Lead halide perovskites (LHPs) have forcefully emerged as a promising materials class for next-generation solar cells. The high efficiencies of LHP-based photovoltaics are underpinned by their outstanding optoelectronic properties, including long carrier lifetimes, long carrier diffusion lengths, high radiative efficiencies, and long-lived hot carriers. In conventional semiconductors, high efficiencies are achieved by stringent control over defect densities; higher purity diminishes the number of carrier scattering events and yields better optoelectronic properties. Given the high defect densities of LHPs, these observed behaviors indicate that LHPs are defect-tolerant and disobey this paradigm via dynamic screening of charge carriers. In order to expand the library of defect-tolerant semiconductors, we must elucidate the carrier-lattice interactions that lead to dynamic screening. LHP lattices are highly anharmonic and dynamically disordered, which must play a role in this screening mechanism. This anharmonicity demands a departure from the conventional Fröhlich interaction, which considers the harmonic coupling of a carrier to one phonon, to a picture that incorporates anharmonic phonon-phonon couplings. The objective of this thesis is to investigate the ultrafast anharmonic lattice response associated with dynamic screening of charge carriers. We probe the formation of large polarons in CH3NH3PbBr3 and CsPbBr3 using time-resolved optical Kerr effect spectroscopy. We further investigate the coupling of phonon modes in a model system, CsPbBr3, in the presence of charge carriers using ultrafast coherent phonon spectroscopy.

Thesis (Ph. D.)--University of Hong Kong, 2007. / Title proper from title frame. Also available in printed format.

The work presented in this thesis was motivated by the large amount of experimental investigations of the phonons in quasicrystals. The generalized vibrational density of states (GVDOS) was measured for many quasicrystalline phases and in some cases at different temperatures [suck et al (1997), Dugain et al (1997)]. The progress achieved in the structure determination of approximants to some quasicrystals was a legitimate motivation for numerical investigations of lattice dynamics in these structures. Two different types of interatomic interactions were used: the spring model and the ab-initio pair potentials. The investigations explained the shape of some experimentally measured GVDOS (d-AlNiCo, o-Al13Co4 and i-ZnMgY) via the calculation of the partial vibrational densities of states. Both calculated and measured GVDOS of the d-AlNiCo phase showed an intensity excess at low energies relatively to the ideal Debye behaviour. This excess was found to be a consequence of the existence of special modes at theses energies which are called ``quasi-localized modes''. These modes seem to be characteristic of the lattice dynamics in the complex Al-TM structures. To calculate the frequency shift due to the shift of the GVDOS through low energies observed experimentally at high temperatures, a new method based on a Monte-Carlo simulation was developed. It was shown that the quasi-localized modes introduce large frequency shifts at low energies. Finally, the vibrational entropy was also investigated, and it was found that it contributes to the stabilization of the complex structures over the relatively simple structures at high temperatures. / Die Arbeit, die in dieser Dissertation präsentiert wird, wurde durch eine Vielzahl von experimentellen Beobachtungen von Phononen in Quasikristallen motiviert. Die verallgemeinerte vibrationelle Zustandsdichte (GVDOS, generalized vibrational density of states) wurde für viele quasikristalline Phasen gemessen und für einige auch bei verschiedener Temperatur [Suck et al. (1997), Dugain et al. (1997)]. Der Fortschritt, der in der Bestimmung von Näherungen für einige Quasikristalle erreicht wurde war eine legitime Motivation für numerische Untersuchungen der Gitterdynamik auf diesen Strukturen. Es wurden zwei unterschiedliche interatomare Wechselwirkungen verwendet: Das Federmodell und die ab-initio Paar Potentiale. Die Untersuchungen erklärten die Form einiger experimenteller GVDOS-Messungen (d-AlNiCo, o-Al13Co4 und i-ZnMgY) mittels der Berechnung der partiellen vibrationellen Zustandsdichte. Beide, berechnete und gemessene, GVDOS der d-AlNiCo Phase zeigten einen Intensitätsanstieg bei kleinen Energien relativ zum idealen Debye Verhalten. Dieser Anstieg stellte sich als Konsequenz der Existenz von besonderen Moden bei diesen Energien heraus, die quasi-lokalisierte Moden genannt werden. Diese Moden scheinen charakteristisch für die Gitterdynamik in den komplexen Al-TM Strukturen zu sein. Um die experimentell beobachtete Frequenzverschiebung aufgrund der Verschiebung der GVDOS durch niedrige Energien zu berechnen, wurde eine neue, auf Monte-Carlo Simulation beruhende, Methode entwickelt. Es wurde gezeigt, daß die quasi-lokalisierten Moden große Frequenzverschiebungen bei kleinen Energien hervorrufen. Letzt-lich wurde auch die vibrationelle Entropie untersucht und es stellte sich heraus, daß sie bei hohen Temperaturen dazu beiträgt die komplexen Strukturen gegenüber den relativ einfachen zu stabilisieren.

