146,845 research outputs found
Nonuniversal Effects in the Homogeneous Bose Gas
Effective field theory predicts that the leading nonuniversal effects in the
homogeneous Bose gas arise from the effective range for S-wave scattering and
from an effective three-body contact interaction. We calculate the leading
nonuniversal contributions to the energy density and condensate fraction and
compare the predictions with results from diffusion Monte Carlo calculations by
Giorgini, Boronat, and Casulleras. We give a crude determination of the
strength of the three-body contact interaction for various model potentials.
Accurate determinations could be obtained from diffusion Monte Carlo
calculations of the energy density with higher statistics.Comment: 24 pages, RevTex, 5 ps figures, included with epsf.te
Study of excitation transfer in laser dye mixtures by direct measurement of fluorescence lifetime
By directly measuring the donor fluorescence lifetime as a function of acceptor concentration in the laser dye mixture Rhodamine 6G-Cresyl violet, we found that the Stern-Volmer relation is obeyed, from which the rate of excitation transfer is determined. The experimental results indicate that the dominant mechanism responsible for the efficient excitation transfer is that of resonance transfer due to long range dipole-dipole interaction
System implications of large radiometric array antennas
Current radiometric earth and atmospheric sensing systems in the centimeter wavelength range generally employ a directive antenna connected through a single terminal pair to a Dicke receiver. It is shown that this approach does not lend itself to systems with greatly increased spatial resolution. Signal to noise considerations relating to antenna efficiency force the introduction of active elements at the subarray level; thus, if Dicke switching is to be used, it must be distributed throughout the system. Some possible approaches are suggested. The introduction of active elements at the subarray level is found to ease the design constraints on time delay elements, necessary for bandwidth, and on multiple beam generation, required in order to achieve sufficient integration time with high resolution
On the Survival of Short-Period Terrestrial Planets
The currently feasible method of detection of Earth-mass planets is transit
photometry, with detection probability decreasing with a planet's distance from
the star. The existence or otherwise of short-period terrestrial planets will
tell us much about the planet formation process, and such planets are likely to
be detected first if they exist. Tidal forces are intense for short-period
planets, and result in decay of the orbit on a timescale which depends on
properties of the star as long as the orbit is circular. However, if an
eccentric companion planet exists, orbital eccentricity () is induced and
the decay timescale depends on properties of the short-period planet, reducing
by a factor of order if it is terrestrial. Here we examine the
influence companion planets have on the tidal and dynamical evolution of
short-period planets with terrestrial structure, and show that the relativistic
potential of the star is fundamental to their survival.Comment: 13 pages, 2 figures, accepted for publication in Ap
Shape optimization of damping layers
Shape optimization of unconstrained and constrained damping layers is completed. The specific problem analyzed is a cantilever beam loaded at its tip by a harmonic force. Finite element modeling and mathematical programming techniques are used to obtain the solution. Performance measures are taken to be reduction of maximum diplacement and increase in fatigue lifetime. Results include the improvement, over the uniform treatment case, of these measures when the profile of the damping layer is optimized
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