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Areas of Concentration/Áreas de Concentração
Physics applied to Natural and Social Systems
Description: Nowadays, there is a growing application of concepts and techniques typical of Physics in the study of different situations involving pure and applied sciences. In particular, various researches related to natural and social systems have employed concepts of probability and statistics common in investigations of statistical mechanics. In these studies, models have been analyzed and data have been considered in the direction of increasing our understanding of a wide range of systems. An example is the use of experimental and theoretical aspects related to usual and anomalous diffusive processes when researching natural and social systems. In the study of anomalous diffusion, from a theoretical point of view, it is common to consider equations that, in some sense, generalize those used for the usual diffusion. These investigative aspects are part of our efforts.
Generalized Statistical Mechanics
Description: The Boltzmann-Gibbs statistical mechanics (usual) is especially useful to describe systems that involve aspects such as short-range interactions and short-term memory. If facets such as these are violated, the success of the usual statistical mechanics application is compromised. In such situations, some sort of generalized statistical mechanics may be considered. In this sense, generalizations of the most diverse results in the usual statistical mechanics have been revisited considering the use of entropies generalizing that of Shannon. In particular, many efforts have been directed to the use of Tsallis entropy and related aspects. Moreover, several of these generalizations have been the object of comparison with experimental data and simulations. This investigative scenario is part of our interests.
Complex Systems
Description: In general, complex systems are composed of many interacting parts, presenting collective patterns that could hardly be inferred from their interactions. In addition, non-linear interaction between the parties, emergency of patterns, adaptability, self-organization and unpredictability have been commonly related to these systems. It should also be noted that several investigations aimed at complex systems indicate that they may have common features to those involved in phase transitions. Universality and scale invariance are examples of these aspects. The various studies on complex systems involve broad theoretical tools. A non-exhaustive list of such tools includes aspects of nonlinear dynamics, stochastic dynamics, time series, fractals, and complex networks. Studying all types of complex systems is one of our goals.
Photoacoustic Spectroscopy
Description: Study and development of techniques to analyze materials with spectroscopy, with emphasis on UV-VIS and IV absorption bands close to various materials such as polymers, glass, organic materials, whether impregnated or doped.
Phototermal Spectroscopy
Description: Study and development of techniques to analyze materials with spectroscopy thermooptically and thermal variation of the refractive index, focusing on various materials such as polymers, glass, biological materials, biodiesel, pharmaceuticals, among others.
Infrared Spectroscopy
Description: Study of integrated emittance in the infrared range of 4 – 14μm for the study of integrated radiance polymers, ceramics and composite materials which are polystyrene-based, either doped or codoped. Measurement of emissivity integrated by lock-in radiometer with heating via Halogen or Xenon arc lamp.
Luminescence and solid spectroscopy
Description: Preparation of polymer blends such as PC/PMMA, TEG and PET/ZnO, DTEG and doping with rare earth, measurements of thermal and spectroscopic properties of absorption and emission, study of the doping and co-doping effect in the emission.
Optical and Spectroscopic Properties of Materials
Description: Study of the absorption spectroscopy in soils and its thermal properties, optical study by visible reflectance and thermophysical measurements, characterization of bioactive composites of hydroxyapatite powder / niobium pentoxide, diffusivity and thermal conductivity.
Thermal Properties
Description: Measurements of thermal analysis (DSC / DTA / TGA), calorimetry and thermal diffusivity as a function of temperature to determine phase transitions in materials. They are mainly applied in studies of structural phase transitions, glass transitions, crystallization and decomposition or degradation of biomaterials.
Thermo-optical Properties
Description: Study of the optical path variation of transparent or semi-transparent materials as a function of temperature and / or composition. This property depends on the refractive index and its thermal variation, as well as the dimensional change of the material, which can thus be used to determine phase transitions, sintering effects, and doping and codoping.
Densitometry and Rheology
Description: The density (ρ) is an important thermodynamic parameter and reflects changes in the molecular structure (or micelle), in particular in the area near a phase transition. The density is discontinuous in a first order transition and changes continuously (the second order or continuous transition). The thermal expansion coefficient (β) is an important parameter in this research line, obtained from the density data. The critical exponent reflecting the progress of these parameters near the transitions of second-order nature (or weakly of first-order) finds resonance in the context of critical phenomena theories. Rheology is a line of research (not restricted to liquid crystals) and has attracted the interest of researchers from other fields.
