arXiv subjects

L. Rosales

Publications and source records attributed to L. Rosales.

29 records · Page 2Linked to original sources

arXiv2011

Self-similar and charged spheres in the free-streaming approximation

We evolve nonadiabatic charged spherical distributions of matter. Dissipation is described by the free-streaming approximation. We match a self-similar interior solution with the Reissner-Nordström-Vaidya exterior solution. The transport mechanism is decisive to the fate of the gravitational collapse. Almost a half of the total initial mass is radiated away. The transport mechanism determines the way in which the electric charge is redistributed.

gr-qc↗

arXiv2010

Nonadiabatic charged spherical evolution in the postquasistatic approximation

We apply the postquasistatic approximation, an iterative method for the evolution of self-gravitating spheres of matter, to study the evolution of dissipative and electrically charged distributions in General Relativity. We evolve nonadiabatic distributions assuming an equation of state that accounts for the anisotropy induced by the electric charge. Dissipation is described by streaming out or diffusion approximations. We match the interior solution, in noncomoving coordinates, with the Vaidya-Reissner-Nordström exterior solution. Two models are considered: i) a Schwarzschild-like shell in the diffusion limit; ii) a Schwarzschild-like interior in the free streaming limit. These toy models tell us something about the nature of the dissipative and electrically charged collapse. Diffusion stabilizes the gravitational collapse producing a spherical shell whose contraction is halted in a short characteristic hydrodynamic time. The streaming out radiation provides a more efficient mechanism for emission of energy, redistributing the electric charge on the whole sphere, while the distribution collapses indefinitely with a longer hydrodynamic time scale.

gr-qc↗

arXiv2010

Bound states in the continuum in graphene quantum dot structures

The existence of bound states in the continuum was predicted at the dawn of quantum mechanics by von Neumann and Wigner. In this work we discuss the mechanism of formation of these exotic states and the feasibility to observe them experimentally in symmetrical heterostructures composed by segments of graphene ribbons with different widths forming a graphene quantum dot. We identify the existence of bound states in the continuum in these graphene quantum dot systems by means of local density of states and electronic conductance calculations.

cond-mat.mes-hall↗

arXiv2010

Heat flow in the postquasistatic approximation

We apply the postquasistatic approximation to study the evolution of spherically symmetric fluid distributions undergoing dissipation in the form of radial heat flow. For a model which corresponds to an incompressible fluid departing from the static equilibrium, it is not possible to go far from the initial state after the emission of a small amount of energy. Initially collapsing distributions of matter are not permitted. Emission of energy can be considered as a mechanism to avoid the collapse. If the distribution collapses initially and emits one hundredth of the initial mass only the outermost layers evolve. For a model which corresponds to a highly compressed Fermi gas, only the outermost shell can evolve with a shorter hydrodynamic time scale.

gr-qc↗

arXiv2010

Shear Viscosity in the Post-quasistatic Approximation

We apply the post-quasi--static approximation, an iterative method for the evolution of self-gravitating spheres of matter, to study the evolution of anisotropic non-adiabatic radiating and dissipative distributions in General Relativity. Dissipation is described by viscosity and free-streaming radiation, assuming an equation of state to model anisotropy induced by the shear viscosity. We match the interior solution, in non-comoving coordinates, with the Vaidya exterior solution. Two simple models are presented, based on the Schwarzschild and Tolman VI solutions, in the non--adiabatic and adiabatic limit. In both cases the eventual collapse or expansion of the distribution is mainly controlled by the anisotropy induced by the viscosity.

gr-qc↗

arXiv2007

Transport properties of graphene nanoribbon heterostructures

We study the electronic and transport properties of heterostructures formed by armchair graphene nanoribbons with intersections of finite length. We describe the system by a tight-binding model and calculate the density of states and the conductance within the Green's function formalism based on real-space renormalization techniques. We show the apparition of interface states and bound states in the continuum which present a strong dependence of the heterostructure geometry. We investigate the effects on the conductance of an external perturbation applied on the edges atoms of the intersection region.

