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

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

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

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

Electronic Properties of Topological Materials: Optical Excitations in Moebius Conjugated Polymers

Electronic structures and optical excitations in Moebius conjugated polymers are studied theoretically. Periodic and Moebius boundary conditions are applied to the tight binding model of poly(para-phenylene), taking exciton effects into account. We discuss that oligomers with a few structural units are more effective than polymers for observations of effects of discrete wave numbers that are shifted by the change in boundary condition. Next, calculations of optical absorption spectra are reported. Certain components of optical absorption for an electric field perpendicular to the polymer axis mix with absorption spectra for an electric field parallel to the polymer axis. Therefore, the polarization dependences of an electric field of light enable us to detect whether conjugated polymers have the Moebius boundary.Comment: 10 pages, 6 figures, to be published in J. Phys. Soc. Jpn., Vol. 74 No. 2 (February, 2005), Letter sectio

A Theoretical Study on Spin-Dependent Transport of "Ferromagnet/Carbon Nanotube Encapsulating Magnetic Atoms/Ferromagnet" Junctions with 4-Valued Conductances

As a novel function of ferromagnet (FM)/spacer/FM junctions, we theoretically investigate multiple-valued (or multi-level) cell property, which is in principle realized by sensing conductances of four states recorded with magnetization configurations of two FMs; (up,up), (up,down), (down,up), (down,down). In order to sense all the states, 4-valued conductances corresponding to the respective states are necessary. We previously proposed that 4-valued conductances are obtained in FM1/spin-polarized spacer (SPS)/FM2 junctions, where FM1 and FM2 have different spin polarizations, and the spacer depends on spin [J. Phys.: Condens. Matter 15, 8797 (2003)]. In this paper, an ideal SPS is considered as a single-wall armchair carbon nanotube encapsulating magnetic atoms, where the nanotube shows on-resonance or off-resonance at the Fermi level according to its length. The magnitude of the obtained 4-valued conductances has an opposite order between the on-resonant nanotube and the off-resonant one, and this property can be understood by considering electronic states of the nanotube. Also, the magnetoresistance ratio between (up,up) and (down,down) can be larger than the conventional one between parallel and anti-parallel configurations.Comment: 10 pages, 4 figures, accepted for publication in J. Phys.: Condens. Matte

Optical absorption spectra of A6C60 and A6C70: Reduction of effective Coulomb interactions in Frenkel excitons

We theoretically investigate optical absorption spectra of \soc^{6-} and \rug^{6-}, and discuss relations with the optical properties of alkali metal doped fullerides A_6\soc and A_6\rug. This is a valid approach for systems where Frenkel exciton effects are dominant. We use a tight binding model with long ranged Coulomb interactions and bond disorder. Optical spectra are obtained by the Hartree-Fock approximation and the configuration interaction method. We find that the Coulomb interaction parameters, which are relevant to the optical spectra of A_6\soc (A_6\rug) in order to explain the excitation energies and relative oscillator strengths of absorption peaks, are almost the half of those of the neutral \soc (\rug). The reduction of the effective Coulomb interactions is concluded for the heavily doped case of \soc and \rug. This finding is closely related with the experimental fact that dielectric constants of fullerides which are maximumly doped with alkali metals become about twice as large as those of the neutral systems.Comment: Note: A full preprint with figures should be requested to the author. It will be sent by air-mail.; E-mail: [email protected]

Theoretical study of a localized quantum spin reversal by the sequential injection of spins in a spin quantum dot

This is a theoretical study of the reversal of a localized quantum spin induced by sequential injection of spins for a spin quantum dot that has a quantum spin. The system consists of ``electrode/quantum well(QW)/dot/QW/electrode" junctions, in which the left QW has an energy level of conduction electrons with only up-spin. We consider a situation in which up-spin electrons are sequentially injected from the left electrode into the dot through the QW and an exchange interaction acts between the electrons and the localized spin. To describe the sequentially injected electrons, we propose a simple method based on approximate solutions from the time-dependent Schr$\ddot{\rm o}$dinger equation. Using this method, it is shown that the spin reversal occurs when the right QW has energy levels of conduction electrons with only down-spin. In particular, the expression of the reversal time of a localized spin is derived and the upper and lower limits of the time are clearly expressed. This expression is expected to be useful for a rough estimation of the minimum relaxation time of the localized spin to achieve the reversal. We also obtain analytic expressions for the expectation value of the localized spin and the electrical current as a function of time. In addition, we found that a system with the non-magnetic right QW exhibits spin reversal or non-reversal depending on the exchange interaction.Comment: 12 pages, 12 figures, to be published in Phys. Rev. B, typos correcte