Order and non-linear Parameters
Description: The order parameter (S) of uniaxial nematic phases can be determined by means of absorption coefficients and refractive indices or directly (NMR experiment). The parallel absorption coefficients (kII) and perpendicular (k /) to the direction of polarization of the incident light is obtained through the respective optical transmittance via spectrophotometry. The interest in this study is directed to the field of nematic phases (lyotropic and thermotropic) and the vicinity of their transitions to the isotropic ones. The Z scan technique targets the study and determination of parameters associated with non-linear optics, in particular, the nonlinear refractive index (n2) of different materials. In the liquid-crystal systems, a problem has been observed in the behavior of n2 in the nematic lyotropic phase (between two isotropic phases) compared to n2 of a typical nematic thermotropic one. This research still needs to be further investigated.
Electrical Properties
Description: The impedance spectroscopy is a powerful technique aimed at the study and determination of electrical properties of solid materials, liquid crystals and other complex flows. The characterization of a transition between nematic thermotropic phases is underway via the behavior of electrical parameters derived from the potentiality of the aforementioned technique.
Optical properties of uniaxial and biaxial phases
Description: The description of uniaxial and biaxial liquid crystal phases, from the point of view of linear optics, requires measures of two indices of extraordinary and ordinary refraction (uniaxial phase) and three indices in the biaxial case. This line of research performs, at first, the characterization of liquid-crystalline textures using polarized light on the optical microscope (image processing and birefringence optical accessories) and high-resolution refractometer. In this experimental context, two refraction indices of the uniaxial / biaxial medium are determined and the third (biaxial phase) index can be obtained by theoretical models of phase transition. The layers mentioned above can be characterized from the point of view of the optical conoscopy. For this purpose, an Amici-Bertrand lens is positioned in the optical microscope system. The characteristics and behavior of interference figures (conoscopic images) observed through the eyepiece (microscope) are usually used in the identification of uniaxiality / biaxility of these materials and related fields.
Thermo-optical Characterization and Material Mechanics
Description: The characterization of physical properties of materials attracts broad interest from the industry and in the development of new materials as well. An example is the characterization of luminescence property of glasses and crystals in the production of lasers and white light. Thermal and mechanical properties such as diffusivity, thermal expansion, stress effects also attract interest in various areas of science. Our group has been working on the development of various techniques used in characterizing thermo, optical and mechanical transparent and opaque solids. Among these techniques, we highlight the heat mirror technique introduced by the group and which has been improved both from a theoretical and experimental point of view (Applied Physics Letters 91, 191908, 2007, Applied Physics Letters 92, 131903, 2008, Journal of Applied Physics 104, 053 520, 2008 J. opt. Soc. Am. B 28, 1735, 2011). In this same sense, we have developed the time-resolved mirage effect technique (Applied Physics Letters 100, 091908, 2012, J. Appl. Phys. 111, 093502 (2012)). These techniques have been applied in the characterization of different materials, such as the study of resonant absorption of excited states and relaxation processes in aluminum silicate glasses doped with Tb3 + (optics Letters 38, 4667, 2013), the laser-induced optical cooling (Applied Physics Letters 102, 141910, 2013) and optical materials in general (Optical Materials 35, 1129, 2013).
Effects of radiation pressure
Description: The deformation of the surface produced by radiation pressure forces and electrostriction has been controversial for more than a century. Commonly accepted theories proposed by Minkowski and Abraham for the energy-momentum tensor lead to different results if you do not have the correct interpretation for the moment of the photon in dielectric media. We have worked on numerical simulation to identify the effects separately assuming non-absorbing and absorbing dielectric media. The thermos-elastic deformation equations are solved numerically and surface deformation profiles are obtained. Finally, a pump-probe method is proposed to detect the radiation pressure forces and electrostriction in liquid dielectrics and transparent solids (Applied Physics Letters 102, 231903, 2013). The effects of the interaction of light with matter have applications in areas of manipulating molecules and atoms by optical tweezers aiming at the science of Nano materials and biological systems. In this sense, we are interested in the theoretical-numerical and experimental points of view when understanding and detecting radiation pressure effects. Extension for the studies with beams presenting orbital angular momentum are also open and are a potential wide field of application.
