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On the present work it is presented a new category of continued fraction functions (CFF) of a real variable $$\lambda$$ , named “the $$\epsilon$$ class”, such that $$\lambda$$ enters in the CFF through small or infinitesimal contributions in the partial quotients. These CFF emerge on the secular equation of the Sturm–Liouville equation (SLE) by a finite difference treatment. Also, it was obtained the eigenvalues of the one-dimensional Schrodinger equation (a particular case of SLE) from the CFF secular equation. The case of the two-dimensional Schrodinger equation was partially studied.

The present work describes the preparation and the investigation of the room temperature electrical and optical properties of a series of liquid nanocomposites (lnC) prepared with different concentrations of multiwalled carbon nanotubes (MWCNT) in a variety of liquid matrices: glycerin, Vaseline, glucose, propylene glycol and silicone oil (SIO). Special attention is deserved to the SIO matrix, owing to its convenient electrical properties for our purposes. We verified that a small percent fraction of MWCNT dispersed along the SIO matrix is capable of improving the electrical conductivity of the matrix by orders of magnitude, indicating that the MWCNT strongly participates in the electrical conduction mechanism. Also, the application of an external electric field to this lnC resulted in large changes in the optical transmittance, that were interpreted as a consequence of the fieldinduced MWCNT alignment into the liquid matrix. The characteristics of such a new category of nanocomposite in the liquid state suggest further studies.

Nesta dissertação utilizamos as técnicas de fotoluminescência (PL, photoluminescence) e de microscopia de força atômica (AFM, Atomic Force Microscopy) para estudar pontos quânticos (QD, Quantum Dots) crescidos pela técnica de epitaxia de feixes moleculares (MBE, Molecular Beam Epitaxy). Estudamos cinco amostras de pontos quânticos de arseneto de índio (InAs) depositado sobre substrato de arseneto de gálio (GaAs), de espessuras nominais 1,0 - 1,5 - 1,75 - 2,0 e 2,5 monocamadas (MC) atômicas de InAs. Nossas investigações por AFM exploraram a processa da formação e da evolução dos pontos quânticos com o progressivo aumento da quantidade de InAs (que define a espessura nominal da camada). Utilizando um programa computacional realizamos um levantamento estatístico do tamanho médio (altura e base) e da densidade dos pontos quânticos em cada amostra. Verificamos que todas as estruturas morfológicas reveladas por AFM também foram evidenciadas pelas medidas de fotoluminescência. Realizamos também medidas de fotoluminescência em função da potência de excitação e da temperatura da amostra. Com base nestes dados, analisamos os processos de recombinação nos pontos quânticos. Verificamos o aparecimento sucessivo de diversos picos associados com as emissões dos estados excitados cujas intensidades saturam com o aumento da potência de excitação. Um importante resultado de migração de portadores entre pontos quânticos. Este processo é mediado por uma estreita camada de InAs (denominada wetting layer, WL) adjacente aos pontos quânticos. Por fim, dedicamos atenção ao estudo de amostras cujas espessuras nominais (1,0 e 1,5 MC) situavam-se abaixo do mínimo valor usualmente atribuido para a formação espontânea de pontos quânticos (na faixa de 1,6-1,7 MC). Na amostra com 1,5 MC observamos as emissões provenientes de diversas estruturas de confinamento: a WL, diferentes famílias de pontos quânticos e aglomerados de pequena altura. ) Este resultado é inédito, já que é a primeira vez que todas as emissões provenientes destas estruturas são observadas nos espectros de PL simultaneamente. Nossos resultados em função da temperatura para esta amostras revelaram-se complexos mecanismos de difusão e captura de portadores entre estas estruturas de confinamento. Process of recombination in quantum dots in samples with different thicknesses of InAs layers using the photoluminescence technique

