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Neutron scattering study on electron–hole doping symmetry of high-Tc superconductivity
PCS 1674
Journal of Physics and Chemistry of Solids 60 (1999) 1025–1030
Neutron scattering study on electron–hole doping symmetry of high-Tc superconductivity K. Yamada a,*, K. Kurahashi b, Y. Endoh b, R.J. Birgeneau c, G. Shirane d a Institute for Chemical Research, Kyoto University, Uji 611-0011, Japan Department of Physics Tohoku University, Aramaki Aoba, Sendai 980-77, Japan c Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA d Department of Physics, Brookhaven National Laboratory, Upton, NY 11973, USA b
Abstract We review recent results of neutron scattering experiment on spin correlations for both hole-doped La22xSrxCuO41y and electron-doped Nd1.85Ce0.15CuO41y superconductors. For the former, hole-doping induces an incommensurate spin density modulation along the Cu–O bonds. For the latter, we observed well-defined spin fluctuations in the superconducting phase (Tc < 18 K) for the first time. In contrast to the hole-doping, the electron-doping induces no well-defined incommensurate spin modulation; the magnetic signal appears at (p, p) with a q-width broader than in the as-grown antiferromagnetic phase. For both systems a static magnetic order was observed in the superconducting state. q 1999 Published by Elsevier Science Ltd. All rights reserved. Keywords: A. Magnetic materials; A. Superconductors; C. Neutron scattering; D. Superconductivity; D. Spin-density waves
1. Introduction Soon after the discovery of hole-doped high-Tc superconductor La22xBaxCuO4 by Bednortz and Mu¨ller [1], Tokura and his coworkers synthesized an electron-doped superconductor Nd22xCexCuO4 [2]. The crystal structures of both systems are similar; CuO2 planes commonly exist between the so-called block layers with rare-earth ions. Non-doped insulating phases of both systems also show a similar quasi two-dimensional antiferromagnetism. Moreover, as shown in Fig. 1, the phase diagram is approximately symmetric as a function of doping-rate for two types of carrier, electron and hole. Therefore, although the mechanism of high-Tc superconductivity is not fully understood at present, existence of a common mechanism of superconductivity for both systems has been widely believed. In order to elucidate such a mechanism the interplay between the magnetic fluctuations and the superconductivity is one of the central issues. However, a large number of important measurements have been missing for the electron-doped system possibly due to the difficulty in growing single crystal and in * Corresponding author.
preparing the superconducting sample by the post-growth heat treatment. Hence many fundamental properties on the superconductivity such as pairing symmetry and superconducting gap are not understood. Another experimental disadvantage in electron system is the large magnetic moments on rare-earth ions which prohibit many techniques from studying the magnetism on CuO2 planes. However, neutron scattering is one of the unique techniques to distinguish signals from Cu and rare-earth ions due to the difference of the structure factor. However, for the hole system, comprehensive studies have been performed and revealed an indispensable role of magnetic fluctuations for the superconductivity. Since the doping-rate can be tuned finely over a wide doping region, Sr-doped 2–1–4 system is one of the typical system for studying the interplay between the magnetic fluctuations and the superconductivity. For example, a spatial spin density modulation along the Cu–O bonds was observed to appear beyond the lower critical doping of superconductivity and rapidly degrade when approaching the normal metallic region [3]. Quite recently, the incommensurate spin fluctuations re-attract remarkable attention triggered by neutron scattering work on Y1–2–3 and Bi2–2–1–2 systems [4] which
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Fig. 1. A schematic phase diagram of both hole-doped La22xSrxCuO4 and electron-doped Nd22xCexCuO4. Details are not clarified near the boundary between the antiferromagnetic and the superconducting phases for the electron-doped system.
