Inspired by graphene, I investigate the properties of several different real and artificial Dirac materials. Firstly, I consider a two-dimensional honeycomb lattice of metallic nanoparticles, each supporting localised surface plasmons, and study the quantum properties of the collective plasmons resulting from the near field dipolar interaction between the nanoparticles. I analytically investigate the dispersion, the effective Hamiltonian and the eigenstates of the collective plasmons for an arbitrary orientation of the individual dipole moments. When the polarisation points close to normal to the plane the spectrum presents Dirac cones, similar to those present in the electronic band structure of graphene. I derive the effective Dirac Hamiltonian for the collective plasmons and show that the corresponding spinor eigenstates represent chiral Dirac-like massless bosonic excitations that present similar effects to those of electrons in graphene, such as a non-trivial Berry phase and the absence of backscattering from smooth inhomogeneities. I further discuss how one can manipulate the Dirac points in the Brillouin zone and open a gap in the collective plasmon dispersion by modifying the polarisation of the localized surface plasmons, paving the way for a fully tunable plasmonic analogue of graphene. I present a phase diagram of gapless and gapped phases in the collective plasmon dispersion depending on the dipole orientation. When the inversion symmetry of the honeycomb structure is broken, the collective plasmons become gapped chiral Dirac modes with an energy-dependent Berry phase. I show that this concept can be generalised to describe many real and artificial graphene-like systems, labeling them Dirac materials with a linear gapped spectrum. I also show that biased bilayer graphene is another Dirac material with an energy dependent Berry phase, but with a parabolic gapped spectrum. I analyse the relativistic phenomenon of Klein Tunneling in both types of system. The Klein paradox is one of the most counter-intuitive results from quantum electrodynamics but it has been seen experimentally to occur in both monolayer and bilayer graphene, due to the chiral nature of the Dirac quasiparticles in these materials. The non-trivial Berry phase of pi in monolayer graphene leads to remarkable effects in transmission through potential barriers, whereas there is always zero transmission at normal incidence in unbiased bilayer graphene in the npn regime. These, and many other 2D materials have attracted attention due to their possible usefulness for the next generation of nano-electronic devices, but some of their Klein tunneling results may be a hindrance to this application. I will highlight how breaking the inversion symmetry of the system allows for results that are not possible in these system's inversion symmetrical counterparts.

Das Verstaendnis dafuer, welche Freiheitsgrade fuer das Eingeschlossensein der Quarks von Bedeutung sind, ist ein altbekanntes Problem. Da weithin angenommen wird, dass das Zentrum der Eichgruppe eine bedeutende Rolle spielt, ist es interessant, eine Theorie mit einem trivialen Zentrum zu untersuchen. Das einfachste Modell, um dieses Problem zu untersuchen, ist eine Theorie mit ungeradzahliger Dimension der Darstellung der Eichgruppe SU(2). Theorien mit einem trivialen Zentrum werden schon seit langer Zeit in zwei verschiedenen Diskretisierungen untersucht: die adjungierte Wilson-Wirkung und die Villain-Wirkung. Es stellte sich heraus, dass sie aus zweierlei Gruenden problematisch sind. Zunaechst zeigte sich, dass in beiden Fällen ein bulk-Phasenuebergang den physikalischen Phasenuebergang bei endlicher Temperatur ueberschattet. Darueberhinaus erwies es sich im Falle der Villain-Theorie, dass die Anwesenheit von Twist-Sektoren fuer die Konstruktion eines ergodischen Algorithmus