6,043 research outputs found
Ultracold polarized Fermi gas at intermediate temperatures
We consider non-zero temperature properties of the polarized two-component
Fermi gas. We point out that stable polarized paired states which are more
stable than their phase separated counterparts with unpolarized superfluid
region can exist below the critical temperature. We also solve the system
behavior in a trap using the local density approximation and find gradually
increasing polarization in the center of the system as the temperature is
increased. However, in the strongly interacting region the central polarization
increases most rapidly close to the mean-field critical temperature, which is
known to be substantially higher than the critical temperature for
superfluidity. This indicates that most of the phase separation occurs in the
fluctuation region prior to superfluidity and that the polarization in the
actual superfluid is modest.Comment: Final published versio
Changes in Behavior, Movement, and Home Ranges of Largemouth Bass Following Large-scale Hydrilla Removal in Lake Seminole, Georgia
About 1,200 ha of hydrilla (
Hydrilla verticillata
L.f. Royle)
was eliminated in the Spring Creek embayment of Lake Seminole,
Georgia, using a drip-delivery application of fluridone
(1- methyl-3-phenyl-5-[3-(trifluoromethl) phenyl]-4(1H)-pyridinone)
in 2000 and 2001.
Two groups of 15 and 20 largemouth
bass (Micropterus salmoides Lacepede) were implanted
with 400-day radio tags in February 2000 and 2001 to determine
changes in movement and behavior before and after
hydrilla reduction.(PDF contains 8 pages.
Essays on Debt in Macroeconomics
My dissertation consists of three chapters, where the common theme among them is debt and saving. My work contributes to our understanding of how debt markets function for entrepreneurs, large corporations and households.
The first chapter studies how entrepreneurs used personal borrowing to fund their businesses during the Great Recession. One of the defining characteristics of this period was a “credit crunch” during which the supply of credit dropped for all borrowers. I show that changes in the finances of entrepreneurs between 2007 and 2009 are consistent with entrepreneurs using personal assets to secure lending for their businesses and overcome this credit crunch. In particular, I find that home equity loan balances increased by 10%, despite a 12% drop in the value of aggregate housing stock. Entrepreneurs were responsible for 76% of the increase in home equity loan balances, while they only represent 13% of the population.
In the second chapter, I study the use of credit ratings by large corporations in the bond market. Over the last 25 years, there has been a drastic change in the distribution of corporate bond ratings: between 1985 and 2010 the number of firms issuing AAA or AA-rated debt dropped by 70%, while the number of firms issuing debt with lower ratings increased. I propose a mechanism whereby investors learn about firms through credit ratings and publicly available financial information and develop a model that incorporates this mechanism such that firms must devote resources to improving their rating. Under general conditions, the number of high-rated firms decreases in response to an increase in public signal accuracy.
The third chapter explores the role of financial market access on household consumption inequality. The standard macroeconomic models of consumption and saving with stochastic income processes have failed to match the rise in consumption inequality between 1980 and 2004. I present a model with idiosyncratic earnings risk and endogenous market segmentation between incomplete and complete markets for financial assets. This model improves upon the qualitative predictions regarding between-group and within-group consumption inequality of a standard incomplete markets model and a standard complete markets model with limited commitment
The 2S(+) - 2P separation in KO
The 2S(+) - 2P separation in KO is investigated using large basis sets and high levels of correlation treatment. Relativistic effects are included at the Dirac-Fock level and reduce the separation only slightly. The basis set superposition error is considered in detail. On the basis of these calculations, our best estimate places the 2p sub 3/2 state about 200 cm(exp -1) above the ground 2 sigma(+) state in agreement with our previous estimate
Bright soliton trains of trapped Bose-Einstein condensates
We variationally determine the dynamics of bright soliton trains composed of
harmonically trapped Bose-Einstein condensates with attractive interatomic
interactions. In particular, we obtain the interaction potential between two
solitons. We also discuss the formation of soliton trains due to the quantum