observed the same incommensurate spin fluctuations as those of Sr-doped or oxygen-doped La2–1–4 system. The spatial symmetry as well as the doping dependence of periodicity of spin modulation are found to be almost identical among these different systems. Therefore, the incommensurate spin modulation is concluded to be one of the inherent magnetic response to the hole-doping in high-Tc cuprates. Before the present study, Kojima et al. have succeeded in growing large single crystal of Nd22xCexCuO4 by using a traveling-solvent-floating-zone (TSFZ) method and preparing the superconducting sample with Tc < 23 K [5,6]. Then neutron scattering measurements have been performed in our group using their single crystals. Although well-defined magnetic excitation was observed in the antiferromagnetic phase, no well-defined signal was found in the heat-treated superconducting phase of the same crystal [7]. If the magnetic fluctuations play an indispensable role for the superconductivity, the missing of magnetic signal in the electron-doped superconducting phase causes a serious difficulty for the common superconducting mechanism in both systems. Then we restarted comprehensive work on single crystal growth, heat treatment and neutron scattering for Nd1.85Ce0.15CuO4 to search for magnetic fluctuations in the superconducting phase. Finally, we found well-defined magnetic fluctuations in the superconducting sample of Nd1.85Ce0.15CuO4. In this report, we introduce a part of neutron scattering results on Nd1.85Ce0.15CuO4 and compare the magnetic fluctuations for both hole and electron systems. One of the important issues is the spatial correlation of the spin fluctuations. The other is the coexistence of the long-range magnetic order with the superconducting phase which is recently discussed for hole-doped 2–1–4 cuprates around x 1/8 [8,9]. Magnetic long-range order is also observed in the electron-doped superconducting phase. However, it has been believed to be macroscopically phase-separated with the superconducting phase due to the insufficient heat treatment.
2. Experimental results We have been growing sizable single crystals of La22xSrxCuO4 and Nd1.85Ce0.15CuO4 by using the TSFZ method. Details of the crystal growth and the characterization of the grown single crystals are described in Refs. [10,3] for hole-doped system, and in Ref. [11] for the electron-doped one. Further, results of neutron scattering experiments on La22xSrxCuO4 are described in separate papers [3,9,12,13]. Therefore, here we only review the fundamentals of electron-doped system and present experimental details and results for Nd1.85Ce0.15CuO4. For the electron-doped system, even the doping mechanism is not fully understood. The as-grown crystal is an antiferromagnetic insulator with the Ne´el temperature TN of around 160–125 K which probably depends on the oxygen partial pressure during the crystal growth. Superconductivity appears by heat treatment under reduced atmosphere. It is believed that in the as-grown crystal there exist excess oxygen ions which occupy a part of apical oxygen sites in the so-called T-type structure such as La2CuO4. Then the annealing is considered to play a role to remove the excess oxygen ions. For the present crystal bulk superconductivity appears at Tc < 18 K which is lower compared with 23 K in the previously grown Nd1.85Ce0.15CuO4. Quite recently, we prepared a sample with Tc < 23 K annealed under a different condition. However at present, we carried out neutron scattering experiments only on the sample with Tc < 18 K. Neutron scattering experiments have been performed by using the thermal neutron three-axis spectrometer TOPAN installed in JRR-3M in JAERI. The measurements were done for both the antiferromagnetic (AF) insulating and the superconducting (SC) phases in [h,k,0] zone. In order to reduce the absorption of neutron beam in the crystal a long rod of single crystal was cut into three rods, which were stacked vertically. For the AF sample, we essentially reconfirmed the results
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Fig. 2. Scans through (1/2,1/2,0) at 3 meV for the as-grown antiferromagnet ic Nd1.85Ce0.15CuO4. Inset shows peak intensity of magnetic Bragg (3/2,1/2,0) as a function of temperature.
by the previous measurements in [h,h,l] zone [7]. As shown in Fig. 2, a sharp commensurate magnetic peak was observed at (p,p) or (1/2,1/2,0) in the I4/mmm tetragonal notation. We monitored (3/2,1/2,0) magnetic reflection to study the long-range antiferromagnetic order. Thus we determined TN to be t140 K. At a low temperature below around 10 K the intensity of (3/2,1/2,0) reflection starts to increase rapidly due to the participation of Nd spins in the magnetic order. Even in the SC phase, we observed a well-defined peak at (3/2,1/2,0) though the intensity is weaker by factor about 10
Fig. 3. Scans through (1/2,1/2,0) at 3 meV for the reduced superconducting Nd1.85Ce0.15CuO4. Inset shows temperature dependence of diamagnetic susceptibility in the zero-field-cooled superconducting sample.
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than that in the AF phase of as-grown sample. The q-width of the elastic peak at (3/2,1/2,0) is resolution limited for both phases. Due to the smooth increase of the magnetic intensity upon cooling it is difficult to determine TN in the superconducting phase but it is obviously lower than that of the as-grown sample. In contrast to the previous work, as shown in Fig. 3 we newly observed a commensurate inelastic peak at (p,p) for the SC sample. Compared with Fig. 2, the q-width of the peak is substantially broader than that of the AF phase. Furthermore, the q-integrated peak intensities are comparable each other except at low temperatures. If the observed commensurate peak originates from the phase-separated or residual AF phase with the reduced volume and no magnetic intensity exists in the SC phase as in the previous work, it is very unlikely to observe comparable magnetic intensity as in the as-grown AF phase. Therefore, the commensurate peak is considered to be associated with the SC phase. As shown in Fig. 4, the temperature dependence of the spin fluctuations is quite different between two phases. According to the previous results, the dynamical susceptibility x 00 (Q,v ) around (p,p) for AF phase exhibits a broad peak around TN [7]. The present result of AF phase is consistent
Fig. 4. Temperature dependence of x 00 (Q,v ) at Q (1/2,1/2,0). Data points shown by open (closed) symbols were obtained from antiferromagnetic (superconducting) phase. Closed squares were obtained from the peak intensity at (1/2,1/2,0) by assuming a constant q-width. Other points were obtained from the q-spectrum.