problematisch sein kann. Die Gitter-Artefakte, die den bulk-Phasenuebergang verursachen, wurden mit Z(2) Monopolen identifiziert. Diese Monopole koennen mit Hilfe eines entsprechenden chemischen Potentials unterdrueckt werden. Eine erste Untersuchung des Phasenuebergangs bei endlicher Temperatur durch andere Autoren wurde nur im Falle der Villain-Wirkung durchgefuehrt, wobei in dieser Untersuchung die Twist-Sektoren ohne Beruecksichtigung blieben. In der vorliegenden Arbeit untersuchen wir nichtstoerungstheoretisch die Wilson-Wirkung in der adjungierten Darstellung der Eichgruppe SU(2) mit einem chemischen Potential, welches die Z(2)-Monopole bei nicht verschwindender Temperatur und bei Temperatur Null unterdrueckt. Wir untersuchen hierbei die Auswirkungen des chemischen Potentials lambda auf einige Observable. Fuer hinreichend grosse lambda zeigen die Observablen keine Diskontinuitaet in der adjungierten Kopplung. In diesem Gebiet des Phasendiagramms untersuchen wir, meist eingeschraenkt auf den trivialen Twist-Sektor, die Existenz eines Phasenuebergangs bei endlicher Temperatur. Um diesen Phasenuebergang zu identifizieren, gelingt es uns, einen neuen Ordungsparameter zu definieren, den wir erfolgreich auch in der fundamentalen Darstellung der SU(2) testen. Ferner analysieren wir die raeumliche Verteilung der fundamentalen Polyakov-loop-Variable und des Pisaer Unordnungs-Operators, welcher die Kondensation magnetischer Ladungen beschreibt. Die Ergebnisse, die wir mit diesen Untersuchungsmethoden erhielten, lassen auf einen vom bulk-Phasenuebergang entkoppelten Phasenuebergang bei endlicher Temperatur oder einen cross-over schliessen. / The understanding of which degrees of freedom are relevant for the confinement of quarks is a long standing problem. Since it is widely believed that the center of the gauge group plays an important role, it is interesting to study a theory with a trivial center. The simplest model to investigate this problem is provided by a theory in an odd-dimensional representation of the gauge group SU(2). Center-blind theories were studied long time ago in two different discretizations, the adjoint Wilson and the Villain action, and they turned out to be problematic for two reasons. In both cases a bulk phase transition was shown to overshadow the physical finite temperature one. Another feature, pointed out in the Villain case, was the presence of twist sectors, which could cause difficulties in the construction of an ergodic algorithm. The lattice artifacts responsible for the bulk phase transition were identified with Z(2) monopoles and they could be suppressed by the use of an appropriate chemical potential. A preliminary investigation of the finite temperature phase transition by other authors was done only in the Villain case and without taking care of the twist sectors. In this thesis we perform a lattice study of the Wilson action in the adjoint representation of the gauge group SU(2) with a chemical potential, which suppresses the Z(2) monopoles at zero and non-zero temperature. We investigate the effects of the chemical potential lambda on some observables. For large enough lambda at vanishing temperature the observables do not show any discontinuity in the adjoint coupling. In this region we study the existence of a finite temperature phase transition restricting ourselves mainly to the trivial twist sector. In order to detect this phase transition we are able to define a new order parameter, which we successfully test also for the case of the fundamental representation of SU(2). Furthermore we analyze the spatial distribution of the fundamental Polyakov loop and the Pisa disorder operator which detects the condensation of magnetic charges. These different tools provide an indication for a finite temperature phase transition or crossover decoupled from the bulk phase transition.

