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Magnetic susceptibility of normal state and superconductivity of La2−xSrxCuO4
PhysicaC 166 (1990) 417-422 North-Holland
MAGNETIC SUSCEPTIBILITY Laz_$3rXCu04 R. YOSHIZAKI,
N. ISHIKAWA
OF NORMAL STATE AND SUPERCONDUCTIVITY
OF
a, H. SAWADA ‘**, E. KITA a and A. TASAKI a
Institute ofApplied Physics and Cryogenics Center, University of Tsukuba. Tsukuba, Ibaraki 305, Japan a Institute ofApplied Physics, University of Tsukuba, Tsukuba, Ibaraki 305. Japan
Received 24 January 1990
The two-dimensional spin-correlation energy, J, was studied in La2_~rxCu04 by means of magnetic susceptibility measurements. We found that J, deduced from the susceptibility peak temperature, T_, changed drastically with the character of the doped holes, localized or itinerant. In the low carrier density region (O~x~O.O8), J was higher than 1000 K, representing the nearest neighbor interaction due to the localization of holes. A sudden drop of J and the appearance of bulk superconductivity were observed at x=0.09, which was ascribed to the itinerant holes. The Zn substitution effect for Cu was investigated near the superconductor-to-normal-metal transition. We found that only 1% Zn substitution resulted in 30% decrease of both T, and T_, and T,,,, disappeared as T, became zero. Those results give us direct evidence that the spin correlation relates to the appearance of the superconductivity.
1. Introduction One of the common features of the copper-based oxide superconductors is that the superconducting phase lies near an antiferromagnetic insulating phase [ 11. So, the novel mechanisms of the high-temperature superconductivity have been approached theoretically with the relevance to the spin fluctuation [ 21. From the experimental point of view, it is of importance to study the behavior of Cu spins from the insulating phase to the superconducting phase. These studies were carried out in Laz_$rXCu04 (LSCO) because the compound was fundamental among the copper-based oxide superconductors. Neutron scattering experiments elucidated the information about quantum spin fluid profiles of the two-dimensional spin system [ 3 1, the x-dependence of the spin correlation length [ 41 and the dispersion of magnons [ 5 1. Two-magnon Raman-scattering experiments also gave us the information about the xdependence of the spin correlation energy, J, [ 6,7 1. In the magnetic susceptibility measurements, the l
Present address: R&D Laboratories-I, Central R&D Bureau, Nippon Steel Corporation, Eda, Nakahara-ku, Kawasaki 2 11, Japan.
092 l-4534/90/$03.50 (North-Holland )
0 Elsevier Science Publishers B.V.
susceptibility peak of the normal state was discussed in relation to the two-dimensional spin correlation [ 8,9 1. The x-dependence of the peak temperature was studied in the vicinity of the disappearance of the superconducting phase for the heavily doped samples with x=0.2-0.25 [ 10-121. We measured the x-dependence of the normal state susceptibility for a wide range of x from 0 to 0.2. We will devote ourselves in the present paper to the discussion about the temperature for the susceptibility peak. We found an abrupt drop of the peak temperature at x=0.08, corresponding to the transformation of the hole character from localized to itinerant. The susceptibility peak was also investigated for varying the Zn content, substituting for Cu, while maintaining the hole density constant. It was found that the superconducting transition temperature, T,, approached zero as the susceptibility peak temperature became zero as a function of the Zn content.
2. Experimental The LSCO ceramic samples were prepared from the hot-press technique [ 13 1. The quality of these samples is excellent, and the superconducting pro-
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files, T, and the fractional volume, were reported elsewhere [ 14 1. Magnetic susceptibility measurements were carried out by employing a magnetic balance in the temperature range above room temperature to 1200 K. Below room temperature, we adopted a SQUID magnetometer (Quantum Design ) . The carrier density of holes was estimated from the iodometic titration. The results showed fairly good correlation between the nominal concentration of Sr, x, and the observed effective excess charge of Cu*+, p, as reported by Torrance et al. [ 15 1. We checked the deficiency of oxygen for the samples after the high-temperature magnetic-balance measurements.