Essential oils are complex mixtures of organic substances with large commercial importance in the pharmaceutical, food, fragrance, and cosmetic industries due to their organoleptic and biological properties. Also, these materials are also luminescent what has taken several studies about its potential uses for the detection and quality control of essential oils, imaging, and for the investigation of the synergies of their constituents. Concerning this, the present work is dedicated to studying the optical properties of selected essential oils: citronella ( Cymbopogon winterianus), Japanese mint ( Mentha arvensis), clove bud ( Syzygium aromaticum), and bergamot ( Citrus bergamia). We carried out a comparative study of the photoluminescence and the ultraviolet–visible optical absorption (abs-UV–Vis) of these essential oils with their typical constituents. To inspect the effects of the intermolecular interactions on the optical response of these systems, mixtures between the essential oils constituents following the expected average percent mass fraction were also studied. From these experiments, the main results were bathochromic effects in the abs-UV–Vis spectra; excimer formation in citral, isopulegol, isomenthone, eugenol, and eugenyl acetate; excimer emission enhancing and specific solvent effect in the essential oils photoluminescence spectra. These results contribute to the knowledge of essential oils’ applications, especially in the evaluation of components’ interactions through a simple abs-UV–Vis assay.

In the present work, it is carried out a study of the optical emission of AlxGa1-xAs/GaAs quantum wells by photoluminescence (PL). A detailed analysis of the thermal redshift of the (PL) peaks showed that strength of the electron-phonon interaction is influenced by the degree of binding of the electron in the conduction band (free or localized excitonic state, bulk or quantum size confined state).

The class of carbonaceous chondrite meteorites has carbon in their structures, similarly to the terrestrial calcite and dolomite rocks, and contains the group =CO3 linked to Ca and/or Mg, which may be, in principle, more susceptible to the action of the spatial gamma radiation (γ–R) due to the presence of these light-atom elements. On the present work, we used a variety of optical techniques to investigate the possible effects of γ–R produced by an artificial 192Ir source in terrestrial calcite and dolomite, which may allow to understand the effect of the spatial radiation in that celestial bodies of the solar system. As a result, we verified that the γ–R irradiation caused the effect of untrapping of electrons from deep color centers, that spatially migrate to other color centers on the samples, resulting on the change of the electron energetic configuration.

Following the “All With One (AW1)” technology proposed in our earlier work, the present study is focused on the production and the characterization of write once read many (WORM) memory devices with a carbon nanoparticle/epoxy resin nanocomposite, where the main advantage stands on the ease of production of the nanostructured phase. The results revealed that short 100-ns, low-voltage (5.0-V) electric pulses are enough to record a bit, and the bit one to bit zero current state ratio $$I_\mathrm{{ON}}/I_\mathrm{{OFF}}$$ reaches 10 $$^{7}$$ .

The production of organic memory devices comprises the preparation of the substrate, deposition of the active layer and the electrodes, and the final capsuling. Owing to the low cost and robustness of the epoxy resin – a typical encapsulant – compared to other polymers employed on the active layer, we propose the development epoxy based memories. We successfully produced epoxy resin/carbon spheres nanocomposite-based organic WORM memories with an I ON / I OFF ratio of the order of 10 5 , low recording voltages of 2.5 V, and thermally and electrically stable during a test period of 30 days. For the concentration 0.4 wt% and the film thickness 1.3 μm a short 100 ns electric pulse was sufficient for the recording, which is lower than the average minimum of 200 ns reported in the literature. Both the high I ON / I OFF ratio and the short recording time we attribute to a fast and efficient mechanism of formation of conducting filaments inside the nanocomposite, due to the action of the external applied electrical field. These results point to the possibility of the development of a “All With One” (AW1) technique, where the overall device is based on a single main material – from the substrate and the active layer to the capsuling.

In the present work, the conductivity of multi-walled carbon nanotube (MWCNT) and epoxy resin matrix nanocomposites produced under the action of a 100-V/cm external sinusoidal electric field is studied, for five MWCNT concentration values. The results showed that the AC electrical conductivities of the nanocomposite samples exhibited a large increase compared to those of pure epoxy resin samples. The conductivity at 40 Hz increased from ~ 10−9 S/m for the pure resin samples to 10−2–10−3 S/m for materials with 0.30 wt% MWCNT concentration, while a further increase of up to ~ 10−1 S/m was achieved on the samples produced under the effects of a sinusoidal field. This phenomenon was attributed to a spatial alignment of the MWCNT inside the epoxy matrix and/or the bending of close MWCNT due to the electrostatic force, which created efficient electric current conduction paths along the MWCNT. A proposed new method estimated the value of the current percolation threshold, yielding a result of 0.016 wt% for the sample that was not subjected to the electric field, which then decreased by approximately one order of magnitude for the samples subjected to the electric field.

