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ESR studies of K-doped C60
Volume 203, number 4
CHEMICAL PHYSICS LETTERS
26 February 1993
ESR studies of K-doped CGO M. Kosaka, IL Tanigaki, I. Hirosawa, Y. Shimakawa, S. Kuroshima, T.W. Ebbesen, J. Mizuki and Y. Kubo Fundamental Research Laboratories, NEC Corporation, 34 Miyukigaoka. Tsukuba 305. Japan
Received 27 October 1992; in final form 11 December 1992
Precise electron spin resonance (ESR) spectroscopic studies have been done for KxCM).Three ESR peaks corresponding to the three important crystal phases of K,C, (face-centered cubic K&, body-centered tetragonal KJ& and body-centered cubic KJ&,) are reported. From the temperature dependencies of the ESR intensity for these three phases, it is confirmed that the superconducting K,C, phase is metallic, and that both of the non-superconducting K.& and K,&,, phases are semiconducting or insulating. The electronic properties of K,C, are discussed in comparison with those of Rb&-, and K&s.
1. Introduction The discovery of superconductivity in potassiumdoped CeO [ 1 ] has aroused great interest in fullerenes. The superconducting transition temperature ( T,) of alkali-metal-doped CsOwas raised from 18 K for K3Ch0[ 1] to 33 K for Rb,Cs& [ 21 ( 28 K for Rb&, [ 31, 3 1 K for Rb2Cs,C60 [ 21). The superconducting phase of alkali-metal-doped Cso ( MxC&, where M is the alkali-metal, has been confirmed to be M$&, with face-centered cubic (fee) structure [ 4 1. In addition, body-centered tetragonal (bet ) M&, [ 5 ] and body-centered cubic (bee) M,Ca [ 61 phases have also been confirmed. They have been shown to be non-superconducting [ 7 1. This new group of superconductors is molecular (C,) based and therefore should show some properties different to those of copper oxide high T, superconductors. In this regard, ESR is considered to supply important information about the structural and electronic characteristics for these new materials, although it may not be effective for the latter case. However, so far, the number of ESR studies is limited. Previous reports [&lo] showed that KxCbo exhibits strong ESR absorption at low temperatures and that the linewidth broadened with temperature. However, those studies were not able to clearly elucidate which phase contributes to the absorption. Here we report the three ESR peaks related to the
predominant three phases (fee K3Cm,bet KoCbo,bee K.&6o) which have not been separately observed so far. The electronic properties of these three crystal phases are discussed based on the temperature dependencies of ESR intensity and of its linewidth,
2. Experimental High quality Ceo was obtained by purification of a crude C.&C,,, mixture (Texas Fullerene) with an aluminum column using hexane/toluene eluents. CeO (20 mg) was put in a 5 mm diameter ESR tube and the stoichiometric amount of dopant K ( K3Ch0:3.3 mg, &CGo:6.5 mg) was introduced into C6,,in a glove box. The samples were degassed to 10e2 Torr and sealed. These were heated in a furnace first at 200°C for 3 d and then at 430” C for 9 d. Then each sample was transferred to another clean ESR tube, degassed and sealed in order to remove the effect of ESR detectable color-centers in the tube, which were caused by reaction with K. K.,Cbowas prepared from the mixture of K$& with &Cao and heated at 250°C for 30 d. ESR absorption was measured using a JEOL JES-RE2X electron spin resonance spectrometer operating at 9.1 GHz with a field modulation frequency of 100 kHz. Mnf was used as a marker of a reference of ESR intensity and TEMPOL was used for estimation of spin concentration. A JEOL ES-
0009-2614/93/$ 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.
429
Volume203,number 4
CHEMICALPHYSICSLETTERS
LTRSX cryostat allowed the temperature variation from 5 to 300 K within 2 0.1 K precision. After the ESR measurements, the samples were divided into two. One sample (10 mg) was used for the temperature dependence measurement of the magnetic shielding under 10 Oe using a SQUID magnetometer (Quantum design MPMS), from which the superconducting phase fraction was estimated. The other sample was transferred into a thin capillary tube for X-ray analyses to determine the crystal structures.