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3. Results and discussion Temperature dependence of the magnetic susceptibility of the normal state for the x=0 sample is shown in fig. 1 (a) for the measurement in low-pressure exchange gas of helium. In the heating process, denoted by open circles, the tail structure of the susceptibility peak associated with the three-dimensional Neel state at TN was observed in the vicinity of 300 K. The susceptibility increased gradually up to 1000 K and decreased steeply above 1100 K. In the subsequent cooling process, denoted by solid circles, the susceptibility was restored below 1000 K, showing a hysteresis. This hysteresis was confirmed to be associated with the desorption and the absorption of oxygen atoms by the gravitational weight measurement of the sample carried out at the same time. As seen in the figure, the restoration of the oxygen atoms was almost complete even in the lowpressure helium atmosphere, and the tail of the susceptibility peak observed at 300 K shifted slightly to the higher temperature side due to the incomplete restoration. It must be emphasized that the desorption of oxygen atoms results in the decrease of the susceptibility. This profile is consistent with the feature that the susceptibility peak above TN is associated with the two-dimensional spin correlation in the Cu02 sheets, since the oxygen deficiency occurs from the comer sites of the Cu04 basal plane structure in the LSCO [ 161. In order to prevent the oxygen desorption as small
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Fig. 1. High-temperature magnetic susceptibility of the LSCO measured in (a) helium exchange gas for the x=0 sample and (b) low pressure oxygen gas for the x=0 and 0.01 samples. The data taken in the heating process are denoted by open circles and those in the subsequent cooling process by solid circles for x=0. The data for the x=0.01 sample are expressed by open squares.
as possible, the high-temperature susceptibility was measured in the low-pressure oxygen atmosphere for the samples x=0 and 0.01, which is shown in fig. 1(b). We can confirm the doping of holes from the shift of the tail structure toward the low temperature side. The susceptibility at high temperatures, however, did not change by doping, increasing monotonically up to the experimental limit of 1200 K. Thus the susceptibility peak due to the two-dimensional spin correlation must be located above 1200 K for those lightly doped or non-doped samples. This is consistent with the results of other experiments. A spin correlation energy J of about 1400 K was obtained for the x=0 sample from the neutron scat-
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tering experiment [ 5 1, and J= 1700- 1500 K was estimated from the Raman scattering experiment for the x= O-0.07 samples, respectively [ 7 1. With increasing hole concentrations from x=0, the susceptibility peak suddenly appeared to be observed below 1000 K for the x=0.09 sample. The susceptibility versus temperature profiles are shown in fig. 2 for the samples from x=0.04 to 0.14. For the lightly doped samples with x50.08, the susceptibility increased monotonically up to 1000 K. Above that temperature, it was expected that the oxygen deficiency might affect the susceptibility. It should be mentioned, however, that the monotonic increase of the susceptibility means for the expected peak to be above 1000 K even if oxygen desorbed from the sample, since the desorption of oxygen causes the decrease of the susceptibility as shown in fig. 1(a). In the range of 0 5 x5 0.08, we measured eight samples with different x, and it was checked that the susceptibility peak was not observed and T,,,,, exceeded 1000-1200 K. For the heavily doped samples with x2 0.09, on the other hand, the susceptibility showed a broad peak below 700 K as shown in fig. 2. Thus,
the temperature corresponding to the susceptibility peak, T,-, changed drastically from Tmax>1000 K to 700 K between x=0.08 and 0.09. The oxygen-deficiency effect on the sample quality was investigated by means of the superconducting-property measurement after the thermal cycle due to the high-temperature susceptibility measurement. The result for the x= 0.08 sample is exhibited in fig. 3. As seen in the figure, the transition temperature T,did not change after the thermal cycle within the experimental error of 0.1 K and the magnitude of the low temperature superconducting diamagnetism decreased slightly by about 10%. Referring to the x dependence of T,and the fractional volume of the superconductor in the LSCO [ 141, the variation of holes due to the thermal cycle was estimated and expressed as the variation of Sr concentration dx, which was dxcO.01. is plotted as a function of x in The obtained T,,,,, fig. 4. The ambiguity of x discussed above is expressed by the horizontal error-bars on the data. It must be emphasized again that T,, decreases rapidly from x=0.08 to 0.09, and this tendency is indicated by the solid curve as a guide for the eye. The present results coincide fairly well with the previous
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Fig. 3. Superconducting diamagnetic susceptibility for the x=0.08 sample observed before and after the heating for the high-temperature susceptibility measurement.
