The doping concentration and resistivity of tin doped Gallium arsenide nanowires (GaAs NWs) have been investigated using Hall effect-, 4-probe-, transmission line-, and field effect measurements. Single nanowires were contacted using electron beam lithography followed by thermal evaporation of Au/Ti (900/100 Å). The Sn precursor (TESn) molar ratios of the investigated nanowires were 8.5·10-7, 1.7·10-6, 3.4·10-6 and 6.8·10-6 resulting in doping concentrations ranging from 4.64·1013 to 2.11·1017 cm-3 and resistivities from ~0.01 to ~1 Ωcm. The yield of the device fabrication was 2.4-7.1 % and evaluation of additional samples should be done in order to establish the validity of the results. The contact material was proved to work well with the higher doped samples but non-ohmic, highly resistive behavior was seen in the lower doped devices. A resistivity gradient along the length of the nanowires was found to be present, most likely the result of a doping gradient. The sample series with TESn molar ratio 1.7·10-6 showed more tapering than the other series possibly leading to a highly doped shell, which was indicated by 4-probe measurements.

The design, fabrication and performance of an Extraordinary Magnetoresistance (EMR) and a Vertical Mesa Hall Sensor (VMHS) are studied. EMR devices have been fabricated from a 2DEG InAs/GaSb structures which exhibit a low carrier density and high mobility that achieve the best performance. The general electrical magneto-transport properties are given. The experiments investigate mainly different metallic patterns, which are Rectangular, Triangular and Tip pattern between 4-300 K. Probe configurations and the enhancement of relative size of metallic patterns are described. EMR effect is due to current deflection around the metal-semiconductor interface. The results are metallic pattern dependent. Using finite element analysis, good agreement between experimental and theoretical results was found. The best performance sensor is a symmetrical metallic Tip pattern. It is enhanced by the length of the Tip’s point and the large metallic area. This pattern when combines with an asymmetrical probe configuration, exhibits the highest EMR of 900% at –0.275T measured by inner probes and the best sensitivity of 54Ω/T at room temperature. The second study presents in-plane Hall effect sensors made from InSb. A simple device geometry has been used in which current flows in a plane perpendicular to the device surface. Device sensitivity depends on its geometry and a series of different contacts are used to investigate the geometry of the current flow distribution. The structures produced are only sensitive to the presence of one in-plane field component, and they also demonstrate good angular selectivity. Multi-electrodes were used to investigate biasing current from both mesa and substrate condition. We are able to examine the Hall voltage as a function of contact positions and also to create multiple VMHS. Offset reduction of devices has been achieved by moving the ground contacts to re-balance the current distribution under the mesa surface.

The most common type of electric propulsion in space exploration is the Hall Effect Thruster (HET), mainly due to its high specific impulse and high thrust to power ratio. However, uncertainties about the thruster's lifetime prediction have prevented widespread integration of HETs. Among these limitations, wall erosion of acceleration channel is of greatest concern. The experimental methods of erosion are time consuming and costly, and they are often limited to one single configuration. Hence, developing a computational model not only decreases the costs but also shortens the design time of a HET. This thesis investigates the feasibility of a uid erosion modelling with a multi-physics software (COMSOL) to further decrease the time and the development cost. First of all, this thesis provides an overview of available plasma modelling techniques and the physics behind the erosion phenomenon. Moreover, the effective parameters and available modules in the multiphysics software as well as their theoretical background were studied and discussed in detail. The Electron Anomalous phenomenon and pressure instability are determined as the main limiting factors for such a model. A non-magnetized model is included to find an optimal value for pressure and to reduce the probability of pressure instability occurrence in magnetized model. To fulfill this task, several simulations for various pressure values (0.005 Torr, 0.05 Torr, and 0.5 Torr) were conducted. Next, the simulation of magnetized/full model has been carried out with addition of magnetic coils in non-magnetized model. To avoid the Electron Anomalous phenomenon, the Bohm diffusion approach was implemented. In addition, a full Particle-In-Cell (PIC) simulation of a typical HET (SPT-100) with the similar input parameters as in fluid model was conducted, and the results were compared and validated using experimental data. The PIC model was intended to be utilized to investigate the accuracy of erosion model in multiphysics software. The results of this thesis indicate that current application of erosion model in COMSOL is not possible whilst high accuracy of the erosion model based on PIC approach can be achieved. Finally, the application of semi-empirical method through direct input of magnetic field data can allow short time simulation of a HET in COMSOL to gain insight about the preliminary behaviour of plasma, however, the simulation of an erosion model requires either a built-in PIC algorithm in COMSOL or a PIC based code.

