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Innovative multifunctional materials are essential to many new sensor applications. Piezoresistive nano-composites make up a promising class of such materials that have the potential to provide a measurable response to strain over a much wider range than typical strain gages. Commercial strain gages are currently dominated by metallic sensors with a useable range of a few percent strain at most. There are, however, many applications that would benefit from a reliable wide-range sensor. These might include the study of explosive behavior, instrumentation of flexible components, motion detection for compliant mechanisms and hinges, human-technology interfaces, and a wide variety of bio-mechanical applications where structural materials may often be approximated as elastomeric. In order to quantify large strains, researchers often use optical methods which are tedious and difficult. This thesis proposes a new material and technique for quantifying large strain (up to 40%) by use of piezoresistive nano-composite strain gages. The nano-composite strain gage material is manufactured by suspending nickel nano-strands within a biocompatible silicone matrix. Study and design iteration on the strain gage material requires an improved understanding of the electrical behavior and conduction path within the material when strained. A percolation model has been suggested for numerical approximations, but has only provided marginal results for lack of data. Critical missing information in the percolation model is the nano-strand cluster size, and how that size changes in response to strain. These data are gathered using a dynamic technique in the scanning electron microscope called voltage contrast. Cluster sizes were found to vary in size by approximately 6% upon being strained to 10%. A feasibility study is also conducted on the nano-composite to show its usability as a strain gage. High Displacement Strain Gages (HDSGs) were manufactured from the nano-composite. HDSGs measured the strain of bovine ligament under prescribed loading conditions. Results demonstrate that HDSGs are an accurate means for measuring ligament strains across a broad spectrum of applied deformations.

Spin-vortices in vertically spaced pairs of thin elliptical Permalloy nanoparticles are investigated. The two vortex cores with parallel out-of-plane magnetization exhibit a strong monopole-like attraction through the spacer much thinner than the core length, thus forming a bound core-core pair. The material of the spacer is designed to suppress both direct and indirect exchange interactions, so the remaining inter-vortex coupling is purely dipolar. In the investigated vortex pairs, the in-plane magnetization in the vortex periphery, outside the vortex cores, curl in opposite directions (have opposite chirality). As a result, the two cores move in opposite directions in response to an in-plane magnetic field, the Zeeman effect of which acts to decouple the core-core pair. This leads to unique dynamics of the spin-vortex parallel-core/antiparallel-chirality pair, which strongly depend on whether the pair is coupled or decoupled. In the coupled state, the cores are held close together by the core-core attraction, which results in short-radius oscillations and a resonance frequency of about 2 GHz for the main rotational eigen-mode. In the decoupled state, the cores are separated by a distance much greater than the core length and gyrate independently with a resonance frequency of the order of 100 MHz. The dynamics of the vortex pair are investigated at 77 K, where there is a bistability between the coupled and decoupled core states. Resonant excitations are used to decouple the cores with pulses of ∼10 Oe in amplitude and ∼100 ps in duration. The ability to decouple a vortex pair using such fast low-power pulses can be useful for multifrequency oscillators and vortex based memory. A search for quantum effects is undertaken at sub-Kelvin temperatures using a dilution refrigerator. Square pulses of 100 ns duration and amplitudes of the order of 1 Oe are applied in-plane to bring the system closer to decoupling, giving the cores a chance to tunnel through the barrier between the coupled and decoupled states. The amplitude required for decoupling is measured as a function of temperature and a leveling off in the decoupling probability is seen below 400 mK, giving some indication of core tunneling. Macroscopic quantum tunneling of magnetization is interesting from the fundamental physics point of view, e.g., as a model system for studying the measurement paradox in quantum mechanics, as well as for current and future computer technology in terms of understanding the ultimate limitations of miniaturizing magnetic memory