45 research outputs found
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
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
Photoassociative Production and Trapping of Ultracold KRb Molecules
We have produced ultracold heteronuclear KRb molecules by the process of
photoassociation in a two-species magneto-optical trap. Following decay of the
photoassociated KRb*, the molecules are detected using two-photon ionization
and time-of-flight mass spectroscopy of KRb. A portion of the metastable
triplet molecules thus formed are magnetically trapped. Photoassociative
spectra down to 91 cm below the K(4) + Rb (5) asymptote have
been obtained. We have made assignments to all eight of the attractive Hund's
case (c) KRb* potential curves in this spectral region.Comment: 4 pages, 4 figure
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
Dynamics of trapped two-component Fermi gas: temperature dependence of the transition from collisionless to collisional regime
We develop a numerical method to study the dynamics of a two-component atomic
Fermi gas trapped inside a harmonic potential at temperature T well below the
Fermi temperature Tf. We examine the transition from the collisionless to the
collisional regime down to T=0.2 Tf and find good qualitative agreement with
the experiments of B. DeMarco and D.S. Jin [Phys. Rev. Lett. vol. 88, 040405
(2002)]. We demonstrate a twofold role of temperature on the collision rate and
on the efficiency of collisions. In particular we observe an hitherto
unreported effect, namely that the transition to hydrodynamic behavior is
shifted towards lower collision rates as temperature decreases.Comment: 4 pages, 3 figure
The effect of pH, aluminum, and chelator manipulations on the growth of acidic and circumneutral species of Asterionella
The growth rates of two diatoms, acidophilic Asterionella ralfsii and circumneutral A. formosa , were differentially affected by varying pH, Al, and EDTA in chemically defined media. Free Al ion concentration increased as pH and EDTA concentration decreased. Free trace metal ion concentration decreased as EDTA levels increased but increased by orders of magnitude upon addition of Al. pH had an overriding species specific effect on growth rate; at low pH A. ralfsii had higher growth rates than A. formosa and vice versa at high pH. For both species higher EDTA levels depressed growth rates. Moderate additions of Al generally resulted in growth stimulation. The growth rate stimulations, especially at 200 and 400 μg L −1 Al additions, correlate to increases in free trace metal ion concentrations. The EDTA-AI interaction effects on growth rate were both pH and concentration dependent: at pH 7 both species were stimulated by addition of Al at all EDTA levels (except A. ralfsii at 5.0 mM EDTA and A. formosa at 0.5 mNM EDTA); at pH 6 Al addition either stimulated or had no effect on the growth rates of both species (except at low EDTA and high Al levels); at pH 5 A. formosa did not grow and additions of 200 μg L −1 Al stimulated growth of A. ralfsii . It is likely that the effect of pH, Al, and EDTA on speciation of essential or toxic trace metals affects growth rates of these diatoms in a species specific manner.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43905/1/11270_2004_Article_BF00282626.pd
Collisionless and hydrodynamic excitations of trapped boson-fermion mixtures
Within a scaling ansatz formalism plus Thomas-Fermi approximation, we
investigate the collective excitations of a harmonically trapped boson-fermion
mixture in the collisionless and hydrodynamic limit at low temperature. Both
the monopole and quadrupole modes are considered in the presence of spherical
as well as cylindrically symmetric traps. In the spherical traps, the frequency
of monopole mode coincides in the collisionless and hydrodynamic regime,
suggesting that it might be undamped in all collisional regimes. In contrast,
for the quadrupole mode, the frequency differs largely in these two limits. In
particular, we find that in the hydrodynamic regime the quadrupole oscillations
with equal bosonic and fermionic amplitudes generate an exact eigenstate of the
system, regardless of the boson-fermion interaction. This resembles the Kohn
mode for the dipole excitation. We discuss in some detail the behavior of
monopole and quadrupole modes as a function of boson-fermion coupling at
different boson-boson interaction strength. Analytic solutions valid at weak
and medium fermion-boson coupling are also derived and discussed.Comment: 29 pages + 7 figures, resubmitted to Physical Review