In the present work we show the results of the investigation of the properties of bismuth nanoparticles (NPs) obtained by the laser ablation in water. The samples were characterized by various techniques: UV/vis spectroscopy, dynamic light scattering, transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, nuclear magnetic resonance and zeta potential. The NPs were metallic and surface oxidated, exhibiting absorption peak at 257 nm due to the surface plasmon resonance. The interaction between the NPs and various amino acids was studied and revealed a selective sensibility to cysteine on a range of concentration from 80 to 720 μM, which we attribute to a dimerization of cysteine to cystine probably complexed to the nanoparticle surface by electrostatic interactions, leading to the modification of the NP absorption spectrum.

O polipropileno com carga ou reforço de fibras de sisal (PP Sisal) é um composto desenvolvido para ser aplicado no processo de injeção de peças plásticas automotivas. Compósitos de polipropileno reforçados com cargas minerais (talco) ou fibras de vidro têm sido largamente aplicados nesse segmento. Porém, fibras naturais são uma importante alternativa considerando-se os aspectos de sustentabilidade, reciclabilidade, abundância e baixo custo, se comparadas a fibras de vidro e ao talco industrial. Neste trabalho escolheu-se o sisal como reforço em função dos resultados obtidos em testes de tração, impacto, homogeneidade, resistência a riscos, a não formação de pontas quando fraturado e ao menor peso específico da peça final. Este trabalho conduziu ensaios e testes para comparar o desempenho do novo composto aos compostos atualmente em uso. Resultados mostraram que o PP - sisal apresenta melhor rigidez e maior resistência do que os seus concorrentes, ainda contando com o apelo natural e ecológico do composto. Outro aspecto muito importante neste estudo foi o uso de polipropileno reciclado, o que transformou o resultado final num produto ainda mais ecológico e rentável. Reinforced with sisal fibers, polypropylene (PP - Sisal) is a compound developed to be applied in the injection process of plastic automotive parts. Polypropylene compounds reinforced with mineral fillers (talc) or fiber glass have been widely applied in this segment. However, natural fibers are an important alternative considering the following aspects: sustainability, recyclability, abundant availability and low cost, if compared with fiber glass and mineral fillers. This work chose sisal as reinforcement in function of the results achieved in the tests of traction, impact, homogeneity and scratch resistance. Besides that, no sharp is formed when broken and specific weight is light in the final part. We conducted try-outs and test to compare the performance of the new composition with that of current ones, and results have shown that the PP - sisal presents better rigidity and greater resistance than its competitors, still counting with natural and ecological appeal. Another very important aspect in this study was the use of recycled polypropylene, which made final results more ecological and profitable.

The density of states of electron or hole systems under the application of external magnetic fields comprises an infinite sum of Landau levels (LLs), which in practical calculations is approximated by a finite sum. Here it is shown a mathematical expression that can be employed in place of these sums with the advantage of shorter computational time. A simplified expression for the corresponding magnetoresistance is also derived.

As is well known, the heat exchange between bodies at different temperatures enclosured at vacuum without thermal contact can be described by the Stefan–Boltzmann law of radiation, where each body receives a fraction of radiant heat from the others depending on their distances and individual temperatures, geometrical shapes, emissivities, and absorptivities. However, when these bodies are surrounded by a gaseous atmosphere conductive and convective phenomena enter on the scenario, leading to a complex mechanism of heat exchange. Here, we study an experimental realization of such situation for two bodies employing a kind of vacuum gauge, on a range of surrounding air pressure between 26 and 1035 mbar, and analyze the heat exchange on the framework of Stefan–Boltzmann law. It is verified empirically that the ratio between the thermal power irradiated by the bodies is independent on their individual heat radiances, and depends only on the surrounding gas pressure.

The classical and the quantum expressions of the energy of the electromagnetic radiation are compared. This comparison shows that, contrary to the commonly expected, the dependency of the energy [Formula: see text] of the photon with respect to the frequency [Formula: see text] on the Einstein–Planck relation [Formula: see text] is obtainable from classical electrodynamics, a fact that may provide a new historic perspective of the birth of quantum mechanics. It is suggested that the photon is a composite particle, constituted by quanta of electric and magnetic field fluxes maybe in an entangled state — being these two quanta either real or only as pseudo particles. It is also suggested a modification on the de Broglie’s fusion principle for the photon for the specific case in consideration.

