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Electronic state of a new organic conductor (TTM-TTP)I3 with a one-dimensional half-filled band
Journal of Physics and Chemistry of Solids 62 (2001) 405±407
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Electronic state of a new organic conductor (TTM-TTP)I3 with a one-dimensional half-®lled band M. Onuki a,*, K. Hiraki a, T. Takahashi a, D. Jinno b, T. Kawamoto b, T. Mori b, K. Tanaka c, Y. Misaki c a
Department of Physics, Gakushuin University, Mejiro 1-5-1, Toshima-ku, Tokyo 171-8588, Japan Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Megro-ku, Tokyo 152-8552, Japan c Department of Molecular Engineering, Kyoto University, Yoshida, Kyoto 606-8501, Japan
b
Abstract 13 C NMR measurements have been carried out to investigate the nature of the electronic state of the one-dimensional half®lled band system, (TTM-TTP)I3. Metal±insulator transition at around 120 K was observed through the temperature dependence of NMR relaxation rate, 1/T1. From the analysis of NMR spectra in the insulating state, it is strongly suggested that charge disproportionation with the valences of the TTM-TTP molecules 2-0-2-0 exists along the stacking axis. q 2000 Elsevier Science Ltd. All rights reserved.
Keywords: A. Organic compounds; C. Nuclear magnetic resonannce (NMR); D. Magnetic properties; D. Phase transition
1. Introduction (TTM-TTP)I3, where TTM-TTP is 2,5-bis(4,5-bis(methyltio)-1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene, is a charge transfer salt with the donor to acceptor ratio of 1:1 [1]. The TTM-TTP molecules form one-dimensional (1D) stacks along the crystallographic c axis. Considering the charge neutrality, it is expected that (TTM-TTP)I3 has a 1D half-®lled band. It behaves metallic in the higher temperature region and exhibits a resistivity minimum at 120±160 K at ambient pressure [1]. This may be due to the small on-site Coulomb repulsion, U: TTM-TTP is a large donor molecule as compared with conventional molecules such as TMTCF and ET, which form charge transfer complexes [2]. The electronic state in the insulating phase of the system has not been clari®ed though magnetic susceptibility measurements [3±5], and X-ray studies [4,5] have been carried out. In the present study, we have performed 13C NMR measurements on a sample selectively substituted with 13 C-isotope.
samples 1 and 2 were crystallized with the 1-site, and 6-sites substituted molecules shown in Fig. 1(a) and (b), respectively. The NMR relaxation rate, 1/T1, were measured in the temperature range 40 # T # 300 K on sample 2 because the signal intensity was expected to be large. The observed 13 C signal comes from all 13C-sites; there exist two 13C-sites per molecule, crystallographically different from each other. Measurements of NMR lineshape were carried out on sample 1, where all 13C sites are crystallographically equivalent. All measurements were performed in the temperature range, 5 # T # 300 K, on polycrystalline samples (sample 1 , 10, sample 2 , 20 mg) with the conventional pulsed NMR system. The applied ®eld was 8.2 T. The NMR spectra were obtained by the fast Fourier transformation of spin echo signals. 1/T1 were obtained by measuring the recovery of the nuclear magnetization after saturation comb pulses. Magnetic susceptibility of 1-site substituted sample (,6.4 mg) was measured from room temperature to 2 K with a SQUID magnetometer (Quantum design) at 1 T.
2. Experimental
3. Results and discussions
Two kinds of
13
C-substituted molecules were prepared;
* Corresponding author.
Temperature dependence of the magnetic susceptibility is shown in Fig. 2. The diamagnetism of core contributions has not been subtracted, while the Curie term observed in the
0022-3697/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S 0022-369 7(00)00176-1
406
M. Onuki et al. / Journal of Physics and Chemistry of Solids 62 (2001) 405±407
Fig. 1. (a) 1-site substituted, and (b) 6-sites substituted TTM-TTP molecules.
Fig. 2. Temperature dependence of the static magnetic susceptibility. (The Curie term was subtracted.)
