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Fernando Moraes

Publications and source records attributed to Fernando Moraes.

At least 37 records · Page 2Linked to original sources
arXiv2017

Using torsion to manipulate spin currents

We address the problem of quantum particles moving on a manifold characterised by the presence of torsion along a preferential axis. In fact, such a torsion may be taylored by the presence of a single screw dislocation, whose Burgers vector measures the torsion amplitude. The problem, first treated in the relativistic limit describing fermions that couple minimally to torsion, is then analysed in the Pauli limit We show that torsion induces a geometric potential and also that it couples generically to the phase of the wave function, giving rise to the possibility of using torsion to manipulate spin currents in the case of spinor wave functions. These results emerge as an alternative strategy for using screw dislocations in the design of spintronic-based devices.

cond-mat.mes-hall↗
arXiv2017

Torsion effects on Condensed Matter: like a magnetic field but not so much

In this work, we study the effects of torsion due to a uniform distribution of topological defects (screw dislocations) on free spin/carrier dynamics in elastic solids. When a particle moves in such a medium, the effect of the torsion associated to the defect distribution is analogous to that of an applied magnetic field but with subtle differences. Analogue Landau levels are present in this system but they cannot be confined to two dimensions. In the case of spinless carriers, zero modes, which do not appear in the magnetic Landau levels, show up for quantized values of the linear momentum projected on the defects axis. Particles with spin are subjected to a Zeeman-like coupling between spin and torsion, which is insensitive to charge. This suggests the possibility of spin resonance experiments without a magnetic field for charged carriers or quasiparticles without electrical charge, like triplet excitons, for instance.

cond-mat.mes-hall↗
arXiv2016

Retrieving the saddle-splay elastic constant $K_{24}$ of nematic liquid crystals from an algebraic approach

The physics of light interference experiments is well established for nematic liquid crystals. Using well-known techniques, it is possible to obtain important quantities, such as the differential scattering cross section and the saddl-splay elastic constant $K_{24}$. However, the usual methods to retrieve the latter involves an adjusting of computational parameters through the visual comparisons between the experimental light interference pattern or a $^{2}H-NMR$ spectral pattern produced by an escaped-radial disclination, and their computational simulation counterparts. To avoid such comparisons, we develop an algebraic method for obtaining of saddle-splay elastic constant $K_{24}$. Considering an escaped-radial disclination inside a capillary tube with radius $R_{0}$ of tens of micrometers, we use a metric approach to study the propagation of the light (in the scalar wave approximation), near to the surface of the tube and to determine the light interference pattern due to the defect. The latter is responsible for the existence of a well-defined interference peak associated to a unique angle $ϕ_{0}$. Since this angle depends on factors such as refractive indexes, curvature elastic constants, anchoring regime, surface anchoring strength and radius $R_{0}$, the measurement of $ϕ_{0}$ from the interference experiments involving two different radii allows us to algebraically retrieve $K_{24}$. Our method allowed us to give the first reported estimation of $K_{24}$ for the lyotropic chromonic liquid crystal Sunset Yellow FCF: $K_{24}=2.1\ pN$.

cond-mat.soft↗
arXiv2016

Modeling Kleinian cosmology with electronic metamaterials

This paper deals with the propagation of Klein-Gordon particles in flat background spacetime exhibiting discontinuous metric changes from a Lorentzian signature (-,+,+,+) to a Kleinian signature (-,+,+,-). A formal analogy with the propagation of electrons at a junction between an anisotropic semiconductor and an electronic metamaterial is presented. From that analogy, we study the dynamics of these particles falling onto planar boundary interfaces between these two families of media and show a mirror-like behavior for the particle flux. Finally, the case of a double junction of finite thickness is examined and the possibility of tunneling through it is discussed. A physical link between the metamaterial and the Kleinian slabs is found by calculating the time of flight of the respective traversing particles.

gr-qc↗
arXiv2016

Generation of optical vorticity from topological defects

The propagation of an electromagnetic wave in a medium with a screw dislocation is studied. Adopting the formalism of differential forms, it is shown that torsion is responsible for quantized modes. Moreover, it is demonstrated that the modes thus obtained have well defined orbital angular momentum, opening the possibility to design liquid-crystal-based optical tweezers.

cond-mat.soft↗
arXiv2016

Thermal diode made by nematic liquid crystal

This work investigates how a thermal diode can be designed from a nematic liquid crystal confined inside a cylindrical capillary. In the case of homeotropic anchoring, a defect structure called escaped radial disclination arises. The asymmetry of such structure causes thermal rectification rates up to 3.5\% at room temperature, comparable to thermal diodes made from carbon nanotubes. Sensitivity of the system with respect the heat power supply, the geometry of the capillary tube and the molecular anchoring angle is also discussed.

cond-mat.soft↗
arXiv2016

Relativistic Quantum Dynamics on a Double Cone

In this paper, we study the relativistic quantum problem of a particle constrained to a double cone surface. For this purpose, we build the Dirac equation in a curved space using the tetrads formalism. Two cases are analysed. First, we consider a free particle on a double cone surface, and then we add an uniform magnetic field. The energy spectrum is obtained and the instability of the motion is discussed. We show that the magnetic field breaks the nappe degeneracy, inducing different energy spectra for each nappe. The results obtained here can be applied, for instance, in he investigation of the electronic and transport properties of condensed matter systems that can be described by an effective Dirac equation, such as graphene and topological insulators.

quant-ph↗
arXiv2016

Geometric effects in the electronic transport of deformed nanotubes

Quasi-two-dimensional systems may exibit curvature, which adds three-dimensional influence to their internal properties. As shown by da Costa \cite{dacosta}, charged particles moving on a curved surface experience a curvature-dependent potential which greatly influence their dynamics. In this paper, we study the electronic ballistic transport in deformed nanotubes. The one-electron Schrödinger equation with open boundary conditions is solved numerically with a flexible MAPLE code made available as Supplementary Data. We find that the curvature of the deformations have indeed strong effects on the electron dynamics suggesting its use in the design of nanotube-based electronic devices.