Utilizando a técnica da matriz de transferência e as ideias de finite-size scaling, de renormalização fenomenológica e de invariância conforme estudamos dois modelos de polímeros interagentes na rede quadrada. Em ambos, a atividade de um monômero pertencente ao polímero vale x = e. Quando as interações são entre as ligações primeiras-vizinhas que pertencem ao polímero definimos um fator de Boltzmann associado à interação como sendo = e-l, onde l é a energia de interação entre ligações. Se as interações são entre os monômeros que pertencem a sítios primeiros-vizinhos mas não-consecutivos o fator de Boltzmann associado à interação é z = e-m , sendo m a energia de interação entre os monômeros. Através do estudo de pares de tiras de larguras L-L obtivemos estimativas para os diagramas de fases dos dois modelos. Ambos apresentam três fases: uma fase não-polimerizada, uma fase polimerizada usual e uma fase polimerizada densa, na qual o polímero se encontra colapsado. Nessa fase, o polímero assume uma configuração que maximiza o número de interações, de modo que a densidade de sítios ocupados pelo polímero tende a 1 e a densidade de interações assume valores muito próximos de 1. Os diagramas de fases dos dois modelos são qualitativamente semelhantes, havendo diferenças quantitativas, já esperadas. As transições entre a fase não polimerizada e a fase polimerizada densa são de primeira ordem. A transição entre a fase não polimerizada e a fase polimerizada usual é de segunda ordem, e um ponto dessa fronteira, que corresponde ao modelo sem interações ( = 1 ou z = 1, dependendo do modelo), é bem conhecido [1-3] e vale xc = 0,37905227 ± 0,00000012. A transição entre as fases polimerizadas é de primeira ordem para valores pequenos de x, mas muda para uma transição de segunda ordem quando x aumenta. Nessa fronteira existe um ponto tri crítico, que foi estimado em ( xPTC = 1,5 ± 0,1, yPTC = 1,1 ± 0,1 ), para o modelo de ligações interagentes. No caso do modelo de monômeros interagentes, não foi possível obter uma estimativa conclusiva a respeito da localização do ponto tri crítico. No encontro das três fronteiras existe um ponto crítico terminal, no qual terminam a linha de transições de segunda ordem entre a fase não polimerizada e a fase polimerizada usual, a linha de transições descontínuas entre as duas fases polimerizadas e a linha de transições de primeira ordem entre as fases não polimerizadas e a polimerizada densa. Os valores estimados por nós são (xPCT = 0,244 ± 0,002, yPCT = 3,86 ± 0,03) para o modelo de ligações interagentes e (xPCT = 0,345 ± 0,001, zPCT = 1,52 ± 0,001) para monômeros interagentes. O ponto , no qual termina a fronteira de segunda ordem entre a fase não-polimeriza e a fase polimerizada usual e onde ocorre pela primeira vez a transição de colapso é um ponto crítico terminal em ambos os modelos. Os expoentes críticos e associados à fronteira entre a fase não polimerizada e polimerizada usual também foram calculados, e encontramos os valores = 0,7507 ± 0,0008 e = 0,2082 ± 0,0004, para y = 1, e = 0,7498 ± 0,0004 e = 0,205 ± 0,003, para y = 1,2, para o modelo de ligações interagentes. Para o modelo de monômero interagentes, os dados foram = 0,7507 ± 0,0007 e = 0,2089 ± 0,0009, para z = 1, e = 0,7500 ± 0,0004 e = 0,205 ± 0,008, para z = 1,2. Observando os valores dos expoentes, vemos que eles ficam constantes dentro das barras de erros, de modo que a transição é uma transição de segunda ordem usual. Os valores concordam muito bem com os valores esperados, que são (exatamente) = 3/4 e = 5/24 [4]. / Using the transfer matrix technique, finite-size scaling, phenomenological renormalization group, and conformal invariance ideas, we studied the thermodynamic behavior of two interacting models of polymers on the square lattice. In both models one monomer that belongs to the polymer has an activity x = e. When the interactions are between first neighbor bonds that belong to the polymer, we define a Boltzmann factor y = e-l, where l is the interaction energy between two bonds. If the interactions are between monomers located at first neighbor but nonconsecutive sites, the associated Boltzmann factor is z = e-m, where m is the interaction energy between two monomers. We consider pairs of strips of widths L-L\' and found estimates for the phase diagrams of both models. They have three phases: a non-polymerized phase, an usual polymerized phase and a dense phase, in which the polymer is colapsed. In this phase, the configuration of the polymer is that maximizes the number of interactions, and the density of sites occupied by the polymer goes to 1, while the density of interactions is very close to 1. The phase diagrams of two models are qualitatively similar, but. there are quantitative differences between them, as we already expected. The transition between non polymerized phase and dense phase is of first order. The transition between non-polymerized phase and usual polymerized