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Electrons near the Fermi level behaving as massless Dirac fermions in graphene in (1+2)-D relativistic spacetime have been confirmed by an experiment. Using this aspect, a myriad of novel and interesting devices can be sought. In this paper, we laid out the theory for using a monolayer graphene sheet as an electron diffractometer, aiming at the determination of surface properties in materials. The key ingredient is the Mott scattering of electrons by screened Coulomb scatterers in (1+2)-D spacetime. The specific array of scatterers provided by a given surface placed in contact with a graphene sheet will induce an angular distribution for the electron scattering events, which can be properly measured through the electric current flowing to external electrodes. It can provide an in situ technique for characterizing quantum dot superlattices with a resolution of a few nanometers. [ABSTRACT FROM AUTHOR]

Copyright of Caderno Brasileiro de Ensino de Física is the property of Caderno Brasileiro de Ensino de Fisica and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Caderno Brasileiro de Ensino de Física is the property of Caderno Brasileiro de Ensino de Fisica and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In this paper we analyze the main transport properties of a simple resonant magnetic tunnel junction (FM-IS-METAL-IS-FM structure) taking into account both elastic and magnon-assisted tunneling processes at low voltages and temperatures near 0° K. We show the possibility of magnetoresistance inversion as a consequence of inelastic processes and spin-dependent transmission coefficients. Resonant tunneling can also explain the effect of scattering by impurities located inside an insulating barrier. [ABSTRACT FROM AUTHOR]

In this article we analyze different contributions to the magnetoresistance of magnetic tunneling junctions at low voltages. A substantial fraction of the resistance drop with voltage can be ascribed to variations of the density of states and the barrier transmission with the bias. However, we found that the anomaly observed at zero bias and the magnetoresistance behavior at very small voltages, point to the contribution of inelastic magnon-assisted tunneling. The latter is described by a transfer parameter TJ, which is one or two orders of magnitude smaller than Td, the direct transmission for elastic currents. Our theory is in excellent agreement with experimental data, yielding estimated values of TJ which are of the order of Td/TJ∼40. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Copyright of Caderno Brasileiro de Ensino de Física is the property of Caderno Brasileiro de Ensino de Fisica and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

A similarity transformation is an equivalence relation between square matrices which preserves determinant, trace and eigenvalues, playing a key role in quantum mechanics in simplifying complex hamiltonian systems and improving analytical results attainable from the use of perturbation theory. As a prototypical example, the conventional BCS theory of superconductivity is usually derived from a similarity transformation of the original electron-phonon hamiltonian, written in second quantized version. Here we discuss the general method for writing the similarity transformation operator in second quantized form, allowing one to recast a hamiltonian describing an interacting fermion-boson system into an effective theory in which only the desired degrees of freedom are kept after the transformation. [ABSTRACT FROM AUTHOR]

Graphene is an allotropic form of carbon made by a monolayer of carbon atoms arranged in a honeycomb crystalline lattice. It can be experimentally obtained through the use of several methods, being the exfoliation of graphite the easiest way. Graphene can be considered the first genuinely two-dimensional material, since it is constituted of a single atomic layer, in contrast to several material structures, such as thin films, which behave as two-dimensional ones, due to the fact that one of the three space dimensions is highly reduced in comparison to the others, but still consisting of many atomic layers. The most striking theoretical prediction, experimentally confirmed, is that electrons in graphene behave as massless Dirac fermions in a "relativistic" (1+2)-D space-time, consisting of one temporal dimension and two space dimensions, near the so-called Dirac points, opening the possibility of emulating high-energy physics through the use of a low-energy condensed matter system, beside the promise to revolutionize the high-speed electronics. The aim of the present contribution is to put forward the fundamentals of Dirac fermions, explaining how the effective model emerges in graphene. We understand that there is a poor literature on the subject in Portuguese. Also, graphene provides a useful introduction to relativistic field theory and to the second quantization methods. [ABSTRACT FROM AUTHOR]

Three-phase electric power systems can transmit large amounts of power, typically in the range of 2000 MVA with a voltage of 765 kV. Under such extreme conditions, the air surrounding the transmission lines is ionized, which significantly increases the conductivity. The present contribution proposes a simple model which allows one to estimate the ionization and the electric conductivity of the air as a function of the power density flowing in the power line system. Near the transmission lines, it is shown that the air conductivity is increased by several orders of magnitude relative to normal conditions. In this scenario, the dielectric breakdown is favored by avalanche processes and lightning stroke probability is significantly increased. [ABSTRACT FROM AUTHOR]