515 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
Effects of initial compression stress on wave propagation in carbon nanotubes
An analytical method to investigate wave propagation in single- and double-
walled carbon nanotubes under initial compression stress is presented. The
nanotube structures are treated within the multilayer thin shell approximation
with the elastic properties taken to be those of the graphene sheet. The
governing equations are derived based on Flugge equations of motion. Frequency
equations of wave propagation in single and double wall carbon nanotubes are
described through the effects of initial compression stress and van der Waals
force. To show the effects of Initial compression stress on the wave
propagation in nanotubes, the symmetrical mode can be analyzed based on the
present elastic continuum model. It is shown that the wave speed are sensitive
to the compression stress especially for the lower frequencies.Comment: 12 pages, 4 figure
Electronic states of metallic and semiconducting carbon nanotubes with bond and site disorder
Disorder effects on the density of states in carbon nanotubes are analyzed by
a tight binding model with Gaussian bond or site disorder. Metallic armchair
and semiconducting zigzag nanotubes are investigated. In the strong disorder
limit, the conduction and valence band states merge, and a finite density of
states appears at the Fermi energy in both of metallic and semiconducting
carbon nanotubes. The bond disorder gives rise to a huge density of states at
the Fermi energy differently from that of the site disorder case. Consequences
for experiments are discussed.Comment: Phys. Rev. B: Brief Reports (to be published). Related preprints can
be found at http://www.etl.go.jp/~harigaya/NEW.htm
Nonlinear optical response in doped conjugated polymers
Exciton effects on conjugated polymers are investigated in soliton lattice
states. We use the Su-Schrieffer-Heeger model with long-range Coulomb
interactions. The Hartree-Fock (HF) approximation and the single-excitation
configuration- interaction (single-CI) method are used to obtain optical
absorption spectra. The third-harmonic generation (THG) at off-resonant
frequencies is calculated as functions of the soliton concentration and the
chain length of the polymer. The magnitude of the THG at the 10 percent doping
increases by the factor about 10^2 from that of the neutral system. This is
owing to the accumulation of the oscillator strengths at the lowest exciton
with increasing the soliton concentration. The increase by the order two is
common for several choices of Coulomb interaction strengths.Comment: Accepted for publication in J. Phys.: Condens. Matte
Competition between spin and charge polarized states in nanographene ribbons with zigzag edges
Effects of the nearest neighbor Coulomb interaction on nanographene ribbons
with zigzag edges are investigated using the extended Hubbard model within the
unrestricted Hartree-Fock approximation. The nearest Coulomb interaction
stabilizes a novel electronic state with the opposite electric charges
separated and localized along both edges, resulting in a finite electric dipole
moment pointing from one edge to the other. This charge-polarized state
competes with the peculiar spin-polarized state caused by the on-site Coulomb
interaction and is stabilized by an external electric field.Comment: 4 pages; 4 figures; accepted for publication in Phys. Rev. B; related
Web site: http://staff.aist.go.jp/k.harigaya/index_E.htm
Optical absorption spectra in fullerenes C60 and C70: Effects of Coulomb interactions, lattice fluctuations, and anisotropy
Effects of Coulomb interactions and lattice fluctuations in the optical
absorption spectra of C60 and C70 are theoretically investigated by using a
tight binding model with long-range Coulomb interaction and bond disorder.
Anisotropy effects in C70 are also considered. Optical spectra are calculated
by using the Hartree-Fock approximation followed by the configuration
interaction method. The main conclusions are as follows: (1) The broad peaks at
excitation energies, 3.7eV, 4.7eV, and 5.7eV, observed in experiments of C60
molecules in a solution are reasonably described by the present theory. Peak
positions and relative oscillator strengths are in overall agreement with the
experiments. The broadening of peaks by lattice fluctuations is well simulated
by the bond disorder model. (2) The optical gap of C70 is larger when the
electric field of light is parallel to the long axis of the molecule. The shape
of the frequency dispersion also depends on the orientation of the molecule.
