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Inelastic neutron scattering study on NaxCoO2(x∼0.3)
Physica C 470 (2010) S691–S692
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Inelastic neutron scattering study on NaxCoO2 ðx 0:3Þ Seiko Ohira-Kawamura a,*, Takashi Nagata b, Keiko Takeda c, Hideki Yoshizawa d, Hazuki Kawano-Furukawa c a
Academic and Information Board, Ochanomizu University, 2-1-1 Bunkyo-ku, Tokyo 112-8610, Japan Department of Physics, Ochanomizu University, 2-1-1 Bunkyo-ku, Tokyo 112-8610, Japan c Graduate School of Humanities and Sciences, Ochanomizu University, 2-1-1 Bunkyo-ku, Tokyo 112-8610, Japan d Neutron Science Laboratory, I.S.S.P., The University of Tokyo, Tokai, Ibaraki 319-1106, Japan b
a r t i c l e
i n f o
Article history: Accepted 21 December 2009 Available online 28 December 2009 Keywords: Superconductivity Sodium cobalt oxide Neutron scattering
a b s t r a c t Inelastic neutron scattering experiments were performed on single crystals of NaxCoO2 ðx 0:3Þ. The results reveal that an inelastic scattering signal appears at DE 5:5 meV and q (0 1 0) in the orthorhombic notation. Intensity of the signal decreases with increasing temperature, and disappears above 100 K. The origin of this signal is of magnetic. However, we also found the signal is absent in samples from a different batch at the same T and Q conditions. This indicates that the signal is very sensitive to condition in sample preparation. It might be attributed to Na ion ordering (disordering). Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Discovery of superconductivity in NaxCoO2yH2O ðx 0:3; y 1:3Þ has attracted much attention to the sodium cobalt oxide (NaxCoO2) and its water-intercalated derivatives [1]. NaxCoO2 consists of a two-dimensional triangular lattice of CoO2. The magnetic and transport properties of NaxCoO2 strongly depend on the number of charge carriers introduced by a change of the Na content x [2]. The undoped CoO2 layer (x ¼ 1) is composed of nonmagnetic Co3+ (S ¼ 0: a low-spin state in strong crystal field limit), and the number of Co4+ (S ¼ 1=2) ions increases with decreasing x. Water intercalation between the CoO2 layers enhances the layer separation to lead to higher two-dimensionality [1]. The superconductivity in the water-intercalated NaxCoO2yH2O system has been expected to be most likely coupled with the spin degrees of freedom in the CoO2 triangular lattice with geometrical frustration. Neutron scattering measurement can directly give information for the spin fluctuations. Indeed, the existence of magnetic fluctuations has been investigated by the inelastic neutron scattering measurements on several systems with different x [3–6]. We performed neutron scattering measurements on NaxCoO2 ðx 0:3Þ, which is the parent material of the optimal doped superconductor Na0.3CoO21:3H2O, to investigate the dominant magnetic interactions. An inelastic scattering peak was observed at q ¼ G=2 below 100 K (G is a nuclear Bragg point). This peak is, however, absent
* Corresponding author. Present address: J-PARC Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan. Tel.: +81 29 284 3830; fax: +81 29 284 3889. E-mail address: [email protected] (S. Ohira-Kawamura). 0921-4534/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2009.12.046
in other Na0.3CoO2 samples from a different batch. The reason for this difference might be some ordering (or disordering), which occurs in a deintercalation process of the Na ions or is due to impurities arranged with a periodicity of the crystal, if any. 2. Experiments Na0.3CoO2 crystals were obtained by removing excess Na ions by putting single crystals of Na0.7CoO2, which were grown by the floating zone method, in the solution of Br2 and CH3CN. The Na concentration was checked by a change of the weight and lattice parameters. After calcining the crystals at 600 °C, an impurity phase of Co3O4 whose volume fraction was less than 4% was detected by X-ray diffraction and magnetization measurements. Neutron scattering experiments were carried out at the triple axis spectrometer GPTAS installed at the JRR-3 research reactor in Japan Atomic Energy Agency. Neutrons with a fixed final momentum of kf ¼ 3:82 or 2.68 Å1 were used. The configuration of collimators 400 —400 —400 —800 were chosen, and a pyrolytic graphite filter was placed after the sample position to eliminate higher order contaminations. Five single crystals with the total volume of 2 cm3 were mounted in an aluminum can and cooled down to 10 K by a closed-cycle refrigerator. We chose an inplane reciprocal zone shown in Fig. 1 as a scattering plane. 3. Results and discussion We performed energy scans at several Q positions. Fig. 2a shows the constant Q profiles at Q = (0 1 0) at various temperatures. Here I
S692
S. Ohira-Kawamura et al. / Physica C 470 (2010) S691–S692
2 1
1
1
0
2
G (Nuclear Bragg) G/2 Fig. 1. Reciprocal lattice space of NaxCoO2 in the (h k 0) zone.
