5,396 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
Coherent macroscopic quantum tunneling in boson-fermion mixtures
We show that the cold atom systems of simultaneously trapped Bose-Einstein
condensates (BEC's) and quantum degenerate fermionic atoms provide promising
laboratories for the study of macroscopic quantum tunneling. Our theoretical
studies reveal that the spatial extent of a small trapped BEC immersed in a
Fermi sea can tunnel and coherently oscillate between the values of the
separated and mixed configurations (the phases of the phase separation
transition of BEC-fermion systems). We evaluate the period, amplitude and
dissipation rate for Na and K-atoms and we discuss the
experimental prospects for observing this phenomenon.Comment: 4 pages, 3 figure
Zero sound in a single component fermion - Bose Einstein Condensate mixture
The resonant dynamics of mediated interactions supports zero-sound in a cold
atom degenerate mixture of a single component fermion gas and a Bose-Einstein
condensate (BEC). We characterize the onset of instability in the phase
separation of an unstable mixture and we find a rich collective mode structure
for stable mixtures with one undamped mode that exhibits an avoided crossing
and a Landau-damped mode that terminates.Comment: 4 pages, 2 figure
Molecule formation as a diagnostic tool for second order correlations of ultra-cold gases
We calculate the momentum distribution and the second-order correlation
function in momentum space, for molecular dimers
that are coherently formed from an ultracold atomic gas by photoassociation or
a Feshbach resonance. We investigate using perturbation theory how the quantum
statistics of the molecules depend on the initial state of the atoms by
considering three different initial states: a Bose-Einstein condensate (BEC), a
normal Fermi gas of ultra-cold atoms, and a BCS-type superfluid Fermi gas. The
cases of strong and weak coupling to the molecular field are discussed. It is
found that BEC and BCS states give rise to an essentially coherent molecular
field with a momentum distribution determined by the zero-point motion in the
confining potential. On the other hand, a normal Fermi gas and the unpaired
atoms in the BCS state give rise to a molecular field with a broad momentum
distribution and thermal number statistics. It is shown that the first-order
correlations of the molecules can be used to measure second-order correlations
of the initial atomic state.Comment: revtex, 15 pages,8 figure
Self-localized impurities embedded in a one dimensional Bose-Einstein condensate and their quantum fluctuations
We consider the self-localization of neutral impurity atoms in a
Bose-Einstein condensate in a 1D model. Within the strong coupling approach, we
show that the self-localized state exhibits parametric soliton behavior. The
corresponding stationary states are analogous to the solitons of non-linear
optics and to the solitonic solutions of the Schroedinger-Newton equation
(which appears in models that consider the connection between quantum mechanics
and gravitation). In addition, we present a Bogoliubov-de-Gennes formalism to
describe the quantum fluctuations around the product state of the strong
coupling description. Our fluctuation calculations yield the excitation
spectrum and reveal considerable corrections to the strong coupling
description. The knowledge of the spectrum allows a spectroscopic detection of
the impurity self-localization phenomenon.Comment: 7 pages, 5 figure
Small polarons in dilute gas Bose-Einstein condensates
A neutral impurity atom immersed in a dilute Bose-Einstein condensate (BEC)
can have a bound ground state in which the impurity is self-localized. In this
small polaron-like state, the impurity distorts the density of the surrounding
BEC, thereby creating the self-trapping potential minimum. We describe the
self-localization in a strong coupling approach
High Temperature Superfluid and Feshbach Resonance
We study an effective field theory describing cold fermionic atoms near a
Feshbach resonance. The theory gives a unique description of the dynamics in
the limit that the energy of the Feshbach resonance is tuned to be twice that
of the Fermi surface. We show that in this limit the zero temperature
superfluid condensate is of order the Fermi energy, and obtain a critical
temperature Comment: 9 pages, 3 figures, RevTe
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