The work presented in this thesis concerns the dynamics in π-conjugated hydrocarbon systems. Due to the molecular bonding structure of these systems there exists a coupling between the electronic system and the phonons of the lattice. If this interaction, which is referred to as the electron-phonon (e-ph) coupling, is sufficiently strong it may cause externally introduced charge carriers to self-localize in a polarization cloud of lattice distortions. These quasi-particles are, if singly charged, termed polarons, the localization length of which, aside from the e-ph coupling strength, also depend upon the structural and energetic disorder of the system. In disordered systems localization is strong and transport is facilitated by nonadiabatic hopping of charge carriers from one localized state to the next, whereas in well-ordered systems, where extended states are formed, adiabatic transport models apply.Despite great academic efforts a unified model for charge transport in π-conjugated systems is still lacking and further investigations are necessary to uncover the basic physics at hand in these systems. The call for such efforts has been the main guidelines for the work presented in this thesis and are related to the topics of papers I-IV. In order to capture the coupled electron-lattice dynamics, we use a methodological approach where we obtain the time-dependence of the electronic degrees of freedom from the solutions to the time-dependent Schrödinger equation and determine the ionic motion in the evolving charge density distribution by simultaneously solving the lattice equation of motion within the potential field of the ions. The Hamiltonian used to describe the system is derived from an extension of the famous Su-Schrieffer-Heeger (SSH) model extended to three-dimensional systems.In papers I-III we explore the impact of phenylene ring torsion on delocalization and transport properties in poly(para-phenylene vinylene) (PPV). The physics that we are particularly interested in relates to the reduced electron transfer integral strength across the interconnection between the phenylene rings and the vinylene groups upon ring torsion. Keeping this in mind, we demonstrate in paper I the impact of static ring torsion on intrachain mobility and provide a detailed analysis of the influence of the potential barriers (due to consecutive ring torsion) on the nature of charge carrier propagation. In paper II we extend our initial approach to include also the dynamics of ring torsion. We show that without any externally applied electric field, this type of dynamics is the dominant property controlling intrachain propagation, but that when an external electric field is applied, charge carriers may traverse the potential barriers through a process that involves nonadiabatic effects and a temporary delocalization of the polaron state. Finally, in paper III we study the impact of the lattice dynamics on the electron localization properties in PPV and show that the phenylene ring torsion modes couples strongly to the electronic wave function which gives rise to electron localization at room temperature.In papers IV and V we focus on the dynamics of molecular crystals using a stack of pentacene molecules in the single crystal configuration as a model system, but study, in paper IV, the transport as a function of the intermolecular interaction strength, J. We observe a smooth transition from a nonadiabatic to an adiabatic polaron drift process over the regime 20<J<120 meV. For intermolecular interaction strengths above J≈120 meV the polaron is no longer stable and transport becomes band-like. In paper V, finally, we study the internal conversion processes in these systems, which is the dominant relaxation channel from higher lying states. This process involves the transfer of energy from the electronic system to the lattice. Our results show that this process is strongly nonadiabatic and that the relaxation time associated with large energy excitations is limited by transitions made between states of different bands. / I dagens samhälle är elektroniken ett allt viktigare och större inslag i vår vardag. Vi ser på TV, talar i mobiltelefoner, och arbetar på datorer. I hjärtat av denna teknologi finner vi diskreta komponenter och integrerade kretsar utformade främst för att styra strömmen av elektroner genom halvledande material. Traditionellt sett har kisel eller olika former av legeringar använts som det aktiva materialet i dessa komponenter och kretsar, men under de senaste 20 åren har såväl transistorer som solceller och lysdioder realiserats där det aktiva materialet är organiskt, d.v.s., kolbaserat.Vi befinner oss för tillfället mitt uppe i det kommersiella genombrottet för organisk elektronik. Redan idag säljs många MP3-spelare och mobiltelefoner med små skärmar där varje pixelelementen utgörs av organiska ljusemitterande dioder (OLEDs), men teknologin håller redan på att introduceras i mer storskaliga produkter som datorskärmar och TV-apparater som därigenom skulle kunna göras energieffektivare, tunnare, flexiblare och på sikt också billigare. Andra tekniska tillämpningsområden för organisk elektronik som förutspås en lysande framtid är RFID-märkning, organiska solceller, och elektronik tryckt på papper, men även smarta textiler och bioelektronik har stor utvecklingspotential.Den kanske största utmaningen kvarstår dock, att skapa elektroniska kretsar och komponenter uppbyggda kring enskilda molekyler, s.k. molekylär elektronik. Mycket snart närmar vi oss den fysikaliska gränsen för hur små komponenter som vi kan realisera med traditionella icke-organiska material som kisel och en stor drivkraft bakom forskningen på halvledande organiska material har varit just visionen om molekylär elektronik som inte är mer än några miljondelars milimeter stora. För detta ändamål krävs en mycket nogrann kontroll av tillverkningsprocesserna liksom en detaljförståelse för hur molekylerna leder ström och hur denna förmåga kan manipuleras för att realisera såväl traditionella som nya komponenter.I denna avhandling presenteras en översikt av den fysik som möjliggör ledningsförmåga hos särskilda klasser av organiska material, s.k. π-konjugerade system, samt de forskningsresultat som utgör mitt bidrag till denna disciplin. En av utmaningarna på området är den komplexitet som de organiska materialen erbjuder: laddningsprocesserna påverkas nämligen av en rad olika faktorer såsom laddningstäthet, temperatur, pålagd spänning, samt molekylernas former och inbördes struktur. I detta arbete har jag utifrån en vidareutveckling av existerande modeller genom numeriska datasimuleringar undersökt effekten av de senare tre faktorerna på elektronstrukturen, laddnigstransporten och energidissipation i denna klass av material. / Center of Organic Electronics (COE)