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Fig. 4. T,,,,, obtained in the LSCO is plotted as a function of x. The solid curve is a guide to the eye based on the data (see text ) The J value observed by neutron scattering (ref. [ 51) is denoted by a solid circle. J values estimated from Raman scattering (ref. [ 71) are expressed by open squares.
work done by Takagi et al. [ 91 and Oda et al. [ lo] in the heavily doped range of x 2 0.15. In a quantum spin system of S= f, the magnetic susceptibility peak was calculated from a linear-chain model by Bonner and Fischer [ 17 1, and the result showed kBTmax= 1.3J, where k, is the Boltzman constant. In the two-dimensional spin system, k,T,,,= 0.95 was obtained by Line [ 18 1. In any case, T, T,,, is close to J, and we assume kBTmax= J in this paper. Under the assumption, we can reread the ordinate of fig. 4 as the energy scale of J/ks. Then, the correlation energies observed from other experiments, neutron and Raman scattering, can be plotted in the same figure by kelvin units. They are expressed by solid circles and open squares, respectively, in fig. 4. To interpret the strange behavior of J against x, it is useful to take the itinerant character of the holes into account. For the lightly doped samples (xs 0.08 ) , the holes have essentially localized character due to the random substitution of Sr for La. Holes are supposed to be localized near the doped Sr ions. Then the correlation energy J to be observed in the susceptibility measurement must be of the order of the one observed for the x= 0 sample; that is 1400-
1700 K from the neutron and Raman experiments [ 5-7 1. Therefore it is natural that we could not observe the susceptibility peak up to 1200 K as described above. With much more doping of holes, the insulator-to-metal transition occurs in the hole system at x=0.08, and the holes can itinerate on the CuOz sheets. Then the susceptibility-peak temperature means no more the nearest-neighbor interaction for an antiferromagnetic insulator but expresses a well-defined effective-correlation energy, Jcn. The occurrence of the metal-insulator transition at x= 0.08 can be conlirmed from the bulk conductivity of holes in the LX0 as shown in fig. 1 (b) of ref. [ 12 ] ; the conductivity at 300 K becomes sizable at about x=0.08 and increases linearly with x. It is noted that, since the resistivity of the LSCO is linear to the temperature up to 1000 K [ 191, the metalinsulator transition at x=0.08 estimated from the 300 K conductivity can be scaled to the high temperature where the susceptibility peak was observed. In the heavily doped samples (x20.09), Jeff seems to decrease linearly with increasing x as shown in fig. 4. When the doped holes can itinerate from x= 0.09, the superconductivity is supposed to be observed from that concentration. In the previous work, the superconductivity was observed from x= 0.06 in the LSCO, but the fractional volume of the superconductor became bulk scale from x=0.09 [ 141. This transformation can be seen in the other results [ 15 1. Thus the superconductivity observed hitherto below x= 0.08 is considered to be not the bulk one but the filamentary one, that may be observed from the inhomogeneity of Sr. It has been shown that the substitution of non-spin element Zn for Cu destroys the superconductivity as quickly as 3% replacement [20]. In such systems, the NCel state was recently studied by Chakraborty et al. for the x= 0 sample [ 2 11. On the other hand, the substitution effect of spin-element Ni for Cu was investigated by Fujishita et al. [ 221 in the LSCO. In order to clarify the role of spin fluctuations in the appearance of superconductivity, we studied the Znsubstitution effect for Cu in the vicinity of the transition in superconductor-to-normal-metal Laz_,SrXCu, _,Zn,O, (LSCZO). We have observed the susceptibility peak of the normal state for the heavily doped samples with x=0.8 or 0.20. The ob-
R. Yoshizakiet al. /Magnetic susceptibilityof Laa,_.,SrxCuOd
tamed dependence of T,,, and T, on the Zn concentration y is shown in figs. 5 (a) and 5 (b), respectively. As seen in the figures, the correlation between T,, versus y and T, versus y holds sufficiently for both the samples; T,, and T, decrease with increasing y, and they seemed to approach zero at the same y. This fact indicates that T,,, is strongly cor-
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Fig. 5. The dependence of T,, (a) and r, (b) on the Zn concentration y for the samples with constant hole density x= 0.18 or 0.20 in the LSCZO.