Fig. 4. 13C NMR spectra of the 1-site substituted sample at various temperatures.
Fig. 3. Temperature dependence of the 13C NMR relaxation rate, 1/T1.
low temperature region has been subtracted. The susceptibility gradually decreases with decreasing T down to 120 K, and begins to decrease more rapidly below that temperature. It suggests a transition into a nonmagnetic ground state at 120 K. This behavior was also observed by the other research groups[4,5]. Temperature dependence of 1/T1 on sample 2 is shown in Fig. 3. In the higher temperature region above 120 K, the relaxation rate obeys a Korringa relation T1 T21 const: which is expected in a metallic state. Korringa constant (T1T ) 21 , 0.0025 s 21K 21 is small as compared with the other donor type organic conductors; e.g. ET2X T1 T21 , 0:05 s21 K21 : This may re¯ect a wide conduction band in the system. Below 120 K, 1/T1 starts to decrease exponentially. This is due to a transition into a spin-singlet state. We have measured 13C NMR spectrum to obtain further information about the electronic state in the insulating state. Fig. 4 shows the 13C NMR lineshapes at various temperatures. The horizontal axis shows the shift from
M. Onuki et al. / Journal of Physics and Chemistry of Solids 62 (2001) 405±407
the resonance position of the standard sample [tetramethylsilane (TMS)]. The single line observed in the higher temperature region indicates that all the 13C atoms are in a crystallographically equivalent site. On the other hand, the lineshape becomes asymmetric and broader below 120 K, and the peak splits into two lines with almost the same signal intensity below about 80 K. This indicates that the 13C sites are no more equivalent. Considering a half-®lled band and two-fold periodicity reported at low temperature, it is expected that the distribution of the electron is modulated with a 2kF wave number. The line separation is about 45 ppm. This value is too large to explain as caused by pure 2kF lattice distortion. This separation should be made not only by the lattice distortion but also by the electronic modulation. The most probable situation, we suppose, is the alternative stacking of molecules with rich and poor electron density with two-fold periodicity along the c axis. The insulating state of this system is nonmagnetic. Therefore, the degree of the charge disproportionation is considered as schematically TTM-TTP0-TTM-TTP21. Both the neutral and divalent molecules are nonmagnetic. We consider that this type of ground state is stable when the onsite Coulomb U is small and the inter-site Coulomb V is large. To con®rm the present model, the comparison with the NMR shift of TTM-TTP0 and TTM-TTP 21 should be crucial. This is under way. It is necessary that the chemical shift values of each valence of molecules are identi®ed. 4. Concluding remarks In conclusion, the result of
13
C NMR measurements on
407
(TTM-TTP)I3 indicates the charge ordering below about 120 K. We propose that the neutral and divalent TTMTTP molecules have been arranged alternately along the stacking direction below 80 K. This model re¯ects well the feature of small on-site Coulomb repulsion U due to the large size of the TTM-TTP molecule.
Acknowledgements This work was supported in part by the ªResearch for Futureº Project no.JSPS-REFTF97P00105, by the Japan Society for Promotion of Science; and Grant-in-Aid for Scienti®c Research no. 11740211 from the Ministry of Education, Science, Sports and Culture, Japan.
References [1] T. Mori, H. Inokuchi, Y. Misaki, T. Yamabe, H. Mori, S. Tanaka, Bull. Chem. Soc. Jpn 67 (1994) 61. [2] L.K. Montgomery, in: J.-P Farges (Ed.), Organic Conductors, Marcel Dekker, New York, 1994, p. 115. [3] T. Mori, T. Kawamoto, J. Yamaura, T. Enoki, Y. Misaki, T. Yamabe, H. Mori, S. Tanaka, Phys. Rev. Lett. 79 (1997) 1702. [4] M. Maesato, Y. Sasou, S. Kagoshima, T. Mori, T. Kawamoto, Y. Misaki, T. Yamabe, Synth. Met. 103 (1999) 2109. [5] N. Fujimura, A. Namba, T. Kambe, Y. Nogami, K. Oshima, T. Mori, T. Kawamoto, Y. Misaki, T. Yamabe, Synth. Met. 103 (1999) 2111.
