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Trellis-coded modulation (TCM) is a known technique to increase the data rate without increasing the channel bandwidth when implementing error correction coding. TCM is a combination of M-ary modulation and error correction coding. This thesis investigates the performance of a low spectral efficiency TCM system, which is compared with three alternative systems having comparable bandwidth. The three alternative systems are all non-TCM systems and consist of QPSK with independent r=1/2 error correction coding on the in-phase and quadrature components, 8-ary biorthogonal keying (8-BOK) with r=2/3 error correction coding, and 16-BOK with r=3/4 error correction coding. The effects of both additive white Gaussian noise (AWGN) and pulse-noise interference (PNI) are considered. The TCM system shows much better than expected performance and significant resistance to PNI, and performance improves as the number of memory element increases. The alternative QPSK system with soft decision decoding (SDD) experiences significant degradation with PNI. The 8-BOK with r=2/3 error correction and 16-BOK with r=3/4 error correction systems occupy approximately the same bandwidth as the TCM system and show better performance in PNI than the alternative QPSK system.

Monolayers and heterostructures of two-dimensional (2D) electronic materials with spin-orbit interactions offer the promise of observing many novel physical effects. While theoretical predictions of 2D layered materials based on density functional theory (DFT) are many, the DFT approach is limited to small simulation sizes (several nanometers), and thus inhomogeneous strain and boundary effects that are often observed experimentally cannot be simulated within a reasonable time. The aim of this thesis is (i) to study effects of strain on 2D materials beyond graphene using first-principles and tight-binding methods and (ii) to investigate the effects of cuts--"kirigami"-- on 2D materials using molecular dynamics and machine learning approach. The first half of this thesis focuses on the effects of strain on manipulating spin and valley degrees of freedom for two classes of 2D materials--monochalcogenide and lead chalcogenide monolayers--using DFT. A tight-binding (TB) approach is developed to describe the electronic changes in lead chalcogenide monolayers due to strains that often persist in real devices. The strain-dependent TB model allows one to establish a relationship between the Rashba field and the out-of-plane strain or electric polarization from a microscopic view, a connection that is not well understood in the ferroelectric Rashba materials. This framework connecting strain fields and electronic changes is important to overcome the size and computational limitations associated with DFT. The second part of the thesis focuses on defect engineering and design of 2D materials via the "kirigami" technique of introducing different patterns of cuts. A machine learning (ML) approach is presented to provide physical insights and an effective model to describe the physical system. We demonstrate that a machine learning model based on a convolutional neural network is able to find the optimal design from a training data set that is much smaller than the design space.

Les matériaux multiferroïques sont des matériaux multifonctionnels qui possèdent simultanément des propriétés magnétiques et ferroélectriques. Les perspectives d’applications sont ainsi très nombreuses dans les domaines de l’électronique (mémoires, dispositifs spintroniques et hyperfréquences). Le nombre restreint de matériaux multiferroïques monophasés a conduit au développement de nanostructures multiferroïques artificielles constituées d'oxydes ferroélectriques et ferrimagnétiques. Ce travail de thèse est axé sur l'effet magnétoélectrique (ME), obtenu pour de telles hétérostructures via la contrainte, qui permet de manipuler la polarisation spontanée ou l’aimantation par l’application d’un champ magnétique (effet ME direct) et d’un champ électrique (effet ME converse) respectivement. Les effets ME peuvent être observés à température ambiante grâce aux effets d’interfaces et de contraintes dans les nanocomposites multiferroïques. La combinaison de matériaux piézoélectriques PbZr0.52Ti0.48O3 (PZT), Ba0.7Sr0.3TiO3 (BSTO), BaTiO3 (BTO) et de matériaux magnétostrictifs CoFe2O4 (CFO) a été largement exploitée pour l’élaboration de nanocomposites multiferroïques. Les