1,905 research outputs found
Spectroscopic Properties and STM Images of Carbon Nanotubes
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
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 -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
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
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
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
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 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
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
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
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