This thesis deals with various functional materials from first-principles methods and is divided into two major parts according to the underlying atomic structure of the system under study. The first part of the thesis deals with the temperature-induced structural phase transitions in metallic  β'-AuZn and perovskite oxide LiOsO3. The former one, i.e. binary AuZn, belongs to a class of shape-memory alloys that regain their initial shape due to a reversible martensitic phase transformation. Here, by means of density functional and density functional perturbation theories, we show that the martensitic transition is due to coupling between the Fermi surface nesting and anomalies in the phonon dispersion relations. The other metallic system, perovskite LiOsO3, exhibits a ferroelectric-like transition and is currently the first and sole realization of the Anderson and Blount idea. By means of ab initio molecular dynamics simulations, we investigate the mechanism behind this structural phase transformation. Another part of the thesis is dedicated to modelling and characterization of topologically disordered materials on atomic level. The structural and electronic properties of amorphous W-S-N are addressed regarding its outstanding tribological properties, i.e. almost vanishing friction coefficient. Molecular dynamics “melt-and-quench” technique has been employed in order to construct a model structure of amorphous W-S-N. Further analysis of the atomic structure revealed a formation of quasi-free N2 molecules trapped in S cages, which, together with the complex atomic structure of W-S-N, is the key to ultra-low-friction in this functional material. In the last chapter of the thesis a magnetic class of amorphous materials is addressed. Magnetic order in amorphous Gd-Fe ferrimagnet has been shown to undergo magnezation switching driven by a femtosecond laser pulse. Here, we combine first-principles density functional theory and atomistic spin dynamics simulations to explore this phenomena. A possible mechanism behind magnetization reversal in Gd-Fe based on a combination of the Dzyaloshinskii-Moriya interaction and exchange frustration is proposed.

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The advent of rapid manufacturing has enabled the realization of countless products that have heretofore been infeasible. From customized clear braces to jet fighter ducts and one-off dental implants, rapid manufacturing allows for increased design complexity and decreased manufacturing costs. The manufacturing capabilities of this process have evolved to the point that they have surpassed current design capabilities. Meso-scale lattice structures can now be built that contain more lattice struts than it is reasonable to efficiently define. This work has attempted to create a method for designing such lattice structures that is efficient enough to allow for the design of large or complex problems. The main hindrance to the design of complex meso-scale lattice problems is essentially the need to define the strut diameters. While it is obvious that a large design would contain more struts than can be specified by hand, designs also quickly surpass the current capabilities of computational optimization routines. To overcome this problem, a design method has been developed that uses a unit-cell library correlated to finite element analysis of the bounding geometry to tailor the structure to the anticipated loading conditions. The unit-cell library is a collection of base lattice primitives, or unit-cells, that have been specialized for certain applications. In this case, primitives have been created that perform best under the types of stress analyzed by finite element analysis. The effectiveness of this process has been demonstrated through several example problems. In all cases, the unit-cell library approach was able to create structures in less time than current methods. The resulting structures had structural performance slightly lower than similar models created through optimization methods, although the extent of this degradation was slight. The method developed in this work performs extremely well, and is able to create designs for even the most complex lattice structures. There is room for future development, however, in the streamlining of the design process and consideration of higher-order affects within unit-cells.