Mathematical Modelling of the light-matter interaction effect applied to experimental techniques
Description: One of the features that led to the formation of the group is the engagement of theoretical treatment with experimental results, which has become a differential over the last years. The theoretical description of the thermoelastic effects induced by the interaction of laser light with matter and the consequent detection using photothermal techniques such as Thermal Lens, Thermal Mirror and Mirage Effect has allowed a qualitative advance in the characterization of physical properties of materials. The models used in these techniques employed a large number of approaches that were often difficult to be experimentally satisfied. The effect as thermal coupling between the sample and fluid adjacent (Journal of Applied Physics 107, 053104, 2010, Journal of Applied Physics 107, 083512, 2010, Applied Spectroscopy 65, 99, 2011, Applied Spectroscopy 66, 1461, 2012), the effect of finite sample sizes (J. opt. Soc. Am. B 28, 1735, 2011), the correct description of the variation of the optical path induced by thermoelastic effects (J. opt. Soc. Am. B. 29, 1772, 2012, J. opt. Soc. Am. B., 29 3355, 2012), population lens effect in the thermal lens (Optics Letters 38, 422, 2013) are examples of results obtained recently. In this direction, the group has a permanent interest in the theoretical treatment of experimental challenges that arise during the development of the various experimental techniques.
Nanoparticle generation by laser ablation
Description: Laser Ablation is the process of removing material from a solid surface using irradiation with a laser beam. At low laser flux, the material is heated by the absorbed laser energy and evaporates. At high laser flux, the material is typically converted to a plasma. Usually, laser ablation refers to removing material with a pulsed laser, but it is possible to ablate material with a continuous wave laser beam if the laser intensity is high enough. The depth over which the laser energy is absorbed, and thus the amount of material removed by a single laser pulse, depends on the material’s optical properties and the laser wavelength and pulse length. Laser pulses can vary over a very wide range of duration (milliseconds to femtoseconds) and fluxes, and can be precisely controlled. This makes laser ablation very valuable for both research and industrial applications. One application is the colloidal solution synthesis of nanoparticles in a variety of solvents. In this method, nanoparticles are generated during condensation of a plasma column formed by the laser ablation of a metal plate immersed in a liquid solution. This process has emerged as a reliable alternative to traditional methods of chemical reduction to obtain nanoparticles of noble metals. This technique is also considered to be a green technique to synthesize nanoparticles in water or organic solvents because they do not require chemical stabilizers. Our laboratory has the necessary equipment to generate, characterize and study the applications of these nanoparticles.
Photochemical reactions and laser-induced mass diffusion
Description: Photodegradation is the alteration of a molecule caused by the absorption of photons, particularly in wavelengths found in sunlight, such as infrared radiation, visible light and ultraviolet light. Photodegradation includes photodissociation and irreversible change of a molecule shape, such as protein denaturation and the addition of other atoms or molecules. A common photodegradation reaction is oxidation, which is used to treat water and pollutants. The study of photobleaching induced by the interaction of light with matter has applications in many areas of science, ranging from physical-chemical analysis to biological systems. The extensive use of laser has been advantageous due to the high quality of the beam, with well-defined wavelength, intensity and spatial distribution. However, in the case of fluids, the localized excitation may induce additional effects as mass diffusion and thermophoresis. We have applied the thermal lens technique in mode-mismatched configuration, monitoring the signal behavior with the excitation laser switched on and off so as to identify and quantify laser induced chemical reaction in aqueous ionic solutions of Fe (II) –TPTZ, and in the study of degradation and fossil fuels (J. Phys. Chem. B 115, 9417, 2011, Applied Physics Letters 95, 191902, 2009, Optics Letters 34, 3460 2009). We have also developed a method to detect and discriminate the contribution of the Soret effect and photoreaction for the thermal lens technique. (Optics Express 19, 4047, 2011). We are interested in the theoretical description of these effects and possible applications in experimental techniques that allow the monitoring of the dynamics and scale of such processes.