421
related to T,, and the result is consistent with that of the Ni-substituted LSCO system [22]. The correlation is excellent in particular for the x= 0.18 sample. We believe the observed T, for the x=0.20 sample is slightly affected by the oxygen deficiency or the inhomogeneity of oxygen content. Contrary to the T,,, versus T, correlation, when we take x as a variable, T,,, seemed to approach zero before T, became zero in the Zn-free system of the LSCO. The inhomogeneity of hole densities must be taken into account for the more definitive discussion. However, the present experiments exhibit that the superconductivity (T,) correlated strongly to the spin correlation (Jeff) rather than to the hole density (x) near the superconductor-to-normal-metal transition. To understand those results, we have assumed the simple situation that only the spins at the Cu sites which were replaced by Zn atoms became zero under the constant hole density. Then, the fact that 30% decreases of Jeff by 1% destruction of spins suggests that the spin-spin correlation observed by the susceptibility peak is probably due to widely spread interactions, extending over about 30 spins. This estimation is consistent with the result obtained by the neutron scattering experiment. The spin-con-elation length 1 was expressed by 1=3.8/~‘/~ in A units. Following the relation, we obtain 1~9 8, for x=0.18, which covers about twenty Cu sites. It is noted, moreover, that the spin correlation length is of the order of the coherence length of the superconductivity in the Cu02 plane. Remember that the appearance of Jeff due to the itinerant holes corresponded to the appearance of the superconducting phase and that the disappearance of Jeff corresponded to the disappearance of the superconductivity, and the antiferromagnetic spin correlation has relevance to the appearance of the superconductivity. Thus, the present results give us direct evidence that the spin-correlating system accompanied by the itinerant holes is closely related to the appearance of the superconducting phase.
4. Summary We have measured the magnetic-susceptibility peak of the normal state in Laz_SrXCu04 (Osx$O.20) and Laz_&,Cul _,Zn,,O, (y= 0 to 0.03 for x= 0.18
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or 0.20). In the LSCO, we found the susceptibility peak-temperature dropped from the temperature above 1200 to 700 K at x=0.08 to 0.09. This result was explained by a model that the itinerant holes make the nearest-neighbor spin-correlation energy J small compared to the effective one, Jefl. The spread of the spin correlation for Jeff was estimated from a similar experiment with varying spin density by substituting Zn for Cu. We found in the LSCZO that the disappearance of Jeff correlated with the disappearance of the superconductivity for the samples near the superconductor-to-normal-metal transition. Those facts indicate directly that the spin correlation plays an important role for the appearance of the high-T, superconductivity.
Acknowledgements The authors thank H. Ikeda, N. Kuroda and their colleagues for technical assistance and supporting the experiment. One of the authors (R.Y. ) expresses his gratitude to M. Inoue and K. Ueda for the occasional valuable discussions and the encouragements. This work is partly supported by Grant-in-Aid for Scientific Research on Priority Areas, “Mechanisms of Superconductivity”, from the Ministry of Education, Science and Culture.
References [ 1 ] T. Fujita, Y. Aoki, Y. Maeno, J. Sakurai, H. Fukuda and H. Fujii, Jpn. J. Appl. Phys. 26 (1987) L368. [ 21 See for example, Mechanisms of High Temperature Superconductivity, eds. H. Kamimura and A. Oshiyama (Springer, Berlin, 1989).
[3]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. 41 R.J. Birgeneau, Phys. Rev. B38 (1988) 6614. ] Y. Endoh. R.J. Biraeneau. D.R. Gabbe. Y. Hidaka. H.P. _ Jenssen, T. Murakami, M. Oda, P.J. Picone, G. Shirane, M. Suzuki, T.R. Thurston and K Yamada, Mechanisms of High Temperature Superconductivity, eds. H. Kamimura and A. Oshiyama (Springer, Berlin, 1989) p. 129 and refs. therein. K.B. Lyons, P.A. Fleury, L.F. Schneemeyer and J.V. Waszczak, Phys. Rev. Lett. 60 (1988) 732. S. Sugai, S. Shamoto and M. Sato, Phys. Rev. B38 ( 1988) 6436. [8] D.C. Johnston and S.K. Sinha, Physica C 153-155 (1988) 572. [9] H. Takagi, T. Ido, S. Ishibashi, M. Uota, S. Uchida and Y. Tokura, Phys. Rev. B40 (1989) 2254. [ 101 M. Oda, T. Ohguro, N. Yamada and M. Ido, J. Phys. Sot. Jpn. 58 (1989) 1137; M. Oda, T. Ohguro, H. Matsuki, N. Yamada and M. Ido, Phys. Rev. B., to be published. [ 111 Y. Ando, M. Sera, S. Yamagata, S. Kondoh, M. Onoda and M. Sato, Solid State Commun. 