10,233 research outputs found
Three-dimensional Dirac oscillator in a thermal bath
The thermal properties of the three-dimensional Dirac oscillator are
considered. The canonical partition function is determined, and the
high-temperature limit is assessed. The degeneracy of energy levels and their
physical implications on the main thermodynamic functions are analyzed,
revealing that these functions assume values greater than the one-dimensional
case. So that at high temperatures, the limit value of the specific heat is
three times bigger.Comment: 9 pages, 4 figures. Text improved, references added. Revised to match
accepted version in Europhysics Letters
Treating some solid state problems with the Dirac equation
The ambiguity involved in the definition of effective-mass Hamiltonians for
nonrelativistic models is resolved using the Dirac equation. The multistep
approximation is extended for relativistic cases allowing the treatment of
arbitrary potential and effective-mass profiles without ordering problems. On
the other hand, if the Schrodinger equation is supposed to be used, our
relativistic approach demonstrate that both results are coincidents if the
BenDaniel and Duke prescription for the kinetic-energy operator is implemented.
Applications for semiconductor heterostructures are discussed.Comment: 06 pages, 5 figure
Electronic transport through bilayer graphene flakes
We investigate the electronic transport properties of a bilayer graphene
flake contacted by two monolayer nanoribbons. Such a finite-size bilayer flake
can be built by overlapping two semiinfinite ribbons or by depositing a
monolayer flake onto an infinite nanoribbon. These two structures have a
complementary behavior, that we study and analyze by means of a tight-binding
method and a continuum Dirac model. We have found that for certain energy
ranges and geometries, the conductance of these systems oscillates markedly
between zero and the maximum value of the conductance, allowing for the design
of electromechanical switches. Our understanding of the electronic transmission
through bilayer flakes may provide a way to measure the interlayer hopping in
bilayer graphene.Comment: 11 pages, 8 figure
Performance Evaluation of Laboratory Wi-Fi ieee 802.11g wpa Point-to-Point Links Using TCP, UDP and FTP
AbstractWireless communications, e.g. Wi-Fi, have been increasingly important in the context of networked and virtual organizations and enterprise information systems. Performance is an issue of crucial importance, resulting in more reliable and efficient communications, therefore improving enterprise information system yield. Security is equally important. Laboratory measurements are made about several performance aspects of Wi-Fi IEEE 802.11g WPA point-to-point links. A contribution is given to performance evaluation of this technology under WPA encryption, using available equipments (DAP-1522 access points from D-Link and WPC600N adapters from Linksys). New detailed results are presented and discussed, namely at OSI levels 4 and 7, from TCP, UDP and FTP experiments, permitting measurements of TCP throughput, jitter, percentage datagram loss and FTP transfer rate. Comparisons are made to corresponding results obtained for open links. Conclusions are drawn about the comparative performance of the links
Lorentzian Vacuum Transitions in Ho\v{r}ava-Lifshitz Gravity
The vacuum transition probabilities for a
Friedmann-Lema\^itre-Robertson-Walker universe with positive curvature in
Ho\v{r}ava-Lifshitz gravity in the presence of a scalar field potential in the
Wentzel-Kramers-Brillouin approximation are studied. We use a general procedure
to compute such transition probabilities using a Hamiltonian approach to the
Wheeler-DeWitt equation presented in a previous work. We consider two
situations of scalar fields, one in which the scalar field depends on all the
spacetime variables and other in which the scalar field depends only on the
time variable. In both cases analytic expressions for the vacuum transition
probabilities are obtained and the infrared and ultraviolet limits are
discussed for comparison with the result obtained by using general relativity.
For the case in which the scalar field depends on all spacetime variables we
obtain that in the infrared limit it is possible to obtain a similar behavior
as in general relativity, however in the ultraviolet limit the behavior found
is completely opposite. Some few comments about possible phenomenological
implications of our results are given. One of them is a plausible resolution of
the initial singularity. On the other hand for the case in which the scalar
field depends only on the time variable, the behavior coincides with that of
general relativity in both limits, although in the intermediate region the
probability is slightly altered.Comment: 26 pages, 2 figures. Some references adde
Scalar field inflation driven by a modification of the Heisenberg algebra
We study the modifications induced on scalar field inflation produced by
considering a general modification of the Heisenberg algebra. We proceed by
modifying the Poisson brackets on the classical theory whenever the
corresponding quantum commutator is modified. We do not restrict ourselves to a
specific form for such modification, instead we constrain the functions
involved by the cosmological behaviour of interest. We present whenever
possible the way in which inflation can be realized approximately via three
slow roll Hubble parameters that depend on the standard slow roll parameters in
a very different form than in the usual case and that can be less restrictive.
Furthermore we find a general analytical solution describing an expanding
universe with constant Hubble parameter that generalizes the standard
cosmological constant case by restricting the form of the modification of the
Heisenberg algebra. It is found that even if such modification can be neglected
in some limit and the cosmological constant is set to zero in that limit, the
exponential expansion is present when the modification is important. Thus an
appropriate modification of the Heisenberg algebra is sufficient to produce an
exponentially expanding universe without the need of any other source.Comment: 34 pages, 1 figur
Direct observation of melting in a 2-D superconducting vortex lattice
Topological defects such as dislocations and disclinations are predicted to
determine the twodimensional (2-D) melting transition. In 2-D superconducting
vortex lattices, macroscopic measurements evidence melting close to the
transition to the normal state. However, the direct observation at the scale of
individual vortices of the melting sequence has never been performed. Here we
provide step by step imaging through scanning tunneling spectroscopy of a 2-D
system of vortices up to the melting transition in a focused-ion-beam
nanodeposited W-based superconducting thin film. We show directly the
transition into an isotropic liquid below the superconducting critical
temperature. Before that, we find a hexatic phase, characterized by the
appearance of free dislocations, and a smectic-like phase, possibly originated
through partial disclination unbinding. These results represent a significant
step in the understanding of melting of 2-D systems, with impact across several
research fields, such as liquid crystal molecules, or lipids in membranes.Comment: Submitted to Nature Physic
Determination of phytoextraction potential of plant speciesfor toxic elements in soils of abandoned sulphide-mining areas
This study has determined contamination levels in soils and plants from the Sa˜o Domingos mining area, Portugal, by k0-INAA. Total concentrations of As, Sb, Cr, Hg, Cu, Zn and Fe in soils were very high, exceeding the maximum limits in Portuguese legislation. Concentrations of toxic elements like As, Sb and Zn were highest in roots of Erica andevalensis, Juncus acutus, Agrostis castellana and Nicotiana glauca. Additionally, As, Br, Cr, Fe, Sb and Zn in all organs of most plants were above toxicity levels.
Those species that accumulated relatively high concentrations of toxic elements in roots (and tops) may be cultivated for phytostabilisation of similar areas
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