3. Results and discussion ESR spectra for the three nominal compositions of KxC6,, were measured at 5 K as shown in fig. 1. In the sample with the nominal composition K&, a strong broad peak (peak A in fig. 1a) was observed. In the nominal composition K&, sample in fig. I b, A ii
a)
!(i
I; / i
;
26 February 1993
a narrow peak B was observed together with a small peak A and a broad peak C. In Ihe nominal composition KJ6,, sample shown in fig. lc, a broad peak was observed as well as small A and B peaks. g-factors were estimated to be g,=2.0014, g,= 2.0004 and g,= 1.9952. From the X-ray diffraction patterns and SQUID measurements described in the following paragraph, the observed three peaks can be assigned as follows: peak A corresponds to fee K3C&, the peak B to bet K4CS0, and peak C to bee KJZsO. The X-ray diffraction patterns of these crystal structures confirm that the nominal composition K,C,, sample is fee K,C,,. Furthermore, the nominal composition K&, is clearly bee I&&,. The nominal composition K&, sample can be best ascribed to bet K&o phase together with a little amount of K&,, and K&, phases. The temperature dependencies of the magnetic susceptibility of the prepared samples were also studied to confirm the observed ESR assignment, as shown in fig. 2. The superconducting fraction in the nominal composition K&, sampte is estimated to be 78Oh, in the nominal K.,Cho sample 6% and in the nominal K&,, sample 0.3%. It is known that only fee K3& shows superconductivity [ 41. Therefore, the superconducting fraction estimated from the magnetic shielding should correspond to the percentage of the K,Ceo phase present in each sample. The observed per-
b)
0
5
10
15
20
25
T(K) 3250
3270
3290
H(G)
Fig. I. ESRspectrameasuredat 5 K. (a) The nominal composition K3Cmsample, (b) the nominal K.,C, sample, (c) the nominal K&sample.
430
Fig. 2. The temperaturedependenciesof the magneticsusceptibilityby SQUIDfor the nominal composition K&sample ( l ), for the nominal K&, sample (0 ).
sample (A ) and for the nominal KJ&,
Volume 203, number 4
CHEMICAL PHYSICS LETTERS
centage of the superconducting fractions clearly supports the assignment of the three ESR peaks to the corresponding crystal phases. Although the susceptibilities from the present SQUID measurements under low magnetic fields do not allow the precise estimation of the Pauli paramagnetism, high field experiments gave the density of states in K&, to be 14 state/eV spin CbO[ It]. Next the electronic properties of these three phases are discussed from the viewpoint of the ESR intensity and linewidth change as a function of temperature. The temperature dependence of the peak intensities for the three crystal phases is shown in fig. 3a and that of the linewidths is shown in fig. 3b. The ESR intensity for K,C,, is roughly constant in the high temperature range but increases abruptly in the low temperature range. The Pauli-like behavior observed in the high temperature range indicates the existence of conduction electrons in the K3C6,,phase and that the I&&,, phase is metallic. In the observed ESR spectrum for K#&,, a small distortion of the line shape was observed. This is probably related to the skin depth effect which is a typical phenomenon observed in metallic samples. The significant increase in the ESR signal intensity in the low temperature range would not be expected from the conduction electrons. Under superconducting states, since some of the conduction electrons near the edge of the Fermi level will make
b) 20
0.00
0.10
1/T(K)
0.20
0
100
200
300
T(K)
Fig. 3. The temperature dependencies of the ESR intensity and of its linewidth for the IQ& ( 0 ), K.&, (A ) and K.& ( q ) phases.
26 February 1993
superconducting paired electrons, the ESR intensity must decrease below T,. Therefore, the observed ESR intensity change in the low temperature range could be related to a ferromagnetic spin behavior, which has been reported for the TDAE-C60complex system
[Ql. On the other hand, the ESR intensities for IL& and K&,, showed Curie-like behavior. The spin concentrations are estimated to be 0.01 spins per K.,C6oand 0.02 spins per IL&,. Since the perfect crystal of I(6&,, does not have free spins, the spins observed for the prepared K&u sample would relate to the K-deficient nonstoichiometric defects in the crystal. The spin concentration for the prepared K.&,, sample was the same amount as that for K&n, Therefore, the spins observed for K&, would also be related to the defects in the crystal. The absence of Pauli-like behavior in K&, and K&o indicates that both phases are semiconducting or insulating. According to the rigid band model for C6,,solid, the lower edge of the conduction band consists of three band surfaces with t,, symmetry, indicating that I(4Cbomight be metallic. The experimental result that I(4C&is not metallic would imply that CsOunder this state would be deformed by the Jahn-Teller effect. In this case, the three tr, bands would be either split into two lower energy bands and one higher energy band, or completely split. The ESR linewidth of K3r& decreases with decreasing temperature down to T, and then increases below T, as shown in fig. 3b. The obtained change above T, can be interpreted in terms of the spin relaxation time. The increase in relaxation time with decreasing temperature will explain the observed line shape change. The linewidth increase phenomenon below T, might be due to strong electron correlation in the ferro-magnetic spin state [ 121. The fact that the linewidth for I&C, decreases with decreasing temperature (fig. 3b) would also be attributed to the spin relaxation time. On the other hand, the linewidth of IL+& is narrow and nearly independent of temperature (fig. 3b). This is unusual and it is being studied further. We also successfully observed three ESR peaks for Rb,Cso, which correlate with the three kinds of crystal phases, fee, bet and bee. These peak shapes, g-factors and temperature dependencies of ESR intensity and of linewidth have the same tendency as those for 431
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CHEMICAL PHYSICS LETTERS
I&&,. However, K,& showed different characteristics from both K&,, and Rb&,. In the nominal composition K3C&sample, only one narrow peak was observed in our experiments and its temperature dependence of the ESR intensity exhibits a Curie-like behavior. This result would imply that the K& phase is not metallic.
