1,410 research outputs found
Tkachenko modes as sources of quasiperiodic pulsar spin variations
We study the long wavelength shear modes (Tkachenko waves) of triangular
lattices of singly quantized vortices in neutron star interiors taking into
account the mutual friction between the superfluid and the normal fluid and the
shear viscosity of the normal fluid. The set of Tkachenko modes that propagate
in the plane orthogonal to the spin vector are weakly damped if the coupling
between the superfluid and normal fluid is small. In strong coupling, their
oscillation frequencies are lower and are undamped for small and moderate shear
viscosities. The periods of these modes are consistent with the observed
~100-1000 day variations in spin of PSR 1828-11.Comment: 7 pages, 3 figures, uses RevTex, v2: added discussion/references,
matches published versio
Vortex lattice stability and phase coherence in three-dimensional rapidly rotating Bose condensates
We establish the general equations of motion for the modes of a vortex
lattice in a rapidly rotating Bose-Einstein condensate in three dimensions,
taking into account the elastic energy of the lattice and the vortex line
bending energy. As in two dimensions, the vortex lattice supports Tkachenko and
gapped sound modes. In contrast, in three dimensions the Tkachenko mode
frequency at long wavelengths becomes linear in the wavevector for any
propagation direction out of the transverse plane. We compute the correlation
functions of the vortex displacements and the superfluid order parameter for a
homogeneous Bose gas of bounded extent in the axial direction. At zero
temperature the vortex displacement correlations are convergent at large
separation, but at finite temperatures, they grow with separation. The growth
of the vortex displacements should lead to observable melting of vortex
lattices at higher temperatures and somewhat lower particle number and faster
rotation than in current experiments. At zero temperature a system of large
extent in the axial direction maintains long range order-parameter correlations
for large separation, but at finite temperatures the correlations decay with
separation.Comment: 10 pages, 2 figures, Changes include the addition of the particle
density - vortex density coupling and the correct value of the shear modulu
Rapidly rotating Bose-Einstein condensates in anharmonic potentials
Rapidly rotating Bose-Einstein condensates confined in anharmonic traps can
exhibit a rich variety of vortex phases, including a vortex lattice, a vortex
lattice with a hole, and a giant vortex. Using an augmented Thomas-Fermi
variational approach to determine the ground state of the condensate in the
rotating frame -- valid for sufficiently strongly interacting condensates -- we
determine the transitions between these three phases for a
quadratic-plus-quartic confining potential. Combining the present results with
previous numerical simulations of small rotating condensates in such anharmonic
potentials, we delineate the general structure of the zero temperature phase
diagram.Comment: 5 pages, 5 figure
Vortices in Spatially Inhomogeneous Superfluids
We study vortices in a radially inhomogeneous superfluid, as realized by a
trapped degenerate Bose gas in a uniaxially symmetric potential. We show that,
in contrast to a homogeneous superfluid, an off-axis vortex corresponds to an
anisotropic superflow whose profile strongly depends on the distance to the
trap axis. One consequence of this superflow anisotropy is vortex precession
about the trap axis in the absence of an imposed rotation. In the complementary
regime of a finite prescribed rotation, we compute the minimum-energy vortex
density, showing that in the rapid-rotation limit it is extremely uniform,
despite a strongly inhomogeneous (nearly) Thomas-Fermi condensate density
. The weak radially-dependent contribution () to the vortex distribution, that vanishes with the
number of vortices as , arises from the interplay between
vortex quantum discretness (namely their inability to faithfully support the
imposed rigid-body rotation) and the inhomogeneous superfluid density. This
leads to an enhancement of the vortex density at the center of a typical
concave trap, a prediction that is in quantitative agreement with recent
experiments (cond-mat/0405240). One striking consequence of the inhomogeneous
vortex distribution is an azimuthally-directed, radially-shearing superflow.Comment: 22 RevTeX pages, 20 figures, Submitted to PR
Influence of equation of state on interpretation of electrical conductivity measurements in strongly coupled tungsten plasma
We study the influence of equation-of-state (EOS) model on the interpretation
of electrical conductivity measurements in strongly coupled plasma of tungsten
by Korobenko et al. (2002 Plasma Physics Reports 28(12) 1008--1016). Three
different semiempirical EOS models for tungsten are used. Discrepancies in
obtained thermodynamic parameters and specific resistivity values as compared
with calculation results of Korobenko et al. are analysed.Comment: 11 pages, 5 Postscript figures, accepted for publication in J. Phys.