travaux issus de la littérature montrent l’existence d’un fort couplage magnétoélectrique à température ambiante dans des films minces épitaxiés (systèmes de connectivité 2-2), mais un verrou est l’effet de « bride » (clamping effect) induit par le substrat. La conception d'architectures innovantes est un défi dans le domaine des nanocomposites multiferroïques. Ce travail est axé sur les composites de type (1-3) au sein desquelles des nanostructures ferrimagnétiques CoFe2O4 unidimensionnelles (1) sont incorporées dans des couches tridimensionnelles PZT, BTO et BSTO (3). De nouvelles approches ont été envisagées pour concevoir trois types de matériaux: i) des réseaux de nanofils CFO unidirectionnels entourés de nanotubes PZT imprégnés dans des membranes d'alumine; ii) des nanopilliers CFO incorporés dans des couches minces de BTO, BSTO et PZT; ii) des réseaux de nanofils CFO interconnectés 3-D intégrés dans une matrice PZT. Nos principaux objectifs visent i) la maîtrise de l’étape d’oxydation des nanofils et des nanopilliers métalliques CoFe2 afin de contrôler la morphologie et la densité des nanostructures CFO, ii) le contrôle des caractéristiques diélectriques des nanocomposites, iii) l’augmentation du couplage magnétoélectrique en optimisant la densité d’interfaces entre les deux phases ferroïques.La première architecture développée est un dépôt par imprégnation sol-gel de nanotubes PZT dans des membranes d'alumine poreuses autosupportées, suivie d'une électrodéposition des nanofils CoFe2 dans les nanotubes PZT et de leur oxydation par traitement thermique. La deuxième architecture repose sur un dépôt par pulvérisation cathodique magnétron en radiofréquence de couches BSTO et BTO et sur un dépôt par sol-gel de couches PZT, sur des réseaux de nanopilliers CoFe2 et CoFe2O4 alignés verticalement sur des substrats Si. L'oxydation de CoFe2 est réalisée in situ lors du dépôt par pulvérisation cathodique de BSTO et BTO. Les réseaux de nanopilliers CoFe2 sont obtenus par électrodéposition dans des structures nanoporeuses en alumine anodisée qui sont ensuite dissoutes. La dernière architecture proposée est obtenue en combinant l'électrodéposition des nanofils CoFe2 dans des membranes polymères poreuses, et le procédé sol-gel. Les nanostructures PZT-CFO sont préparées par imprégnation sol-gel de couches épaisses PZT dans des réseaux de nanofils CoFe2 et leur oxydation simultanée au cours de la cristallisation des couches PZT.Une attention particulière a été accordée aux effets d’interfaces par le biais des études microstructurales et morphologiques des nanocomposites (XRD, HRSEM, TEM et EDX). Les caractérisations magnétiques, diélectriques, ferroélectriques et magnétoélectriques ont permis d’évaluer les performances des différents nanocomposites élaborés. / Multiferroic materials including magnetoelectric materials that combine magnetic and ferroelectric orders have attracted great attention due to a possible strain-mediated coupling leading to potential applications in memories, sensors, detectors, spintronic and microwave devices. The number of single-phase multiferroic materials operating at room temperature being limited, we are exploring artificially designed multiferroic nanostructures consisting of ferroelectric and ferrimagnetic oxides. Current work is focused on strain-mediated magnetoelectric effect, which allows to generate a spontaneous polarization or magnetization by an applied magnetic field (direct ME effect) and electric field (converse ME effect) respectively. ME effects can be observed at room temperature through interface and strain interaction in two-phase multiferroic nanocomposites. The combination of piezoelectric materials PbZr0.52Ti0.48O3 (PZT), Ba0.7Sr0.3TiO3 (BSTO), BaTiO3 (BTO) and magnetostrictive CoFe2O4 (CFO) materials have been intensively studied in multiferroic nanocomposites. The community has been able to demonstrate large magnetoelectric coupling at room temperature in epitaxial thin films, so called 2-2 connectivity system, but a key limitation in epitaxially grown thin films is a substrate imposed clamping effect limiting thin film’s strain. Designing innovative architectures is a challenge in the field of multiferroic nanocomposites. Our work is focused on vertically aligned multiferroic nanostructures, so called (1-3) connectivity nanocomposites, where one-dimensional ferrimagnetic CoFe2O4 nanostructures (1) are embedded into three-dimensional PZT, BTO and BSTO layers (3). New