6
6
(a) E (meV)
4 3 2 1
3
0
(c)
3
2
I. I. (a.u.)
ki / kf (a.u.) I / (1+n)
10 K 30 K 50 K 100 K
4
1
0
(b)
5
5
0
2
4
6
E (meV)
8
10
2 1 0 0
100
200
T (K)
Fig. 2. (a) Temperature dependence of I=ð1 þ nÞ ki =kf . Strong intensity below 2 meV is incoherent scattering. The fitted curves are guides to the eye. (b) and (c) Temperature dependences of the peak position and the integrated intensity.
is the observed intensity, ð1 þ nÞ a Bose factor, and ki ðkf Þ the initial (final) neutron wave numbers. At 10 K, a clear inelastic scattering peak is observed at around 5.5 meV. The peak is suppressed with raising temperature, accompanying a slight shift of its position toward lower energy. The temperature dependences of the peak position and the integrated intensity are summarized in Fig. 2b and c. The peak appears below 100 K at 4 meV. Note that the constant energy scan at E ¼ 5 meV along the (0 k 0) direction also shows a clear peak at Q = (0 1 0) and that this peak shows a clear six-fold symmetry (not shown). From the obtained results, it is suggested that the observed inelastic scattering peak is of magnetic origin and the dominant spin coupling in this system is antiferromagnetic. Recently Moyoshi et al. reported that NaxCoO2yD2O showed an inelastic neutron scattering peak at Q = (1/2 0 2.8) and E ¼ 3 meV in the rhombic notation [6], where Q = (1/2 0 0) corresponds to (0 1 0) in our orthorhombic notation. Their result was interpreted as an antiferromagnetic fluctuations.
We also found that this peak is absent in other Na0.3CoO2 samples from a different batch at the same T and Q conditions. The only difference between the samples from these batches is that the sample which exhibited the peak contains a tiny fraction of Co3O4 impurity (4%). This volume fraction is too small to create such an intense inelastic signal. Moreover, the observed peak shows a clear six-fold symmetry, while Co3O4 should be contained randomly in the crystal. The facts let us attribute the observed inelastic scattering peak to Na0.3CoO2 itself, though the reason for this difference is not understood at present. A conceivable scenario for the occurrence of the sample dependence is due to Na ordering (disordering). Recently Na trimer ordering on the NaxCoO2 surface was reported [7], where the three main phases (kagome, hexagonal and honeycomb phases) were observed in the ultrahigh vacuum scanning tunneling microscopy measurement at room temperature, depending on the Na concentration. The driving force of the Na ordering has been thought to be electrostatics, which maximizes Na–Na separation, and then the Na ordering has been believed to affect physical properties of the CoO2 layers [8–10]. As mentioned above, the Na0.3CoO2 crystals are prepared by deintercalating the Na ions from Na0.7CoO2 in the solution of Br2 and CH3CN. A slight difference in this process may affect the Na arrangement and so different magnetic fluctuations. The magnetic fluctuations are believed to significantly contribute to the appearance of the superconductivity in the waterintercalated system. The impurities arranged with a periodicity of the crystal are also a candidate (if any), but such a phenomenon is not reported at present. Thus, the present results indicate that the Na ordering (disordering) might be an important key to discuss the mechanism of the superconductivity in NaxCoO2yH2O. Acknowledgments We would like to acknowledge Prof. N. Furukawa for valuable discussions. T.N. and H.Y. were supported by a Grant-In-Aid from the Ministry of Education, Culture, Sports, Science and Technology, Japan. References [1] K. Takada, H. Sakurai, E. Takayama-Muromachi, F. Izumi, R.A. Dilanian, T. Sasaki, Nature 422 (2003) 53. [2] M.L. Foo, Y. Wang, S. Watauchi, H.W. Zandbergen, T. He, R.J. Cava, N.P. Ong, Phys. Rev. Lett. 92 (2004) 247001. [3] A.T. Boothroyd, R. Coldea, D.A. Tennant, D. Prabhakaran, L.M. Helme, C.D. Frost, Phys. Rev. Lett. 92 (2004) 197201. [4] L.M. Helme, A.T. Boothroyd, R. Coldea, D. Prabhakaran, A. Stunault, G.J. McIntyre, N. Kernavanois, Phys. Rev. B 73 (2006) 054405. [5] T. Moyoshi, Y. Yasui, M. Soda, Y. Kobayashi, M. Sato, K. Kakurai, J. Phys. Soc. Jpn. 75 (2006) 074705. [6] T. Moyoshi, Y. Yasui, Y. Kobayashi, M. Sato, K. Kakurai, J. Phys. Soc. Jpn. 77 (2008) 073709. [7] W.W. Pai, S.H. Huang, Ying S. Meng, Y.C. Chao, C.H. Liu, H.L. Liu, F.C. Chou, Phys. Rev. Lett. 100 (2008) 206404. [8] P. Zhang, R.B. Capaz, M.L. Cohen, S.G. Louie, Phys. Rev. B 71 (2005) 153102. [9] C.A. Marianetti, G. Kotliar, Phys. Rev. Lett. 98 (2007) 176405. [10] M. Roger et al., Nature (London) 631 (2007) 631.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Inelastic neutron scattering study on NaxCoO2 ðx 0:3Þ Seiko Ohira-Kawamura a,*, Takashi Nagata b, Keiko Takeda c, Hideki Yoshizawa d, Hazuki Kawano-Furukawa c a
Academic and Information Board, Ochanomizu University, 2-1-1 Bunkyo-ku, Tokyo 112-8610, Japan Department of Physics, Ochanomizu University, 2-1-1 Bunkyo-ku, Tokyo 112-8610, Japan c Graduate School of Humanities and Sciences, Ochanomizu University, 2-1-1 Bunkyo-ku, Tokyo 112-8610, Japan d Neutron Science Laboratory, I.S.S.P., The University of Tokyo, Tokai, Ibaraki 319-1106, Japan b
a r t i c l e
i n f o
Article history: Accepted 21 December 2009 Available online 28 December 2009 Keywords: Superconductivity Sodium cobalt oxide Neutron scattering
a b s t r a c t Inelastic neutron scattering experiments were performed on single crystals of NaxCoO2 ðx 0:3Þ. The results reveal that an inelastic scattering signal appears at DE 5:5 meV and q (0 1 0) in the orthorhombic notation. Intensity of the signal decreases with increasing temperature, and disappears above 100 K. The origin of this signal is of magnetic. However, we also found the signal is absent in samples from a different batch at the same T and Q conditions. This indicates that the signal is very sensitive to condition in sample preparation. It might be attributed to Na ion ordering (disordering). Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Discovery of superconductivity in NaxCoO2yH2O ðx 0:3; y 1:3Þ has attracted much attention to the sodium cobalt oxide (NaxCoO2) and its water-intercalated derivatives [1]. NaxCoO2 consists of a two-dimensional triangular lattice of CoO2. The magnetic and transport properties of NaxCoO2 strongly depend on the number of charge carriers introduced by a change of the Na content x [2]. The undoped CoO2 layer (x ¼ 1) is composed of nonmagnetic Co3+ (S ¼ 0: a low-spin state in strong crystal field limit), and the number of Co4+ (S ¼ 1=2) ions increases with decreasing x. Water intercalation between the CoO2 layers enhances the layer separation to lead to higher two-dimensionality [1]. The superconductivity in the water-intercalated NaxCoO2yH2O system has been expected to be most likely coupled with the spin degrees of freedom in the CoO2 triangular lattice with geometrical frustration. Neutron scattering measurement can directly give information for the spin fluctuations. Indeed, the existence of magnetic fluctuations has been investigated by the inelastic neutron scattering measurements on several systems with different x [3–6]. We performed neutron scattering measurements on NaxCoO2 ðx 0:3Þ, which is the parent material of the optimal doped superconductor Na0.3CoO21:3H2O, to investigate the dominant magnetic interactions. An inelastic scattering peak was observed at q ¼ G=2 below 100 K (G is a nuclear Bragg point). This peak is, however, absent
