub.z. Here, z represents 
the amount of oxygen in the substance, and it varies depending on the 
sintering conditions so as to produce a variety of final products. 
Static magnetic susceptibility or magnetization itself was measured in weak 
fields up to H.perspectiveto.50 Oe by using a microwave SQUID 
(Superconducting Quantum Interference Device) at 9 GHz as shown in FIG. 2A 
and FIG. 2B. The system was normally operated in the mode locked to the 
Q-pattern as shown in FIG. 2C. The bandwidth of the system covered from 
d.c. to 200 kHz, and the slew rate was set at about 10.sup.4 .phi..sub.0 
/sec. 
Spectral responses were studied by selecting the wavelength .lambda. from a 
dye laser with a resolution .DELTA..lambda.-1 nm while using an 
appropriate normalization procedure for incident power from the laser and 
spectral sensitivity of the pyroelectric detector. A possibility of 
heating of the specimens by light excitation was minimized and estimated 
to be negligible. Photocarrier density was of the order of (10.sup.6 to 
10.sup.8)/cm.sup.3 averaged over a specimen. All photosignals were 
normally detected in the synchronized mode by using the Boxcar integrator. 
EXPERIMENTAL RESULTS 
The inventors observed definite signals of photoconductivity of specimen 
No. S22 Y.sub.3 -Cu.sub.6 -O.sub.z and all specimens Y.sub.3-x -Ba.sub.x 
-Cu.sub.3 -O.sub.z except No. S20 (with x=2) by using the transient pulse 
technique described above. 
Firstly, the dependence of photoconductivity Q(.lambda.,T,E,H) on E was 
found to be almost linear up to E.perspectiveto.5 kV/cm at 77K. No 
appreciable magnitude of the transverse and longitudinal 
magneto-resistance in Q(.lambda.,T,E,H) has been observed up to 
H.perspectiveto.15 kOe at 77K. FIG. 3A illustrates a typical spectrum of 
pulse-excited transient photoresponse Q(.lambda.,T) of specimen No. S22 
Y.sub.3 -Cu.sub.6 -O.sub.z (z being about 10.5) over wavelengths 460 to 
640 nm, FIG. 3B illustrates a similar spectrum of specimen No. S61 Y.sub.3 
-Cu.sub.3 -O.sub.z (z being about 7.5), and FIG. 3C illustrates a similar 
spectrum of specimen No. S21 Y.sub.2 -Ba.sub.1 -Cu.sub.3 -O.sub.z (z being 
about 6.6). The inset of FIG. 3A illustrates well established reference 
data of photocurrent in Cu.sub.2 O reported by Gross. 
Secondly, it was confirmed that the magnetizations M(T,H) of specimen No. 
S22 Y.sub.3 -Cu.sub.6 -O.sub.z (z being about 10.5) and specimen No. S61 
Y.sub.3 -Cu.sub.3 -O.sub.z (z being about 7.5) were clearly paramagnetic 
or at least positive at 4.2K and H=48 Oe, as shown in FIG. 4. Most 
remarkably, the values of magnetizations M(T,H) of specimen No. S20 
Y.sub.1 -Ba.sub.2 -Cu.sub.3 -O.sub.z and particularly even specimen No. 
S21 Y.sub.2 -Ba.sub.1 -Cu.sub.3 -O.sub.z (z being about 6.6) at 4.2K 
definitely indicated negative diamagnetic signals which were 
characteristic to superconducting specimens. The magnitude of 
magnetization of specimen No. S21 was about one thirtieth (1/30) of that 
of specimen No. S20. 
