Structuring Language and Community in an Online Space

Massively multiplayer online games (MMOs) present an interesting area for linguistic and social research, being a setting for of computer-mediated communication (CMC) that is task-oriented in nature and often requires high level of cooperation between players. This study investigates how Spanish-speaking players of the MMO Final Fantasy XIV: A Realm Reborn use the linguistic phenomena of discourse markers, laughter and politeness to structure their communication and their community. Through analysis of in-game conversations gathered from a community of said players, this study demonstrates how each of these phenomena work together to build a community based on inclusive language and positive reinforcement

Everything is imperfect: Studies on point defects in insulators

Point defects can be detrimental or absolutely necessary for the performance of a material. Due to the implications defectshave on a material, defects have been widely studied over the last century. However, there are plenty of open questions that concernsthe effect of point defects in materials but also properties of defects that are common across materials. This thesis consists of studies on equilibrium properties of oxygen vacancies and optical lineshapes of transitions on point defects. Using first principles calculations, general properties of oxygen vacancies in wide band gap oxides are revealed. The induced defect level of the oxygen vacancy is found to be constant in a number of materials. It is often located in the middle of the band gap and associated with a large structural distortion upon a change in charge state. Due to the large structural distortion, oxygen vacancies may play a role in luminescence quenching in oxide based phosphors and recombination processes in general. A possible recombination mechanism involving oxygen vacancies is proposed in the oxide phosphor Ce:YAG, which could explain the luminescence quenching. In connection with luminescence quenching in Ce:YAG, the origin of the phonon side bands of the optical transition is elucidated. Due to the connection between the optical lineshape and the geometry of a point defect, the optical lineshape can be used to identify point defects. The point defect emission in hexagonal boron nitride is addressed and several point defects can be matched to observed emission lines

Optical properties of point defects in insulators and of transition metal dichalcogenides

There is a need for new or modified materials, both to improve current devices and to create novel functionalities.Engineering materials to target specific functionalities requires a better understanding of how microscopic processes impact materials properties.In this thesis, the effects of defects, mixing of materials, and twisting of parts of the material are explored via first-principles calculations. In particular, the properties of defects in a range of technologically important insulators and of mixing and twisting of transition metal dichalcogenides (TMDs) are addressed, with an emphasis on optical properties. In the part of the thesis that pertains to defects, the commonality of oxygen vacancies is considered. It is shown that oxygen vacancies exhibit properties that extend beyond specific insulating oxides and that there is a strong interplay between lattice geometry and oxygen vacancy character. The coupling between defect states and lattice vibrations is subsequently accounted for and used to identify the contribution of specific defects to relaxation mechanisms. It is shown that oxygen vacancies may be detrimental to the performance of the oxide phosphor YAG, playing a key role in a reaction mechanism that leads to luminescence quenching. The part of this thesis that is concerned with defects is concluded by an analysis of the optical signatures and phonon sidebands of defects in h-BN and SiC.On the topic of TMDs, the properties of twisted bilayers are explored, in particular in connection to excitons. It is shown that for very small twist angles, excitons become localized. In addition, the twist-induced potential is dissected and it is shown that the purely electrostatic component of this potential decays with increasing twist angle. Finally, a high-throughput study on TMD alloys was performed for which mixing properties and band edge alignments are presented

Phylogeny and Historical Ecology of Rhodocoma (Restionaceae) from the Cape Floristic Region

A macroevolutionary analysis of macroecological relationships in Rhodocoma revealed a complex history of rapid ecological divergence, as well as genetic isolation via shifts in flowering times. The rate and extent of divergence observed among even the youngest of species pairs indicated that the selective forces driving these processes are strong enough to effect substantial amounts of ecological change in relatively short periods of time, and are potentially important factors promoting the origin and persistence of species diversity not only in Rhodocoma, but also the African Restionaceae as a whole. These results also suggest that the rate and extent of ecological differentiation can vary between lineages, and this may be a consequence of variations in the intensities of selective regimes or phylogenetic constraints that different lineages experience. Investigation into the nature of this differentiation revealed that much of it has occurred along altitudinal gradients, but in tandem with substantial shifts in other ecological parameters such as rainfall and fire survival. This multidimensionality of ecological differentiation increases the number of possible combinations of ecological parameters and may allow for a more precise partitioning of niche space

Luminescence Quenching via Deep Defect States: A Recombination Pathway via Oxygen Vacancies in Ce-Doped YAG

Luminescence quenching via nonradiative recombination channels limits the efficiency of optical materials such as phosphors and scintillators and therefore has implications for conversion efficiency and device lifetimes. In materials such as Ce-doped yttrium aluminum garnet (YAG:Ce), quenching shows strong dependence on both temperature and activator concentration, limiting the fabrication of high-intensity white-light emitting diodes with high operating temperatures. Here, we reveal by means of first-principles calculations an efficient recombination mechanism in YAG:Ce that involves oxygen vacancies and gives rise to thermally activated concentration quenching. We demonstrate that the key requirements for this mechanism to be active are localized states with strong electron-phonon coupling. These conditions are commonly found for intrinsic defects such as anion vacancies in wide band gap materials. The present findings are therefore relevant to a broad class of optical materials and shine light on thermal quenching mechanisms in general

High-Throughput Characterization of Transition Metal Dichalcogenide Alloys: Thermodynamic Stability and Electronic Band Alignment

Alloying offers a way to tune many of the properties of the transition metal dichalcogenide (TMD) monolayers. While these systems in many cases have been thoroughly investigated previously, the fundamental understanding of critical temperatures, phase diagrams, and band edge alignment is still incomplete. Based on first-principles calculations and alloy cluster expansions, we compute the phase diagrams of 72 TMD monolayer alloys and classify the mixing behavior. We show that ordered phases in general are absent at room temperature but that there exist some alloys that have a stable Janus phase at room temperature. Furthermore, for a subset of these alloys, we quantify the band edge bowing and show that the band edge positions for the mixing alloys can be continuously tuned in the range set by the boundary phases

A climatology of the Middle Atlantic Bight shelfbreak front

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 1996.Includes bibliographical references (p. 93-95).by Christopher Andrew Linder.M.S

Exciton landscape in van der Waals heterostructures

van der Waals heterostructures consisting of vertically stacked transition-metal dichalcogenides (TMDs) exhibit a rich landscape of bright and dark intra- and interlayer excitons. In spite of a growing literature in this field of research, the type of excitons dominating optical spectra in different van der Waals heterostructures has not yet been well established. The spectral position of exciton states depends strongly on the strength of hybridization and energy renormalization due to the periodic moire potential. Combining exciton density-matrix formalism and density-functional theory, we shed light on the exciton landscape in TMD homo- and heterobilayers at different stackings. This allows us to identify on a microscopic footing the energetically lowest-lying exciton state for each material and stacking. Furthermore, we disentangle the contribution of hybridization and layer polarization-induced alignment shifts of dark and bright excitons in photoluminescence spectra. By revealing the exciton landscape in van der Waals heterostructures, our work provides the basis for further studies of the optical, dynamical, and transport properties of this technologically promising class of nanomaterials