14,428 research outputs found
Atom-Molecule Laser Fed by Stimulated Three-Body Recombination
Using three-body recombination as the underlying process, we propose a method
of coherently driving an atomic Bose-Einstein condensate (BEC) into a molecular
BEC. Superradiant-like stimulation favors atom-to-molecule transitions when two
atomic BECs collide at a resonant kinetic energy, the result being two
molecular BEC clouds moving with well defined velocities. Potential
applications include the construction of a molecule laser.Comment: 4 pgs, 3 figs, RevTeX4, submitted to PRL; Corrected numerical
example
Energy efficient engine sector combustor rig test program
Under the NASA-sponsored Energy Efficient Engine program, Pratt & Whitney Aircraft has successfully completed a comprehensive combustor rig test using a 90-degree sector of an advanced two-stage combustor with a segmented liner. Initial testing utilized a combustor with a conventional louvered liner and demonstrated that the Energy Efficient Engine two-stage combustor configuration is a viable system for controlling exhaust emissions, with the capability to meet all aerothermal performance goals. Goals for both carbon monoxide and unburned hydrocarbons were surpassed and the goal for oxides of nitrogen was closely approached. In another series of tests, an advanced segmented liner configuration with a unique counter-parallel FINWALL cooling system was evaluated at engine sea level takeoff pressure and temperature levels. These tests verified the structural integrity of this liner design. Overall, the results from the program have provided a high level of confidence to proceed with the scheduled Combustor Component Rig Test Program
Gapped tunneling spectra in the normal state of PrCeCuO
We present tunneling data in the normal state of the electron doped cuprate
superconductor PrCeCuO for three different values of the doping
. The normal state is obtained by applying a magnetic field greater than the
upper critical field, for . We observe an anomalous normal
state gap near the Fermi level. From our analysis of the tunneling data we
conclude that this is a feature of the normal state density of states. We
discuss possible reasons for the formation of this gap and its implications for
the nature of the charge carriers in the normal and the superconducting states
of cuprate superconductors.Comment: 7 pages ReVTeX, 11 figures files included, submitted to PR
Asymptotic Statistics of Poincar\'e Recurrences in Hamiltonian Systems with Divided Phase Space
By different methods we show that for dynamical chaos in the standard map
with critical golden curve the Poincar\'e recurrences P(\tau) and correlations
C(\tau) asymptotically decay in time as P ~ C/\tau ~ 1/\tau^3. It is also
explained why this asymptotic behavior starts only at very large times. We
argue that the same exponent p=3 should be also valid for a general chaos
border.Comment: revtex, 4 pages, 3 ps-figure
Water Clouds in Y Dwarfs and Exoplanets
The formation of clouds affects brown dwarf and planetary atmospheres of
nearly all effective temperatures. Iron and silicate condense in L dwarf
atmospheres and dissipate at the L/T transition. Minor species such as sulfides
and salts condense in mid-late T dwarfs. For brown dwarfs below Teff=450 K,
water condenses in the upper atmosphere to form ice clouds. Currently over a
dozen objects in this temperature range have been discovered, and few previous
theoretical studies have addressed the effect of water clouds on brown dwarf or
exoplanetary spectra. Here we present a new grid of models that include the
effect of water cloud opacity. We find that they become optically thick in
objects below Teff=350-375 K. Unlike refractory cloud materials, water ice
particles are significantly non-gray absorbers; they predominantly scatter at
optical wavelengths through J band and absorb in the infrared with prominent
features, the strongest of which is at 2.8 microns. H2O, NH3, CH4, and H2 CIA
are dominant opacity sources; less abundant species such as may also be
detectable, including the alkalis, H2S, and PH3. PH3, which has been detected
in Jupiter, is expected to have a strong signature in the mid-infrared at 4.3
microns in Y dwarfs around Teff=450 K; if disequilibrium chemistry increases
the abundance of PH3, it may be detectable over a wider effective temperature
range than models predict. We show results incorporating disequilibrium
nitrogen and carbon chemistry and predict signatures of low gravity in
planetary- mass objects. Lastly, we make predictions for the observability of Y
dwarfs and planets with existing and future instruments including the James
Webb Space Telescope and Gemini Planet Imager.Comment: 23 pages, 20 figures, Revised for Ap
Near threshold rotational excitation of molecular ions by electron-impact
New cross sections for the rotational excitation of H by electrons are
calculated {\it ab initio} at low impact energies. The validity of the
adiabatic-nuclei-rotation (ANR) approximation, combined with -matrix
wavefunctions, is assessed by comparison with rovibrational quantum defect
theory calculations based on the treatment of Kokoouline and Greene ({\it Phys.
Rev. A} {\bf 68} 012703 2003). Pure ANR excitation cross sections are shown to
be accurate down to threshold, except in the presence of large oscillating
Rydberg resonances. These resonances occur for transitions with
and are caused by closed channel effects. A simple analytic formula is derived
for averaging the rotational probabilities over such resonances in a 3-channel
problem. In accord with the Wigner law for an attractive Coulomb field,
rotational excitation cross sections are shown to be large and finite at
threshold, with a significant but moderate contribution from closed channels.Comment: 3 figures, a5 page
Trans-Planckian signals from the breaking of local Lorentz invariance
This article examines how a breakdown of a locally Lorentz invariant,
point-like description of nature at tiny space-time intervals would translate
into a distinctive set of signals in the primordial power spectrum generated by
inflation. We examine the leading irrelevant operators that are consistent with
the spatial translations and rotations of a preferred, isotropically expanding,
background. A few of the resulting corrections to the primordial power spectrum
do not have the usual oscillatory factor, which is sometimes taken to be
characteristic of a "trans-Planckian" signal. Perhaps more interestingly, one
of these leading irrelevant operators exactly reproduces a correction to the
power spectrum that occurs in effective descriptions of the state of the field
responsible for inflation.Comment: 11 pages, no figures, uses ReVTe
Universal diffusion near the golden chaos border
We study local diffusion rate in Chirikov standard map near the critical
golden curve. Numerical simulations confirm the predicted exponent
for the power law decay of as approaching the golden curve via principal
resonances with period (). The universal
self-similar structure of diffusion between principal resonances is
demonstrated and it is shown that resonances of other type play also an
important role.Comment: 4 pages Latex, revtex, 3 uuencoded postscript figure
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