Thesis (Ph. D.)--University of Chicago, Department of Physics, December 2003. / Includes bibliographical references. Also available on the Internet.

This dissertation focuses on two areas of garnet porphyroblast crystallization that have until now remained largely uninvestigated: epitaxial nucleation of garnet porphyroblasts and yttrium and rare earth (Y+REE) uptake in metamorphic garnet. The mechanism of epitaxial nucleation is explored as a step towards determining which aspects of interfaces are significant to interfacial energies and nucleation rates. Garnet from the aureole of the Vedrette di Ries tonalite, Eastern Alps, shows a clear case of epitaxial nucleation in which garnet nucleated on biotite with (110)grt / (001)bt with [100]grt / [100]bt. The occurrence is remarkable for the clear genetic relationships revealed by the microstructures and for its preservation of the mica substrate, which allows unambiguous determination of the coincident lattice planes and directions involved in the epitaxy. Not all epitaxial nucleation is conspicuous; to increase the ability to document epitaxial relationships between garnet and micas, I develop and apply a method for determining whether evidence for epitaxial nucleation of garnet is present in porphyroblasts containing an included fabric. Although the magnitude of uncertainties in orientation measurements for garnets from Passo del Sole (Switzerland), the Nevado Filabride Complex (Spain), and Harpswell Neck (USA) preclude definitive identification of epitaxial relationships, the method has potential to become a viable technique for creating an inventory of instances and orientations of epitaxial nucleation with appropriate sample selection. Using lattice-dynamics simulations, I explore the most commonly documented epitaxial relationship, (110)grt / (001)ms. The range of interfacial energies resulting from variations in the intracrystalline layer within garnet at the interface, the initial atomic arrangement at the interface, and the rotational orientation of the garnet structure relative to the muscovite structure shows that the intracrystalline layer within garnet has the greatest effect on interfacial energy. A complete understanding of the role of intergranular diffusion for yttrium and rare-earth-element uptake in porphyroblastic garnet is critical because the complexities of Y+REE zoning in garnets and the mechanisms of Y+REE uptake have implications for petrologic interpretations and garnet-based geochronology. Y+REE distributions in garnets from the Picuris Mountains (USA), Passo del Sole (USA), and the Franciscan Complex (USA) imply diverse origins linked to differing degrees of mobility of these elements through the intergranular medium during garnet growth. / text

Not available / text

We use normal mode decomposition to obtain phonon properties from quasi-harmonic lattice dynamics calculations and classical molecular dynamics simulations in unstrained Lennard-Jones argon superlattices with perfect and mixed interfaces. Debye scaling of phonon lifetimes at low frequencies in both perfect and mixed superlattices and Rayleigh scaling for intermediate frequencies in mixed superlattices is observed. For short period mixed superlattices, lifetimes below the Ioffe-Regel limit are observed. The relaxation-time approximation of the Boltzmann transport equation is used to predict cross-plane and in-plane thermal conductivity. We find that using a dispersion relation which includes the secondary periodicity is required to predict thermal conductivity. The assumption of perturbative disorder, where Tamura elastic mass defect scattering theory can be applied, was found to be valid for predicting cross-plane thermal conductivities but not in-plane thermal conductivities in mixed superlattices.

We use normal mode decomposition to obtain phonon properties from quasi-harmonic lattice dynamics calculations and classical molecular dynamics simulations in unstrained Lennard-Jones argon superlattices with perfect and mixed interfaces. Debye scaling of phonon lifetimes at low frequencies in both perfect and mixed superlattices and Rayleigh scaling for intermediate frequencies in mixed superlattices is observed. For short period mixed superlattices, lifetimes below the Ioffe-Regel limit are observed. The relaxation-time approximation of the Boltzmann transport equation is used to predict cross-plane and in-plane thermal conductivity. We find that using a dispersion relation which includes the secondary periodicity is required to predict thermal conductivity. The assumption of perturbative disorder, where Tamura elastic mass defect scattering theory can be applied, was found to be valid for predicting cross-plane thermal conductivities but not in-plane thermal conductivities in mixed superlattices.

No description available.

Thesis (Ph. D.)--University of Oregon, 2007. / Typescript. Includes vita and abstract. Includes bibliographical references (leaves 209-231). Also available for download via the World Wide Web; free to University of Oregon users.