This thesis presents the results from Monte Carlo calculations on classical vector spins in face-centered-cubic (FCC) lattices. The goal of the study was to understand the effect of interlayer coupling, dilution of magnetic atoms in the lattice, and symmetry-changing strain. Experimental work by T. M. Giebultowicz et al. and J. A. Borchers et al. greatly inspired my work [1, 2]. J. A. Borchers's group studied NiO/CoO superlattices and observed that the magnetic order of CoO persisted above its Neel temperature due to the effect of interlayer coupling with NiO, which has a higher Neel temperature than CoO [1]. Simulating on a model of NiO/CoO bilayer reproduced the experimental results from Borchers et al. [1]. I concluded that exchange pinning on the NiO/CoO interface preserves the magnetic order of CoO above its Neel temperature significantly. Building on this initial result, a ferromagnet/antiferromagnet/ferromagnet (FM/AFM/FM) trilayer model was studied, where the ferromagnetic (FM) layers were antiferromagnetically coupled. First, I calculated the strength of the AF coupling as a function of the number of antiferromagnetic (AFM) spacer monolayers and concluded that the strength of AFM coupling decreases as the number of AFM spacer monolayers increases. Secondly, I added a uniaxial anisotropy to the model and obtained magnetization curves which exhibited hysteresis-like features with an external field and a first order magnetic transition. Lastly, I diluted the AFM spacer layer in the FM/AFM/FM trilayer by replacing magnetic spins with zero spins in the model. The dilution of AFM spacer layer caused fluctuations in the magnetization curves with external field but the strength of AFM coupling decreases as the number of AFM monolayers increases as in the nondiluted cases. The experimental results from T.M. Giebultowicz's group on MnSe/ZnTe superlattices by neutron scattering showed incommensurate helical spin order in MnSe, where MnSe layers were under tensile strain due to a small mismatching in the lattice parameter [2]. In addition, they observed that the pitch of the spin helix increased as the temperature increased [2]. I modeled the MnSe/ ZnTe system with Monte Carlo method and found that the pitch of the spin helix increased with temperature. In fact, the dependence of helix pitch on temperature was present regardless of the thickness of the sample, so I concluded that this pitch increase is not from the weakening of coupling of surface spins / Graduation date: 2013 / Access restricted to the OSU Community at author's request from Jan. 18, 2013 - July 18, 2013

Thesis (Ph. D.)--University of Texas at Austin, 2004. / Supervisor: Daniel J. Heinzen. Vita. Includes bibliographical references. Also available from UMI.

Thesis (Ph. D.)--Ohio State University, 2005. / Title from first page of PDF file. Document formatted into pages; contains xxiii, 261 p.; also includes graphics. Includes bibliographical references (p. 256-261). Available online via OhioLINK's ETD Center

