Interaction between U/UO2 bilayers and hydrogen studied by in-situ X-ray diffraction

This paper reports experiments investigating the reaction of H$_{2}$ with uranium metal-oxide bilayers. The bilayers consist of $\leq$ 100 nm of epitaxial $\alpha$-U (grown on a Nb buffer deposited on sapphire) with a UO$_{2}$ overlayer of thicknesses of between 20 and 80 nm. The oxides were made either by depositing via reactive magnetron sputtering, or allowing the uranium metal to oxidise in air at room temperature. The bilayers were exposed to hydrogen, with sample temperatures between 80 and 200 C, and monitored via in-situ x-ray diffraction and complimentary experiments conducted using Scanning Transmission Electron Microscopy - Electron Energy Loss Spectroscopy (STEM-EELS). Small partial pressures of H$_{2}$ caused rapid consumption of the U metal and lead to changes in the intensity and position of the diffraction peaks from both the UO$_{2}$ overlayers and the U metal. There is an orientational dependence in the rate of U consumption. From changes in the lattice parameter we deduce that hydrogen enters both the oxide and metal layers, contracting the oxide and expanding the metal. The air-grown oxide overlayers appear to hinder the H$_{2}$-reaction up to a threshold dose, but then on heating from 80 to 140 C the consumption is more rapid than for the as-deposited overlayers. STEM-EELS establishes that the U-hydride layer lies at the oxide-metal interface, and that the initial formation is at defects or grain boundaries, and involves the formation of amorphous and/or nanocrystalline UH$_{3}$. This explains why no diffraction peaks from UH$_{3}$ are observed. {\textcopyright British Crown Owned Copyright 2017/AWE}Comment: Submitted for peer revie

Charge Dynamics of InAs Quantum Dots Under Resonant and Above-Band Excitation

Research involving light-matter interactions in semiconductor nanostructures has been an interesting topic of investigation for decades. Many systems have been studied for not only probing fundamental physics of the solid state, but also for direct development of technological advancements. Research regarding self-assembled, epitaxially grown quantum dots (QDs) has proven to be prominent in both regards. The development of a reliable, robust source for the production of quantum bits to be utilized in quantum information protocols is a leading venture in the world of condensed matter and solid-state physics. Fluorescence from resonantly driven QDs is a promising candidate for the production of single, indistinguishable photons to be utilized in quantum information protocols, and the material/sample currently leading the research in regards to this are indium-arsenide (InAs) QDs. However, a few obstacles exist inhibiting InAs QDs’ ability to be an efficient and reliable source of single, indistinguishable photons. The root sources of these problems are mostly associated with the dynamic electrical environment in the vicinity of the QDs. The electrical environment is complex due to inevitable emergence of defects and impurities in the bulk host material during epitaxial growth. The presence of these defects results in a complicated network through which charges can migrate around, into, and out of the QDs, resulting in time-dependent perturbations to the electric potential by which QDs confine charge carriers. Inevitably, this results in time-dependent fluctuations in the optical frequency of the emitted fluorescence, and ultimately a broadening of the time-averaged absorption and emission spectra, dubbed spectral diffusion. Additionally, blinking can occur, which is fluctuations of the fluorescence intensity on time scales that are large relative to the lifetime of confined excited states. Both contribute to a loss of applicability to use these samples as an efficient source of single, indistinguishable photons. The broadening of the time-averaged emission spectrum via spectral diffusion results in a loss of indistinguishability amongst photons emitted at different times, whereas blinking results in an abatement of a consistent single photon source. Understanding the exact electrical environment in which the QDs reside, as well as the complex environment through which carriers migrate can help future implementation of both growth and excitation techniques to minimize these undesirable effects. In this dissertation we explore the electric environment of our sample, the complex pathways through which carriers migrate, and how the resulting charge dynamics affect the intensity and indistinguishability of the emitted fluorescence from resonantly driven InAs QDs

Transforming gender relations in an ageing world : a policy discussion paper

This policy discussion paper explores the way in which intersecting inequalities affect life courses and gender relations in older age. It argues for a gendered lifecourse perspective within the Sustainable Development Goals framework