25 research outputs found
Helical spin textures in dipolar Bose-Einstein condensates
We numerically study elongated helical spin textures in ferromagnetic spin-1
Bose-Einstein condensates subject to dipolar interparticle forces. Stationary
states of the Gross-Pitaevskii equation are solved and analyzed for various
values of the helical wave vector and dipolar coupling strength. We find two
helical spin textures which differ by the nature of their topological defects.
The spin structure hosting a pair of Mermin-Ho vortices with opposite mass
flows and aligned spin currents is stabilized for a nonzero value of the
helical wave vector.Comment: 7 pages, 6 figure
Dynamically stable multiply quantized vortices in dilute Bose-Einstein condensates
Multiquantum vortices in dilute atomic Bose-Einstein condensates confined in
long cigar-shaped traps are known to be both energetically and dynamically
unstable. They tend to split into single-quantum vortices even in the ultralow
temperature limit with vanishingly weak dissipation, which has also been
confirmed in the recent experiments [Y. Shin et al., Phys. Rev. Lett. 93,
160406 (2004)] utilizing the so-called topological phase engineering method to
create multiquantum vortices. We study the stability properties of multiquantum
vortices in different trap geometries by solving the Bogoliubov excitation
spectra for such states. We find that there are regions in the trap asymmetry
and condensate interaction strength plane in which the splitting instability of
multiquantum vortices is suppressed, and hence they are dynamically stable. For
example, the doubly quantized vortex can be made dynamically stable even in
spherical traps within a wide range of interaction strength values. We expect
that this suppression of vortex-splitting instability can be experimentally
verified.Comment: 5 pages, 6 figure
Elementary excitations in dipolar spin-1 Bose-Einstein condensates
We have numerically solved the low-energy excitation spectra of ferromagnetic
Bose-Einstein condensates subject to dipolar interparticle interactions. The
system is assumed to be harmonically confined by purely optical means, thereby
maintaining the spin degree of freedom of the condensate order parameter. Using
a zero-temperature spin-1 model, we solve the Bogoliubov excitations for
different spin textures, including a spin-vortex state in the absence of
external magnetic fields and a rapidly rotating polarized spin texture in a
finite homogeneous field. In particular, we consider the effect of dipolar
interactions on excitations characteristic of ferromagnetic condensates. The
energies of spin waves and magnetic quadrupole modes are found to increase
rapidly with the dipolar coupling strength, whereas the energies of density
oscillations change only slightly.Comment: 7 pages, 4 figure
Spin textures in condensates with large dipole moments
We have solved numerically the ground states of a Bose-Einstein condensate in
the presence of dipolar interparticle forces using a semiclassical approach.
Our motivation is to model, in particular, the spontaneous spin textures
emerging in quantum gases with large dipole moments, such as 52Cr or Dy
condensates, or ultracold gases consisting of polar molecules. For a
pancake-shaped harmonic (optical) potential, we present the ground state phase
diagram spanned by the strength of the nonlinear coupling and dipolar
interactions. In an elongated harmonic potential, we observe a novel helical
spin texture. The textures calculated according to the semiclassical model in
the absence of external polarizing fields are predominantly analogous to
previously reported results for a ferromagnetic F = 1 spinor Bose-Einstein
condensate, suggesting that the spin textures arising from the dipolar forces
are largely independent of the value of the quantum number F or the origin of
the dipolar interactions.Comment: 9 pages, 6 figure
Stable Fractional Vortices in the Cyclic States of Bose-Einstein Condensates
We propose methods to create fractional vortices in the cyclic state of an F
= 2 spinor Bose-Einstein condensate by manipulating its internal spin structure
using pulsed microwave and laser fields. The stability of such vortices is
studied as a function of the rotation frequency of the confining harmonic trap
both in pancake and cigar shaped condensates. We find a range of parameters for
which the so-called 1/3-vortex state is energetically favorable. Such
fractional vortices could be created in condensates of 87Rb atoms using current
experimental techniques facilitating probing of topological defects with
non-Abelian statistics.Comment: 5 pages, 2 figure
Splitting times of doubly quantized vortices in dilute Bose-Einstein condensates
Recently, the splitting of a topologically created doubly quantized vortex
into two singly quantized vortices was experimentally investigated in dilute
atomic cigar-shaped Bose-Einstein condensates [Y. Shin et al., Phys. Rev. Lett.
93, 160406 (2004)]. In particular, the dependency of the splitting time on the
peak particle density was studied. We present results of theoretical
simulations which closely mimic the experimental set-up. Contrary to previous
theoretical studies, claiming that thermal excitations are the essential
mechanism in initiating the splitting, we show that the combination of
gravitational sag and time dependency of the trapping potential alone suffices
to split the doubly quantized vortex in time scales which are in good agreement
with the experiments. We also study the dynamics of the resulting singly
quantized vortices which typically intertwine--especially, a peculiar vortex
chain structure appears for certain parameter values.Comment: 5 pages, 5 figure
Tunable single-photon heat conduction in electrical circuits
We build on the study of single-photon heat conduction in electronic circuits
taking into account the back-action of the
superconductor--insulator--normal-metal thermometers. In addition, we show that
placing capacitors, resistors, and superconducting quantum interference devices
(SQUIDs) into a microwave cavity can severely distort the spatial current
profile which, in general, should be accounted for in circuit design. The
introduction of SQUIDs also allows for in situ tuning of the photonic power
transfer which could be utilized in experiments on superconducting quantum
bits
Stabilization and pumping of giant vortices in dilute Bose-Einstein condensates
Recently, it was shown that giant vortices with arbitrarily large quantum
numbers can possibly be created in dilute Bose-Einstein condensates by
cyclically pumping vorticity into the condensate. However, multiply quantized
vortices are typically dynamically unstable in harmonically trapped nonrotated
condensates, which poses a serious challenge to the vortex pump procedure. In
this theoretical study, we investigate how the giant vortices can be stabilized
by the application of a Gaussian potential peak along the vortex core. We find
that achieving dynamical stability is feasible up to high quantum numbers. To
demonstrate the efficiency of the stabilization method, we simulate the
adiabatic creation of an unsplit 20-quantum vortex with the vortex pump.Comment: 8 pages, 6 figures; to be published in J. Low Temp. Phys., online
publication available at http://dx.doi.org/10.1007/s10909-010-0216-