Neutron scattering study of incommensurate elastic magnetic peaks in La1.88Sr0.12CuO4

PCS 1681

Journal of Physics and Chemistry of Solids 60 (1999) 1067–1070

Neutron scattering study of incommensurate elastic magnetic peaks in La1.88Sr0.12CuO4 H. Kimura a,*, H. Matsushita a, K. Hirota a, Y. Endoh a, K. Yamada b, G. Shirane c, Y.S. Lee d, M.A. Kastner d, R.J. Birgeneau d a Department of Physics, Tohoku University, Sendai 980-8578, Japan Institute for Chemical Research, Kyoto University, Gokasho, Uji 610-0011, Japan c Department of Physics, Brookhaven National Laboratory, Upton, NY 11973, USA d Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA b

Abstract Elastic magnetic peaks around (p ,p ) have been studied in La22xSrxCuO4 (LSCO) by neutron scattering. Recently in superconducting LSCO of x ˆ 0.12, we found that the incommensurate elastic peak (IC peak) appears around 31 K which is very close to Tc. The line width reaches almost resolution limit, which indicates that the static magnetic correlation length exceeds ˚ 21 isotropically in the CuO2 planes. Furthermore recently in the course of studying the IC peak, we found the new 200 A experimental condition where the signals can be observed more clearly. The signal-to-noise ratio (S/N) is much more improved (,1.4) than that at the previous experiment (,0.6). q 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Superconductors; C. Neutron scattering; D. Magnetic properties; D. Superconductivity

Recent discoveries of the incommensurate elastic peak (IC peak) in La22xSrxCuO4 is now providing a new-key feature in high-Tc superconductivity. As for the nonsuperconducting La-cuprates, early elastic measurements on neutron scattering in La2CuO41d [1,2] and La1.96Sr0.04CuO4 [3] reported important phenomena at low temperature. In both cases, commensurate elastic peaks located at (p ,p ) were observed below 40 K, where intensities increase progressively with decreasing temperature. However, IC peaks were first discovered in La1.62xNd0.4SrxCuO4 (LNSCO) with x ˆ 0.12, 0.15, and 0.20 [4,5]. The appearing temperatures of the IC peaks which we defined as T* were determined to be ,50 K, 46 K, and 15 K respectively. Tranquada and coworkers [4,5] speculate that the dynamical spin correlation is pinned by the in-plane-oxygen displacement due to structural phase transition from LTO (low temperature orthorhombic) to LTT (low temperature tetragonal) phase, which corresponds to the emergence of the elastic peak. In fact, each T* is not close to Tc (,4 K,11 K,15 K,

* Corresponding author.

respectively) but very close to the structural-phasetransition temperature Ts. Recently, Suzuki et al. [6] obtained evidence for the incommensurate magnetic order at low temperature in superconducting La1.88Sr0.12CuO4. They estimated the T* of x ˆ 0.12 to be 45 K and found that T* is almost identical to the temperature where the structural instability starts accelerating, which corresponds to a sign of the LTO to LTT structural transition. Therefore they claimed that the magnetic order may be originated from the microscopic LTT phase or the structural instability which exists essentially around x ˆ 1=8: However the estimated T* still has uncertainty because no true elastic neutron scattering measurements were carried out by them. Clearly the discoveries of static antiferromagnetic correlations in the superconducting LSCO which has no macroscopic LTT phase is very important to clarify the essential features for the static correlations in the superconducting state. In this paper, we report the summary of elastic neutron scattering studies on La1.88Sr0.12Cu12yZnyO4 for superconducting y ˆ 0.00 and nonsuperconducting y ˆ 0.03 using the cold neutron source, which is previously reported by Kimura et al. [7]. Furthermore we also report more

0022-3697/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S0022-369 7(99)00049-9

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Fig. 1. Temperature dependence of the peak intensity at Q ˆ ((1/2) 2 e 1/2 0) for the Zn-free (y ˆ 0.00) and the Zn-substituted (y ˆ 0.03) LSCO. Plots for y ˆ 0.00 and y ˆ 0.03 with filled circles and filled triangles are shown in (a) and (b), respectively. Short dashed lines in each figure indicate estimated backgrounds. Long dashed lines are guides to the eye. The insets in each figure are scans, through the peak position Q, below and above T*. The data of (a) and (b) were obtained with HER and SPINS spectrometers, respectively.

