Influence of nearest-neighbor Coulomb interactions on the phase diagram of the ferromagnetic Kondo model

The influence of a nearest-neighbor Coulomb repulsion of strength V on the properties of the ferromagnetic Kondo model is analyzed using computational techniques. The Hamiltonian studied here is defined on a chain using localized S = 1/2 spins, and one orbital per site. Special emphasis is given to the influence of the Coulomb repulsion on the regions of phase separation recently discovered in this family of models, as well as on the double-exchange-induced ferromagnetic ground state. When phase separation dominates at V= 0, the Coulomb interaction breaks the large domains of the two competing phases into small islands of one phase embedded into the other. This is in agreement with several experimental results, as discussed in the text. Vestiges of the original phase separation regime are found in the spin structure factor as incommensurate peaks, even at large values of V. In the ferromagnetic regime close to density n = 0.5, the Coulomb interaction induces tendencies to charge ordering without altering the fully polarized character of the state. This regime of charge-ordered ferromagnetism may be related with experimental observations of a similar phase by Chen and Cheong [Phys. Rev. Lett. 76, 4042 (1996)]. Our results reinforce the recently introduced notion [see, e.g., S. Yunoki et al., Phys. Rev. Lett. 80, 845 (1998)] that in realistic models for manganites analyzed with unbiased many-body techniques, the ground state properties arise from a competition between ferromagnetism and phase-separation - charge-ordering tendencies. ©1999 The American Physical Society.

Liquid-gas phase transition in Bose-Einstein condensates with time evolution

We study the effects of a repulsive three-body interaction on a system of trapped ultracold atoms in a Bose-Einstein condensed state. The stationary solutions of the corresponding s-wave nonlinear Schrödinger equation suggest a scenario of first-order liquid-gas phase transition in the condensed state up to a critical strength of the effective three-body force. The time evolution of the condensate with feeding process and three-body recombination losses has a different characteristic pattern. Also, the decay time of the dense (liquid) phase is longer than expected due to strong oscillations of the mean-squared radius.

Pressure-induced phase transitions in Zr-rich PbZr1-xTixO3 ceramics

A Raman study of structural changes in the Zr-rich PbZr1-x TixO3 (PZT) system under hydrostatic pressures up to 5.0 GPa is presented. We observe that externally applied pressure induces several phase transitions in PZT ceramics among phases with orthorhombic (Ao), rhombohedral low-temperature (RLT), and rhombohedral high-temperature (RHT) symmetries (all found in PZT at ambient pressure and room temperature). Each of the compositions investigated (0.02 ≤ x ≤ 0.14) exhibits a high-pressure phase with orthorhombic (OI′) symmetry. We further report a detailed study of the pressure dependence of Raman frequencies to elucidate the phase transitions and to provide a set of pressure coefficients for the high-pressure phases.

Raman scattering study of the PbZr1-xTixO3 system: Rhombohedral-monoclinic-tetragonal phase transitions

In this paper we present a Raman-scattering study of the phase transitions in the PbZr1-xTixO3 systems around the morphotropic phase boundary over a wide temperature range. The boundary between rhombohedral and monoclinic phases was found to be a quasivertical line between x = 0.46 and x = 0.47. We also studied the monoclinic-tetragonal phase boundary and our spectroscopic results agree very well with those reported by using x-ray diffraction.

Small-angle x-ray scattering study of the structural evolution of the drying of sonogels with the liquid phase exchanged by acetone

The structural evolution on the drying of wet sonogels of silica with the liquid phase exchanged by acetone, obtained from tetraethoxisilane sonohydrolysis, was studied in situ by small-angle x-ray scattering (SAXS). The periods associated to the structural evolution as determined by SAXS are in agreement with those classical ones established on basis of the features of the evaporation rate of the liquid phase in the obtaining of xerogels. The wet gel can be described as formed by primary particles (microclusters), with characteristic length a ∼ 0.67 nm and surface which is fractal, linking together to form mass fractal structures with mass fractal dimension D=2.24 in a length scale ξ∼6.7 nm. As the network collapses while the liquid/vapor meniscus is kept out of the gel volume, the mass fractal structure becomes more compacted by increasing D and decreasing ξ, with smoothing of the fractal surface of the microclusters. The time evolution of the density of the wet gels was evaluated exclusively from the SAXS parameters ξ, D, and a. The final dried acetone-exchanged gel presents Porod's inhomogeneity length of about 2.8 nm and apparently exhibits an interesting singularity D →3, as determined by the mass fractal modeling used to fit the SAXS intensity data for the obtaining of the parameters ξ and D.