routes were considered to design three kinds of materials: i) vertically aligned CFO nanowire arrays surrounded by PZT nanotubes embedded into alumina membranes; ii) vertically aligned CFO nanopillar arrays embedded in thin BTO, BSTO and PZT layers supported on Si substrates; ii) 3-D interconnected CFO nanowire networks embedded in a thick PZT matrix. The objectives of the present work are to control the oxidation of metallic CoFe2 nanowires and nanopillars to control the morphology and density of CFO nanostructures, to control the resistivity and dielectric losses of the nanocomposites at the interface region, and to increase the magnetoelectric coupling of the multiferroic nanocomposites by increasing the interfacial surface area between the two ferroic phases.The first geometry we are developing is a deposition by sol-gel dip impregnation of PZT nanotube arrays into self-supported porous alumina membranes, followed by an electrodeposition and thermal oxidation of CoFe2 nanowire arrays within PZT nanotubes. The second architecture we are focusing on is a deposition by RF magnetron sputtering of BSTO and BTO layers and by sol-gel dip coating of PZT layers onto vertically aligned CoFe2 and CoFe2O4 nanopillar arrays supported on Si substrates. The CoFe2 oxidation is conducted in-situ during the BSTO and BTO sputter deposition. Free-standing CoFe2 nanopillar arrays are obtained by electrodeposition into anodized alumina nanoporous structures and chemical dissolution of alumina templates. The last geometry is prepared using a combination of electrodeposition into self-supported porous polymer membranes and sol-gel processes. The PZT-CFO nanostructures are prepared using impregnation of thick PZT layers into self-supported CoFe2 3D nanowire networks on Si substrates by sol-gel method and their simultaneous oxidation during PZT layers crystallization. Specific attention was focused on interfaces through microstructural and morphological evaluations of nanocomposites using XRD, HRSEM, TEM and EDS characterizations. The performances of the nanocomposites were evaluated using magnetic, dielectric, ferroelectric and ME measurements, an alternating gradient magnetometer, impedance analyser, PFM and the ME susceptometer operated inside PPMS were utilized, respectively.

Magnetic and ferroelectric materials have both had a very important impact in our society, not only because of the fascinating science behind the two phenomena, but also as a result of their use in many technological applications. The coupling and coexistence of these two order parameters within the same material opens the door to exiting new functional devices. Materials where magnetism and ferroelectricity coexist are known as multiferroic materials. In this thesis, muon spectroscopy and other complementary experimental techniques, including neutron scattering and resonant ultrasound spectroscopy, are used to investigate two di↵erent multiferroics. Muon and total neutron scattering studies have been performed on BiFeO3, one of the most studied multiferroic materials. Muon measurements reveal an anomaly in the temperature region of 200 - 220 K with a sudden and abrupt change in the muon’s precession frequency that corresponds to a process of muon di↵usion throughout the entire sample. The pair distribution function, calculated from total neutron scattering experiments on the compound, suggest that a change in the local structure of the material involving the bismuth-oxygen bond, in the same temperature region as the muon di↵usion sets in, is a strong indicative that there is a link between two in terms of the muon di↵usion being triggered by these local changes. Also, an extensive analysis and characterisation of the magnetic and ferroelectric properties of Ba4Dy0.87Nb10O30, an entirely new tetragonal tungsten bronze magnetoelectric material, is given. Neutron scattering and dielectric measurements are used to show that this material becomes ferroelectric below 470 K. We use muon spectroscopy and magnetic susceptibility measurements to investigate the magnetic properties of the material. Muon measurements under an applied electric field indicate that there is a strong coupling between the magnetism and ferroelectricity in the material. Resonant ultrasound spectroscopy is use to investigate whether the source of this coupling could be related to strain e↵ects. Magnetic neutron scattering measurements show that there is no long range ordering in the material.

