R. M. Rubinger

• PhD
• Professor (Full) at Federal University of Itajubá

O primeiro material bidimensional descoberto foi o grafeno em 2004. A partir de então, estudos foram desenvolvidos com outros materiais e, um dos primeiros semicondutores a ser isolado foi o dissulfeto de molibdênio, o MoS2. Ao contrário do grafeno, o MoS2 tem gap de energia, e na forma de monocamada o gap passa de indireto para direto. Essa mudanç...

In this research, a Concentrated Solar Power (CSP) as a Parabolic Trough Collector (PTC), using Peltier cooling modules for power generation was analyzed by the Cross-Entropy method. When comparing conventional solar electric generators with this system, we have the advantage that it is compact and lightweight and can be easily assembled and used a...

The syntheses of TiO2 and Pd/TiO2 nanoparticles were carried by hydrolysis of alkoxide precursors using the reverse micelles method using titanium tetrabutoxide and palladium acetylacetonate as precursor metals. The method was modified by precipitation with the addition of n-butanol and then washed with an ethanol/water solution at a ratio of 1:1....

Polyaniline (PANI) samples were deposited through chemical and electrochemical routes onto macroporous silicon (MPS). Their chemical and electrical features were investigated. For this, Fourier transforms infrared, Raman, and energy dispersive spectroscopies were used to show the successful incorporation of PANI into the pores and the formation of...

Due to the large reduction in fossil fuel reservoirs, the consequent cost increase of deepwater extraction, and the emission of pollutants, there is a constant search for alternative ways to obtain clean energy at a lower cost. Among those sources, we focus on the energy produced by thermoelectric materials. In this work, we present a new system fo...

In this work we developed a didactic linear parabolic solar concentrator for use with Peltier cooling modules for power generation. The main idea is to maximize the temperature gradient between the hot and cold sides to provide maximum power for this system and use that in class for experimental explanations. The cold temperature is provided by a f...

This work reports the electrical characterization of ZnSn_x Ge_1-xN_2 (ZTGN) layers $(10\% < x < 90\%)$ deposited on glass by combinatorial sputtering and further assesses the performance of silicon heterojunction (SHJ) solar cells featuring them as electron-selective contacts. Bandgap, dark conductivity, and the activation energy of the latter w...

Silicon dioxide (SiO2) is a material that is abundant in nature and has wide application in semiconductor and insulating devices. In this work, a set of six SiO2 samples were grown on a Sigma-Aldrich Silicon substrate, varying the growth time and temperature. This set of samples were grown using times of 10 and 12h and temperatures of 800, 900, and...

This manuscript presents the study of electronic transport on a set of five multilayer molecular beam epitaxy-grown InAs/GaAs semiconductor samples. We developed an automated switch system to carry out electronic transport measurements of mobility and carrier concentration using the van der Pauw technique. Measurements were carried out as a functio...

Atualmente, as ferritas têm sido tema de muitas pesquisas devido as suas vantagens e propriedades que podem ser facilmente manipuladas, sendo assim de grande interesse tecnológico e científico. Para tanto, a distribuição dos cátions e interações magnéticas destacam um papel importante nestes materiais e, portanto, têm sua importância científica. As...

Neste trabalho apresentamos os resultados da construção de um equipamento, de baixo custo, para medidas medida Hall, utilizando a técnica de van der Pauw. Para isso, foi desenvolvido um sistema composto por uma placa eletrônica controlada por uma placa DAQ que juntamente com um software em Labview realiza a automação de todas as medidas necessárias...

Ferrites have received a renewed technological and scientific interest in recent years, due to their low cost of production and the diversity of applications. Nickel-zinc ferrites are well known as soft magnetic materials with good properties, such as high electrical resistivity, low loss of eddy current at high frequencies and induction heating pr...

This work studied the effects of noises on four metaheuristic algorithms, namely the Self-Adaptive Differential Evolution (DE), Performance Guided JAYA (PGJAYA), Self-Adaptive Teaching-Learning Based Optimization (SATLBO) and Biogeography-based Heterogeneous Cuckoo Search (BHCS) for extracting the PV parameters from I-V curves considering the singl...

Low resistive electrodes based on Co and Mo co-doped SnO2 were prepared by the conventional solid-state reaction and sintered at 1250 °C for 2 h. Concentration of Co2O3 precursor was unchanged (1 mol%), while MoO3 was varied (0.25, 0.50 to 0.75 mol%) to promote conductivity. The structural and microstructural characterization revealed that the samp...

Lead and copper ceramics are technologically important materials due to their high magnetic permeability, low losses and high resistivity. In this paper, we present the production of a set of samples through the ceramic process, involving solid state and high temperature reactions between the constituent oxides, and the characterization of Pbx Cu1–...