phase is of second order, and one point of this frontier, which corresponds to the non-interacting model (y = 1 ou z = 1, depending on the specific model), is well known [1-3J and has the value xc = 0,37905227 ± 0,00000012. The transition between the two polymerized phases is of first order for small values of x, and it changes to a second order transition when x increases. At this frontier there is a tri critical point, and we found ( xTCP = 1,5 ± 0,1, yTCP = 1,1 ± 0,1 ) for the interacting bond model. It was not possible to obtain a conclusive estimation of the location of the tri critical point for the model of interacting monomers. At the point that all transition lines meet there is a critical endpoint, in which the second order transition line between non polymerized phase and usual polymerized phase, the first order transition line between polymerized phases and the first order transition line between non polymerized phase and dense polymerized phases finish. We found (xCEP = 0,244 ± 0,002, yCEP = 3,86 ± 0,03) for the interacting bond model and (xCEP = 0,345 ± 0,001, zCEP = 1,52 ± 0,001) for the interacting monomer model. The point is where ends the second order transition between non polymerized phase and usual polymerized phase and at this point the collapse transition happens at the first time. Then, in our models, the point is a critical endpoint. We also found the critical exponents and of the second order transition line between non polymerized phase and usual 1\'olymerizcd phase. The values we obtained are = 0,7507 ± 0,0008 and = 0,2082 ± 0,0004, for y = 1, and = 0,7498 ± 0,0004 and = 0,205 ± 0,003, for y = 1,2, to the model of interacting bonds. The interacting monomers model has = 0,7507 ± 0,0007 and = 0,2089 ± 0,0009, for z = 1, and = 0,7500 ± 0,0004 and = 0,205 ± 0,008, for z = 1,2. Looking at these results we can see that the exponents remain constant within error bars, thus the transition is a usual second order transition. Furthermore, these values are in a very good agreement with the expected values, which are = 3/4 and = 5/24 [4].

Orientador: Eduardo Miranda / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física "Gleb Wataghin" / Made available in DSpace on 2018-09-24T13:52:11Z (GMT). No. of bitstreams: 1 HerazoWarnes_JesusMaria_D.pdf: 4836710 bytes, checksum: 5b7290f1db20bc31b153f3e7202fff39 (MD5) Previous issue date: 2011 / Resumo: A questão da natureza das transições de fases de sistemas de redes de bósons tem se tornado cada vez mais urgente à vista da capacidade de carregamento de átomos ultrafrios em redes ópticas. Nesta tese, tentamos avançar este conhecimento através do estudo de 3 modelos básicos de redes de bósons interagentes. Inicialmente, determinamos o diagrama de fases c as propriedades físicas do modelo bosônico de impureza única de Anderson. Este modelo é interessante tanto em si mesmo quanto por causa de sua relação com outras abordagens teóricas tais como a teoria dinâmica de campo médio bosônica. Usamos como estratégia a inclusão de um pequeno campo externo acoplado ao parâmetro de ordem superfluido, que quebra a simetria global de calibre do modelo. Desta forma, foi possível estudar a transição de condensação de Base-Einstein através do critério de quebra espontânea de simetria global de calibre. Outras quantidades como a ocupação da impureza, o desvio padrão da ocupação e a susceptibilidade com respeito ao campo externo também foram calculadas, caracterizando a transição de fase do modelo. Alguns desses resultados foram comparados com aqueles já obtidos na literatura através do grupo de renormalização numérico. Encontramos bom acordo entre os dois métodos. O segundo estudo realizado nesta tese refere-se ao comportamento crítico do modelo de Bose-Hubbard desordenado através da chamada teoria de campo médio estocástica. O objeto central dessa teoria de campo médio é a distribuição de parâmetros de ordem P(?). Estudos numéricos estabelecem que perto da linha crítica que separa as fases superfluida e vidro de Base do modelo, essa distribuição exibe uma grande região com comportamento de lei de potência P(?) ~ ? ^-(1+ß_c), onde ß_c < 1. Usando esse comportamento como tentativa, obtivemos analiticamente tanto a fronteira de fases quanto o valor do expoente crítico da lei de potência ß_c , encontrando um razoável acordo com os resultados numéricos e avançando o entendimento da natureza da transição de fase específica ao modelo desordenado. Finalmente, o modelo de Bose-Hubbard desordenado para partículas de spin-1 foi estudado dentro da teoria de campo médio estocástica. As distribuições de probabilidade de várias quantidades físicas como o parâmetro de ordem superfluido, o desvio padrão da ocupação por sítio, a fração do condensado, o quadrado do operador de spin, bem como seus valores médios, foram determinados para as três fases do modelo, a saber, o superfluido polar, o isolante de Mott e o vidro de Bose. Uma completa