8,824 research outputs found
On Dirac-like Monopoles in a Lorentz- and CPT-violating Electrodynamics
We study magnetic monopoles in a Lorentz- and CPT-odd electrodynamical
framework in (3+1) dimensions. This is the standard Maxwell model extended by
means of a Chern-Simons-like term, (
constant), which respects gauge invariance but violates both Lorentz and CPT
symmetries (as a consequence, duality is also lost). Our main interest concerns
the analysis of the model in the presence of Dirac monopoles, so that the
Bianchi identity no longer holds, which naively yields the non-conservation of
electric charge. Since gauge symmetry is respected, the issue of charge
conservation is more involved. Actually, the inconsistency may be circumvented,
if we assume that the appearance of a monopole induces an extra electric
current. The reduction of the model to (2+1) dimensions in the presence of both
the magnetic sources and Lorentz-violating terms is presented. There, a
quantization condition involving the scalar remnant of , say, the mass
parameter, is obtained. We also point out that the breaking of duality may be
associated with an asymmetry between electric and magnetic sources in this
background, so that the electromagnetic force experienced by a magnetic pole is
supplemented by an extra term proportional to , whenever compared to the
one acting on an electric charge.Comment: 10 pages, no figures, typed in te
Graphene as an electronic membrane
Experiments are finally revealing intricate facts about graphene which go
beyond the ideal picture of relativistic Dirac fermions in pristine two
dimensional (2D) space, two years after its first isolation. While observations
of rippling added another dimension to the richness of the physics of graphene,
scanning single electron transistor images displayed prevalent charge
inhomogeneity. The importance of understanding these non-ideal aspects cannot
be overstated both from the fundamental research interest since graphene is a
unique arena for their interplay, and from the device applications interest
since the quality control is a key to applications. We investigate the membrane
aspect of graphene and its impact on the electronic properties. We show that
curvature generates spatially varying electrochemical potential. Further we
show that the charge inhomogeneity in turn stabilizes ripple formation.Comment: 6 pages, 11 figures. Updated version with new results about the
re-hybridization of the electronic orbitals due to rippling of the graphene
sheet. The re-hybridization adds the next-to-nearest neighbor hopping effect
discussed in the previous version. New reference to recent STM experiments
that give support to our theor
Dirac Fermion Confinement in Graphene
We study the problem of Dirac fermion confinement in graphene in the presence
of a perpendicular magnetic field B. We show, analytically and numerically,
that confinement leads to anomalies in the electronic spectrum and to a
magnetic field dependent crossover from \sqrt{B}, characteristic of
Dirac-Landau level behavior, to linear in B behavior, characteristic of
confinement. This crossover occurs when the radius of the Landau level becomes
of the order of the width of the system. As a result, we show that the
Shubnikov-de Haas oscillations also change as a function of field, and lead to
a singular Landau plot. We show that our theory is in excellent agreement with
the experimental data.Comment: 4 pages, 6 figure
Riemann-Cartan Space-times of G\"odel Type
A class of Riemann-Cartan G\"odel-type space-times are examined in the light
of the equivalence problem techniques. The conditions for local space-time
homogeneity are derived, generalizing previous works on Riemannian G\"odel-type
space-times. The equivalence of Riemann-Cartan G\"odel-type space-times of this
class is studied. It is shown that they admit a five-dimensional group of
affine-isometries and are characterized by three essential parameters : identical triads () correspond to locally
equivalent manifolds. The algebraic types of the irreducible parts of the
curvature and torsion tensors are also presented.Comment: 24 pages, LaTeX fil
Theoretical Analysis of Cold-formed Steel Battened Double Angle Members under Compression
In Brazil, battened double angle system is one of the systems most used in light truss, however, there are not any specific studies on its behavior, resulting in the fact that the standard procedures do not provide subsidies for the design of this section. Moreover, cold-formed steel si mple angles under compression, mostly with slender legs, have an interesting structural behavior compared to other cold-formed steel shapes. Two critical modes are observed in the elastic stability analysis: (i) global flexural mode in the case of longer members and (ii) a coincident local-plate/global flexural-torsional mode, which is critical for shorter members. Studying the behavior of double angle members is interesting, because in this case, besides the critical modes of the single angle, they also show critical modes, due to the presence of the batten plates that sometimes interfere with the behavior of the syst em. In this work, a nonlinear numerical analysis on the behavior of double angle in battened system is presented. The number of batten plates was varied studying their effectiveness in the nominal axial strength. The sensitivity of the members to initial geometric imperfections was also analyzed. Except for the thin angle specimen (t = 1.5 mm) the results obtained from the nonlinear analysis sh owed that the presence of the batten plates significantly increased the nomin al axial strength of the members. However for an increased number of batten plates the nominal axial strength of the members remained almost constant. It was observed that the members were more sensitive to initial geometric imperfections increasing that to the number of batten plates
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