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Effect of oxygen vacancy on magnetic and superconducting properties in La1.82Sr0.18CuO4
PHYSICA
Physica C 199 (1992) 143-148 North-Holland
Effect of oxygen vacancy on magnetic and superconducting
properties in La1.82Sro.18CUO4 R. Yoshizaki a, N. Kuroda b, S. Nakamura b and N. Ishikawa b,1 a Institute of Applied Physics and Cryogenics Center, University ofTsukuba, Tsukuba, lbaraki 305, Japan b Institute of Applied Physics, University ofTsukuba, Tsukuba, Ibaraki 305, Japan Received 14 May 1992
The role of oxygen vacancies in the CuO2 plane was investigated in Lal.g2Sro.18CuO4 by measuring the normal-state magnetic susceptibility. We found that the oxygen vacancies hardly affected the two-dimensional spin correlation when the reduction of the hole density was taken into account. In contrast, the superconductivity broke down much faster than the reduction rate of holes.
1. Introduction All of the high temperature superconductors in the copper oxides so far discovered have the CuO2 planes in their crystal structures [ 1,2 ] and the carriers in the CuO2 planes are believed to sustain the superconductivity [3 ]. As another characteristic of the CuO2 plane, Cu E÷ ions form a two-dimensional (2D) antiferromagnetic (AF) quantum spin system [ 4 ]. Introduction of a few % holes in the plane leads the three-dimensional AF spin correlation to abrupt destruction [5,6 ] and 2D spin correlation to gradual decay with further increase of hole density [ 7-10 ]. Those behaviors have been well studied in the LaE_xSrxCuO4 (LSCO) system because of the simplicity of its crystal structure. From the point of the correlation between the CuO2 planes and the superconductivity, the substitution effect has been examined by replacing the Cu atom with 3d transition elements and sp elements. The result is quick destruction of the superconductivity with the doping of only 2-5% [ 11-13]. As for the behaviour of the Cu spins, recently it has been revealed from the DC magnetic-susceptibility measurement [ 14 ] that there are two aspects concerning the effects in the normalstate magnetic properties by the substitution for Cu. Present address: Department of Physics, Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki 319-11, Japan.
One is the decrease or increase of the 2D spin correlation energy depending on the occupancy of the 3d levels of the dopant. The other is the presence of the induced localized spin moment at low temperature, which is explained by the picture of the rigid spin order system [ 14 ]. As for the role of the oxygen atom in the CuO2 plane, on the other hand, pioneering work on the correlation between the oxygen deficiencies and the magnetic susceptibility in the normal state was done by Johnston et al. [ 15] and Takagi et al. [ 7 ]. They have revealed the role of oxygen deficiencies as reducing hole densities effectively. More extensive studies were carried out and recently reported by Muromachi and Rice [ 16 ]. They insisted that the destruction of the superconductivity was mainly due to the effect of the induced randomness in the CuO2 planes. In the present paper, the effect of the oxygen deficiency on the magnetic properties of the normal state has been investigated by means of the magnetic-susceptibility measurement. The result will be compared with those observed in the substitution experiments for Cu.
2. Experimental In the present experiment, the Sr concentration was fixed at x = 0.18 in order to yield the measurement of the spin correlation peak in the normal-state mag-
0921-4534/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
144
R. Yoshizaki et al. /Effect of oxygen vacancy in Lal s2Sro lsCu04
netic susceptibility which was observed at about 260 K for the sample without oxygen deficiencies [ 9,10 ]. The compounds, La~.82Sro.~8CuO4, were prepared by using a hot-press method [17]: the appropriate amounts of La203, SrCO3 and CuO were well mixed and calcined at 970°C for 12 h in air. The calcined powder was ground, pressed into a pellet and sintered at 1070°C for 10 h in flowing oxygen. Then the products were ground, pressed into a pellet again and sintered at 1070°C for 2.5 h under a compression of about 300 kgf/cm 2. The samples were annealed at 820 °C in flowing oxygen and subsequently at 650 °C for 40 h. Oxygen-reduced samples were prepared by annealing the oxidized samples at 800 ° C-950 °C for 10-24 h in flowing nitrogen atmosphere [13 ]. As a result we obtained samples with the chemical formula of La L82Sro.18CUO4_a whose oxygen content was varied as 5=0, 0.006, 0.013, 0.028, 0.033 and 0.043. Any trace of the secondary phases was observed in the powder X-ray diffraction spectra even after the annealing process for the oxygen reduction. The oxygen content, 4 - & was determined by iodometric titration with a resolution of 0.007, and the hole concentration per Cu, p, was calculated from the charge neutrality condition to be p = 0 . 1 8 - 25 by assuming La 3+, Sr 2+ and 0 2 - . The electrical-resistivity measurements were carried out using a standard fourprobe method. The temperature dependence of the susceptibility in the normal state was observed under a field of 1 T using a SQUID magnetometer. As reference materials we observed the magnetic and transport properties of LSCO and La1.82Sro.18Cul - y Z n y O 4 .
