6,240 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
Star product and the general Leigh-Strassler deformation
We extend the definition of the star product introduced by Lunin and
Maldacena to study marginal deformations of N=4 SYM. The essential difference
from the latter is that instead of considering U(1)xU(1) non-R-symmetry, with
charges in a corresponding diagonal matrix, we consider two Z_3-symmetries
followed by an SU(3) transformation, with resulting off-diagonal elements. From
this procedure we obtain a more general Leigh-Strassler deformation, including
cubic terms with the same index, for specific values of the coupling constants.
We argue that the conformal property of N=4 SYM is preserved, in both beta-
(one-parameter) and gamma_{i}-deformed (three-parameters) theories, since the
deformation for each amplitude can be extracted in a prefactor. We also
conclude that the obtained amplitudes should follow the iterative structure of
MHV amplitudes found by Bern, Dixon and Smirnov.Comment: 21 pages, no figures, JHEP3, v2: references added, v3: appendix A
added, v4: clarification in section 3.
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
Superspace calculation of the four-loop spectrum in N=6 supersymmetric Chern-Simons theories
Using N=2 superspace techniques we compute the four-loop spectrum of single
trace operators in the SU(2) x SU(2) sector of ABJM and ABJ supersymmetric
Chern-Simons theories. Our computation yields a four-loop contribution to the
function h^2(\lambda) (and its ABJ generalization) in the magnon dispersion
relation which has fixed maximum transcendentality and coincides with the
findings in components given in the revised versions of arXiv:0908.2463 and
arXiv:0912.3460. We also discuss possible scenarios for an all-loop function
h^2(\lambda) that interpolates between weak and strong couplings.Comment: LaTeX, feynmp, 34 pages; v2: typos corrected, formulations improved,
references adde
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
- …