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Neutron-scattering study of spin correlations in electron-doped Pr0.89LaCe0.11CuO4 single crystals
Physica C 392–396 (2003) 130–134 www.elsevier.com/locate/physc
Neutron-scattering study of spin correlations in electron-doped Pr0:89LaCe0:11CuO4 single crystals M. Fujita a
a,*
, S. Kuroshima a, M. Matsuda b, K. Yamada
a
Institute for Chemical Research, Kyoto university, Gokasyo, Uji, Kyoto 611-0011, Japan b Advanced Science Research Center, JAERI, Tokai, Ibaraki 319-1195, Japan Received 13 November 2002; accepted 31 January 2003
Abstract We have performed neutron-scattering measurements on the single crystals of electron-doped cuprate, Pr0:89 LaCe0:11 CuO4d in order to study the relationship between spin correlations and the superconductivity. In the nonsuperconducting as-grown x ¼ 0:11 sample, clear magnetic Bragg peak appears below 200 K, while no evidence of antiferromagnetic order was found in the oxygen-reduced superconducting one (Tc ¼ 25:5 K). These results imply that superconductivity concomitantly appears with the degradation of a magnetic ordering by oxygen-reduced procedure. Furthermore, we observed clear commensurate low-energy spin fluctuations, at the antiferromagnetic zone center in both as-grown and reduced samples, analogous to the result for Nd1:85 Ce0:15 CuO4 . Therefore, commensurate spin fluctuations are robust against the electron-doping and is commonly exist in the electron-doped superconductors. Ó 2003 Elsevier B.V. All rights reserved. PACS: 74.72.Jt; 61.12.)q; 74.25.Ha; 75.40.Gb Keywords: Electron-doped superconductor; Neutron scattering; Spin fluctuations
1. Introduction Mechanism of high-Tc superconductivity mediated by spin fluctuations is one of the central topics in highly correlated electron systems. Studies of spin correlations in the hole-doped high-Tc cuprates such as La2x Srx CuO4 (LSCO) have intensively been carried out by neutron-scattering experiments, and show an intimate relation between spin correlations and superconductivity [1].
*
Corresponding author. Tel.: +81-774383111; fax: +81774383118. E-mail address: [email protected] (M. Fujita).
However, comprehensive study on the electrondoped system is lacking due to the difficulties in growing single crystal and post-anneal treatment for inducing superconductivity. Quite recently, spin correlations in the electrondoped Nd1:85 Ce0:15 CuO4 (NCCO) has been investigated by neutron-scattering measurements [2]. It is revealed that commensurate spin fluctuation exists in the superconducting (SC) phase as seen in the AF phase. Furthermore, evidence of competitive relation between antiferromagnetic (AF) order and superconductivity was shown from the comparison between the Neel temperature and the superconducting transition temperature (Tc ) for the as-grown and the annealed samples. These are
0921-4534/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-4534(03)00750-0
M. Fujita et al. / Physica C 392–396 (2003) 130–134
crucial results for understanding the common role of the spin correlations for the mechanism of highTc superconductivity irrespective of carrier type. However, to investigate the universal nature in the electron-doped system further systematical study using other electron-doped cuprates is important. Especially sample with negligible effects from large rare-earth moment is highly required since such as Nd3þ moment would impede the detailed study of inherent nature of Cu2þ spins that will play important role for superconductivity. We therefore performed systematic susceptibility and neutron-scattering measurements on the Pr0:89 LaCe0:11 CeO4d (PLCCO) system in which the effect of rare-earth moment is much reduced compared with that in NCCO. Furthermore, in the PLCCO system, larger amount of oxygen than in NCCO needs to be removed for the appearance of bulk superconductivity [3]. This feature allows us to control the amount of oxygen contents in order to study the role and mechanism of oxygen-reduction for the appearance of electron-doped superconductivity. In this paper, we present the change in the static and dynamical spin correlations and in the superconducting property in the PLCCO system caused by the oxygen reduction. We also discuss the relation between spin correlations and superconductivity.
131
conducting shielding signal with a SQUID magnetometer. Elastic and inelastic neutron-scattering experiments were performed using a series of single crystals on triple-axis spectrometers TAS-1 and TAS-2, installed in the JRR-3M at the JAERI in Tokai, Japan. We selected the horizontal collimation sequences and incident neutron energy (Ei) of 800 –400 –400 –800 and 13.7 meV at TAS-1, and 170 – 200 –200 –800 and 14.7 meV at TAS-2. Pyrolytic graphite filters were used to eliminate contamination from higher-order wavelengths in the neutron beams. Crystals were mounted in the (h k 0) zone. All crystallographic indices in this paper are described in the orthorhombic notation.