cond-mat.mes-hall↗
arXiv2015

Optics near an hyperbolic defect

We examine the properties of a new family of defects called hyperbolic disclinations, and discuss their possible use for the design of perfect optical absorbers. In hyperbolic metamaterials, the ratio of ordinary and extraordinary permittivities is negative, which leads to an effective metric of Kleinian signature (two timelike coordinates). Considering a disclination in the hyperbolic nematic host matrix, we show that the timelike geodesics are Poinsot spirals, i.e. whatever the impact parameter of an incident light beam, it is confined and whirls about the defect core. The trapping effect does not require light to be coherent. This property also remains in the wave formalism, which may be the sign for many potential applications.

cond-mat.mes-hall↗
arXiv2010

Flowing Liquid Crystal Simulating the Schwarzschild Metric

We show how to simulate the equatorial section of the Schwarzschild metric through a flowing liquid crystal in its nematic phase. Inside a liquid crystal in the nematic phase, a traveling light ray feels an effective metric, whose properties are linked to perpendicular and parallel refractive indexes, $n_o$ e $n_e$ respectively, of the rod-like molecule of the liquid crystal. As these indexes depend on the scalar order parameter of the liquid crystal, the Beris-Edwards hydrodynamic theory is used to connect the order parameter with the velocity of a liquid crystal flow at each point. This way we calculate a radial velocity profile that simulates the equatorial section of the Schwarzschild metric, in the region outside of Schwarzschild's radius, in the nematic phase of the liquid crystal. In our model, the higher flow velocity can be of the order of some meters per second.

gr-qc↗
arXiv2008

On the deflection of light by topological defects in nematic liquid crystals

The influence of controlable parameters like temperature and wavelength on the trajectories of light in a nematic liquid crystal with topological defects is studied through a geometric model. The model incorporates phenomenological details as how the refractive indices depend on such parameters. The deflection of light by the topological defect is then shown to be greater at lower temperatures and shorter wavelengths.

cond-mat.soft↗
arXiv2007

Exploring NoC Mapping Strategies: An Energy and Timing Aware Technique

Complex applications implemented as Systems on Chip (SoCs) demand extensive use of system level modeling and validation. Their implementation gathers a large number of complex IP cores and advanced interconnection schemes, such as hierarchical bus architectures or networks on chip (NoCs). Modeling applications involves capturing its computation and communication characteristics. Previously proposed communication weighted models (CWM) consider only the application communication aspects. This work proposes a communication dependence and computation model (CDCM) that can simultaneously consider both aspects of an application. It presents a solution to the problem of mapping applications on regular NoCs while considering execution time and energy consumption. The use of CDCM is shown to provide estimated average reductions of 40% in execution time, and 20% in energy consumption, for current technologies.

cs.AR↗
arXiv2007

MultiNoC: A Multiprocessing System Enabled by a Network on Chip

The MultiNoC system implements a programmable on-chip multiprocessing platform built on top of an efficient, low area overhead intra-chip interconnection scheme. The employed interconnection structure is a Network on Chip, or NoC. NoCs are emerging as a viable alternative to increasing demands on interconnection architectures, due to the following characteristics: (i) energy efficiency and reliability; (ii) scalability of bandwidth, when compared to traditional bus architectures; (iii) reusability; (iv) distributed routing decisions. An external host computer feeds MultiNoC with application instructions and data. After this initialization procedure, MultiNoC executes some algorithm. After finishing execution of the algorithm, output data can be read back by the host. Sequential or parallel algorithms conveniently adapted to the MultiNoC structure can be executed. The main motivation to propose this design is to enable the investigation of current trends to increase the number of embedded processors in SoCs, leading to the concept of "sea of processors" systems.

cs.AR↗
arXiv2006

Lensing effects in a nematic liquid crystal with topological defects

Light traveling through a liquid crystal with disclinations perceives a geometrical background which causes lensing effects similar to the ones predicted for cosmic objects like global monopoles and cosmic strings. In this article we explore the effective geometry as perceived by light in such media. The comparison between both systems suggests that experiments can be done in the laboratory to simulate optical properties, like gravitational lensing, of cosmic objects.

cond-mat.soft↗
arXiv2006

Geometric Phases in Graphitic Cones

In this article we use a geometric approach to study geometric phases in graphitic cones. The spinor that describes the low energy states near the Fermi energy acquires a phase when transported around the apex of the cone, as found by a holonomy transformation. This topological result can be viewed as an analogue of the Aharonov-Bohm effect. The topological analysis is extended to a system with $n$ cones, whose resulting configuration is described by an effective defect.

cond-mat.mtrl-sci↗
arXiv2005

Symmetric flows and Darcy's law in Curved Spaces

We consider the problem of existence of certain symmetrical solutions of Stokes equation on a three-dimensional manifold $M$ with a general metric possessing symmetry. These solutions correspond to unidirectional flows. We have been able to determine necessary and sufficient conditions for their existence. Symmetric unidirectional flows are fundamental for deducing the so-called Darcy's law, which is the law governing fluid flow in a Hele-Shaw cell embedded in the environment $M$. Our main interest is to depart from the usual, flat background environment, and consider the possibility of an environment of arbitrary constant curvature $K$ in which a cell is embedded. We generalize Darcy's law for particular models of such spaces obtained from $\real^3$ with a conformal metric. We employ the calculus of differential forms for a simpler and more elegant approach to the problems herein discussed.

math-ph↗