A: Math. Ge
Two-component Bose-Einstein Condensates with Large Number of Vortices
We consider the condensate wavefunction of a rapidly rotating two-component
Bose gas with an equal number of particles in each component. If the
interactions between like and unlike species are very similar (as occurs for
two hyperfine states of Rb or Na) we find that the two components
contain identical rectangular vortex lattices, where the unit cell has an
aspect ratio of , and one lattice is displaced to the center of the
unit cell of the other. Our results are based on an exact evaluation of the
vortex lattice energy in the large angular momentum (or quantum Hall) regime.Comment: 4 pages, 2 figures, RevTe
Theory of vortex-lattice melting in a one-dimensional optical lattice
We investigate quantum and temperature fluctuations of a vortex lattice in a
one-dimensional optical lattice. We discuss in particular the Bloch bands of
the Tkachenko modes and calculate the correlation function of the vortex
positions along the direction of the optical lattice. Because of the small
number of particles in the pancake Bose-Einstein condensates at every site of
the optical lattice, finite-size effects become very important. Moreover, the
fluctuations in the vortex positions are inhomogeneous due to the inhomogeneous
density. As a result, the melting of the lattice occurs from the outside
inwards. However, tunneling between neighboring pancakes substantially reduces
the inhomogeneity as well as the size of the fluctuations. On the other hand,
nonzero temperatures increase the size of the fluctuations dramatically. We
calculate the crossover temperature from quantum melting to classical melting.
We also investigate melting in the presence of a quartic radial potential,
where a liquid can form in the center instead of at the outer edge of the
pancake Bose-Einstein condensates.Comment: 17 pages, 17 figures, submitted to Phys. Rev. A, references update
Tkachenko oscillations and the compressibility of a rotating Bose gas
The elastic oscillations of the vortex lattice of a cold Bose gas (Tkachenko
modes) are shown to play a crucial role in the saturation of the
compressibility sum rule, as a consequence of the hybridization with the
longitudinal degrees of freedom. The presence of the vortex lattice is
responsible for a behavior of the static structure factor at small
wavevectors , which implies the absence of long range order in 2D
configurations at zero temperature. Sum rules are used to calculate the
Tkachenko frequency in the presence of harmonic trapping. Results are derived
in the Thomas-Fermi regime and compared with experiments as well as with
previous theoretical estimates.Comment: 4 pages, 2 figure
Coulomb Charging Effects in an Open Quantum Dot
Low-temperature transport properties of a lateral quantum dot formed by
overlaying finger gates in a clean one-dimensional channel are investigated.
Continuous and periodic oscillations superimposed upon ballistic conductance
steps are observed, when the conductance G of the dot changes within a wide
range 0<G<6e^2/h. Calculations of the electrostatics confirm that the measured
periodic conductance oscillations correspond to successive change of the total
charge of the dot by . By modelling the transport it is shown that the
progression of the Coulomb oscillations into the region G>2e^2/h may be due to
suppression of inter-1D-subband scattering. Fully transmitted subbands
contribute to coherent background of conductance, while sequential tunneling
via weakly transmitted subbands leads to Coulomb charging of the dot.Comment: 12 pages, RevTeX, 15 eps figures included, submitted to Phys. Rev.
Collective Oscillations of Vortex Lattices in Rotating Bose-Einstein Condensates
The complete low-energy collective-excitation spectrum of vortex lattices is
discussed for rotating Bose-Einstein condensates (BEC) by solving the
Bogoliubov-de Gennes (BdG) equation, yielding, e.g., the Tkachenko mode
recently observed at JILA. The totally symmetric subset of these modes includes
the transverse shear, common longitudinal, and differential longitudinal modes.
We also solve the time-dependent Gross-Pitaevskii (TDGP) equation to simulate
the actual JILA experiment, obtaining the Tkachenko mode and identifying a pair
of breathing modes. Combining both the BdG and TDGP approaches allows one to
unambiguously identify every observed mode.Comment: 5 pages, 4 figure
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