3. Results In Fig. 1, magnetic susceptibility for x ¼ 0:11 samples with different d is shown. The as-grown (d ¼ 0) and slightly oxygen-reduced (d ¼ 0:03) samples do not show the superconductivity. However, in the sample with d ¼ 0:05, bulk superconductivity appears at the onset temperature of 25.5 K. These results clearly demonstrate that there exists a critical value of oxygen loss, dc , for the appearance of superconductivity in the sample with a given Ce concentration, similar to the case of NCCO [4].
2. Experimental Single crystals of PLCCO with x ¼ 0:11 were grown by a traveling-solvent floating-zone method. As-grown single crystalline rod in the shape of columnar, 100 mm in length and 6 mm in diameter, is cut into 30 mm long for the neutronscattering measurement. In order to remove the relative oxygen contents per unit formula, d, of 0.03 and 0.05, we annealed crystals under argon gas flow at 900 and 960 °C for 12 h, respectively. Here, as-grown sample is assumed to be stoichiometric (d ¼ 0) and the values for annealed sample were determined by measuring the weight loss of the sample after the heat treatment. Furthermore, the edge of the annealed crystals was sliced into 1 mm thick pieces in order to measure the super-
Fig. 1. Magnetic susceptibility measured for single crystal samples of Pr0:89 LaCe0:11 CuO4 with (a) d ¼ 0, (b) 0.03 and (c) 0.05 after the zero-field-cooling process.
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Fig. 3. Temperature dependence of volume corrected peak intensity of (1 2 0) magnetic Bragg position measured for Pr0:89 LaCe0:11 CuO4 with d ¼ 0, 0.03 and 0.05. The solid lines are the results of a fit to a power low Cð1 T =TN Þ2b with TN ¼ 200 5 K, b ¼ 0:370 0:015, and TN ¼ 97 10 K, b ¼ 0:330 0:040 for d ¼ 0 and 0.03 samples, respectively.
Fig. 2. Peak-profiles through (1 2 0) magnetic Bragg peak for Pr0:89 LaCe0:11 CuO4 with (a) d ¼ 0, (b) 0.03 and (c) 0.05. Scan trajectories are shown in the upper panel.
We next investigated the magnetic properties in these samples by neutron-scattering measurements. Fig. 2 shows the peak-profiles through the (1 2 0) magnetic Bragg position. Profiles were measured along diagonal direction to the orthorhombic h-axis for x ¼ 0 sample, while scan trajectory for x ¼ 0:03 and 0.05 samples was parallel to the h-axis. Thus, the profiles are plotted in q 1 ), which is defined as q ¼ Q Q (A ð1 2 0Þ , in order to compare with each profile. Here Q and Qð1 2 0Þ are the momentum transfer and the wave vector from origin to (1 2 0) Bragg position, respectively. In the d ¼ 0 and 0.03 samples, intensities are enhanced at low temperature, indicating the existence of three-dimensional long-range AF order. On the other hand, no clear difference between the peak-profiles measured at 4 and 80 K was observed in the superconducing d ¼ 0:05 sample.
Note that the peak-profile through (1 2 0) position is temperature independent. Appearance of the temperature independent superlattice peak in the annealed sample is similar to the result in the NCCO system for which the origin is discussed from the viewpoint of atomic displacement caused by the heat treatment [4]. In order to check the absence of magnetic order in d ¼ 0:05 sample and to determine Neel temperature (TN ) for d ¼ 0 and 0.03 samples, we examine the temperature dependence of (1 2 0) peak intensity as shown in Fig. 3. Intensities for three samples are measured under the identical experimental setups and normalized by the effective sample volume after subtracting the background estimated at high temperatures. Comparing to the results for d ¼ 0 and 0.03, it is obvious that both peak-intensity and TN decreases with increasing d. No clear evidence of magnetic order was observed in the superconducting sample with d ¼ 0:05. Therefore, the AF order is degraded by the oxygen-reduction with decreasing ordered moment and/or region and TN with increasing d. Upon further oxygen-reduction, SC phase concomitantly appears with the disappearance of AF phase, suggesting the competitive relation between superconductivity and magnetic order.
