496 research outputs found
Magnetic response of carbon nanotubes from ab initio calculations
We present {\it ab initio} calculations of the magnetic susceptibility and of
the C chemical shift for carbon nanotubes, both isolated and in bundles.
These calculations are performed using the recently proposed gauge-including
projector augmented-wave approach for the calculation of magnetic response in
periodic insulating systems. We have focused on the semiconducting zigzag
nanotubes with diameters ranging from 0.6 to 1.6 nm. Both the susceptibility
and the isotropic shift exhibit a dependence with the diameter (D) and the
chirality of the tube (although this dependence is stronger for the
susceptibility). The isotropic shift behaves asymptotically as , where is a different constant for each family of nanotubes.
For a tube diameter of around 1.2 nm, a value normally found in experimental
samples, our results are in excellent agreement with experiments. Moreover, we
calculated the chemical shift of a double-wall tube. We found a diamagnetic
shift of the isotropic lines corresponding to the atoms of the inner tube due
to the effect of the outer tube. This shift is in good agreement with recent
experiments, and can be easily explained by demagnetizing currents circulating
the outer tube.Comment: 7 pages, 4 figure
Nonlocal pseudopotentials and magnetic fields
We show how to describe the coupling of electrons to non-uniform magnetic
fields in the framework of the widely used norm-conserving pseudopotential
appro ximation for electronic structure calculations. Our derivation applies to
magnetic fields that are smooth on the scale of the core region. The method is
validated by application to the calculation of the magnetic susceptibility of
molecules. Our results are compared with high quality all electron quantum
chemical results, and another recently proposed formalism.Comment: 4 pages, submitted to Physical Review Letter
Anharmonic phonon spectra of PbTe and SnTe in the self-consistent harmonic approximation
At room temperature, PbTe and SnTe are efficient thermoelectrics with a cubic
structure. At low temperature, SnTe undergoes a ferroelectric transition with a
critical temperature strongly dependent on the hole concentration, while PbTe
is an incipient ferroelectric. By using the stochastic self-consistent harmonic
approximation, we investigate the anharmonic phonon spectra and the occurrence
of a ferroelectric transition in both systems. We find that vibrational spectra
strongly depends on the approximation used for the exchange-correlation kernel
in density functional theory. If gradient corrections and the theoretical
volume are employed, then the calculation of the free energy Hessian leads to
phonon spectra in good agreement with experimental data for both systems. In
PbTe, we reproduce the transverse optical mode phonon satellite detected in
inelastic neutron scattering and the crossing between the transverse optical
and the longitudinal acoustic modes along the X direction. In the case
of SnTe, we describe the occurrence of a ferroelectric transition from the high
temperature Fmm structure to the low temperature R3m one.Comment: 12 pages, 15 Picture
Kohn Anomalies and Electron-Phonon Interaction in Graphite
We demonstrate that graphite phonon dispersions have two Kohn anomalies at
the Gamma-E_2g and K-A'1 modes. The anomalies are revealed by two sharp kinks.
By an exact analytic derivation, we show that the slope of these kinks is
proportional to the square of the electron-phonon coupling (EPC). Thus, we can
directly measure the EPC from the experimental dispersions. The Gamma-E_2g and
K-A'1 EPCs are particularly large, whilst they are negligible for all the other
modes at Gamma and K.Comment: 4 pages, 2 figure
Electron-phonon coupling and phonon self-energy in MgB: do we really understand MgB Raman spectra ?
We consider a model Hamiltonian fitted on the ab-initio band structure to
describe the electron-phonon coupling between the electronic bands and
the phonon E mode in MgB. The model allows for analytical
calculations and numerical treatments using very large k-point grids. We
calculate the phonon self-energy of the E mode along two high symmetry
directions in the Brillouin zone. We demonstrate that the contribution of the
bands to the Raman linewidth of the E mode via the
electron-phonon coupling is zero. As a consequence the large resonance seen in
Raman experiments cannot be interpreted as originated from the mode at
. We examine in details the effects of Fermi surface singularities in
the phonon spectrum and linewidth and we determine the magnitude of finite
temperature effects in the the phonon self-energy. From our findings we suggest
several possible effects which might be responsible for the MgB Raman
spectra.Comment: 10 pages, 9 figure
Phonon Linewidths and Electron Phonon Coupling in Nanotubes
We prove that Electron-phonon coupling (EPC) is the major source of
broadening for the Raman G and G- peaks in graphite and metallic nanotubes.
This allows us to directly measure the optical-phonon EPCs from the G and G-
linewidths. The experimental EPCs compare extremely well with those from
density functional theory. We show that the EPC explains the difference in the
Raman spectra of metallic and semiconducting nanotubes and their dependence on
tube diameter. We dismiss the common assignment of the G- peak in metallic
nanotubes to a Fano resonance between phonons and plasmons. We assign the G+
and G- peaks to TO (tangential) and LO (axial) modes.Comment: 5 pages, 4 figures (correction in label of fig 3
Electron-phonon coupling and electron self-energy in electron-doped graphene: calculation of angular resolved photoemission spectra
We obtain analytical expressions for the electron self-energy and the
electron-phonon coupling in electron-doped graphene using electron-phonon
matrix elements extracted from density functional theory simulations. From the
electron self-energies we calculate angle resolved photoemission spectra. We
demonstrate that the measured kink at eV from the Fermi level is
actually composed of two features, one at eV due to the
twofold degenerate E mode, and a second one at eV due to
the A mode. The electron-phonon coupling extracted from the kink
observed in ARPES experiments is roughly a factor of 5.5 larger than the
calculated one. This disagreement can only be partially reconciled by the
inclusion of resolution effects. Indeed we show that a finite resolution
increases the apparent electron-phonon coupling by underestimating the
renormalization of the electron velocity at energies larger than the kinks
positions. The discrepancy between theory and experiments is thus reduced to a
factor of 2.2. From the linewidth of the calculated ARPES spectra we
obtain the electron relaxation time. A comparison with available experimental
data in graphene shows that the electron relaxation time detected in ARPES is
almost two orders of magnitudes smaller than what measured by other
experimental techniques.Comment: 9 pages, 7 figures, see also Matteo Calandra and Francesco Mauri,
arXiv:0707.149
Giant non-adiabatic effects in layer metals: Raman spectra of intercalated graphite explained
The occurrence of non-adiabatic effects in the vibrational properties of
metals have been predicted since the 60's, but hardly confirmed experimentally.
We report the first fully \emph{ab initio} calculations of non-adiabatic
frequencies of a number of layer and conventional metals. We suggest that
non-adiabatic effects can be a feature of the vibrational Raman spectra of any
bulk metal, and show that they are spectacularly large (up to 30% of the phonon
frequencies) in the case of layer metals, such as superconducting ,
and other graphite intercalated compounds. We develop a framework
capable to estimate the electron momentum-relaxation time of a given system,
and thus its degree of non-adiabaticity, in terms of the experimentally
observed frequencies and linewidths.Comment: 4 pages, 3 figures, 1 tabl
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