with the previous one. For the SC phase however, at 3 meV there exists a clear drop in x 00 (Q,v ) upon cooling below T < 10 K slightly lower than Tc. However, at 4.5 meV such a drop disappears. These results suggests an energy-gap of around 4 meV in the superconducting spin fluctuations. It is noted that for the hole-doped La22xSrxCuO4 a similar temperature dependence in x 00 (Q,v ) is also observed but the peak locates around Tc. The reason for the peak position lower than Tc in Nd1.85Ce0.15CuO4 may be interpreted by the smaller size of the energy-gap compared to the measured energy. In fact, a preliminary measurement on the same crystal at 1 meV reveals a peak near Tc.
3. Discussion Concerning the missing of magnetic signal in the previous neutron scattering measurement, several differences appear between the present and previous measurements. (1) Scattered neutron intensity is higher by factor about 5 in the present work due to the larger crystal volume and smaller attenuation of neutron beam. (2) The measurements were done at different zones. (3) Superconducting temperature is lower for the present crystal. The previous sample shows the superconductivity at Tc < 23 K higher than the present sample, Tc < 18 K. Relating with the last point, we recently found at least three different superconducting phases in Nd1.85Ce0.15CuO41y with different Tc: phase A; Tc < 18 K, phase B; Tc < 20 K and phase C; Tc < 23 K. We observed spin fluctuations probably in phase A and previous measurement was done in phase C. In these three phases, the amount of excess oxygen and/or types of oxygen order should be different. However, no well-defined oxygen order is so far observed. If the spatial coherency of the spin fluctuations couples with the carrier concentration, the missing of spin fluctuations in the previous measurement can be attributed to the substantial peak-broadening due to the higher dopingrate. In fact, even in the present sample the q-width of the SC phase is broader than in the AF phase suggesting the peak-broadening by electron-doping. We note such degradation of the spatial coherence of spin fluctuations occurs in the overdoped region for hole-doped case [3]. The most prominent contrast in the spin correlation between the hole- and electron-doped system is the spatial spin modulation. For the former, incommensurate spin modulation occurs in the superconducting phase. For the latter on the other hand, spatial spin modulation is the same as the antiferromagnetic insulator except the difference in the peak-width. What causes such difference in the spatial spin modulation for both systems; commensurate or incommensurate? We predict the difference in the orbital or the site character between the doped hole and electron is one of the possible candidates. Most of holes are well known to go into the 2pxy orbital of in-plane oxygen ions. However, doped-electrons predominantly occupy 3dx2 2y2 orbitals of Cu ions. For the former, frustrated magnetic interactions
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between the neighboring Cu spins may modify the spatial correlation over a wide region around the holes and induce incommensurate spatial correlation beyond a critical doping. The stripe model is one of the model to describe the origin of the incommensurate spatial correlation where the doped holes segregate microscopically into the periodic striped order [14]. However, for the latter, only the Cu-spins near the doped site are ‘‘diluted’’ by nonmagnetic ions with 3d 10 configuration. Therefore, the effect of doping on the magnetic correlation is more moderate compared to the case of hole doping. Such an interpretation will be supported by the weak doping dependence of TN as shown in Fig. 1. Finally, we discuss coexistence of long-range magnetic order for both hole and electron systems. Quite recently, a static long-range magnetic order was observed in the superconducting state of hole-doped La2–1–4 system around 1/8 doping. Tranquada et al. [8] firstly argued the possible coexistence of the incommensurate magnetic order and the superconductivity. They also showed a competitive relation between the static magnetic order and the superconductivity. Neutron scattering on the superconducting Nd-free La2–1–4 system has conducted by Suzuki et al. on La1.88Sr0.12CuO4 [9]. They found sharp incommensurate peaks at the same positions around (p,p) as those of the