elements. / I detta arbete studeras spinnvirvlar i elliptiska skivor av Permalloy ordnade i vertikala par. Kärnor av parallell vertikal magnetisering attraherar varandra likt monopoler genom en film mycket tunnare än kärnorna och bildar därmed ett sammankopplat par. Materialet i filmen mellan virvlarna är designat för att förhindra både direkt och indirekt utbytesväxelverkan och lämnar endast kärnornas dipolväxelverkan. I de virvelpar som studeras går den plana magnetiseringen i virvlarnas periferi runt kärnorna åt olika håll (de har motsatt kiralitet). På grund av detta rör sig kärnorna åt olika håll vid applikation av magnetfält i planet (Zeeman effekten) vilket kan leda till att de kopplas isär. Detta ger virvelpar med parallella kärnor och antiparallell kiralitet unika dynamiska egenskaper som ändras med deras tillstånd, sammankopplade eller isärkopplade. I det sammankopplade tillståndet hålls kärnorna ihop av monopolattraktionen vilket gör att de bara kan röra sig i små banor kring sitt magnetiska masscentrum, med en resonansfrekvens på circa 2 GHz. I det isärkopplade tillståndet är kärnorna separerade med ett avstånd som är mycket större än kärnornas diameter, och de rör sig oberoende av varandra med en resonansfrekvens i storleksordningen 100 MHz. Virvelparets dynamik undersöks vid 77 K, där det finns en bistabilitet mellan det sammankopplade och det isärkopplade tillståndet. Pulser med längd ∼100 ps och styrka ∼10 Oe i resonans med det sammankopplade tillståndet används för att koppla isär kärnorna. Att kunna koppla isär dem med så korta lågeffektspulser kan vara användbart för virvelbaserade minnen och multifrekvensoscillatorer. Ett sökande efter kvanteffekter påbörjas vid temperaturer under 1 K med hjälp av en utspädningskyl. Fyrkantsvågor med en längd på 100 ns och en styrka i storleksordningen 1 Oe, orienterade i planet, används för att ge kärnorna en chans att tunnla genom barriären mellan det sammankopplade och det isärkopplade tillståndet. Den vågamplitud som krävs för att koppla isär kärnorna plottas mot temperaturen och kan ses plana ut under 400 mK, vilket ger viss indikation av tunnling. Dessa undersökningar av makroskopisk kvanttunnling av magnetisering kan vara användbar i grundforskning för att studera mätparadoxen i kvantmekanik, men också i modern datorteknologi för att förstå de absoluta begränsningarna i hur små magnetiska minneselement kan göras.

CoordenaÃÃo de AperfeÃoamento de Pessoal de NÃvel Superior / Os avanÃos nas tÃcnicas de crescimento tornaram possÃvel a fabricaÃÃo de estruturas semicondutoras quase-unidimensionais em escalas nanomÃtricas, chamadas pontos, fios, poÃos e anÃis quÃnticos. Interesse nessas estruturas tem crescido consideravelmente, nÃo sà devido Ãs suas possÃveis aplicaÃÃes em dispositivos eletrÃnicos e à sua manipulaÃÃo quÃmica fÃcil, mas tambÃm porque eles oferecem a possibilidade de explorar experimentalmente vÃrios aspectos de confinamento quÃntico, espalhamento e fenÃmenos de interferÃncia. Em particular, neste trabalho, investigamos as propriedades eletrÃnicas e de transporte em poÃos quÃnticos, fios e anÃis, cujas dimensÃes podem ser alcanÃados experimentalmente. Para isto, resolvemos a equaÃÃo de SchrÃdinger dependente do tempo utilizando o mÃtodo Split-operator em duas dimensÃes. Nesta tese, abordamos quatro trabalhos, sendo o primeiro uma analogia ao Paradoxo de Braess para um sistema mesoscÃpico. Para isso, utilizamos um anel quÃntico com um canal adicional na regiÃo central, alinhado com os canais de entrada e saÃda. Este canal extra faz o papel do caminho adicional em uma rede de trÃfego na teoria dos jogos, similar ao caso do paradoxo de Braess. Calculamos as auto-energias e a evoluÃÃo temporal para o anel quÃntico. Surpreendentemente, o coeficiente de transmissÃo para algumas larguras do canal extra diminuiu, semelhante ao que acontece com redes de trÃfego, onde a presenÃa de uma via extra nÃo necessariamente melhora o fluxo total. Com a analise dos resultados obtidos, foi possÃvel determinar que neste sistema o paradoxo ocorre devido a efeitos de interferÃncia e de espalhamento quÃntico. No segundo trabalho, foi feita uma extensÃo do primeiro, (i) aplicando-se um campo magnÃtico, onde foi possÃvel obter o efeito Aharonov-Bohm para pequenos valores do canal extra e controlar efeitos de interferÃncia responsÃveis pelo paradoxo mencionado, e (ii) fazendo tambÃm a aplicaÃÃo de um potencial que simula a ponta de um microscÃpio de forÃa atÃmica (AFM) interagindo com a amostra - este potencial à repulsivo e simula um possÃvel fechamento do caminho em que o pacote de onda se propaga. Assim, neste trabalho, realizamos uma contra-prova do primeiro, onde observamos que com o posicionamento da ponta do AFM sobre canal extra, se diminui o efeito de reduÃÃo de corrente