4. Conclusions
We studied K,C, ESR spectra in detail and observed three ESR peaks correlating with the three kinds of crystal phases (fee K&,, bet K&,,, and bee K&m). From the temperature dependencies of the ESR intensity for these three phases, it was confirmed that the superconducting K3CG0phase is metallic, and that both K& and &C& phases are semiconducting or insulating. The fact that IQ&, is not metallic suggests a Jahn-Teller effect in this Cm state.
Acknowledgement
We would like to thank Mr. Mitsuta and Mr. Mizuta of JEOL Corporation for their assistance with the ESR measurements.
432
26 February 1993
References
[I ] A.F. Hebard, M.J. Rosseinsky, R.C. Haddon, D.W. Murphy, S.H. Glarum, T.T.M. Palstra, A.P. Ramirez and A.R. Kortan, Nature 350 ( 1991) 600. [2] K. Tanigaki, T.W. Ebbesen, S. Saito, J. Mizuki, J.S. Tsai, Y. Kubo and S. Kuroshima, Nature 352 (1991) 222. [ 31 M.J. Rosseinsky,A.P. Ramirez, S.H. Glarum, D.W. Murphy, R.C. Haddon, A.F. Hebard, T.T.M. Palstra, A.R. Kortan, S.M. Zahurak and A.V. Makhija, Phys. Rev. Letters 66 (1991) 21. [4]P.W. Stephens, L. Mihaly, P.L. Lee, R.L. Whetten, S.M. Huang, R. Kaner, F. Diederich and K. Holczer, Nature 35 I (1991) 632. [ 51K. Holczer, 0. Klein, S.M. Huang, R.B. Kaner, K.J. Fu, R.L. Wbetten and F. Diederich, Science 252 ( 1991) 1154. [6] 0. Zhou, J.E. Fischer, N. Coustel, S. Kycia, Q. Zhu, A.R. McGhie, W.J. Romanow, J.P. McCauley Jr, A.B. Smith III and D.E. Cox, Nature 35 1 ( 199I ) 462. [7]R.M. Fleming, M.J. Rosseinsky, A.P. Ramirez, D.W. Murphy, J.C. T&y, R.C. Haddon, T. Siegrist, R. Tycko, S.H. Glarum, P. Marsh, G. Dabbagh, SM. Zaburak, A.V. Makhija and C. Hampton, Nature 352 ( 199I ) 70 1. [8] A.A. Zakhidov, A. Ugawa, K. Imaeda, K. Yakushi, H. Inokuchi, K. Kikuchi, I. Ikemoto, S. Suzuki and Y. Achiba, Solid State Commun. 79 ( 1991) 939. [9] S.H. Glarum, S.J. Duclos and R.C. Haddon, J. Am. Chem. Sot. 114 (1992) 1996. [IO] P. Petit and K. Holczer, Synth. Met., Proceedings of the IntemationaI Conference on Synthetic Metals ‘92, Synth. Met., ( 1992) p. 330, to be published. [ 111A.P. Ramirez, M.J. Rosseinsky, D.W. Murphy and R.C. Haddon, Phys. Rev. Letters 69 (1992) 1687. [ 121P.M. Allemand, K.C. Khemani, A. Koch, F. Wudl, K. Holczer, S. Donovan, G. Gnmer and J.D. Thompson, Seienee253 (1991) 301.
