Nesta dissertação, estudamos cadeias unidimensionais antiferromagnéticas de spins 1/2 modeladas pelo Hamiltoniano de Heisenberg na presença de inomogeneidades causadas principalmente pela introdução de ligações substitucionais (defeitos nas ligações) e por efeitos de borda. Interessados então em determinar a energia do estado fundamental de sistemas com quaisquer distribuições das ligações, utilizamos o formalismo da Teoria do Funcional da Densidade (DFT) desenvolvido para o modelo de Heisenberg. O formalismo da DFT permite a estimativa da energia do estado fundamental de sistemas não-homogêneos conhecendo-se o sistema homogêneo. Construímos funcionais na aproximação da ligação local (LBA), proposta recentemente em analogia à já conhecida LSA (aproximação local para o spin). A obtenção dos funcionais se baseou no estudo do modelo de uma cadeia de spins em que as ligações são alternadas, isto é, a interação de troca se alterna em valor de sítio para sítio. Isso originou um funcional não-local na interação de troca da cadeia. Apesar disso, continuamos utilizando a nomenclatura LBA. Todos os resultados fornecidos pelos funcionais são comparados a dados provenientes de diagonalização numérica exata. / In this dissertation, we use the Heisenberg model to describe inhomogeneous antiferromagnetic spin 1/2 chains. The translational invariance is broken mainly due to the non-uniform distribution of bond interactions (defects) and the presence of boundaries. Interested in obtaining the ground-state energy of systems with any distribution of exchange couplings (Jij), we use the density-functional theory (DFT) formalism, developed for the Heisenberg model. The DFT formalism allows an estimate of the ground-state energy of inhomogeneous systems based on the homogeneous systems. We build functionals for the ground-state energy using a local bond approximation (LBA), recently proposed in analogy to the already known LSA (local spin approximation). To obtain the functionals we studied a model that describes an alternating chain, in which the exchange coupling alternates from site-to-site. This resulted in non-local functionals on the spin-spin exchange interaction. Nevertheless, we still call them LBA functionals. All the results from the functionals are compared with exact numerical data.

abstract: Attaining a sufficiently large critical current density (Jc) in magnetic-barrier Josephson junctions has been one of the greatest challenges to the development of dense low-power superconductor memories. Many experimentalists have used various combinations of superconductor (S) and ferromagnetic (F) materials, with limited success towards the goal of attaining a useful Jc. This trial-and-error process is expensive and time consuming. An improvement in the fundamental understanding of transport through the ferromagnetic layers and across the superconductor-ferromagnetic interface could potentially give fast, accurate predictions of the transport properties in devices and help guide the experimental studies. In this thesis, parameters calculated using density functional methods are used to model transport across Nb/0.8 nm Fe/Nb/Nb and Nb/3.8 nm Ni /Nb/Nb Josephson junctions. The model simulates the following transport processes using realistic parameters from density functional theory within the generalized gradient approximation: (a) For the first electron of the Cooper pair in the superconductor to cross the interface- conservation of energy and crystal momentum parallel to the interface (kll). (b) For the second electron to be transmitted coherently- satisfying the Andreev reflection interfacial boundary conditions and crossing within a coherence time, (c) For transmission of the coherent pair through the ferromagnetic layer- the influence of the exchange field on the electrons’ wavefunction and (d) For transport through the bulk and across the interfaces- the role of pair-breaking from spin-flip scattering of the electrons. Our model shows the utility of using realistic electronic-structure band properties of the materials used, rather the mean-field exchange energy and empirical bulk and interfacial material parameters used by earlier workers. [Kontos et al. Phys. Rev Lett, 93(13), 137001. (2004); Demler et al. Phys. Rev. B, 55(22), 15174. (1997)]. The critical current densities obtained from out model for Nb/0.8 nm Fe/Nb is 104 A/cm2 and for Nb/3.8 nm Ni/Nb is 7.1*104 A/cm2. These values fall very close to those observed experimentally- i.e. for Nb/0.8 nm Fe/Nb is 8*103 A/cm2 [Robinson et al" Phys. Rev. B 76, no. 9, 094522. (2007)] and for Nb/3.8 nm of Ni/Nb is 3*104 A/cm2 [Blum et al Physical review letters 89, no. 18, 187004. (2002). This indicates that our approach could potentially be useful in optimizing the properties of ferromagnetic-barrier structures for use in low-energy superconducting memories. / Dissertation/Thesis / Masters Thesis Materials Science and Engineering 2018