www.elsevier.nl/locate/jpcs
Electronic state of a new organic conductor (TTM-TTP)I3 with a one-dimensional half-®lled band M. Onuki a,*, K. Hiraki a, T. Takahashi a, D. Jinno b, T. Kawamoto b, T. Mori b, K. Tanaka c, Y. Misaki c a
Department of Physics, Gakushuin University, Mejiro 1-5-1, Toshima-ku, Tokyo 171-8588, Japan Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Megro-ku, Tokyo 152-8552, Japan c Department of Molecular Engineering, Kyoto University, Yoshida, Kyoto 606-8501, Japan
b
Abstract 13 C NMR measurements have been carried out to investigate the nature of the electronic state of the one-dimensional half®lled band system, (TTM-TTP)I3. Metal±insulator transition at around 120 K was observed through the temperature dependence of NMR relaxation rate, 1/T1. From the analysis of NMR spectra in the insulating state, it is strongly suggested that charge disproportionation with the valences of the TTM-TTP molecules 2-0-2-0 exists along the stacking axis. q 2000 Elsevier Science Ltd. All rights reserved.
Keywords: A. Organic compounds; C. Nuclear magnetic resonannce (NMR); D. Magnetic properties; D. Phase transition
1. Introduction (TTM-TTP)I3, where TTM-TTP is 2,5-bis(4,5-bis(methyltio)-1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene, is a charge transfer salt with the donor to acceptor ratio of 1:1 [1]. The TTM-TTP molecules form one-dimensional (1D) stacks along the crystallographic c axis. Considering the charge neutrality, it is expected that (TTM-TTP)I3 has a 1D half-®lled band. It behaves metallic in the higher temperature region and exhibits a resistivity minimum at 120±160 K at ambient pressure [1]. This may be due to the small on-site Coulomb repulsion, U: TTM-TTP is a large donor molecule as compared with conventional molecules such as TMTCF and ET, which form charge transfer complexes [2]. The electronic state in the insulating phase of the system has not been clari®ed though magnetic susceptibility measurements [3±5], and X-ray studies [4,5] have been carried out. In the present study, we have performed 13C NMR measurements on a sample selectively substituted with 13 C-isotope.
samples 1 and 2 were crystallized with the 1-site, and 6-sites substituted molecules shown in Fig. 1(a) and (b), respectively. The NMR relaxation rate, 1/T1, were measured in the temperature range 40 # T # 300 K on sample 2 because the signal intensity was expected to be large. The observed 13 C signal comes from all 13C-sites; there exist two 13C-sites per molecule, crystallographically different from each other. Measurements of NMR lineshape were carried out on sample 1, where all 13C sites are crystallographically equivalent. All measurements were performed in the temperature range, 5 # T # 300 K, on polycrystalline samples (sample 1 , 10, sample 2 , 20 mg) with the conventional pulsed NMR system. The applied ®eld was 8.2 T. The NMR spectra were obtained by the fast Fourier transformation of spin echo signals. 1/T1 were obtained by measuring the recovery of the nuclear magnetization after saturation comb pulses. Magnetic susceptibility of 1-site substituted sample (,6.4 mg) was measured from room temperature to 2 K with a SQUID magnetometer (Quantum design) at 1 T.
2. Experimental
3. Results and discussions
Two kinds of
13
C-substituted molecules were prepared;
* Corresponding author.
Temperature dependence of the magnetic susceptibility is shown in Fig. 2. The diamagnetism of core contributions has not been subtracted, while the Curie term observed in the
0022-3697/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S 0022-369 7(00)00176-1
406
M. Onuki et al. / Journal of Physics and Chemistry of Solids 62 (2001) 405±407
Fig. 1. (a) 1-site substituted, and (b) 6-sites substituted TTM-TTP molecules.
Fig. 2. Temperature dependence of the static magnetic susceptibility. (The Curie term was subtracted.)
Fig. 4. 13C NMR spectra of the 1-site substituted sample at various temperatures.