M. Fujita et al. / Physica C 392–396 (2003) 130–134
In the PLCCO, since the negligible effect from the Pr moment is expected [5], only Cu moment is dominantly responsible for the magnetic Bragg intensity. Temperature dependence of intensity for d ¼ 0 and 0.03 samples shows a simple power-low behavior as seen in the Mott insulator, La2 CuO4 [6]. Here we fit the data to power low function of I ¼ Cð1 T =TN Þ2b in order to extract the critical exponent b and TN and to characterize the critical phenomena, where C is scale factor. For d ¼ 0 and 0.03 samples, the parameters are found to be TN ¼ 200 5 K, b ¼ 0:370 0:015, and TN ¼ 97 10 K, b ¼ 0:330 0:040, respectively. b evaluated for as-grown sample is close to the value of 0.365 for 3D Heisenberg model. Here we assumed the magnetically ordered region in each sample is independent of temperature below TN . Indeed, our lSR measurements on PLCCO detected a AF volume fraction, which is nearly constant below the ordering temperature [5]. The difference in b between two samples is possibly due to the smearing of TN originated from the inhomogeneously distributed or defected oxygen atoms. Therefore, Heisenberg interaction seems to remain even thought the magnetic order degrades. In order to study the oxygen-reduction effect on the dynamical spin fluctuations, we systematically measured the constant energy profile at x ¼ 4 meV for three samples. Fig. 4 shows the longitudinal scan through (1 0 0) position, which corresponds to AF zone center of ðp; pÞ, at temperature below 4 K. Well-defined commensurate peak was observed in both AF and SC phases, consistent with the results for NCCO system [2]. Thus, present result combined with the previous one for NCCO suggests that the commensurate spin fluctuation commonly exists in the electron-doped superconductors. Furthermore, the peak-width in the SC phase with d ¼ 0:05 is substantially boarder than that in the AF phase with d ¼ 0 and 0.03, while the volume corrected integrated intensities are comparable. Assuming the Gaussian function for the peak-shape, we evaluated the width in half1 ), with taking the width at half-maximum j, (A experimental resolution into account. j for three samples is summarized in the Table 1 together with the Tc and TN .
133
Fig. 4. Constant-energy profiles at x ¼ 4 meV though (1 0 0) AF zone center for Pr0:89 LaCe0:11 CuO4 with (a) d ¼ 0, (b) 0.03 and (c) 0.05. Scan trajectory is shown in the upper panel. Solid lines are results fitted with a single Gaussian function by convoluting the experimental resolution.
Table 1 Neel temperature (TN ), onset temperature of superconducting transition (Tc ) and peak-width in half-width at half-maximum j 1 ) for Pr0:89 LaCe0:11 CuO4d with d ¼ 0, 0.03 and 0.05 (A 1 ) TN (K) j (x ¼ 4 meV) (A d Tc (K) 0 0.03 0.05
– – 25.5
200 5 97 10 –
0.003 0.001 0.004 0.001 0.026 0.002
4. Discussion and conclusion Present study clearly shows the concomitant appearance of bulk superconductivity with the disappearance of AF order. Phase diagram of the electron-doped system for Ce-concentration,
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which shows the SC phase adjacent to the broad AF ordered phase, also suggests the competitive relation between two phases [2]. Therefore, the competitive relation is commonly seen in the electron-doped system around the phase boundary. Decrease of magnetic Bragg peak intensity at low temperatures with increasing d indicates the reduction of either the magnetic moment or magnetically ordered region when the magnetic structure is same. Our lSR measurements on the PLCCO system show the reduction of both magnetic volume fraction and amplitude of magnetic moment upon Ce doping. If the similar changes in the magnetic properties occur with increasing Ce concentration and oxygen reduction, present results lead the important consequence. That is, the volume fraction of AF ordered region continuously degrades with increasing d, while SC volume fraction increases abruptly. Therefore, AF ordered region is not simply replaced by the SC region but the bulk superconductivity appears when the magnetically ordered region and/or magnetic moment are sufficiently diminished. On the other hand, spin fluctuations were observed even in the SC sample. Therefore, wellcorrelated spin fluctuations exist in the SC phase irrespective of the types of doped carrier, supporting a common role of the spin fluctuations for the high-Tc superconductivity. However, commensurate spin correlation observed in both AF and SC phase is contrastive to the incommensurate one in the hole-doped LSCO system in which the spin density modulation changes from being diagonal to parallel to the Cu–O bond direction on crossing the insulator–superconductor phase boundary [7]. Difference of the dynamical spin correlations between AF and SC phase is only seen in the spatial coherence length, corresponding to the inverse of peak-width. The coherence length is shorter in the SC conducting sample. This seems to be consistent with the results for low-energy spin fluctuations in LSCO showing the local minimum of j at x 0:12 where the superconductivity is slightly suppressed. In summary, we studied the superconductivity and the spin correlations in the electron-doped Pr0:89 LaCe0:11 CuO4d by magnetic susceptibility and neutron-scattering measurements. Competi-
tive relation between AF and SC phases is revealed by the systematic control of the oxygen content in the samples. Well-defined commensurate spin fluctuations are observed in both AF and SC phases, analogous to the results for NCCO but contrastive to the ones for the hole-doped system. In order to clarify common mechanism of the high-Tc superconductivity, further similarities and differences between electron and hole-doped system needs to be investigated.