These properties are common in the free electron model and the model with
Coulomb interactions. (3) The spectrum of C70 averaged over bond disorder and
random orientations is compared with experiments in a solution. There is an
overall agreement about the spectral shape. Differences in the spectra of C60
and C70 are discussed in connection with the symmetry reduction from a
soccerball to a rugbyball.Comment: PACS numbers: 78.66.Qn, 78.20.Dj, 71.35.+z, 31.20.Tz; LaTeX, 15
pages, 5 figures (Physical Review B); Note: Please request figures to
Authors. They will be sent via snail mai
Dimerization structures on the metallic and semiconducting fullerene tubules with half-filled electrons
Possible dimerization patterns and electronic structures in fullerene tubules
as the one-dimensional pi-conjugated systems are studied with the extended
Su-Schrieffer-Heeger model. We assume various lattice geometries, including
helical and nonhelical tubules. The model is solved for the half-filling case
of -electrons. (1) When the undimerized systems do not have a gap, the
Kekule structures prone to occur. The energy gap is of the order of the room
temperatures at most and metallic properties would be expected. (2) If the
undimerized systems have a large gap (about 1eV), the most stable structures
are the chain-like distortions where the direction of the arranged
trans-polyacetylene chains is along almost the tubular axis. The electronic
structures are ofsemiconductors due to the large gap.Comment: submitted to Phys. Rev. B, pages 15, figures 1
Lattice distortion and energy level structures in doped C_{60} and C_{70} studied with the extended Su-Schrieffer-Heeger model: Polaron excitations and optical absorption
We extend the Su-Schrieffer-Heeger model of polyacetylene to C_{60} and
C_{70} molecules, and solve numerically. The calculations of the undoped
systems agree well with the known results. When the system (C_{60} or C_{70})
is doped with one or two electrons (or holes), the additional charges
accumulate almost along an equatorial line of the molecule. The dimerization
becomes weaker almost along the same line. Two energy levels intrude largely in
the gap. The intrusion is larger in C_{70} than in C_{60}. Therefore,
``polarons'' are predicted in doped buckminster- fullerenes. We calculate
optical absorption coefficient for C_{60} in order to look at how ``polarons''
will be observed. It is predicted that there appears a new peak at the lower
energy than the intergap transition peaks. It is also found that C_{60} and
C_{70} are related mutually with respect to electronical structures as well as
lattice geometries. (to be published in Phys. Rev. B 45, June 15 issue)Comment: 21 page
Elastic anomaly of heavy fermion systems in a crystalline field
An elastic anomaly, observed in the heavy fermi liquid state of Ce alloys
(for example, CeCu and CeTe), is analyzed by using the infinite-
Anderson lattice model. The four atomic energy levels are assumed for
f-electrons. Two of them are mutually degenerate. A small crystalline splitting
is assumed between two energy levels. The fourfold degenerate
conduction bands are also considered in the model. We solve the model using the
mean field approximation to slave bosons, changing the Fermi energy in order to
keep the total electron number constant. The nonzero value of the mean field of
the slave bosons persists over the temperatures much higher than the Kondo
temperature. This is the effect of the constant electron number. Next, the
linear susceptibility with respect to is calculated in order to obtain
the renomalized elastic constant. The resulting temperature dependence of the
constant shows the downward dip. We point out the relation of our finding with
the experimental data.Comment: submitted to J. Phys.: Condens. Matter, please request figure copies
to [email protected]
Polaron Excitations in Doped C60: Effects of Disorders
Effects on C by thermal fluctuations of phonons, misalignment of
C molecules in a crystal, and other intercalated impurities (remaining
C, oxygens, and so on) are simulated by disorder potentials. The
Su-Schrieffer-Heeger--type electron-phonon model for doped C is solved
with gaussian bond disorders and also with site disorders. Sample average is
performed over sufficient number of disorder configurations. The distributions
of bond lengths and electron densities are shown as functions of the disorder
strength and the additional electron number. Stability of polaron excitations
as well as dimerization patterns is studied. Polarons and dimerizations in
lightly doped cases (C) are relatively stable against disorders,
indicated by peak structures in distribution functions. In more heavily doped
cases, the several peaks merge into a single peak, showing the breakdown of
polaron structures as well as the decrease of the dimerization strength.
Possibility of the observation of polaronic lattice distortions and electron
structures in doped C is discussed.Comment: Note: This manusript was accepted for publication in Physical Review
B. Figures will be sent to you via snail (conventional) mai
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