Fig. 3. Temperature dependence of the 13C NMR relaxation rate, 1/T1.
low temperature region has been subtracted. The susceptibility gradually decreases with decreasing T down to 120 K, and begins to decrease more rapidly below that temperature. It suggests a transition into a nonmagnetic ground state at 120 K. This behavior was also observed by the other research groups[4,5]. Temperature dependence of 1/T1 on sample 2 is shown in Fig. 3. In the higher temperature region above 120 K, the relaxation rate obeys a Korringa relation T1 T21 const: which is expected in a metallic state. Korringa constant (T1T ) 21 , 0.0025 s 21K 21 is small as compared with the other donor type organic conductors; e.g. ET2X T1 T21 , 0:05 s21 K21 : This may re¯ect a wide conduction band in the system. Below 120 K, 1/T1 starts to decrease exponentially. This is due to a transition into a spin-singlet state. We have measured 13C NMR spectrum to obtain further information about the electronic state in the insulating state. Fig. 4 shows the 13C NMR lineshapes at various temperatures. The horizontal axis shows the shift from
M. Onuki et al. / Journal of Physics and Chemistry of Solids 62 (2001) 405±407
the resonance position of the standard sample [tetramethylsilane (TMS)]. The single line observed in the higher temperature region indicates that all the 13C atoms are in a crystallographically equivalent site. On the other hand, the lineshape becomes asymmetric and broader below 120 K, and the peak splits into two lines with almost the same signal intensity below about 80 K. This indicates that the 13C sites are no more equivalent. Considering a half-®lled band and two-fold periodicity reported at low temperature, it is expected that the distribution of the electron is modulated with a 2kF wave number. The line separation is about 45 ppm. This value is too large to explain as caused by pure 2kF lattice distortion. This separation should be made not only by the lattice distortion but also by the electronic modulation. The most probable situation, we suppose, is the alternative stacking of molecules with rich and poor electron density with two-fold periodicity along the c axis. The insulating state of this system is nonmagnetic. Therefore, the degree of the charge disproportionation is considered as schematically TTM-TTP0-TTM-TTP21. Both the neutral and divalent molecules are nonmagnetic. We consider that this type of ground state is stable when the onsite Coulomb U is small and the inter-site Coulomb V is large. To con®rm the present model, the comparison with the NMR shift of TTM-TTP0 and TTM-TTP 21 should be crucial. This is under way. It is necessary that the chemical shift values of each valence of molecules are identi®ed. 4. Concluding remarks In conclusion, the result of
13
C NMR measurements on
407
(TTM-TTP)I3 indicates the charge ordering below about 120 K. We propose that the neutral and divalent TTMTTP molecules have been arranged alternately along the stacking direction below 80 K. This model re¯ects well the feature of small on-site Coulomb repulsion U due to the large size of the TTM-TTP molecule.
Acknowledgements This work was supported in part by the ªResearch for Futureº Project no.JSPS-REFTF97P00105, by the Japan Society for Promotion of Science; and Grant-in-Aid for Scienti®c Research no. 11740211 from the Ministry of Education, Science, Sports and Culture, Japan.
References [1] T. Mori, H. Inokuchi, Y. Misaki, T. Yamabe, H. Mori, S. Tanaka, Bull. Chem. Soc. Jpn 67 (1994) 61. [2] L.K. Montgomery, in: J.-P Farges (Ed.), Organic Conductors, Marcel Dekker, New York, 1994, p. 115. [3] T. Mori, T. Kawamoto, J. Yamaura, T. Enoki, Y. Misaki, T. Yamabe, H. Mori, S. Tanaka, Phys. Rev. Lett. 79 (1997) 1702. [4] M. Maesato, Y. Sasou, S. Kagoshima, T. Mori, T. Kawamoto, Y. Misaki, T. Yamabe, Synth. Met. 103 (1999) 2109. [5] N. Fujimura, A. Namba, T. Kambe, Y. Nogami, K. Oshima, T. Mori, T. Kawamoto, Y. Misaki, T. Yamabe, Synth. Met. 103 (1999) 2111.