dynamical spin fluctuations. In contrast to the Nd-doped system the onset temperature of the peak almost coincides with Tc. Similar coincidence of both temperatures was also observed in oxygen-staged La2CuO41y [15]. Recent neutron scattering study [12] as well as ultrasonic study [16] predict a microscopic coexistence of the incommensurate magnetic order with the bulk superconductivity. Is the magnetic order observed in the electron-doped superconducting state coupled or decoupled with the superconductivity? As shown in the previous work [7], insufficient heat treatment definitely causes a macroscopic phaseseparation. However, there is no experimental evidence to reject the microscopic coexistence for both states, which appear at similar temperature region. Moreover, there exist some experimental facts, which cannot be interpreted by simple macroscopic phase-separation. For example, the observed sharp superconducting transition and the broad Ne´el transition are difficult to understand. It is hard to expect that the doping is done inhomogeneously in the AF phase but homogeneously in the SC phase. In the case of oxygendoped La2CuO41y where the macroscopic phase-separation into the hole-poor AF phase and the hole-rich SC phase occurs, there both transition temperatures are well defined [17]. Higher q-resolution measurement is required to distinguish the additional sharp peak of AF phase on the broad peak of SC phase in the case of macroscopic phaseseparation. In summary, we finally proved by neutron scattering the coexistence of dynamical spin fluctuations with high-Tc superconductivity irrespective of types of doped carriers, hole or electron. Then it is worth searching for a common mechanism of the high-Tc superconductivity from a view
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point of magnetic mechanism. However, there are some different aspects between the two systems. First, no welldefined incommensurate spatial spin modulation was detected in the electron-doped Nd1.85Ce0.15CuO4. Second, in the electron-doped case, the shortening of the spatial coherence length could rise Tc which seems to be in conflict with the result of hole-doped system. More detailed neutron scattering study in the phase C (Tc < 23 K) is highly required. Concerning the coexistence of the magnetic order with the superconductivity many experiments suggest the coexistence for hole-doped system. For the electron-doped system on the other hand, there is no conclusive experimental evidence. However, it is important to search for two types of AF phases, one is phase-separated and the other is coexisting with the superconductivity. Such a situation is quite similar to the oxygen-doped La2CuO41y where the phase-separated AF phase is commensurate and the coexisting AF phase is incommensurate.
Acknowledgements The authors acknowledge K. Nemoto and M. Onodera for their technical assistance at JAERI and Tohoku University. We wish to thank Y. Kojima, I. Tanaka, S. Hosoya, K. Hirota, S. Wakimoto, H. Kimura, T. Suzuki, T. Fukase, M. Greven, M.A. Kastner, Y.J. Kim, Y.S. Lee, S. H. Lee for their valuable discussions. The present work in part was supported by the US–Japan Cooperative Research Program on Neutron Scattering and a Grant-In-Aid for Scientific Research from the Ministry of Education, Science, Culture and Sports of Japan and by a Grant for the Promotion of Science from the Science and Technology Agency and by CREST. Work at Brookhaven National Laboratory was carried out under Contract No. DE-AC02-98-CH10886, Division of Material Science, U.S. Department of Energy. The research at Massachusetts Institute of Technology was supported by the National Science Foundation under Grant No. DMR97-04532 and by MRSEC Program of the National Science Foundation under Award No. DMR94-00334.
References [1] J.G. Bednorz, K.A. Mu¨ller, Z. Phys. B64 (1986) 189. [2] Y. Tokura, H. Takagi, S. Uchida, Nature (London) 337 (1989) 345. [3] K. Yamada, C.H. Lee, K. Kurahashi, J. Wada, Y. Kimura, S. Ueki, Y. Endoh, S. Hosoya, G. Shirane, R.J. Birgeneau, M. Greven, M.A. Kastner, Y.J. Kim, Phys. Rev. B57 (1998) 6165. [4] H.A. Mook, P.Dai, S.M. Hayden, A. Aeppli, T.G. Perring, F. Dogan, submitted for publication. [5] I. Tanaka, H. Kojima, Nature (London) 337 (1989) 345. [6] Y. Inoue, Master Thesis (in Japanese), Yamanashi University, 1994. [7] M. Matsuda, Y. Endoh, K. Yamada, H. Kojima, I. Tanaka, R.J.
