1,644 research outputs found
Frequency and damping of the Scissors Mode of a Fermi gas
We calculate the frequency and damping of the scissors mode in a classical
gas as a function of temperature and coupling strength. Our results show good
agreement with the main features observed in recent measurements of the
scissors mode in an ultracold gas of Li atoms. The comparison between
theory and experiment involves no fitting parameters and thus allows an
identification of non-classical effects at and near the unitarity limit.Comment: 4 pages, 2 figure
Shear viscosity and damping for a Fermi gas in the unitarity limit
The shear viscosity of a two-component Fermi gas in the normal phase is
calculated as a function of temperature in the unitarity limit, taking into
account strong-coupling effects that give rise to a pseudogap in the spectral
density for single-particle excitations. The results indicate that recent
measurements of the damping of collective modes in trapped atomic clouds can be
understood in terms of hydrodynamics, with a decay rate given by the viscosity
integrated over an effective volume of the cloud.Comment: 7 pages, 3 figures. Discussion significantly extended. Appendix
added. To appear in PR
Effects of the trapping potential on a superfluid atomic Fermi Gas
We examine a dilute two-component atomic Fermi gas trapped in a harmonic
potential in the superfluid phase. For experimentally realistic parameters, the
trapping potential is shown to have crucial influence on various properties of
the gas. Using an effective hamiltonian, analytical results for the critical
temperature, the temperature dependence of the superfluid gap, and the energy
of the lowest collective modes are derived. These results are shown to agree
well with numerical calculations. We furthermore discuss in more detail a
previous proposed method to experimentally observe the superfluid transition by
looking at the collective mode spectrum. Our results are aimed at the present
experimental effort to observe a superfluid phase transition in a trapped
atomic Fermi gas.Comment: 2. revised version. Minor mistakes in equation references corrected.
To appear in Phys. Rev.
Viscous relaxation and collective oscillations in a trapped Fermi gas near the unitarity limit
The viscous relaxation time of a trapped two-component gas of fermions in its
normal phase is calculated as a function of temperature and scattering length,
with the collision probability being determined by an energy-dependent s-wave
cross section. The result is used for calculating the temperature dependence of
the frequency and damping of collective modes studied in recent experiments,
starting from the kinetic equation for the fermion distribution function with
mean-field effects included in the streaming terms.Comment: 10 pages, 9 figures; proof version, corrected typo in Eq. (23);
accepted for publication in PR
Viscosity and Thermal Relaxation for a resonantly interacting Fermi gas
The viscous and thermal relaxation rates of an interacting fermion gas are
calculated as functions of temperature and scattering length, using a many-body
scattering matrix which incorporates medium effects due to Fermi blocking of
intermediate states. These effects are demonstrated to be large close to the
transition temperature to the superfluid state. For a homogeneous gas in
the unitarity limit, the relaxation rates are increased by nearly an order of
magnitude compared to their value obtained in the absence of medium effects due
to the Cooper instability at . For trapped gases the corresponding ratio
is found to be about three due to the averaging over the inhomogeneous density
distribution. The effect of superfluidity below is considered to leading
order in the ratio between the energy gap and the transition temperature.Comment: 7 pages, 3 figure
Cooper pairing and single particle properties of trapped Fermi gases
We calculate the elementary excitations and pairing of a trapped atomic Fermi
gas in the superfluid phase. The level spectra and pairing gaps undergo several
transitions as the strength of the interactions between and the number of atoms
are varied. For weak interactions, the Cooper pairs are formed between
particles residing in the same harmonic oscillator shell. In this regime, the
nature of the paired state is shown to depend critically on the position of the
chemical potential relative to the harmonic oscillator shells and on the size
of the mean field. For stronger interactions, we find a region where pairing
occur between time-reversed harmonic oscillator states in different shells
also.Comment: Slightly revised version: Mistakes in equation references in figures
corrected. Accepted for Phys. Rev.
Low energy monopole Modes of a Trapped atomic Fermi Gas
We consider the low energy collective monopole modes of a trapped weakly
interacting atomic Fermi gas in the collisionless regime. The spectrum is
calculated for varying coupling strength and chemical potential. Using an
effective Hamiltonian, we derive analytical results that agree well with
numerical calculations in various regimes. The onset of superfluidity is shown
to lead to effects such as the vanishing of the energy required to create a
Cooper molecule at a critical coupling strength and to the emergence of pair
vibration excitations. Our analysis suggests ways to experimentally detect the
presence of the superfluid phase in trapped atomic Fermi gases.Comment: 5 pages & 1 figure. Accepted for Phys. Rev. Let
Pairing of fermions in atomic traps and nuclei
Pairing gaps for fermionic atoms in harmonic oscillator traps are calculated
for a wide range of interaction strengths and particle number, and compared to
pairing in nuclei. Especially systems, where the pairing gap exceeds the level
spacing but is smaller than the shell splitting , are studied
which applies to most trapped Fermi atomic systems as well as to finite nuclei.
When solving the gap equation for a large trap with such multi-level pairing,
one finds that the matrix elements between nearby harmonic oscillator levels
and the quasi-particle energies lead to a double logarithm of the gap, and a
pronounced shell structure at magic numbers. It is argued that neutron and
proton pairing in nuclei belongs to the class of multi-level pairing, that
their shell structure follows naturally and that the gaps scale as - all in qualitative agreement with odd-even staggering of nuclear
binding energies. Pairing in large systems are related to that in the bulk
limit. For large nuclei the neutron and proton superfluid gaps approach the
asymptotic value in infinite nuclear matter: MeV.Comment: 11 pages, 5 figure
Far-Field Plasmonic Resonance Enhanced Nano-Particle Image Velocimetry within a Micro Channel
In this paper, a novel far-field plasmonic resonance enhanced
nanoparticle-seeded Particle Image Velocimetry (nPIV) has been demonstrated to
measure the velocity profile in a micro channel. Chemically synthesized silver
nanoparticles have been used to seed the flow in the micro channel. By using
Discrete Dipole Approximation (DDA), plasmonic resonance enhanced light
scattering has been calculated for spherical silver nanoparticles with
diameters ranging from 15nm to 200nm. Optimum scattering wavelength is
specified for the nanoparticles in two media: water and air. The
diffraction-limited plasmonic resonance enhanced images of silver nanoparticles
at different diameters have been recorded and analyzed. By using standard PIV
techniques, the velocity profile within the micro channel has been determined
from the images.Comment: submitted to Review of Scientific Instrument
Vortex Tubes in Turbulence Velocity Fields at Reynolds Numbers 300-1300
The most elementary structures of turbulence, i.e., vortex tubes, are studied
using velocity data obtained in a laboratory experiment for boundary layers
with microscale Reynolds numbers 295-1258. We conduct conditional averaging for
enhancements of a small-scale velocity increment and obtain the typical
velocity profile for vortex tubes. Their radii are of the order of the
Kolmogorov length. Their circulation velocities are of the order of the
root-mean-square velocity fluctuation. We also obtain the distribution of the
interval between successive enhancements of the velocity increment as the
measure of the spatial distribution of vortex tubes. They tend to cluster
together below about the integral length and more significantly below about the
Taylor microscale. These properties are independent of the Reynolds number and
are hence expected to be universal.Comment: 8 pages, to appear in Physical Review
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