3. Results and discussion The temperature dependence of resistivity for the sample with varying 6 is shown in fig. I. With increasing oxygen deficiency, the metallic behavior of the resistivity versus temperature profile changed to a semiconducting one. However, the change was not scaled only by the reduction of the hole density due to the introduction of oxygen deficiencies; e.g. the resistivity is fully semiconducting below room temperature for the sample with 5=0.028 in fig. l, while the temperature dependence of the resistivity is still metallic for the sample, L a l . 9 S r o . l C u O 4 , which has
8.0 E
La~ ~2Srox~CuO4
O
c; 6.0
o 8=0.0 ° 0.00(~ + 0.013
uOu
[] % o >
4.0 D
O9
2.0 []
0.0
----
III
[
0
I
-
1 O0
200
300
TEMPERAT[ 1RE (K) Fig. 1. Temperature dependence of resistivity for the oxygen reduced samples.
~
2.0
0 C
.L"
0.0
~-
-2.0
-,,,,,
~
°
°
° ~=0.0
4.0 0 oo
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-8.0 0
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i
i
i
i
10
20
30
4O
50
T E M P E R A T U R E (K) Fig. 2. Superconducting magnetic susceptibility measured in a field cooling process for the oxygen reduced samples.
less hole density than x = 0.18 and 5 = 0.028. In this comparison we assumed a homogeneous distribution of the oxygen deficiencies, which will be shown in the following experimental results. In fact, the deficient oxygen was confirmed to be the oxygen atoms in the CuOz plane by magnetic measurement for the La2CuO4 and LSCO samples with small x [ 10 ] and by neutron diffraction measurement in LSCO [ 18 ]. We determined Tc from the Meissner-effect measurement under a field of 1 mT in the field-cooling process. The results are shown in fig. 2. Tc was regarded as the intersection of the steepest slope of the transition curve in magnetic susceptibility, X, with Z= 0. The systematic change of the transition curves
R. Yoshizaki et aL /Effect of oxygen vacancy in La l.seSro.lsCuO4
will rule out the possibility of inhomogeneous oxygen deficiencies being, for example, localized at the grain surfaces. T~ is plotted as a function o f p in fig. 3, where the variation of Tc in La2_xSrxCuO4 is also plotted for comparison by assuming x = p [ 18,16 ]. As seen from the figure, the variation of T~ for the sample with reducing oxygen does not follow the T~ for the sample with reducing x but indicates a much faster decrease than the latter, which is in good agreement with the result of transport measurement mentioned above and previous results [ 16 ]. The reduction rate of T~ with oxygen deficiencies is compared with that of T~ for the case of Zn substitution for Cu. In case of Zn doping, the Hall number was not varied with doping and the hole density was retained constant. The reduction rate of Tc, d T d dy, became small with increasing hole concentration p as reported in the case of Ni substitution for Cu [12]. Then we can plot Tc as functions of p and y and get a phase diagram of the superconductivity, which is schematically shown in fig. 4(a). (The superconducting phase is truncated at p = 0.2. ) In case of deficient oxygens, on the other hand, the hole density was varied as p = x - 2 8 . In the present case of x = 0.18, the observed T¢s are plotted along the line o f p = 0.18 - 2~ in p - ~ space. They are represented by solid circles in fig. 4 ( b ) . In the figure the T¢ surface for the Zn-doped case is simultaneously plotted by 50 40
oO •
o
~d
•
[]
30
o
Q
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20
u Our work o Takagi et al.