Com os desenvolvimentos tecnológicos na área da nanociência e nanotecnologia pode se realizar novas pesquisas de fronteira. Entres estas pesquisas uma de muito interesse é sobre as interações nas escalas nanométricas e micrométricas. E com o desenvolvimento de novos equipamentos para esta área conseguimos observar e entender interações como entre a luz e materiais condutores como os metais. Esta interação produz ondas densas de elétrons, estas ondas são conhecidas como plásmons. Os estudos acerca dos plásmons estão sendo desenvolvidos desde os anos 80, e com as novas tecnologias podemos cada vez mais refinar e potencializar os resultados sobre estes assuntos. As pesquisas mais atuais sobre este tema veem gerando inúmeros desenvolvimentos nas áreas de gravação magneto-ótica, microscopia, detectores moleculares biológicos, entre outras. Os plasmons são oscilações que se confinam na superfície dos materiais, que têm como característica serem ondas evanescentes, por isso eles precisam ser observados em campo próximo. Com o intuito de observar e compreender a propagação destes plasmons de superfície (SPs) foi utilizado um microscópio ótico de varredura em campo próximo (SNOM). Este equipamento permite imagens óticas de campo próximo simultaneamente com imagens topográficas da superfície do material. Em trabalhos anteriores foi possível caracterizar a propagação dos SP em filmes de Ag e Au. Uma característica observada foi um padrão de oscilação ótica dentro da propagação dos SP. Visando entender esta oscilação, estudamos a influência da rugosidade dos filmes nesta oscilação dentro da propagação dos SP. Com o SNOM foi possível analisar a influência da rugosidade sobre a oscilação na propagação dos SP. Este tipo de oscilação já havia sido mostrado em artigos na literatura, porém nunca antes foi analisado em profundidade ou foi dada uma explicação clara para sua existência. Visando produzir filmes com rugosidades diferentes, foi utilizado um sistema de \"magnetron sputtering\" disponível no Laboratório de Materiais Magneticos da USP. Usando-se diferentes temperaturas de deposição e materiais codepositados se produziu filmes de rugosidades distintas. As rugosidades variaram de 1 nm à 40 nm, com isto se pode constatar que existe uma oscilação dentro da propagação dos SP que fica mais evidente, e sofre perturbações, conforme o filme é mais rugoso. Porém também foi observado que mesmo com filmes muito lisos esta oscilação permanece, podendo ser algo intrínseco da própria propagação do SP, e quanto maior rugosidade superficial mais intensa e irregular esta oscilação. / New border technology researches can be made from the technological developments in nanocience and nanotechnology, among which, the research on nanometric and micrometric scales is of great interest. With the development of new equipment for this area, we are able to observe and understand interactions between light and conductive materials, such as metals, for example. This interaction produces dense electron waves, these waves are known as Plasmon. Studies on Plasmon have been developed since the 1980\'s and, with the new technologies we can constantly refine and potentialize the results on this matter. The more recent researches on the theme have been generating innumerous developments in the areas of magneto-optical recording, microscopy, biological molecular detectors, among others. Plasmon\'s consist in confined oscillations in the surfaces of materials, characterized for being evanescent waves, for that reason they have to be observed in near field. In order to observe and comprehend the propagation of these Surface Plasmon (SP) a scanning near-field optical microscope (SNOM) was used. This equipment allows near field images to be formed concomitantly with topographic images of the material\'s surface. In previous works it was possible to characterize the propagation of the SP in Ag and Au films. One observed characteristic was an oscillation pattern within the SP propagation. Aiming to understand this oscillation, we studied the influence of the films rugosity on the oscillation within the propagation of the SP. With SNOM it was possible to analyze the influence of the rugosity on the oscillation within the propagation of the SP. This type of oscillation had already been observed in the literature, but it had never before been analyzed in depth or a clear explanation for its existence has been given. Aiming to produce films with different rugosities, a magnetron sputtering system was used. Using different deposition temperatures and co-deposited materials we produced films with different rugosities. The rugosities vary from 1 nm to 40 nm, with this we could note that there is an oscillation within the SP propagation which are more evident and suffer disturbances, as the film\'s rugosity increases. Although it was observed that even in very smooth films this oscillation remains, which can indicate an intrinsic character of the SP propagation and also, the larger the rugosity, more intense and irregular this oscillation.