The accurate knowledge of the solar cells parameters dependence on irradiance and temperature is of vital importance for the performance assessment of photovoltaic modules and development of new devices, and many works have been published so far to understand the aforementioned dependence, but none employed a metaheuristic technique. To understand...

To obtain a copper and niobium ferrite with the structure of spinel samples were produced by solid state reaction process stoichiometry NbxCu1-xFe2O4 (x = 0.0, 0.10 and 0.20) and sintered at 1000°C for 8 h and subsequent characterization of its microstructure and determine their magnetic properties. The structural characterizations, microstructural...

The aim of this work was to investigate the magnetic properties and the electrical conductivity temperature dependence associated to the potential barrier between the crystallites model. Gallium and magnesium containing spinel ceramic has low magnetic coercivity and high electrical resistivity. MgGa2-xFexO4 samples (x= 0.01, 0.05, 0.15, 0.25, 0.35)...

We report high-resolution measurements that experimentally confirm a spiral cascade structure and a scaling relationship of shrimps in the Chua's circuit. Circuits constructed using this component allow for a comprehensive characterization of the circuit behaviors through high resolution parameter spaces. To illustrate the power of our technologica...

In the present work, mixed manganese-copper ferrite of composition MnxCu1-xFe2O4 (within x=0.40, 0.42, 0.44, 0.46, 0.48 and 0.50) have been investigated for their electric and magnetic properties such as dc resistivity, Curie temperature, saturation magnetization. MnxCu1-xFe2O4 ferrite samples were prepared by uniaxial pressing from the oxide mixtu...

The aim of this work was to investigate the magnetic properties and the electrical conductivity temperature dependence associated to the potential barrier between the crystallites model. Gallium and magnesium containing spinel ceramic has low magnetic coercivity and high electrical resistivity. MgGa2-xFex O4 samples (x= 0.01, 0.05, 0.15, 0.25, 0.35...

In this paper, we present the results of the electrical characterization of semiconductor mutli-quantum-well of InAs/GaAs. The growth by molecular beam epitaxy may induce the formation of other structures such as dislocations and point defects and also quantum dots due to the stress that arise in the lattice caused by mismatches in lattice constant...

Comparative and quantitative analysis of white light-emitting diodes and other lamps used for home illumination Abstract. We compared the photometric and radiometric quantities in the visible, ultraviolet, and infrared spectra of white light-emitting diodes (LEDs), incandescent light bulbs and a compact fluorescent lamp used for home illumination....

This work describes the preparation and electrical characterization of conducting carbon black on polyurethane and polyvinyl commercially available screen printing inks. Resistivity dependence with temperature for samples containing 3.0, 4.4, 5.0, 8.0 and 10.7 % filler mass concentration were investigated from 5 to 310 K. By using the differential...

In this work; resistive moisture sensors were obtained by dip coating sulfonated polycarbonate (SPC) onto silver interdigitated electrodes. Commercial polycarbonate was sulfonated with acetyl sulphate at two different sulfonation degrees corresponding to 9.0 and 18.0 mole %. Impedance spectroscopy was used to investigate the humidity sensing proper...

The amount of information exchanged per unit of time between two nodes in a dynamical network or between two data sets is a powerful concept for analysing complex systems. This quantity, known as the mutual information rate (MIR), is calculated from the mutual information, which is rigorously defined only for random systems. Moreover, the definitio...

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Magnetoresistance measurements were performed on an illuminated semi-insulating GaAs sample with intrinsic deep level defects. Electrical carriers were photo-generated under light excitation and positive magnetoresistance for B<0.2T was observed in the whole range of temperatures measured (220-315K). Using the model developed by H. Fukuyama and K....

Electrical resistivity measurements were performed on p-type Pb{sub 1-x}Eu{sub x}Te films with Eu content x = 4%, 5%, 6%, 8%, and 9%. The well-known metal-insulator transition that occurs around 5% at room temperature due to the introduction of Eu is observed, and we used the differential activation energy method to study the conduction mechanisms...

This study describes the preparation and electrical characterisation of conducting carbon black-filled/styrene–butadiene–styrene tri-block copolymer. Composites containing 5 and 10 vol% of carbon black were studied. The carbon black distribution, conductive mechanism and electrical properties of the composites were investigated from 8 to 304 K. For...

We have measured electrical resistivity in the range of 100–500 K on six semi-insulating low-temperature grown molecular-beam epitaxy GaAs samples that were exposed to annealing treatment. Samples 2, 3, 4, 5 and 6 were annealed with temperatures of 350 °C, 400 °C, 450 °C, 500 °C and 550 °C, while sample 1 was not exposed to the annealing process. W...