caracterização das propriedades físicas dessas fases e das transições de fase entre elas foi estabelecida / Abstract: The question of the nature of phase transitions of systems of lattice bosons has become increasingly more pressing in view of the capability of loading ultracold atoms in opticallattices. In this thesis we try to advance this understanding through the study of 3 basic models of interacting lattice bosons. Initially, we determined the phase diagram and physical properties of the bosonic singleimpurity Anderson model. This model is interesting both in its own right and because of its relation to other theoretical approaches such as the bosonic dynamical field theory method. We used as strategy the inclusion of a small external field coupled to the superfluid order parameter, which breaks the global gauge symmetry of the model. Thus, it was possible to study the Base-Einstein condensation transition through the criterion of the onset of spontaneous broken global gauge symmetry. Other quantities such as the occupation of the impurity, the standard deviation of the occupation and the susceptibility with respect to the external! Field were calculated characterizing the phase transition in the model. Some of the results were compared with those already reported in the literature, obtained with tic numerical renormalization group. We found good agreement between the two methods. The second study carried out in this thesis concerned the critical behavior of the disordered Bose-Hubbard model within the so-called stochastic mean-field theory. The central object of this mean-field theory is the distribution of order parameters P(?). Numerical studies establish that near the critical line separating the superfluid and Bose glass phases of this model, this distribution shows a wide region of power-law behavior P(?) ~ ? ^-(1+ß_c), where ß_c < 1. Using this behavior as an Ansatz, we obtained analytically both the phase boundary and the value of the critical power-law exponent ß_c, finding a reasonably good agreement with the numerical results and thus shedding new light on the nature of this phase transition specific to disordered model. Finally, the disordered Bose-Hubbard model for spin-1 particles was studied within the stochastic mean-field theory. The probability distributions of various physical quantities, such as the superfluid order parameter, the standard deviation of the occupation per site, the condensate fraction, the square of the spin operator, as well as their average values, were determined for the three phases of the model, namely, the polar superfluid, the Mott insulating and the Bose glass phases. A complete characterization of the physical properties of these phases and the phase transitions between them was then established / Doutorado / Física da Matéria Condensada / Doutor em Ciências

Conselho Nacional de Desenvolvimento Científico e Tecnológico / Neste trabalho, os efeitos de um campo magnético oblíquo externo no modelo de Gross- Neveu (2+1)-dimensional, que inclui as componentes paralela e perpendicular do campo em relação ao sistema, são estudados no contexto da simetria quiral e discreta do modelo. Nosso principal interesse está nos efeitos deste campo sobre o diagrama de fase do sistema, onde também incluímos os efeitos combinados de temperatura e potencial químico. Os diagramas de fase são obtidos através do potencial efetivo a 1 loop para o modelo, derivado em primeira ordem na expansão 1=N. Transições de fase relevantes que podem ser estudadas através deste modelo são, por exemplo, metal-isolante em matéria condensada e na teoria quântica de campos de férmions planares em geral. A relação entre a transição de fase com quebra da simetria quiral e discreta e o surgimento de um gap (ou a presença de um valor esperado no vácuo do campo escalar diferente de zero), como função do campo magnético oblíquo, é analisada em detalhes. / In this work, the effects of an external oblique magnetic field in the (2+1)-dimensional Gross-Neveu model, and that therefore includes both parallel and perpendicular components of the applied field, are studied in the context of the models discrete chiral symmetry. Our main concern is in the effects of such a field in the systems phase diagram and that also includes the combined effects of temperature and chemical potential. The phase diagrams are obtained through the one-loop effective potential for the model, derived in the leading order in the 1=N expansion Relevant phase transitions that can be studied through this model are, for example, metal-insulator ones in condensed matter and in the quantum field theory of planar fermions in general. The relation between the phase transition with (discrete) chiral symmetry breaking and the emergence of a gap (or the presence of a chiral nonvanishing vacuum expectation value) in the planar fermionic system, as a function of the external oblique magnetic field, is analyzed in details.