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[10]
[11]
K. Yamada et al. / Journal of Physics and Chemistry of Solids 60 (1999) 1025–1030 Birgeneau, M.A. Kastner, G. Shirane, Phys. Rev. B45 (1992) 12548. J.M. Tranquada, J.D. Axe, N. Ichikawa, A.R. Moodenbaugh, Y. Nakamura, S. Uchida, Phys. Rev. Lett. 78 (1997) 338. T. Suzuki, T. Goto, T. Shinoda, T. Fukase, H. Kimura, K. Yamada, M. Ohashi, Y. Yamaguchi, Phys. Rev. B 57 (1998) R3229. C.H. Lee, N. Kaneko, S. Hosoya, K. Krahashi, S. Wakimoto, K. Yamada, Y. Endoh, Supercond. Sci. Technol. 11 (1998) 891. K. Kurahashi, PhD Thesis (in Japanese), Tohoku University, 1999.
[12] H. Kimura, K. Hirota, K. Yamada, Y. Endoh, S.H. Lee, C.H. Majkrzak, R. Erwin, G. Shirane, M. Greven, Y.S. Lee, M.A. Kastner, R.J. Birgeneau, submitted for publication to Phys. Rev. B. [13] C.H. Lee, K. Yamada, Y. Endoh, G. Shirane, R.J. Birgeneau, M. Greven, Y.J. Kim, in preparation. [14] J.M. Tranquada, B.J. Sternlieb, J.D. Axe, Y. Nakamura, S. Uchida, Nature (London) 375 (1995) 561. [15] Y.S. Lee et al., in preparation. [16] T. Suzuki, T. Fukase et al., unpublished data. [17] S. Wakimoto, K. Yamada, Y. Endoh, S. Hosoya, J. Phys. Soc. Jpn. 65 (1996) 58.
Journal of Physics and Chemistry of Solids 60 (1999) 1025–1030
Neutron scattering study on electron–hole doping symmetry of high-Tc superconductivity K. Yamada a,*, K. Kurahashi b, Y. Endoh b, R.J. Birgeneau c, G. Shirane d a Institute for Chemical Research, Kyoto University, Uji 611-0011, Japan Department of Physics Tohoku University, Aramaki Aoba, Sendai 980-77, Japan c Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA d Department of Physics, Brookhaven National Laboratory, Upton, NY 11973, USA b
Abstract We review recent results of neutron scattering experiment on spin correlations for both hole-doped La22xSrxCuO41y and electron-doped Nd1.85Ce0.15CuO41y superconductors. For the former, hole-doping induces an incommensurate spin density modulation along the Cu–O bonds. For the latter, we observed well-defined spin fluctuations in the superconducting phase (Tc < 18 K) for the first time. In contrast to the hole-doping, the electron-doping induces no well-defined incommensurate spin modulation; the magnetic signal appears at (p, p) with a q-width broader than in the as-grown antiferromagnetic phase. For both systems a static magnetic order was observed in the superconducting state. q 1999 Published by Elsevier Science Ltd. All rights reserved. Keywords: A. Magnetic materials; A. Superconductors; C. Neutron scattering; D. Superconductivity; D. Spin-density waves
1. Introduction Soon after the discovery of hole-doped high-Tc superconductor La22xBaxCuO4 by Bednortz and Mu¨ller [1], Tokura and his coworkers synthesized an electron-doped superconductor Nd22xCexCuO4 [2]. The crystal structures of both systems are similar; CuO2 planes commonly exist between the so-called block layers with rare-earth ions. Non-doped insulating phases of both systems also show a similar quasi two-dimensional antiferromagnetism. Moreover, as shown in Fig. 1, the phase diagram is approximately symmetric as a function of doping-rate for two types of carrier, electron and hole. Therefore, although the mechanism of high-Tc superconductivity is not fully understood at present, existence of a common mechanism of superconductivity for both systems has been widely believed. In order to elucidate such a mechanism the interplay between the magnetic fluctuations and the superconductivity is one of the central issues. However, a large number of important measurements have been missing for the electron-doped system possibly due to the difficulty in growing single crystal and in * Corresponding author.
preparing the superconducting sample by the post-growth heat treatment. Hence many fundamental properties on the superconductivity such as pairing symmetry and superconducting gap are not understood. Another experimental disadvantage in electron system is the large magnetic moments on rare-earth ions which prohibit many techniques from studying the magnetism on CuO2 planes. However, neutron scattering is one of the unique techniques to distinguish signals from Cu and rare-earth ions due to the difference of the structure factor. However, for the hole system, comprehensive studies have been performed and revealed an indispensable role of magnetic fluctuations for the superconductivity. Since the doping-rate can be tuned finely over a wide doping region, Sr-doped 2–1–4 system is one of the typical system for studying the interplay between the magnetic fluctuations and the superconductivity. For example, a spatial spin density modulation along the Cu–O bonds was observed to appear beyond the lower critical doping of superconductivity and rapidly degrade when approaching the normal metallic region [3]. Quite recently, the incommensurate spin fluctuations re-attract remarkable attention triggered by neutron scattering work on Y1–2–3 and Bi2–2–1–2 systems [4] which
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Fig. 1. A schematic phase diagram of both hole-doped La22xSrxCuO4 and electron-doped Nd22xCexCuO4. Details are not clarified near the boundary between the antiferromagnetic and the superconducting phases for the electron-doped system.