10
0
i
0.0
I
~
0.1
HOLES
I
a
0.2
PER Cu SITE
0.3
p
Fig. 3. Tcvs. hole density per Cu. Tcfor the oxygenreduced samshown by solid circles and the data for x-varied samples by open symbols. ples is
145
assuming y = ½~, and the surface is truncated along p = 0.18 - 2~ in order to compare the reduction of Tc for the two cases. We find that both To-reduction profiles are comparable to each other with on taking into account the uncertainty of Tc for both cases. Similar truncations at various x keeping p = x - 2 ~ give us the expected profile of the T~ decrease with oxygen deficiencies in LSCO, which is qualitatively consistent with the experimental results reported by Muromachi and Rice (see fig. 9 in ref. [16]. Thus both defects in the Cu site and the oxygen site in the CuO2 plane play a similar role in the T~ reduction in LSCO. But the present results seem to suggest that the contribution of defects in a Cu site is different by a factor two compared to those in an oxygen site if we assume y = ½~. As for this point, further investigation will be needed, experimentally as well as theoretically. Figure 5 shows the normal-state magnetic susceptibility with increasing oxygen deficiency. Three prominent features are observed in the spectra. One is the shift of the broad correlation peak to the higher temperature side, a second one is the reduction of the normal-state susceptibility except the sample with ~=0.033 and the third one is the lack of a Curie-like peak at low temperature which was normally observed for the Cu-substituted samples [ 10 ]. The first two characteristics are qualitatively consistent with the results observed in the spectra for the hole density varied samples [ 7-10 ]; namely, the correlation peak shifts to the lower temperature side with increasing hole concentration accompanying the gradual increase of the magnitude of the magnetic susceptibility. Lack of a Curie-like peak suggests that the rigid spin order system model at low temperature [ 14 ] still holds for the oxygen deficiencies in the 2D Cu-spin system. We confirmed the restoring of the correlation peak as well as T~ with reannealing the deoxidized sample in flowing oxygen atmosphere. To exclude the effect of the oxygen deficiencies, we compared the magnetic susceptibilities for the oxygen reduced sample, Z(X=0.18, t~=0.043), and the hole reduced sample, y ( x = 0 . 1 0 , ~ = 0 ) , in fig. 6. T h e subtracted magnetic susceptibility shows almost temperature independent behavior at about 5 X 10- 8 emu/g in the whole temperature range observed. This difference is substantially very small when the ambiguity of J is taken into account and will be ex-
146
R. Yoshizaki et al. / Effect o f oxygen vacancy in La ,.seSro.lsCuO4
(K)
(K) 40'
(a)
4O
(b)
20 I
(
0¢
P
; =p 0.2
i,i
o.oy 0.06/ o
0.08
q ~@~5--
0.04//
Fig. 4. Schematic plot of the variation of Tc as functions of hole density p and Zn content y in (a) functions of p and oxygen deficiencies t~ in (b). The phase is truncated at p=0.2 in (a) and at p = 0 . 1 8 - 2 ~ in (b). The closed circles in (b) are the present results and the dashed line is a guide for the eyes.
5.0
E
4.0
La~.s2Sr0.1sCuOa_ • 6=0.0 • 0.006 + 0.013
4.0
I:1
,.-.I
[] 0.028 o 0.033 • 0.043
0
X(x=O. 1O) Z( 8 =0.043) X( 8 =O.043)~X,(x=O.1O)
G 2.0
3.0
_= ¢J
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r~ l_J
o~
m
#
1.0
0
i
i
i
i
1 O0
200
300
400
0.0 500
T E M P E R A T U R E (K) Fig. 5. Magnetic susceptibility of the normal state for the oxygen reduced samples. The open triangles denote the correlation peak temperature.
p l a i n e d partly by t h e d i f f e r e n c e o f t h e c o r e diam a g n e t i s m o f the t w o substances. T h e r e f o r e , the t e m p e r a t u r e d e p e n d e n t parts o f t h e s u s c e p t i b i l i t y
0
'
1
Ijl,
i
I
I
100
200
300
400
TEMPERATURE (K) Fig. 6. The normal state susceptibilities for the deoxidized sample and Sr reduced samples. The difference between the two is denoted by triangles. w h i c h are a s c r i b e d to t h e 2 D A F spin c o r r e l a t i o n are a l m o s t the s a m e for t h e s a m p l e s w i t h s i m i l a r h o l e c o n c e n t r a t i o n w i t h i n the a c c u r a c y o f the p r e s e n t experiment.
R. Yoshizaki et al. /Effect of oxygen vacancy in Lal.seSro.18Cu04
ation o f hole density was taken into account. In contrast, the reduction o f the superconducting transition t e m p e r a t u r e was decreased with increasing oxygen deficiency, which is not explained simply by a hole concentration effect. Thus the shift o f the correlation peak t e m p e r a t u r e is not directly connected to the change in Tc b u t the d i s o r d e r in a CuO2 plane is playing an i m p o r t a n t role in reducing To.
700
"... "-... "-..
600 0
0 "-...
"...
500
0
400
"-... ".. "-...