Acknowledgements This work was supported in part by the Japanese Ministry of Education, Culture, Sports, Science and Technology, Grant-in-Aid for Scientific Research on Priority Areas (Novel Quantum Phenomena in Transition Metal Oxides), for Scientific Research (A), and for Encouragement of Young Scientists, by the Japan Science and Technology Corporation, the Core Research for Evolutional Science and Technology Project (CREST).
References [1] K. Yamada, C.H. Lee, K. Kurahashi, J. Wada, S. Wakimoto, 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. [2] K. Yamada, K. Kurahashi, Y. Endoh, R.J. Birgeneau, G. Shirane, J. Phys. Chem. Solids 60 (1999) 1025; K. Yamada, K. Kurahashi, T. Uefuji, M. Fujita, S.H. Lee, Y. Endoh, cond-mat/0210536 (unpublished). [3] Y. Koike, A. Kakimoto, M. Yoshida, H. Inuzuka, T. Noji, Y. Saito, Physica B 165–166 (1990) 1665; S.J. Kim, D.R. Gaskell, Physica C 209 (1993) 381. [4] K. Kurahashi, H. Matsushita, M. Fujita, K. Yamada, J. Phys. Soc. Jpn. 71 (2002) 910. [5] M. Fujita, T. Kubo, S. Kuroshima, T. Uefuji, K. Kawashima, K. Yamada, I. Watanabe, K. Nagamine, cond-mat/ 0203320 (unpublished). [6] 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. [7] M. Fujita, K. Yamada, H. Hiraka, P.M. Gehring, S.H. Lee, S. Wakimoto, G. Shirane, Phys. Rev. B 65 (2002) 064505.
Neutron-scattering study of spin correlations in electron-doped Pr0:89LaCe0:11CuO4 single crystals M. Fujita a
a,*
, S. Kuroshima a, M. Matsuda b, K. Yamada
a
Institute for Chemical Research, Kyoto university, Gokasyo, Uji, Kyoto 611-0011, Japan b Advanced Science Research Center, JAERI, Tokai, Ibaraki 319-1195, Japan Received 13 November 2002; accepted 31 January 2003
Abstract We have performed neutron-scattering measurements on the single crystals of electron-doped cuprate, Pr0:89 LaCe0:11 CuO4d in order to study the relationship between spin correlations and the superconductivity. In the nonsuperconducting as-grown x ¼ 0:11 sample, clear magnetic Bragg peak appears below 200 K, while no evidence of antiferromagnetic order was found in the oxygen-reduced superconducting one (Tc ¼ 25:5 K). These results imply that superconductivity concomitantly appears with the degradation of a magnetic ordering by oxygen-reduced procedure. Furthermore, we observed clear commensurate low-energy spin fluctuations, at the antiferromagnetic zone center in both as-grown and reduced samples, analogous to the result for Nd1:85 Ce0:15 CuO4 . Therefore, commensurate spin fluctuations are robust against the electron-doping and is commonly exist in the electron-doped superconductors. Ó 2003 Elsevier B.V. All rights reserved. PACS: 74.72.Jt; 61.12.)q; 74.25.Ha; 75.40.Gb Keywords: Electron-doped superconductor; Neutron scattering; Spin fluctuations
1. Introduction Mechanism of high-Tc superconductivity mediated by spin fluctuations is one of the central topics in highly correlated electron systems. Studies of spin correlations in the hole-doped high-Tc cuprates such as La2x Srx CuO4 (LSCO) have intensively been carried out by neutron-scattering experiments, and show an intimate relation between spin correlations and superconductivity [1].