Here we study the convergence of numeric solutions for the one-dimensional Schrodinger–Poisson problem for electrons confined into a semiconductor quantum well structure. One kind of algorithm that is largely used is based on a simple iterative procedure that is finished when the solution is achieved when particular parameter (for example, an energy) converges. There is also the possibility of the employ of a mixing parameter to control the variation of a particular parameter of the system, or to fix the number of iterations while a particular parameter of the system is gradually increased (for example, the electron density). We show that the two latter algorithms are capable of solving the problem for a wider class of situations if compared to the former iterative without mixing, without significant loss of precision.

Here we present the results of magneto resistance measurements in tilted magnetic field and compare them with calculations. The comparison between calculated and measured spectra for the case of perpendicular fields enable us to estimate the dependence of the valley splitting as a function of the magnetic field and the total Landé g -factor (which is assumed to be independent of the magnetic field). Since both the exchange contribution to the Zeeman splitting as well as the valley splitting are properties associated with the 2D quantum confinement, they depend only on the perpendicular component of the magnetic field, while the bare Zeeman splitting depends on the total magnetic field. This information aided by the comparison between experimental and calculated gray scale maps permits to obtain separately the values of the exchange and the bare contribution to the g -factor.

We report observation of the Hall slope change in wide Al cGa1-cAs parabolic wells in the presence of a quasi-parallel magnetic field. Above the critical magnetic field B > 4T, the Hall resistance becomes temperature dependent and can be described by equation Rxy/costheta ~ (B-B0)/en s, where n s is the electron density, B0=2-2.6 T and theta is the angle between magnetic field and the normal to the well plane. The effect strongly depends on the electron density; it is observed only in parabolic wells, which are almost completely filled by electrons. We attribute the Hall slope change to the unusual behaviour of the effective g factor in such parabolic well, which depends on the Al composition and changes the sign along the well width.

Photoluminescence (PL) spectroscopy and atomic-force microscopy (AFM) were used to investigate the size evolution of InAs quantum dots on GaAs(001) as a function of the amount of InAs material. Different families of islands were observed in the AFM images and unambiguously identified in the PL spectra, together with the signal of the wetting layer. PL measurements carried out at low and intermediate temperatures showed a thermal carrier redistribution among dots belonging to different families. The physical origin of this behavior is explained in terms of the different temperature dependence of the carrier-capture rate into the quantum dots. At high temperatures, an enhancement of the total PL-integrated intensity of the largest-sized quantum dots was attributed to the increase of diffusivity of the photogenerated carriers inside the wetting layer.

The construction of a thermocouple vacuum gauge for the range of pressures from 200 to 800 mbar is reported. The gauge differs from similar ones that cover the medium vacuum region. The main difference is that the thermocouple is not fixed to the filament, as usual, but is placed at some distance from the filament.

Following the line of the history, if by one side the electromagnetic theory was consolidated on the 19th century, the emergence of the special and the general relativity theories on the 20th century opened possibilities of further developments, with the search for the unification of the gravitation and the electromagnetism on a single unified theory. Some attempts to the geometrization of the electromagnetism emerged in this context, where these first models resided strictly on a classical basis. Posteriorly, they were followed by more complete and embracing quantum field theories. The present work reconsiders the classical viewpoint, with the purpose of showing that at first order of approximation the electromagnetism constitutes a geometric structure aside other phenomena as gravitation, and that magnetic monopoles do not exist at least up to this order of approximation. Even though being limited, the model is consistent and offers the possibility of an experimental test of validity., 17 pages, no figures

Shubnikov-de Haas (SdH) and Hall measurements have been used to investigate the electron densities and mobilities in a pair of adjacent two-dimensional electron gases (2DEGs), which were confined in coupled and uncoupled AlxGa1−xAs/AlAs/AlyGa1−yAs double quantum wells (DQWs), as a function of front-gate voltage (Vg). The electron densities and mobilities of the first and second sub-bands were determined using the fast Fourier transform (FFT) in both coupled and uncoupled samples, named S1, S2 and S3, which have different aluminum (Al) content in each well. Sample S1 was illuminated in order to reach the coupled behavior. The electronic properties of the samples have been compared, showing that the electron density and mobility decrease with increasing Al content within the quantum wells (QWs). The last fact was attributed to the alloy scattering in the AlxGa1−xAs quantum well.