Structural and Magnetic Characterization of Materials
Description: Multiple application of techniques for the magnetic and structural characterization of synthetic material (cited above), including steel and nuclear fuel, as well as soils and minerals. The Mössbauer spectroscopy on 57Fe, 151Eu and 155Gd isotopes is widely applied in the geometry of transmission and of the conversion electrons as well (57Fe). The X-ray diffractometry is used in Bragg-Brentano geometry and with beam in oblique angle, when obtaining crystallographic parameters. Refinements by Rietveld method are usually conducted to characterize phases. For the analysis of the magnetic properties, vibrating sample, extraction and SQUID magnetometers are employed. For all the techniques, it is common to use cryogenics practices, with measurements at low temperatures. Thermal analysis and calorimetry are also widely used procedures. Electron microscopy – scanning and transmission – and neutron diffraction are auxiliary characterizations.
Characterization of nuclear materials
Description: Ceramic nuclear (for power plants) and metal (for propulsion reactors) fuels are characterized. They are uranium-based compounds such as (U, Gd) O2 and U-Zr-M (M = Mo, Nb and Gd), studied in conjunction with the Marine Technology Center. Maraging steels are also analyzed (and thermally processed) to be used in super-centrifuges. Here the precipitates formed by “aging” and the magnetic properties of steel are identified.
Studies of magnetic compounds
Description: Preparation and characterization of synthetic compounds such as spinels, garnets, catalysts, perovskites, diluted magnetic semiconductors, sillenites and pyrochlores. Hyperfine and magnetic properties are measured, which are correlated with the structural properties of the compounds. Phenomena such as magnetic orders, critical temperatures, geometric frustration, spin re-orientation, exchange bias and orbital orientation are the object of study.
Material Synthesis by various techniques
Description: To use different synthesis methods such as solid-state reaction, high-energy grinding, arc and induction melting, sol-gel reaction, lyophilization, thermal treatments, irradiation with energetic ions and other processes when preparing ceramics and metal alloys with unique structural and magnetic properties. Semiconductors, thin films, oxidized surfaces, amorphous metals, extended solid solutions, nanoparticles, quasicrystals and granular materials are also prepared.
Development of multifunctional devices
Description: The magnetoelectric (ME) multiferroics represent a class of materials that provides mechanisms for potential applications in multifunction devices such as multi-state memories, magnetoelectric field sensors, transducers for “Energy Harvesting”, tunable solid-state transformers, and tunable microwave devices.
Research of structural and microstructural properties of monolithic polycrystalline multiferroics
Description: Multiferroics are defined as materials exhibiting ferromagnetism (or antiferromagnetism) and ferroelectricity (or antiferroelectricity) at the same temperature and pressure. As most ferroelectric materials are also ferroelastic (they have a hysterical stress-strain behavior), the label “multi-iron” usually includes three coupled order parameters. In this line of research, we study the relationship between structural and microstructural properties of materials and their multiferroic properties.
Research on ferroic (ferroelectric, dielectric, piezoelectric, magnetostrictive, electrostrictive, magnetic and pyroelectric) properties of monolithic polycrystalline multiferroics
Description: From a fundamental point of view, a magnetoelectric response is characteristic of either a change in electrical polarization in response to the application of an external magnetic field or a change of magnetization in response to an external electric field. These materials provide an effective conversion between the energy stored in the electric and magnetic fields. Thus, in this research line, we study the ferroic responses of different materials in order to understand their multiferroic properties.
Simulation and optimization of physical structures and properties of materials using ab initio calculations
Description: In this research line, we conduct structural refinements by Rietveld method using conventional X-ray diffraction data and synchrotron and neutron diffraction. Through the results obtained in the refinements, we perform electron density calculations and associate them to the multiferroic properties of the materials studied. In order to obtain a more precise analysis of structural and electronic behavior of these materials, we also carry out ab initio calculations via Density Functional Theory.
Synthesis of (magnetoelectric, magneto-elastic, electro-elastic, piezoelectric and piezomagnetic) monolithic polycrystalline multiferroics
Description: In this research line, we study different routes of synthesis of powders, and different routes and sintering processes of multiferroics.