O modelo de Ising dipolar bidimensional inclui, além da interação ferromagnética entre os primeiros vizinhos, interações de longo alcance entre os momentos de dipolo magnético dos spins. A presença da interação dipolar muda completamente o sistema, apresentando um rico diagrama de fase, cujas características têm originado inúmeros estudos na literatura. Além disso, a possibilidade de explicar fenômenos observados em filmes magnéticos ultrafinos, os quais possuem diversas aplicações em àreas tecnológicas, também motiva o estudo deste modelo. O estado fundamental ferromagnético do modelo de Ising puro é alterado para uma série de fases do tipo faixas, as quais consistem em domínios ferromagnéticos de largura $h$ com magnetizações opostas. A largura das faixas depende da razao $\\delta$ das intensidades dos acoplamentos ferromagnético e dipolar. Através de simulações de Monte Carlo e técnicas de repesagem em histogramas múltiplos identificamos as temperaturas críticas de tamanho finito para as transições de fase quando $\\delta=2$, o que corresponde a $h=2$. Calculamos o calor específico e a susceptibilidade do parâmetro de ordem, no intervalo de temperaturas onde as transições são observadas, para diferentes tamanhos de rede. As técnicas de repesagem permitem-nos explorar e identificar máximos distintos nessas funções da temperatura e, desse modo, estimar as temperaturas críticas de tamanho finito com grande precisão. Apresentamos evidências numéricas da existência de uma fase nemática de Ising para tamanhos grandes de rede. Em nossas simulações, observamos esta fase para tamanhos de rede a partir de $L=48$. Para verificar o quanto a interação dipolar de longo alcance afeta as estimativas físicas, nós calculamos o tempo de autocorrelação integrado nas séries temporais da energia. Inferimos daí quão severo é o critical slowing down (decaimento lento crítico) para esse sistema próximo às transições de fase termodinâmicas. Os resultados obtidos utilizando um algoritmo de atualização local foram comparados com os resultados obtidos utilizando o algoritmo multicanônico. / Two-dimensional spin model with nearest-neighbor ferromagnetic interaction and long-range dipolar interactions exhibit a rich phase diagram, whose characteristics have been exploited by several studies in the recent literature. Furthermore, the possibility of explain observed phenomena in ultrathin magnetic films, which have many technological applications, also motivates the study of this model. The presence of dipolar interaction term changes the ferromagnetic ground state expected for the pure Ising model to a series of striped phases, which consist of ferromagnetic domains of width $h$ with opposite magnetization. The width of the stripes depends on the ratio $\\delta$ of the ferromagnetic and dipolar couplings. Monte Carlo simulations and reweighting multiple histograms techniques allow us to identify the finite-size critical temperatures of the phase transitions when $\\delta=2$, which corresponds to $h=2$. We calculate, for different lattice sizes, the specific heat and susceptibility of the order parameter around the transition temperatures by means of reweighting techniques. This allows us to identify in these observables, as functions of temperature, the distinct maxima and thereby to estimate the finite-size critical temperatures with high precision. We present numerical evidence of the existence of a Ising nematic phase for large lattice sizes. Our results show that simulations need to be performed for lattice sizes at least as large as $L=48$ to clearly observe the Ising nematic phase. To access how the long-range dipolar interaction may affect physical estimates we also evaluate the integrated autocorrelation time in energy time series. This allows us to infer how severe is the critical slowing down for this system with long-range interaction and nearby thermodynamic phase transitions. The results obtained using a local update algorithm are compared with results obtained using the multicanonical algorithm.

