143 research outputs found
Density Matrix Functional Calculations for Matter in Strong Magnetic Fields: I. Atomic Properties
We report on a numerical study of the density matrix functional introduced by
Lieb, Solovej and Yngvason for the investigation of heavy atoms in high
magnetic fields. This functional describes {\em exactly} the quantum mechanical
ground state of atoms and ions in the limit when the nuclear charge and the
electron number tend to infinity with fixed, and the magnetic field
tends to infinity in such a way that . We have
calculated electronic density profiles and ground state energies for values of
the parameters that prevail on neutron star surfaces and compared them with
results obtained by other methods. For iron at G the ground state
energy differs by less than 2 \% from the Hartree-Fock value. We have also
studied the maximal negative ionization of heavy atoms in this model at various
field strengths. In contrast to Thomas-Fermi type theories atoms can bind
excess negative charge in the density matrix model. For iron at G
the maximal excess charge in this model corresponds to about one electron.Comment: Revtex, 13 pages with 6 eps figures include
Storing and processing optical information with ultra-slow light in Bose-Einstein condensates
We theoretically explore coherent information transfer between ultra-slow
light pulses and Bose-Einstein condensates (BECs) and find that storing light
pulses in BECs, by switching off the coupling field, allows the coherent
condensate dynamics to process optical information. We develop a formalism,
applicable in both the weak and strong probe regimes, to analyze such
experiments and establish several new results. Investigating examples relevant
to Rb-87 experimental parameters we see a variety of novel two-component BEC
dynamics occur during the storage, including interference fringes, gentle
breathing excitations, and two-component solitons. We find the dynamics when
the levels |F=1, M_F=-1> and |F=2, M_F=+1> are well suited to designing
controlled processing of the information. By switching the coupling field back
on, the processed information is rewritten onto probe pulses which then
propagate out as slow light pulses. We calculate the fidelity of information
transfer between the atomic and light fields upon the switch-on and subsequent
output. The fidelity is affected both by absorption of small length scale
features and absorption of regions of the pulse stored near the condensate
edge. In the strong probe case, we find that when the oscillator strengths for
the two transitions are equal the fidelity is not strongly sensitive to the
probe strength, while when they are unequal the fidelity is worse for stronger
probes. Applications to distant communication between BECs, squeezed light
generation and quantum information are anticipated.Comment: 19 pages, 12 figures, submitted to Phys. Rev.
Trapped Particle Stability for the Kinetic Stabilizer
A kinetically stabilized axially symmetric tandem mirror (KSTM) uses the
momentum flux of low-energy, unconfined particles that sample only the outer
end-regions of the mirror plugs, where large favorable field-line curvature
exists. The window of operation is determined for achieving MHD stability with
tolerable energy drain from the kinetic stabilizer. Then MHD stable systems are
analyzed for stability of the trapped particle mode. This mode is characterized
by the detachment of the central-cell plasma from the kinetic stabilizer region
without inducing field-line bending. Stability of the trapped particle mode is
sensitive to the electron connection between the stabilizer and the end plug.
It is found that the stability condition for the trapped particle mode is more
constraining than the stability condition for the MHD mode, and it is
challenging to satisfy the required power constraint. Furthermore a severe
power drain may arise from the necessary connection of low-energy electrons in
the kinetic stabilizer to the central region
Stable two-dimensional solitary pulses in linearly coupled dissipative Kadomtsev-Petviashvili equations
A two-dimensional (2D) generalization of the stabilized Kuramoto -
Sivashinsky (KS) system is presented. It is based on the Kadomtsev-Petviashvili
(KP) equation including dissipation of the generic (Newell -- Whitehead --
Segel, NWS) type and gain. The system directly applies to the description of
gravity-capillary waves on the surface of a liquid layer flowing down an
inclined plane, with a surfactant diffusing along the layer's surface.