E [..-,
300 ob 200 100 0 0.05
147
o La 82Sr0 18CUO4 ¢~ ..... La2.xSrxCuO4
Acknowledgements 0.10
0.15
020
0 25
Fig. 7. The correlation peak temperature, Tmaxis plotted as a function of hole density per Cu, p. The dotted line is the result for the x-varied samples. In fact, the relation between the t e m p e r a t u r e - d e p e n d e n t susceptibility a n d the oxygen deficiency was investigated from the p o i n t o f the correlation energy versus hole density relation. The normal-state spectra in fig. 5 were scaled using the empirical form o f the universal function p r o p o s e d by J o h n s t o n [ 19 ]. The peak t e m p e r a t u r e which represented the correlation energy was evaluated for the oxygen r e d u c e d samples. They are plotted as a function o f p in fig. 7. In the figure the dotted line is the result o f the peak t e m p e r a t u r e o b t a i n e d for the samples with varying Sr content [ 10 ]. The coincidence o f the two results indicates that the spin correlation for the oxygen deficient samples is almost explained by the reduction o f the hole density. Thus the oxygen vacancies in a CuO2 plane themselves have little effect on the 2D spin correlation. This fact is consistent with the lack o f a Curie-like peak at low t e m p e r a t u r e in the normal-state magnetic susceptibility with oxygen deficiencies as m e n t i o n e d above.
4. Conclusion In conclusion we o b s e r v e d the normal-state magnetic susceptibility for L S C O with reducing oxygens from CuO2 plane. T h e results indicate that the twod i m e n s i o n a l spin correlation effect was not affected so much by the oxygen deficiencies as far as the v a i l -
The authors t h a n k E. T a k a y a m a - M u r o m a c h i for showing us the d a t a on the oxygen deficiencies and he annealing t e m p e r a t u r e prior to the publication. They are also grateful to I. N a k a i for assisting in the X-ray diffraction m e a s u r e m e n t a n d to H. Ikeda for the technical assistance in the experiments. This work was s u p p o r t e d by a G r a n t - i n - A i d for Scientific Research on Priority Areas from the Ministry o f Education, Science a n d Culture.
References [ 1] R.M. Hazen, in: Physical Properties of High Temperature Superconductivity II, ed. D.M. Ginsberg (World Scientific, Singapore, 1989) p. 121. [2] R. Beyers and T.M. Shaw, in: Solid State Physics, vol. 42, eds. H. Ehrereich and D. Turnbull (Academic Press, Boston, 1989) p. 135. [3 ] H. Kamimura and A. Oshiyama, Mechanisms of High Temperature Superconductivity (Springer, Berlin, 1987 ). [4] G. Shirane, Y. Endoh, R.J. Birgeneau, M.A. Kastner, Y. Hidaka, M. Oda, M. Suzuki and T. Murakami, Phys. Rev. Lett. 59 (1987) 1613. [5 ] T. Fujita, Y. Aoki, Y. Macho, J. Sakurai, H. Fukuda and H. Fujii, Jpn. J. Appl. Phys. 26 (1987) L368. [6] R.J. Birgeneau and G. Shirane, in: Physical Properties of High Temperature Superconductivity I, ed. D.M. Ginsberg (World Scientific, Singapore, 1989) p. 151. [ 7 ] H. Takagi, T. Ido, S. Ishibashi, M. Uota, S. Uchida and Y. Tokura, Phys. Rev. B 40 (1989) 2254. [8] J.B. Torrance, A. Bezinge, A.I. Nazzal, T.C. Huang, S.S.P. Parkin, D.T. Keane, S.J. LaPlaca, P.M. Horn and G.A. Held, Phys. Rev. B 40 (1989) 8872. [9] M. Oda, T. Ohguro, H. Matsuki, N. Yamada and M. Ido, Phys. Rev. B 41 (1990) 2605. [ 10 ] R. Yoshizaki, N. Ishikawa, H. Sawada, E. Kita and A. Tasaki, Physica C 166 (1990) 417.
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R. Yoshizaki et al. /Effect o f oxygen vacancy in Lal seSro lsCu04
[ 11 ] J.M. Tarascon, L.H. Green, P. Barboux, W.R. McKinnon, G.W. Hull, T.P. Orlando, K.A. Delin, S. Foner and E.J. McNiffJr., Phys. Rev. B 36 (1987) 8393. [ 12] H. Fujishita and M. Sato, Solid State Commun. 72 (1989) 529. [ 13] L.H. Green and B.G. Bagley, in: Physical Properties of High Temperature Superconductivity II, ed. D.M. Ginsberg (World Scientific, Singapore, 1989 ) p. 509. [14]N. Ishikawa, N. Kuroda, H. Ikeda and R. Yoshizaki, preprints. [ 15 ] D.C. Johnston, S.K. Sinha, A.J. Jacobson and J.M. Newsam, Physica C 153-155 (1988) 572.
