3,510 research outputs found
A Fermion-like description of condensed Bosons in a trap
A Bose-Einstein condensate of atoms, trapped in an axially symmetric harmonic
potential, is considered. By averaging the spatial density along the symmetry
direction over a length that preserves the aspect ratio, the system may be
mapped on to a zero temperature noninteracting Fermi-like gas. The ``mock
fermions'' have a state occupancy factor proportional to the ratio of
the coherance length to the ``hard-core'' radius of the atom. The mapping
reproduces the ground state properties of the condensate, and is used to
estimate the vortex excitation energy analytically. The ``mock-fermion''
description predicts some novel collective excitation in the condensed phase.Comment: 11 pages, REVTE
Haldane Exclusion Statistics and the Boltzmann Equation
We generalize the collision term in the one-dimensional Boltzmann-Nordheim
transport equation for quasiparticles that obey the Haldane exclusion
statistics. For the equilibrium situation, this leads to the ``golden rule''
factor for quantum transitions. As an application of this, we calculate the
density response function of a one-dimensional electron gas in a periodic
potential, assuming that the particle-hole excitations are quasiparticles
obeying the new statistics. We also calculate the relaxation time of a nuclear
spin in a metal using the modified golden rule.Comment: version accepted for publication in J. of Stat. Phy
Ground state fluctuations in finite Fermi and Bose systems
We consider a small and fixed number of fermions (bosons) in a trap. The
ground state of the system is defined at T=0. For a given excitation energy,
there are several ways of exciting the particles from this ground state. We
formulate a method for calculating the number fluctuation in the ground state
using microcanonical counting, and implement it for small systems of
noninteracting fermions as well as bosons in harmonic confinement. This exact
calculation for fluctuation, when compared with canonical ensemble averaging,
gives considerably different results, specially for fermions. This difference
is expected to persist at low excitation even when the fermion number in the
trap is large.Comment: 20 pages (including 1 appendix), 3 postscript figures. An error was
found in one section of the paper. The corrected version is updated on
Sep/05/200
Analytical flutter investigation of a composite propfan model
A theoretical model and an associated computer program for predicting subsonic bending-torsion flutter in propfans are presented. The model is based on two-dimensional unsteady cascade strip theory and three-dimensional steady and unsteady lifting surface aerodynamic theory in conjunction with a finite element structural model for the blade. The analytical results compare well with published experimental data. Additional parametric studies are also presented illustrating the effects on flutter speed of steady aeroelastic deformations, blade setting angle, rotational speed, number of blades, structural damping, and number of modes
Ion composition and drift observations in the nighttime equatorial ionosphere
The first in situ measurements of ion composition in the nighttime equatorial E and F region ionospheres (90-300 km) are presented and discussed. These profiles were obtained by two rocket-borne ion mass spectrometers launched from Thumba, India on March 9-10, 1970 at solar zenith angles of 112 deg and 165 deg. Ionosonde data established that the composition was measured at times bounding a period of F region downward drift. During this period the ions O(+) and N(+) were enhanced by one to three orders of magnitude between 220 and 300 km. Below the drift region (200 km), O(+) ceased to be the major ionic constituent, but the concentrations of O(+) and N(+) remained larger than predicted from known radiation sources and loss processes. Here also, both the O2(+) and NO(+) profiles retained nearly the same shape and magnitude throughout the night in agreement with theories assuming scattered UV radiation to be the maintaining source. Light metallic ions including Mg(+), Na(+) and possibly Si(+) were observed to altitude approaching 300 km, while the heavier ions Ca(+) and K(+) were seen in reduced quantity to 200 km. All metal ion profiles exhibited changes which can be ascribed to vertical drifting
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