Compósitos cerâmicos planares de titanato zirconato de bário, BaTi1-xZrxO3, foram produzidos e os efeitos da quantidade de Zr4+ em suas propriedades funcionais foram estudados. As amostras foram fabricadas pelo método convencional de processamento cerâmico e pela técnica de deposição de fitas cerâmicas a partir de BaTi1-xZrxO3 com x=0, 0,05, 0,1, 0,15 e 0,2 sintetizados pelo método hidrotermal e pelo método dos precursores poliméricos. Foram realizadas caracterizações estrutural (difração de Raios X pelo método do pó e espectroscopia Raman), composicional (espectroscopia por dispersão de energia), microestrutural (microscopia eletrônica de varredura, ensaios de dilatometria) e funcional (permissividade elétrica, coeficiente piezoelétrico d33, coeficiente piroelétrico e histerese ferroelétrica). Além disso, a regra da mistura de fase foi utilizada para prever a permissividade elétrica dos compósitos e as tensões mecânicas internas e seus efeitos na permissividade elétrica e na temperatura de Curie de policristais de BaTiO3 foram simulados. As análises dos dados e as discussões foram realizadas considerando o modelo fenomenológico de Devonshire, a modificação de Forsbergh deste modelo para incluir efeitos de tensões mecânicas bidimensionais e o modelo de policristal tetragonal de BaTiO3 sugerido por Buessem. Os resultados mostram a existência de tensões residuais bidimensionais que surgem após o resfriamento dos compósitos devido às diferenças nos coeficientes de expansão térmica das fases constituintes. Os métodos de obtenção das amostras afetaram o tamanho final dos grãos e as espessuras das interfaces dos compósitos, sendo que, em geral, eles são menores e mais finos nas fitas cerâmicas homogêneas do que nas correspondentes cerâmicas homogêneas. Os compósitos apresentaram deslocamento da Tc para maiores temperaturas e aumento do grau de difusividade da transição. A presença das tensões mecânicas residuais e as características microestruturais, juntamente com os modelos utilizados, explicam qualitativamente a permissividade elétrica obtida. Concluí-se que a quantidade de Zr4+ modifica o comportamento das camadas durante a sinterização e altera o coeficiente de dilatação. Estas mudanças geram tensões mecânicas residuais que afetam a microestrutura e as propriedades funcionais dos compósitos. Portanto, a produção de compósitos cerâmicos ferroelétricos deve considerar a correlação existente entre microestrutura e tensões residuais para que suas propriedades sejam otimizadas. / Planar BaTi1-xZrxO3 ceramic composites had their functional properties investigated. These composites were obtained by the conventional ceramic processing technique and by tape casting technique. Furthermore, BaTi1-xZrxO3 x=0, 0.05, 0.1, 0.15 and 0.2 were synthesized by the polymeric precursors method and by the hydrothermal synthesis to study the synthesis influences and to study the effect of Zr4+ on the functional properties of the composites. Structural characterization (X-Ray powder diffraction and Raman spectroscopy), compositional analysis (energy dispersive X-Ray spectroscopy), microestrutural evaluation (scanning electron microscopy, dilatometry measurements) and functional properties characterization (electrical permittivity, piezoelectric coefficient d33, pyroelectric coefficient and ferroelectric hysteresis) were performed. Furthermore, the composites electrical permittivity was predicted by the simple mixture and the internal mechanical stress distribution and it´s effects on both, electrical permittivity and Curie´s temperature (Tc) of BaTiO3 polycrystals, were simulated. The analyses and discussions were supported by 1- Devonshire´s phenomenological theory, 2- Forsbergh´s modification to Devonshire´s theory to include the two-dimensional stress effects on Tc and 3- Buessem´s BaTiO3 tetragonal polycrystal model. The results show that the sintered composites present two-dimensional residual stresses after cooling due to the constrained sintering of the layers and their thermal expansion coefficient mismatch. The methods of sample preparation led to differences in grain size and interface thickness, with the homogeneous ceramic tapes presenting smaller grain sizes and thinner thickness than the conventional homogeneous ceramics. On the other hand, the composites showed a Tc shift to higher temperatures, a more diffuse phase transition and residual mechanical stresses. Concluding, the Zr4+ substitution of Ti4+ modifies the layer´s sintering behavior and their thermal expansion coefficient. These changes lead to microstructure modifications that affect the functional properties of planar BaTi1-xZrxO3 composites. Thus, the design of ferroelectric composites should take into consideration the correlations between microstructure and residual stresses in order to optimize their functional properties.