70 ( 1989) 303. [ 121 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. B40 (1989) 8872. [ 131 R. Yoshizaki, T. Iwazumi, H. Sawada, H. Ikeda and E. Matsuura, Jpn. J. Appl. Phys. 26 (1987) L311; T. Iwazumi, R. Yoshizaki, H. Sawada, H. Uwe, T. Sakudo and E. Matsuura, Jpn. J. Appl. Phys. 26 (1987) L386. [ 141 R. Yoshizaki and I. Nakai, Res. Rep. on Mechanism of Superconductivity, Science Research on Priority Areas No. 03 1, Ministry of Education, Science and Culture ( 1989) p. 89. [ 15 ] J.B. Torrance, Y. Tokura, AI. Nazzal, A. Bezinge, T.C. Huangand S.S.P. Parkin, Phys. Rev. Lett. 61 (1988) 1127. [ 161 E. Muromachi, private communications. [ 171 J.C. Bonner and M.E. Fischer, Phys. Rev. 135 ( 1964) A640. [ 181 M.E. Lines, J. Phys. Chem. Solids 3 1 ( 1970) 101. [ 191 M. Gurvich and A. Fiory, Phys. Rev. Lett. 59 ( 1987) 1337. [20] C.V.N. Rao, B. Jayaram, S.K. Agarwal and A.V. Narlikar, Physica C 152 ( 1988) 479. [2 1 ] A. Chakraborty, A.J. Epstein, M. Jarrel and E.M. McCarron, Phys. Rev. B40 (1989) 5296. [22] H. Fujishita and M. Sato, Solid State Commun. 72 ( 1989) 529.
MAGNETIC SUSCEPTIBILITY Laz_$3rXCu04 R. YOSHIZAKI,
N. ISHIKAWA
OF NORMAL STATE AND SUPERCONDUCTIVITY
OF
a, H. SAWADA ‘**, E. KITA a and A. TASAKI a
Institute ofApplied Physics and Cryogenics Center, University of Tsukuba. Tsukuba, Ibaraki 305, Japan a Institute ofApplied Physics, University of Tsukuba, Tsukuba, Ibaraki 305. Japan
Received 24 January 1990
The two-dimensional spin-correlation energy, J, was studied in La2_~rxCu04 by means of magnetic susceptibility measurements. We found that J, deduced from the susceptibility peak temperature, T_, changed drastically with the character of the doped holes, localized or itinerant. In the low carrier density region (O~x~O.O8), J was higher than 1000 K, representing the nearest neighbor interaction due to the localization of holes. A sudden drop of J and the appearance of bulk superconductivity were observed at x=0.09, which was ascribed to the itinerant holes. The Zn substitution effect for Cu was investigated near the superconductor-to-normal-metal transition. We found that only 1% Zn substitution resulted in 30% decrease of both T, and T_, and T,,,, disappeared as T, became zero. Those results give us direct evidence that the spin correlation relates to the appearance of the superconductivity.
1. Introduction One of the common features of the copper-based oxide superconductors is that the superconducting phase lies near an antiferromagnetic insulating phase [ 11. So, the novel mechanisms of the high-temperature superconductivity have been approached theoretically with the relevance to the spin fluctuation [ 21. From the experimental point of view, it is of importance to study the behavior of Cu spins from the insulating phase to the superconducting phase. These studies were carried out in Laz_$rXCu04 (LSCO) because the compound was fundamental among the copper-based oxide superconductors. Neutron scattering experiments elucidated the information about quantum spin fluid profiles of the two-dimensional spin system [ 3 1, the x-dependence of the spin correlation length [ 41 and the dispersion of magnons [ 5 1. Two-magnon Raman-scattering experiments also gave us the information about the xdependence of the spin correlation energy, J, [ 6,7 1. In the magnetic susceptibility measurements, the l
Present address: R&D Laboratories-I, Central R&D Bureau, Nippon Steel Corporation, Eda, Nakahara-ku, Kawasaki 2 11, Japan.
092 l-4534/90/$03.50 (North-Holland )
0 Elsevier Science Publishers B.V.
susceptibility peak of the normal state was discussed in relation to the two-dimensional spin correlation [ 8,9 1. The x-dependence of the peak temperature was studied in the vicinity of the disappearance of the superconducting phase for the heavily doped samples with x=0.2-0.25 [ 10-121. We measured the x-dependence of the normal state susceptibility for a wide range of x from 0 to 0.2. We will devote ourselves in the present paper to the discussion about the temperature for the susceptibility peak. We found an abrupt drop of the peak temperature at x=0.08, corresponding to the transformation of the hole character from localized to itinerant. The susceptibility peak was also investigated for varying the Zn content, substituting for Cu, while maintaining the hole density constant. It was found that the superconducting transition temperature, T,, approached zero as the susceptibility peak temperature became zero as a function of the Zn content.