Os fotodetectores infravermelhos baseados em pontos quânticos (Quantum-dot Infrared Photodetectors, QDIPs) surgiram recentemente como uma nova tecnologia para a detecção de radiação infravermelha. Comparados com fotodetectores mais convencionais baseados em poços quânticos (Quantum-well Infrared Photodetectors, QWIPs), as suas vantagens se originam no confinamento tridimensional de portadores e incluem a sensibilidade intrínseca à incidência normal de luz, um maior tempo de vida dos portadores fotoexcitados e uma baixa corrente de escuro, que devem permitir o funcionamento dos dispositivos acima das temperaturas criogênicas. No presente trabalho, a técnica de epitaxia por feixe molecular (Molecular-Beam Epitaxy - MBE) foi usada para crescer várias amostras de QDIPs de InAs/GaAs com o objetivo de estudar a inuência dos parâmetros estruturais destes dispositivos. Após o crescimento, as amostras foram processadas em pequenas mesas quadradas por técnicas de litografia convencional e, então, caracterizadas. As propriedades ópticas e eletrônicas dos dispositivos foram verificadas para temperaturas a partir de 10 K. Com o objetivo de realizar medidas eletrônicas de alta qualidade, janelas de Ge e cabos com conectores de baixo ruído para baixa temperatura foram empregados. As curvas de corrente de escuro, as curvas de responsividade com corpo negro (fotocorrente), as medições do ruído com uma analisador de sinais e as respostas espectrais por FTIR (Fourier Transform Infrared) forneceram um conjunto completo de informações sobre os dispositivos. As figuras de mérito dos nossos melhores dispositivos permitiram também, determinar a probabilidade de captura e o ganho fotocondutivo. Com o intuito de compreender a relação entre as dimensões físicas dos pontos quânticos e as características de funcionamento dos QDIPs, desenvolveu-se um cálculo dos estados eletrônicos de da função de onda de um elétron confinado em um ponto quântico de InxGa1-xAs em formato de lente, envolvido em uma matriz de GaAs, com massas efetivas dependentes da posição. Esse modelo leva em conta o efeito da tensão assim como o gradiente de In dentro do ponto quântico, resultante do forte efeito de segregação presente em um sistema de InxGa1-xAs/GaAs. Diferentes perfis de segregação foram testados com o nosso modelo teórico com vista a proporcionar o melhor ajuste os nossos dados experimentais. / Quantum-dot Infrared Photodetectors (QDIPs) recently emerged as a new technology for detecting infrared radiation. Compared to more conventional photodetectors based on quantum wells (QWIPs), their advantages originate from the three-dimensional confinement of carriers and include an intrinsic sensitivity to normal incidence of light, a longer lifetime of the photoexcited carriers and a lower dark current which should hopefully allow their operation close to room temperature. In the present work, molecular-beam epitaxy (MBE) was used to grow several InAs/GaAs QDIP samples in order to analyse the influence the structural properties of such devices. After the growth, the samples were processed into small squared mesas by conventional lithography techniques and fully characterized. The optical and electrical properties of the devices were checked as a function of temperature using Ge optical windows and all the connectors and low-temperature/low-noise cables needed to perform high quality low-level electrical measurements. Dark-current curves, Responsivity (photocurrent) data with a black body, noise measurements with a signal analyzer and spectral responses by FTIR provided a full set of information about the devices. The figures of merit of our best devices allowed us also to determine the capture probability and the photoconductive gain. In order to understand the relationship between the physical dimensions of the quantum dots and the operating characteristics of the QDIPs, we developed a position-dependent effective-mass calculation of the bound energy levels and wave function of the electrons confined in lensshaped InxGa1-xAs quantum dots embedded in GaAs, taking into account the strain as well as the In gradient inside the quantum dots which is due to the strong In segregation and intermixing present in the InxGa1-xAs/GaAs system. Different In profiles inside the quantum dots were tested with our new theoretical model in order to provide the best _t to our experimental data.