Actually, the model is quite general, offering a simple way to stabilize
nonlinear waves in media combining the weakly-2D dispersion of the KP type with
gain and NWS dissipation. Parallel to this, another model is introduced, whose
dissipative terms are isotropic, rather than of the NWS type. Both models
include an additional linear equation of the advection-diffusion type, linearly
coupled to the main KP-NWS equation. The extra equation provides for stability
of the zero background in the system, opening a way to the existence of stable
localized pulses. The consideration is focused on the case when the dispersive
part of the system of the KP-I type, admitting the existence of 2D localized
pulses. Treating the dissipation and gain as small perturbations and making use
of the balance equation for the field momentum, we find that the equilibrium
between the gain and losses may select two 2D solitons, from their continuous
family existing in the conservative counterpart of the model (the latter family
is found in an exact analytical form). The selected soliton with the larger
amplitude is expected to be stable. Direct simulations completely corroborate
the analytical predictions.Comment: a latex text file and 16 eps files with figures; Physical Review E,
in pres
Mode-coupling and nonlinear Landau damping effects in auroral Farley-Buneman turbulence
The fundamental problem of Farley-Buneman turbulence in the auroral
-region has been discussed and debated extensively in the past two decades.
In the present paper we intend to clarify the different steps that the auroral
-region plasma has to undergo before reaching a steady state. The
mode-coupling calculation, for Farley-Buneman turbulence, is developed in order
to place it in perspective and to estimate its magnitude relative to the
anomalous effects which arise through the nonlinear wave-particle interaction.
This nonlinear effect, known as nonlinear ``Landau damping'' is due to the
coupling of waves which produces other waves which in turn lose energy to the
bulk of the particles by Landau damping. This leads to a decay of the wave
energy and consequently a heating of the plasma. An equation governing the
evolution of the field spectrum is derived and a physical interpration for each
of its terms is provided
Transverse instability and its long-term development for solitary waves of the (2+1)-Boussinesq equation
The stability properties of line solitary wave solutions of the
(2+1)-dimensional Boussinesq equation with respect to transverse perturbations
and their consequences are considered. A geometric condition arising from a
multi-symplectic formulation of this equation gives an explicit relation
between the parameters for transverse instability when the transverse
wavenumber is small. The Evans function is then computed explicitly, giving the
eigenvalues for transverse instability for all transverse wavenumbers. To
determine the nonlinear and long time implications of transverse instability,
numerical simulations are performed using pseudospectral discretization. The
numerics confirm the analytic results, and in all cases studied, transverse
instability leads to collapse.Comment: 16 pages, 8 figures; submitted to Phys. Rev.
Evidence for topological nonequilibrium in magnetic configurations
We use direct numerical simulations to study the evolution, or relaxation, of
magnetic configurations to an equilibrium state. We use the full single-fluid
equations of motion for a magnetized, non-resistive, but viscous fluid; and a
Lagrangian approach is used to obtain exact solutions for the magnetic field.
As a result, the topology of the magnetic field remains unchanged, which makes
it possible to study the case of topological nonequilibrium. We find two cases
for which such nonequilibrium appears, indicating that these configurations may
develop singular current sheets.Comment: 10 pages, 5 figure
Conductivity in quasi two-dimensional systems
The conductivity in quasi two-dimensional systems is calculated using the
quantum kinetic equation. Linearizing the Lenard-Balescu collision integral
with the extension to include external field dependences allows one to
calculate the conductivity with diagrams beyond the GW approximation including
maximally crossed lines. Consequently the weak localization correction as an
interference effect appears here from the field dependence of the collision
integral (the latter dependence sometimes called intra-collisional field
effect). It is shown that this weak localization correction has the same origin
as the Debye-Onsager relaxation effect in plasma physics. The approximation is
applied to a system of quasi two-dimensional electrons in hetero-junctions
which interact with charged and neutral impurities and the low temperature
correction to the conductivity is calculated analytically. It turns out that
the dynamical screening due to charged impurities leads to a linear temperature
dependence, while the scattering from neutral impurities leads to the usual
Fermi-liquid behavior. By considering an appropriate mass action law to
determine the ratio of charged to neutral impurities we can describe the
experimental metal-insulator transition at low temperatures as a Mott-Hubbard
transition.Comment: 7 pages 7 pages appendix 11 figure
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