* Corresponding author. Present address: J-PARC Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan. Tel.: +81 29 284 3830; fax: +81 29 284 3889. E-mail address: [email protected] (S. Ohira-Kawamura). 0921-4534/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2009.12.046
in other Na0.3CoO2 samples from a different batch. The reason for this difference might be some ordering (or disordering), which occurs in a deintercalation process of the Na ions or is due to impurities arranged with a periodicity of the crystal, if any. 2. Experiments Na0.3CoO2 crystals were obtained by removing excess Na ions by putting single crystals of Na0.7CoO2, which were grown by the floating zone method, in the solution of Br2 and CH3CN. The Na concentration was checked by a change of the weight and lattice parameters. After calcining the crystals at 600 °C, an impurity phase of Co3O4 whose volume fraction was less than 4% was detected by X-ray diffraction and magnetization measurements. Neutron scattering experiments were carried out at the triple axis spectrometer GPTAS installed at the JRR-3 research reactor in Japan Atomic Energy Agency. Neutrons with a fixed final momentum of kf ¼ 3:82 or 2.68 Å1 were used. The configuration of collimators 400 —400 —400 —800 were chosen, and a pyrolytic graphite filter was placed after the sample position to eliminate higher order contaminations. Five single crystals with the total volume of 2 cm3 were mounted in an aluminum can and cooled down to 10 K by a closed-cycle refrigerator. We chose an inplane reciprocal zone shown in Fig. 1 as a scattering plane. 3. Results and discussion We performed energy scans at several Q positions. Fig. 2a shows the constant Q profiles at Q = (0 1 0) at various temperatures. Here I
S692
S. Ohira-Kawamura et al. / Physica C 470 (2010) S691–S692
2 1
1
1
0
2
G (Nuclear Bragg) G/2 Fig. 1. Reciprocal lattice space of NaxCoO2 in the (h k 0) zone.
6
6
(a) E (meV)
4 3 2 1
3
0
(c)
3
2
I. I. (a.u.)
ki / kf (a.u.) I / (1+n)
10 K 30 K 50 K 100 K
4
1
0
(b)
5
5
0
2
4
6
E (meV)
8
10
2 1 0 0
100
200
T (K)
Fig. 2. (a) Temperature dependence of I=ð1 þ nÞ ki =kf . Strong intensity below 2 meV is incoherent scattering. The fitted curves are guides to the eye. (b) and (c) Temperature dependences of the peak position and the integrated intensity.
is the observed intensity, ð1 þ nÞ a Bose factor, and ki ðkf Þ the initial (final) neutron wave numbers. At 10 K, a clear inelastic scattering peak is observed at around 5.5 meV. The peak is suppressed with raising temperature, accompanying a slight shift of its position toward lower energy. The temperature dependences of the peak position and the integrated intensity are summarized in Fig. 2b and c. The peak appears below 100 K at 4 meV. Note that the constant energy scan at E ¼ 5 meV along the (0 k 0) direction also shows a clear peak at Q = (0 1 0) and that this peak shows a clear six-fold symmetry (not shown). From the obtained results, it is suggested that the observed inelastic scattering peak is of magnetic origin and the dominant spin coupling in this system is antiferromagnetic. Recently Moyoshi et al. reported that NaxCoO2yD2O showed an inelastic neutron scattering peak at Q = (1/2 0 2.8) and E ¼ 3 meV in the rhombic notation [6], where Q = (1/2 0 0) corresponds to (0 1 0) in our orthorhombic notation. Their result was interpreted as an antiferromagnetic fluctuations.