Thirdly, temperature dependence of the pulse-excited transient 
photoresponse Q(.lambda.,T) in the region between 
.lambda..perspectiveto.420-640 nm, i.e., the visible wavelength region, 
were studied both for an insulating specimen No. S61 and for a 
superconducting specimen No. S21 as illustrated in FIG. 5A and FIG. 5B, 
respectively. Surprisingly, there definitely exists a remarkable 
similarity between general features of the transient photoresponse 
Q(.lambda.,T) for No. S61 and No. S21, regardless of the huge difference 
in dark resistivity P(T) as illustrated only for No. S21 in FIG. 5C. Dark 
resistivity .rho.(T) of No. S61 is too large to be shown. As the 
temperature decreases, it is clearly recognized that "photoconductivity" 
starts to reveal in the proximity of 100K, assumes maximum value at 70-80K 
and maintains a comparatively flat level until about 10K, and then the 
transient photoresponse Q(.lambda.,T) rapidly decreases for 
superconducting specimen No. 21, whereas it still increases for insulating 
specimen No. 61. 
Finally, the resistivity .rho.(T) in the dark of the Superconducting 
specimen No. S21 Y.sub.2 -Ba.sub.1 -Cu.sub.3 -O.sub.z and that of specimen 
No. S20 Y.sub.1 -Ba.sub.2 -Cu.sub.3 -O.sub.z are displayed in FIG. 5C as 
functions of temperature T(K). One immediately notices that both of those 
specimens become superconductive below T.sub.c =50-90K. No photoconductive 
signals of photoresponse Q(.lambda.,T) have been observed for the specimen 
No. S20. 
It is by no means easy to interpret these facts. At 300K, the specimen No. 
S22 Y.sub.3 -Cu.sub.6 -O.sub.z is clearly an insulator and the specimen 
No. S21 Y.sub.2 -Ba.sub.1 -Cu.sub.3 -O.sub.z is semiconductive. In the 
specimen No. S21, the photoconductivity observed with the blocking 
electrodes is compatible with superconductivity as illustrated in FIG. 5B 
probably due to the insulating part of specimen No. S21 as also noted via 
the value of magnetization. Surprisingly, there exists an occurrence of 
photoconductivity potentially correlative with superconductivity 
underlying even in insulating specimen No. S61 as displayed in FIG. 5A. 
DISCUSSION 
It is a widely recognized fact that the specimens Y.sub.3-x -Ba.sub.x 
-Cu.sub.3 -O.sub.z such as No. S21 usually have dark green colors. A 
specimen of Y.sub.3 -Cu.sub.6 -O.sub.z such as No. S22 looks blue green. 
The spectral response of photoconductivity Q(.lambda.,T) in FIGS. 3A, 3B 
and 3C strongly suggests that there exists a region of the Cu.sub.2 O-like 
state at least in the specimen of Y.sub.3 -Cu.sub.6 -O.sub.z and possibly 
even in those of Y.sub.3-x -Ba.sub.x -Cu.sub.3 -O.sub.z, if not atomic 
layers. As the signals Q(.lambda.,T) are clearly observed, one has to 
recognize that either conduction electrons or positive holes or even both 
are mobile in insulating Y-Cu-O and Y-Ba Cu-O specimens. The sign of the 
photoresponse in the Dember effect indicates that the dominant 
photocarriers are positive holes. 
Optical absorption and photoconductivity of Cu.sub.2 O have been thoroughly 
analyzed in terms of the exciton theory. We can recognize a few fine 
structures due to the excitons in the spectra of Q(.lambda.) both of 
Y.sub.3 -Cu.sub.6 -O.sub.z and of Y.sub.3-x -Ba.sub.x -Cu.sub.3 -O.sub.z 
similar to those of Cu.sub.2 O. Thus, we can reasonably conceive that 
there exists at least a finite fraction of the Cu.sub.2 O-like phase which 
cannot be ignored in the Y Cu-O and Y-Ba-Cu-O systems where the 
photoexcited electrons and holes are definitely mobile, irrespective of a 
certain difference of the crystal structures. Data obtained by experiments 
of XPS (X-ray Photoelectronic Spectrography), EXAFS (Extended X-ray 
Absorption Fine Structures) and XANES (X-ray Absorption Near Edge 
Structures) revealed the existence of monovalent Cu.sup.1+ ions besides 
divalent Cu.sup.2+ ions, and trivalent Cu.sup.3+ ions. Results of the 
energy band calculation and an evaluation of the density of states (DOS) 
also indicate similar tendencies. 