Yuen Chi Yan = 有關光折變晶體中空間孤子、偏振轉動以及霍爾效應的研究 / 阮志仁. / Thesis submitted in: July 2004. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2005. / Includes bibliographical references (leaves 81-82). / Text in English; abstracts in English and Chinese. / Yuen Chi Yan = You guan guang zhe bian jing ti zhong kong jian gu zi, pian zhen zhuan dong yi ji Huoer xiao ying de yan jiu / Ruan Zhiren. / Acknowledgments --- p.i / Abstract --- p.ii / Table of Contents --- p.v / Chapter Chapter 1 --- Photorefractive Spatial Soliton --- p.1 / Chapter 1.1 --- Introduction --- p.1 / Chapter 1.2 --- Bright Spatial Soliton --- p.3 / Chapter 1.2.1 --- Experiment --- p.4 / Chapter 1.2.2 --- Results and Discussion --- p.6 / Chapter 1.2.2.1 --- Expansion --- p.6 / Chapter 1.2.2.2 --- Contraction --- p.10 / Chapter 1.3 --- Dark Spatial Soliton --- p.15 / Chapter 1.3.1 --- Experiment --- p.15 / Chapter 1.3.2 --- Results and Discussion --- p.20 / Chapter Chapter 2 --- Polarization Rotation --- p.23 / Chapter 2.1 --- Introduction --- p.23 / Chapter 2.2 --- Experiment --- p.24 / Chapter 2.3 --- Results and Discussion --- p.30 / Chapter 2.3.1 --- Effect of varying pump beam power --- p.30 / Chapter 2.3.2 --- Effect of different polarizations of signal beam --- p.41 / Chapter 2.3.3 --- Effect of signal beam size --- p.43 / Chapter 2.3.4 --- Effect of applied E-field --- p.46 / Chapter 2.3.5 --- Effect of signal beam and pump beam separation and perpendicularly --- p.52 / Chapter 2.3.6 --- Investigation of Δne using interferometer --- p.60 / Chapter 2.3.7 --- Computer Simulation --- p.69 / Chapter Chapter 3 --- Hall Effect --- p.72 / Chapter 3.1 --- Introduction --- p.72 / Chapter 3.2 --- Experiment --- p.75 / Chapter 3.3 --- Results and Discussion --- p.76 / Conclusion and Possible Further Works --- p.79 / References --- p.81

Orientador: Eudes Borges de Araújo / Banca: Rafael Zadorosny / Banca: Jesiel Freitas Carvalho / Resumo: O titanato zirconato de chumbo dopado com lantânio, dado convencionalmente pela fórmula estequiométrica Pb1− x Lax ( Zry ; Ti1− y ) 1− x / 4 O3 , com x = 0,09 e y = 0,65, também conhecido como PLZT 9/65/35, é um importante sistema ferroelétrico relaxor devido as suas propriedades dielétricas, elétricas e eletroóticas. Sendo um ferroelétrico, exibe, também, propriedades tais como piezo e piroeletricidade, dependendo apenas das proporções em que são preparados. Logo, esse sistema é bastante interessante para uma gama de aplicações tecnológicas. Na forma de filmes finos, a composição PLZT 9/65/35 tem sido amplamente estudada e preparada pelos mais diversos métodos. Neste trabalho propõe-se a síntese de filmes finos ferroelétricos da composição PLZT 9/65/35 pelo método dos precursores óxidos, a fim de se compreender a dinâmica dos processos de cristalização e, também, avaliar suas propriedades físicas, como permissividade elétrica e histerese ferroelétrica. A intenção, assim, é colaborar com as informações presentes na literatura sobre as propriedades de filmes finos de PLZT 9/65/35 / Abstract: The lead zirconate titanate doped with lanthanum, conventionally given by stoichiometric formula Pb1− x Lax ( Zry ; Ti1− y ) 1− x / 4 O3 , with x=0,09 and y=0,65, also known as PLZT 9/65/35, is an important relaxor ferroelectric system due to its dielectric, electrical and electrooptical properties. Being a ferroelectric material exhibits also properties such as piezo- and piroelectricity, depending upon the extent to which they are prepared. Therefore, this system is very interesting for a range of technological applications. In the thin films format, the composition PLZT 9/65/35 has been widely studied and prepared by several methods. In this project it is proposed the synthesis of thin films of such material by the oxide precursor method in order to understand the dynamics of crystallization process and also to evaluate their physical properties like electrical permittivity and ferroelectric hysteresis. The intention, thus, is collaborate with the information presented in the literature about the properties of PLZT 9/65/35 thin films / Mestre

