1,318 research outputs found
Rapidly Rotating Fermi Gases
We show that the density profile of a Fermi gas in rapidly rotating potential
will develop prominent features reflecting the underlying Landau level like
energy spectrum. Depending on the aspect ratio of the trap, these features can
be a sequence of ellipsoidal volumes or a sequence of quantized steps.Comment: 4 pages, 1 postscript fil
Energy non-equipartition in systems of inelastic, rough spheres
We calculate and verify with simulations the ratio between the average
translational and rotational energies of systems with rough, inelastic
particles, either forced or freely cooling. The ratio shows non-equipartition
of energy. In stationary flows, this ratio depends mainly on the particle
roughness, but in nonstationary flows, such as freely cooling granular media,
it also depends strongly on the normal dissipation. The approach presented here
unifies and simplifies different results obtained by more elaborate kinetic
theories. We observe that the boundary induced energy flux plays an important
role.Comment: 4 pages latex, 4 embedded eps figures, accepted by Phys Rev
Dynamics of inelastically colliding rough spheres: Relaxation of translational and rotational energy
We study the exchange of kinetic energy between translational and rotational
degrees of freedom for inelastic collisions of rough spheres. Even if
equipartition holds in the initial state it is immediately destroyed by
collisions. The simplest generalisation of the homogeneous cooling state allows
for two temperatures, characterizing translational and rotational degrees of
freedom separately. For times larger than a crossover frequency, which is
determined by the Enskog frequency and the initial temperature, both energies
decay algebraically like with a fixed ratio of amplitudes, different
from one.Comment: 5 pages, RevTeX, 2 eps figures, slightly expanded discussion, new
figures with dimensionless units, added references, accepted for publication
in PRE as a Rapid Com
Granular cooling of hard needles
We have developed a kinetic theory of hard needles undergoing binary
collisions with loss of energy due to normal and tangential restitution. In
addition, we have simulated many particle systems of granular hard needles. The
theory, based on the assumption of a homogeneous cooling state, predicts that
granular cooling of the needles proceeds in two stages: An exponential decay of
the initial configuration to a state where translational and rotational
energies take on a time independent ratio (not necessarily unity), followed by
an algebraic decay of the total kinetic energy . The simulations
support the theory very well for low and moderate densities. For higher
densities, we have observed the onset of the formation of clusters and shear
bands.Comment: 7 pages, 8 figures; major changes, extended versio
First and Second Sound Modes of a Bose-Einstein Condensate in a Harmonic Trap
We have calculated the first and second sound modes of a dilute interacting
Bose gas in a spherical trap for temperatures () and for
systems with to particles. The second sound modes (which exist
only below ) generally have a stronger temperature dependence than the
first sound modes. The puzzling temperature variations of the sound modes near
recently observed at JILA in systems with particles match
surprisingly well with those of the first and second sound modes of much larger
systems.Comment: a shorten version, more discussions are given on the nature of the
second sound. A long footnote on the recent work of Zaremba, Griffin, and
Nikuni (cond-mat/9705134) is added, the spectrum of the (\ell=1, n_2=0) mode
is included in fig.
Quantum Criticality of 1D Attractive Fermi Gas
We obtain an analytical equation of state for one-dimensional strongly
attractive Fermi gas for all parameter regime in current experiments. From the
equation of state we derive universal scaling functions that control whole
thermodynamical properties in quantum critical regimes and illustrate physical
origin of quantum criticality. It turns out that the critical properties of the
system are described by these of free fermions and those of mixtures of
fermions with mass and . We also show how these critical properties of
bulk systems can be revealed from the density profile of trapped Fermi gas at
finite temperatures and can be used to determine the T=0 phase boundaries
without any arbitrariness.Comment: extended version, 9 pages, 7 eps figures, corrections of few typo
Transport Coefficients for Granular Media from Molecular Dynamics Simulations
Under many conditions, macroscopic grains flow like a fluid; kinetic theory
pred icts continuum equations of motion for this granular fluid. In order to
test the theory, we perform event driven molecular simulations of a
two-dimensional gas of inelastic hard disks, driven by contact with a heat
bath. Even for strong dissipation, high densities, and small numbers of
particles, we find that continuum theory describes the system well. With a bath
that heats the gas homogeneously, strong velocity correlations produce a
slightly smaller energy loss due to inelastic collisions than that predicted by
kinetic theory. With an inhomogeneous heat bath, thermal or velocity gradients
are induced. Determination of the resulting fluxes allows calculation of the
thermal conductivity and shear viscosity, which are compared to the predictions
of granular kinetic theory, and which can be used in continuum modeling of
granular flows. The shear viscosity is close to the prediction of kinetic
theory, while the thermal conductivity can be overestimated by a factor of 2;
in each case, transport is lowered with increasing inelasticity.Comment: 14 pages, 17 figures, 39 references, submitted to PRE feb 199
Homogeneous cooling of rough, dissipative particles: Theory and simulations
We investigate freely cooling systems of rough spheres in two and three
dimensions. Simulations using an event driven algorithm are compared with
results of an approximate kinetic theory, based on the assumption of a
generalized homogeneous cooling state. For short times , translational and
rotational energy are found to change linearly with . For large times both
energies decay like with a ratio independent of time, but not
corresponding to equipartition. Good agreement is found between theory and
simulations, as long as no clustering instability is observed. System
parameters, i.e. density, particle size, and particle mass can be absorbed in a
rescaled time, so that the decay of translational and rotational energy is
solely determined by normal restitution and surface roughness.Comment: 10 pages, 10 eps-figure
Fragmented and Single Condensate Ground States of Spin-1 Bose Gas
We show that the ground state of a spin-1 Bose gas with an antiferro-
magnetic interaction is a fragmented condensate in uniform magnetic fields. The
number fluctuations in each spin component change rapidly from being enormous
(order ) to exceedingly small (order 1) as the magnetization of the system
increases. A fragmented condensate can be turned into a single condensate state
by magnetic field gradients. The conditions for existence and the method of
detecting fragmented states are presented.Comment: 4 pages, no figure
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