On considère un analogue électronique de l'interféromètre de Hong-Ou-Mandel (HOM), dans lequel deux électrons uniques propagent selon des états de bords chiraux opposés et rentrent en collision au niveau d'un point contact quantique. En étudiant le bruit de courant, on montre qu'en raison des interactions entre les canaux co-propageant, le degré d'indistinctibilité entre deux paquets d'ondes électroniques est dramatiquement réduit, ce qui résulte en un contraste réduit pour le signal HOM. Ce phénomène de décohérence dépend fortement de la résolution en énergie des paquets. Étant donné que les interactions provoquent la fractionalisation de la charge, on montre que le mode de charge et le mode neutre interfèrent l'un avec l'autre, ce qui crée des creux ou des pics satellites dans le bruit de courant. Nos calculs expliquent de récents résultats expérimentaux qui révèlent un signal électronique HOM avec un contraste réduit. / We consider an electronic analog of the Hong-Ou-Mandel (HOM) interferometer, where two single electrons travel along opposite chiral edge states and collide at a Quantum Point Contact. Studying the current noise, we show that because of interactions between co-propagating edge states, the degree of indistinguishability between the two electron wavepackets is dramatically reduced, leading to reduced contrast for the HOM signal. This decoherence phenomenon strongly depends on the energy resolution of the packets. Insofar as interactions cause charge fractionalization, we show that charge and neutral modes interfere with each other, leading to satellite dips or peaks in the current noise. Our calculations explain recent experimental results where an electronic HOM signal with reduced contrast was observed.

The two dimensional electron gas subjected to a perpendicular magnetic field is a model system that supports a variety of electronic phases. Perhaps the most well-known are the fractional quantum Hall states, but in recent years there has been an upsurge of interest in the charge ordered phases commonly referred to as electron solids. These solids are a consequence of electron-electron interactions in a magnetic field. While some solid phases form in the lowest Landau level, the charged ordered phases are most abundant in the higher Landau levels. Examples of such phases include the Wigner solids, electronic bubble phases and stripe or nematic phases. Open questions surround the exact role of disorder, confinement potential, temperature and the Landau level index in determining the stability and competition of these phases with other ground states. <div>The interface of GaAs/AlGaAs remains the cleanest host for the two-dimensional electron gas due to the extremely high quality of materials available and the advancement in molecular beam epitaxy growth techniques. As a result, exceptionally high electron mobilities in this system have been instrumental in the discovery of numerous electron solids. </div><div>In this Thesis, I discuss the discovery and properties of several electron solids that develop in such state-of-the-art two dimensional electron gases. These electron solids often develop at ultra low temperatures, in the milliKelvin temperature range. After an introduction to the physics of the quantum Hall effect in two dimensions, in chapter 3, I discuss electron solids developing in the N=1 Landau level. While these solids have been known for some time, details of the competition of these phases xiii with the nearby fractional quantum Hall states remains elusive. A number of reports observe new fractional quantum Hall states at filling factors where electron solids are found in other experiments. We undertook a systematic study to answer some of these unsettled questions. We see evidence for incipient fractional quantum Hall states at 2+2/7 and 2+5/7 at intermediate temperatures which are overtaken by the electronic bubble phases at lower temperatures. Several missing fractional states including those at filling factors 2+3/5, 2+3/7, 2+4/9 highlight the relative stability of the electronic solids called the bubble phases in the vicinity in our sample. </div><div>In chapter 4, I discuss a newly seen electron crystal which manifests itself in transport measurements as a reentrant integer quantum Hall state. Reentrant integer behavior is common in high Landau levels, but so far it was not observed in the lowest Landau level in narrow quantum well samples. In contrast to high Landau levels, where such reentrant integer behavior was associated with electronic bubbles, we believe that the same signature in the N=0 Landau level is due to an electronic Wigner crystal. The filling factors at which we observe such reentrance reveal that it is a crystal of holes, rather than electrons. The discovery of this reentrant integer state paints a complex picture of the interplay of the Wigner crystal and fractional quantum Hall states. </div><div>Finally, in chapter 5, I discuss the observation of a novel phenomenon, that of reentrant fractional quantum Hall effect. In the lowest Landau level, we observe a fractional quantum Hall state, but as the field is increased, we see a deviation and then a return to quantization in the Hall resistance. Such a behavior indicates a novel electron solid. In contrast to the collective localization of electrons evidenced by the reentrant integer quantum Hall effect, such reentrance to a fractional Hall resistance clearly points to the involvement of composite fermion quasiparticles. This property thus distinguishes the ground state we observed as a solid formed of composite fermions. Such a solid phase is evidence for exotic electron-electron correlations at play which are clearly different from those in the traditional Wigner solid of electrons.<br></div>

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.