CHEMICAL PHYSICS LETTERS
26 February 1993
ESR studies of K-doped CGO M. Kosaka, IL Tanigaki, I. Hirosawa, Y. Shimakawa, S. Kuroshima, T.W. Ebbesen, J. Mizuki and Y. Kubo Fundamental Research Laboratories, NEC Corporation, 34 Miyukigaoka. Tsukuba 305. Japan
Received 27 October 1992; in final form 11 December 1992
Precise electron spin resonance (ESR) spectroscopic studies have been done for KxCM).Three ESR peaks corresponding to the three important crystal phases of K,C, (face-centered cubic K&, body-centered tetragonal KJ& and body-centered cubic KJ&,) are reported. From the temperature dependencies of the ESR intensity for these three phases, it is confirmed that the superconducting K,C, phase is metallic, and that both of the non-superconducting K.& and K,&,, phases are semiconducting or insulating. The electronic properties of K,C, are discussed in comparison with those of Rb&-, and K&s.
1. Introduction The discovery of superconductivity in potassiumdoped CeO [ 1 ] has aroused great interest in fullerenes. The superconducting transition temperature ( T,) of alkali-metal-doped CsOwas raised from 18 K for K3Ch0[ 1] to 33 K for Rb,Cs& [ 21 ( 28 K for Rb&, [ 31, 3 1 K for Rb2Cs,C60 [ 21). The superconducting phase of alkali-metal-doped Cso ( MxC&, where M is the alkali-metal, has been confirmed to be M$&, with face-centered cubic (fee) structure [ 4 1. In addition, body-centered tetragonal (bet ) M&, [ 5 ] and body-centered cubic (bee) M,Ca [ 61 phases have also been confirmed. They have been shown to be non-superconducting [ 7 1. This new group of superconductors is molecular (C,) based and therefore should show some properties different to those of copper oxide high T, superconductors. In this regard, ESR is considered to supply important information about the structural and electronic characteristics for these new materials, although it may not be effective for the latter case. However, so far, the number of ESR studies is limited. Previous reports [&lo] showed that KxCbo exhibits strong ESR absorption at low temperatures and that the linewidth broadened with temperature. However, those studies were not able to clearly elucidate which phase contributes to the absorption. Here we report the three ESR peaks related to the
predominant three phases (fee K3Cm,bet KoCbo,bee K.&6o) which have not been separately observed so far. The electronic properties of these three crystal phases are discussed based on the temperature dependencies of ESR intensity and of its linewidth,
2. Experimental High quality Ceo was obtained by purification of a crude C.&C,,, mixture (Texas Fullerene) with an aluminum column using hexane/toluene eluents. CeO (20 mg) was put in a 5 mm diameter ESR tube and the stoichiometric amount of dopant K ( K3Ch0:3.3 mg, &CGo:6.5 mg) was introduced into C6,,in a glove box. The samples were degassed to 10e2 Torr and sealed. These were heated in a furnace first at 200°C for 3 d and then at 430” C for 9 d. Then each sample was transferred to another clean ESR tube, degassed and sealed in order to remove the effect of ESR detectable color-centers in the tube, which were caused by reaction with K. K.,Cbowas prepared from the mixture of K$& with &Cao and heated at 250°C for 30 d. ESR absorption was measured using a JEOL JES-RE2X electron spin resonance spectrometer operating at 9.1 GHz with a field modulation frequency of 100 kHz. Mnf was used as a marker of a reference of ESR intensity and TEMPOL was used for estimation of spin concentration. A JEOL ES-
0009-2614/93/$ 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.
429
Volume203,number 4
CHEMICALPHYSICSLETTERS
LTRSX cryostat allowed the temperature variation from 5 to 300 K within 2 0.1 K precision. After the ESR measurements, the samples were divided into two. One sample (10 mg) was used for the temperature dependence measurement of the magnetic shielding under 10 Oe using a SQUID magnetometer (Quantum design MPMS), from which the superconducting phase fraction was estimated. The other sample was transferred into a thin capillary tube for X-ray analyses to determine the crystal structures.
