547 research outputs found
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 and the extracted
higher fullerenes -- C, C, C, and C -- 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. 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 or , 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
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