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 $\pi$-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$_6$ and CeTe), is analyzed by using the infinite-$U$ Anderson lattice model. The four atomic energy levels are assumed for f-electrons. Two of them are mutually degenerate. A small crystalline splitting $2\Delta$ 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 $\Delta$ 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$_{60}$ by thermal fluctuations of phonons, misalignment of C$_{60}$ molecules in a crystal, and other intercalated impurities (remaining C$_{70}$, oxygens, and so on) are simulated by disorder potentials. The Su-Schrieffer-Heeger--type electron-phonon model for doped C$_{60}$ 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$_{60}^{-1,-2}$) 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$_{60}$ is discussed.Comment: Note: This manusript was accepted for publication in Physical Review B. Figures will be sent to you via snail (conventional) mai