Parabolic quantum wells (PQWs) have been studied by temperature dependent photoluminescence (PL). Two kind of samples have been studied. Concerning the undoped sample, the dominant luminescences were the bulk GaAs and the fundamental transition of the PQW. The evolution on temperature of the energy position of both PL emissions follows the well known Varshing formula. For the doped samples strong radiative recombination of the electron gas with photogenerated holes was observed. At low temperature strong Fermi level enhancement occurs in the luminescence as a result of the multi-electron-hole scattering, which is smear out increasing the temperature. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this article we investigated structural and vibrational properties of an amorphous alloy of the In-Se system. The amorphous InSe9 alloy was produced by mechanical alloying and it was studied using Raman spectroscopy and extended X-ray absorption fine structure spectroscopy techniques, and from them several properties, such as vibrational modes, average coordination numbers and interatomic distances, structural and thermal disorders, assymetry of the partial distribution functions g ij (r), thermal expansions and Einstein temperatures were determined and compared to the values found in other alloys of the In-Se system. This comparison indicated some important differences among our alloy produced by mechanical alloying and alloys produced by other techniques, which is a clear indication of the influence of the fabrication technique in their physical properties. The main differences occur in the vibrational spectrum, with the presence of new vibrational modes, and in the structure, since Se-Se pairs are found in a higher quantity than in other In-Se alloys.

Mn ions were implanted in p-type cubic GaN at doses from 0.6 to 2.4×1016cm−2 at 200 keV energy. A 200-nm-thick epitaxial layer, grown by molecular beam epitaxy on GaAs(001) substrate, is used for the Mn implantation. The Mn implanted samples were subjected to an annealing at 950 °C for 1–5 min. The structural quality of the samples was investigated by high resolution x-ray diffraction and Raman spectroscopy. The annealing procedure leads to a significant increasing of the crystalline quality of the samples. Hysteresis loops were observed for all cubic GaMnN annealed samples and ferromagnetism was detected up to room temperature.

is the Landau index number, in the tilted magneticfield.At high filling factors, the Shubnikov-de Haas oscillations exhibit beating effects at certain tilt angles. Weattribute such behavior to oscillations of the tunneling gap due to Aharonov-Bohm interference effect betweencyclotron orbits in different layers. The interplay between quantum and quasiclassical regimes is established.DOI: 10.1103/PhysRevB.78.155320 PACS number s : 73.50.Rb, 73.40.Lq

We have studied the quantum Hall effect in parabolic ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ and square GaAs quantum wells with two occupied subbands in magnetic fields $B$ tilted by an angle $\ensuremath{\Theta}$ with respect to the normal to the sample. We built the density--magnetic field ${n}_{s}\text{\ensuremath{-}}B$ and angle-magnetic field $\ensuremath{\Theta}\text{\ensuremath{-}}B$ topological diagrams for the longitudinal resistivity ${\ensuremath{\rho}}_{xx}$ and observed that the latter shows a similarity with a simulation based on a single-particle harmonic potential approximation.

We have studied the quantum Hall effect in a parabolic well with two occupied subbands. The ringlike structure is found in the density‐magnetic field phase diagram for longitudinal and Hall resistances. In addition we produce a tilt angle‐magnetic field phase diagram, which also demonstrates a set of loop‐like structures. Both phase diagrams are different from the early observations in samples with lower mobility and density.