Os medidores de vazão contribuem de forma significante para a otimização industrial onde o deslocamento contínuo de massa é parte integrante do sistema produtivo, sendo, portanto, cruciais para a indústria permitindo assim padronizar produtos e automatizar ações reduzindo a força de trabalho humana e minimizando erros. Este é o caso da produção de palatabilizantes, substâncias produzidas a partir da hidrólise enzimática da proteína animal para conferir aroma e sabor aos alimentos, onde existe um deslocamento continuo de massa com dosagem de fluidos acionadas em várias etapas do processo. Podem-se destacar duas etapas principais: a etapa onde ocorre a dosagem de soluções alcalinas para a hidrólise enzimática e a etapa onde ocorre a acidificação para garantir a segurança microbiológica do produto. No trabalho em questão foi desenvolvido e analisado um medidor de vazão de turbina por sensoriamento via efeito Hall, para determinar a vazão volumétrica de fludos no processo produtivo de palatabilizantes. O sistema utilizado na avaliação do medidor de vazão foi desenvolvido em escala piloto. A calibração do medidor permitiu-se calcular o fator K (coeficiente do medidor) tanto para água, onde o coeficiente foi de 5,5, quanto para palatabilizantes, onde o coeficiente calculado foi de 7. A partir dos dados obtidos pelo medidor de vazão pode-se observar um alto nível de correlação dos dados. O medidor de vazão apresentou-se eficaz para os fluidos testados (água e palatabilizante); e, com uma precisão de ± 2%, mostra-se uma alternativa viável para o processo de produção de palatabilizantes. / Flowmeters contribute significantly to the industrial optimization where mass continuous displacement is an integral part of the production system, is therefore crucial for the industry allowing standardize products and automate actions reducing human labor and minimizing errors. This is the case of the production of flavors, substances produced from the enzymatic hydrolysis of animal protein to impart flavor and aroma to food, where there is a continuous mass displacement with fluids dosage triggered in several process steps. They can point out two main steps: a step where dosing occurs alkaline solutions for enzymatic hydrolysis and acidification stage which takes place to ensure the microbiological safety of the product. At work in question was developed and analyzed a turbine flowmeter for sensing via Hall effect to determine the volumetric flow fludos in the production of palatability process. The system used to assess the flow meter was developed on a pilot scale. The meter calibration allowed to calculate the K factor (the ratio meter) for both water where the ratio was 5.5, and for palatability, where the coefficient was calculated 7. From the data obtained by the flowmeter one can observe a high degree of correlation of the data. The flowmeter is made effective for the tested fluids (water and palatability); and with an accuracy of ± 2%, there is shown a viable alternative for the production of flavors process.