We report numerical results on the existence of periodic structures embedded in chaotic and hyperchaotic regions on the Lyapunov exponent diagrams of a 4-dimensional Chua system. The model was obtained from the 3-dimensional Chua system by the introduction of a feedback controller. Both the largest and the second largest Lyapunov exponents were con...

We report a numerical bifurcation study on the Chua's circuit with parallel resistor. Through the largest Lyapunov exponent, we constructed a two-dimensional parameter space of the model. We also implemented the experimental circuit to show the similarities between the model and the experimental data. With that modification we discuss the effect of...

We report a Periodicity-Detection algorithm, implemented in a LabVIEW routine for real-time data analysis on experimental chaos, to evaluate the periodicity P of experimental time series. The Periodicity-Detector (PD) algorithm was applied to the forced Chua’s circuit with the aim to build the Periodicity-parameter-space (P-parameter-space). As res...

We have derived equations to calculate upper and lower bounds for the rate of information exchanged between two nodes (or two groups of nodes) in a dynamical network, the mutual information per unit of time (MIR), without having to calculate probabilities but rather Lyapunov exponents or expansion rates. Since no probabilities need to be calculated...

A high-resolution codimension-two parameter space showing the abundance of complex periodic structures of an experimental chaotic circuit is reported. Such resolution was propitiated by the use of a 0.5 mV step dc voltage source as one of the control parameters. Those complex periodic structures organize themselves in a period-adding bifurcation ca...

Low frequency current oscillations have been usually investigated under the influence of the external applied voltage, temperature and illumination. A parallel applied magnetic field has been used in the present work in order to obtain odd periodic windows and bifurcations inside them in a two dimension parameter space for a semi-insulating GaAs sa...

Investigation of crystal structure, dielectric, magnetic and local ferroelectric properties of the diamagnetically substituted Bi1−xAxFeO3−x/2 (A=Ca, Sr, Pb, Ba; x=0.2, 0.3) polycrystalline samples has been carried out. It has been shown that the heterovalent A2+ substitution result in the formation of oxygen vacancies in the host lattice. The soli...

We report the existence of complex periodic structures, forming intricate periodic networks, embedded in chaotic regions of bi-dimensional bifurcation diagrams (parameter spaces) of a chaotic circuit model. Such circuit is composed by a negative differential conductivity (NDC) device coupled with a three variable RLC tank circuit. Particularly, we...

Spin resonance of electron states in a Ge/Si heterosystem with Ge quantum dots has been investigated. Electron localization in the strain-induced potential wells in Si in the vicinity of the Ge dots is confirmed by an analysis of the obtained g-tensor values. A well pronounced anisotropy of the linewidth is explained in terms of an effective magnet...

Conducting blends of polyaniline (PAni) and sulfonated polystyrene (PSS) were obtained by in situ polymerization. We prepared PAni/PSS blends with molar ratio of 1:2 and PSS sulfonated at the degrees of 13, 21 and 30mol%. The blends were investigated by d.c. conductivity and scanning electron microscopy (SEM). Increasing sulfonation degree increase...

CaCu3Ti4O12 ceramic samples were prepared by solid state conventional route using stoichiometric amounts of CuO, TiO2, and CaCO3. Afterward the material was doped with GeO2 with concentrations up to 6% by weight and sintered at 1050 °C for 12 h. The influence of doping on the microstructure, vibrational modes, and dielectric properties of the mater...

In order to investigate the formation of Mn/Sb clusters embedded in crystalline silicon, sequential ion implantation with fluences of 1 × 1016 at cm−2 and 2 × 1016 at cm−2, respectively, was used to incorporate Mn and Sb ions at high concentrations into Si(0 0 1). Based on investigations with Rutherford backscattering spectroscopy (RBS) and corresp...

Thermal, morphological and dielectric properties of conductive thermoplastic blends based on polystyrene, styrene–butadiene–styrene block copolymer and doped polyaniline have been investigated with special emphasis on the effect of doped polyaniline concentration. Blends were prepared by melt blending using an internal mixer. Morphological studies...

In a 2D parameter-space of an inductorless Chua's circuit model, we carried out numerical investigations and observed self-similar stability structures embedded in a sea of chaos, known until recently just in discrete-time models, namely, shrimps. We showed that those structures are self-similar and organize themselves in a period-adding bifurcatio...