A conduction electron or a positive hole in standard types of Cu.sub.2 O 
crystals has been reported to have a rather weak coupling constant .alpha. 
with the LO-phonons (.alpha.=0.14-0.18, being rather large as compared 
with that of regular III, IV group semiconductors) in the polaron 
formation, which results in a "large polaron". Actually, the cyclotron 
resonances of conduction electrons and positive holes have been observed 
at 1.2-20K. On the other hand, however, it must have a much larger 
effective dielectric constant .kappa. and a coupling constant .alpha. in 
the Cu.sub.2 O-like part, possibly enhanced due to an order of oxygen 
vacancy in the Y.sub.3 -Cu.sub.6 -O.sub.z and Y.sub.3-x -Ba.sub.x 
-Cu.sub.3 -O.sub.z specimens as a series of ferroelectric materials with a 
large static dielectric constant. Originally, Muller with Bednorz started 
to study oxides which are likely to have strong electron-phonon 
interaction and are good candidates for polaron formation due to lattice 
deformation caused by the Jahn-Teller effect. The dynamical effects of the 
polaron, whether it is a "large polaron" with the LO-phonons, a "small 
polaron" due to the Jahn-Teller effect or possibly an intermediate one due 
to both effects, they must be substantial as well as the "electronic 
polaron effect". They, i.e., those polarons due to phonons and due to 
electrons, are probably effective in a coherently hybridized form of these 
elementary excitations. 
Therefore, we may reasonably conceive of potential roles of an ensemble of 
polarons, whether large or small, and excitons in the phenomena of 
superconductivity here. The ensemble of united polarons and excitons are 
probably a set of bipolarons, polaronic excitons and/or excitonic polarons 
due to the dynamical electron-phonon and electron correlation effects. 
These polarons and excitons had yielded out of the optical transition from 
the hybridized 2p-Oxygen and 3d-Cu valence bands leaving (3d).sup.9 
positive holes to the 4s-Cu conduction band creating a (4s).sup.1 
conduction electron together with the LO-phonon interaction. A polaron can 
be created either by the optical excitation here or substitution of Y by 
Ba especially in the Y-Cu-O system. As indicated in FIG. 5A, photosignals 
or photoresponse Q(.lambda.,T) in the Y-Cu-O system reflect the occurrence 
of superconductivity in the Y-Ba-Cu-O system. Similar phenomena have been 
observed also in the La-Cu-O systems. Consequently, the inventors believe 
that these studies of elementary excitations here must reveal the nature 
of the superconducting ground state. To the best knowledge of the 
inventors, this is the first clear experimental indications of the polaron 
and exciton mechanism, i.e., "the exiton mediated bipolaron mechanism", 
displayed in the high-T.sub.c superconductivity and the perfect 
diamagnetism. 
EFFECT OF THE INVENTION 
As described in detail in the foregoing, the inventors have observed, for 
the first time, an unexpected accordance of the onsets of profound 
significance, i.e., "correlation of photoconductivity with 
superconductivity and perfect diamagnetism", at least in substance with a 
chemical formula of Y.sub.3-x -Ba.sub.x -Cu.sub.y -O.sub.z, x being 0 to 
2, y being 3 to 6, and z being 6 to 12 (but excluding superconductive 
compositions with x=2, y=3 and z=6.75-6.97). A method for producing the 
above substance has been also developed. 
It should be noted that the invention is an outcome of theoretical and 
experimental studies on the "dynamical mechanism of polarons and excitons" 
for "high-temperature superconductivity" as proposed by the inventors. The 
proposed substance of the invention will open a new scientific and 
industrial field, to be named as "Superconductive Opto-Electronics", 
wherein superconductivity is directly controlled by light. 
Although the invention has been described with a certain degree of 
particularity, it must be understood that the present disclosure has been 
made only by way of example and that further numerous changes in details 
may be resorted to without departing from the scope of the invention as 
hereinafter claimed.