2,297 research outputs found
Electron capture rates in a plasma
A new general expression is derived for nuclear electron capture rates within
dense plasmas. Its qualitative nature leads us to question some widely accepted
assumptions about how to calculate the effects of the plasma on the rates. A
perturbative evaluation, though not directly applicable to the strongly
interacting case, appears to bear out these suspicions.Comment: 9 page
Dynamics of rapidly rotating Bose-Einstein condensates in a harmonic plus quartic trap
A two-dimensional rapidly rotating Bose-Einstein condensate in a harmonic
plus quartic trap is expected to have unusual vortex states that do not occur
in a pure harmonic trap. At a critical rotation speed , a central
hole appears in the condensate, and at some faster rotation speed ,
the system undergoes a transition to a giant vortex state with pure
irrotational flow. Using a time-dependent variational analysis, we study the
behavior of an annular condensate with a single concentric ring of vortices.
The transition to a giant vortex state is investigated by comparing the energy
of the two equilibrium states (the ring of vortices and the giant vortex) and
also by studying the dynamical stability of small excitation modes of the ring
of vortices.Comment: 12pages, 4figure
Energy and Vorticity in Fast Rotating Bose-Einstein Condensates
We study a rapidly rotating Bose-Einstein condensate confined to a finite
trap in the framework of two-dimensional Gross-Pitaevskii theory in the strong
coupling (Thomas-Fermi) limit. Denoting the coupling parameter by 1/\eps^2
and the rotational velocity by , we evaluate exactly the next to
leading order contribution to the ground state energy in the parameter regime
|\log\eps|\ll \Omega\ll 1/(\eps^2|\log\eps|) with \eps\to 0. While the TF
energy includes only the contribution of the centrifugal forces the next order
corresponds to a lattice of vortices whose density is proportional to the
rotational velocity.Comment: 19 pages, LaTeX; typos corrected, clarifying remarks added, some
rearrangements in the tex
An efficient method for the Quantum Monte Carlo evaluation of the static density-response function of a many-electron system
In a recent Letter we introduced Hellmann-Feynman operator sampling in
diffusion Monte Carlo calculations. Here we derive, by evaluating the second
derivative of the total energy, an efficient method for the calculation of the
static density-response function of a many-electron system. Our analysis of the
effect of the nodes suggests that correlation is described correctly and we
find that the effect of the nodes can be dealt with
Random-phase-approximation-based correlation energy functionals: Benchmark results for atoms
The random phase approximation (RPA) for the correlation energy functional of
density functional theory has recently attracted renewed interest. Formulated
in terms of the Kohn-Sham (KS) orbitals and eigenvalues, it promises to resolve
some of the fundamental limitations of the local density and generalized
gradient approximations, as for instance their inability to account for
dispersion forces. First results for atoms, however, indicate that the RPA
overestimates correlation effects as much as the orbital-dependent functional
obtained by a second order perturbation expansion on the basis of the KS
Hamiltonian. In this contribution, three simple extensions of the RPA are
examined, (a) its augmentation by an LDA for short-range correlation, (b) its
combination with the second order exchange term, and (c) its combination with a
partial resummation of the perturbation series including the second order
exchange. It is found that the ground state and correlation energies as well as
the ionization potentials resulting from the extensions (a) and (c) for closed
sub-shell atoms are clearly superior to those obtained with the unmodified RPA.
Quite some effort is made to ensure highly converged RPA data, so that the
results may serve as benchmark data. The numerical techniques developed in this
context, in particular for the inherent frequency integration, should also be
useful for applications of RPA-type functionals to more complex systems.Comment: 11 pages, 7 figure
Two-fluid model for a rotating trapped Fermi gas in the BCS phase
We investigate the dynamical properties of a superfluid gas of trapped
fermionic atoms in the BCS phase. As a simple example we consider the reaction
of the gas to a slow rotation of the trap. It is shown that the currents
generated by the rotation can be understood within a two-fluid model similar to
the one used in the theory of superconductors, but with a position dependent
ratio of normal and superfluid densities. The rather general result of this
paper is that already at very low temperatures, far below the critical one, an
important normal-fluid component appears in the outer regions of the gas. This
renders the experimental observation of superfluidity effects more difficult
and indicates that reliable theoretical predictions concerning other dynamical
properties, like the frequencies of collective modes, can only be made by
taking into account temperature effects.Comment: 6 pages, 4 figure
Kelvin mode of a vortex in a nonuniform Bose-Einstein condensate
In a uniform fluid, a quantized vortex line with circulation h/M can support
long-wavelength helical traveling waves proportional to e^{i(kz-\omega_k t)}
with the well-known Kelvin dispersion relation \omega_k \approx (\hbar k^2/2M)
\ln(1/|k|\xi), where \xi is the vortex-core radius. This result is extended to
include the effect of a nonuniform harmonic trap potential, using a quantum
generalization of the Biot-Savart law that determines the local velocity V of
each element of the vortex line. The normal-mode eigenfunctions form an
orthogonal Sturm-Liouville set. Although the line's curvature dominates the
dynamics, the transverse and axial trapping potential also affect the normal
modes of a straight vortex on the symmetry axis of an axisymmetric Thomas-Fermi
condensate. The leading effect of the nonuniform condensate density is to
increase the amplitude along the axis away from the trap center. Near the ends,
however, a boundary layer forms to satisfy the natural Sturm-Liouville boundary
conditions. For a given applied frequency, the next-order correction
renormalizes the local wavenumber k(z) upward near the trap center, and k(z)
then increases still more toward the ends.Comment: 9 pages, 1 figur
The QCD string tension curve, the ferromagnetic magnetization, and the quark-antiquark confining potential at finite Temperature
We study the string tension as a function of temperature, fitting the SU(3)
lattice QCD finite temperature free energy potentials computed by the Bielefeld
group. We compare the string tension points with order parameter curves of
ferromagnets, superconductors or string models, all related to confinement. We
also compare the SU(3) string tension with the one of SU(2) Lattice QCD. With
the curve providing the best fit to the finite temperature string tensions, the
spontaneous magnetization curve, we then show how to include finite
temperature, in the state of the art confining and chiral invariant quark
models.Comment: 9 pages, 12 figure
Mechanism of d_{x^2-y^2}-wave superconductivity based on doped hole induced spin texture in high T_c cuprates
A mechanism of d_{x^2-y^2}-wave superconductivity is proposed for the
high-T_c cuprates based on a spin texture with non-zero topological density
induced by doped holes through Zhang-Rice singlet formation. The pairing
interaction arises from the magnetic Lorentz force like interaction between the
holes and the spin textures. The stability of the pairing state against the
vortex-vortex interaction and the Coulomb repulsion is examined. The mechanism
suggests appearance of a p-wave pairing component by introducing anisotropy in
the CuO_2 plane.Comment: 9 pages, 3 figures; added references, corrected minor error
Electron and phonon correlations in systems of one-dimensional electrons coupled to phonons
Electron and phonon correlations in systems of one-dimensional electrons
coupled to phonons are studied at low temperatures by emphasizing on the effect
of electron-phonon backward scattering. It is found that the -wave
components of the electron density and phonon displacement field share the same
correlations. Both correlations are quasi-long-ranged for a single conducting
chain coupled to one-dimensional or three-dimensional phonons, and they are
long-ranged for repulsive electron-electron interactions for a
three-dimensional array of parallel one-dimensional conducting chains coupled
to three-dimensional phonons
- …