1,887 research outputs found
Limits to Sympathetic Evaporative Cooling of a Two-Component Fermi Gas
We find a limit cycle in a quasi-equilibrium model of evaporative cooling of
a two-component fermion gas. The existence of such a limit cycle represents an
obstruction to reaching the quantum ground state evaporatively. We show that
evaporatively the \beta\mu ~ 1. We speculate that one may be able to cool an
atomic fermi gas further by photoassociating dimers near the bottom of the
fermi sea.Comment: Submitted to Phys. Rev
Intrapulmonary expression of PPAR in a rat model of pulmonary hypertension [abstract]
Abstract only availableFaculty Mentor: Jeffrey Skimming MD and Vincent DeMarco PhD, Child HealthActivation of peroxisome proliferator-activated receptor-gamma (PPARγ) inhibits vascular smooth muscle proliferation and neointimal formation associated with pulmonary hypertension. Recently, our laboratory developed a model of pulmonary hypertension incorporating both pnuemonectomy (PNX) of the left lung and exposure to monocrotaline (MCT) in rats. Together, PNX and MCT caused inflammation and vascular remodeling within the right lung including smooth muscle proliferation and neointimal formation. Therefore, we tested the hypothesis that PNX/MCT dual insult induces pulmonary hypertension by decreasing PPARγ expression. METHODS: Sixteen rats were randomly divided into four groups: 1) PNX/MCT, 2) PNX/Phosphate Buffer Solution (PBS), 3) Sham Surgery (SS)/MCT, and 4) SS/PBS. Seven days after surgery, we injected the animals with either monocrotaline (60 mg/kg, s.q.) or PBS. Three weeks after those injections, right ventricular systolic pressures (RVSP) were measured. Lung tissue was harvested for analysis of PPARγ protein expression and histopathology. Right ventricular to left ventricular plus septum ratios (RVR) were also determined. RESULTS: PNX/MCT animals exhibited higher RVSPs and RVRs than the other treatment groups. Also, morphometric analysis revealed medial hypertrophy and neointimal formation within the resistance vessels of PNX/MCT rat lungs. In our study, neither PNX nor MCT alone had an effect on intrapulmonary expression of PPARγ protein. Surprisingly, however, the dual insult induced PPARγ expression (
Resonant control of elastic collisions in an optically trapped Fermi gas of atoms
We have loaded an ultracold gas of fermionic atoms into a far off resonance
optical dipole trap and precisely controlled the spin composition of the
trapped gas. We have measured a magnetic-field Feshbach resonance between atoms
in the two lowest energy spin-states, |9/2, -9/2> and |9/2, -7/2>. The
resonance peaks at a magnetic field of 201.5 plus or minus 1.4 G and has a
width of 8.0 plus or minus 1.1 G. Using this resonance we have changed the
elastic collision cross section in the gas by nearly 3 orders of magnitude.Comment: 4 pages, 3 figure
Cooper Pairing in Ultracold K-40 Using Feshbach Resonances
We point out that the fermionic isotope K-40 is a likely candidate for the
formation of Cooper pairs in an ultracold atomic gas. Specifically, in an
optical trap that simultaneously traps the spin states |9/2,-9/2> and
|9/2,-7/2>, there exists a broad magnetic field Feshbach resonance at B = 196
gauss that can provide the required strong attractive interaction between
atoms. An additional resonance, at B = 191 gauss, could generate p-wave pairing
between identical |9/2,-7/2> atoms. A Cooper-paired degenerate Fermi gas could
thus be constructed with existing ultracold atom technology.Comment: 4 pages, 2 figs, submitted to Phys. Rev.
Collisionless collective modes of fermions in magnetic traps
We present a Random-Phase-Approximation formalism for the collective spectrum
of two hyperfine species of dilute 40K atoms, magnetically trapped at zero
temperature and subjected to a repulsive s-wave interaction between atoms with
different spin projections. We examine the density-like and the spin-like
oscillation spectra, as well as the transition density profiles created by
external multipolar fields. The zero sound spectrum is always fragmented and
the density and spin channels become clearly distinguishable if the trapping
potentials acting on the species are identical. Although this distinction is
lost when these confining fields are different, at selected excitation
frequencies the transition densities may display the signature of the channel.Comment: 10 pages, 9 figure
Evaporative cooling of trapped fermionic atoms
We propose an efficient mechanism for the evaporative cooling of trapped
fermions directly into quantum degeneracy. Our idea is based on an electric
field induced elastic interaction between trapped atoms in spin symmetric
states. We discuss some novel general features of fermionic evaporative cooling
and present numerical studies demonstrating the feasibility for the cooling of
alkali metal fermionic species Li, K, and Rb. We also
discuss the sympathetic cooling of fermionic hyperfine spin mixtures, including
the effects of anisotropic interactions.Comment: to be publishe
Scattering of short laser pulses from trapped fermions
We investigate the scattering of intense short laser pulses off trapped cold
fermionic atoms. We discuss the sensitivity of the scattered light to the
quantum statistics of the atoms. The temperature dependence of the scattered
light spectrum is calculated. Comparisons are made with a system of classical
atoms who obey Maxwell-Boltzmann statistics. We find the total scattering
increases as the fermions become cooler but eventually tails off at very low
temperatures (far below the Fermi temperature). At these low temperatures the
fermionic degeneracy plays an important role in the scattering as it inhibits
spontaneous emission into occupied energy levels below the Fermi surface. We
demonstrate temperature dependent qualitative changes in the differential and
total spectrum can be utilized to probe quantum degeneracy of trapped Fermi gas
when the total number of atoms are sufficiently large . At smaller
number of atoms, incoherent scattering dominates and it displays weak
temperature dependence.Comment: updated figures and revised content, submitted to Phys.Rev.
The Present and Future of Planetary Nebula Research. A White Paper by the IAU Planetary Nebula Working Group
We present a summary of current research on planetary nebulae and their
central stars, and related subjects such as atomic processes in ionized
nebulae, AGB and post-AGB evolution. Future advances are discussed that will be
essential to substantial improvements in our knowledge in the field.Comment: accepted for publication in RMxAA; 37 page
Hydrodynamic excitations of trapped dipolar fermions
A single-component Fermi gas of polarized dipolar particles in a harmonic
trap can undergo a mechanical collapse due to the attractive part of the
dipole-dipole interaction. This phenomenon can be conveniently manipulated by
the shape of the external trapping potential. We investigate the signatures of
the instability by studying the spectrum of low-lying collective excitations of
the system in the hydrodynamic regime. To this end, we employ a time-dependent
variational method as well as exact numerical solutions of the hydrodynamic
equations of the system.Comment: 4 pages, 2 eps figures, final versio
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