99 research outputs found
Spin critical opalescence in zero temperature Bose-Einstein Condensates
Cold atom developments suggest the prospect of measuring scaling properties
and long-range fluctuations of continuous phase transitions at
zero-temperature. We discuss the conditions for characterizing the phase
separation of Bose-Einstein condensates of boson atoms in two distinct
hyperfine spin states. The mean-field description breaks down as the system
approaches the transition from the miscible side. An effective spin description
clarifies the ferromagnetic nature of the transition. We show that a difference
in the scattering lengths for the bosons in the same spin state leads to an
effective internal magnetic field. The conditions at which the internal
magnetic field vanishes (i.e., equal values of the like-boson scattering
lengths) is a special point. We show that the long range density fluctuations
are suppressed near that point while the effective spin exhibits the long-range
fluctuations that characterize critical points. The zero-temperature system
exhibits critical opalescence with respect to long wavelength waves of impurity
atoms that interact with the bosons in a spin-dependent manner.Comment: 6 pages, 2 figure
Driving superfluidity with photoassociation
We theoretically examine photoassociation of a two-component Fermi degenerate
gas. Our focus is on adjusting the atom-atom interaction, and thereby
increasing the critical temperature of the BCS transition to the superfluid
state. In order to avoid spontaneous decay of the molecules, the
photoassociating light must be far-off resonance. Very high light intensities
are therefore required for effective control of the BCS transition.Comment: 7 (preprint) pages; submitted to Optics Expres
Reentrant stability of BEC standing wave patterns
We describe standing wave patterns induced by an attractive finite-ranged
external potential inside a large Bose-Einstein Condensate (BEC). As the
potential depth increases, the time independent Gross-Pitaevskii equation
develops pairs of solutions that have nodes in their wavefunction. We elucidate
the nature of these states and study their dynamical stability. Although we
study the problem in a two-dimensional BEC subject to a cylindrically symmetric
square-well potential of a radius that is comparable to the coherence length of
the BEC, our analysis reveals general trends, valid in two and three
dimensions, independent of the symmetry of the localized potential well, and
suggestive of the behavior in general, short- and large-range potentials. One
set of nodal BEC wavefunctions resembles the single particle n node bound state
wavefunction of the potential well, the other wavefunctions resemble the n-1
node bound-state wavefunction with a kink state pinned by the potential. The
second state, though corresponding to the lower free energy value of the pair
of n node BEC states, is always unstable, whereas the first can be dynamically
stable in intervals of the potential well depth, implying that the standing
wave BEC can evolve from a dynamically unstable to stable, and back to unstable
status as the potential well is adiabatically deepened, a phenomenon that we
refer to as "reentrant dynamical stability".Comment: 13 pages, 9 figures; revised discussion in Sec.
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