Thesis advisor: Ziqiang Wang / Exotic phases and associated phase transitions in low dimensions have been a fascinating frontier and a driving force in modern condensed matter physics since the 80s. Due to strong correlation effect, they are beyond the description of mean-field theory based on a single-particle picture and Landau's symmetry-breaking theory of phase transitions. These new phases of matter require new physical quantities to characterize them and new languages to describe them. This thesis is devoted to the study on exotic phases of correlated electrons in two spatial dimensions. We present the following efforts in understanding two-dimensional exotic phases: (1) Using Zn vertex algebra, we give a complete classification and characterization of different one-component fractional quantum Hall (FQH) states, including their ground state properties and quasiparticles. (2) In terms of a non-unitary transformation, we obtain the exact form of statistical interactions between composite fermions in the lowest Landau level (LLL) with v=1/(2m), m=1,2... By studying the pairing instability of composite fermions we theoretically explains recently observed FQHE in LLL with v=1/2,1/4. (3) We classify different Z2 spin liquids (SLs) on kagome lattice in Schwinger-fermion representation using projective symmetry group (PSG). We propose one most promising candidate for the numerically discovered SL state in nearest-neighbor Heisenberg model on kagome lattice}. (4) By analyzing different Z2 spin liquids on honeycomb lattice within PSG classification, we find out the nature of the gapped SL phase in honeycomb lattice Hubbard model, labeled sublattice pairing state (SPS) in Schwinger-fermion representation. We also identify the neighboring magnetic phase of SPS as a chiral-antiferromagnetic (CAF) phase and analyze the continuous phase transition between SPS and CAF phase. For the first time we identify a SL called 0-flux state in Schwinger-boson representation with one (SPS) in Schwinger-fermion representation by a duality transformation. (5) We show that when certain non-collinear magnetic order coexists in a singlet nodal superconductor, there will be Majorana bound states in vortex cores/on the edges of the superconductor. This proposal opens a window for discovering Majorana fermions in strongly correlated electrons. (6) Motivated by recent numerical discovery of fractionalized phases in topological flat bands, we construct wavefunctions for spin-polarized fractional Chern insulators (FCI) and time reversal symmetric fractional topological insulators (FTI) by parton approach. We show that lattice symmetries give rise to different FCI/FTI states even with the same filling fraction. For the first time we construct FTI wavefunctions in the absence of spin conservation which preserve all lattice symmetries. The constructed wavefunctions also set up the framework for future variational Monte Carlo simulations. / Thesis (PhD) — Boston College, 2011. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Le sujet de cette thèse concerne le transport en alternatif à basse fréquence dans les états de bords de l'effet Hall quantique. L'objet d'étude est la partie imaginaire de l'admittance, inductive ou capacitive, des échantillons de Hall. Dans un gaz d'électrons bidimensionnel, le transport à très basse température sous champ magnétique quantifiant, se fait dans des canaux de bord unidimensionnels. Ces canaux ont une capacité quantique propre et sont par ailleurs couplés par l'interaction électrostatique.Nos échantillons sont fabriqués à partir d'hétérojonctions GaAlAs/GaAs et n'ont pas de grille pour n'exhiber que leur propriétés capacitives intrinsèques. Deux topologies sont étudiées dans cette thèse : les barres de Hall et les Corbinos, de géométrie circulaire. La topologie permet de distinguer 2 comportements dans le transport en alternatif.Sont d'abord présentées les mesures à l'impédancemètre de la partie imaginaire de l'impédance et les spectres de réactance, pour les barres de Hall en fonction de la fréquence. Nous utilisons des mesures 3 points ou bien 2 points, mais toujours en 4 fils, et révélons la nature inductive des état de bords. Nous montrons que l'inductance est inversement proportionnelle à la densité d'état, comme le prévoit le calcul de l'inductance quantique d'un canal 1D. Le formalisme de Buttiker permet en outre de comprendre cette dépendance en terme de capacité mutuelle des états de bords.Nous mesurons également la partie imaginaire de l'admittance et traçons les spectres de susceptance des Corbinos, qui s'avèrent être des capacités parfaites. Nos