4,918 research outputs found
The FIR-absorption of short period quantum wires and the transition from one to two dimensions
We investigate the FIR-absorption of short period parallel quantum wires in a
perpendicular quantizing magnetic field. The external time-dependent electric
field is linearly polarized along the wire modulation. The mutual Coulomb
interaction of the electrons is treated self-consistently in the ground state
and in the absorption calculation within the Hartree approximation. We consider
the effects of a metal gate grating coupler, with the same or with a different
period as the wire modulation, on the absorption. The evolution of the
magnetoplasmon in the nonlocal region where it is split into several Bernstein
modes is discussed in the transition from: narrow to broad wires, and isolated
to overlapping wires. We show that in the case of narrow and not strongly
modulated wires the absorption can be directly correlated with the underlying
electronic bandstructure.Comment: 15 pages, 9 figures, Revtex, to appear in Phys. Rev.
Neutrinos and Nucleosynthesis in Supernova
The type II supernova is considered as a candidate site for the production of
heavy elements. The nucleosynthesis occurs in an intense neutrino flux, we
calculate the electron fraction in this environment.Comment: RevTex4 style, 3 pages including 1 figure. Presented at Mexican
School of Astrophysics 2002, Guanajuato, Mexico, 31 Jul - 7 Aug 2002. Final
version to appear in the Proceedings of IX Mexican Workshop on Particles and
Fields Physics Beyond the Standard Model, Colima Col. Mexico, November 17-22,
200
Level densities and -ray strength functions in Yb
Level densities and radiative strength functions in Yb and Yb
nuclei have been measured using the
Yb(He,He)Yb and
Yb(He,)Yb reactions. New data on Yb
are compared to a previous measurement for Yb from the
Yb(He,)Yb reaction. Systematics of level
densities and radiative strength functions in Yb are
established. The entropy excess in Yb relative to the even-even nuclei
Yb due to the unpaired neutron quasiparticle is found to be
approximately 2. Results for the radiative strength function from the two
reactions lead to consistent parameters characterizing the ``pygmy''
resonances. Pygmy resonances in the Yb populated by the
(He,) reaction appear to be split into two components for both of
which a complete set of resonance parameters are obtained.Comment: 8 pages, 7 figure
Remarks on abelian dominance
We used a renormalisation group based smoothing to address two questions
related to abelian dominance. Smoothing drastically reduces short distance
fluctuations but it preserves the long distance physical properties of the
SU(2) configurations. This enabled us to extract the abelian heavy-quark
potential from time-like Wilson loops on Polyakov gauge projected
configurations. We obtained a very small string tension which is inconsistent
with the string tension extracted from Polyakov loop correlators. This shows
that the Polyakov gauge projected abelian configurations do not have a
consistent physical meaning. We also applied the smoothing on SU(2)
configurations to test how sensitive abelian dominance in the maximal abelian
gauge is to the short distance fluctuations. We found that on smoothed SU(2)
configurations the abelian string tension was about 30% smaller than the SU(2)
string tension which was unaffected by smoothing. This suggests that the
approximate abelian dominance found with the Wilson action is probably an
accident and it has no fundamental physical relevance.Comment: 13 pages, LaTeX, 3 eps figure
The Cosmological Constant is Back
A diverse set of observations now compellingly suggest that Universe
possesses a nonzero cosmological constant. In the context of quantum-field
theory a cosmological constant corresponds to the energy density of the vacuum,
and the wanted value for the cosmological constant corresponds to a very tiny
vacuum energy density. We discuss future observational tests for a cosmological
constant as well as the fundamental theoretical challenges---and
opportunities---that this poses for particle physics and for extending our
understanding of the evolution of the Universe back to the earliest moments.Comment: latex, 8 pages plus one ps figure available as separate compressed
uuencoded fil
A Biopersistence Study following Exposure to Chrysotile Asbestos Alone or in Combination with Fine Particles
In designing a study to evaluate the inhalation biopersistence of a chrysotile asbestos that was used as a component of a joint-compound, a feasibility study was initiated to evaluate the short-term biopersistence of the chrysotile alone and of the chrysotile in combination witht the sanded reformulated joint-compound. Two groups of Wistar rats were exposed to either 7RF3 chrysotile (Group 2) or to 7RF3 chrysotile combined with aerosolized sanded joint-compound (Group 3). In addition, a control group was exposed to flltered-air. The chrysotile used in the Ready Mix joint compound is rapidly removed from the lung. The chrysotile alone exposure group had a clearance half-time of fibers L > 20 μm of 2.2 days; in the chrysotile plus sanded exposure group the clearance half-time of fibers L > 20 μm was 2.8 days. However, across all size ranges there was approximately an order of magnitude decrease in the mean number of fibers remaining in the lungs of Group 3 as compared to Group 2 despite similiar aerosol exposures. Histopathological examination showed that the chrysotile exposed lungs had the same appearance as the flltered-air controls. This study uniquely illustrates that additional concurrent exposure to an aerosol of the sanded joint-compound, with large numbers of fine-particles depositing in the lungs, accelerates the recruitment of macrophages, resulting in a tenfold decrease in the number of fibers remaining in the lung. The increased number of macrophages in the chrysotile/sanded joint exposure group was confirmed histologically, with this being the only exposure-related histological finding reported
Neutral Pion Photoproduction on Nuclei in Baryon Chiral Perturbation Theory
Threshold neutral pion photoproduction on light nuclei is studied in the
framework of baryon chiral perturbation theory. We obtain a general formula for
the electric dipole amplitude in the special case of neutral pion
photoproduction on a nucleus. To third order in small momenta, the amplitude is
a sum of 2- and 3-body interactions with no undetermined parameters. With
reasonable input from the single nucleon sector, our result for neutral pion
photoproduction on the deuteron is in agreement with experiment.Comment: 24 pages, 4 uuencoded postscript figures, uses LaTex and epsf.tex.
Added footnote and references. Minor changes in text and forma
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