7,390 research outputs found
Universal Properties of the Ultra-Cold Fermi Gas
We present some general considerations on the properties of a two-component
ultra-cold Fermi gas along the BEC-BCS crossover. It is shown that the
interaction energy and the ground state energy can be written in terms of a
single dimensionless function , where and
. The function incorporates all the many-body physics
and naturally occurs in other physical quantities as well. In particular, we
show that the RF-spectroscopy shift \bar{\d\o}(\xi,\tau) and the molecular
fraction in the closed channel can be expressed in terms of
and thus have identical temperature dependence. The conclusions
should have testable consequences in future experiments
Laser cooling all the way down to molecular condensate
Numerical simulations show that laser cooling of fermions on the repulsive
side of the Feshbach resonance can sympathetically cool molecules well below
their condensation temperature.Comment: 7 pages, 2 .eps figure
BEC-BCS Crossover with Feshbach Resonance for a Three-Hyperfine-Species Model
We consider the behavior of an ultracold Fermi gas across a narrow Feshbach
resonance, where the occupation of the closed channel may not be negligible.
While the corrections to the single-channel formulae associated with the
nonzero chemical potential and with particle conservation have been considered
in the existing literature, there is a further effect, namely the
"inter-channel Pauli exclusion principle" associated with the fact that a
single hyperfine species may be common to the two channels. We focus on this
effect and show that, as intuitively expected, the resulting corrections are of
order , where is the Fermi energy of the gas in the absence of
interactions and is the Zeeman energy difference between the two
channels. We also consider the related corrections to the fermionic excitation
spectrum, and briefly discuss the collective modes of the system
BCS-BEC Crossover in Atomic Fermi Gases with a Narrow Resonance
We determine the effects on the BCS-BEC crossover of the energy dependence of
the effective two-body interaction, which at low energies is determined by the
effective range. To describe interactions with an effective range of either
sign, we consider a single-channel model with a two-body interaction having an
attractive square well and a repulsive square barrier. We investigate the
two-body scattering properties of the model, and then solve the Eagles-Leggett
equations for the zero temperature crossover, determining the momentum
dependent gap and the chemical potential self-consistently. From this we
investigate the dependence of the crossover on the effective range of the
interaction.Comment: 12 pages, 14 figure
BCS-BEC crossover and quantum phase transition for 6Li and 40K atoms across Feshbach resonance
We systematically study the BCS-BEC crossover and the quantum phase
transition in ultracold 6Li and 40K atoms across a wide Feshbach resonance. The
background scattering lengths for 6Li and 40K have opposite signs, which lead
to very different behaviors for these two types of atoms. For 40K, both the
two-body and the many-body calculations show that the system always has two
branches of solutions: one corresponds to a deeply bound molecule state; and
the other, the one accessed by the current experiments, corresponds to a weakly
bound state with population always dominantly in the open channel. For 6Li,
there is only a unique solution with the standard crossover from the weakly
bound Cooper pairs to the deeply bound molecules as one sweeps the magnetic
field through the crossover region. Because of this difference, for the
experimentally accessible state of 40K, there is a quantum phase transition at
zero temperature from the superfluid to the normal fermi gas at the positive
detuning of the magnetic field where the s-wave scattering length passes its
zero point. For 6Li, however, the system changes continuously across the zero
point of the scattering length. For both types of atoms, we also give detailed
comparison between the results from the two-channel and the single-channel
model over the whole region of the magnetic field detuning.Comment: 7 pages, 6 figure
Revealing the Condensate and Non-Condensate Distributions in the Inhomogeneous Bose-Hubbard Model
We calculate the condensate fraction and the condensate and non-condensate
spatial and momentum distribution of the Bose-Hubbard model in a trap. From our
results, it is evident that using approximate distributions can lead to
erroneous experimental estimates of the condensate. Strong interactions cause
the condensate to develop pedestal-like structures around the central peak that
can be mistaken as non-condensate atoms. Near the transition temperature, the
peak itself can include a significant non-condensate component. Using
distributions generated from QMC simulations, experiments can map their
measurements for higher accuracy in identifying phase transitions and
temperature.Comment: 5 pages, 3 figures, 1 tabl
Cooper pairs as bosons
Although BCS pairs of fermions are known not to obey Bose-Einstein (BE)
commutation relations nor BE statistics, we show how Cooper pairs (CPs),
whether the simple original ones or the CPs recently generalized in a many-body
Bethe-Salpeter approach, being clearly distinct from BCS pairs at least obey BE
statistics. Hence, contrary to widespread popular belief, CPs can undergo BE
condensation to account for superconductivity if charged, as well as for
neutral-atom fermion superfluidity where CPs, but uncharged, are also expected
to form.Comment: 8 pages, 2 figures, full biblio info adde
Surface-enhanced pair transfer in quadrupole states of neutron-rich Sn isotopes
We investigate the neutron pair transfer modes associated with the low-lying
quadrupole states in neutron-rich Sn isotopes by means of the quasiparticle
random phase approximation based on the Skyrme-Hartree-Fock-Bogoliubov mean
field model. The transition strength of the quadrupole pair-addition mode
feeding the state is enhanced in the Sn isotopes with . The
transition density of the pair-addition mode has a large spatial extension in
the exterior of nucleus, reaching far to fm. The quadrupole
pair-addition mode reflects sensitively a possible increase of the effective
pairing interaction strength in the surface and exterior regions of
neutron-rich nuclei.Comment: 14 page
Towards an Understanding of the Atmospheres of Cool White Dwarfs
Cool white dwarfs with Teff < 6000 K are the remnants of the oldest stars
that existed in our Galaxy. Their atmospheres, when properly characterized, can
provide valuable information on white dwarf evolution and ultimately star
formation through the history of the Milky Way. Understanding the atmospheres
of these stars requires joined observational effort and reliable atmosphere
modeling. We discuss and analyze recent observations of the near-ultraviolet
(UV) and near-infrared (IR) spectrum of several cool white dwarfs including
DQ/DQp stars showing carbon in their spectra. We present fits to the entire
spectral energy distribution (SED) of selected cool stars, showing that the
current pure-hydrogen atmosphere models are quite reliable, especially in the
near-UV spectral region. Recently, we also performed an analysis of the coolest
known DQ/DQp stars investigating further the origin of the C2 Swan bands-like
spectral features that characterize the DQp stars. We show that the carbon
abundances derived for DQp stars fit the trend of carbon abundance with Teff
seen in normal cool DQ stars. This further supports the recent conclusion of
Kowalski A&A (2010) that DQp stars are DQ stars with pressure distorted Swan
bands. However, we encounter some difficulties in reproducing the IR part of
the SED of stars having a mixed He/H atmosphere. This indicates limitations in
current models of the opacity in dense He/H fluids.Comment: 6 pages, 4 figures, to appear in the proceedings of the "18th
European White Dwarf Workshop" in Krakow, Poland (2012
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