[16] E. Takayama-Muromachi and D.E. Rice, Physica C 177 (1991) 195. [17] R. Yoshizaki, T. Iwazumi, H. Sawada, H. Ikeda and E. Matsuura, Jpn. J. Appl. Phys. 26 (1987) L311. [ 18 ] T. Kamiyama, F. Izumi, H. Asano, H. Takagi, S. Uchida, Y. Tokura, E. Takayama-Muromachi, M. Matsuda, K. Yamada, Y. Endoh and Y. Hidaka, Physica C 172 (1990) 120. [19] D.C. Johnston, Phys. Rev. Lett. 62 (1989) 957.
Physica C 199 (1992) 143-148 North-Holland
Effect of oxygen vacancy on magnetic and superconducting
properties in La1.82Sro.18CUO4 R. Yoshizaki a, N. Kuroda b, S. Nakamura b and N. Ishikawa b,1 a Institute of Applied Physics and Cryogenics Center, University ofTsukuba, Tsukuba, lbaraki 305, Japan b Institute of Applied Physics, University ofTsukuba, Tsukuba, Ibaraki 305, Japan Received 14 May 1992
The role of oxygen vacancies in the CuO2 plane was investigated in Lal.g2Sro.18CuO4 by measuring the normal-state magnetic susceptibility. We found that the oxygen vacancies hardly affected the two-dimensional spin correlation when the reduction of the hole density was taken into account. In contrast, the superconductivity broke down much faster than the reduction rate of holes.
1. Introduction All of the high temperature superconductors in the copper oxides so far discovered have the CuO2 planes in their crystal structures [ 1,2 ] and the carriers in the CuO2 planes are believed to sustain the superconductivity [3 ]. As another characteristic of the CuO2 plane, Cu E÷ ions form a two-dimensional (2D) antiferromagnetic (AF) quantum spin system [ 4 ]. Introduction of a few % holes in the plane leads the three-dimensional AF spin correlation to abrupt destruction [5,6 ] and 2D spin correlation to gradual decay with further increase of hole density [ 7-10 ]. Those behaviors have been well studied in the LaE_xSrxCuO4 (LSCO) system because of the simplicity of its crystal structure. From the point of the correlation between the CuO2 planes and the superconductivity, the substitution effect has been examined by replacing the Cu atom with 3d transition elements and sp elements. The result is quick destruction of the superconductivity with the doping of only 2-5% [ 11-13]. As for the behaviour of the Cu spins, recently it has been revealed from the DC magnetic-susceptibility measurement [ 14 ] that there are two aspects concerning the effects in the normalstate magnetic properties by the substitution for Cu. Present address: Department of Physics, Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki 319-11, Japan.
One is the decrease or increase of the 2D spin correlation energy depending on the occupancy of the 3d levels of the dopant. The other is the presence of the induced localized spin moment at low temperature, which is explained by the picture of the rigid spin order system [ 14 ]. As for the role of the oxygen atom in the CuO2 plane, on the other hand, pioneering work on the correlation between the oxygen deficiencies and the magnetic susceptibility in the normal state was done by Johnston et al. [ 15] and Takagi et al. [ 7 ]. They have revealed the role of oxygen deficiencies as reducing hole densities effectively. More extensive studies were carried out and recently reported by Muromachi and Rice [ 16 ]. They insisted that the destruction of the superconductivity was mainly due to the effect of the induced randomness in the CuO2 planes. In the present paper, the effect of the oxygen deficiency on the magnetic properties of the normal state has been investigated by means of the magnetic-susceptibility measurement. The result will be compared with those observed in the substitution experiments for Cu.