Neste trabalho, apresentamos o resultado de nossas investigações em amostras de poços quânticos parabólicos (PQW) de AlGaAs crescidas em substratos de GaAs por MBE (Molecular Beam Epitaxy). Nossos estudos se concentram nas implicações da variação do fator g de Landé ao longo da estrutura dos PQW, a qual ocorre em virtude da dependência dessa grandeza com respeito ao conteúdo de Al na liga AlGaAs. Essas implicações são analisadas através de medidas de transporte eletrônico (medidas de Hall e do efeito Shubnikov-de Haas). As medidas de Subnikov-de Haas a temperaturas da ordem de dezenas a centenas de milikelvin com variação do ângulo de inclinação se mostram um eficiente método para a determinação do fator g. Distinguimos não só o fator g determinado pelas propriedades da liga, como também uma contribuição oriunda de efeitos de muitos corpos (contribuição de troca). Por outro lado, as medidas de Hall nos revelam um comportamento anômalo, que mostramos não ter origem no conhecido "efeito Hall anômalo" presente em materiais ferromagnéticos, nem em efeitos de ocupação de múltiplas sub-bandas. Atribuímos o fenômeno a um efeito "válvula de spin", conseqüente da variação espacial do fator g. Nossas observações nos permitem a idealização de um transistor "válvula de spin", prescindindo do emprego de materiais magnéticos. We present the results of our investigations concerning MBE grown AlGaAs/GaAs parabolic quantum well (PQW) samples. We focused on the variation of the Landé g factor along the structure of the PQWs, which occur as a consquence of its dependence on the Al content on the alloy AlGaAs. The implications are studied by Hall and Shubnikov-de Haas measurements. Shubnikov-de Haas measurements at temperatures of the order of tenths to hundreds of milikelvin with variation of the tilt angle are shown to be an efficient method for the determination of the g factor. We could distinguish not only the alloy g factor, but its many body contribution (exchange contribution). On the other hand, Hall measurements exhibit an unusual behavior, which we prooved it has no relation neither to the well known "anomalous Hall effect", a characteristic of ferromagnetic materials, nor to a multi subband occupation effect. We atribute such behavior to a "spin valve effect", caused by the spatial variation of the g factor. Our observations allow us to idealize a "spin valve" transistor, without any ferromagnetic material in its structure.

We compare the transport properties for triangular, parabolic and cubic quantum wells. We calculate the transport mobility for electrons belonging to the different subbands. We obtain the energy spacing between first and second subbands from the electron sheet density and compare results for different potential profiles. We find that experimental results are in quantitative agreements with our calculations.

The Shubnikov de Haas oscillations in 500 A parabolic well are studied in the tilted magnetic field. The electric field displaces the electron wave function along Z -axis and leads to the strong variation of the average bare g-factor in such system. From the measurements of the filling factor ν c at which the spin gap collapse occurs, we deduce the total Zeeman energy, which consists of the bare Zeeman energy and exchange-correlation term. By investigating of the variation of ν c in tilted field we reliably extract the bare g-factor as a function of the gate voltage.

We study the Hall effect in wide AlcGac−1As parabolic wells in the presence of the tilted magnetic field. The Hall resistance is described by equations Rxy / cos Q=−B /ens at B,4 T, and Rxy / cos Q=−A3 sB−B0d /ens at B.4 T, where ns is the electron density, B0=2–2.6 T, A is the temperature dependent coefficient, and Q is the angle between the magnetic field and the normal to the well plane. The effective g factor in such materials depends on the Al composition and changes the sign along the well width. In the presence of the strong tilted magnetic field electron moving along the z direction acquires a spin flip process, which is strongly suppressed at low temperatures, and leads to the change of the Hall effect slope.

In the present paper we report on the experimental electron sheet density vs. magnetic field diagram for the magnetoresistance Rxx of a two-dimensional electron system (2DES) with two occupied subbands. For magnetic fields above 9?T, we found fractional quantum Hall levels centered around the filing factor ?=3/2 in both the two occupied electric subbands. We focused specially on the fractional levels of the second subband, whose experimental values of the magnetic field B of their minima do not obey a periodicity law in 1/|B?Bc|, where Bc is the critical field at the filling factor ?=3/2, and we explain this fact entirely in the framework of the composite fermions theory. We use a simple theoretical model to give a possible explanation for the fact.

In this work the electronic structure of undoped AlGaAs/GaAs wide parabolic quantum wells (PQWs) with different well widths (1000 A and 3000 A) were investigated by means of photoluminescence (PL) measurements. Due to the particular potential shape, the sample structure confines photocreated carriers with almost three-dimensional characteristics. Our data show that depending on the well width thickness it is possible to observe very narrow structures in the PL spectra, which were ascribed to emissions associated to the recombination of confined 1s-excitons of the parabolic potential wells. From our measurements, the exciton binding energies (of a few meV) were estimated. Besides the exciton emission, we have also observed PL emissions associated to electrons in the excited subbands of the PQWs.

