32,442 research outputs found
Growth of ZnO nanostructures on Si by means of plasma immersion ion implantation and deposition
Crystalline zinc oxide (ZnO) nanostructures have been grown on Si substrates by means of Plasma Based Ion Implantation and Deposition (PIII&D) at temperature of about 300 0C and in the presence of an argon glow discharge. In the process a crucible filled with small pieces of metallic zinc plays the role of the anode of the discharge itself, being polarized by positive DC voltage of about 400V. Electrons produced by thermionic emission by an oxide cathode (Ba, Sr, Ca)O impact this crucible, causing its heating and vaporization of Zn. Partial ionization of Zn atoms takes place due to collisions with plasma particles. High negative voltage pulses (7 kv/40μs/250Hz) applied to the sample holder cause the implantation of metallic zinc into Si surface, while Zn deposition happens between pulses. After annealing at 700 0C, strong UV and various visible photoluminescence bands are observed at room temperature, as well as the presence of ZnO nanoparticles. The coated surface was characterized in detail using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), atomic force microscopy (AFM) and photoluminescence (PL) spectroscopy. XRD indicated the presence of only ZnO peaks after annealing. The composition analysis by EDS revealed distinct Zn/O stoichiometry relation depending on the conditions of the process. AFM images showed the formation of columns in the nanoscale range. Topography viewed by SEM showed the formation of structures similar to cactus with nanothorns. Depth analysis performed by XPS indicated an increase of concentration of metallic Zn with increasing depth and the exclusive presence of ZnO for outer regions. PIII&D allowed to growing nanostructures of ZnO on Si without the need of a buffer layer
On the integrability of halo dipoles in gravity
We stress that halo dipole components are nontrivial in core-halo systems in
both Newton's gravity and General Relativity. To this end, we extend a recent
exact relativistic model to include also a halo dipole component. Next, we
consider orbits evolving in the inner vacuum between a monopolar core and a
pure halo dipole and find that, while the Newtonian dynamics is integrable, its
relativistic counterpart is chaotic. This shows that chaoticity due only to
halo dipoles is an intrinsic relativistic gravitational effect.Comment: 9 pages, REVTEX, two postscript figures include
Information entropy of classical versus explosive percolation
We study the Shannon entropy of the cluster size distribution in classical as
well as explosive percolation, in order to estimate the uncertainty in the
sizes of randomly chosen clusters. At the critical point the cluster size
distribution is a power-law, i.e. there are clusters of all sizes, so one
expects the information entropy to attain a maximum. As expected, our results
show that the entropy attains a maximum at this point for classical
percolation. Surprisingly, for explosive percolation the maximum entropy does
not match the critical point. Moreover, we show that it is possible determine
the critical point without using the conventional order parameter, just
analysing the entropy's derivatives.Comment: 6 pages, 6 figure
Signatures of the self-modulation instability of relativistic proton bunches in the AWAKE experiment
We investigate numerically the detection of the self-modulation instability
in a virtual detector located downstream from the plasma in the context of
AWAKE. We show that the density structures, appearing in the temporally
resolving virtual detector, map the transverse beam phase space distribution at
the plasma exit. As a result, the proton bunch radius that appears to grow
along the bunch in the detector results from the divergence increase along the
bunch, related with the spatial growth of the self-modulated wakefields. In
addition, asymmetric bunch structures in the detector are a result of
asymmetries of the bunch divergence, and do not necessarily reflect asymmetric
beam density distributions in the plasma.Comment: Accepted for publication in NIM-A for the proceedings of the 3rd
European Advanced Accelerator Workshop. 5 pages, 2 figure
A simple deterministic self-organized critical system
We introduce a new continuous cellular automaton that presents self-organized
criticality. It is one-dimensional, totally deterministic, without any kind of
embedded randomness, not even in the initial conditions. This system is in the
same universality class as the Oslo rice pile system, boundary driven interface
depinning and the train model for earthquakes. Although the system is chaotic,
in the thermodynamic limit chaos occurs only in a microscopic level.Comment: System slightly modified. New results on Liapunov exponents.
Submitted for publication (8 pages
Chaos in Periodically Perturbed Monopole + Quadrupole Like Potentials
The motion of a particle that suffers the influence of simple inner (outer)
periodic perturbations when it evolves around a center of attraction modeled by
an inverse square law plus a quadrupole-like term is studied. The equations of
motion are used to reduce the Melnikov method to the study of simple graphics.Comment: 12 pages, 6 Postscript figure
Comparison of the Spherical Averaged Pseudopotential Model with the Stabilized Jellium Model
We compare Kohn-Sham results (density, cohesive energy, size and effect of
charging) of the Spherical Averaged Pseudopotential Model with the Stabilized
Jellium Model for clusters of sodium and aluminum with less than 20 atoms. We
find that the Stabilized Jellium Model, although conceptually and practically
more simple, gives better results for the cohesive energy and the elastic
stiffness. We use the Local Density Approximation as well as the Generalized
Gradient Approximation to the exchange and correlation energies.Comment: 13 pages, latex, 8 figures, compressed postscript version available
at http://www.fis.uc.pt/~vieir
Chaos and Synchronized Chaos in an Earthquake Model
We show that chaos is present in the symmetric two-block Burridge-Knopoff
model for earthquakes. This is in contrast with previous numerical studies, but
in agreement with experimental results. In this system, we have found a rich
dynamical behavior with an unusual route to chaos. In the three-block system,
we see the appearance of synchronized chaos, showing that this concept can have
potential applications in the field of seismology.Comment: To appear in Physical Review Letters (13 pages, 6 figures
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