observed the same incommensurate spin fluctuations as those of Sr-doped or oxygen-doped La2–1–4 system. The spatial symmetry as well as the doping dependence of periodicity of spin modulation are found to be almost identical among these different systems. Therefore, the incommensurate spin modulation is concluded to be one of the inherent magnetic response to the hole-doping in high-Tc cuprates. Before the present study, Kojima et al. have succeeded in growing large single crystal of Nd22xCexCuO4 by using a traveling-solvent-floating-zone (TSFZ) method and preparing the superconducting sample with Tc < 23 K [5,6]. Then neutron scattering measurements have been performed in our group using their single crystals. Although well-defined magnetic excitation was observed in the antiferromagnetic phase, no well-defined signal was found in the heat-treated superconducting phase of the same crystal [7]. If the magnetic fluctuations play an indispensable role for the superconductivity, the missing of magnetic signal in the electron-doped superconducting phase causes a serious difficulty for the common superconducting mechanism in both systems. Then we restarted comprehensive work on single crystal growth, heat treatment and neutron scattering for Nd1.85Ce0.15CuO4 to search for magnetic fluctuations in the superconducting phase. Finally, we found well-defined magnetic fluctuations in the superconducting sample of Nd1.85Ce0.15CuO4. In this report, we introduce a part of neutron scattering results on Nd1.85Ce0.15CuO4 and compare the magnetic fluctuations for both hole and electron systems. One of the important issues is the spatial correlation of the spin fluctuations. The other is the coexistence of the long-range magnetic order with the superconducting phase which is recently discussed for hole-doped 2–1–4 cuprates around x 1/8 [8,9]. Magnetic long-range order is also observed in the electron-doped superconducting phase. However, it has been believed to be macroscopically phase-separated with the superconducting phase due to the insufficient heat treatment.
2. Experimental results We have been growing sizable single crystals of La22xSrxCuO4 and Nd1.85Ce0.15CuO4 by using the TSFZ method. Details of the crystal growth and the characterization of the grown single crystals are described in Refs. [10,3] for hole-doped system, and in Ref. [11] for the electron-doped one. Further, results of neutron scattering experiments on La22xSrxCuO4 are described in separate papers [3,9,12,13]. Therefore, here we only review the fundamentals of electron-doped system and present experimental details and results for Nd1.85Ce0.15CuO4. For the electron-doped system, even the doping mechanism is not fully understood. The as-grown crystal is an antiferromagnetic insulator with the Ne´el temperature TN of around 160–125 K which probably depends on the oxygen partial pressure during the crystal growth. Superconductivity appears by heat treatment under reduced atmosphere. It is believed that in the as-grown crystal there exist excess oxygen ions which occupy a part of apical oxygen sites in the so-called T-type structure such as La2CuO4. Then the annealing is considered to play a role to remove the excess oxygen ions. For the present crystal bulk superconductivity appears at Tc < 18 K which is lower compared with 23 K in the previously grown Nd1.85Ce0.15CuO4. Quite recently, we prepared a sample with Tc < 23 K annealed under a different condition. However at present, we carried out neutron scattering experiments only on the sample with Tc < 18 K. Neutron scattering experiments have been performed by using the thermal neutron three-axis spectrometer TOPAN installed in JRR-3M in JAERI. The measurements were done for both the antiferromagnetic (AF) insulating and the superconducting (SC) phases in [h,k,0] zone. In order to reduce the absorption of neutron beam in the crystal a long rod of single crystal was cut into three rods, which were stacked vertically. For the AF sample, we essentially reconfirmed the results
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Fig. 2. Scans through (1/2,1/2,0) at 3 meV for the as-grown antiferromagnet ic Nd1.85Ce0.15CuO4. Inset shows peak intensity of magnetic Bragg (3/2,1/2,0) as a function of temperature.
by the previous measurements in [h,h,l] zone [7]. As shown in Fig. 2, a sharp commensurate magnetic peak was observed at (p,p) or (1/2,1/2,0) in the I4/mmm tetragonal notation. We monitored (3/2,1/2,0) magnetic reflection to study the long-range antiferromagnetic order. Thus we determined TN to be t140 K. At a low temperature below around 10 K the intensity of (3/2,1/2,0) reflection starts to increase rapidly due to the participation of Nd spins in the magnetic order. Even in the SC phase, we observed a well-defined peak at (3/2,1/2,0) though the intensity is weaker by factor about 10
Fig. 3. Scans through (1/2,1/2,0) at 3 meV for the reduced superconducting Nd1.85Ce0.15CuO4. Inset shows temperature dependence of diamagnetic susceptibility in the zero-field-cooled superconducting sample.
