1,621 research outputs found
Rapid rotation of a Bose-Einstein condensate in a harmonic plus quartic trap
A two-dimensional rapidly rotating Bose-Einstein condensate in an anharmonic
trap with quadratic and quartic radial confinement is studied analytically with
the Thomas-Fermi approximation and numerically with the full time-independent
Gross-Pitaevskii equation. The quartic trap potential allows the rotation speed
to exceed the radial harmonic frequency . In the regime
, the condensate contains a dense vortex array
(approximated as solid-body rotation for the analytical studies). At a critical
angular velocity , a central hole appears in the condensate.
Numerical studies confirm the predicted value of , even for
interaction parameters that are not in the Thomas-Fermi limit. The behavior is
also investigated at larger angular velocities, where the system is expected to
undergo a transition to a giant vortex (with pure irrotational flow).Comment: 14 pages, 5 figure
Oscillations of a Bose-Einstein condensate rotating in a harmonic plus quartic trap
We study the normal modes of a two-dimensional rotating Bose-Einstein
condensate confined in a quadratic plus quartic trap. Hydrodynamic theory and
sum rules are used to derive analytical predictions for the collective
frequencies in the limit of high angular velocities, , where the vortex
lattice produced by the rotation exhibits an annular structure. We predict a
class of excitations with frequency in the rotating frame,
irrespective of the mode multipolarity , as well as a class of low energy
modes with frequency proportional to . The predictions are in good
agreement with results of numerical simulations based on the 2D
Gross-Pitaevskii equation. The same analysis is also carried out at even higher
angular velocities, where the system enters the giant vortex regime.Comment: 4 pages, 2 figure
An Active-Sterile Neutrino Transformation Solution for r-Process Nucleosynthesis
We discuss how matter-enhanced active-sterile neutrino transformation in both
neutrino and antineutrino channels could enable the production of the rapid
neutron capture (r-process) nuclei in neutrino-heated supernova ejecta. In this
scheme the lightest sterile neutrino would be heavier than the electron
neutrino and split from it by a vacuum mass-squared difference roughly between
3 and 70 eV and vacuum mixing angle given by .Comment: 27 pages plus twelve figures. Submitted to Phys. Rev.
The s-wave pion-nucleus optical potential
We calculate the s-wave part of the pion-nucleus optical potential using a
unitarized chiral approach that has been previously used to simultaneously
describe pionic hydrogen and deuterium data as well as low energy pi N
scattering in the vacuum. This energy dependent model allows for additional
isoscalar parts in the potential from multiple rescattering. We consider Pauli
blocking and pion polarization in an asymmetric nuclear matter environment.
Also, higher order corrections of the pi N amplitude are included. The model
can accommodate the repulsion required by phenomenological fits, though the
theoretical uncertainties are bigger than previously thought. At the same time,
we also find an enhancement of the isovector part compatible with empirical
determinations.Comment: 31 pages, 27 figure
Vortex line in a neutral finite-temperature superfluid Fermi gas
The structure of an isolated vortex in a dilute two-component neutral
superfluid Fermi gas is studied within the context of self-consistent
Bogoliubov-de Gennes theory. Various thermodynamic properties are calculated
and the shift in the critical temperature due to the presence of the vortex is
analyzed. The gapless excitations inside the vortex core are studied and a
scheme to detect these states and thus the presence of the vortex is examined.
The numerical results are compared with various analytical expressions when
appropriate.Comment: 8 pages, 6 embedded 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
Description of double beta decay within continuum-QRPA
A method to calculate the nuclear double beta decay (- and
-) amplitudes within the continuum random phase approximation
(cQRPA) is formulated. Calculations of the transition amplitudes
within the cQRPA are performed for ^{76}Ge, ^{100}Mo and ^{130}Te. A rather
simple nuclear Hamiltonian consisting of phenomenological mean field and
zero-range residual particle-hole and particle-particle interaction is used.
The calculated M^{2\nu} are almost not affected when the single-particle
continuum is taken into account. At the same time, a regular suppression of the
-amplitude is found that can be associated with additional
ground state correlations due to collective states in the continuum. It is
expected that future inclusion of the nucleon pairing in the single-particle
continuum will somewhat compensate the suppression.Comment: 20 pages, 1 figure, published versio
Normal Modes of a Vortex in a Trapped Bose-Einstein Condensate
A hydrodynamic description is used to study the normal modes of a vortex in a
zero-temperature Bose-Einstein condensate. In the Thomas-Fermi (TF) limit, the
circulating superfluid velocity far from the vortex core provides a small
perturbation that splits the originally degenerate normal modes of a
vortex-free condensate. The relative frequency shifts are small in all cases
considered (they vanish for the lowest dipole mode with |m|=1), suggesting that
the vortex is stable. The Bogoliubov equations serve to verify the existence of
helical waves, similar to those of a vortex line in an unbounded weakly
interacting Bose gas. In the large-condensate (small-core) limit, the
condensate wave function reduces to that of a straight vortex in an unbounded
condensate; the corresponding Bogoliubov equations have no bound-state
solutions that are uniform along the symmetry axis and decay exponentially far
from the vortex core.Comment: 15 pages, REVTEX, 2 Postscript figures, to appear in Phys. Rev. A. We
have altered the material in Secs. 3B and 4 in connection with the normal
modes that have |m|=1. Our present treatment satisfies the condition that the
fundamental dipole mode of a condensate with (or without) a vortex should
have the bare frequency $\omega_\perp
Theory and it ab initio calculation of radiative lifetime of excitons in semiconducting carbon nanotubes
We present theoretical analysis and first-principles calculation of the
radiative lifetime of excitons in semiconducting carbon nanotubes. An intrinsic
lifetime of the order of 10 ps is computed for the lowest optically active
bright excitons. The intrinsic lifetime is however a rapid increasing function
of the exciton momentum. Moreover, the electronic structure of the nanotubes
dictates the existence of dark excitons nearby in energy to each bright
exciton. Both effects strongly influence measured lifetime. Assuming a thermal
occupation of bright and dark exciton bands, we find an effective lifetime of
the order of 10 ns at room temperature, in good accord with recent experiments.Comment: 12 pages, 3 figure
Conductivity of the classical two-dimensional electron gas
We discuss the applicability of the Boltzmann equation to the classical
two-dimensional electron gas. We show that in the presence of both the
electron-impurity and electron-electron scattering the Boltzmann equation can
be inapplicable and the correct result for conductivity can be different from
the one obtained from the kinetic equation by a logarithmically large factor.Comment: Revtex, 3 page
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