875 research outputs found
Instabilities of wave function monopoles in Bose-Einstein condensates
We present analytic and numerical results for a class of monopole solutions
to the two-component Gross-Pitaevski equation for a two-species Bose condensate
in an effectively two-dimensional trap. We exhibit dynamical instabilities
involving vortex production as one species pours through another, from which we
conclude that the sub-optical sharpness of potentials exerted by matter waves
makes condensates ideal tools for manipulating condensates. We also show that
there are two equally valid but drastically different hydrodynamic descriptions
of a two-component condensate, and illustrate how different phenomena may
appear simpler in each.Comment: 4 pages, 9 figures (compressed figures become legible when zoomed or
when paper is actually printed
Winding up by a quench: vortices in the wake of rapid Bose-Einstein condensation
A second order phase transition induced by a rapid quench can lock out
topological defects with densities far exceeding their equilibrium expectation
values. We use quantum kinetic theory to show that this mechanism, originally
postulated in the cosmological context, and analysed so far only on the mean
field classical level, should allow spontaneous generation of vortex lines in
trapped Bose-Einstein condensates of simple topology, or of winding number in
toroidal condensates.Comment: 4 pages, 2 figures; misprint correcte
Dynamics of Quantum Phase Transition in an Array of Josephson Junctions
We study the dynamics of the Mott insulator-superfluid quantum phase
transition in a periodic 1D array of Josephson junctions. We show that crossing
the critical point diabatically i.e. at a finite rate with a quench time
induces finite quantum fluctuations of the current around the loop
proportional to . This scaling could be experimentally verified
with in array of weakly coupled Bose-Einstein condensates or superconducting
grains.Comment: 4 pages in RevTex, 3 .eps figures; 2 references added; accepted for
publication in Phys.Rev.Let
Interpolating between the Bose-Einstein and the Fermi-Dirac distributions in odd dimensions
We consider the response of a uniformly accelerated monopole detector that is
coupled to a superposition of an odd and an even power of a quantized, massless
scalar field in flat spacetime in arbitrary dimensions. We show that, when the
field is assumed to be in the Minkowski vacuum, the response of the detector is
characterized by a Bose-Einstein factor in even spacetime dimensions, whereas a
Bose-Einstein as well as a Fermi-Dirac factor appear in the detector response
when the dimension of spacetime is odd. Moreover, we find that, it is possible
to interpolate between the Bose-Einstein and the Fermi-Dirac distributions in
odd spacetime dimensions by suitably adjusting the relative strengths of the
detector's coupling to the odd and the even powers of the scalar field. We
point out that the response of the detector is always thermal and we, finally,
close by stressing the apparent nature of the appearance of the Fermi-Dirac
factor in the detector response.Comment: RevTeX, 7 page
Decoherence of electron beams by electromagnetic field fluctuations
Electromagnetic field fluctuations are responsible for the destruction of
electron coherence (dephasing) in solids and in vacuum electron beam
interference. The vacuum fluctuations are modified by conductors and
dielectrics, as in the Casimir effect, and hence, bodies in the vicinity of the
beams can influence the beam coherence. We calculate the quenching of
interference of two beams moving in vacuum parallel to a thick plate with
permittivity . In case of an
ideal conductor or dielectric the dephasing is suppressed
when the beams are close to the surface of the plate, because the random
tangential electric field , responsible for dephasing, is zero at the
surface. The situation is changed dramatically when
or are finite. In this case there exists a layer near
the surface, where the fluctuations of are strong due to evanescent
near fields. The thickness of this near - field layer is of the order of the
wavelength in the dielectric or the skin depth in the conductor, corresponding
to a frequency which is the inverse electron time of flight from the emitter to
the detector. When the beams are within this layer their dephasing is enhanced
and for slow enough electrons can be even stronger than far from the surface
Tkachenko modes and quantum melting of Josephson junction type of vortex array in rotating Bose-Einstein condensate
Using path integral formalism, we show that the Abrikosov-Tkachenko vortex
lattice may equivalently be understood as an array of Josephson junctions. The
Tkachenko modes are found to be basically equivalent to the low energy
excitations (Goldstone modes) of an ordered state. The calculated frequencies
are in very good agreement with recent experimental data. Calculations of the
fluctuations of the relative displacements of the vortices show that vortex
melting is a result of quantum fluctuations around the ordered state due to the
low energy excitations (Tkachenko modes)and occurs when the ratio of the
kinectic energy to the potential energy of the vortex lattice is 0.001.Comment: revised paper 11 pages with 2 figures, all in Pdf forma
Nonlinear dynamics for vortex lattice formation in a rotating Bose-Einstein condensate
We study the response of a trapped Bose-Einstein condensate to a sudden
turn-on of a rotating drive by solving the two-dimensional Gross-Pitaevskii
equation. A weakly anisotropic rotating potential excites a quadrupole shape
oscillation and its time evolution is analyzed by the quasiparticle projection
method. A simple recurrence oscillation of surface mode populations is broken
in the quadrupole resonance region that depends on the trap anisotropy, causing
stochastization of the dynamics. In the presence of the phenomenological
dissipation, an initially irrotational condensate is found to undergo damped
elliptic deformation followed by unstable surface ripple excitations, some of
which develop into quantized vortices that eventually form a lattice. Recent
experimental results on the vortex nucleation should be explained not only by
the dynamical instability but also by the Landau instability; the latter is
necessary for the vortices to penetrate into the condensate.Comment: RevTex4, This preprint includes no figures. You can download the
complete article and figures at
http://matter.sci.osaka-cu.ac.jp/bsr/cond-mat.htm
Decoherence from a Chaotic Environment: An Upside Down "Oscillator" as a Model
Chaotic evolutions exhibit exponential sensitivity to initial conditions.
This suggests that even very small perturbations resulting from weak coupling
of a quantum chaotic environment to the position of a system whose state is a
non-local superposition will lead to rapid decoherence. However, it is also
known that quantum counterparts of classically chaotic systems lose exponential
sensitivity to initial conditions, so this expectation of enhanced decoherence
is by no means obvious. We analyze decoherence due to a "toy" quantum
environment that is analytically solvable, yet displays the crucial phenomenon
of exponential sensitivity to perturbations. We show that such an environment,
with a single degree of freedom, can be far more effective at destroying
quantum coherence than a heat bath with infinitely many degrees of freedom.
This also means that the standard "quantum Brownian motion" model for a
decohering environment may not be as universally applicable as it once was
conjectured to be.Comment: RevTeX, 29 pages, 5 EPS figures. Substantially rewritten analysis,
improved figures, additional references, and errors fixed. Final version (to
appear in PRA
The Josephson plasmon as a Bogoliubov quasiparticle
We study the Josephson effect in alkali atomic gases within the two-mode
approximation and show that there is a correspondence between the Bogoliubov
description and the harmonic limit of the phase representation. We demonstrate
that the quanta of the Josephson plasmon can be identified with the Bogoliubov
excitations of the two-site Bose fluid. We thus establish a mapping between the
Bogoliubov approximation for the many-body theory and the linearized pendulum
Hamiltonian.Comment: 9 pages, LaTeX, submitted to J. Phys.
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