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than that in the AF phase of as-grown sample. The q-width of the elastic peak at (3/2,1/2,0) is resolution limited for both phases. Due to the smooth increase of the magnetic intensity upon cooling it is difficult to determine TN in the superconducting phase but it is obviously lower than that of the as-grown sample. In contrast to the previous work, as shown in Fig. 3 we newly observed a commensurate inelastic peak at (p,p) for the SC sample. Compared with Fig. 2, the q-width of the peak is substantially broader than that of the AF phase. Furthermore, the q-integrated peak intensities are comparable each other except at low temperatures. If the observed commensurate peak originates from the phase-separated or residual AF phase with the reduced volume and no magnetic intensity exists in the SC phase as in the previous work, it is very unlikely to observe comparable magnetic intensity as in the as-grown AF phase. Therefore, the commensurate peak is considered to be associated with the SC phase. As shown in Fig. 4, the temperature dependence of the spin fluctuations is quite different between two phases. According to the previous results, the dynamical susceptibility x 00 (Q,v ) around (p,p) for AF phase exhibits a broad peak around TN [7]. The present result of AF phase is consistent
Fig. 4. Temperature dependence of x 00 (Q,v ) at Q (1/2,1/2,0). Data points shown by open (closed) symbols were obtained from antiferromagnetic (superconducting) phase. Closed squares were obtained from the peak intensity at (1/2,1/2,0) by assuming a constant q-width. Other points were obtained from the q-spectrum.
with the previous one. For the SC phase however, at 3 meV there exists a clear drop in x 00 (Q,v ) upon cooling below T < 10 K slightly lower than Tc. However, at 4.5 meV such a drop disappears. These results suggests an energy-gap of around 4 meV in the superconducting spin fluctuations. It is noted that for the hole-doped La22xSrxCuO4 a similar temperature dependence in x 00 (Q,v ) is also observed but the peak locates around Tc. The reason for the peak position lower than Tc in Nd1.85Ce0.15CuO4 may be interpreted by the smaller size of the energy-gap compared to the measured energy. In fact, a preliminary measurement on the same crystal at 1 meV reveals a peak near Tc.
3. Discussion Concerning the missing of magnetic signal in the previous neutron scattering measurement, several differences appear between the present and previous measurements. (1) Scattered neutron intensity is higher by factor about 5 in the present work due to the larger crystal volume and smaller attenuation of neutron beam. (2) The measurements were done at different zones. (3) Superconducting temperature is lower for the present crystal. The previous sample shows the superconductivity at Tc < 23 K higher than the present sample, Tc < 18 K. Relating with the last point, we recently found at least three different superconducting phases in Nd1.85Ce0.15CuO41y with different Tc: phase A; Tc < 18 K, phase B; Tc < 20 K and phase C; Tc < 23 K. We observed spin fluctuations probably in phase A and previous measurement was done in phase C. In these three phases, the amount of excess oxygen and/or types of oxygen order should be different. However, no well-defined oxygen order is so far observed. If the spatial coherency of the spin fluctuations couples with the carrier concentration, the missing of spin fluctuations in the previous measurement can be attributed to the substantial peak-broadening due to the higher dopingrate. In fact, even in the present sample the q-width of the SC phase is broader than in the AF phase suggesting the peak-broadening by electron-doping. We note such degradation of the spatial coherence of spin fluctuations occurs in the overdoped region for hole-doped case [3]. The most prominent contrast in the spin correlation between the hole- and electron-doped system is the spatial spin modulation. For the former, incommensurate spin modulation occurs in the superconducting phase. For the latter on the other hand, spatial spin modulation is the same as the antiferromagnetic insulator except the difference in the peak-width. What causes such difference in the spatial spin modulation for both systems; commensurate or incommensurate? We predict the difference in the orbital or the site character between the doped hole and electron is one of the possible candidates. Most of holes are well known to go into the 2pxy orbital of in-plane oxygen ions. However, doped-electrons predominantly occupy 3dx2 2y2 orbitals of Cu ions. For the former, frustrated magnetic interactions