2. Experimental The LSCO ceramic samples were prepared from the hot-press technique [ 13 1. The quality of these samples is excellent, and the superconducting pro-
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files, T, and the fractional volume, were reported elsewhere [ 14 1. Magnetic susceptibility measurements were carried out by employing a magnetic balance in the temperature range above room temperature to 1200 K. Below room temperature, we adopted a SQUID magnetometer (Quantum Design ) . The carrier density of holes was estimated from the iodometic titration. The results showed fairly good correlation between the nominal concentration of Sr, x, and the observed effective excess charge of Cu*+, p, as reported by Torrance et al. [ 15 1. We checked the deficiency of oxygen for the samples after the high-temperature magnetic-balance measurements.
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3. Results and discussion Temperature dependence of the magnetic susceptibility of the normal state for the x=0 sample is shown in fig. 1 (a) for the measurement in low-pressure exchange gas of helium. In the heating process, denoted by open circles, the tail structure of the susceptibility peak associated with the three-dimensional Neel state at TN was observed in the vicinity of 300 K. The susceptibility increased gradually up to 1000 K and decreased steeply above 1100 K. In the subsequent cooling process, denoted by solid circles, the susceptibility was restored below 1000 K, showing a hysteresis. This hysteresis was confirmed to be associated with the desorption and the absorption of oxygen atoms by the gravitational weight measurement of the sample carried out at the same time. As seen in the figure, the restoration of the oxygen atoms was almost complete even in the lowpressure helium atmosphere, and the tail of the susceptibility peak observed at 300 K shifted slightly to the higher temperature side due to the incomplete restoration. It must be emphasized that the desorption of oxygen atoms results in the decrease of the susceptibility. This profile is consistent with the feature that the susceptibility peak above TN is associated with the two-dimensional spin correlation in the Cu02 sheets, since the oxygen deficiency occurs from the comer sites of the Cu04 basal plane structure in the LSCO [ 161. In order to prevent the oxygen desorption as small
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Fig. 1. High-temperature magnetic susceptibility of the LSCO measured in (a) helium exchange gas for the x=0 sample and (b) low pressure oxygen gas for the x=0 and 0.01 samples. The data taken in the heating process are denoted by open circles and those in the subsequent cooling process by solid circles for x=0. The data for the x=0.01 sample are expressed by open squares.
as possible, the high-temperature susceptibility was measured in the low-pressure oxygen atmosphere for the samples x=0 and 0.01, which is shown in fig. 1(b). We can confirm the doping of holes from the shift of the tail structure toward the low temperature side. The susceptibility at high temperatures, however, did not change by doping, increasing monotonically up to the experimental limit of 1200 K. Thus the susceptibility peak due to the two-dimensional spin correlation must be located above 1200 K for those lightly doped or non-doped samples. This is consistent with the results of other experiments. A spin correlation energy J of about 1400 K was obtained for the x=0 sample from the neutron scat-
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tering experiment [ 5 1, and J= 1700- 1500 K was estimated from the Raman scattering experiment for the x= O-0.07 samples, respectively [ 7 1. With increasing hole concentrations from x=0, the susceptibility peak suddenly appeared to be observed below 1000 K for the x=0.09 sample. The susceptibility versus temperature profiles are shown in fig. 2 for the samples from x=0.04 to 0.14. For the lightly doped samples with x50.08, the susceptibility increased monotonically up to 1000 K. Above that temperature, it was expected that the oxygen deficiency might affect the susceptibility. It should be mentioned, however, that the monotonic increase of the susceptibility means for the expected peak to be above 1000 K even if oxygen desorbed from the sample, since the desorption of oxygen causes the decrease of the susceptibility as shown in fig. 1(a). In the range of 0 5 x5 0.08, we measured eight samples with different x, and it was checked that the susceptibility peak was not observed and T,,,,, exceeded 1000-1200 K. For the heavily doped samples with x2 0.09, on the other hand, the susceptibility showed a broad peak below 700 K as shown in fig. 2. Thus,
the temperature corresponding to the susceptibility peak, T,-, changed drastically from Tmax>1000 K to 700 K between x=0.08 and 0.09. The oxygen-deficiency effect on the sample quality was investigated by means of the superconducting-property measurement after the thermal cycle due to the high-temperature susceptibility measurement. The result for the x= 0.08 sample is exhibited in fig. 3. As seen in the figure, the transition temperature T,did not change after the thermal cycle within the experimental error of 0.1 K and the magnitude of the low temperature superconducting diamagnetism decreased slightly by about 10%. Referring to the x dependence of T,and the fractional volume of the superconductor in the LSCO [ 141, the variation of holes due to the thermal cycle was estimated and expressed as the variation of Sr concentration dx, which was dxcO.01. is plotted as a function of x in The obtained T,,,,, fig. 4. The ambiguity of x discussed above is expressed by the horizontal error-bars on the data. It must be emphasized again that T,, decreases rapidly from x=0.08 to 0.09, and this tendency is indicated by the solid curve as a guide for the eye. The present results coincide fairly well with the previous
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Fig. 3. Superconducting diamagnetic susceptibility for the x=0.08 sample observed before and after the heating for the high-temperature susceptibility measurement.