*
Corresponding author. Tel.: +81-774383111; fax: +81774383118. E-mail address: [email protected] (M. Fujita).
However, comprehensive study on the electrondoped system is lacking due to the difficulties in growing single crystal and post-anneal treatment for inducing superconductivity. Quite recently, spin correlations in the electrondoped Nd1:85 Ce0:15 CuO4 (NCCO) has been investigated by neutron-scattering measurements [2]. It is revealed that commensurate spin fluctuation exists in the superconducting (SC) phase as seen in the AF phase. Furthermore, evidence of competitive relation between antiferromagnetic (AF) order and superconductivity was shown from the comparison between the Neel temperature and the superconducting transition temperature (Tc ) for the as-grown and the annealed samples. These are
0921-4534/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-4534(03)00750-0
M. Fujita et al. / Physica C 392–396 (2003) 130–134
crucial results for understanding the common role of the spin correlations for the mechanism of highTc superconductivity irrespective of carrier type. However, to investigate the universal nature in the electron-doped system further systematical study using other electron-doped cuprates is important. Especially sample with negligible effects from large rare-earth moment is highly required since such as Nd3þ moment would impede the detailed study of inherent nature of Cu2þ spins that will play important role for superconductivity. We therefore performed systematic susceptibility and neutron-scattering measurements on the Pr0:89 LaCe0:11 CeO4d (PLCCO) system in which the effect of rare-earth moment is much reduced compared with that in NCCO. Furthermore, in the PLCCO system, larger amount of oxygen than in NCCO needs to be removed for the appearance of bulk superconductivity [3]. This feature allows us to control the amount of oxygen contents in order to study the role and mechanism of oxygen-reduction for the appearance of electron-doped superconductivity. In this paper, we present the change in the static and dynamical spin correlations and in the superconducting property in the PLCCO system caused by the oxygen reduction. We also discuss the relation between spin correlations and superconductivity.
131
conducting shielding signal with a SQUID magnetometer. Elastic and inelastic neutron-scattering experiments were performed using a series of single crystals on triple-axis spectrometers TAS-1 and TAS-2, installed in the JRR-3M at the JAERI in Tokai, Japan. We selected the horizontal collimation sequences and incident neutron energy (Ei) of 800 –400 –400 –800 and 13.7 meV at TAS-1, and 170 – 200 –200 –800 and 14.7 meV at TAS-2. Pyrolytic graphite filters were used to eliminate contamination from higher-order wavelengths in the neutron beams. Crystals were mounted in the (h k 0) zone. All crystallographic indices in this paper are described in the orthorhombic notation.
3. Results In Fig. 1, magnetic susceptibility for x ¼ 0:11 samples with different d is shown. The as-grown (d ¼ 0) and slightly oxygen-reduced (d ¼ 0:03) samples do not show the superconductivity. However, in the sample with d ¼ 0:05, bulk superconductivity appears at the onset temperature of 25.5 K. These results clearly demonstrate that there exists a critical value of oxygen loss, dc , for the appearance of superconductivity in the sample with a given Ce concentration, similar to the case of NCCO [4].
2. Experimental Single crystals of PLCCO with x ¼ 0:11 were grown by a traveling-solvent floating-zone method. As-grown single crystalline rod in the shape of columnar, 100 mm in length and 6 mm in diameter, is cut into 30 mm long for the neutronscattering measurement. In order to remove the relative oxygen contents per unit formula, d, of 0.03 and 0.05, we annealed crystals under argon gas flow at 900 and 960 °C for 12 h, respectively. Here, as-grown sample is assumed to be stoichiometric (d ¼ 0) and the values for annealed sample were determined by measuring the weight loss of the sample after the heat treatment. Furthermore, the edge of the annealed crystals was sliced into 1 mm thick pieces in order to measure the super-
Fig. 1. Magnetic susceptibility measured for single crystal samples of Pr0:89 LaCe0:11 CuO4 with (a) d ¼ 0, (b) 0.03 and (c) 0.05 after the zero-field-cooling process.
