42 research outputs found
Kelvon-roton instability of vortex lines in dipolar Bose-Einstein condensates
The physics of vortex lines in dipolar condensates is studied. Due to the
nonlocality of the dipolar interaction, the 3D character of the vortex plays a
more important role in dipolar gases than in typical short-range interacting
ones. In particular, the dipolar interaction significantly affects the
stability of the transverse modes of the vortex line. Remarkably, in the
presence of a periodic potential along the vortex line, a roton minimum may
develop in the spectrum of transverse modes. We discuss the appropriate
conditions at which this roton minimum may eventually lead to an instability of
the straight vortex line, opening new scenarios for vortices in dipolar gases.Comment: 4 pages, 3 eps figure
Vortex dynamics of rotating dipolar Bose-Einstein condensates
We study the influence of dipole-dipole interaction on the formation of
vortices in a rotating dipolar Bose-Einstein condensate (BEC) of Cr and
Dy atoms in quasi two-dimensional geometry. By numerically solving the
corresponding time-dependent mean-field Gross-Pitaevskii equation, we show that
the dipolar interaction enhances the number of vortices while a repulsive
contact interaction increases the stability of the vortices. Further, an
ordered vortex lattice of relatively large number of vortices is found in a
strongly dipolar BEC.Comment: 15 pages, 10 figures, 1 tabl
Interlayer superfluidity in bilayer systems of fermionic polar molecules
We consider fermionic polar molecules in a bilayer geometry where they are
oriented perpendicularly to the layers, which permits both low inelastic losses
and superfluid pairing. The dipole-dipole interaction between molecules of
different layers leads to the emergence of interlayer superfluids. The
superfluid regimes range from BCS-like fermionic superfluidity with a high
to Bose-Einstein (quasi-)condensation of interlayer dimers, thus
exhibiting a peculiar BCS-BEC crossover. We show that one can cover the entire
crossover regime under current experimental conditions.Comment: 4 pages, 4 figure
Superfluidity of flexible chains of polar molecules
We study properties of quantum chains in a gas of polar bosonic molecules
confined in a stack of N identical one- and two- dimensional optical lattice
layers, with molecular dipole moments aligned perpendicularly to the layers.
Quantum Monte Carlo simulations of a single chain (formed by a single molecule
on each layer) reveal its quantum roughening transition. The case of finite
in-layer density of molecules is studied within the framework of the J-current
model approximation, and it is found that N-independent molecular superfluid
phase can undergo a quantum phase transition to a rough chain superfluid. A
theorem is proven that no superfluidity of chains with length shorter than N is
possible. The scheme for detecting chain formation is proposed.Comment: Submitted to Proceedings of the QFS2010 satellite conference "Cold
Gases meet Many-Body Theory", Grenoble, August 7, 2010. This is the expanded
version of V.
Static spectroscopy of a dense superfluid
Dense Bose superfluids, as HeII, differ from dilute ones by the existence of
a roton minimum in their excitation spectrum. It is known that this roton
minimum is qualitatively responsible for density oscillations close to any
singularity, such as vortex cores, or close to solid boundaries. We show that
the period of these oscillations, and their exponential decrease with the
distance to the singularity, are fully determined by the position and the width
of the roton minimum. Only an overall amplitude factor and a phase shift are
shown to depend on the details of the interaction potential. Reciprocally, it
allows for determining the characteristics of this roton minimum from static
"observations" of a disturbed ground state, in cases where the dynamics is not
easily accessible. We focus on the vortex example. Our analysis further shows
why the energy of these oscillations is negligible compared to the kinetic
energy, which limits their influence on the vortex dynamics, except for high
curvatures.Comment: 14 pages, 4 figures, extended version, published in J. Low Temp. Phy