728 research outputs found
Thermodynamics of the BCS-BEC crossover
We present a self-consistent theory for the thermodynamics of the BCS-BEC
crossover in the normal and superfluid phase which is both conserving and
gapless. It is based on the variational many-body formalism developed by
Luttinger and Ward and by DeDominicis and Martin. Truncating the exact
functional for the entropy to that obtained within a ladder approximation, the
resulting self-consistent integral equations for the normal and anomalous Green
functions are solved numerically for arbitrary coupling. The critical
temperature, the equation of state and the entropy are determined as a function
of the dimensionless parameter , which controls the crossover from the
BCS-regime of extended pairs to the BEC-regime of tightly bound molecules. The
tightly bound pairs turn out to be described by a Popov-type approximation for
a dilute, repulsive Bose gas. Even though our approximation does not capture
the critical behaviour near the continuous superfluid transition, our results
provide a consistent picture for the complete crossover thermodynamics which
compare well with recent numerical and field-theoretic approaches at the
unitarity point.Comment: published versio
Brief increases in corticosterone affect morphology, stress responses, and telomere length, but not post-fledging movements, in a wild songbird
Organisms are frequently exposed to challenges during development, such as
poor weather and food shortage. Such challenges can initiate the hormonal
stress response, which involves secretion of glucocorticoids. Although the
hormonal stress response helps organisms deal with challenges, long-term
exposure to high levels of glucocorticoids can have morphological, behavioral,
and physiological consequences, especially during development. Glucocorticoids
are also associated with reduced survival and telomere shortening. To
investigate whether brief, acute exposures to glucocorticoids can also produce
these phenotypic effects in free-living birds, we exposed wild tree swallow
(Tachycineta bicolor) nestlings to a brief exogenous dose of cort once per day
for five days and then measured their morphology, baseline and stress-induced
corticosterone levels, and telomere length. We also deployed radio tags on a
subset of nestlings, which allowed us to determine the age at which tagged
nestlings left the nest (fledged) and their pattern of presence and absence at
the natal site during the post-breeding period. Corticosterone-treated
nestlings had lower mass, higher baseline and stress-induced corticosterone,
and reduced telomeres; other metrics of morphology were affected weakly or not
at all. Our treatment resulted in no significant effect on survival to
fledging, fledge age, or age at first departure from the natal site, and we
found no negative effect of corticosterone on inter-annual return rate. These
results show that brief acute corticosterone exposure during development can
have measurable effects on phenotype in free-living tree swallows.
Corticosterone may therefore mediate correlations between rearing environment
and phenotype in developing organisms, even in the absence of prolonged
stressors.Comment: 35 pages, 4 figures, 1 appendi
Spectral Properties of the Attractive Hubbard Model
Deviations from Fermi liquid behavior are well documented in the normal state
of the cuprate superconductors, and some of these differences are possibly
related to pre-formed pairs appearing at temperatures above T_c. In order to
test these ideas we have investigated the attractive Hubbard model within a
self-consistent, conserving ladder approximation. In this version of the
theory, no feature is present which can be related to the pseudo gap found in
the high-T_c materials. Further, the interactions between two-particle bound
states change the physics of the superconducting instability in a profound
fashion, and lead to a completely different phenomenology that one predicts
based on the non-self-consistent version of the same theory.Comment: 4 pages including 2 figures, to appear in the proceedings of the
SNS'9
Criticality and Superfluidity in liquid He-4 under Nonequilibrium Conditions
We review a striking array of recent experiments, and their theoretical
interpretations, on the superfluid transition in He in the presence of a
heat flux, . We define and evaluate a new set of critical point exponents.
The statics and dynamics of the superfluid-normal interface are discussed, with
special attention to the role of gravity. If is in the same direction as
gravity, a self-organized state can arise, in which the entire sample has a
uniform reduced temperature, on either the normal or superfluid side of the
transition. Finally, we review recent theory and experiment regarding the heat
capacity at constant . The excitement that surrounds this field arises from
the fact that advanced thermometry and the future availability of a
microgravity experimental platform aboard the International Space Station will
soon open to experimental exploration decades of reduced temperature that were
previously inaccessible.Comment: 16 pages, 9 figures, plus harvard.sty style file for references
Accepted for publication in Colloquia section of Reviews of Modern Physic
Liquid 4He near the superfluid transition in the presence of a heat current and gravity
The effects of a heat current and gravity in liquid 4He near the superfluid
transition are investigated for temperatures above and below T_lambda. We
present a renormalization-group calculation based on model F for the Green's
function in a self-consistent approximation which in quantum many-particle
theory is known as the Hartree approximation. The approach can handle a zero
average order parameter above and below T_lambda and includes effects of
vortices. We calculate the thermal conductivity and the specific heat for all
temperatures T and heat currents Q in the critical regime. Furthermore, we
calculate the temperature profile. Below T_lambda we find a second correlation
length which describes the dephasing of the order parameter field due to
vortices. We find dissipation and mutual friction of the superfluid-normal
fluid counterflow and calculate the Gorter-Mellink coefficient A. We compare
our theoretical results with recent experiments.Comment: 26 pages, 9 figure
Effect of Ibuprofen Administration on Postural Changes in Post-Parturient Sows
Loss of piglets due to crushing is an unnecessary loss that creates both a financial and welfare concern for the swine industry. Sows in the wild lie more quietly after farrowing compared with sows in farrowing crates. The restless behavior of the sow in a farrowing crate increases the chances that she will crush her offspring. Data collected during this study indicate that the discomfort a sow feels associated with lying on a hard surface after farrowing can not be relieved with ibuprofen. This discomfort causes her to change positions more frequently, increasing the chances that she will crush her piglets. Emphasis needs to focus on allowing the post-parturient sow to rest quietly if a decrease in piglet crushing is to be realized
Cooper pair dispersion relation in two dimensions
The Cooper pair binding energy {\it vs.} center-of-mass-momentum dispersion
relation for Bose-Einstein condensation studies of superconductivity is found
in two dimensions for a renormalized attractive delta interaction. It crosses
over smoothly from a linear to a quadratic form as coupling varies from weak to
strong.Comment: 2 pages, 1 figure, new version published in Physica
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