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0.30 x
Fig. 4. T,,,,, obtained in the LSCO is plotted as a function of x. The solid curve is a guide to the eye based on the data (see text ) The J value observed by neutron scattering (ref. [ 51) is denoted by a solid circle. J values estimated from Raman scattering (ref. [ 71) are expressed by open squares.
work done by Takagi et al. [ 91 and Oda et al. [ lo] in the heavily doped range of x 2 0.15. In a quantum spin system of S= f, the magnetic susceptibility peak was calculated from a linear-chain model by Bonner and Fischer [ 17 1, and the result showed kBTmax= 1.3J, where k, is the Boltzman constant. In the two-dimensional spin system, k,T,,,= 0.95 was obtained by Line [ 18 1. In any case, T, T,,, is close to J, and we assume kBTmax= J in this paper. Under the assumption, we can reread the ordinate of fig. 4 as the energy scale of J/ks. Then, the correlation energies observed from other experiments, neutron and Raman scattering, can be plotted in the same figure by kelvin units. They are expressed by solid circles and open squares, respectively, in fig. 4. To interpret the strange behavior of J against x, it is useful to take the itinerant character of the holes into account. For the lightly doped samples (xs 0.08 ) , the holes have essentially localized character due to the random substitution of Sr for La. Holes are supposed to be localized near the doped Sr ions. Then the correlation energy J to be observed in the susceptibility measurement must be of the order of the one observed for the x= 0 sample; that is 1400-
1700 K from the neutron and Raman experiments [ 5-7 1. Therefore it is natural that we could not observe the susceptibility peak up to 1200 K as described above. With much more doping of holes, the insulator-to-metal transition occurs in the hole system at x=0.08, and the holes can itinerate on the CuOz sheets. Then the susceptibility-peak temperature means no more the nearest-neighbor interaction for an antiferromagnetic insulator but expresses a well-defined effective-correlation energy, Jcn. The occurrence of the metal-insulator transition at x= 0.08 can be conlirmed from the bulk conductivity of holes in the LX0 as shown in fig. 1 (b) of ref. [ 12 ] ; the conductivity at 300 K becomes sizable at about x=0.08 and increases linearly with x. It is noted that, since the resistivity of the LSCO is linear to the temperature up to 1000 K [ 191, the metalinsulator transition at x=0.08 estimated from the 300 K conductivity can be scaled to the high temperature where the susceptibility peak was observed. In the heavily doped samples (x20.09), Jeff seems to decrease linearly with increasing x as shown in fig. 4. When the doped holes can itinerate from x= 0.09, the superconductivity is supposed to be observed from that concentration. In the previous work, the superconductivity was observed from x= 0.06 in the LSCO, but the fractional volume of the superconductor became bulk scale from x=0.09 [ 141. This transformation can be seen in the other results [ 15 1. Thus the superconductivity observed hitherto below x= 0.08 is considered to be not the bulk one but the filamentary one, that may be observed from the inhomogeneity of Sr. It has been shown that the substitution of non-spin element Zn for Cu destroys the superconductivity as quickly as 3% replacement [20]. In such systems, the NCel state was recently studied by Chakraborty et al. for the x= 0 sample [ 2 11. On the other hand, the substitution effect of spin-element Ni for Cu was investigated by Fujishita et al. [ 221 in the LSCO. In order to clarify the role of spin fluctuations in the appearance of superconductivity, we studied the Znsubstitution effect for Cu in the vicinity of the transition in superconductor-to-normal-metal Laz_,SrXCu, _,Zn,O, (LSCZO). We have observed the susceptibility peak of the normal state for the heavily doped samples with x=0.8 or 0.20. The ob-
R. Yoshizakiet al. /Magnetic susceptibilityof Laa,_.,SrxCuOd