Enhanced multi-wavelength holographic profilometry by laser mode selection

The application of multi-wavelength holography for surface shape measurement is presented. In our holographic setup a Bi12TiO 20 (BTO) photorefractive crystal was the holographic recording medium and a multimode diode laser emitting in the red region was the light source in a two-wave mixing scheme. The holographic imaging with multimode lasers results in multiple holograms in the BTO. By employing such lasers the resulting holographic image appears covered of interference fringes corresponding to the object relief and the interferogram spatial frequency is proportional to the diode laser free spectral range (FSR). We used a Fabry-Perot étalon at the laser output for laser mode selection. Thus, larger effective values of the laser FSR were achieved, leading to higher-spatial frequency interferograms and therefore to more sensitive and accurate measurements. The quantitative evaluation of the interferograms was performed through the phase stepping technique (PST) and the phase map unwrapping was carried out through the Cellular-Automata method. For a given surface, shape measurements with different interferogram spatial frequencies were performed and compared, concerning measurement noise and visual inspection.

Visualizing the ac magnetic susceptibility of superconducting films via magneto-optical imaging

We have established a link between the global ac response and the local flux distribution of superconducting films by combining magnetic ac susceptibility, dc magnetization, and magneto-optical measurements. The investigated samples are three Nb films: a plain specimen, used as a reference sample, and other two films patterned with square arrays of antidots. At low temperatures and small ac amplitudes of the excitation field, the Meissner screening prevents penetration of flux into the sample. Above a certain ac drive threshold, flux avalanches are triggered during the first cycle of the ac excitation. The subsequent periodic removal, inversion, and rise of flux occurs essentially through the already-created dendrites, giving rise to an ac susceptibility signal weakly dependent on the applied field. The intradendrite flux oscillation is followed, at higher values of the excitation field, by a more drastic process consisting of creation of new dendrites and antidendrites. In this more invasive regime, the ac susceptibility shows a clear field dependence. At higher temperatures a smooth penetration occurs, and the flux profile is characteristic of a critical state. We have also shown that the regime dominated by vortex avalanches can be reliably identified by ac susceptibility measurements. © 2011 American Physical Society.

Lamb mode diversity imaging for non-destructive testing of plate-like structures

Several Lamb wave modes can be coupled to a particular structure, depending on its geometry and transducer used to generate the guided waves. Each Lamb mode interacts in a particular form with different types of defects, like notches, delamination, surface defects, resulting in different information which can be used to improve damage detection and characterization. An image compounding technique that uses the information obtained from different propagation modes of Lamb waves for non-destructive testing of plate-like structures is proposed. A linear array consisting of 16 piezoelectric elements is attached to a 1 mm thickness aluminum plate, coupling the fundamental A0 and S0 modes at the frequencies of 100 kHz and 360 kHz, respectively. For each mode two images are obtained from amplitude and phase information: one image using the Total Focusing Method (TFM) and one phase image obtained from the Sign Coherence Factor (SCF). Each TFM image is multiplied by the SCF image of the respective mode to improve contrast and reduce side and grating lobes effects. The high dispersive characteristic of the A0 mode is compensated for adequate defect detection. The information in the SCF images is used to select one of the TFM mode images, at each pixel, to obtain the compounded image. As a result, dead zone is reduced, resolution and contrast are improved, enhancing damage detection when compared to the use of only one mode. © 2013 Elsevier Ltd.

Dynamical properties for a mixed Fermi accelerator model

The behavior of the decay of velocity in a semi-dissipative one-dimensional Fermi accelerator model is considered. Two different kinds of dissipative forces were considered: (i) F-v and; (ii) F-v2. We prove the decay of velocity is linear for (i) and exponential for (ii). During the decay, the particles move along specific corridors which are constructed by the borders of the stable manifolds of saddle points. These corridors organize themselves in a very complicated way in the phase space leading the basin of attraction of the sinks to be seemingly of fractal type. © 2013 Elsevier B.V. All rights reserved.

Bilhares dependentes do tempo: um mecanismo para suprimir aceleração de Fermi

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Comportamento dinâmico não-linear de um sistema mecânico com vibrações associadas a uma transição brusca na rigidez

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

CRS seismic data imaging system: a case study for basib reevaluation

Este trabalho apresenta resultados práticos de uma atenção sistemática dada ao processamento e à interpretação sísmica de algumas linhas terrestres do conjunto de dados do gráben do Tacutu (Brasil), sobre os quais foram aplicadas etapas fundamentais do sistema WIT de imageamento do empilhamento CRS (Superfície de Reflexão Comum) vinculado a dados. Como resultado, esperamos estabelecer um fluxograma para a reavaliação sísmica de bacias sedimentares. Fundamentado nos atributos de frente de onda resultantes do empilhamento CRS, um macro-modelo suave de velocidades foi obtido através de inversão tomográfica. Usando este macro-modelo, foi realizado uma migração à profundidade pré- e pós-empilhamento. Além disso, outras técnicas baseadas no empilhamento CRS foram realizadas em paralelo como correção estática residual e migração de abertura-limitada baseada na zona de Fresnel projetada. Uma interpretação geológica sobre as seções empilhadas e migradas foi esboçada. A partir dos detalhes visuais dos painéis é possível interpretar desconformidades, afinamentos, um anticlinal principal falhado com conjuntos de horstes e grábens. Também, uma parte da linha selecionada precisa de processamento mais detalhado para evidenciar melhor qualquer estrutura presente na subsuperfície.