We also found that this peak is absent in other Na0.3CoO2 samples from a different batch at the same T and Q conditions. The only difference between the samples from these batches is that the sample which exhibited the peak contains a tiny fraction of Co3O4 impurity (4%). This volume fraction is too small to create such an intense inelastic signal. Moreover, the observed peak shows a clear six-fold symmetry, while Co3O4 should be contained randomly in the crystal. The facts let us attribute the observed inelastic scattering peak to Na0.3CoO2 itself, though the reason for this difference is not understood at present. A conceivable scenario for the occurrence of the sample dependence is due to Na ordering (disordering). Recently Na trimer ordering on the NaxCoO2 surface was reported [7], where the three main phases (kagome, hexagonal and honeycomb phases) were observed in the ultrahigh vacuum scanning tunneling microscopy measurement at room temperature, depending on the Na concentration. The driving force of the Na ordering has been thought to be electrostatics, which maximizes Na–Na separation, and then the Na ordering has been believed to affect physical properties of the CoO2 layers [8–10]. As mentioned above, the Na0.3CoO2 crystals are prepared by deintercalating the Na ions from Na0.7CoO2 in the solution of Br2 and CH3CN. A slight difference in this process may affect the Na arrangement and so different magnetic fluctuations. The magnetic fluctuations are believed to significantly contribute to the appearance of the superconductivity in the waterintercalated system. The impurities arranged with a periodicity of the crystal are also a candidate (if any), but such a phenomenon is not reported at present. Thus, the present results indicate that the Na ordering (disordering) might be an important key to discuss the mechanism of the superconductivity in NaxCoO2yH2O. Acknowledgments We would like to acknowledge Prof. N. Furukawa for valuable discussions. T.N. and H.Y. were supported by a Grant-In-Aid from the Ministry of Education, Culture, Sports, Science and Technology, Japan. References [1] K. Takada, H. Sakurai, E. Takayama-Muromachi, F. Izumi, R.A. Dilanian, T. Sasaki, Nature 422 (2003) 53. [2] M.L. Foo, Y. Wang, S. Watauchi, H.W. Zandbergen, T. He, R.J. Cava, N.P. Ong, Phys. Rev. Lett. 92 (2004) 247001. [3] A.T. Boothroyd, R. Coldea, D.A. Tennant, D. Prabhakaran, L.M. Helme, C.D. Frost, Phys. Rev. Lett. 92 (2004) 197201. [4] L.M. Helme, A.T. Boothroyd, R. Coldea, D. Prabhakaran, A. Stunault, G.J. McIntyre, N. Kernavanois, Phys. Rev. B 73 (2006) 054405. [5] T. Moyoshi, Y. Yasui, M. Soda, Y. Kobayashi, M. Sato, K. Kakurai, J. Phys. Soc. Jpn. 75 (2006) 074705. [6] T. Moyoshi, Y. Yasui, Y. Kobayashi, M. Sato, K. Kakurai, J. Phys. Soc. Jpn. 77 (2008) 073709. [7] W.W. Pai, S.H. Huang, Ying S. Meng, Y.C. Chao, C.H. Liu, H.L. Liu, F.C. Chou, Phys. Rev. Lett. 100 (2008) 206404. [8] P. Zhang, R.B. Capaz, M.L. Cohen, S.G. Louie, Phys. Rev. B 71 (2005) 153102. [9] C.A. Marianetti, G. Kotliar, Phys. Rev. Lett. 98 (2007) 176405. [10] M. Roger et al., Nature (London) 631 (2007) 631.