tamed dependence of T,,, and T, on the Zn concentration y is shown in figs. 5 (a) and 5 (b), respectively. As seen in the figures, the correlation between T,, versus y and T, versus y holds sufficiently for both the samples; T,, and T, decrease with increasing y, and they seemed to approach zero at the same y. This fact indicates that T,,, is strongly cor-
400
I
I
a
La*&SrxCul.YZn,Or
0
0 0
I 0.0 Zn
0
P
1
0.02
CONCENTRATION
003 y
I
40
b
e
La&rxCu,.tZnyOL
I
t
30
b
x=0.18
0
x=0.20
0
Y
-1
0
l
20
0
0
Zn
CONCENTRATION
y
Fig. 5. The dependence of T,, (a) and r, (b) on the Zn concentration y for the samples with constant hole density x= 0.18 or 0.20 in the LSCZO.
421
related to T,, and the result is consistent with that of the Ni-substituted LSCO system [22]. The correlation is excellent in particular for the x= 0.18 sample. We believe the observed T, for the x=0.20 sample is slightly affected by the oxygen deficiency or the inhomogeneity of oxygen content. Contrary to the T,,, versus T, correlation, when we take x as a variable, T,,, seemed to approach zero before T, became zero in the Zn-free system of the LSCO. The inhomogeneity of hole densities must be taken into account for the more definitive discussion. However, the present experiments exhibit that the superconductivity (T,) correlated strongly to the spin correlation (Jeff) rather than to the hole density (x) near the superconductor-to-normal-metal transition. To understand those results, we have assumed the simple situation that only the spins at the Cu sites which were replaced by Zn atoms became zero under the constant hole density. Then, the fact that 30% decreases of Jeff by 1% destruction of spins suggests that the spin-spin correlation observed by the susceptibility peak is probably due to widely spread interactions, extending over about 30 spins. This estimation is consistent with the result obtained by the neutron scattering experiment. The spin-con-elation length 1 was expressed by 1=3.8/~‘/~ in A units. Following the relation, we obtain 1~9 8, for x=0.18, which covers about twenty Cu sites. It is noted, moreover, that the spin correlation length is of the order of the coherence length of the superconductivity in the Cu02 plane. Remember that the appearance of Jeff due to the itinerant holes corresponded to the appearance of the superconducting phase and that the disappearance of Jeff corresponded to the disappearance of the superconductivity, and the antiferromagnetic spin correlation has relevance to the appearance of the superconductivity. Thus, the present results give us direct evidence that the spin-correlating system accompanied by the itinerant holes is closely related to the appearance of the superconducting phase.
4. Summary We have measured the magnetic-susceptibility peak of the normal state in Laz_SrXCu04 (Osx$O.20) and Laz_&,Cul _,Zn,,O, (y= 0 to 0.03 for x= 0.18
422
R. Yoshizaki et al. /Magnetic susceptibility of La,_$-xCuO,
or 0.20). In the LSCO, we found the susceptibility peak-temperature dropped from the temperature above 1200 to 700 K at x=0.08 to 0.09. This result was explained by a model that the itinerant holes make the nearest-neighbor spin-correlation energy J small compared to the effective one, Jefl. The spread of the spin correlation for Jeff was estimated from a similar experiment with varying spin density by substituting Zn for Cu. We found in the LSCZO that the disappearance of Jeff correlated with the disappearance of the superconductivity for the samples near the superconductor-to-normal-metal transition. Those facts indicate directly that the spin correlation plays an important role for the appearance of the high-T, superconductivity.
Acknowledgements The authors thank H. Ikeda, N. Kuroda and their colleagues for technical assistance and supporting the experiment. One of the authors (R.Y. ) expresses his gratitude to M. Inoue and K. Ueda for the occasional valuable discussions and the encouragements. This work is partly supported by Grant-in-Aid for Scientific Research on Priority Areas, “Mechanisms of Superconductivity”, from the Ministry of Education, Science and Culture.
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