cond-mat.mes-hall↗

arXiv2007

Transport modulation of graphene nanoribbons with side-attached organic molecules

In this work we address the effects on the conductance of graphene nanoribbons (GNRs) at which organic molecules are side-attached on the ribbon ends. For simplicity, only armchair (AGNRs) and zigzag (ZGNRs) nanoribbons are considered and quasi one-dimensional molecules, such as linear poly-aromatic hydrocarbon (LPHC) and poly(para-phenylene), are chosen. The conductance of the GNRs exhibit a particular behavior as a function of the length of the organic molecules: the energy spectrum of the quasi one-dimensional system is clearly reflected in the conductance curves of the GNRs. The results suggest that GNRs can be used as an spectrograph-sensor device. An even-odd parity effect, as a function of the length of the attached molecules, can be observed in the conductance of these system. The nanostructures are described using a single-band tight binding Hamiltonian and the electronic conductance and the density of states of the systems are calculated within the Green's function formalism based on real-space renormalization techniques.

cond-mat.mes-hall↗

arXiv2006

Self--similar and charged radiating spheres: An anisotropic approach

Considering charged fluid spheres as anisotropic sources and the diffusion limit as the transport mechanism, we suppose that the inner space--time admits self--similarity. Matching the interior solution with the Reissner--Nordström--Vaidya exterior one, we find an extremely compact and oscillatory final state with a redistribution of the electric charge function and non zero pressure profiles.

gr-qc↗

arXiv2006

Magnetic-field effects on transport in carbon nanotube junctions

Here we address a theoretical study on the behaviour of electronic states of heterojunctions and quantum dots based on carbon nanotubes under magnetic fields. Emphasis is put on the analysis of the local density of states, the conductance, and on the characteristic curves of current versus voltage. The heterostructures are modeled by joining zigzag tubes through single pentagon-heptagon pair defects, and described within a simple tight binding calculation. The conductance is calculated using the Landauer formula in the Green functions formalism. The used theoretical approach incorporates the atomic details of the topological defects by performing an energy relaxation via Monte Carlo calculation. The effect of a magnetic field on the conductance gap of the system is investigated and compared to those of isolated constituent tubes. It is found that the conductance gap of the studied CNHs exhibits oscillations as a function of the magnetic flux. However, unlike the pristine tubes case, they are not Aharonov-Bohm periodic oscillations.

cond-mat.mes-hall↗

arXiv2005

Equation of state and transport processes in self--similar spheres

We study the effect of transport processes (diffusion and free--streaming) on a collapsing spherically symmetric distribution of matter in a self--similar space--time. A very simple solution shows interesting features when it is matched with the Vaidya exterior solution. In the mixed case (diffusion and free--streaming), we find a barotropic equation of state in the stationary regime. In the diffusion approximation the gravitational potential at the surface is always constant; if we perturb the stationary state, the system is very stable, recovering the barotropic equation of state as time progresses. In the free--streaming case the self--similar evolution is stationary but with a non--barotropic equation of state.

gr-qc↗

arXiv2004

The 3-dimensional Einstein-Klein-Gordon system in characteristic numerical relativity

We incorporate a massless scalar field into a 3-dimensional code for the characteristic evolution of the gravitational field. The extended 3-dimensional code for the Einstein--Klein--Gordon system is calibrated to be second order convergent. It provides an accurate calculation of the gravitational and scalar radiation at infinity. As an application, we simulate the fully nonlinear evolution of an asymmetric scalar pulse of ingoing radiation propagating toward an interior Schwarzschild black hole and compute the backscattered scalar and gravitational outgoing radiation patterns. The amplitudes of the scalar and gravitational outgoing radiation modes exhibit the predicted power law scaling with respect to the amplitude of the initial data. For the scattering of an axisymmetric scalar field, the final ring down matches the complex frequency calculated perturbatively for the $\ell=2$ quasinormal mode.

gr-qc↗