Ferromagnetic resonance (FMR) is used to probe the magnetic properties of Co / Ag superlattices (SLs) with ultrathin Co layers (2–6 Å ) . Different series of 5×[ Ag / Co ] multilayers have been grown by molecular beam epitaxy on Si(111) substrates, monitoring the growth by reflection high energy electron diffraction. Cross-section transmission elec...

MnP nanowhiskers have been grown by molecular beam epitaxy technique on the InP(100) surface. The measurements of the magnetization revealed that samples with MnP nanowhiskers exhibit ferromagnetic behavior up to room temperature. The investigation of temperature and angular dependences of ferromagnetic resonance has shown the existence of 90° anis...

Polycrystalline samples with the nominal composition Bi0.8Pb0.2FeO3 have been studied via x-ray diffraction, Mössbauer spectroscopy, dielectric, magnetic, and local ferroelectric measurements. It has been found that the heterovalent Pb2+ substitution in Bi0.8Pb0.2FeOy is realized through the formation of oxygen vacancies. The crystal structure of t...

Polyaniline (PANI) was obtained by in situ polymerization in a polystyrene sulfonic acid (PSS) solution. We prepared a series of PANI/PSS blends with molar ratio of 1:2, and PSS sulfonated at the degrees of 13mol%, 21mol% and 30mol%. A control sample of polystyrene, at an intermediate sulfonation degree of 21%, without polyaniline, was also conside...

Manganese monophosphate MnP single crystal deserves attention due to its rich magnetic phase diagram, which is quite different depending on the direction of the applied magnetic field. Generally speaking, it has a Curie temperature around 291K and several other magnetic arrangements at low temperatures (cone-, screw-, fan-, and ferromagnetic-type s...

Chaos control has stimulated a large amount of work. We have studied the effect of an external parallel magnetic field on the low-frequency current oscillations observed on a molecular beam epitaxy GaAs sample grown at 265 deg. C, and we have shown that it can be efficiently used for chaos control. The study of the magnetoresistance indicates that...

The electrical and magnetic properties of GaSb:Mn layers deposited on (100)GaAs substrates from a laser plasma in vacuum have been studied. It is shown that the films deposited at 200–440 °C are mosaic single crys-talline and epitaxial to the substrate, with p-type conduction. Manganese-doped layers had a hole concentration higher than 1 × 10 19 cm...

In this paper we have analyzed the time series obtained from an experimental realization of an inductorless version of Chua’s circuit. We have also compared the experimental results with the results emerging by analysis of the time series obtained by numerical simulations of the respective model equations. Lyapunov exponent spectra, the Kaplan–York...

Commercial polystyrene was sulfonated with sulfonic acid groups to a degree of 22mol%. Polystyrene and sulfonated polystyrene were analyzed with respect to its chemical and physical properties trough titration, FT-IR, microprobe analysis and SEM. These techniques allowed us to determine the effectiveness of the sulfonation reaction and the changes...

We have implemented an operational amplifier inductorless realization of the Chua's circuit. We have registered time series from its dynamical variables with the resistor R as the control parameter and varying from 1300 Ω to 2000 Ω. Experimental time series at fixed R were used to reconstruct attractors by the delay vector technique. The flow attr...

In this work, we have implemented an operational amplifier inductorless realization of the Chua's circuit. We have registered time series from its dynamical variables with the resistor R as the control parameter and varying from 1300Ω to 2000Ω. The measurements were carried with a 16bit data acquisition interface. Attractors were reconstructed by t...

We measured resistivity in the range of 30–390 K on four semi-insulating low-temperature grown molecular-beam epitaxy GaAs samples. The growth temperature range was from 215 °C to 315 °C. Arrhenius fittings with T−1 and hopping fitting with T−1/4 do not permit us the definition of the temperature ranges controlled by band and hopping conduction, re...

Semi-insulating GaAs samples show low-frequency current oscillations (LFO) under high dc voltage. These spontaneous oscillations are caused by traveling high-electric-field domains and can be explained based on the well-accepted model of electron trapping in deep levels by multiphonon emission processes. This paper aims to explain today the most im...

We investigated the effect of adding the field-dependent recombination process, namely field-enhanced trapping, to the generation-recombination processes of charge carriers that model current oscillations in semiconductors. The main new features arising from this modification are identified in bifurcation diagrams with the electric field as the con...

We present an experimental study of bifurcation diagrams from low frequency current oscillations (LFO) measurements obtained from semi-insulating GaAs samples grown by low temperature molecular beam Epitaxy (LT-MBE). The considered growth temperatures were 215ºC and 265ºC. LFO are considered to be spontaneously generated oscillations under constant...