Electro-optical devices
Description: Electro-optical devices (or displays) are mostly cells with controlled thickness and filled with some type of liquid crystal that modulates the light of each individual unit of the device, called pixel. Liquid crystals are used because they flow (hence they can fill these cells), manipulate the light (they are birefringent) and respond to an applied electric field (they are also dielectrically anisotropic). From a fundamental point of view, it is necessary to understand the mechanisms that make such materials important for these devices, and thus to optimize these mechanisms. For example, the response or relaxation time of a nematic liquid crystal determines how quickly an image can be displayed by the device. This time depends on several physical parameters of the phase and this calculation is not always trivial. We have recently shown that an analytic way can be found for nematic liquid crystals depending, besides viscosity and elastic constant, on the thickness, anchoring energy and surface viscosity (Ann. Of Phys. 346, 14, 2014). Several other types of studies, including interactions with the surface, are possible. Another material, besides the nematic one, which is explored by the group is the reflective cholesteric phase. In these types of displays, there is a transition from the planar texture to a focal-conic texture by applying an electric field. There are two mechanisms for this: nucleation of the so-called oily streaks (defective lines) or through the Helfrich instabilities, where the cholesteric layers bend until they break into domains. Generally, the nucleation process has a critical field a little lower and it predominates, although both mechanisms are observed simultaneously. In this case, by nucleation, the process is slow and inconsistent with the operation of a display. One way to speed up this process was proposed by adding nanoparticles (J. Soc. Inf. Disp. 19, 410, 2011) and employing the active matrix method (SID 11 Digest, 392, 2011). However, this is still a point that should be developed. On the other hand, a display running through Helfrich instabilities was proposed and its functioning analyzed, given different boundary conditions (J. of Appl. Phys., 112, 124513, 2012). However, the physical mechanism of this display’s dynamics, capable of transitions in the order of 5 ms, is still unknown and must be explored by the group. Other studies such as the angle on the surface, surface viscosity and other types of doping are still open and must be explored by the group.
Impedance spectroscopy of isotropic and anisotropic fluids
Description: The impedance spectroscopy technique is used in the electrochemical characterization of systems of interest describing molecular relaxation and changes in various physico-chemical properties, as the frequency of an electric field applied to the sample varies. In our group, the technique is used to analyze liquid-crystalline samples, vegetable oils (oxidation study) and samples containing other biological fluids. From a theoretical point of view, the group has also been recognized in recent years for diffusive models that use the fractional diffusion equations considering various boundary conditions to be followed by the system, so as to represent situations as close as possible to the experiments and respond to the enormous complexity of the electrode behavior. This type of analysis is of particular importance to the physics of liquid crystals, which is still the flagship of the group’s research. It is necessary to emphasize that the use of fractional derivative formalism in this context is a pioneer of group contribution to international literature devoted to these problems. It should be mentioned, in particular, the new version of the Poisson-Nernst-Planck model (PNP) with the incorporation of anomalous diffusion phenomena to the description of the problems (Model PNPA) successfully held by the group.
Usual and anomalous diffusive phenomena
Description: The diffusive phenomena and the understanding of the formal aspects related to them have attracted the attention of many researchers due to the large number of applications in various scientifical fields and, in particular, in view of the applications associated with anomalous diffusion. Inserted in this general context, we look into solutions for diffusion equations, their extensions and other physical situations that can be related to them; for example, the problem of molecular reorientation in nematic liquid crystals under the action of external fields; the dynamics of ionic adsorption or neutral particles in liquid crystals; and the fractional distribution in connection with the results of impedance spectroscopy in liquids and complex fluids.
Liquid crystal mixtures containing polymers: stabilized phases, microscopy and applications
Description: The process of adding liquid crystal polymer produces several important results in both basic and applied research. There are, basically, two types of organization: when the amount of polymer is large (~70%), there is a phase separation between the polymer and the liquid crystal forming an emulsion of liquid crystal droplets. These are called PDLC. In general, these systems are interesting from the optical point of view since they scatter light and the liquid crystal display presents different orientations and anchoring, depending on the specific type of polymer. The second type of mixture utilizes a low polymer concentration (~ 10%); the polymer network stabilizes the liquid crystal. This is because, a priori, the monomers are dissolved in the liquid crystal and then photo-polymerized. During this process, the polymer network grows to have the same shape as the liquid crystalline organization (J. of Appl. Phys., 112, 124,513, 2012 and J. of Disp. Tech. 7: 11, 2011). In our group, we intend to use these techniques coupled with electron microscopy to visualize patterns such as the interface between cholesteric liquid crystals and the isotropic phase. Furthermore, the system can be used to study charges adsorbed by the network that induce an asymmetrical step in the cholesteric phase. The group also uses these systems as a laboratory of new phases, new polymers and the application and improvement of electro-optical devices.
