11,664 research outputs found
Collision-Dependent Atom Tunnelling Rate in Bose-Einstein Condensates
We show that the interaction (cross-collision) between atoms trapped in
distinct sites of a double-well potential can significantly increase the atom
tunneling rate for special trap configurations leading to an effective linear
Rabi regime of population oscillation between the trap wells. The inclusion of
cross-collisional effects significantly extends the validity of the two-mode
model approach allowing it to be alternatively employed to explain the recently
observed increase of tunneling rates due to nonlinear interactions.Comment: 4 pages, 2 figures. Replaced with improved versio
Optimal Conditions for Atomic Homodyne Detection on Bose-Einstein Condensates
The dynamics of a two-mode Bose-Einstein condensate trapped in a double-well
potential results approximately in an effective Rabi oscillation regime of
exchange of population between both wells for sufficiently strong overlap
between the modes functions. Facing this system as a temporal atomic beam
splitter we show that this regime is optimal for a nondestructive atom-number
measurement allowing an atomic homodyne detection, thus yielding indirect
relative phase information about one of the two-mode condensates.Comment: 9 pages, 5 figure
The Origin of Fluorine: Abundances in AGB Carbon Stars Revisited
Revised spectroscopic parameters for the HF molecule and a new CN line list
in the 2.3 mu region have been recently available, allowing a revision of the F
content in AGB stars. AGB carbon stars are the only observationally confirmed
sources of fluorine. Nowadays there is not a consensus on the relevance of AGB
stars in its Galactic chemical evolution. The aim of this article is to better
constrain the contribution of these stars with a more accurate estimate of
their fluorine abundances. Using new spectroscopic tools and LTE spectral
synthesis, we redetermine fluorine abundances from several HF lines in the
K-band in a sample of Galactic and extragalactic AGB carbon stars of spectral
types N, J and SC spanning a wide range of metallicities. On average, the new
derived fluorine abundances are systematically lower by 0.33 dex with respect
to previous determinations. This may derive from a combination of the lower
excitation energies of the HF lines and the larger macroturbulence parameters
used here as well as from the new adopted CN line list. Yet, theoretical
nucleosynthesis models in AGB stars agree with the new fluorine determinations
at solar metallicities. At low metallicities, an agreement between theory and
observations can be found by handling in a different way the
radiative/convective interface at the base of the convective envelope. New
fluorine spectroscopic measurements agree with theoretical models at low and at
solar metallicity. Despite this, complementary sources are needed to explain
its observed abundance in the solar neighbourhood.Comment: 9 pages, 4 figures, accepted in A&
A semiquantitative approach to the impurity-band-related transport properties of GaMnAs nanolayers
We investigate the spin-polarized transport of GaMnAs nanolayers in which a
ferromagnetic order exists below a certain transition temperature. Our
calculation for the self-averaged resistivity takes into account the existence
of an impurity band determining the extended ("metallic" transport) or
localized (hopping by thermal excitation) nature of the states at and near the
Fermi level. Magnetic order and resistivity are inter-related due to the
influence of the spin polarization of the impurity band and the effect of the
Zeeman splitting on the mobility edge. We obtain, for a given range of Mn
concentration and carrier density, a "metallic" behavior in which the transport
by extended carriers dominates at low temperature, and is dominated by the
thermally excited localized carriers near and above the transition temperature.
This gives rise to a conspicuous hump of the resistivity which has been
experimentally observed and brings light onto the relationship between
transport and magnetic properties of this material
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