The conduction-electron spin resonance of the superconducting fulleride compound, ${\mathrm{K}}_{3}{\mathrm{C}}_{60}$ ${(T}_{c}=19$ K) has been observed between 2.5 and 800 K at several frequencies from 9 to 225 GHz. Between ${T}_{c}$ and ${0.7T}_{c}$ the spin lifetime of normal excitations increases as predicted by the theory of Yafet for scattering from nonmagnetic structural defects. At lower temperatures the CESR arises from states bound to vortices; here the spectrum is inhomogeneously broadened and an anomalous g shift is found. The anomalous temperature dependence of the g factor and linewidth in the normal state reported by Petit et al. is most important between 50 and 250 K, suggesting that these are related to the rotation of ${\mathrm{C}}_{60}$ ions.

We present a 13C and 133Cs NMR investigation of the CsC60 cubic quenched phase. Previous ESR measurements suggest that this phase is metallic, but NMR reveals contrasting electronic behavior on the local scale. The 13C spin-lattice relaxation time (T1) exhibits a typical metallic behavior down to 50 K, but indicates that a partial spin-gap opens for T<50 K. Unexpectedly, 133Cs NMR shows that there are two inequivalent Cs sites. For one of these sites, the NMR shift and (T1T)^{-1} follow an activated law, confirming the existence of a spin-gap. We ascribe this spin-gap to the occurrence of localized spin-singlets on a small fraction of the C60 molecules.

We present a discussion of the electronic properties of the quenched cubic phase of CsC60, based on our recent investigation by C-13 and Cs-133 NMR. We conclude that the phase is metallic but that localized spin-singlets are formed on a small fraction of the C-60 molecules. We discuss possible mechanisms for the stabilization of these spin-singlets, related to the occurrence of Jahn-Teller distortions of the C-60 molecule.

The low temperature behavior of the CsC60 polymer was investigated by NMR measurements on C-13 and Cs-133 nuclei at ambient and under hydrostatic pressure up to 9 kbar. The existence of a second-order structural phase transition to a spin-singlet (nonmagnetic) ground state at T-s = 13.8 K is clearly established at ambient pressure. This state coexists with the magnetic order that develops at T-N = 30 K. The application of pressure first suppresses the magnetic order and a homogeneous nonmagnetic ground state is stabilized at 5 kbar.

Alkali metal doped fullerides reveal a polymerized crystal structure with parallel chains of covalently bounded C60 molecules. The electronic structure shows a narrow π-electron band. The existence of strong electron-electron correlations in this fulleride is supported by the establishment of an antiferromagnetic ground state at TN=30K. We report the measurements of 133Cs and 13C-NMR relaxation rates on CsC60 polymers which have been performed versus pressure and temperature. Our results show that the hydrostatic pressure suppresses the antiferromagnetic order and stabilizes a spin-singlet long range order at 5kbar. A study of the quadrupolar spin-echo of 133Cs performed at ambient pressure reveals a second order structural transition at TS=13.8K towards a spin-singlet ground state which coexists with the preexisting antiferromagnetic order.

Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation Natasa Čegar, Ferenc Simon, Gabriel Baumgartner, Andrzej Sienkiewicz, László Forró, Barbara Ruzicka, Leonardo Degiorgi, László Mihály; Electronic properties of the Na2AC60 family (A=K, Rb, Cs). AIP Conf. Proc. 30 September 1999; 486 (1): 64–68. https://doi.org/10.1063/1.59838 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAIP Publishing PortfolioAIP Conference Proceedings Search Advanced Search |Citation Search

The conduction electron spin resonance (CESR) of K3C60 single crystal and powder samples was studied from temperatures well below the superconducting transition (Tc=19 K) to 800 K. Several ESR frequencies were used with corresponding magnetic fields up to 8 T. We observed an anomalous change of the g factor with temperature in the superconducting state which may arise from an anisotropy of the superconducting energy gap. We proved that the change of the g factor is not caused by diamagnetic screening.

Alkali metal doped fullerides reveal a polymerized crystal structure with pal allel chains of covalently bounded C-60 molecules. The electronic structure shows a narrow p-electron band. The existence of strong electron-electron correlations in this fuller ide is supported by the establishment of an antiferromagnetic ground state at low temperature. We have performed Cs-133-NMR relaxation time T-1 measurements on (CsC60)(n) versus pressure and temperature. The salient and most unexpected result of our work is the fact that T-1 is not sensitive to pressure (up to 9kbar) in the temperature range 100 and 300K unlike T-1 measurements performed under pressure on C-13 in the parent coumpound (RbC60)(n). This finding allows to rule out a purely magnetic relaxation mechanism for the alkali atom. However, the relaxation becomes strongly pressure dependent at low temperature (T<100K). In particular a new ground state (found to be non magnetic and possibly spin-Peierls) establishes above 5kbar with the concomitant activation of 1/T-1 below 30K.