A Na, K-ATPase é uma proteína que realiza o transporte ativo de cátions, se encontra na membrana plasmática de praticamente todas células animais e é formada por três subunidades: (110 kDa), (50 kDa) e (10 kDa). Neste trabalho, realizou-se a extração da proteína Na,K-ATPase de rim de coelho que foi preparada em 3 diferentes condições (i) fração de membrana rica em Na,K-ATPase; (ii) solubilizada e purificada em C12E8 e (iii) reconstituída em DPPC: DPPE lipossomo (1:1 lipídio:lipídio, 1:3 lipídio:proteína). Através de medidas de Espalhamento de Luz Dinâmico (DLS), Espectroscopia de Absorção (ABS) e Espalhamento de Raio-X a Baixos Ângulos (SAXS), associadas à medidas de atividade enzimática, constatou-se que a amostra de Na,K-ATPase solubilizada e purificada em C12E8 é constituida por diferentes agregados/oligômeros em solução. Com o intuito de eliminar os grandes agregados/oligômeros da amostra realizou-se a filtração (poro de 220 nm) e a adição do surfactante dodecil sulfato de sódio (SDS) e ambos procedimentos foram capazes de eliminar as populações de grandes agregados e/ou grandes oligômeros. A retirada destas populações pelo filtro promoveu um aumento de atividade específica da enzima. Já o SDS deve promover alterações conformacionais na estrutura da proteína que causam a inativação da mesma. Investigou-se variações de atividade da Na, K-ATPase através da irradiação da proteína presente em fração de membrana e reconstituída em lipossomo por meio de três lasers de baixa intensidade com comprimentos de onda diferentes: = 532 nm (5 mW), = 650 nm (50 mW) e = 780 nm (50 mW). Demonstrou-se que a variação da atividade enzimática depende do valor de dose de energia depositada, independe do comprimento de onda estudado neste intervalo e retorna para o nível basal após 6 horas. / The Na, K-ATPase is an active cation transporter protein, which is found in the plasma membrane of virtually all animal cells and it is comprised of three subunits: (110 kDa), (50 kDa) and (10 kDa). In this work, we performed the extraction of protein Na, K-ATPase from the kidney of adult rabbit for three different enzyme preparations (i) membrane-bound fraction; (ii) C12E8 solubilized and purified and (iii) reconstituted in DPPC: DPPE liposome (1: 1 - lipid: lipid, 1:3 - lipid:protein). Dynamic Light Scattering (DLS), Absorption Spectroscopy (ABS) and Small Angle X-ray Scattering (SAXS) were employed, associated with enzyme activity measurements. The results revealed that Na, K-ATPase C12E8-solubilized and purified is composed by different aggregates/oligomers. With the aim of eliminating large aggregates/oligomers from the protein sample, filtration (pore size 220 nm) and surfactant sodium dodecyl sulfate (SDS) addition were used. Both procedures were able to eliminate populations composed of large aggregates and/or large oligomers. The removal of these populations by the filter promoted an increase in the specific activity of the enzyme. On the other hand, SDS must promote conformational changes in the protein structure that inactivate thereof. Finally, here we also investigated variations of Na, K-ATPase activity present in the membrane-bound fraction and reconstituted in liposome under irradiation of three low-intensity lasers with different wavelengths: = 532 nm (5mW), = 650 nm (50 mW) and = 780 nm (50 mW). The results give support to the conclusion that the change in the enzymatic activity depends upon the amount of energy dose deposited, it is independent of the wavelength in the studied range and returns to the basal level after 6 hours.

