Thesis (Ph. D.)--University of Missouri-Columbia, 2002. / Typescript. Vita. Includes bibliographical references (leaf 158). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2002. / Typescript. Vita. Includes bibliographical references (leaf 158). Also available on the Internet.

A comprehensive study of transport properties of lightly electron-doped manganese oxides with perovskite structure is presented. Two similar classes are investigated in both their both paramagnetic and antiferromagnetically ordered states: Ca1-xLaxMnO3 and Ca1-ySryMnO3. Much simpler than their widely-studied hole-doped counterparts, these compounds are model systems for investigating the issue of magnetic polaron formation in perovskite manganites. Our measurements sustain the phase segregation scenario both above and below the magnetic ordering temperature in Ca1-xLaxMnO3, but it is found that for T>Tn, the small-polaron theory (successful in describing the colossal magnetoresistance compounds) is incompatible with our results and a large polaron theory should be used instead. Particularly interesting are the nominally undoped, semiconducting Ca1-ySryMnO3 compounds with a very small electron concentration associated with native oxygen vacancies. At low temperatures, electron bound near vacancies are mobilized in weak applied electric field (F<100 V/cm). This internal current source allows for a distinction between self-trapped and bound magnetic polarons and provides a new tool for studying strongly correlated electron systems with a tunable mobile carrier density.

Thesis (Ph. D.)--Lehigh University, 2001. / Includes vita. Includes bibliographical references (leaves 121-126).

No description available.

Le niobate de lithium (LN), en raison de ses propriétés, électro-optiques, optiques non linéaires et photoréfractives (PR) , s'impose comme un matériau de choix pour des applications en modulation, filtrage, stockage holographique ou conversion de fréquence optique. L'effet PR est gouverné à la fois par les centres profonds extrinsèques (Fe2+ et Fe3+ le plus souvent) et par les antisites niobium en site (Nbli5+), qui constituent des pièges préférentiels pour les électrons arrachés aux donneurs profonds et forment ainsi des polarons liés Nbli4+, caractérisables par une large bande d'absorption photo-induite (API) s"étendant dans le rouge et le proche infrarouge, ainsi que par une bande de photoluminescence (PL) légèrement décalée en longueur d'onde par rapport à la précédente. Nous présentons ici une étude expérimentale de la PL polaron dans le LN congruent dopé fer, fonction de la température et de l'intensité lumineuse excitatrice, en régime continu et en régime pulsé, ainsi qu'un modèle phénoménologique à trois centres permettent d'interpréter toutes les caractéristiques observées. Nous montrons que la PL permet en principe de doser les donneurs profonds dans le LN congruent en qualité infime, que soit leur nature. Pour les ions Fe2+, la détectivité est typiquement de 0,25 ppm à l'ambiante, ce qui est bien meilleur que la spectroscopie d'absorption. La PL résolue spatialement permet en outre, contrairement aux autres techniques, de cartographier la concentration de donneurs profonds à l'échelle micrométrique, d'où son potentiel pour la caractérisation de guides d'onde, de composants optiques intégrés ou autres microstructures / Because of their electro-optical, non-linear optical and photorefractive effet, lithium niobate crystals (LN) are used in many applications such as modulation, filtering, holographic storage or frequency conversion. Its known that the photorefractive effect is influenced by both extrincic deep centers (Fe2+ and Fe3+ in most cases) and by niobium antisites (Nbli5+) which constitute preferential sites to trap an electron hence giving a small bound polaron (Nbli4+). This defect is characterized by a photo_indiced obsorption (API) broad band in the NIR range, as well as slightly Stokes shifted photoluminescence band (PL). We presnt nerein an experime,tal study of the polaron related PL in iron doped congruent lithium niobate as a function of temperature and incident intensity in CW and pulsed regime. We also propose a phenomenological threecenter model as an interpretation of all observed results. We show that the PL permits one to determine the concentration of deep centres in congruent LN in trace amounts, whatever their nature. For Fe 2+ ions, the sensitivity is typically around 0,25 ppm at RT, which is better than absorption spectroscopy. In addition, unlike other techniques, the PL is spatially resolved (micrometer scale) which allows to maps the concentration of deep donors, hence its potential for characterizating waveguides, integrated optical components and other microstructures

No description available.

Lead halide perovskites are a class of soft ionic semiconductors characterized by strong excitonic absorption and long carrier lifetimes. Recent studies suggest that electrons and holes in these materials interact with longitudinal optical phonons to form large polarons on subpicosecond time-scales. The same interaction is responsible for hot electron cooling via phonon emission and is thought to be screened by large polaron formation resulting in the long-lived hot electrons observed in methylammonium lead iodide perovskite. Time-resolved two-photon photoemission is used to follow the initial hot electron cooling and large polaron formation dynamics in single-crystal cesium lead bromide perovskite at 80 K and 300 K. The initial relaxation rates are found to be weakly temperature-dependent and are attributed to the cooling of unscreened hot electrons by the emission of longitudinal optical phonons. The large polaron formation times, however, are inferred to be approximately three times faster at 300 K. The decrease in polaron formation time with temperature is correlated with the broadening in phonon linewidths, suggesting that disorder can assist large polaron formation. In addition, the initial electron relaxation is faster than large polaron formation explaining the absence of long-lived hot electrons in cesium lead bromide perovskite as opposed to methylammonium lead iodide perovskite where the two processes are competitive. The second part of this thesis focuses on the strong light-matter interaction in nanowire waveguide geometries of single-crystal lead halide perovskites which are well known for their emission tunability and low lasing thresholds under pulsed optical excitation. Using fluorescence microscopy, it is found that the luminescence from single-crystal cesium lead bromide perovskite nanowires is dominated by sub-bandgap modes called exciton-polaritons, i.e. hybridized exciton-photon states. A one-dimensional exciton-polariton model reproduces the observed modes at the bottleneck of the lower polariton branch with a Rabi splitting of about 200 milli-electron volts. As the power density increases under continuous excitation, the exciton-polaritons undergo Bose-stimulated scattering and a super-linear increase in mode intensity is observed. This is the first demonstration of continuous-wave lasing in lead halide perovskite nanowires and reveals an inherently strong light-matter interaction in lead halide perovskites that can be used for continuous-wave optoelectronic applications. These findings corroborate the role of dynamic screening in unifying these two regimes of carrier-carrier interactions responsible for the strong absorption and subsequent carrier protection. We also demonstrate the viability of lead halide perovskite nanowires for future optoelectronics.

Th.--Sci. phys.--Grenoble 1, 1984. N°: 127.

No description available.