The discovery of ferroelectricity at the nanoscale has incited a lot of interest in perovskite ferroelectrics not only for their potential in device application but also for their potential to expand fundamental understanding of complex phenomena at very small size scales. Unfortunately, not much is known about the dynamics of ferroelectrics at this scale. Many of the widely held theories for ferroelectric materials are based on bulk dynamics which break down when applied to smaller scales. In an effort to increase understanding of nanoscale ferroelectric materials we use atomistic resolution computational simulations to investigate the dynamics of polar perovskites. Within the framework of a well validated effective Hamiltonian model we are able to accurately predict many of the properties of ferroelectric materials at the nanoscale including the response of the soft mode to mechanical boundary conditions and the polarization reversal dynamics of ferroelectric nanowires. Given that the focus of our study is the dynamics of ferroelectric perovskites we begin by developing an effective Hamiltonian based model that could simultaneously describe both static and dynamic properties of such materials. Our study reveals that for ferroelectric perovskites that undergo a sequence of phase transitions, such as BaTiO3. for example, the minimal parameter effective Hamiltonian model is unable to reproduce both static and dynamical properties simultaneously. Nevertheless we developed two sets of parameters that accurately describes the static properties and dynamic properties of BaTiO3 independently. By creating a tool that accurately models the dynamical properties of perovskite ferroelectrics we are able to investigate the frequencies of the soft modes in the perovskite crystal. The lowest energy transverse optical soft modes in perovskite ferroelectrics are known to be cause of the ferroelectric phase transition in these materials and affect a number of electrical properties. The performance of a ferroelectric device is therefore directly influenced by the dynamics of the soft mode. Interestingly, however, little study has been done on the effect of mechanical boundary conditions on the soft modes of perovskites. Understanding the effect of mechanical forces on the soft modes is critical to device applications as complicated growth structures often are the cause of pressures, stresses and strains. Using classical molecular dynamics we study the effect of hydrostatic pressure, uniaxial stress, biaxial stress and biaxial strain on the soft modes of the ferroelectric PbTiO3. The results of this study indicate the existence of Curie-Weiss laws for not only hydrostatic pressure, which is well known, but also for uniaxial stress, biaxial stress and biaxial strain. The mode frequencies are also seen to respond very differently to these mechanical forces and lead to a more complete picture of the behavior of nanoscale ferroelectrics. One nanoscale geometry of perovskite ferroelectrics is the pseudo one-dimensional nanowire. These structures have very unique properties that are highly attractive for use as interconnects, nanoscale sensors or more directly in computer memory devices. Perovskite nanowires have only recently been synthesized and the techniques are not well developed. While progress has been made towards consistently fabricating uniform, high quality nanowires experimental investigation of their properties is prohibitively difficult. Of immediate interest is the polarization reversal dynamics of ferroelectric nanowires. The reading and writing of bits of information stored in a wire's polarization state is done by switching the polarization. Again using classical molecular dynamics we study the polarization reversal dynamics in ferroelectric nanowires made of Pb(Ti1-xZrx)O3 disordered alloy. We find that there are two competing mechanisms for polarization reversal and that the interplay of these mechanisms is dependent on electric field strength. The dynamics in nanowires also sheds light on long standing theories about polarization reversal mechanisms in thin film and bulk geometries.

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