abstract: Wide bandgap (WBG) semiconductors GaN (3.4 eV), Ga2O3 (4.8 eV) and AlN (6.2 eV), have gained considerable interests for energy-efficient optoelectronic and electronic applications in solid-state lighting, photovoltaics, power conversion, and so on. They can offer unique device performance compared with traditional semiconductors such as Si. Efficient GaN based light-emitting diodes (LEDs) have increasingly displaced incandescent and fluorescent bulbs as the new major light sources for lighting and display. In addition, due to their large bandgap and high critical electrical field, WBG semiconductors are also ideal candidates for efficient power conversion. In this dissertation, two types of devices are demonstrated: optoelectronic and electronic devices. Commercial polar c-plane LEDs suffer from reduced efficiency with increasing current densities, knowns as “efficiency droop”, while nonpolar/semipolar LEDs exhibit a very low efficiency droop. A modified ABC model with weak phase space filling effects is proposed to explain the low droop performance, providing insights for designing droop-free LEDs. The other emerging optoelectronics is nonpolar/semipolar III-nitride intersubband transition (ISBT) based photodetectors in terahertz and far infrared regime due to the large optical phonon energy and band offset, and the potential of room-temperature operation. ISBT properties are systematically studied for devices with different structures parameters. In terms of electronic devices, vertical GaN p-n diodes and Schottky barrier diodes (SBDs) with high breakdown voltages are homoepitaxially grown on GaN bulk substrates with much reduced defect densities and improved device performance. The advantages of the vertical structure over the lateral structure are multifold: smaller chip area, larger current, less sensitivity to surface states, better scalability, and smaller current dispersion. Three methods are proposed to boost the device performances: thick buffer layer design, hydrogen-plasma based edge termination technique, and multiple drift layer design. In addition, newly emerged Ga2O3 and AlN power electronics may outperform GaN devices. Because of the highly anisotropic crystal structure of Ga2O3, anisotropic electrical properties have been observed in Ga2O3 electronics. The first 1-kV-class AlN SBDs are demonstrated on cost-effective sapphire substrates. Several future topics are also proposed including selective-area doping in GaN power devices, vertical AlN power devices, and (Al,Ga,In)2O3 materials and devices. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2019

Dans ce manuscrit de thèse, nous revisitons d'abord la physique des états de Yu-Shiba-Rusinov, en nous concentrant sur leur polarisation en spin. Nous commençons par montrer théoriquement que nous pouvons extraire beaucoup d'informations sur le supraconducteur hôte, en analysant la densité locale d'états électroniques liée à la présence d'impuretés magnétiques. Tout d'abord, nous démontrons que le couplage spin-orbite peut être lu directement et sans ambiguïté par la spectroscopie par effet tunnel résolu en spin dans les systèmes bidimensionnels et unidimensionnels, qu’ils soient supraconducteurs ou métalliques. Nous analysons les oscillations induites par les impuretés dans la densité d'états électroniques. En particulier, nous nous concentrons sur la transformation de Fourier (TF) des oscillations de Friedel et nous notons que les caractéristiques à haute intensité apparaissent pour un vecteur d'onde donné par deux fois la longueur inverse du spin-orbite. Ensuite, nous montrons qu'il est possible de déterminer le mécanisme d’appariement dominant, qu’il soit en ondes s ou en ondes p, dans les supraconducteurs non conventionnels en analysant la structure spectrale résolue en spin des états liés de Yu-Shiba-Rusinov. De manière frappante, nous démontrons qu'une analyse minutieuse de la densité d'états électroniques polarisée en spin ne permet pas seulement de caractériser sans équivoque le degré d’appariement de type triplet, mais également son orientation, a.k.a. le vecteur d. Enfin, nous proposons et discutons deux approches différentes d'ingénierie et de contrôle des phases topologiques à l’aide