The identity of dark matter is one of the key outstanding problems in both particle and astrophysics. In this thesis, I describe a number of complementary searches for particle dark matter. I discuss how the impact of dark matter on stars can constrain its interaction with nuclei, focussing on main sequence stars close to the Galactic Centre, and on the first stars as seen through the upcoming James Webb Space Telescope. The mass and annihilation cross-section of dark matter particles can be probed with searches for gamma rays produced in astronomical targets. Dwarf galaxies and ultracompact, primordially-produced dark matter minihalos turn out to be especially promising in this respect. I illustrate how the results of these searches can be combined with constraints from accelerators and cosmology to produce a single global fit to all available data. Global fits in supersymmetry turn out to be quite technically demanding, even with the simplest predictive models and the addition of complementary data from a bevy of astronomical and terrestrial experiments; I show how genetic algorithms can help in overcoming these challenges. / At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 5: Accepted. Paper 6: Submitted.

The thesis consists of two parts. Part 1 deals with the preparation of thin films and sub-micron powders of complex metal oxides by nebulized spray pyrolysis (NSP) and Part 2 consists of Brillouin scattering studies of solid materials exhibiting interesting phase transitions. The simple technique of NSP has been employed to prepare thin films of A12O3, PbTiO3, Pb(Zr0.5Ti0.5)O3 (PZT) and PbZrO3 on single crystal substrate. The films were characterized by various techniques for their composition, structure, morphology and dielectric properties. Ferroelectric (FE) films of the configuration FE/LaNiO3/SiO2/Si (FE = PbTiO3 and PZT), wherein the LaNiO3 barrier electrode was also deposited on the SiO2/Si substrate by NSP, have been investigated. The films exhibit satisfactory ferroelectric properties. PbZrO3 films deposited on LaNiO3/SiO2/Si substrates show good features, including a reversible AFE ↔ FE transition. Sub-micron particles of TiO2, ZrO2, Pb(Zr0.5Ti0.5)O3, Al2O3, S1O2 and mullite have been prepared by NSP and characterized by various techniques. Brillouin scattering has been used, for the first time, not only to characterize the Peierls transition but also the incommensurate to commensurate transition in the one-dimensional blue bronze, K0.3M0O3. The charge density wave transition in NbSe2 has also been investigated by Brillouin scattering. The charge ordering and antiferromag-netic transitions in single crystals of the rare earth manganates, Nd0.5Ca0.5MnO3 and Pr0.63Ca 0.37MnO3, have been investigated by Brillouin scattering. It is noteworthy that the temperature variation of the Brillouin shift and intensity parallel to that of the magnetization, thereby throwing light on magnetic excitations in charge-ordered state. Brillouin scattering investigations of C60 and C70 films have yielded values of the elastic moduli.

