13,346 research outputs found
Quadratic Maps in Two Variables on Arbitrary Fields
Let be a field of characteristic different from and , and
let be a vector space of dimension over . The generic
classification of homogeneous quadratic maps under the action
of the linear group of , is given and efficient computational criteria to
recognize equivalence are provided.Comment: 12 pages, no figure
A Cellular Automaton Model for the Traffic Flow in Bogota
In this work we propose a car cellular automaton model that reproduces the
experimental behavior of traffic flows in Bogot\'a. Our model includes three
elements: hysteresis between the acceleration and brake gaps, a delay time in
the acceleration, and an instantaneous brake. The parameters of our model were
obtained from direct measurements inside a car on motorways in Bogot\'a. Next,
we simulated with this model the flux-density fundamental diagram for a
single-lane traffic road and compared it with experimental data. Our
simulations are in very good agreement with the experimental measurements, not
just in the shape of the fundamental diagram, but also in the numerical values
for both the road capacity and the density of maximal flux. Our model
reproduces, too, the qualitative behavior of shock waves. In addition, our work
identifies the periodic boundary conditions as the source of false peaks in the
fundamental diagram, when short roads are simulated, that have been also found
in previous works. The phase transition between free and congested traffic is
also investigated by computing both the relaxation time and the order
parameter. Our work shows how different the traffic behavior from one city to
another can be, and how important is to determine the model parameters for each
city.Comment: 14 pages and 13 figures (gzipped tar file). Submitted to
Int.J.Mod.Phys.C. Minor changes, specially at references and typoes, plus a
clearer summary of the CA rule
A 300 GHz "Always-in-Focus" Focusing System for Target Detection
A focusing system for a 300 GHz radar with 5 m target distance and 10 mm diameter spot size resolution is proposed. The focusing system is based on a Gaussian telescope scheme and its main parameters have been deÂŹsigned using Gaussian beam quasi-optical propagation theory with an in-house developed MATLABÂź based analysis tool. Then, this approach has been applied to a real focusing system based on two elliptical mirrors in order to reduce the distortion and cross-polar level and a plane mirror to provide scanning capabilities. The overÂŹall system has been simulated with a full-wave electromagÂŹnetic simulator and its behavior is presented. With this approach, the focusing system always works "in-focus" since the only mirror that is rotated when scanning is the output plane mirror, so the beam is almost not distorted. The design process, although based in the well-known Gaussian beam quasi-optical propagation theory, provides a fast and accurate method and minimizes the overall size of the mirrors. As a consequence, the size of the focusing system is also reduced
Molecular gas in low-metallicity starburst galaxies: Scaling relations and the CO-to-H conversion factor
We study the molecular content and the star formation efficiency of 21 Blue
Compact Dwarfs (BCDs). We present CO(1-0) and (2-1) observations, further
supplemented with additional CO measurements and multiwavelength ancillary data
from the literature. We find the CO luminosity to be correlated with the
stellar and HI masses, SFR tracers, the size of the starburst and its
metallicity. BCDs appear offset from the Schmidt-Kennicutt (SK) law, showing
extremely low (0.1 Gyr) H2 and H2+HI depletion timescales. The
departure from the SK law is smaller when considering H2+HI rather than H2
only, and is larger for BCDs with lower metallicity and higher specific SFR.
Thus, the molecular fraction and H2 depletion timescale of BCDs is found to be
strongly correlated with metallicity. Using this and assuming that the
empirical correlation found between the specific SFR and galaxy-averaged H2
depletion timescale of more metal-rich galaxies extends to lower masses, we
derive a metallicity-dependent CO-to-H2 conversion factor , with in qualitative agreement
with previous determinations, dust-based measurements, and recent model
predictions. Our results suggest that in vigorously star-forming dwarfs the
fraction of H2 traced by CO decreases by a factor of about 40 from to , leading to a strong underestimation of
the H2 mass in metal-poor systems when a Galactic is
considered. Adopting we find that departures from the SK law
are partially resolved. Our results suggest that starbursting dwarfs have
shorter depletion gas timescales and lower molecular fractions compared to
normal late-type disc galaxies even accounting for the molecular gas not traced
by CO emission in metal-poor environments, raising additional constraints to
model predictions (Abridged).Comment: 18 pages, 14 Figures, 4 Tables: Accepted for publication in A&
First clear evidence of quantum chaos in the bound states of an atomic nucleus
We study the spectral fluctuations of the Pb nucleus using the
complete experimental spectrum of 151 states up to excitation energies of
MeV recently identified at the Maier-Leibnitz-Laboratorium at Garching,
Germany. For natural parity states the results are very close to the
predictions of Random Matrix Theory (RMT) for the nearest-neighbor spacing
distribution. A quantitative estimate of the agreement is given by the Brody
parameter , which takes the value for regular systems and
for chaotic systems. We obtain which
is, to our knowledge, the closest value to chaos ever observed in experimental
bound states of nuclei. By contrast, the results for unnatural parity states
are far from RMT behavior. We interpret these results as a consequence of the
strength of the residual interaction in Pb, which, according to
experimental data, is much stronger for natural than for unnatural parity
states. In addition our results show that chaotic and non-chaotic nuclear
states coexist in the same energy region of the spectrum.Comment: 9 pages, 1 figur
Solid flow drives surface nanopatterning by ion-beam irradiation
Ion Beam Sputtering (IBS) is known to produce surface nanopatterns over
macroscopic areas on a wide range of materials. However, in spite of the
technological potential of this route to nanostructuring, the physical process
by which these surfaces self-organize remains poorly under- stood. We have
performed detailed experiments of IBS on Si substrates that validate dynamical
and morphological predictions from a hydrodynamic description of the
phenomenon. Our results elucidate flow of a nanoscopically thin and highly
viscous surface layer, driven by the stress created by the ion-beam, as a
description of the system. This type of slow relaxation is akin to flow of
macroscopic solids like glaciers or lead pipes, that is driven by defect
dynamics.Comment: 12 pages, 4 figure
Spin splitting in a polarized quasi-two-dimensional exciton gas
We have observed a large spin splitting between "spin" and
heavy-hole excitons, having unbalanced populations, in undoped GaAs/AlAs
quantum wells in the absence of any external magnetic field. Time-resolved
photoluminescence spectroscopy, under excitation with circularly polarized
light, reveals that, for high excitonic density and short times after the
pulsed excitation, the emission from majority excitons lies above that of
minority ones. The amount of the splitting, which can be as large as 50% of the
binding energy, increases with excitonic density and presents a time evolution
closely connected with the degree of polarization of the luminescence. Our
results are interpreted on the light of a recently developed model, which shows
that, while intra-excitonic exchange interaction is responsible for the spin
relaxation processes, exciton-exciton interaction produces a breaking of the
spin degeneracy in two-dimensional semiconductors.Comment: Revtex, four pages; four figures, postscript file Accepted for
publication in Physical Review B (Rapid Commun.
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