20,897 research outputs found
Near-infrared Fabry-Perot imaging of Herbig-Haro energy sources: Collimated, small-scale H2 jets and wide-angled winds
Peer reviewe
Electronic dummy for acoustical testing
Electronic Dummy /ED/ used for acoustical testing represents the average male torso from the Xiphoid process upward and includes an acoustic replica of the human head. This head simulates natural flesh, and has an artificial voice and artificial ears that measure sound pressures at the eardrum or the entrance to the ear canal
Decoherence and Recoherence in Model Quantum Systems
We discuss the various manifestations of quantum decoherence in the forms of
dephasing, entanglement with the environment, and revelation of "which-path"
information. As a specific example, we consider an electron interference
experiment. The coupling of the coherent electrons to the quantized
electromagnetic field illustrates all of these versions of decoherence. This
decoherence has two equivalent interpretations, in terms of photon emission or
in terms of Aharonov-Bohm phase fluctuations. We consider the case when the
coherent electrons are coupled to photons in a squeezed vacuum state. The
time-averaged result is increased decoherence. However, if only electrons which
are emitted during selected periods are counted, the decoherence can be
suppressed below the level for the photon vacuum. This is the phenomenon of
recoherence. This effect is closely related to the quantum violations of the
weak energy condition, and is restricted by similar inequalities. We give some
estimates of the magnitude of the recoherence effect and discuss prospects for
observing it in an electron interferometry experiment.Comment: 8 pages, 3 figures, talk presented at the 7th Friedmann Seminar, Joao
Pessoa, Brazil, July 200
Dual Instantons
We show how to map the Belavin-Polyakov instantons of the O(3)-nonlinear
model to a dual theory where they then appear as nontopological
solitons. They are stationary points of the Euclidean action in the dual
theory, and moreover, the dual action and the O(3)-nonlinear model
action agree on shell.Comment: 13 page
Composite Fermions in Modulated Structures: Transport and Surface Acoustic Waves
Motivated by a recent experiment of Willett et al. [Phys. Rev. Lett. 78, 4478
(1997)], we employ semiclassical composite-fermion theory to study the effect
of a periodic density modulation on a quantum Hall system near Landau level
filling factor nu=1/2. We show that even a weak density modulation leads to
dramatic changes in surface-acoustic-wave (SAW) propagation, and propose an
explanation for several key features of the experimental observations. We
predict that properly arranged dc transport measurements would show a structure
similar to that seen in SAW measurements.Comment: Version published in Phys. Rev. Lett. Figures changed to show SAW
velocity shift. LaTeX, 5 pages, two included postscript figure
Supersymmetric Extension of the Snyder Algebra
We obtain a minimal supersymmetric extension of the Snyder algebra and study
its representations. The construction differs from the general approach given
in Hatsuda and Siegel ({\tt hep-th/0311002}), and does not utilize super-de
Sitter groups. The spectra of the position operators are discrete, implying a
lattice description of space, and the lattice is compatible with supersymmetry
transformations.Comment: 14 page
Spectroscopic confirmation of a galaxy cluster associated with 7C1756+6520 at z=1.416
We present spectroscopic follow-up of an overdensity of galaxies
photometrically selected to be at 1.4<z<2.5 found in the vicinity of the radio
galaxy 7C1756+6520 at z=1.4156. Using the DEIMOS optical multi-object
spectrograph on the Keck 2 telescope, we observed a total of 129 BzK-selected
sources, comprising 82 blue, star-forming galaxy candidates (sBzK) and 47 red,
passively-evolving galaxy candidates (pBzK*), as well as 11 mid-infrared
selected AGN candidates. We obtain robust spectroscopic redshifts for 36 blue
galaxies, 7 red galaxies and 9 AGN candidates. Assuming all foreground
interlopers were identified, we find that only 16% (9%) of the sBzK (pBzK*)
galaxies are at z<1.4. Therefore, the BzK criteria are shown to be relatively
robust at identifying galaxies at moderate redshifts. Twenty-one galaxies,
including the radio galaxy, four additional AGN candidates and three red galaxy
candidates are found with 1.4156 +/- 0.025, forming a large scale structure at
the redshift of the radio galaxy. Of these, eight have projected offsets <2Mpc
relative to the radio galaxy position and have velocity offsets <1000km/s
relative to the radio galaxy redshift. This confirms that 7C1756+6520 is
associated with a high-redshift galaxy cluster. A second compact group of four
galaxies is found at z~1.437, forming a sub-group offset by Dv~3000km/s and
approximately 1.5' east of the radio galaxy.Comment: 9 pages, 6 figures, 2 tables, accepted for publication in A&A
Infrared absorption and Raman scattering on coupled plasmon--phonon modes in superlattices
We consider theoretically a superlattice formed by thin conducting layers
separated spatially between insulating layers. The dispersion of two coupled
phonon-plasmon modes of the system is analyzed by using Maxwell's equations,
with the influence of retardation included. Both transmission for the finite
plate as well as absorption for the semi-infinite superlattice in the infrared
are calculated. Reflectance minima are determined by the longitudinal and
transverse phonon frequencies in the insulating layers and by the density-state
singularities of the coupled modes. We evaluate also the Raman cross section
from the semi-infinite superlattice.Comment: 20 pages,14 figure
IR Kuiper Belt Constraints
We compute the temperature and IR signal of particles of radius and
albedo at heliocentric distance , taking into account the
emissivity effect, and give an interpolating formula for the result. We compare
with analyses of COBE DIRBE data by others (including recent detection of the
cosmic IR background) for various values of heliocentric distance, ,
particle radius, , and particle albedo, . We then apply these
results to a recently-developed picture of the Kuiper belt as a two-sector disk
with a nearby, low-density sector (40<R<50-90 AU) and a more distant sector
with a higher density. We consider the case in which passage through a
molecular cloud essentially cleans the Solar System of dust. We apply a simple
model of dust production by comet collisions and removal by the
Poynting-Robertson effect to find limits on total and dust masses in the near
and far sectors as a function of time since such a passage. Finally we compare
Kuiper belt IR spectra for various parameter values.Comment: 34 pages, LaTeX, uses aasms4.sty, 11 PostScript figures not embedded.
A number of substantive comments by a particularly thoughtful referee have
been addresse
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