d’impuretés scalaires et magnétiques. Nous commençons par fournir une théorie microscopique des réseaux d'impuretés scalaires sur les supraconducteurs chiraux. Nous montrons que pour un supraconducteur topologique de type chiral, les impuretés scalaires donnent lieu à une hiérarchie complexe de phases non triviales distinctes avec des nombres de Chern élevés. Deuxièmement, nous proposons et étudions théoriquement une nouvelle plate-forme prometteuse que nous appelons «la chaîne dynamique de Shiba», c'est-à-dire une chaîne d'impuretés magnétiques classiques dans un supraconducteur en ondes s avec des spins qui précessent. Nous montrons que cette approche peut être utilisée non seulement pour créer une phase supraconductrice topologique, mais surtout pour contrôler les transitions de phase topologiques au moyen de la dynamique de la texture de la magnétisation. Ce manuscrit est organisé comme suit. Dans la première partie, les informations d'introduction essentielles sur la supraconductivité, les oscillations de Friedel et les états de Yu-Shiba-Rusinov sont fournies. La deuxième partie est consacrée à la polarisation en spin des états Yu-Shiba-Rusinov et aux propriétés qui pourraient être extraites au moyen de la microscopie par effet tunnel résolu en spin. Dans la dernière partie, deux configurations proposées pour l'ingénierie de phases topologiques, basées sur les états induits par les impuretés, sont présentées, suivies de conclusions, d’un bref résumé des réalisations de cette thèse et enfin d’une discussion de possibles directions futures. / In this manuscript we first revisit the physics of Yu-Shiba-Rusinov subgap states, focusing on their spin polarisation. We start by showing theoretically that we can extract a considerable amount of information about the host superconductor, by analysing spin-polarised local density of states related to the presence of magnetic impurities. First, we demonstrate that the spin-orbit coupling in two-dimensional and one-dimensional systems, both superconducting and metallic, can be read-off directly and unambiguously via spin-resolved STM. We analyse the impurity-induced oscillations in the local density of states. In particular, we focus on the Fourier transform (FT) of the Friedel oscillations and we note that high-intensity FT features appear at a wave vector given by twice the inverse spin-orbit length. Second, in unconventional superconductors with both s-wave and p-wave pairing, by analysing the spin-resolved spectral structure of the Yu-Shiba-Rusinov states it is possible to determine the dominating pairing mechanism. Most strikingly, we demonstrate that a careful analysis of spin-polarised density of states allows not only to unambiguously characterise the degree of triplet pairing, but also to define the orientation of the triplet pairing vector, also known as the d-vector.Finally, we discuss two different ways of engineering and controlling topological phases with both scalar and magnetic impurities. We start with providing a microscopic theory of scalar impurity structures on chiral superconductors. We show that given a non-trivial chiral superconductor, the scalar impurities give rise to a complex hierarchy of distinct non-trivial phases with high Chern numbers. Second, we propose and study theoretically a new promising platform that we call 'dynamical Shiba chain', i.e. a chain of classical magnetic impurities in an s-wave superconductor with precessing spins. We have shown that it can be employed not only for engineering a topological superconducting phase, but most remarkably for controlling topological phase transitions by means of magnetisation texture dynamics.This manuscript is organised as follows. In the first part, the essential introductory information on superconductivity, Friedel oscillations and Yu-Shiba-Rusinov states is provided. The second part is dedicated to spin polarisation of Yu-Shiba-Rusinov states and the properties that could be extracted by means of spin-resolved STM measurements. In the last part, two setups proposed for topological phase engineering based on impurity-induced states are presented, followed by conclusions with a brief summary of the thesis achievements and further directions to pursue.

