21 research outputs found
Study of the topological Hall effect on simple models
Recently, a chirality-driven contribution to the anomalous Hall effect has
been found that is induced by the Berry phase and does not directly involve
spin-orbit coupling. In this paper, we will investigate this effect numerically
in a two-dimensional electron gas with a simple magnetic texture model. Both
the adiabatic and non-adiabatic regimes are studied, including the effect of
disorder. By studying the transition between both regimes the discussion about
the correct adiabaticity criterium in the diffusive limit is clarified.Comment: 8 pages, 7 figures, revtex
Disorder-induced pseudodiffusive transport in graphene nanoribbons.
We study the transition from ballistic to diffusive and localized transport in graphene nanoribbons in the presence of binary disorder, which can be generated by chemical adsorbates or substitutional doping. We show that the interplay between the induced average doping (arising from the nonzero average of the disorder) and impurity scattering modifies the traditional picture of phase-coherent transport. Close to the Dirac point, intrinsic evanescent modes produced by the impurities dominate transport at short lengths giving rise to a regime analogous to pseudodiffusive transport in clean graphene, but without the requirement of heavily doped contacts. This intrinsic pseudodiffusive regime precedes the traditional ballistic, diffusive, and localized regimes. The last two regimes exhibit a strongly modified effective number of propagating modes and a mean free path which becomes anomalously large close to the Dirac point
Casimir forces in modulated systems
For the first time we present analytical results for the contribution of
electromagnetic fluctuations into thermodynamic properties of modulated
systems, like cholesteric or smectic liquid crystalline films. In the case of
small dielectric anisotropy we have derived explicit analytical expressions for
the chemical potential of such systems. Two limiting cases were specifically
considered: (i) the Van der Waals (VdW) limit, i.e., in the case when the
retardation of the electromagnetic interactions can be neglected; and (ii) the
Casimir limit, i.e. when the effects of retardation becomes considerable. It
was shown that in the Casimir limit, the film chemical potential oscillates
with the thickness of the film. This non-monotonic dependence of the chemical
potential on the film thickness can lead to step-wise wetting phenomena,
surface anchoring reorientation and other important effects. Applications of
the results may concern the various systems in soft matter or condensed matter
physics with multilayer or modulated structures.Comment: 13 page
The Magnetic Casimir Effect
The Casimir effect results from alterations of the zero-point electromagnetic
energy introduced by boundary-conditions. For ferromagnetic layers separated by
vacuum (or a dielectric) such boundary-conditions are influenced by the
magneto-optical Kerr effect. We will show that this gives rise to a long-range
magnetic interaction and discuss the effect for two different configurations
(magnetization parallel and perpendicular to the layers). Analytical
expressions are derived for two models and compared to numerical calculations.
Numerical calculations of the effect for Fe are also presented and the
possibility of an experimental observation of the Casimir magnetic interaction
is discussed
Observational Constraints on the Angular and Spectral Distributions of Photons in Gamma-Ray Burst Sources
The typical spectra of gamma-ray bursts (GRBs) are discussed in the context
of the compactness problem for GRB sources and how it is resolved in the
popular fireball model. In particular, observational (model-independent)
constraints on the collimation of the gamma-rays and the dependence of the
collimation angle on the photon energy are considered. The fact that the
threshold for the creation of pairs depends on the angle between
the momenta of the annihilating photons in the GRB source provides an
alternative solution to the compactness problem. A new approach to explaining
GRBs, taking into account the angular dependence for pair creation, is
proposed, and the main features of a scenario describing a GRB source with a
total (photon) energy smaller or of the order of erg are laid out.
Thus, we are dealing with an alternative to an ultra-relativistic fireball, if
it turns out (as follows from observations) that all "long" GRBs are associated
with normal (not peculiar) core-collapse supernovae. The effects of radiation
pressure and the formation of jets as a consequence of even a small amount of
anisotropy in the total radiation field in a (compact) GRB source are examined
in this alternative model. Possible energy release mechanisms acting in regions
smaller or of the order of cm in size (a compact model for a GRB) are
discussed. New observational evidence for such compact energy release in the
burst source is considered.Comment: 15 pages, no figures, no table
Numerical analysis of the resistance behavior of an electrostatically-induced graphene double junction
We present a numerical approach that we have developed in order to reproduce and explain the resistance behavior recently observed, as a function of the backgate voltage and of the position of a biased scanning probe, in a graphene flake in which a double p-n junction has been electrostatically induced. A simplified electrostatic model has been adopted to simulate the effect of gate voltages on the potential landscape, assuming for it a slow variation in space and using a simple capacitive model for the coupling between the electrodes and the graphene sheet. The transport analysis has then been performed with a solution of the Dirac equation in the reciprocal space coupled with a recursive scattering matrix approach. The efficiency of the adopted numerical procedure has allowed us to explore a wide range of possible potential landscapes and bias points, with the result of achieving a good agreement with available experimental data