This thesis deals with the conformation and phase-transitions in anchored alkyl chain bilayer assemblies in organic-inorganic hybrids. The alkyl chain bilayers in organic-inorganic hybrids bear a striking resemblances to the lipid bilayers that are an integral part of biomembranes. However, unlike the lipid bilayer where individual lipid molecules can undergo the total absence of translational mobility. The anchored bilayer are, therefore, the simplest model system for understanding the structure, organization and thermal behaviour of alkyl-chain assemblies. The anchored bilayer in the organic-inorganic hybrids also offer the advantage that unlike the lipid bilayers that are essentially fluid like, these are solids and therefore, amenable to study by a variety of solid-state spectroscopic techniques. The objective of the present works was to determine the organization, conformation and thermal behaviour of alkyl chains in these class of materials- the alkyl ammonium layered pervoskites and in zinc soaps of saturated and unsaturated fatty acids. The preparation, conformation and orientation of alkyl chains assemblies in the layered(CH3CH2)nNH3)2PbI4 is described in chapter 2 and the phase-transitions and associated changes in conformation in presented in chapter 3. In chapter 4 the preparation, structure, conformation and phase-transitions of alkyl chains in the m = 2 member of the layered (CH3NH3)m-1(CH3CH2)nNH3)2PbmI3m+1 Ruddleson-Popper series is discussed. The thermal behavior of zinc soaps of saturated fatty acids is discussed in chapter 5 and formation and phase-transitions in solid solution between saturated zinc soaps of differing chain lengths, zinc stearate and zinc myristate is presented in chapter 6. The effect of a rigid link or double bond on the evolution of conformational disorder and phase-transitions of other-wise flexible hydrocarbon chains is explored in chapter 7 by comparing the thermal behavior of zinc oleate and zine elaidate with that of zinc stearate. A unique feature of the zinc soaps is that they form solid solutions over the entire composition range feature of the zinc soaps is that they form solid solutions over the entire composition range between soaps of saturated and unsaturated fatty acids. The formation and conformation of alkyl chains in the solid solution of zinc stearate and zinc oleate is discussed in chapter 8.

Die Betrachtung von Schwere-Fermionen-Systemen stellt ein wichtiges Themengebiet im Bereich der Festkörperphysik dar. Das Verhalten von Schwere-Fermionen-Systemen wird durch die starken Korrelationen der magnetischen Momente der ungepaarten Spins der f-Elektronen bestimmt. Experimentell zugängliche Messgrößen sind dadurch bei tiefen Temperaturen stark erhöht, so dass sich diese Systeme besonders gut zur Untersuchung von Grundzustandseigenschaften eignen. Zentrales Thema dieser Arbeit ist die Untersuchung zweier intermetallischer Seltenerd-Verbindungen in Bezug auf Quantenphasenübergänge. Diese treten am absoluten Nullpunkt der Temperatur als Funktion eines anderen Parameters wie Magnetfeld, Druck oder chemischer Substitution auf und sind bei endlicher Temperatur durch Abweichungen physikalischer Messgrößen von der durch L. D. Landau aufgestellten Theorie der Fermi-Flüssigkeiten nachzuweisen. Zu diesem Zweck wurden Tieftemperaturexperimente bis hinab zu 20mK und in Magnetfeldern bis zu 18T durchgeführt. Es wurden elektrischer Widerstand, magnetische Wechselfeldsuszeptibilität, Magnetostriktion und thermische Ausdehnung gemessen. / The investigation of heavy-fermion systems marks an important subject in the research field of solid state physics. The behaviour of heavy-fermion systems is dominated by the strong correlations of the magnetic moments of the unpaired f-electron spins. At low temperatures, experimentally accessible variables are strongly enhanced so that these systems are especially suited to analyse ground state properties. The central topic of this thesis is the investigation of two intermetallic rare-earth compounds with regard to quantum phase transitions. The latter occur at zero temperature as a function of parameters such as magnetic field, pressure or chemical substitution. They are traceable at finite temperature due to deviations of physical variables from the theory of Fermi liquids established by L. D. Landau. For this purpose, low-temperature experiments were performed down to 20mK and in magnetic fields up to 18T. Electrical resistivity, magnetic ac susceptibility, magnetostriction and thermal expansion were measured.