devido ao paradoxo de Braess. No terceiro trabalho, realizamos uma anÃlise de tunelamento entre dois fios quÃnticos separados por uma certa distÃncia e calculamos qual a menor distÃncia para qual ocorre tunelamento significativo nesse sistema eletrÃnico. Este trabalho à de fundamental importÃncia para o manufaturamento de dispositivos nanoestruturados, porque nos permite investigar qual a distÃncia mÃnima para a construÃÃo de um circuito eletrÃnico sem que haja interferÃncias nas transmissÃes das informaÃÃes. No quarto e Ãltimo trabalho desta tese, investigamos a energia de ligaÃÃo do elÃtron-impureza em GaN/HfO2 para um poÃo quÃntico. Consideramos simultaneamente as contribuiÃÃes de todas as interaÃÃes das auto-energias devido ao descasamento das constantes dielÃtricas entre os materiais. Foram estudados poÃos largos e estreitos, comparando os resultados para diferentes posiÃÃes da impureza e a contribuiÃÃo da auto-energia para o sistema. / Advances in growth techniques have made possible the fabrication of quasi one-dimensional semiconductor structures on nanometric scales, called quantum dots, wires, wells and rings. Interest in these structures has grown considerably not only due to their possible applications in electronic devices and to their easy chemical manipulation, but also because they offer the possibility of experimentally exploring several aspects of quantum confinement, scattering and interference phenomena. In particular, in this work, we investigate the electronic and transport properties in quantum wells, wires and rings, whose dimensions can be achieved experimentally. For this purpose, we solve the time-dependent SchrÃdinger equation using the split-operator method in two dimensions. We address four different problems: in the first one, the electronic transport properties of a mesoscopic branched out quantum ring are discussed in analogy to the Braess Paradox of game theory, which, in simple words, states that adding an extra path to a traffic network does not necessarily improves its overall flow. In this case, we consider a quantum ringindex{Quantum ring} with an extra channel in its central region, aligned with the input and output leads. This extra channel plays the role of an additional path in a similar way as the extra roads in the classical Braess paradox. Our results show that in this system, surprisingly the transmission coefficient decreases for some values of the extra channel width, similarly to the case of traffic networks in the original Braess problem. We demonstrate that such transmission reduction in our case originates from both quantum scattering and interference effects, and is closely related to recent experimental results in a similar mesoscopic system. In the second work of this thesis, we extend the first system by considering different ring geometries, and by investigating the effects of an external perpendicular magnetic field and of obstructions to the electrons pathways on the transport properties of the system. For narrow widths of the extra channel, it is possible to observe Aharonov-Bohm oscillations in the transmission probability. More importantly, the Aharonov-Bohm phase acquired by the wave function in the presence of the magnetic field allows one to verify in which situations the transmission reduction induced by the extra channel is purely due to interference. We simulate a possible closure of one of the paths by applying a local electrostatic potential, which can be seen as a model for the charged tip of an atomic force microscope (AFM). We show that positioning the AFM tip in the extra channel suppresses the transmission reduction due to the Braess paradox, thus demonstrating that closing the extra path improves the overall transport properties of the system. In the third work, we analyze the tunneling of wave packets between two semiconductor quantum wires separated by a short distance. We investigate the smallest distance at which a significant tunneling between the semiconduting wires still occur. This work is of fundamental importantance for the manufacturing of future nanostructured devices, since it provides information on the minimum reasonable distances between the electron channels in miniaturized electronic circuits, where quantum tunnelling and interference effects will start to play a major role. In the last work of this thesis, we investigate the binding energy of the electron-impurity pair in a GaN/HfO2 quantum well. We consider simultaneously the contributions of all interactions in the self-energy due to the dielectric constant mismatch between materials. We investigate the electron-impurity bound states in quantum wells of several widths, and compared the results for different impurity positions.

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