218,843 research outputs found
Influence of Induced Interactions on the Superfluid Transition in Dilute Fermi Gases
We calculate the effects of induced interactions on the transition
temperature to the BCS state in dilute Fermi gases. For a pure Fermi system
with 2 species having equal densities, the transition temperature is suppressed
by a factor , and for fermion species, the
transition temperature is increased by a factor . For mixtures of fermions and bosons the exchange of boson density
fluctuations gives rise to an effective interaction, and we estimate the
increase of the transition temperature due to this effect.Comment: 4 pages, 3 figure
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
The structure of correlation tensors in homogeneous anisotropic turbulence
The study of turbulence with spatially homogeneous but anisotropic statistical properties has applications in space physics and laboratory plasma physics. The first step in the systematic study of such fluctuations is the elucidation of the kinematic properties of the relevant statistical objects, which are the correlation tensors. The theory of isotropic tensors, developed by Robertson, Chandrasekhar and others, is reviewed and extended to cover the general case of turbulence with a pseudo-vector preferred direction, without assuming mirror reflection invariance. Attention is focused on two point correlation functions and it is shown that the form of the decomposition into proper and pseudo-tensor contributions is restricted by the homogeneity requirement. It is also shown that the vector and pseudo-vector preferred direction cases yield different results. An explicit form of the two point correlation tensor is presented which is appropriate for analyzing interplanetary magnetic fluctuations. A procedure for determining the magnetic helicity from experimental data is presented
Experimental evaluation of outer case blowing or bleeding of single stage axial flow compressor. Part 3 - Performance of blowing insert configuration no. 1
Experimental evaluation of outer case blowing or bleeding of single stage axial flow compressor, and performance tests using distorted or undistorted inlet flo
Kinetic Theory of Collective Modes in Atomic Clouds above the Bose-Einstein Transition Temperature
We calculate frequencies and damping rates of the lowest collective modes of
a dilute Bose gas confined in an anisotropic trapping potential above the
Bose-Einstein transition temperature. From the Boltzmann equation with a
simplified collision integral we derive a general dispersion relation that
interpolates between the collisionless and hydrodynamic regimes. In the case of
axially symmetric traps we obtain explicit expressions for the frequencies and
damping rates of the lowest modes in terms of a phenomenological collision
time. Our results are compared with microscopic calculations and experiments.Comment: Nordita preprint NORDITA-1999/46 C
The efficiency and the demagnetization field of a general Halbach cylinder
The maximum magnetic efficiency of a general multipole Halbach cylinder of
order is found as function of . The efficiency is shown to decrease for
increasing absolute value of . The optimal ratio between the inner and outer
radius, i.e. the ratio resulting in the most efficient design, is also found as
function of and is shown to tend towards smaller and smaller magnet sizes.
Finally, the demagnetizing field in a general -Halbach cylinder is
calculated, and it is shown that demagnetization is largest either at or . For the common case of a Halbach cylinder
the maximum values of the demagnetizing field is either at at
the outer radius, where the field is always equal to the remanence, or at at the inner radius, where it is the magnitude of the field in the
bore. Thus to avoid demagnetization the coercivity of the magnets must be
larger than these values.Comment: 5 pages, 5 figure
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