mechanical phase fluctuations of a one-dimensional condensate.Comment: 4 pages, 2 figures, submitted to PR
All-electron molecular Dirac-Hartree-Fock calculations: The group 4 tetrahydrides CH4, SiH4, GeH4, SnH4 and PbH4
A basis-set-expansion Dirac-Hartree-Fock program for molecules is described. Bond lengths and harmonic frequencies are presented for the ground states of the group 4 tetrahydrides, CH4, SiH4, GeH4, SnH4, and PbH4. The results are compared with relativistic effective core potential (RECP) calculations, first-order perturbation theory (PT) calculations and with experimental data. The bond lengths are well predicted by first-order perturbation theory for all molecules, but non of the RECP's considered provides a consistent prediction. Perturbation theory overestimates the relativistic correction to the harmonic frequencies; the RECP calculations underestimate the correction
Deformation of a Trapped Fermi Gas with Unequal Spin Populations
The real-space densities of a polarized strongly-interacting two-component
Fermi gas of Li atoms reveal two low temperature regimes, both with a
fully-paired core. At the lowest temperatures, the unpolarized core deforms
with increasing polarization. Sharp boundaries between the core and the excess
unpaired atoms are consistent with a phase separation driven by a first-order
phase transition. In contrast, at higher temperatures the core does not deform
but remains unpolarized up to a critical polarization. The boundaries are not
sharp in this case, indicating a partially-polarized shell between the core and
the unpaired atoms. The temperature dependence is consistent with a tricritical
point in the phase diagram.Comment: Accepted for publication in Physical Review Letter
Response to Comment on "Pairing and Phase Separation in a Polarized Fermi Gas"
Zwierlein and Ketterle rely on subjective arguments and fail to recognize
important differences in physical parameters between our experiment and theirs.
We stand by the conclusions of our original report
Bose-Einstein Condensation and Spin Mixtures of Optically Trapped Metastable Helium
We report the realization of a BEC of metastable helium-4 atoms (4He*) in an
all optical potential. Up to 10^5 spin polarized 4He* atoms are condensed in an
optical dipole trap formed from a single, focused, vertically propagating far
off-resonance laser beam. The vertical trap geometry is chosen to best match
the resolution characteristics of a delay-line anode micro-channel plate
detector capable of registering single He* atoms. We also confirm the
instability of certain spin state combinations of 4He* to two-body inelastic
processes, which necessarily affects the scope of future experiments using
optically trapped spin mixtures. In order to better quantify this constraint,
we measure spin state resolved two-body inelastic loss rate coefficients in the
optical trap
The Evolution of L and T Dwarfs in Color-Magnitude Diagrams
We present new evolution sequences for very low mass stars, brown dwarfs and
giant planets and use them to explore a variety of influences on the evolution
of these objects. We compare our results with previous work and discuss the
causes of the differences and argue for the importance of the surface boundary
condition provided by atmosphere models including clouds.
The L- to T-type ultracool dwarf transition can be accommodated within the
Ackerman & Marley (2001) cloud model by varying the cloud sedimentation
parameter. We develop a simple model for the evolution across the L/T
transition. By combining the evolution calculation and our atmosphere models,
we generate colors and magnitudes of synthetic populations of ultracool dwarfs
in the field and in galactic clusters. We focus on near infrared color-
magnitude diagrams (CMDs) and on the nature of the ``second parameter'' that is
responsible for the scatter of colors along the Teff sequence. Variations in
metallicity and cloud parameters, unresolved binaries and possibly a relatively
young population all play a role in defining the spread of brown dwarfs along
the cooling sequence. We find that the transition from cloudy L dwarfs to
cloudless T dwarfs slows down the evolution and causes a pile up of substellar
objects in the transition region, in contradiction with previous studies. We
apply the same model to the Pleiades brown dwarf sequence. Taken at face value,
the Pleiades data suggest that the L/T transition occurs at lower Teff for
lower gravity objects. The simulated populations of brown dwarfs also reveal
that the phase of deuterium burning produces a distinctive feature in CMDs that
should be detectable in ~50-100 Myr old clusters.Comment: Accepted for publication in the ApJ. 52 pages including 20 figure
- …