improved studies of the elastic peak of y ˆ 0.00 using the thermal neutron source. The sample preparations for y ˆ 0.00 and y ˆ 0.03 and characterizations, including Tc, Ts, etc., are described in detail elsewhere [7–9]. Throughout this paper, we use reciprocal lattice units for the high-temperature tetragonal (I4/ mmm) coordinate system, where the two short axes are defined as the distance between the nearest-neighbor Cu atoms along the in-plane Cu–O bond. In this notation, the zone boundary point (p ,p ) corresponds to …1=2 1=2 0†: We carried out the true elastic neutron scattering in y ˆ 0.00 and y ˆ 0.03 using HER in JAERI and SPINS in NIST triple-axis-spectrometers with the cold neutron beam, which can derive the high resolution in not only momentum but also energy spaces. Detailed experimental conditions are described elsewhere [7]. The IC peak in y ˆ 0.00 was clearly observed at four positions of ……1=2† 2 1 1=2 0†; …1=2 …1=2† 2 1 0†: The incommensurability e is essentially identical to that of the corresponding

dynamical spin fluctuations in La22xSrxCuO4 with the same x [8]. The linewidth is very close to the resolution limit and the minute analyses convoluted with the instrumental resolution give the intrinsic linewidth k in the momentum space, which is estimated to be less than ˚ 21. Then the correlation length j which corresponds 0.005 A ˚. to the inverse of k can be determined to be more than 200 A Furthermore we carried out the scans along several q-directions about one IC peak in order to study static correlations in the CuO2 plane. Then we found that the line width in any direction reaches almost resolution limit, therefore the static antiferromagnetic correlation extends at quite long-range and is almost isotropic in CuO2 planes. More surprisingly, as shown in Fig. 1(a), the T* of y ˆ 0.00 coincides with the onset of Tc to within the statistical errors. In addition to Znfree x ˆ 0.12, we tried to measure the IC peak for superconducting LSCO with x ˆ 0.10 and x ˆ 0.15. Weak elastic peaks were observed for x ˆ 0.10 with lower T* than that of x ˆ 0.12, while for x ˆ 0.15 any elastic scattering is below the detectable limit. As for the nonsuperconducting y ˆ 0.03 sample, elastic peaks were also observed. However, the linewidth is broader than that of y ˆ 0.00 as clearly shown in the insets of Fig. 1. Furthermore Fig. 1 indicates that the T* of y ˆ 0.03 is lower than that of y ˆ 0.00. In this case, the Zn-substitution on x ˆ 0.12 degrades the antiferromagnetic order. We discuss these surprising experimental results below. First, we found the coincidence of the T* with the onset of Tc in x ˆ 0.12. With this result, it might be speculated that the static antiferromagnetic correlations and superconducting states are not in competition but in cooperation with each other. Lee et al. recently observed in superconducting La2CuO41d [10] that the IC peak starts appearing around 40 K which is almost identical to Tc of this material. This result strongly supports the above speculation. Secondly, we found that x ˆ 0.12 remains in the LTO phase down to the lowest temperature and could estimate the volume fraction of the LTT phase as an upper limit, which must be present in concentrations below 1% level. Furthermore, La2CuO41d also remains in the LTO phase. Therefore these imply that the incommensurate magnetic ordering in the superconducting state is an intrinsic feature which does not depend on a specific microscopic structure. We can further speculate that the magnetic order does not exist locally but is uniform in the CuO2 plane and might coexist with the superconductivity. From this point of view, the exact calculation of the magnetic moment for the Cu 21 atom in the superconducting state is very important. The studies using local probe such as mSR [11] and NMR [12] measurements reported that the estimated magnetic moments are about 0.08m B/Cu 21 and 0.3m B/Cu 21 respectively. Furthermore we previously estimated [7] the moment to be 0.1m B/Cu 21 whose value is overlapped with the local probe measurements. However, this value had large errors since there were many ambiguities, such as an extinction effect of nuclear Bragg scattering

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JAERI. It is clearly seen that the signal-to-noise ratio (S/N) measured at TOPAN is much more improved than that at HER, which is in excess of the factor of two. Furthermore we noticed that the IC peaks are not exactly on the [100] or [010] axis, but shift away from those axes. Similar behavior has been already discovered by Lee and his collaborators [10] on La2CuO41d . Further minute studies on this anomalous behavior are required. We are now planning the experiment to determine the accurate position of the IC peaks using new experimental conditions which improve the S/N for the IC peak. Acknowledgements

Fig. 2. Peak profiles of the elastic peaks along the h-direction at low temperature for the superconducting La1.88Sr0.12CuO4. Each scan was performed at (a) HER with the cold neutron beam (Ei ˆ 5 meV) and at (b) TOPAN with the thermal neutron beam (Ei ˆ 14.7 meV). Detailed experimental conditions are shown at the top of each figure.

