2,136 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
Spin Excitations in a Fermi Gas of Atoms
We have experimentally investigated a spin excitation in a quantum degenerate
Fermi gas of atoms. In the hydrodynamic regime the damping time of the
collective excitation is used to probe the quantum behavior of the gas. At
temperatures below the Fermi temperature we measure up to a factor of 2
reduction in the excitation damping time. In addition we observe a strong
excitation energy dependence for this quantum statistical effect.Comment: 4 pages, 3 figure
Two-species magneto-optical trap with 40K and 87Rb
We trap and cool a gas composed of 40K and 87Rb, using a two-species
magneto-optical trap (MOT). This trap represents the first step towards cooling
the Bose-Fermi mixture to quantum degeneracy. Laser light for the MOT is
derived from laser diodes and amplified with a single high power semiconductor
amplifier chip. The four-color laser system is described, and the
single-species and two-species MOTs are characterized. Atom numbers of 1x10^7
40K and 2x10^9 87Rb are trapped in the two-species MOT. Observation of trap
loss due to collisions between species is presented and future prospects for
the experiment are discussed.Comment: 4 pages, 4 figures; accepted for publication in Physical Review
Low energy collective excitations in a superfluid trapped Fermi gas
We study low energy collective excitations in a trapped superfluid Fermi gas,
that describe slow variations of the phase of the superfluid order parameter.
Well below the critical temperature the corresponding eigenfrequencies turn out
to be of the order of the trap frequency, and these modes manifest themselves
as the eigenmodes of the density fluctuations of the gas sample. The latter
could provide an experimental evidence of the presence of the superfluid phase.Comment: 5 pages, REVTeX, referencies correcte
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
Pauli Blocking of Collisions in a Quantum Degenerate Atomic Fermi Gas
We have produced an interacting quantum degenerate Fermi gas of atoms
composed of two spin-states of magnetically trapped K. The relative
Fermi energies are adjusted by controlling the population in each spin-state.
Measurements of the thermodynamics reveal the resulting imbalance in the mean
energy per particle between the two species, which is as large as a factor of
1.4 at our lowest temperature. This imbalance of energy comes from a
suppression of collisions between atoms in the gas due to the Pauli exclusion
principle. Through measurements of the thermal relaxation rate we have directly
observed this Pauli blocking as a factor of two reduction in the effective
collision cross-section in the quantum degenerate regime.Comment: 11 pages, 4 figure
Phonon spectrum and dynamical stability of a quantum degenerate Bose-Fermi mixture
We calculate the phonon excitation spectrum in a zero-temperature
boson-fermion mixture. We show how the sound velocity changes due to the
boson-fermion interaction and we determine the dynamical stability regime of a
homogeneous mixture. We identify a resonant phonon-exchange interaction between
the fermions as the physical mechanism leading to the instability.Comment: 4 pages, 3 figure
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 (
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.
Transition from Collisionless to Hydrodynamic Behaviour in an Ultracold Atomic Gas
Relative motion in a two-component, trapped atomic gas provides a sensitive
probe of interactions. By studying the lowest frequency excitations of a two
spin-state gas confined in a magnetic trap, we have explored the transition
from the collisionless to the hydrodynamic regime. As a function of collision
rate, we observe frequency shifts as large as 6% as well as a dramatic,
non-monotonic dependence of the damping rate. The measurements agree
qualitatively with expectations for behavior in the collisionless and
hydrodynamic limits and are quantitatively compared to a classical kinetic
model.Comment: 5 pages, 4 figure
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