Photoluminescence measurements at different temperatures have been performed to investigate the optical response of a two-dimensional electron gas in n-type wide parabolic quantum wells. A series of samples with different well widths in the range of 1000–3000A was analyzed. Many-body effects, usually observed in the recombination process of a two-dimensional electron gas, appear as a strong enhancement in the photoluminescence spectra at the Fermi level at low temperature only in the thinnest parabolic quantum wells. The suppression of the many-body effect in the thicker quantum wells was attributed to the decrease of the overlap between the wavefunctions of the photocreated holes and the two-dimensional electrons belonging to the highest occupied electron subband.

Nesta tese, apresentamos estudos de magnetotransporte em poços quânticos duplos (DQWs) a campos magnéticos de baixo e sob a aplicação de um campo elétrico externo (potencial de porta). Medidas de magnetorresistência foram realizadas tanto no regime linear quanto no regime não linear. Relatamos a observação de oscilações magnéticas de inter-sub-banda (MIS) pela primeira vez. Estas oscilações MIS já foram estudadas em poços quânticos simples (QWs) com duas sub-bandas ocupadas; um DQW´e o sistema mais apropriado para o estudo das oscilações MIS. As oscilações MIS são atribuídas ao espalhamento inter-sub-banda, e a intensidade delas depende da largura da barreira (relacionada ao gap de energia entre as duas sub-bandas ocupadas, SAS). O estudo das oscilações MIS é uma ferramenta importante para poder acessar ao tempo de vida quântico dos elétrons a temperaturas onde as oscilações Shubnikov-de Haas (SdH) já não são observadas. Em nossas amostras, as oscilações MIS persistem até 25 K. Explicamos estes resultados num modelo teórico considerando um potencial de espalhamento de curto alcance com uma contribuição significativa do tempo de espalhamento elástico dos elétrons e uma contribuição do espalhamento elétron-elétron (e-e) com o aumento da temperatura. A aplicação de um campo elétrico externo (correntes dc) modifica fortemente as oscilações MIS. Descrevemos este efeito não linear causado pelo campo elétrico dc com uma função de distribuição oscilatória. Considerando o aquecimento dos elétrons pelo campo elétrico, é extraído o tempo de espalhamento inelástico. Para correntes dc grandes são encontradas discrepâncias entre o experimento e a teoria. Finalmente, consideramos medidas de magnetotransporte como função de potenciais de porta (porta na superficie) levando ao desbalance do DQW. Encontramos que as contribuições clássica e quântica são necessárias para a descrição teórica da magnetorresistência. Descrevemos as contribuições da magnetorresistência em termos das taxas de espalhamento inter e intra sub-banda utilizando uma função gaussiana como função da correlação do potencial. In this thesis we present studies of magnetotransport in double quantum wells (DQWs) in low magnetic fields and under application of an external electric field (gate potential). Measurements of magnetoresistance have been carried out in both linear and non-linear regime. We report on the observation of magneto-intersubband (MIS) oscillations for the first time. These MIS oscillations have been studied already in quantum wells (QWs) with two occupied subbands, DQW is the most convenient system for studies of MIS oscillations. They are attributed to intersubband scattering and the strength of MIS oscillations depends on the barrier width (´delta´SAS). Analysis of MIS oscillations is an important tool to access quantum lifetime of electrons at high temperatures where Shubnikov-de Haas (SdH) oscillations are already absent. For our samples, MIS oscillations still exist up to 25 K. We explain these results in a theoretical model considering short-range scattering potential with a significant contribution of el´astic scattering time of electrons and a contribution of electron-electron (e-e) scattering if one increases temperature. Application of an external electric field (here a dc currents) strongly modifies the MIS oscillations. We describe this non-linear effect caused by a dc electric field with nonequilibrium part of the electron distribution function. Including the heating of electrons by the electric field, we are able to extract inelastic scattering time. For a strong dc current, a discrepancy between experiment and theory is found. Finally, we consider gate-dependent (top gate) magnetotransport measurements and drive de DQWs out of balance. We find that both cl´assical and quantum contributions are necessary for theoretical description of the magnetoresistance. We express both contributions in terms of inter and intrasubband scattering rates using a gaussian function as correlation function of the potential.