25 research outputs found
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
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-
Controlled creation of a singular spinor vortex by circumventing the Dirac belt trick
Persistent topological defects and textures are particularly dramatic consequences of superfluidity. Among the most fascinating examples are the singular vortices arising from the rotational symmetry group SO(3), with surprising topological properties illustrated by Dirac’s famous belt trick. Despite considerable interest, controlled preparation and detailed study of vortex lines with complex internal structure in fully three-dimensional spinor systems remains an outstanding experimental challenge. Here, we propose and implement a reproducible and controllable method for creating and detecting a singular SO(3) line vortex from the decay of a non-singular spin texture in a ferromagnetic spin-1 Bose–Einstein condensate. Our experiment explicitly demonstrates the SO(3) character and the unique spinor properties of the defect. Although the vortex is singular, its core fills with atoms in the topologically distinct polar magnetic phase. The resulting stable, coherent topological interface has analogues in systems ranging from condensed matter to cosmology and string theory
On the analysis of the contact angle for impacting droplets using a polynomial fitting approach
ractical considerations on the measurement of the dynamic contact angle and the spreading diameter of impacting droplets are discussed in this paper. The contact angle of a liquid is commonly obtained either by a polynomial or a linear fitting to the droplet profile around the triple phase point. Previous works have focused on quasi-static or sessile droplets, or in cases where inertia does not play a major role on the contact angle dynamics. Here, we study the effect of droplet shape, the order of the fitting polynomial, and the fitting domain, on the measurement of the contact angle on various stages following droplet impact where the contact line is moving. Our results, presented in terms of the optical resolution and the droplet size, show that a quadratic fitting provides the most consistent results for a range of various droplet shapes. As expected, our results show that contact angle values are less sensitive to the fitting conditions for the cases where the droplet can be approximated to a spherical cap. Our experimental conditions include impact events with liquid droplets of different sizes and viscosities on various substrates. In addition, validating past works, our results show that the maximum spreading diameter can be parameterised by the Weber number and the rapidly advancing contact angle