3. Results and discussion ESR spectra for the three nominal compositions of KxC6,, were measured at 5 K as shown in fig. 1. In the sample with the nominal composition K&, a strong broad peak (peak A in fig. 1a) was observed. In the nominal composition K&, sample in fig. I b, A ii
a)
!(i
I; / i
;
26 February 1993
a narrow peak B was observed together with a small peak A and a broad peak C. In Ihe nominal composition KJ6,, sample shown in fig. lc, a broad peak was observed as well as small A and B peaks. g-factors were estimated to be g,=2.0014, g,= 2.0004 and g,= 1.9952. From the X-ray diffraction patterns and SQUID measurements described in the following paragraph, the observed three peaks can be assigned as follows: peak A corresponds to fee K3C&, the peak B to bet K4CS0, and peak C to bee KJZsO. The X-ray diffraction patterns of these crystal structures confirm that the nominal composition K,C,, sample is fee K,C,,. Furthermore, the nominal composition K&, is clearly bee I&&,. The nominal composition K&, sample can be best ascribed to bet K&o phase together with a little amount of K&,, and K&, phases. The temperature dependencies of the magnetic susceptibility of the prepared samples were also studied to confirm the observed ESR assignment, as shown in fig. 2. The superconducting fraction in the nominal composition K&, sampte is estimated to be 78Oh, in the nominal K.,Cho sample 6% and in the nominal K&,, sample 0.3%. It is known that only fee K3& shows superconductivity [ 41. Therefore, the superconducting fraction estimated from the magnetic shielding should correspond to the percentage of the K,Ceo phase present in each sample. The observed per-
b)
0
5
10
15
20
25
T(K) 3250
3270
3290
H(G)
Fig. I. ESRspectrameasuredat 5 K. (a) The nominal composition K3Cmsample, (b) the nominal K.,C, sample, (c) the nominal K&sample.
430
Fig. 2. The temperaturedependenciesof the magneticsusceptibilityby SQUIDfor the nominal composition K&sample ( l ), for the nominal K&, sample (0 ).
sample (A ) and for the nominal KJ&,
Volume 203, number 4
CHEMICAL PHYSICS LETTERS
centage of the superconducting fractions clearly supports the assignment of the three ESR peaks to the corresponding crystal phases. Although the susceptibilities from the present SQUID measurements under low magnetic fields do not allow the precise estimation of the Pauli paramagnetism, high field experiments gave the density of states in K&, to be 14 state/eV spin CbO[ It]. Next the electronic properties of these three phases are discussed from the viewpoint of the ESR intensity and linewidth change as a function of temperature. The temperature dependence of the peak intensities for the three crystal phases is shown in fig. 3a and that of the linewidths is shown in fig. 3b. The ESR intensity for K,C,, is roughly constant in the high temperature range but increases abruptly in the low temperature range. The Pauli-like behavior observed in the high temperature range indicates the existence of conduction electrons in the K3C6,,phase and that the I&&,, phase is metallic. In the observed ESR spectrum for K#&,, a small distortion of the line shape was observed. This is probably related to the skin depth effect which is a typical phenomenon observed in metallic samples. The significant increase in the ESR signal intensity in the low temperature range would not be expected from the conduction electrons. Under superconducting states, since some of the conduction electrons near the edge of the Fermi level will make
b) 20
0.00
0.10
1/T(K)
0.20
0
100
200
300
T(K)
Fig. 3. The temperature dependencies of the ESR intensity and of its linewidth for the IQ& ( 0 ), K.&, (A ) and K.& ( q ) phases.
26 February 1993
superconducting paired electrons, the ESR intensity must decrease below T,. Therefore, the observed ESR intensity change in the low temperature range could be related to a ferromagnetic spin behavior, which has been reported for the TDAE-C60complex system
[Ql. On the other hand, the ESR intensities for IL& and K&,, showed Curie-like behavior. The spin concentrations are estimated to be 0.01 spins per K.,C6oand 0.02 spins per IL&,. Since the perfect crystal of I(6&,, does not have free spins, the spins observed for the prepared K&u sample would relate to the K-deficient nonstoichiometric defects in the crystal. The spin concentration for the prepared K.&,, sample was the same amount as that for K&n, Therefore, the spins observed for K&, would also be related to the defects in the crystal. The absence of Pauli-like behavior in K&, and K&o indicates that both phases are semiconducting or insulating. According to the rigid band model for C6,,solid, the lower edge of the conduction band consists of three band surfaces with t,, symmetry, indicating that I(4Cbomight be metallic. The experimental result that I(4C&is not metallic would imply that CsOunder this state would be deformed by the Jahn-Teller effect. In this case, the three tr, bands would be either split into two lower energy bands and one higher energy band, or completely split. The ESR linewidth of K3r& decreases with decreasing temperature down to T, and then increases below T, as shown in fig. 3b. The obtained change above T, can be interpreted in terms of the spin relaxation time. The increase in relaxation time with decreasing temperature will explain the observed line shape change. The linewidth increase phenomenon below T, might be due to strong electron correlation in the ferro-magnetic spin state [ 121. The fact that the linewidth for I&C, decreases with decreasing temperature (fig. 3b) would also be attributed to the spin relaxation time. On the other hand, the linewidth of IL+& is narrow and nearly independent of temperature (fig. 3b). This is unusual and it is being studied further. We also successfully observed three ESR peaks for Rb,Cso, which correlate with the three kinds of crystal phases, fee, bet and bee. These peak shapes, g-factors and temperature dependencies of ESR intensity and of linewidth have the same tendency as those for 431
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CHEMICAL PHYSICS LETTERS
I&&,. However, K,& showed different characteristics from both K&,, and Rb&,. In the nominal composition K3C&sample, only one narrow peak was observed in our experiments and its temperature dependence of the ESR intensity exhibits a Curie-like behavior. This result would imply that the K& phase is not metallic.
