Y. Iwasa

Measurement of Complex Electrical Conductivity of Metallic Powders in a Sealed Tube

We studied the method of complex-electrical-conductivity measurement for powders placed in a sealed tube at microwave and millimeter-wave frequencies, using a cavity perturbation technique. We applied this technique to a molecular superconductor K3C60, whose electrical conduction is well known in single crystals, and reproduced the metallic conductivity with the magnitude of 0.5–1.2 mΩcm at 20 K which is in good agreement with the data for single crystals. From this result, it was found that the cavity perturbation technique works well for the conductivity measurement of anaerobic powders, with respect to metallic materials.

Rotational tunneling of ammonia in (NH3)K3C60

The rotational dynamics of NH3 in the ammoniated fulleride (NH3)K3C60 are studied by the inelastic neutron scattering technique. Well-defined excitations appear in the low-energy spectra at 1.5 K as a broad band centered at ∼350 μeV with shoulders at ∼240 and 590 μeV. The temperature dependence of their intensities, energies, and widths is consistent with their assignment to rotational tunneling transitions of the NH3 molecule. The estimated barrier of the threefold hindrance potential to rotation of the H atoms about the K–N axis is ∼5.7 meV. The existence of minority hindering sites with both larger (∼8 meV) and smaller (∼2 meV) torsional barriers shows that orientational disorder effects survive in (NH3)K3C60 even at low temperatures.

Formation Mechanism of Polymeric Fullerenes

Abstract The energetics and reaction paths of polymeric forms of fullerenes have been investigated by means of calorimetry experiment and semiempirical molecular orbital calculations. Energy schemes of three types of single phase C60 polymers i.e., dimer, one-and two-dimensional polymers have been experimentally established. This energy scheme, as well as the dramatic difference in experimental conditions for the polymerization and the depolymerization, is reasonably explained by a calculated energy contour map taking the distortion of fullerene molecules into account.

Magnetic properties of metal endohedral fullerene crystals La@C82.(Cs2)1.5

Abstract We report a magnetic investigation of the endohedral metallofullerene La@C82.(CS2)1.5. Firstly, pure La@CC82 was prepared and isolated, then single crystals were grown by a slow evaporation method. The crystals were screened by x-ray diffraction measurements and the structure was confirmed to be a body centered cubic lattice a = 25.62Å with space group I-43d. Optical absorption measurement revealed the compound to be a localised insulating or semi-conducting with a band gap around 2500cm−1. Temperature dependent EPR and SQUID measurements indicate that the high temperature magnetic behavior is that of a Curie-Weiss paramagnet, C = 0.4(+/−0.1)cm3mol−1K, which corresponds to a single spin localised on the C82 cage. Furthermore, a large negative Weiss constant, 0 = −150(+/−20)K, indicates the presence of strong antiferromagnetic interactions. EPR measurements show a sharp decrease in the line width occuring below around 160K, indicating a phase transition. The Curie moment appears to be lost at temperatures below 80K.

Band filling and width control in fullerides

Abstract Highly doped states of fullerene C 60 solids have been investigated. A single phase of binary Ba 4 C 60 was first synthesized and confirmed to be a noncubic ambient pressure superconductor. By cointercalation of alkali metals and Ba, an isostructural series of alkali-Ba-C 60 , ternaries have been synthesized. This new class of compounds allows us to vary the band-filling and band-filling with keeping the body-centered-cubic (bcc) structure unchanged. Systematic synthesis study of these Ba-including fullerides uncovered various aspects of the highly doped fullerides. Of particular interest is that the criteria for the occurrence of superconductivity are considerably different between the low and high reduction states. Superconductivity at the low reduction state is strictly limited at the half-filling of the t 1u band and the fcc-related structures, while that at the high reduction state appears at several valence states irrespective of crystal structures.

Temperature dependence of Raman scattering in K3Ba3C60

Raman spectra at different temperatures are reported for K3Ba3C60. A systematic change of the Raman spectra with temperature is observed. The development of the linewidth and intensity for all modes with decreasing temperature is in sharp contrast to the case for pure C60. For the two lowest-frequency Hg modes, the appearance of the low-frequency components and an increase in their intensity are observed as temperature decreases. This could be related to an internal strain between the doped part of the crystal and the undoped part of the crystal, and the Jahn-Teller effect. An anomalously large upshift of 9 cm-1 between 20 K and room temperature for the pinch Ag(2) mode suggests the existence of hybridization between the C60 molecules and the intercalants. The doublet of Ag(1) at low temperature could arise from the splitting of the crystal field.