NH3K3C60 undergoes a magnetic ordering phase transition below 40K which is probably the reason that this system is not a superconductor at ambient pressure. We measured the magnetic field dependence (H=0.3, 2.7, 5.4 and 8.1 T) of the electron spin resonance of NH3K3C60. Below 40 K the resonance broadens and shifts as the magnetic order develops. The broadening and shift are much larger at 0.3 T than at higher fields, but do not follow the 1/H dependence expected for an antiferromagnetic order. The magnetic spin susceptibility measured by the ESR intensity changes only little, if at all, through the transition. At 9 GHz (0.3 T) results depend on the cooling rate from ambient temperatures to temperatures below 50K.

Alkali metal doped fullerides reveal a polymerized crystal structure with parallel chains of covalently bounded C60 molecules. The electronic structure shows a narrow p-electron band. The existence of strong electron-electron correlations in this fulleride is supported by the establishment of an antiferromagnetic ground state at low temperature. We have performed 133Cs-NMR relaxation time T1 measurements on (CsC60)n versus pressure and temperature. The salient and most unexpected result of our work is the fact that T1 is not sensitive to pressure (up to 9kbar) in the temperature range 100 and 300K unlike T1 measurements performed under pressure on 13C in the parent coumpound (RbC60)n. This finding allows to rule out a purely magnetic relaxation mechanism for the alkali atom. However, the relaxation becomes strongly pressure dependent at low temperature (T<100 K). In particular a new ground state (found to be non magnetic and possibly spin-Peierls) establishes above 5kbar with the concomitant activation of 1/T1 below 30K.

We have shown that the two-dimensional polymer of Na4C60 transforms into a monomeric phase above similar to 500 K. The polymer-->monomer phase transition is reversible and the magnitude of the formation enthalpy corroborates the polymeric structure. The high-temperature tetragonal phase is isometric to other A(4)C(60) (A = K, Rb, Cs) compounds but with an extremely short interfullerene distance. Na4C60 is the only metal among its A4C60 analogs and this uniqueness can be rationalized in the Mott-Hubbard picture. [S0163-1829(98)04226-X].

The long-missing stoichiometric phase of Na4C60 has been identified by Rietveld analysis of synchrotron powder diffraction data. Its monoclinic structure is based on polymer planes of C-60 where each molecule forms four ''single'' bonds within the plane. This compound is net only the first fulleride polymer with such bonds, but also the first two-dimensional polymer which is naturally intercalated with alkali ions and can be synthesized at ambient pressure. Na4C60 is a metal and is expected to be a prototype structure where electronic overlap between the planes can be tuned by the alkali cation size.

Antiferromagnetic resonance has been observed in powders of the conducting alkali fulleride linear polymers, RbC60 and CsC60, at high frequencies (75, 150, and 225 GHz). This is proof for an antiferromagnetically ordered ground state and shows that these systems are not spin glasses. The sublattice magnetization is independent of applied magnetic field up to at least 8 T. Magnetic fluctuations are observed between 35 and 50 K. Comparison with the spin-density-wave system (TMTSeF)(2)PF6 clearly shows that these polymers are also quasi-1D spin-density-wave systems with 3D ordering at low temperatures, as suggested previously.

We report Hall coefficient (R(H)) and magnetoresistance (Delta rho/rho) measurements on thin films of aligned carbon nanotubes. R(H) is positive in the whole temperature range (1.7-300 K) showing the predominance of hole transport in the electronic conduction. The upper limit of the carrier concentration is 1.6x10(19) cm(-3). The resistance of the thin-film samples is governed mainly by the loose tube-tube contacts and this shows up in the Delta rho/rho<0 at low fields, suggesting a noncoherent transport between the nanotubes.

We report the results of magnetic measurements on the orthorhombic alkalimetal fulleride Rb 1 C 60 . Measurements of the NMR relaxation T 1 and ESR spin susceptibility under pressure (P ≤ 5 kbar) provide clear evidence for the importance of electron correlations in this phase at variance with superconducting phase 3 fullerides. We show that spin fluctuations (ferromagnetic) persisting up to room temperature order antiferromagnetically at low temperature and give rise to long range order below 15 K. These results support the band structure calculation describing this polymerised phase by a three-dimensional model. The magnetic ground state is suppressed by a pressure exceeding 6 kbar and gives rise to a conducting phase at 12 kbar whose susceptibility is still possibly enhanced by magnetic fluctuations.