4. Conclusions
We studied K,C, ESR spectra in detail and observed three ESR peaks correlating with the three kinds of crystal phases (fee K&,, bet K&,,, and bee K&m). From the temperature dependencies of the ESR intensity for these three phases, it was confirmed that the superconducting K3CG0phase is metallic, and that both K& and &C& phases are semiconducting or insulating. The fact that IQ&, is not metallic suggests a Jahn-Teller effect in this Cm state.
Acknowledgement
We would like to thank Mr. Mitsuta and Mr. Mizuta of JEOL Corporation for their assistance with the ESR measurements.
432
26 February 1993
References
[I ] A.F. Hebard, M.J. Rosseinsky, R.C. Haddon, D.W. Murphy, S.H. Glarum, T.T.M. Palstra, A.P. Ramirez and A.R. Kortan, Nature 350 ( 1991) 600. [2] K. Tanigaki, T.W. Ebbesen, S. Saito, J. Mizuki, J.S. Tsai, Y. Kubo and S. Kuroshima, Nature 352 (1991) 222. [ 31 M.J. Rosseinsky,A.P. Ramirez, S.H. Glarum, D.W. Murphy, R.C. Haddon, A.F. Hebard, T.T.M. Palstra, A.R. Kortan, S.M. Zahurak and A.V. Makhija, Phys. Rev. Letters 66 (1991) 21. [4]P.W. Stephens, L. Mihaly, P.L. Lee, R.L. Whetten, S.M. Huang, R. Kaner, F. Diederich and K. Holczer, Nature 35 I (1991) 632. [ 51K. Holczer, 0. Klein, S.M. Huang, R.B. Kaner, K.J. Fu, R.L. Wbetten and F. Diederich, Science 252 ( 1991) 1154. [6] 0. Zhou, J.E. Fischer, N. Coustel, S. Kycia, Q. Zhu, A.R. McGhie, W.J. Romanow, J.P. McCauley Jr, A.B. Smith III and D.E. Cox, Nature 35 1 ( 199I ) 462. [7]R.M. Fleming, M.J. Rosseinsky, A.P. Ramirez, D.W. Murphy, J.C. T&y, R.C. Haddon, T. Siegrist, R. Tycko, S.H. Glarum, P. Marsh, G. Dabbagh, SM. Zaburak, A.V. Makhija and C. Hampton, Nature 352 ( 199I ) 70 1. [8] A.A. Zakhidov, A. Ugawa, K. Imaeda, K. Yakushi, H. Inokuchi, K. Kikuchi, I. Ikemoto, S. Suzuki and Y. Achiba, Solid State Commun. 79 ( 1991) 939. [9] S.H. Glarum, S.J. Duclos and R.C. Haddon, J. Am. Chem. Sot. 114 (1992) 1996. [IO] P. Petit and K. Holczer, Synth. Met., Proceedings of the IntemationaI Conference on Synthetic Metals ‘92, Synth. Met., ( 1992) p. 330, to be published. [ 111A.P. Ramirez, M.J. Rosseinsky, D.W. Murphy and R.C. Haddon, Phys. Rev. Letters 69 (1992) 1687. [ 121P.M. Allemand, K.C. Khemani, A. Koch, F. Wudl, K. Holczer, S. Donovan, G. Gnmer and J.D. Thompson, Seienee253 (1991) 301.