2. Experimental In the present experiment, the Sr concentration was fixed at x = 0.18 in order to yield the measurement of the spin correlation peak in the normal-state mag-
0921-4534/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
144
R. Yoshizaki et al. /Effect of oxygen vacancy in Lal s2Sro lsCu04
netic susceptibility which was observed at about 260 K for the sample without oxygen deficiencies [ 9,10 ]. The compounds, La~.82Sro.~8CuO4, were prepared by using a hot-press method [17]: the appropriate amounts of La203, SrCO3 and CuO were well mixed and calcined at 970°C for 12 h in air. The calcined powder was ground, pressed into a pellet and sintered at 1070°C for 10 h in flowing oxygen. Then the products were ground, pressed into a pellet again and sintered at 1070°C for 2.5 h under a compression of about 300 kgf/cm 2. The samples were annealed at 820 °C in flowing oxygen and subsequently at 650 °C for 40 h. Oxygen-reduced samples were prepared by annealing the oxidized samples at 800 ° C-950 °C for 10-24 h in flowing nitrogen atmosphere [13 ]. As a result we obtained samples with the chemical formula of La L82Sro.18CUO4_a whose oxygen content was varied as 5=0, 0.006, 0.013, 0.028, 0.033 and 0.043. Any trace of the secondary phases was observed in the powder X-ray diffraction spectra even after the annealing process for the oxygen reduction. The oxygen content, 4 - & was determined by iodometric titration with a resolution of 0.007, and the hole concentration per Cu, p, was calculated from the charge neutrality condition to be p = 0 . 1 8 - 25 by assuming La 3+, Sr 2+ and 0 2 - . The electrical-resistivity measurements were carried out using a standard fourprobe method. The temperature dependence of the susceptibility in the normal state was observed under a field of 1 T using a SQUID magnetometer. As reference materials we observed the magnetic and transport properties of LSCO and La1.82Sro.18Cul - y Z n y O 4 .
3. Results and discussion The temperature dependence of resistivity for the sample with varying 6 is shown in fig. I. With increasing oxygen deficiency, the metallic behavior of the resistivity versus temperature profile changed to a semiconducting one. However, the change was not scaled only by the reduction of the hole density due to the introduction of oxygen deficiencies; e.g. the resistivity is fully semiconducting below room temperature for the sample with 5=0.028 in fig. l, while the temperature dependence of the resistivity is still metallic for the sample, L a l . 9 S r o . l C u O 4 , which has
8.0 E
La~ ~2Srox~CuO4
O
c; 6.0
o 8=0.0 ° 0.00(~ + 0.013
uOu
[] % o >
4.0 D
O9
2.0 []
0.0
----
III
[
0
I
-
1 O0
200
300
TEMPERAT[ 1RE (K) Fig. 1. Temperature dependence of resistivity for the oxygen reduced samples.
~
2.0
0 C
.L"
0.0
~-
-2.0
-,,,,,
~
°
°
° ~=0.0
4.0 0 oo
-6.0
La~ 82Sro.~sCuO4 a
-8.0 0
D
D 0.006 o 0.013 • 0.028 • 0.033 + 0.043
i
i
i
i
10
20
30
4O
50
T E M P E R A T U R E (K) Fig. 2. Superconducting magnetic susceptibility measured in a field cooling process for the oxygen reduced samples.
less hole density than x = 0.18 and 5 = 0.028. In this comparison we assumed a homogeneous distribution of the oxygen deficiencies, which will be shown in the following experimental results. In fact, the deficient oxygen was confirmed to be the oxygen atoms in the CuOz plane by magnetic measurement for the La2CuO4 and LSCO samples with small x [ 10 ] and by neutron diffraction measurement in LSCO [ 18 ]. We determined Tc from the Meissner-effect measurement under a field of 1 mT in the field-cooling process. The results are shown in fig. 2. Tc was regarded as the intersection of the steepest slope of the transition curve in magnetic susceptibility, X, with Z= 0. The systematic change of the transition curves
R. Yoshizaki et aL /Effect of oxygen vacancy in La l.seSro.lsCuO4
will rule out the possibility of inhomogeneous oxygen deficiencies being, for example, localized at the grain surfaces. T~ is plotted as a function o f p in fig. 3, where the variation of Tc in La2_xSrxCuO4 is also plotted for comparison by assuming x = p [ 18,16 ]. As seen from the figure, the variation of T~ for the sample with reducing oxygen does not follow the T~ for the sample with reducing x but indicates a much faster decrease than the latter, which is in good agreement with the result of transport measurement mentioned above and previous results [ 16 ]. The reduction rate of T~ with oxygen deficiencies is compared with that of T~ for the case of Zn substitution for Cu. In case of Zn doping, the Hall number was not varied with doping and the hole density was retained constant. The reduction rate of Tc, d T d dy, became small with increasing hole concentration p as reported in the case of Ni substitution for Cu [12]. Then we can plot Tc as functions of p and y and get a phase diagram of the superconductivity, which is schematically shown in fig. 4(a). (The superconducting phase is truncated at p = 0.2. ) In case of deficient oxygens, on the other hand, the hole density was varied as p = x - 2 8 . In the present case of x = 0.18, the observed T¢s are plotted along the line o f p = 0.18 - 2~ in p - ~ space. They are represented by solid circles in fig. 4 ( b ) . In the figure the T¢ surface for the Zn-doped case is simultaneously plotted by 50 40
oO •
o
~d
•
[]
30
o
Q
[]
20
u Our work o Takagi et al.