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between the neighboring Cu spins may modify the spatial correlation over a wide region around the holes and induce incommensurate spatial correlation beyond a critical doping. The stripe model is one of the model to describe the origin of the incommensurate spatial correlation where the doped holes segregate microscopically into the periodic striped order [14]. However, for the latter, only the Cu-spins near the doped site are ‘‘diluted’’ by nonmagnetic ions with 3d 10 configuration. Therefore, the effect of doping on the magnetic correlation is more moderate compared to the case of hole doping. Such an interpretation will be supported by the weak doping dependence of TN as shown in Fig. 1. Finally, we discuss coexistence of long-range magnetic order for both hole and electron systems. Quite recently, a static long-range magnetic order was observed in the superconducting state of hole-doped La2–1–4 system around 1/8 doping. Tranquada et al. [8] firstly argued the possible coexistence of the incommensurate magnetic order and the superconductivity. They also showed a competitive relation between the static magnetic order and the superconductivity. Neutron scattering on the superconducting Nd-free La2–1–4 system has conducted by Suzuki et al. on La1.88Sr0.12CuO4 [9]. They found sharp incommensurate peaks at the same positions around (p,p) as those of the dynamical spin fluctuations. In contrast to the Nd-doped system the onset temperature of the peak almost coincides with Tc. Similar coincidence of both temperatures was also observed in oxygen-staged La2CuO41y [15]. Recent neutron scattering study [12] as well as ultrasonic study [16] predict a microscopic coexistence of the incommensurate magnetic order with the bulk superconductivity. Is the magnetic order observed in the electron-doped superconducting state coupled or decoupled with the superconductivity? As shown in the previous work [7], insufficient heat treatment definitely causes a macroscopic phaseseparation. However, there is no experimental evidence to reject the microscopic coexistence for both states, which appear at similar temperature region. Moreover, there exist some experimental facts, which cannot be interpreted by simple macroscopic phase-separation. For example, the observed sharp superconducting transition and the broad Ne´el transition are difficult to understand. It is hard to expect that the doping is done inhomogeneously in the AF phase but homogeneously in the SC phase. In the case of oxygendoped La2CuO41y where the macroscopic phase-separation into the hole-poor AF phase and the hole-rich SC phase occurs, there both transition temperatures are well defined [17]. Higher q-resolution measurement is required to distinguish the additional sharp peak of AF phase on the broad peak of SC phase in the case of macroscopic phaseseparation. In summary, we finally proved by neutron scattering the coexistence of dynamical spin fluctuations with high-Tc superconductivity irrespective of types of doped carriers, hole or electron. Then it is worth searching for a common mechanism of the high-Tc superconductivity from a view
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point of magnetic mechanism. However, there are some different aspects between the two systems. First, no welldefined incommensurate spatial spin modulation was detected in the electron-doped Nd1.85Ce0.15CuO4. Second, in the electron-doped case, the shortening of the spatial coherence length could rise Tc which seems to be in conflict with the result of hole-doped system. More detailed neutron scattering study in the phase C (Tc < 23 K) is highly required. Concerning the coexistence of the magnetic order with the superconductivity many experiments suggest the coexistence for hole-doped system. For the electron-doped system on the other hand, there is no conclusive experimental evidence. However, it is important to search for two types of AF phases, one is phase-separated and the other is coexisting with the superconductivity. Such a situation is quite similar to the oxygen-doped La2CuO41y where the phase-separated AF phase is commensurate and the coexisting AF phase is incommensurate.
Acknowledgements The authors acknowledge K. Nemoto and M. Onodera for their technical assistance at JAERI and Tohoku University. We wish to thank Y. Kojima, I. Tanaka, S. Hosoya, K. Hirota, S. Wakimoto, H. Kimura, T. Suzuki, T. Fukase, M. Greven, M.A. Kastner, Y.J. Kim, Y.S. Lee, S. H. Lee for their valuable discussions. The present work in part was supported by the US–Japan Cooperative Research Program on Neutron Scattering and a Grant-In-Aid for Scientific Research from the Ministry of Education, Science, Culture and Sports of Japan and by a Grant for the Promotion of Science from the Science and Technology Agency and by CREST. Work at Brookhaven National Laboratory was carried out under Contract No. DE-AC02-98-CH10886, Division of Material Science, U.S. Department of Energy. The research at Massachusetts Institute of Technology was supported by the National Science Foundation under Grant No. DMR97-04532 and by MRSEC Program of the National Science Foundation under Award No. DMR94-00334.
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