Au voisinage et au dessous des fréquences térahertz, nous avons investigué la dynamique des modes mous centraux de type Debye dans les super-réseaux ferroélectriques BaTiO3/BaxSr1-xTiO3 dans un large domaine de températures en utilisant la spectroscopie Raman polarisée. La coexistence d'un pic central et du mode mou sous-atténué suggère un caractère complexe ordre-désordre des transitions de phase successives dans ces matériaux. L'apparition du mode central prononcé pourrait expliquer l'anomalie diélectrique de type relaxateur récemment observée dans de tels super-réseaux. Nous avons exploré et comparé la dynamique des super-réseaux à trois couches et à deux couches. Nous avons montré que l'utilisation de couches de compositions chimiques différentes dans des super-réseaux multicouches permet d'obtenir des hétérostructures ayant des caractéristiques souhaitables en raison des effets de déformation entre les couches alternées et de contrôler leurs paramètres.Nous avons établi les diagrammes de phase théoriques "strain-misfit temperature" pour les couches minces de BaxSr1-xTiO3 déposées sur des substrats cubiques orientés (111). Ces diagrammes de phase sont utiles pour des applications pratiques dans l'ingénierie des couches minces. Nous avons aussi réalisé une étude expérimentale sur un film mince de Ba0.8Sr0.2TiO3 déposé sur un substrat MgO (111) pour vérifier nos prédictions théoriques. Nous avons étudié l'inversion de la polarisation induite par le champ dans la phase ferroélectrique orientée suivant l'axe C des films de structure pérovskite sous contrainte. Nous avons montré qu'en plus du mécanisme de commutation longitudinal conventionnel, lorsque le module du vecteur de polarisation orienté selon l'axe c change, les mécanismes longitudinaux-transversaux et transversaux sont aussi possibles lorsque la composante perpendiculaire de la polarisation est activée dynamiquement / We investigate near- and sub-Terahertz dynamics of soft and Debye-type central modes by the polarized Raman spectroscopy in ferroelectric BaTiO3/BaxSr1-xTiO3 superlattices in a broad temperature range. Coexistence of the central peak and the underdamped soft mode suggests complicated order-disorder character of successive phase transitions in these superlattices. The occurrence of the pronounced central mode can explain the recently observed relaxor-like dielectric anomaly in such superlattices. We explore and compare the lattice dynamics of three-layer and two-layer superlattices. We show that the using layers of different chemical compositions in multilayered superlattices one can obtain heterostructures with the desirable characteristics and realize fine tuning of their parameters due to strain effects between alternating layers.We construct the “temperature-misfit strain” theoretical phase diagrams for BaxSr1-xTiO3 thin films grown on (111)-oriented cubic substrates. The phase diagrams are useful for practical applications in thin-film engineering. We experimentally investigate a Ba0.8Sr0.2TiO3 thin film deposited on (111)MgO substrate with the aim to verify our theoretical predictions. We study the field-induced polarization reversal in the c-oriented ferroelectric phase of strained perovskite films. We show that in addition to the conventional longitudinal switching mechanism, when the c-oriented polarization vector changes its modulus, the longitudinal-transversal and transversal mechanisms when the perpendicular component of polarization is dynamically admixed are possible

Les multiferroïques sont entre autres des matériaux possédant à la fois un ordre magnétique et un ordre ferroélectrique, le plus souvent couplés entre eux (couplage magnétoélectrique). Ce caractère multifonctionnel scientifiquement et technologiquement prometteur, rend ces matériaux plus attrayants, d’autant plus que l'aimantation peut être contrôlée par l'application de champ électrique, ou que la polarisation électrique peut être contrôlée par un champ magnétique. D’un point de vue technologique, ces matériaux ouvrent la voie à des applications dans les domaines de l’électronique de spins, des capteurs magnétoélectriques, des mémoires de stockage, … D’un point de vue scientifique, ce sont les questions fondamentales relative à la compréhension des mécanismes gouvernant la présence de l'ordre ferroélectrique dans un matériau magnétique, qui expliquent leur attractivité. / Multiferroics are materials exhibiting in the same phase, at least two ferroics orders such as magnetism and ferroelectricity, which is furthermore extended when these orders are coupled (magnetoelectric coupling). This multifunctionality is scientifically and technologically promising, and makes multiferroics more attractive, especially since the magnetization can be controlled by the application of an electric field, or the polarization can be controlled by a magnetic field. From a technological point of view, these materials open pathways for many applications in spintronics, magnetoelectric sensors, data storage memories, ... From a scientific point of view, their attractiveness is explained by the fact that many fundamental questions related to the mechanisms of the occurrence of ferroelectricity in a magnetic material, are still unanswered.

Materials that are light weight, low cost and have high hydrogen storage capacity are essential for on-board vehicular applications. Some reversible complex hydrides are alanates and amides but they have lower capacity than the DOE target (6.0 wt %) for 2010. High capacity, light weight, reversibility and fast kinetics at lower temperature are the primary desirable aspects for any type of hydrogen storage material. Borohydride complexes as hydrogen storage materials have recently attracted great interest. Understanding the above parameters for designing efficient complex borohydride materials requires modeling across different length and time scales. A direct method lattice dynamics approach using ab initio force constants is utilized to calculate the phonon dispersion curves. This allows us to establish stability of the crystal structure at finite temperatures. Density functional theory (DFT) is used to calculate electronic properties and the direct method lattice dynamics is used to calculate the finite temperature thermodynamic properties. These computational simulations are applied to understand the crystal structure, nature of bonding in the complex borohydrides and mechanistic studies on doping to improve the kinetics and reversibility, and to improve the hydrogen dynamics to lower the decomposition temperature. A combined theoretical and experimental approach can better lead us to designing a suitable complex material for hydrogen storage. To understand the structural, bulk properties and the role of dopants and their synergistic effects on the dehydrogenation and/or reversible rehydrogenation characteristics, these complex hydrides are also studied experimentally in this work.