10
0
i
0.0
I
~
0.1
HOLES
I
a
0.2
PER Cu SITE
0.3
p
Fig. 3. Tcvs. hole density per Cu. Tcfor the oxygenreduced samshown by solid circles and the data for x-varied samples by open symbols. ples is
145
assuming y = ½~, and the surface is truncated along p = 0.18 - 2~ in order to compare the reduction of Tc for the two cases. We find that both To-reduction profiles are comparable to each other with on taking into account the uncertainty of Tc for both cases. Similar truncations at various x keeping p = x - 2 ~ give us the expected profile of the T~ decrease with oxygen deficiencies in LSCO, which is qualitatively consistent with the experimental results reported by Muromachi and Rice (see fig. 9 in ref. [16]. Thus both defects in the Cu site and the oxygen site in the CuO2 plane play a similar role in the T~ reduction in LSCO. But the present results seem to suggest that the contribution of defects in a Cu site is different by a factor two compared to those in an oxygen site if we assume y = ½~. As for this point, further investigation will be needed, experimentally as well as theoretically. Figure 5 shows the normal-state magnetic susceptibility with increasing oxygen deficiency. Three prominent features are observed in the spectra. One is the shift of the broad correlation peak to the higher temperature side, a second one is the reduction of the normal-state susceptibility except the sample with ~=0.033 and the third one is the lack of a Curie-like peak at low temperature which was normally observed for the Cu-substituted samples [ 10 ]. The first two characteristics are qualitatively consistent with the results observed in the spectra for the hole density varied samples [ 7-10 ]; namely, the correlation peak shifts to the lower temperature side with increasing hole concentration accompanying the gradual increase of the magnitude of the magnetic susceptibility. Lack of a Curie-like peak suggests that the rigid spin order system model at low temperature [ 14 ] still holds for the oxygen deficiencies in the 2D Cu-spin system. We confirmed the restoring of the correlation peak as well as T~ with reannealing the deoxidized sample in flowing oxygen atmosphere. To exclude the effect of the oxygen deficiencies, we compared the magnetic susceptibilities for the oxygen reduced sample, Z(X=0.18, t~=0.043), and the hole reduced sample, y ( x = 0 . 1 0 , ~ = 0 ) , in fig. 6. T h e subtracted magnetic susceptibility shows almost temperature independent behavior at about 5 X 10- 8 emu/g in the whole temperature range observed. This difference is substantially very small when the ambiguity of J is taken into account and will be ex-
146
R. Yoshizaki et al. / Effect o f oxygen vacancy in La ,.seSro.lsCuO4
(K)
(K) 40'
(a)
4O
(b)
20 I
(
0¢
P
; =p 0.2
i,i
o.oy 0.06/ o
0.08
q ~@~5--
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Fig. 4. Schematic plot of the variation of Tc as functions of hole density p and Zn content y in (a) functions of p and oxygen deficiencies t~ in (b). The phase is truncated at p=0.2 in (a) and at p = 0 . 1 8 - 2 ~ in (b). The closed circles in (b) are the present results and the dashed line is a guide for the eyes.
5.0
E
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0
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m
#
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0
i
i
i
i
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200
300
400
0.0 500
T E M P E R A T U R E (K) Fig. 5. Magnetic susceptibility of the normal state for the oxygen reduced samples. The open triangles denote the correlation peak temperature.
p l a i n e d partly by t h e d i f f e r e n c e o f t h e c o r e diam a g n e t i s m o f the t w o substances. T h e r e f o r e , the t e m p e r a t u r e d e p e n d e n t parts o f t h e s u s c e p t i b i l i t y
0
'
1
Ijl,
i
I
I
100
200
300
400
TEMPERATURE (K) Fig. 6. The normal state susceptibilities for the deoxidized sample and Sr reduced samples. The difference between the two is denoted by triangles. w h i c h are a s c r i b e d to t h e 2 D A F spin c o r r e l a t i o n are a l m o s t the s a m e for t h e s a m p l e s w i t h s i m i l a r h o l e c o n c e n t r a t i o n w i t h i n the a c c u r a c y o f the p r e s e n t experiment.