Molecular orientation and surface effects in thermotropic and anisotropic liquid crystals
Description: The investigation of interfacial elastic behavior in nematic media is an activity worthy of the best efforts of the scientific community. A classic line of research we have discussed for almost two and a half decades is aimed at determining the equilibrium profile of the stage director from the consideration of molecular orientation favored by the surface. These problems can be affronted by the elastic continuum theory for liquid-crystalline materials, and, form the mathematical point of view, may be formulated as boundary problems and of initial value. Our problem approaches consider various factors such as the inhomogeneity in the distribution of the easy axes on the surface, the presence of external fields, surface geometry effects, wetting, among others, on the properties of a nematic volume. More recently, studies by the group have emphasizes the adsorption phenomena occurring in the crystal-liquid interface – substrate, both of neutral particles and of the moving charges present in the fluid. In the latter case, the study of surface effects in liquid crystals joins the more general study of the electrical responses of electrolytic cells, and has been affronted by various techniques. In particular, anomalous diffusion processes have also been considered in detail in this research line.
Transitions between textures and wetness in chiral nematic
Description: The chiral nematic or cholesteric phase has drawn attention in recent years due to the symmetry breaking induced by the chirality that leads to unique phenomena. An interesting aspect of this phase is that it presents several different textures, which present themselves in certain situations such as anchoring, electric field and type of confinement. It is very important to know the mechanisms behind the transitions between these textures as they can be applied to electro-optical devices. One of the aspects of this line of study is to study these transitions within both experimental and theoretical scopes, determining the factors and usability for each texture. This knowledge can also be applied in the so-called wetting transitions, which occur when the contact angle of two coexisting phases becomes zero with respect to a solid substrate, when the temperature (or the composition) is changed. Recently, our group was pioneer in studying a wetting transition for chiral nematic (Phys. Rev. Lett. 110, 057801, 2013). During the wetting transition, a thin LC layer occurs where wetting force, chirality, elasticity and anchoring act reciprocally, giving rise to the nematic-cholesteric transition and formation of fringes depending on the ratio between elastic constants and the ratio between thickness and pitch. As the wetting layer increases with decreasing temperature, the frequency of fringe changes, and finally there is a rupture of these fringes. The major challenges are still to understand the shape of the interface between the isotropic and the cholesteric phases, and whether, in the interface, domains of “blue-phases” can co-exist for certain steps, studies to be conducted with microscopy (electronic and transmission), analytical and computational calculations via Landau-de-Gennes and Monte Carlo models. Other challenges include the interaction and dynamics of defects in CLCs and explaining the rupture of the fringes observed experimentally.
Biological systems
Description: Characterization of biological tissue by optical illumination and determination of the optical response by absorption. In this line, the highlights are topical applications of drug substances on the skin, in vivo or ex vivo, and impregnation of antibacterial agents on dental tissue in vitro to analyze the permeation profile and heat diffusion properties of the diffused material through tissue and the study of the photostability of drug substances for application in biological systems.
Vibrational Spectroscopy
Description: Study and characterization of materials using vibrational spectroscopy from the techniques by Fourier Transform Infrared Spectroscopy (FTIR), Fourier-transform Raman scattering (FT-Raman) and dispersive scattering Raman coupled to confocal microscope, with which characterization of functional groups through their characteristic frequencies is possible. In this line, we highlight the studies in vitreous systems, polymers and copolymers, drug substances, biomaterials and biological systems, with an emphasis on altered tissue studies.
Production and Characterization of Optical Glasses
Description: Study of absorption spectroscopy, aluminosilicate composites in powder, and melted powders in the glassy state, their thermal properties, optical characterization with rare-earth doping, spectroscopic property measurements of absorption and emission, study of doping effect and co-doping in the emission.