Mayumi Kosaka

ANNEALING EFFECT ON CARBON NANOTUBES AN ESR STUDY

The effect of annealing on the electronic properties of nanotubes has been studied. The conduction ESR intensity is temperature-independent for both annealed and non-annealed nanotubes. However, the g-value of the nanotubes and its temperature dependence change significantly after annealing. These results suggest that some types of defects are present in the nanotubes which are then removed by the annealing process. The ESR studies of the annealed nanotubes indicate that the intrinsic electronic properties of nanotubes are quite different from those of graphite.

Electron spin resonance of carbon nanotubes

Abstract The electron spin resonance (ESR) of carbon nanotubes at various stages of purification have been measured between 4 and 296 K. The conduction electron spin resonance is identified and the results imply that metallic and/or narrow gap semiconducting nanotubes are actually present as predicted by theory. From the temperature dependences of the ESR intensity, linewidth and g -value, the electronic properties of the observed nanotubes are found to be similar to those of graphite.

ESR studies of K-doped C60

Abstract Precise electron spin resonance (ESR) spectroscopic studies have been done for K x C 60 . Three ESR peaks corresponding to the three important crystal phases of K x C 60 (face-centered cubic K 3 C 60 , body-centered tetragonal K 4 C 60 , and body-centered cubic K 6 C 60 ) are reported. From the temperature dependencies of the ESR intensity for these three phases, it is confirmed that the superconducting K 3 C 60 phase is metallic, and that both of the non-superconducting K 4 C 60 and K 6 C 60 phases are semiconducting or insulating. The electronic properties of K x C 60 are discussed in comparison with those of Rb x C 60 and K x C 70 .

Alkali effects on the electronic states of K3C60 and Rb3C60

The electronic states in metallic alkali fullerides K3C60 and Rb3C60, are studied in detail by cw- and pulsed-ESR. The ESR linewidth associated with conduction electrons varies substantially, depending sensitively on the alkali metals used. It is clearly revealed that the linewidth is homogeneous and is strongly controlled by the spin-lattice relaxation times (T1) determined via the spin-orbit interactions of the small fraction of s electrons remaining on the alkali atoms. The results indicate that the electron transfer from the alkali atoms to C60 is not perfectly complete, with the remaining fraction being much greater in the tetrahedral sites than in the octahedral sites of the face-centered cubic lattice.

Electronic States and Superconductivity in Alkali-Intercalated Fullerides: 13C-NMR Study in Na2RbC60, Na2CsC60, K3C60, K2RbC60, K2CsC60, KRbCsC60, Rb2CsC60 and RbCs2C60

A systematic 13 C NMR experiment is reported in alkali-fullerides A 3 C 60 (where A 3 is alkali-metals) between superconducting transition temperature T c and ∼100 K. Careful 13 C NMR spin-lattice relaxation time study clarified the properties of the conduction electron wave functions at Fermi surface. An upper limit of Stoner exchange parameter was estimated to be ∼3 eV/(spin-carbon), which is far from spin-wave instability criterion. Based on these studies, a variation of the density of states at Fermi level, N ( E f ), with T c was deduced. The results were analyzed in terms of McMillan formula, leading to weak or medium electron-phonon coupling constants and relevant phonon energy larger than 600 K. It was strongly suggested that the A 3 C 60 superconductor is described within a framework of the conventional phonon-mediated BCS superconductivity.

mu +SR study of zero-field magnetic ordering in CsC60

We have measured the zero-field muon spin relaxation (ZF mu +SR) spectra of the quasi-one-dimensional fulleride conductor CsC60. Static magnetic order of a random nature develops gradually below about 30 K. The local field distribution at the muon site has a Gaussian shape with a width of 48.3(7) G at 2.8 K. Muon spin rotation (TF mu +SR) measurements at an applied transverse field of 6 kG also show the onset of magnetic order in the same temperature range.

Molecular dynamics of C60 in superconducting Na2CsC60

Abstract We have measured the low-energy inelastic neutron scattering spectra of superconducting Na 2 CsC 60 in the temperature range 100–350 K. Well-defined librational peaks are observed at 2.7(1) meV; their position and width are temperature independent. They survive well above the order-disorder transition at 299 K. A quasielastic feature with a width 1.03(6) meV at an average scattering vector of 1.09 A −1 is observed at 350 K, arising from diffusive motion of the C 60 3− ions. Evidence is found for influence of the Na + ions on the rotational motion of the C 60 3− ions.

NMR Studies on Orientational Ordering Phase Transition in Na2CsC60

Sodium-containing alkali metal fulleride, Na 2 CsC 60 , was studied by 13 C and 23 Na NMR below and above the face-centered cubic ( f c c ) to simple cubic ( s c ) phase transition temperature, T m . It is shown that the C 60 molecules rotate faster than the rotational correlation time of 1.6×10 -5 sec above ∼140 K and 2.4×10 -9 sec around 300 K. Using 23 Na NMR spectra broadened by the second-order quadrupole interaction, the electric field gradient at Na sites, e 2 q Q / h , was estimated to be 940±20 kHz and less than 0.1 kHz below and above T m , respectively. The detailed analysis suggests that Na atoms in s c phase have a covalent character with C 60 molecules. It is also suggested that Na 2 CsC 60 is the first metallic plastic crystal.

ESR and heat capacity studies of phase transition in Rb1C60

Abstract Rb 1 C 60 is studied by ESR as a function of frequency from several tenth MHz to 26 GHz and by heat capacity, to reveal the nature of the phase transition around 50 K. Above the transition temperature, the electronic states are consistent with a metallic state. Below it, the ESR linewidth increases linearly with decreasing T , which is ascribed to a development of short-range order (SRO) based on its frequency dependence. Furthermore, the present analysis of the EPR spectra suggested a distribution of the linewidth that was consistent with broadly distributed transition temperatures of the spin density wave (SDW) or antiferromagnetism. The heat capacity shows a broad anomaly from 50 K down to 30 K The apparent entropy change of such a magnetic anomaly suggests that only one fourth of spins are in the ordered state at 30 K, which is consistent with the reported intensity of AFMR.

The MEM Charge Density Study of Rb2CsC60, K2RbC60, and Li2CsC60 by Synchrotron Radiation Powder Method

A charge density study of alkali metal doped fullerenes, Rb2CsC60, K2RbC60 and Li2CsC60, was carried out at room temperature by the Maximum Entropy Method (MEM) combined with the Rietveld analysis using synchrotron powder data. The MEM charge densities of the C60 molecules show distinct differences related to the superconducting properties. For both K2RbC60 and Rb2CsC60, which are superconductors at low temperature, the C60 molecules were in a merohedral disorder. On the other hand, the C60 molecules in Li2CsC60; non-superconductor even at very low temperature, were in an orientational disorder. In the interatomic region between the C60 molecules and doped metal atoms, any significant overlapped charge densities related to covalent bond were not found for all three materials. This is the clear evidence for that superconductivity of these materials and covalency of doped metals have no correlation at all. Instead, the charge deficiencies of the doped metal atoms which should be associated with charge transfer from the metal atoms to the C60 molecule seems to have a strong correlation to the superconducting transition temperature, Tc, i.e. the higher the Tc, the more the deficiencies.