Residual Spectrum of Power Bounded Operators
The residual spectrum of a power bounded operator lies in the open unit disk.
math.FA↗
arXiv subjects
A. Mello
Publications and source records attributed to A. Mello.
SAMplus: adaptive optics at optical wavelengths for SOAR
Adaptive Optics (AO) is an innovative technique that substantially improves the optical performance of ground-based telescopes. The SOAR Adaptive Module (SAM) is a laser-assisted AO instrument, designed to compensate ground-layer atmospheric turbulence in near-IR and visible wavelengths over a large Field of View. Here we detail our proposal to upgrade SAM, dubbed SAMplus, that is focused on enhancing its performance in visible wavelengths and increasing the instrument reliability. As an illustration, for a seeing of 0.62 arcsec at 500 nm and a typical turbulence profile, current SAM improves the PSF FWHM to 0.40 arcsec, and with the upgrade we expect to deliver images with a FWHM of $\approx0.34$ arcsec -- up to 0.23 arcsec FWHM PSF under good seeing conditions. Such capabilities will be fully integrated with the latest SAM instruments, putting SOAR in an unique position as observatory facility.
astro-ph.IM↗
Power Series Solution to Non-Linear Partial Differential equations of Mathematical Physics
Power Series Solution method has been used traditionally for to solve Linear Differential Equations, in Ordinary and Partial form. But this method has been limited to this kind of problems. We present the solution of problems of Non Linear Partial Differential equations of Physical Mathematical using power series.
cs.SC↗
Magnetic vortex chirality switching in Permalloy nanowires with asymmetric notches
We have investigated the motion of vortex domain walls passing across non symmetric triangular notches in single Permalloy nanowires. We have measured hysteresis cycles using the focused magneto-optical Kerr effect before and beyond the notch, which allowed to probe beyond the notch the occurrence probability of clockwise (CW) and counter-clockwise (CCW) walls in tail-to-tail (TT) and head-to-head (HH) configurations. We present experimental evidence of chirality flipping provided by the vortex -- notch interaction. With a low exit angle the probability of chirality flipping increases and here with the lowest angle of 15$^o$ the probability of propagation of the energetically favored domain wall configuration (CCW for TT or CW for HH walls) is $\approx 75\%$. Micromagnetic simulations reveal details of the chirality reversal dynamics.
cond-mat.mes-hall↗
Interlayer coupling in spin valves studied by broadband ferromagnetic resonance
Magnetization dynamic response of coupled and uncoupled spin valves with structure NiFe(20 nm)/Cu(t_Cu)/NiFe(20 nm)/IrMn(10nm) were probed using broadband ferromagnetic resonance absorption measurements. By varying the Cu thickness t_Cu we were able to tailor the coupling intensity between the free and pinned layers. Broadband spectra show two resonant modes for each measured field. It is noticed that the coupling among NiFe layers modifies the amplitude of the absorption peaks and, moreover, the dispersion relations of the modes, which are distorted specially at the anti-parallel state. A numerical model is proposed and applied to address the underlying physics and the observed properties of these systems.
cond-mat.mes-hall↗
Power Series Method applied to Inverse Analysis in Chemical Kinetics Problem
Power Series Solution Method has been traditionally used to solve Ordinary and Partial Linear Differential Equations. However, despite their usefulness the application of this method has been limited to this particular kind of equations. In this work we use the method of power series to solve nonlinear partial differential equations. The method is applied to solve three versions of nonlinear time-dependent Burgers-Type differential equations in order to demonstrate its scope and applicability.
math.NA↗
Undulation textures at the phase transitions of some alkyloxybenzoic acids
We observed undulated smectic textures for some compounds of the 4,n-alkyloxybenzoic (nOBAC) acid series, at transitions between the smectic and the isotropic phase and between the smectic and nematic phase. Studied compounds were 12OBAC, 16OBAC and a binary mixture of 12- and 16OBAC. The undulations are dressing a usual Schlieren texture. In the case of the binary mixture, an interesting fingerprint pattern is observed too.
cond-mat.soft↗
Growth of toric domains in the smectic phase of oxadiazoles
In a previous paper (Phase Transitions, 80(9), 987, 2007) we discussed the smectic and nematic textures of some mesomorphic oxadiazole compounds with terminal Cl-substituent. Optical microscope investigations showed a very interesting behaviour of smectic and nematic phases; the smectic phase has fan-shaped and toric textures and the nematic phase has spherulitic domains, which disappear as the sample is further heated, the texture changing into a smooth one. Here, we investigate four oxadiazole compounds with the same structure but terminal Br-substituent. The behaviour of the smectic and nematic phases is like that observed in the compounds with Cl. The focus of the paper is on the growth of toric domains from the nematic melt and on the role of defects in the domain structure.
cond-mat.mtrl-sci↗
Fan-shaped and toric textures of mesomorphic oxadiazoles
When a family of non symmetrical heterocycled compounds is investigated, a variety of mesophases can be observed with rather different features. Here we report the behaviour of seven different members among a family of such materials, that consists of mesomorphic oxadiazole compounds. In two of these compounds, the optical microscope investigation shows very interesting behaviours. In their smectic phases, fan-shaped and toric textures, sometimes with periodic instability, are observed. Moreover, the nematic phase displays a texture transition. Texture transitions have been previously observed only inside the nematic phase of some compounds belonging to the families of the oxybenzoic and cyclohexane acids. In these two oxadiazole compounds we can observe what we define as a "toric nematic phase", heating the samples from the smectic phase. The toric nematic texture disappears as the sample is further heated, changing into a smooth texture.
cond-mat.soft↗
Texture transitions in the liquid crystalline alkyloxybenzoic acid 6OBAC
The 4,n-alkyloxybenzoic acid 6OBAC has a very rich variety of crystalline structures and two nematic sub-phases, characterised by different textures. It is a material belonging to a family of liquid crystals formed by hydrogen bonded molecules, the 4,n-alkyloxybenzoic acids indicates the homologue number). The homologues with n ranging from 7 to 13 display both smectic C and N phases. In spite of the absence of a smectic phase, 6OBAC exhibits two sub-phases with different textures, as it happens in other materials of the homologue series which possess the smectic phase. This is the first material that exhibits a texture transition in a nematic phase directly originated from a crystal phase. Here we present the results of an image processing assisted optical investigation to characterise the textures and the transitions between textures. This processing is necessary to discriminate between crystal modifications and nematic sub-phases.
cond-mat.soft↗
Texture transitions in binary mixtures of 6OBAC with compounds of its homologous series
Recently we have observed in compounds of the 4,n-alkyloxybenzoic acid series, with the homologous index n ranging from 6 to 9, a texture transition in the nematic range which subdivides the nematic phase in two sub-phases displaying different textures in polarised light analysis. To investigate a persistence of texture transitions in nematic phases, we prepared binary mixtures of 4,6-alkyloxybenzoic acid (6OBAC) with other members (7-,8-,9-,12-, 16OBAC) of its homologous series. Binary mixtures exhibit a broadening in the temperature ranges of both smectic and nematic phases. A nematic temperature range of 75 C is observed. In the nematic phase, in spite of the microscopic disorder introduced by mixing two components, the polarised light optics analysis of the liquid crystal cells reveals a texture transition. In the case of the binary mixture of 6OBAC with 12OBAC and with 16OBAC, that is of compounds with monomers of rather different lengths, the texture transition temperature is not homogeneous in the cell, probably due to a local variation in the relative concentrations of compounds.
cond-mat.soft↗