Comparação dos filtros de velocidade e do operador WHLP-CRS na atenuação de múltiplas

A motivação geológica deste trabalho reside no imageamento de estruturas de bacias sedimentares da região Amazônica, onde a geração e o acúmulo de hidrocarboneto estão relacionados com a presença de soleiras de diabásio. A motivação sísmica reside no fato de que essas rochas intrusivas possuem grandes contrastes de impedância com a rocha encaixante, o que resulta em múltiplas, externas e internas, com amplitudes semelhantes as das primárias. O sinal sísmico das múltiplas podem predominar sobre o sinal das reflexões primárias oriundas de interfaces mais profundas, o que pode dificultar o processamento, a interpretação e o imageamento da seção sísmica temporal. Neste trabalho, estudamos a atenuação de múltiplas em seções sintéticas fonte-comum (FC) através da comparação de dois métodos. O primeiro método resulta da combinação das técnicas Wiener-Hopf-Levinson de predição (WHLP) e o de empilhamento superfície-de-reflexão-comum (CRS), e denominando WHLP-CRS, onde o operador é desenhado exclusivamente no domínio do tempo-espaço. O segundo método utilizado é o filtro de velocidade (ω-k) aplicado após o empilhamento superfície-de-reflexão (CRS), onde o operador é desenhado exclusivamente no domínio bidimensional de freqüência temporal-espacial. A identificação das múltiplas é feita na seção de afastamento-nulo (AN) simulada com o empilhamento CRS, e utiliza o critério da periodicidade entre primária e suas múltiplas. Os atributos da frente de onda, obtidos através do empilhamento CRS, são utilizados na definição de janelas móveis no domínio tempo-espaço, que são usadas para calcular o operador WHLP-CRS. O cálculo do filtroω-k é realizado no domínio da freqüência temporal-espacial, onde os eventos são selecionados para corte ou passagem. O filtro (ω-k) é classificado como filtro de corte, com alteração de amplitude, mas não de fase, e limites práticos são impostos pela amostragem tempo-espaço. Em termos práticos, concluímos que, para o caso de múltiplas, os eventos separados no domínio x-t não necessariamente se separam no domínio ω-k, o que dificulta o desenho de um operador ω-k semelhante em performance ao operador x-t.

Estudo geofísico para a caracterização geológica e hidrogeológica da subsuperfície rasa em área de deposição de rejeitos sólidos no município de Barcarena-PA

Este estudo foi realizado na Área de Depósito de Rejeitos Sólidos (ADRS) da ALBRAS (Alumínio Brasileiro S.A.), localizada no município de Barcarena, Estado do Pará. Os resultados alcançados permitiram estabelecer um padrão de caracterização da subsuperfície, a partir de medidas geofísicas, fornecendo informações sobre as camadas geológicas e sobre parâmetros hidrogeológicos, tais como direção e velocidade do fluxo subterrâneo. Na realização do trabalho foram utilizados três métodos geofísicos: os métodos elétricos (Potencial Espontâneo e Eletrorresitividade) e eletromagnético (Slingram). A distribuição dos potenciais medidos através do Método do Potencial Espontâneo indicaram o sentido do fluxo subterrâneo local. Durante o estudo, foi montado um experimento de infiltração de solução salina no solo e seu efeito foi monitorado através de imageamento elétrico. Esse experimento permitiu que se estimasse a velocidade e o sentido do fluxo subterrâneo local ao longo da linha de imageamento. Os modelos interpretativos obtidos através da inversão de dados de resistividade aparente obtidos em sondagens elétricas verticais apresentaram uma boa correlação com perfis de raios gama corridos em poços tubulares da área de estudo. Estas Sondagens Elétricas Verticais permitiram que se detalhasse as unidades litológicas rasas através da estimativa de seus valores de resistividade e espessura. A interpretação dos dados eletromagnéticos através da análise de perfis de medidas, sondagens eletromagnéticas e mapas de contornos permitiu identificar a presença de zonas mais condutivas (material argiloso) e zonas mais resistivas (material arenoso). Além disso, observou-se que o mapa de contornos para a freqüência de 14080 Hz apresentou uma boa correlação com o mapa de contornos do Potencial Espontâneo referente a mesma área.

The use of instantaneous phase for improving sparse arrays images

Sparse arrays have pitch larger than half-wavelength (lambda/2) and there is a reduced number of elements in comparison with a full-populated array. Consequently, there is a reduction in cost, data acquisition and processing. However, conventional beamforming techniques result in large side and grating lobes, and consequently in image artifacts. In this work the instantaneous phase of the signals is used in a beamforming technique instead of the instantaneous amplitudes to improve images obtained from sparse arrays configurations. A threshold based on a statistical analysis and the number of signals used for imaging is applied to each pixel, in order to determine if that pixel is related to a defect or not. Three sets of data are used to evaluate the technique, considering medical and non-destructive testing: a simulated point spread function, a medical phantom and an aluminum plate with 2 lambda-, 7 lambda- and lambda-pitch, respectively. The conventional amplitude image is superposed by the image improved by the instantaneous phase, increasing the reflectors detectability and reducing artifacts for all cases, as well as dead zone for the tested plate.