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