17,351 research outputs found
A parity conserving dimer model with infinitely many absorbing states
We propose and study a model where, for the first time, two aspects are
present: parity conservation and infinitely many absorbing states. Whereas
steady-state simulations show that the static critical behaviour is not
affected by the presence of multiple absorbing configurations, the influence of
the initial state associated with the presence of slowly decaying memory
effects is clearly displayed in time dependent simulations. We report results
of a detailed investigation of the dependence of critical spreading exponents
on the initial particle density.Comment: 4 pages, 3 figures.p
Roper Electroproduction Amplitudes in a Chiral Confinement Model
A description of the Roper using the chiral chromodielectric model is
presented and the transverse and the scalar helicity
amplitudes for the electromagnetic Nucleon--Roper transition are obtained for
small and moderate . The sign of the amplitudes is correct but the model
predictions underestimate the data at the photon point. Our results do not
indicate a change of sign in any amplitudes up to GeV. The
contribution of the scalar meson excitations to the Roper electroproduction is
taken into account but it turns out to be small in comparison with the quark
contribution. However, it is argued that mesonic excitations may play a more
prominent role in higher excited states.Comment: 10 pages, 8 figures, uses World Scientific macros. Talk presented at
EMI2001 in Osaka, Japa
N^* electroproduction amplitudes in a model with dynamical confinement
The Roper resonance is described in a chiral version of the chromodielectric
model as a cluster of three quarks in radial-orbital configuration
(1s)(2s), surrounded by and -meson clouds and by a
chromodielectric field which assures quark dynamical confinement. Radial
profiles for all fields are determined self-consistently for each baryon.
Transverse and scalar helicity amplitudes for the
nucleon-Roper transition are calculated. The contribution of glueball and
-meson vibrations is estimated; although small for N(1440), the
contribution can be large for N(1710).Comment: 12 pages, 6 figures, uses elsevier macro
Abrasion of flat rotating shapes
We report on the erosion of flat linoleum "pebbles" under steady rotation in
a slurry of abrasive grit. To quantify shape as a function of time, we develop
a general method in which the pebble is photographed from multiple angles with
respect to the grid of pixels in a digital camera. This reduces digitization
noise, and allows the local curvature of the contour to be computed with a
controllable degree of uncertainty. Several shape descriptors are then employed
to follow the evolution of different initial shapes toward a circle, where
abrasion halts. The results are in good quantitative agreement with a simple
model, where we propose that points along the contour move radially inward in
proportion to the product of the radius and the derivative of radius with
respect to angle
Excitonic effects in the optical properties of CdSe nanowires
Using a first-principle approach beyond density functional theory we
calculate the electronic and optical properties of small diameter CdSe
nanowires.Our results demonstrate how some approximations commonly used in bulk
systems fail at this nano-scale level and how indispensable it is to include
crystal local fields and excitonic effects to predict the unique optical
properties of nanowires. From our results, we then construct a simple model
that describes the optical gap as a function of the diameter of the wire, that
turns out to be in excellent agreement with experiments for intermediate and
large diameters.Comment: submitte
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