résultats s'interprètent en faisant intervenir la capacité quantique des états de bords.Enfin, nous montrons que l'effet Hall dans un échantillon type barre, peut être vu comme l'effet d'apparition d'une inductance, et ceci même à haute température. Cette inductance cinétique est le résultat de l'augmentation du temps de dissipation de l'énergie sur les phonons, avec le champ magnétique. / In a two dimensional electron gas, low energy transport in presence of a magnetic field occurs in chiral 1D channels located on the edges of the sample. In the Buttiker’s description of a.c. quantum transport, the “emittance” determines the amplitude of the imaginary part of the admittance, whose sign and physical meaning are determined by the topology: in the case of an Hall bar the emittance is an inductance, while it is a capacitance in the case of a corbino sample.Emittance is related to the density of states and to the drift velocity of carriers. So quantum capacitances and inductances give access to the velocity of the charge carriers through the transit time/dwell time of charges in the quantum circuit.We performed systematic studies on samples with different topologies : Hall bars and Corbino disks. Our samples have no gate, which makes us able to observe the inner properties of the quantum states. We have measured the ac admittance of quantum Hall samples using standard electrical techniques in the [0.1-100] kHz frequency range, at low temperature under high magnetic field.We measured kinetic inductances of Hall bars with three contacts and showed the perfect inductive nature of edge states. We measured quantum capacitances of Corbino disks as well. In both topologies we observed the close relation between the emittance and the density of states. We show also the proportionality between the emittance and the length of the circuit. We obtain the transit time of electrons through the device, and finally the drift velocity on edge states.At high temperature we still observe an inductive behavior of Hall bars, induced by increase of energy relaxation time with magnetic field.

A partir da teoria de resposta linear (formalismo de Kubo) calculamos a condutividade de spin $\\sigma_^$ para um gás bidimensional de elétrons formado num poço quântico com duas subbandas devido a atuação de um novo tipo de interação spin-órbita [Bernardes et al. \\textit{Phys. Rev. Lett.} \\textbf, 076603 (2007) \\& Calsaverini \\textit{et al}. \\textit{Phys. Rev. B} \\textbf, 155313 (2008)]. Este novo termo é não-nulo mesmo em estruturas simétricas e surge devido ao acoplamento entre os estados confinados no poço de paridades diferentes. Encontramos um valor para $\\sigma_^$ não-nulo e não-universal (dependente da intensidade do acoplamento $\\eta$) quando somente uma das subbandas está ocupada, ao contrário de Rashba. Para encontrarmos valores realistas para $\\sigma_^$, determinamos $\\eta$ via cálculo autoconsistente. Esse cálculo é executado para diferentes valores de densidade eletrônica em poços simples e duplos. Obtivemos que $\\sigma_^$ possui um comportamento não-monótono e sofre inversão de sinal como função da energia de Fermi (densidade de elétrons) conforme ela varia entre as duas subbandas. Contudo nossos resultados indicam que a condutividade Hall de spin é muito pequena $\\left(``\\ll \\frac{8\\pi}\"ight)$ nesses sistemas (poços simples e duplos) e possivelmente não mensurável. / Using the Kubo linear response theory, we investigate spin Hall conductivity $\\sigma_^$ in a two-dimensional electron gas in quantum wells with two subbands, when intersubband-induced spin-orbit coupling is operative [Bernardes et al. \\textit{Phys. Rev. Lett.} \\textbf, 076603 (2007) \\& Calsaverini \\textit{et al}. \\textit{Phys. Rev. B} \\textbf, 155313 (2008)]. This new spin-orbit term is non-zero even in symmetric structures and it arises from the distinct parity of the confined states. We find non-zero and non-universal $\\sigma_^$ (dependent on spin-orbit coupling strength $\\eta$) when only one of the subbands is occupied. This is in contrast to the Rashba spin-orbit interaction for which $\\sigma_^$ is identically zero. To obtain realistic values for $\\sigma_^$, we develop a self-consistent scheme to calculate $\\eta$. We performed this calcultion for different values of the eletronic density in single and double wells. We find that $\\sigma_^$ shows a non-monotonic behavior and a sign change as the Fermi energy (carrier density) varies between the two subband edges. However, our results indicate that $\\sigma_^$ is extremely small $\\left(``\\ll \\frac{8\\pi}\"ight)$ and possibly not measurable.