Hideo Shimoda

Gigantic optical Stark effect and ultrafast relaxation of excitons in single-walled carbon nanotubes

We report a splitting and a shift of exciton absorptions in semiconducting single-walled carbon nanotubes (SWNTs) by resonant and off-resonant excitation using a fs laser pulse, respectively. This is the first unambiguous observation of the optical Stark effect of excitons in SWNTs, which is responsible for their high optical-switching performance. By scrutinizing transient absorption spectra from the mid- to near-infrared region (0.1–0.8 eV), the ultrafast recovery of real carrier excitations characteristic of bundled structures was attributed to the energy- and charge-transfer processes to metallic SWNTs.

Structure and superconductivity in alkali-ammonia complex fullerides

Abstract Alkali ammonia complex fullendes form a rich variety of compounds with unique structural and electronic properties. Intercalation of NH3 into K3C60 produces orthorhombic NH3C60, which undergoes a metal-insulator transition instead of superconductivity that is expected to occur at around 30 K from the empirical relation betweenTc and the lattice parameter. Synthesis in liquid ammonia yields new superconducting fullerides(NH3)xNaA2C60 (0.5

Magnetic resonance in the antiferromagnetic and normal state of NH3K3C60

We report on the magnetic resonance of NH_3K_3C_60 powders in the frequency range of 9 to 225 GHz. The observation of an antiferromagnetic resonance below the phase transition at 40 K is evidence for an antiferromagnetically ordered ground state. In the normal state, above 40 K, the temperature dependence of the spin-susceptibilty measured by ESR agrees with previous static measurements and is too weak to be explained by interacting localized spins in an insulator. The magnetic resonance line width has an unusual magnetic-field dependence which is large and temperature independent in the magnetically ordered state and decreases rapidly above the transition. These observations agree with the suggestion that NH_3K_3C_60 is a metal in the normal state and undergoes a Mott-Hubbard metal to insulator transition at 40 K.

Synthesis and superconductivity of (NH3)xK3C60

Abstract Here we report on the synthesis and superconductivity of (NH3)xK3C60. Reaction of stoichiometric amount of potassium and C60 powders in liquid ammonia produces (NH3)xK3C60 (0

Anomalous high pressure properties in fullerene superconductors

Abstract Correlations among lattice parameter, external pressure, and critical temperature Tc has been surveyed on fullerene superconductors with a wide range of lattice parameters and various valence states. The observed value of dTc/dP for Na2Rb0.5Cs0.5C60 and Li3CsC60, having small interfullerene separations, is about a few times lager than that of K3C60. In contrast, small dTc/dP values are found in fullerides with expanded unit cells, such as (NH3)xNaAA†C60 (A, A† = K, Rb and Cs) and A3Ba3C60. Interestingly, the chemical and physical pressure effects on Tc are considerably different in these fullerides with large interfullerene spacings. Our results suggest that the pressure dependence is scaled by an interfullerene distance when the interfullerene distances is small, but that this scaling fails when the unit cell is expanded.

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.

Current issues of intercalation and superconductivity in solid state fullerenes

Abstract Co-intercalation of alkali metals and neutral molecules into C60 have been investigated. Successful synthesis of this class of materials provides us a new insight into superconductivity and other physical properties; such as criterion for the occurrence of superconductivity and metal-insulator transitions. Based on the recently synthesized ammoniated fullerides, we discuss on the relation between the structure, and electronic properties.

NMR studies of ammoniated alkali fullerides

13C-NMR measurements of (NH3)xK3C60 were carried out for three samples with different NH3 content of x=1.14(#1), x=1.05(#2), and x=0.98(#3). 13C-NMR spectrum for x=0.98(#3) shows an anomalous spectral broadening, whereas a distinct anomaly is hardly observed for the off-stoichiometric sample with x=1.14(#1). In the magnetic ordered state at 4.2 K, although unusual spectral broadening ascribed to the AF order was observed for all sample. We found, however, the linewidth depends on the sample-quality, i.e., stoichiometry of NH3 content (∼0.8, 1.3, and 3 MHz for x=1.14(#1), 1.05(#2), and 0.98(#3), respectively). Present studies reconfirmed that the lines A and B are inherent in AF ordering and the off-stoichiometric ammoniation obstruct the AF ordering.

Structural studies of (NH3)K3C60 at ambient and high pressure

The structural features of the ammoniated fulleride (NH3)K3C60 are studied by neutron and synchrotron X-ray powder diffraction at low temperature and high pressure, respectively. Below 150 K the system adopts an orthorhombic structure in space group Fddd. Rietveld refinements led to the determination of the exact orientation and geometry of both the C60 units (antiferrorotative ordering) and the ammonia molecules in the pseudo octahedral sites (antiferroelectric ordering). We also performed synchrotron X-ray powder diffraction measurements at pressures between ambient and 3.9 GPa. Near ∼1 GPa, (NH3)K3C60 undergoes a structural phase transition from orthorhombic Fmmm (isostructural with the ambient pressure high temperature phase) to orthorhombic Fddd (isostructural with the ambient pressure low temperature phase).

Mott transition with antiferromagnetic ordering in ammoniated alkali C60 superconductors: NMR studies

Abstract 13 C -NMR measurements were performed for ammoniated A 3 C 60 ( A = alkali metals ) superconductors, (NH3)xA3C60. For (NH3)K3C60, the Curie–Weiss-type T dependence of susceptibility and T-independent nuclear relaxation rate, 1/T1=const., were observed at high Ts above 150 K, indicating the localized spin state of C60 molecular spins. The extremely broadened spectra for 13 C -NMR was observed below T N ∼45 K , providing unambiguous evidence for an antiferromagnetic order with a moment of μ 0 ∼1 μ B / C 60 . The present NMR measurements clearly showed that the intercalation of NH3 makes (NH3)K3C60 a Mott–Hubbard insulator with AF order below T N ∼45 K . We also discuss the partially ammoniated A3C60.