Charge transfer excitons in optical absorption spectra of C60-dimers and polymers

Charge-transfer (CT) exciton effects are investigated for the optical absorption spectra of crosslinked C60 systems by using the intermediate exciton theory. We consider the C60-dimers, and the two (and three) molecule systems of the C60-polymers. We use a tight-binding model with long-range Coulomb interactions among electrons, and the model is treated by the Hartree-Fock approximation followed by the single-excitation configuration interaction method. We discuss the variations in the optical spectra by changing the conjugation parameter between molecules. We find that the total CT-component increases in smaller conjugations, and saturates at the intermediate conjugations. It decreases in the large conjugations. We also find that the CT-components of the doped systems are smaller than those of the neutral systems, indicating that the electron-hole distance becomes shorter in the doped C60-polymers.Comment: Figures should be requested to the autho

Local Non-Fermi Liquid Theory of Magnetic Impurity Effects in Carbon Nanotubes

Magnetic impurity effects in carbon nanotubes are studied theoretically. The multi channel Kondo effect is investigated with the band structure of the metallic nanotubes. The local non-Fermi liquid behavior is realized at temperatures lower than the Kondo temperature T_K. The density of states of localized electron has a singularity |omega|^1/2 which gives rise to a pseudo gap at the Kondo resonance in low temperatures. The temperature dependence of the electronic resistivity is predicted as T^1/2, and the imaginary part of dynamical susceptibilities has the |omega|^1/2 dependence.Comment: 3 page

Disorder effects and electronic conductance in metallic carbon nanotubes

Disorder effects on the density of states and electronic conduction in metallic carbon nanotubes are analyzed by a tight binding model with Gaussian bond disorder. Metallic armchair and zigzag nanotubes are considered. We obtain a conductance which becomes smaller by the factor 1/2 ~ 1/3 from that of the clean nanotube. This decrease mainly comes from lattice fluctuations of the width which is comparable to thermal fluctuations. We also find that suppression of electronic conductance around the Fermi energy due to disorder is smaller than that of the inner valence (and conduction) band states. This is a consequence of the extended nature of electronic states around the Fermi energy between the valence and conduction bands, and is a property typical of the electronic structures of metallic carbon nanotubes.Comment: PACS numbers: 72.80.Rj, 72.15.Eb, 73.61.Wp, 73.23.Ps; Figures should be requested to the author. Related preprints could be found at http://www.etl.go.jp/~harigaya/welcome_E.htm

Nonlinear optical response in higher fullerenes

Nonlinear optical properties of extracted higher fullerenes - C70, C76, C78, and C84 - are theoretically investigated. Magnitudes of off-resonant third-harmonic-generation are calculated by the intermediate exciton theory. We find that optical nonlinearities of higher fullerenes are a few times larger than those of C60. The magnitudes of nonlinearity tend to increase as the optical gap decreases in higher fullerenes.Comment: Condensed Matter Theory Group at ETL: http://www.etl.go.jp/Organization/Bussei-kiso

Novel electronic wave interference patterns in nanographene sheets

Superperiodic patterns with a long distance in a nanographene sheet observed by STM are discussed in terms of the interference of electronic wave functions. The period and the amplitude of the oscillations decrease spatially in one direction. We explain the superperiodic patterns with a static linear potential theoretically. In the k-p model, the oscillation period decreases, and agrees with experiments. The spatial difference of the static potential is estimated as 1.3 eV for 200 nm in distance, and this value seems to be reasonable in order that the potential difference remains against perturbations, for example, by phonon fluctuations and impurity scatterings. It turns out that the long-distance oscillations come from the band structure of the two-dimensional graphene sheet.Comment: Published as a LETTER in J. Phys.: Condens. Matter; 8 pages; 6 figures; Online version at http://www.iop.org/EJ/S/3/1256/0hJAmc5sCL6d.7sOO.BtLw/abstract/0953-8984/14/3 6/10

Coulomb interaction effects on nonlinear optical response in C60, C70, and higher fullerenes

Nonlinear optical properties in the fullerene C$_{60}$ and the extracted higher fullerenes -- C$_{70}$, C$_{76}$, C$_{78}$, and C$_{84}$ -- are theoretically investigated by using the exciton formalism and the sum-over-states method. We find that off-resonant third order susceptibilities of higher fullerenes are a few times larger than those of C$_{60}$. The magnitude of nonlinearity increases as the optical gap decreases in higher fullerenes. The nonlinearity is nearly proportional to the fourth power of the carbon number when the onsite Coulomb repulsion is $2t$ or $4t$, $t$ being the nearest neighbor hopping integral. This result, indicating important roles of Coulomb interactions, agrees with quantum chemical calculations of higher fullerenes.Comment: 8 pages; 3 figures; Figures should be requested to the author (E-mail: [email protected]

Long-Range Excitons in Optical Absorption Spectra of Electroluminescent Polymer Poly(para-phenylenevinylene)

The component of photoexcited states with large spatial extent is investigated for poly(para-phenylenevinylene) using the intermediate exciton theory. We find a peak due to long-range excitons at the higher-energy side of the lowest main feature of optical spectra. The fact that the onset of long-range excitons is located near the energy gap is related to the mechanisms of large photocurrents measured in such energy regions. We show that a large value of the hopping integral is realistic for characterizing optical excitations.Comment: To be published in J. Phys. Soc. Jpn. (Letters

Mechanism of magnetism in stacked nanographite: Theoretical study

Nanographite systems, where graphene sheets of the orders of the nanometer size are stacked, show novel magnetic properties, such as, spin-glass like behaviors and the change of ESR line widths in the course of gas adsorptions. We theoretically investigate stacking effects in the zigzag nanographite sheets by using a tight binding model with the Hubbard-like onsite interactions. We find a remarkable difference in the magnetic properties between the simple A-A and A-B type stackings. For the simple stacking, there are not magnetic solutions. For the A-B stacking, we find antiferromagnetic solutions for strong onsite repulsions. The local magnetic moments tend to exist at the edge sites in each layer due to the large amplitude of wavefunctions at these sites. Relations with experiments are discussed.Comment: PACS numbers: 75.30.-m, 75.70.Cn, 75.10.Lp, 75.40.Mg; E-mail: [email protected]; http://www.etl.go.jp/~harigaya/welcome_E.htm