1,904 research outputs found

    Spectroscopic Properties and STM Images of Carbon Nanotubes

    Full text link
    We present a theoretical study of the role of the local environment in the electronic properties of carbon nanotubes: isolated single- and multi-wall nanotubes, nanotube-ropes, tubes supported on gold and cutted to finite length. Interaction with the substrate or with other tubes does not alter the scanning-tunneling-microscopy (STM) patterns observed for isolated tubes. STM-topographic images of topological defects (pentagon/heptagon pair) and tube-caps have also been studied. In both cases the obtained image depends on the sign of the applied voltage and it can be described in terms of the previous catalog of STM-images (interference between electronic waves scattered by the defect). We also have computed the electronic density of states for isolated tubes with different chiralities and radii confirming a correlation between the peak-structure in the DOS and the nanotube diameter, however the metallic plateau in the DOS also depends on the nanotube chirality. Furthermore, the conduction and valence band structures are not fully symmetrical to one another. In contrast to STM images, the interaction with the substrate does modify the energy levels of the nanotube. We observe opening of small pseudogaps around the Fermi level and broadening of the sharp van Hove singularities of the isolated single-walled-nanotubes that can be used to extract useful information about the tube structure and bonding. The combination of STM and spectroscopic studies opens a new technique to address the electronic and structural properties of carbon and composite nanotubes.Comment: 9 pages, 8 eps figures. Applied Physics A (in press

    Band structure of boron doped carbon nanotubes

    Get PDF
    We present {\it ab initio} and self-consistent tight-binding calculations on the band structure of single wall semiconducting carbon nanotubes with high degrees (up to 25 %) of boron substitution. Besides a lowering of the Fermi energy into the valence band, a regular, periodic distribution of the p-dopants leads to the formation of a dispersive ``acceptor''-like band in the band gap of the undoped tube. This comes from the superposition of acceptor levels at the boron atoms with the delocalized carbon π\pi-orbitals. Irregular (random) boron-doping leads to a high concentration of hybrids of acceptor and unoccupied carbon states above the Fermi edge.Comment: 4 pages, 2 figure

    The phonon dispersion of graphite revisited

    Get PDF
    We review calculations and measurements of the phonon-dispersion relation of graphite. First-principles calculations using density-functional theory are generally in good agreement with the experimental data since the long-range character of the dynamical matrix is properly taken into account. Calculations with a plane-wave basis demonstrate that for the in-plane optical modes, the generalized-gradient approximation (GGA) yields frequencies lower by 2% than the local-density approximation (LDA) and is thus in better agreement with experiment. The long-range character of the dynamical matrix limits the validity of force-constant approaches that take only interaction with few neighboring atoms into account. However, by fitting the force-constants to the ab-initio dispersion relation, we show that the popular 4th-nearest-neighbor force-constant approach yields an excellent fit for the low frequency modes and a moderately good fit (with a maximum deviation of 6%) for the high-frequency modes. If, in addition, the non-diagonal force-constant for the second-nearest neighbor interaction is taken into account, all the qualitative features of the high-frequency dispersion can be reproduced and the maximum deviation reduces to 4%. We present the new parameters as a reliable basis for empirical model calculations of phonons in graphitic nanostructures, in particular carbon nanotubes.Comment: 26 pages, 7 figures, to appear in Solid State Com

    Ab initio simulations of excited carrier dynamics in carbon nanotubes

    Get PDF
    Combining time-dependent density functional calculations for electrons with molecular dynamics simulations for ions, we investigate the dynamics of excited carriers in a (3,3) carbon nanotube at different temperatures. Following an hv=6.8 eV photoexcitation, the carrier decay is initially dominated by efficient electron-electron scattering. At room temperature, the excitation gap is reduced to nearly half its initial value after ~230 fs, where coupling to phonons starts dominating the decay. We show that the onset point and damping rate in the phonon regime change with initial ion velocities, a manifestation of temperature dependent electron-phonon coupling.Comment: 8 pages, 3 figures, 1 EPAPS supplementary fil

    Renormalization of Molecular Quasiparticle Levels at Metal-Molecule Interfaces: Trends Across Binding Regimes

    Get PDF
    When an electron or a hole is added into an orbital of an adsorbed molecule the substrate electrons will rearrange in order to screen the added charge. This results in a reduction of the electron addition/removal energies as compared to the free molecule case. In this work we use a simple model to illustrate the universal trends of this renormalization mechanism as a function of the microscopic key parameters. Insight of both fundamental and practical importance is obtained by comparing GW quasiparticle energies with Hartree-Fock and Kohn-Sham calculations. We identify two different polarization mechanisms: (i) polarization of the metal (image charge formation) and (ii) polarization of the molecule via charge transfer across the interface. The importance of (i) and (ii) is found to increase with the metal density of states at the Fermi level and metal-molecule coupling strength, respectively.Comment: 4 pages, 3 figure

    Taming singularities of the diagrammatic many-body perturbation theory

    Full text link
    In a typical scenario the diagrammatic many-body perturbation theory generates asymptotic series. Despite non-convergence, the asymptotic expansions are useful when truncated to a finite number of terms. This is the reason for popularity of leading-order methods such as GWGW approximation in condensed matter, molecular and atomic physics. Emerging higher-order implementations suffer from the appearance of nonsimple poles in the frequency-dependent Green's functions and negative spectral densities making self-consistent determination of the electronic structure impossible. Here a method based on the Pad\'e approximation for overcomming these difficulties is proposed and applied to the Hamiltonian describing a core electron coupled to a single plasmonic excitation. By solving the model purely diagrammatically, expressing the self-energy in terms of combinatorics of chord diagrams, and regularizing the diverging perturbative expansions using the Pad\'e approximation the spectral function is determined self-consistently using 3111 diagrams up to the sixth order

    Long-lived oscillatory incoherent electron dynamics in molecules: trans-polyacetylene oligomers

    Get PDF
    We identify an intriguing feature of the electron-vibrational dynamics of molecular systems via a computational examination of \emph{trans}-polyacetylene oligomers. Here, via the vibronic interactions, the decay of an electron in the conduction band resonantly excites an electron in the valence band, and vice versa, leading to oscillatory exchange of electronic population between two distinct electronic states that lives for up to tens of picoseconds. The oscillatory structure is reminiscent of beating patterns between quantum states and is strongly suggestive of the presence of long-lived molecular electronic coherence. Significantly, however, a detailed analysis of the electronic coherence properties shows that the oscillatory structure arises from a purely incoherent process. These results were obtained by propagating the coupled dynamics of electronic and vibrational degrees of freedom in a mixed quantum-classical study of the Su-Schrieffer-Heeger Hamiltonian for polyacetylene. The incoherent process is shown to occur between degenerate electronic states with distinct electronic configurations that are indirectly coupled via a third auxiliary state by the vibronic interactions. A discussion of how to construct electronic superposition states in molecules that are truly robust to decoherence is also presented

    Smiling under stochastic volatility

    Get PDF
    This paper studies the behavior of the implied volatility function (smile) when the true distribution of the underlying asset is consistent with the stochastic volatility model proposed by Heston (1993). The main result of the paper is to extend previous results applicable to the smile as a whole to alternative degrees of moneyness. The conditions under which the implied volatility function changes whenever there is a change in the parameters associated with Hestons stochastic volatility model for a given degree of moneyness are given.volatility smile, stochastic volatility, skewness, kurtosis, option pricing
    • …
    corecore