This dissertation is concerned with the understanding of physico-chemical properties of energetic materials (EMs). Recently, a substantial amount of work has been directed towards calculations of equations of state and structural changes upon compression of existing EMs, as well as elucidating the underlying chemistry of initiation in detonating EMs. This work contributes to this effort by 1) predicting equations of state and thermo-physical properties of EMs, 2) predicting new phases of novel EMs, and 3) examining the initial stages of chemistry that result in detonation in EMs. The motivation for the first thrust, is to provide thermodynamic properties as input parameters for mesoscale modeling. Such properties are urgently sought for a wide range of temperatures and pressures, and are often difficult or even impossible to obtain from experiment. However, thermo-physical properties are obtained by calculating structural properties and vibration spectra using density function theory and employing the quasi-harmonic approximation. The second thrust is directed towards the prediction and investigation of novel polymorphs of known azide compounds to identify precursor materials for synthesis of polymeric nitrogen EMs. Structural searches are used to identify new polymorphs, while theoretical Raman spectra for these polymorphs are calculated to aid experimentalists in identifying the appearance of these azide compounds under high pressure. The final thrust is concerned with elucidating the initial chemical events that lead to detonation through hypervelocity collision simulations using first-principles molecular dynamics. The chemical mechanisms of initiation are determined from the atomic trajectory data, while heats of reaction are calculated to quantify energy trends of chemical transformations.

Development of the faster and denser magnetic memory storage elements has been an active area of research since early 20th century. The path of research on magnetization manipulation began with firstly changing the magnetization state of a medium in an external magnetic field, then heating of a medium and magnetizing with a permanent magnet was explored, while the latest efforts have been focused on switching the magnetization only by a polarized laser light. Nowadays due to the technological advancement of lasers and material fabrication methods, the search and development process of magnetic memory elements is much faster. The implementation of such technologies, however, relies on finding suitable magnetic materials which would allow for a fast magnetization writing and read-out processes and would remain magnetized, even with the reduced dimensions. Ferrimagnetic rare Earth - transition metal (RE-TM) alloys have been used for fabricating magneto-optical recording media already since the 1990’s. Relatively recently, in 2007, it was demonstrated that the ferrimagnetic GdFeCo alloy magnetization state can be switched using only circularly polarized laser light. Hence, ferrimagnetic RE-TMalloys could be suitable candidates for all-optical light-induced magnetization switching (AOS), without any external magnetic field. Another combination of RE-TM alloys that was shown to exhibit AOS is ferrimagnetic amorphous alloys containing terbium and cobalt (Tb:Co). They have attracted attention due to their strong out-of-plane magnetic anisotropy, high magneto-optical activity and amorphicity, which makes them attractive from a fabrication point of view since a variety of substrates and buffer layers could be used for growing such layers. In this Thesis, TbCo alloys are investigated in order to examine how the magnetic, optical and magneto-optical properties could be tuned by varying the elemental ratio and film thickness. The main question that was addressed here was whether such a system is suitable for fabrication of nanosized magnetic elements as the building blocks for the magnetic memory applications. TbCo alloys were prepared as thin films by magnetron co-sputtering method onto different substrates and buffer layers. Films were characterized using a variety of techniques such as an ion beam analysis, an x-ray reflectivity and diffraction, and magneto-optical characterization techniques. It was observed that the properties of such alloys depend not only on the Tb:Co ratio but also on the film thickness and an underlying buffer layer. Magnetization compensation point, at which the magnetization of a film is zero, as in an antiferromagnet, can be modified depending on the buffer layer. All-optical switching (AOS) of magnetization experiments were performed on the fabricated samples. It was determined that AOS with at least 50-100 laserpulses can be achieved for the films grown directly onto fused silica substrates and with the compositions above the magnetization compensation point at room temperature, in the range of 24 - 30 at.% Tb. In the Outlook, the initial efforts of patterning the films into the arrays of nanosized elements are presented. It is demonstrated that after the lithographic patterning of the films, the resulting nanosized elements remained out-of-plane magnetized. In this work it is shown that the ferrimagnetic TbCo alloy system is a potential candidate material for bothfacilitating AOS and the fabrication of arrays of nanomagnets. Combining the TbCo alloys,which show AOS, together with a suitable buffer layer and patterning the hybrid structure,could enable selective element-by-element magnetization switching for the magnetic memorystorage devices.

