149 research outputs found
Towards improved exact exchange functionals relying on GW quasiparticle methods for parametrization
We use fully self-consistent GW calculations on diamond and silicon carbide
to reparametrize the Heyd-Scuseria-Ernzerhof exact exchange density functional
for use in band structure calculations of semiconductors and insulators. We
show that the thus modified functional is able to calculate the band structure
of bulk Si, Ge, GaAs, and CdTe with good quantitative accuracy at a
significantly reduced computational cost as compared to GW methods. We discuss
the limitations of this functional in low-dimensions by calculating the band
structures of single-layer hexagonal BN and MoS, and by demonstrating
that the diameter scaling of curvature induced band gaps in single-walled
carbon nanotubes is still physically incorrect using our functional; we
consider possible remedies to this problem.Comment: Submitted to Physical Review
Fine-tuning the functional properties of carbon nanotubes via the interconversion of encapsulated molecules
Tweaking the properties of carbon nanotubes is a prerequisite for their
practical applications. Here we demonstrate fine-tuning the electronic
properties of single-wall carbon nanotubes via filling with ferrocene
molecules. The evolution of the bonding and charge transfer within the tube is
demonstrated via chemical reaction of the ferrocene filler ending up as
secondary inner tube. The charge transfer nature is interpreted well within
density functional theory. This work gives the first direct observation of a
fine-tuned continuous amphoteric doping of single-wall carbon nanotubes
Doped carbon nanotubes as a model system of biased graphene
Albeit difficult to access experimentally, the density of states (DOS) is a
key parameter in solid state systems which governs several important phenomena
including transport, magnetism, thermal, and thermoelectric properties. We
study DOS in an ensemble of potassium intercalated single-wall carbon nanotubes
(SWCNT) and show using electron spin resonance spectroscopy that a sizeable
number of electron states are present, which gives rise to a Fermi-liquid
behavior in this material. A comparison between theoretical and the
experimental DOS indicates that it does not display significant correlation
effects, even though the pristine nanotube material shows a Luttinger-liquid
behavior. We argue that the carbon nanotube ensemble essentially maps out the
whole Brillouin zone of graphene thus it acts as a model system of biased
graphene
Electron spin resonance signal of Luttinger liquids and single-wall carbon nanotubes
A comprehensive theory of electron spin resonance (ESR) for a Luttinger
liquid (LL) state of correlated metals is presented. The ESR measurables such
as the signal intensity and the line-width are calculated in the framework of
Luttinger liquid theory with broken spin rotational symmetry as a function of
magnetic field and temperature. We obtain a significant temperature dependent
homogeneous line-broadening which is related to the spin symmetry breaking and
the electron-electron interaction. The result crosses over smoothly to the ESR
of itinerant electrons in the non-interacting limit. These findings explain the
absence of the long-sought ESR signal of itinerant electrons in single-wall
carbon nanotubes when considering realistic experimental conditions.Comment: 5 pages, 1 figur
A lattice dynamical treatment for the total potential energy of single-walled carbon nanotubes and its applications: relaxed equilibrium structure, elastic properties, and vibrational modes of ultra-narrow tubes
In this paper, we proposed a lattice dynamic treatment for the total
potential energy for single-walled carbon nanotubes (SWCNT's) which is, apart
from a parameter for the non-linear effects, extracted from the vibrational
energy of the planar graphene sheet. Based upon the proposal, we investigated
systematically the relaxed lattice configuration for narrow SWCNT's, the strain
energy, the Young's modulus and Poisson ratio, and the lattice vibrational
properties respected to the relaxed equilibrium tubule structure. Our
calculated results for various physical quantities are nicely in consistency
with existing experimental measurements. Particularly, we verified that the
relaxation effect brings the bond length longer and the frequencies of various
optical vibrational modes softer; Our calculation provides the evidence that
the Young's modulus of armchair tube exceeds that of the planar graphene sheet,
and the large diameter limits of the Young's modulus and Poisson ratio are in
agreement with the experimental values of the graphite; The calculated radial
breathing modes for the ultra narrow tubes with diameter range between 0.2 -
0.5 nm coincide the experimental results and the existing {\it ab initio}
calculations with satisfaction; For narrow tubes of diameter 2 nm, the
calculated frequencies of optical modes in tubule tangential plane as well as
those of radial breathing modes are also in good agreement with the
experimental measurement. In addition, our calculation shows that various
physical quantities of relaxed SWCNT's can actually be expanded in terms of the
chiral angle defined for the correspondent ideal SWCNT's.Comment: 9 pages, 7 figure
van der Waals interaction in nanotube bundles : consequences on vibrational modes
We have developed a pair-potential approach for the evaluation of van der
Waals interaction between carbon nanotubes in bundles.
