2,304 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
Thermodynamic properties of a dipolar Fermi gas
Based on the semi-classical theory, we investigate the thermodynamic
properties of a dipolar Fermi gas. Through a self-consistent procedure, we
numerically obtain the phase space distribution function at finite temperature.
We show that the deformations in both momentum and real space becomes smaller
and smaller as one increases the temperature. For homogeneous case, we also
calculate pressure, entropy, and heat capacity. In particular, at low
temperature limit and in weak interaction regime, we obtain an analytic
expression for the entropy, which agrees qualitatively with our numerical
result. The stability of a trapped gas at finite temperature is also explored
Quasiparticle and Optical Properties of Rutile and Anatase TiO
Quasiparticle excitation energies and optical properties of TiO in the
rutile and anatase structures are calculated using many-body perturbation
theory methods. Calculations are performed for a frozen crystal lattice;
electron-phonon coupling is not explicitly considered. In the GW method,
several approximations are compared and it is found that inclusion of the full
frequency dependence as well as explicit treatment of the Ti semicore states
are essential for accurate calculation of the quasiparticle energy band gap.
The calculated quasiparticle energies are in good agreement with available
photoemission and inverse photoemission experiments. The results of the GW
calculations, together with the calculated static screened Coulomb interaction,
are utilized in the Bethe-Salpeter equation to calculate the dielectric
function for both the rutile and anatase structures. The
results are in good agreement with experimental observations, particularly the
onset of the main absorption features around 4 eV. For comparison to low
temperature optical absorption measurements that resolve individual excitonic
transitions in rutile, the low-lying discrete excitonic energy levels are
calculated with electronic screening only. The lowest energy exciton found in
the energy gap of rutile has a binding energy of 0.13 eV. In agreement with
experiment, it is not dipole allowed, but the calculated exciton energy exceeds
that measured in absorption experiments by about 0.22 eV and the scale of the
exciton binding energy is also too large. The quasiparticle energy alignment of
rutile is calculated for non-polar (110) surfaces. In the GW approximation, the
valence band maximum is 7.8 eV below the vacuum level, showing a small shift
from density functional theory results.Comment: Submitted to Physical Review
Spin-dependent Hedin's equations
Hedin's equations for the electron self-energy and the vertex were originally
derived for a many-electron system with Coulomb interaction. In recent years it
has been increasingly recognized that spin interactions can play a major role
in determining physical properties of systems such as nanoscale magnets or of
interfaces and surfaces. We derive a generalized set of Hedin's equations for
quantum many-body systems containing spin interactions, e.g. spin-orbit and
spin-spin interactions. The corresponding spin-dependent GW approximation is
constructed.Comment: 5 pages, 1 figur
Dynamical fermion mass generation by a strong Yukawa interaction
We consider a model with global Abelian chiral symmetry of two massless
fermion fields interacting with a complex massive scalar field. We argue that
the Schwinger-Dyson equations for the fermion and boson propagators admit
ultraviolet-finite chiral-symmetry-breaking solutions provided the Yukawa
couplings are large enough. The fermions acquire masses and the elementary
excitations of the complex scalar field are the two real spin-zero particles
with different masses. As a necessary consequence of the dynamical chiral
symmetry breakdown both in the fermion and scalar sectors, one massless
pseudoscalar Nambu--Goldstone boson appears in the spectrum as a collective
excitation of both the fermion and the boson fields. Its effective couplings to
the fermion and boson fields are calculable.Comment: 9 pages, REVTeX4, uses feynmp, 1 eps figur
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
Loss of superfluidity by fermions in the boson Hubbard model on an optical lattice
The experimentally observed loss of superfluidity by introducing fermions to
the boson Hubbard system on an optical lattice is explained. We show that the
virtual transitions of the bosons to the higher Bloch bands, coupled with the
contact boson-fermion interactions of either sign, result in an effective
increase of the boson on-site repulsion. If this renormalization of the on-site
potential is dominant over the fermion screening of the boson interactions, the
Mott insulating lobes of the Bose-Hubbard phase diagram will be enhanced for
either sign of the boson-fermion interactions. We discuss implications for cold
atom experiments where the expansion of the Mott lobes by fermions has been
conclusively established.Comment: 4 pages, 1 eps figure, minor change
Anomalous Pinning Fields in Helical Magnets: Screening of the Quasiparticle Interaction
The spin-orbit interaction strength g_so in helical magnets determines both
the pitch wave number q and the critical field H_c1 where the helix aligns with
an external magnetic field. Within a standard Landau-Ginzburg-Wilson (LGW)
theory, a determination of g_so in MnSi and FeGe from these two observables
yields values that differ by a factor of 20. This discrepancy is remedied by
considering the fermionic theory underlying the LGW theory, and in particular
the effects of screening on the effective electron-electron interaction that
results from an exchange of helical fluctuations.Comment: 4pp, 2 fig
On the Quantum Kinetic Equation in Weak Turbulence
The quantum kinetic equation used in the study of weak turbulence is
reconsidered in the context of a theory with a generic quartic interaction. The
expectation value of the time derivative of the mode number operators is
computed in a perturbation expansion which places the large diagonal component
of the quartic term in the unperturbed Hamiltonian. Although one is not
perturbing around a free field theory, the calculation is easily tractable
owing to the fact that the unperturbed Hamiltonian can be written solely in
terms of the mode number operators.Comment: 12 pages, LATEX, no figures, to appear in Phys. Rev.
On the Cholesky Decomposition for electron propagator methods: General aspects and application on C60
To treat the electronic structure of large molecules by electron propagator
methods we developed a parallel computer program called P-RICD. The
program exploits the sparsity of the two-electron integral matrix by using
Cholesky decomposition techniques. The advantage of these techniques is that
the error introduced is controlled only by one parameter which can be chosen as
small as needed. We verify the tolerance of electron propagator methods to the
Cholesky decomposition threshold and demonstrate the power of the
P-RICD program for a representative example (C60). All decomposition
schemes addressed in the literature are investigated. Even with moderate
thresholds the maximal error encountered in the calculated electron affinities
and ionization potentials amount to a few meV only, and the error becomes
negligible for small thresholds.Comment: 30 pages, 6 figures submitted to J.Chem. Phy
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