R. Yoshizaki et al. /Effect of oxygen vacancy in Lal.seSro.18Cu04
ation o f hole density was taken into account. In contrast, the reduction o f the superconducting transition t e m p e r a t u r e was decreased with increasing oxygen deficiency, which is not explained simply by a hole concentration effect. Thus the shift o f the correlation peak t e m p e r a t u r e is not directly connected to the change in Tc b u t the d i s o r d e r in a CuO2 plane is playing an i m p o r t a n t role in reducing To.
700
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600 0
0 "-...
"...
500
0
400
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E [..-,
300 ob 200 100 0 0.05
147
o La 82Sr0 18CUO4 ¢~ ..... La2.xSrxCuO4
Acknowledgements 0.10
0.15
020
0 25
Fig. 7. The correlation peak temperature, Tmaxis plotted as a function of hole density per Cu, p. The dotted line is the result for the x-varied samples. In fact, the relation between the t e m p e r a t u r e - d e p e n d e n t susceptibility a n d the oxygen deficiency was investigated from the p o i n t o f the correlation energy versus hole density relation. The normal-state spectra in fig. 5 were scaled using the empirical form o f the universal function p r o p o s e d by J o h n s t o n [ 19 ]. The peak t e m p e r a t u r e which represented the correlation energy was evaluated for the oxygen r e d u c e d samples. They are plotted as a function o f p in fig. 7. In the figure the dotted line is the result o f the peak t e m p e r a t u r e o b t a i n e d for the samples with varying Sr content [ 10 ]. The coincidence o f the two results indicates that the spin correlation for the oxygen deficient samples is almost explained by the reduction o f the hole density. Thus the oxygen vacancies in a CuO2 plane themselves have little effect on the 2D spin correlation. This fact is consistent with the lack o f a Curie-like peak at low t e m p e r a t u r e in the normal-state magnetic susceptibility with oxygen deficiencies as m e n t i o n e d above.
4. Conclusion In conclusion we o b s e r v e d the normal-state magnetic susceptibility for L S C O with reducing oxygens from CuO2 plane. T h e results indicate that the twod i m e n s i o n a l spin correlation effect was not affected so much by the oxygen deficiencies as far as the v a i l -
The authors t h a n k E. T a k a y a m a - M u r o m a c h i for showing us the d a t a on the oxygen deficiencies and he annealing t e m p e r a t u r e prior to the publication. They are also grateful to I. N a k a i for assisting in the X-ray diffraction m e a s u r e m e n t a n d to H. Ikeda for the technical assistance in the experiments. This work was s u p p o r t e d by a G r a n t - i n - A i d for Scientific Research on Priority Areas from the Ministry o f Education, Science a n d Culture.
References [ 1] R.M. Hazen, in: Physical Properties of High Temperature Superconductivity II, ed. D.M. Ginsberg (World Scientific, Singapore, 1989) p. 121. [2] R. Beyers and T.M. Shaw, in: Solid State Physics, vol. 42, eds. H. Ehrereich and D. Turnbull (Academic Press, Boston, 1989) p. 135. [3 ] H. Kamimura and A. Oshiyama, Mechanisms of High Temperature Superconductivity (Springer, Berlin, 1987 ). [4] G. Shirane, Y. Endoh, R.J. Birgeneau, M.A. Kastner, Y. Hidaka, M. Oda, M. Suzuki and T. Murakami, Phys. Rev. Lett. 59 (1987) 1613. [5 ] T. Fujita, Y. Aoki, Y. Macho, J. Sakurai, H. Fukuda and H. Fujii, Jpn. J. Appl. Phys. 26 (1987) L368. [6] R.J. Birgeneau and G. Shirane, in: Physical Properties of High Temperature Superconductivity I, ed. D.M. Ginsberg (World Scientific, Singapore, 1989) p. 151. [ 7 ] H. Takagi, T. Ido, S. Ishibashi, M. Uota, S. Uchida and Y. Tokura, Phys. Rev. B 40 (1989) 2254. [8] J.B. Torrance, A. Bezinge, A.I. Nazzal, T.C. Huang, S.S.P. Parkin, D.T. Keane, S.J. LaPlaca, P.M. Horn and G.A. Held, Phys. Rev. B 40 (1989) 8872. [9] M. Oda, T. Ohguro, H. Matsuki, N. Yamada and M. Ido, Phys. Rev. B 41 (1990) 2605. [ 10 ] R. Yoshizaki, N. Ishikawa, H. Sawada, E. Kita and A. Tasaki, Physica C 166 (1990) 417.
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R. Yoshizaki et al. /Effect o f oxygen vacancy in Lal seSro lsCu04
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