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M. Fujita et al. / Physica C 392–396 (2003) 130–134
Fig. 3. Temperature dependence of volume corrected peak intensity of (1 2 0) magnetic Bragg position measured for Pr0:89 LaCe0:11 CuO4 with d ¼ 0, 0.03 and 0.05. The solid lines are the results of a fit to a power low Cð1 T =TN Þ2b with TN ¼ 200 5 K, b ¼ 0:370 0:015, and TN ¼ 97 10 K, b ¼ 0:330 0:040 for d ¼ 0 and 0.03 samples, respectively.
Fig. 2. Peak-profiles through (1 2 0) magnetic Bragg peak for Pr0:89 LaCe0:11 CuO4 with (a) d ¼ 0, (b) 0.03 and (c) 0.05. Scan trajectories are shown in the upper panel.
We next investigated the magnetic properties in these samples by neutron-scattering measurements. Fig. 2 shows the peak-profiles through the (1 2 0) magnetic Bragg position. Profiles were measured along diagonal direction to the orthorhombic h-axis for x ¼ 0 sample, while scan trajectory for x ¼ 0:03 and 0.05 samples was parallel to the h-axis. Thus, the profiles are plotted in q 1 ), which is defined as q ¼ Q Q (A ð1 2 0Þ , in order to compare with each profile. Here Q and Qð1 2 0Þ are the momentum transfer and the wave vector from origin to (1 2 0) Bragg position, respectively. In the d ¼ 0 and 0.03 samples, intensities are enhanced at low temperature, indicating the existence of three-dimensional long-range AF order. On the other hand, no clear difference between the peak-profiles measured at 4 and 80 K was observed in the superconducing d ¼ 0:05 sample.
Note that the peak-profile through (1 2 0) position is temperature independent. Appearance of the temperature independent superlattice peak in the annealed sample is similar to the result in the NCCO system for which the origin is discussed from the viewpoint of atomic displacement caused by the heat treatment [4]. In order to check the absence of magnetic order in d ¼ 0:05 sample and to determine Neel temperature (TN ) for d ¼ 0 and 0.03 samples, we examine the temperature dependence of (1 2 0) peak intensity as shown in Fig. 3. Intensities for three samples are measured under the identical experimental setups and normalized by the effective sample volume after subtracting the background estimated at high temperatures. Comparing to the results for d ¼ 0 and 0.03, it is obvious that both peak-intensity and TN decreases with increasing d. No clear evidence of magnetic order was observed in the superconducting sample with d ¼ 0:05. Therefore, the AF order is degraded by the oxygen-reduction with decreasing ordered moment and/or region and TN with increasing d. Upon further oxygen-reduction, SC phase concomitantly appears with the disappearance of AF phase, suggesting the competitive relation between superconductivity and magnetic order.
M. Fujita et al. / Physica C 392–396 (2003) 130–134
In the PLCCO, since the negligible effect from the Pr moment is expected [5], only Cu moment is dominantly responsible for the magnetic Bragg intensity. Temperature dependence of intensity for d ¼ 0 and 0.03 samples shows a simple power-low behavior as seen in the Mott insulator, La2 CuO4 [6]. Here we fit the data to power low function of I ¼ Cð1 T =TN Þ2b in order to extract the critical exponent b and TN and to characterize the critical phenomena, where C is scale factor. For d ¼ 0 and 0.03 samples, the parameters are found to be TN ¼ 200 5 K, b ¼ 0:370 0:015, and TN ¼ 97 10 K, b ¼ 0:330 0:040, respectively. b evaluated for as-grown sample is close to the value of 0.365 for 3D Heisenberg model. Here we assumed the magnetically ordered region in each sample is independent of temperature below TN . Indeed, our lSR measurements on PLCCO detected a AF volume fraction, which is nearly constant below the ordering temperature [5]. The difference in b between two samples is possibly due to the smearing of TN originated from the inhomogeneously distributed or defected oxygen atoms. Therefore, Heisenberg interaction seems to remain even thought the magnetic order degrades. In order to study the oxygen-reduction effect on the dynamical spin fluctuations, we systematically measured the constant energy profile at x ¼ 4 meV for three samples. Fig. 4 shows the longitudinal scan through (1 0 0) position, which corresponds to AF zone center of ðp; pÞ, at temperature below 4 K. Well-defined commensurate peak was observed in both AF and SC phases, consistent with the results for NCCO system [2]. Thus, present result combined with the previous one for NCCO suggests that the commensurate spin fluctuation commonly exists in the electron-doped superconductors. Furthermore, the peak-width in the SC phase with d ¼ 0:05 is substantially boarder than that in the AF phase with d ¼ 0 and 0.03, while the volume corrected integrated intensities are comparable. Assuming the Gaussian function for the peak-shape, we evaluated the width in half1 ), with taking the width at half-maximum j, (A experimental resolution into account. j for three samples is summarized in the Table 1 together with the Tc and TN .