Starting from a continuum model, we show that the intertube modes range from
to . Using a non-orthogonal tight-binding approximation
for describing the covalent intra-tube bonding in addition, we confirme a
slight chiral dependance of the breathing mode frequency and we found that this
breathing mode frequency increase by 10 % if the nanotube lie inside a
bundle as compared to the isolated tube.Comment: 5 pages, 2 figure
Quantum Capacitance Extraction for Carbon Nanotube Interconnects
Electrical transport in metallic carbon nanotubes, especially the ones with diameters of the order of a few nanometers can be best described using the Tomanaga Luttinger liquid (TL) model. Recently, the TL model has been used to create a convenient transmission line like phenomenological model for carbon nanotubes. In this paper, we have characterized metallic nanotubes based on that model, quantifying the quantum capacitances of individual metallic single walled carbon nanotubes and crystalline bundles of single walled tubes of different diameters. Our calculations show that the quantum capacitances for both individual tubes and the bundles show a weak dependence on the diameters of their constituent tubes. The nanotube bundles exhibit a significantly large quantum capacitance due to enhancement of density of states at the Fermi level
Ab-initio structural, elastic, and vibrational properties of carbon nanotubes
A study based on ab initio calculations is presented on the estructural,
elastic, and vibrational properties of single-wall carbon nanotubes with
different radii and chiralities. We use SIESTA, an implementation of
pseudopotential-density-functional theory which allows calculations on systems
with a large number of atoms per cell. Different quantities like bond
distances, Young moduli, Poisson ratio and the frequencies of different phonon
branches are monitored versus tube radius. The validity of expectations based
on graphite is explored down to small radii, where some deviations appear
related to the curvature effects. For the phonon spectra, the results are
compared with the predictions of the simple zone-folding approximation. Except
for the known defficiencies of this approximation in the low-frequency
vibrational regions, it offers quite accurate results, even for relatively
small radii.Comment: 13 pages, 7 figures, submitted to Phys. Rev. B (11 Nov. 98
Theory of coherent phonons in carbon nanotubes and graphene nanoribbons
We survey our recent theoretical studies on the generation and detection of coherent radial
breathing mode (RBM) phonons in single-walled carbon nanotubes and coherent radial
breathing like mode (RBLM) phonons in graphene nanoribbons. We present a microscopic
theory for the electronic states, phonon modes, optical matrix elements and electronヨphonon
interaction matrix elements that allows us to calculate the coherent phonon spectrum. An
extended tight-binding (ETB) model has been used for the electronic structure and a valence
force field (VFF) model has been used for the phonon modes. The coherent phonon
amplitudes satisfy a driven oscillator equation with the driving term depending on the
photoexcited carrier density. We discuss the dependence of the coherent phonon spectrum on
the nanotube chirality and type, and also on the graphene nanoribbon mod number and class
(armchair versus zigzag). We compare these results with a simpler effective mass theory
where reasonable agreement with the main features of the coherent phonon spectrum is found.
In particular, the effective mass theory helps us to understand the initial phase of the coherent
phonon oscillations for a given nanotube chirality and type. We compare these results to two
different experiments for nanotubes: (i) micelle suspended tubes and (ii) aligned nanotube
films. In the case of graphene nanoribbons, there are no experimental observations to date. We
also discuss, based on the evaluation of the electronヨphonon interaction matrix elements, the
initial phase of the coherent phonon amplitude and its dependence on the chirality and type.
Finally, we discuss previously unpublished results for coherent phonon amplitudes in zigzag
nanoribbons obtained using an effective mass theory
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