intensity, and static magnetic correlations between the CuO2 planes. Therefore, in order to estimate the Cu 21 moment more precisely, we collected the integrated intensities of as many Bragg reflections as possible using the thermal neutron source and tried to obtain the extinction curve for the x ˆ 0.12 single crystal. Furthermore we measured the integrated intensity of the IC peak in identical experimental conditions with the measurements of Bragg intensities. As a result, we could obtain the Cu 21 moment as 0.04 ^ 0.01m B/ Cu 21. However, it should be noted that the moment was estimated as a lower limit, and therefore we expect the proper value to be much larger. As far as the out-of-plane correlations are concerned, experiments are being planned and will be carried out in the near future. The detailed results of the above experiments will be published elsewhere. In the course of the neutron scattering study using a thermal neutron source, we found a new experimental condition for the IC peak where the signals can be observed more clearly. Fig. 2 shows the peak profile of the IC peak at Q ˆ ((1/2) 2 e 1/2 0), which were measured at (a) HER (cold neutron) and at (b) TOPAN (thermal neutron) in

We thank V. J. Emery, H. Fukuyama, S. Wakimoto, K. Kurahashi, T. Suzuki, and J. M. Tranquada for helpful discussions. We also acknowledge M. Onodera and K. Nemoto for their technical support on the neutron scattering experiments at JAERI. This work was supported in part by a Grant-In-Aid for Scientific Research from the Japanese Ministry of Education, Science, Sports and Culture, by a Grant for the Promotion of Science from the Science and Technology Agency and by CREST. The US–Japan cooperative research program also provided support for the neutron scattering experiment at NIST. Work at Brookhaven National Laboratory was carried out under Contract No. DE-AC02-98-CH10886, Division of Material Science, US Department of Energy. The research at MIT was supported by the National Science Foundation under Grant No. DMR97-04532 and by the MRSEC Program of the National Science Foundation under Award No. DMR94-00334. References [1] Y. Endoh, K. Yamada, R.J. Birgeneau, D.R. Gabbe, H.P. Jenssen, M.A. Kastner, C.J. Peters, P.J. Picone, T.R. Thurston, J.M. Tranquada, G. Shirane, Y. Hidaka, M. Oda, Y. Enomoto, M. Suzuki, T. Murakami, Phys. Rev. B 37 (1988) 7443. [2] C.J. Peters, R.J. Birgeneau, M.A. Kastner, H. Yoshizawa, J.M. Tranquada, G. Shirane, Y. Hidaka, M. Oda, M. Suzuki, T. Murakami, Phys. Rev. B 37 (1988) 9761. [3] B. Keimer, N. Belk, R.J. Birgeneau, A. Cassanho, C.Y. Chen, M. Greven, M.A. Kastner, A. Aharony, Y. Endoh, R.W. Erwin, G. Shirane, Phys. Rev. B 46 (1992) 14034. [4] J.M. Tranquada, J.D. Axe, N. Ichikawa, Y. Nakamura, S. Uchida, B. Nachumi, Phys. Rev. B 54 (1996) 7489. [5] J.M. Tranquada, J.D. Axe, N. Ichikawa, A.R. Moodenbaugh, Y. Nakamura, S. Uchida, B. Nachumi, Phys. Rev. Lett. 78 (1997) 338. [6] T. Suzuki, T. Goto, K. Chiba, T. Shinoda, T. Fukase, H. Kimura, K. Yamada, M. Ohashi, Y. Yamaguchi, Phys. Rev. B 57 (1998) R3229. [7] H. Kimura, K. Hirota, H. Matsushita, K. Yamada, Y. Endoh, S.-H. Lee, C.F. Majkrzak, R. Erwin, G. Shirane, M. Greven, Y.S. Lee, M.A. Kastner, R.J. Birgeneau, Phys. Rev. B., submitted for publication. [8] K. Yamada, C.-H. Lee, K. Kurahashi, J. Wada, S. Wakimoto,

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S. Ueki, H. Kimura, Y. Endoh, S. Hosoya, G. Shirane, R.J. Birgeneau, M. Greven, M.A. Kastner, Y.J. Kim, Phys. Rev. B 57 (1998) 6165. [9] S. Hosoya, C.-H. Lee, S. Wakimoto, K. Yamada, Y. Endoh, Physica C 235–240 (1994) 547.

[10] Y.S. Lee et al., unpublished. [11] K. Kumagai, K. Kawano, I. Watanabe, K. Nishiyama, K. Nagamine, J. Supercond. 7 (1994) 63. [12] T. Goto, S. Kazama, K. Miyagawa, T. Fukase, J. Phys. Soc. Jpn. 63 (1994) 3494.