2,971 research outputs found
Static and dynamic properties of shell-shaped condensates
Static, dynamic, and topological properties of hollow systems differ from
those that are fully filled as a result of the presence of a boundary
associated with an inner surface. Hollow Bose-Einstein condensates (BECs)
naturally occur in various ultracold atomic systems and possibly within neutron
stars but have hitherto not been experimentally realized in isolation on Earth
because of gravitational sag. Motivated by the expected first realization of
fully closed BEC shells in the microgravity conditions of the Cold Atomic
Laboratory aboard the International Space Station, we present a comprehensive
study of spherically symmetric hollow BECs as well as the hollowing transition
from a filled sphere BEC into a thin shell through central density depletion.
We employ complementary analytic and numerical techniques in order to study
equilibrium density profiles and the collective mode structures of condensate
shells hosted by a range of trapping potentials. We identify concrete and
robust signatures of the evolution from filled to hollow structures and the
effects of the emergence of an inner boundary, inclusive of a dip in
breathing-mode-type collective mode frequencies and a restructuring of surface
mode structure across the transition. By extending our analysis to a
two-dimensional transition of a disk to a ring, we show that the collective
mode signatures are an essential feature of hollowing, independent of the
specific geometry. Finally, we relate our work to past and ongoing experimental
efforts and consider the influence of gravity on thin condensate shells. We
identify the conditions under which gravitational sag is highly destructive and
study the mode-mixing effects of microgravity on the collective modes of these
shells.Comment: 26 pages, 13 figure
Tunnelling of topological line defects in strongly coupled superfluids
The geometric theory of vortex tunnelling in superfluid liquids is developed.
Geometry rules the tunnelling process in the approximation of an incompressible
superfluid, which yields the identity of phase and configuration space in the
vortex collective co-ordinate. To exemplify the implications of this approach
to tunnelling, we solve explicitly for the two-dimensional motion of a point
vortex in the presence of an ellipse, showing that the hydrodynamic collective
co-ordinate description limits the constant energy paths allowed for the vortex
in configuration space. We outline the experimental procedure used in helium II
to observe tunnelling events, and compare the conclusions we draw to the
experimental results obtained so far. Tunnelling in Fermi superfluids is
discussed, where it is assumed that the low energy quasiparticle excitations
localised in the vortex core govern the vortex dynamical equations. The
tunnelling process can be dominated by Hall or dissipative terms, respectively
be under the influence of both, with a possible realization of this last
intermediate case in unconventional, high-temperature superconductors.Comment: 51 pages, 15 figures, uses Ann. Phys. (Leipzig) style file; forms
part of author's dissertation, available at
http://xxx.lanl.gov/abs/cond-mat/9909166v
Adiabaticity and spectral splits in collective neutrino transformations
Neutrinos streaming off a supernova core transform collectively by
neutrino-neutrino interactions, leading to "spectral splits" where an energy
E_split divides the transformed spectrum sharply into parts of almost pure but
different flavors. We present a detailed description of the spectral split
phenomenon which is conceptually and quantitatively understood in an adiabatic
treatment of neutrino-neutrino effects. Central to this theory is a
self-consistency condition in the form of two sum rules (integrals over the
neutrino spectra that must equal certain conserved quantities). We provide
explicit analytic and numerical solutions for various neutrino spectra. We
introduce the concept of the adiabatic reference frame and elaborate on the
relative adiabatic evolution. Violating adiabaticity leads to the spectral
split being "washed out". The sharpness of the split appears to be represented
by a surprisingly universal function.Comment: 20 pages, revtex, 13 figure
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