The layered iridates such as Sr2IrO4 and Sr3Ir2O7, have attracted substantial attention due to their novel electronic states originating from strong spin-orbit coupling and electron-correlation. Recent studies have revealed the possibilities of novel phases such as topological insulators, Weyl semimetals, and even a potential high-Tc superconducting state with a d-wave gap. However, there are still controversial issues regarding the fundamental electronic structure of these systems: the origin of the insulating gap is disputed as arising either from an antiferromagnetic ordering, i.e. Slater scheme or electron-correlation, i.e. Mott scheme. Moreover, it is a formidable task to unveil the physics of layered iridates due to the limited number of available materials for experimental characterizations. One way to overcome this limit and extend our investigation of the layered iridates is using metastable materials. These materials which are far from their equilibrium state, often have mechanical, electronic, and magnetic properties that different from their thermodynamically stable phases. However, these materials cannot be synthesized using thermodynamic equilibrium processes. One way to synthesize these materials is by using pulsed laser deposition (PLD). PLD is able to generate nonequilibrium material phases through the use of substrate strain and deposition conditions. Using this method, we have synthesized several thermodynamically metastable iridate thin-films and have investigated their electronic and optical properties. Synthesizing and investigating metastable iridates opens a path to expand the tunability further than the ability of the bulk methods. This thesis consists of four studies on metastable layered iridate thin film systems. In the first study, three-dimensional Mott variable-range hopping transport with decreased characteristic temperatures under lattice strain or isovalent doping has been observed in Sr2IrO4 thin films. Application of lattice strain or isovalent doping exerts metastable chemical pressure in the compounds, which changes both the bandwidth and electronic hopping. The variation of the characteristic temperature under lattice strain or isovalent doping implies that the density of states near the Fermi energy is reconstructed. The increased density of states in the Sr2IrO4 thin films with strain and isovalent doping could facilitate a condition to induce unprecedented electronic properties, opening a way for electronic device applications. In the second study, the effects of tuning the bandwidth via chemical pressure (i.e., Ca and Ba doping) on the optical properties of Sr2IrO4 epitaxial thin films has been investigated. Substitution of Sr by Ca and Ba ions exerts metastable chemical pressure in the system, which changes both the bandwidth and electronic hopping. The optical conductivity results of these thin films suggest that the two-peak-like optical conductivity spectra of the layered iridates originates from the overlap between the optically-forbidden spin-orbit exciton and the inter-site optical transitions within the Jeff = ½ band, which is consistent with the results obtained from a multi-orbital Hubbard model calculation. In the third study, thermodynamically metastable Ca2IrO4 thin- films have been synthesized. Since the perovskite structure of Ca2IrO4 is not thermodynamically stable, its bulk crystals do not exist in nature. We have synthesized the layered perovskite phase Ca2IrO4 thin- films from a polycrystalline hexagonal bulk crystal using an epitaxial stabilization technique. The smaller A-site in this compound compared to Sr2IrO4 and Ba2IrO4, increases the octahedral rotation and tilting, which enhance electron-correlation. The enhanced electron-correlation is consistent with the observation of increased gap energy in this compound. This study suggest that the epitaxial stabilization of metastable-phase thin-films can be used effectively for investigating complex-oxide systems. Finally, structural, transport, and optical properties of tensile strained (Sr1-xLax)3Ir2O7 (x = 0, 0.025, 0.05) thin-films have been investigated. While high-Tc superconductivity is predicted in the system, all of the samples are insulating. The insulating behavior of the La-doped Sr3Ir2O7 thin-films is presumably due to disorder-induced localization and ineffective electron-doping of La, which brings to light the intriguing difference between epitaxial thin films and bulk single crystals of the iridates. These studies thoroughly investigate a wide array of novel electronic and optical phenomena via tuning the relative strengths of electron correlation, electronic bandwidth, and spin-orbit coupling using perturbations such as chemical doping, and the stabilization of metastable phases in the layered iridates.