Electric transport properties measured by Van der Pauw resistivity experiments of Low-Temperature Molecular Beam Epitaxy (LT-MBE) GaAs samples are used to identify a method to improve the resistivity of GaAs material. We present results on five samples grown at 265, 310, 315, 325, and 345 ºC. The electric measurements were carried out at temperatur...

We have analyzed the nonlinear electric field time series theoretically generated by a set of three coupled first-order differential equations in order to interpret the chaotic behavior present in measurements of low-frequency current oscillations in semiconductor samples. In the present work, we carry out a direct investigation of the theoretical...

Low frequency oscillations (LFO) on current I(V, t) curves were studied in a GaAs sample grown by molecular beam epitaxy. Our measurements were carried out at a temperature of 200 K and under infrared illumination. The nonlinear I(V) characteristics give rise to spontaneous oscillations under constant applied bias V. The attractors were constructed...

Polyaniline (PANI) and their charge transfer complexes are engineering materials that continue to attract considerable attention from engineers due to their unique electrochemical behavior, environmental stability and high degree of processability. Extensive works on the preparation of electrically conductive PANI blends containing a polymeric matr...

We have carried out van der Pauw resistivity and Hall effect measurements on a series of Molecular Beam Epitaxy InAs/GaAs superlattice samples containing InAs quantum dots. Three growth parameters were varied, the InAs coverage, the number of repetitions of the InAs/GaAs layers, and the GaAs spacer thickness. The results can be grouped in two sets,...

Studying the transport properties via Hall and resistivity measurements of low-temperature molecular-beam epitaxy (LT-MBE) GaAs samples, the optimal conditions for fabricating high-resistivity material are found. We present results on three LT-MBE GaAs samples grown at 215, 265, and 315 °C. The measurements were carried out at temperatures ranging...

We have observed low frequency current oscillations in a semi-insulating GaAs sample grown by low temperature molecular beam epitaxy. For this, an experimental setup proper to measure high impedance samples with small external noise was developed. Spontaneous oscillations in the current were observed for some bias conditions. Although measurements...

Semi-insulating GaAs samples present N-shaped negative differential conductivity under high-electric fields. This behavior can be associated with two physical processes: Impact ionization (generation) and field-enhanced trapping (recombination), both of which involve trapped and free electrons. We have analyzed the j(E) characteristic curves of a G...

We have carried out our experiments on a semi-insulating GaAs sample grown by low temperature Molecular Beam Epitaxy. Three parameters were used to fine tune the experiments, namely, temperature, illumination, and applied bias. We have used powerful tools of time series analysis in order to assess the embedding dimension through near false nearest...

We have carried out the time, temperature, and illumination dependencies of the current density in a semi-insulating GaAs sample grown at 300 °C under strong electric field. Standard ohmic behavior was observed at room temperature. A negative differential behavior as a function of the applied electric field was observed by lowering the temperature...

We have studied the temperature and illumination dependencies of the current versus voltage characteristics I(V) of a Cr-doped GaAs sample. An S-shaped negative differential conductivity with a threshold at around 1 kV cm-1 was associated with a trap with activation energy of 90 meV. The low-conductivity state in I(V) characteristics is strongly in...

In this work we present studies for the behavior of impurity breakdown of two GaAs samples grown by Molecular Beam Epitaxy at 200°C and 300°C. We vary the temperature and the illumination intensity. For the sample grown at 200°C, the transport mechanism after the breakdown is of the same type of the main free carriers at low electric fields. Below...

We have observed Low Frequency Oscillations (LFO) in a GaAs sample grown by molecular beam epitaxy (MBE) at 300 °C as a function of applied electric field, temperature and illumination intensity. Time series of the oscillations for different values of the applied electric field at room temperature were obtained. We have also obtained the jxE charac...

The photoconductivity of a GaAs sample grown by molecular beam epitaxy was dynamically-measured at low temperature to observed the photoquenching (PQ) effect. As a function of applied electric field, several results were observed. Below about 1.2 kV/cm, the PQ presented a slow decay followed by a fast quenching. The delay time for the onset of the...

Using thermally stimulated current (TSC) spectroscopy we have identified the presence of several deep traps in low temperature grown (LTG) nonintentionally doped bulk molecular beam epitaxy (MBE)-GaAs and silicon planar-doped MBE-GaAs samples. The experiments of TSC spectroscopy were carried out on a LTG MBE-GaAs epilayer grown at 300 ° C and the p...

Through photo-Hall measurements at temperatures below about 120 K, we have observed the presence of a deep donor defect, with characteristics similar to those of the EL2 center, in planar-doped GaAs samples grown by molecular beam epitaxy at 300 °C. We have shown that this EL2-like center can account for the remarkable photoreleasing of electrons a...