133
Fig. 4. Constant-energy profiles at x ¼ 4 meV though (1 0 0) AF zone center for Pr0:89 LaCe0:11 CuO4 with (a) d ¼ 0, (b) 0.03 and (c) 0.05. Scan trajectory is shown in the upper panel. Solid lines are results fitted with a single Gaussian function by convoluting the experimental resolution.
Table 1 Neel temperature (TN ), onset temperature of superconducting transition (Tc ) and peak-width in half-width at half-maximum j 1 ) for Pr0:89 LaCe0:11 CuO4d with d ¼ 0, 0.03 and 0.05 (A 1 ) TN (K) j (x ¼ 4 meV) (A d Tc (K) 0 0.03 0.05
– – 25.5
200 5 97 10 –
0.003 0.001 0.004 0.001 0.026 0.002
4. Discussion and conclusion Present study clearly shows the concomitant appearance of bulk superconductivity with the disappearance of AF order. Phase diagram of the electron-doped system for Ce-concentration,
134
M. Fujita et al. / Physica C 392–396 (2003) 130–134
which shows the SC phase adjacent to the broad AF ordered phase, also suggests the competitive relation between two phases [2]. Therefore, the competitive relation is commonly seen in the electron-doped system around the phase boundary. Decrease of magnetic Bragg peak intensity at low temperatures with increasing d indicates the reduction of either the magnetic moment or magnetically ordered region when the magnetic structure is same. Our lSR measurements on the PLCCO system show the reduction of both magnetic volume fraction and amplitude of magnetic moment upon Ce doping. If the similar changes in the magnetic properties occur with increasing Ce concentration and oxygen reduction, present results lead the important consequence. That is, the volume fraction of AF ordered region continuously degrades with increasing d, while SC volume fraction increases abruptly. Therefore, AF ordered region is not simply replaced by the SC region but the bulk superconductivity appears when the magnetically ordered region and/or magnetic moment are sufficiently diminished. On the other hand, spin fluctuations were observed even in the SC sample. Therefore, wellcorrelated spin fluctuations exist in the SC phase irrespective of the types of doped carrier, supporting a common role of the spin fluctuations for the high-Tc superconductivity. However, commensurate spin correlation observed in both AF and SC phase is contrastive to the incommensurate one in the hole-doped LSCO system in which the spin density modulation changes from being diagonal to parallel to the Cu–O bond direction on crossing the insulator–superconductor phase boundary [7]. Difference of the dynamical spin correlations between AF and SC phase is only seen in the spatial coherence length, corresponding to the inverse of peak-width. The coherence length is shorter in the SC conducting sample. This seems to be consistent with the results for low-energy spin fluctuations in LSCO showing the local minimum of j at x 0:12 where the superconductivity is slightly suppressed. In summary, we studied the superconductivity and the spin correlations in the electron-doped Pr0:89 LaCe0:11 CuO4d by magnetic susceptibility and neutron-scattering measurements. Competi-
tive relation between AF and SC phases is revealed by the systematic control of the oxygen content in the samples. Well-defined commensurate spin fluctuations are observed in both AF and SC phases, analogous to the results for NCCO but contrastive to the ones for the hole-doped system. In order to clarify common mechanism of the high-Tc superconductivity, further similarities and differences between electron and hole-doped system needs to be investigated.
Acknowledgements This work was supported in part by the Japanese Ministry of Education, Culture, Sports, Science and Technology, Grant-in-Aid for Scientific Research on Priority Areas (Novel Quantum Phenomena in Transition Metal Oxides), for Scientific Research (A), and for Encouragement of Young Scientists, by the Japan Science and Technology Corporation, the Core Research for Evolutional Science and Technology Project (CREST).
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