Yield of metastable atoms from a rare gas discharge in a longitudinal magnetic field

Atomic beam experiments are limited by intensity. Intensity limitations are specially critical in the measurements of metastable atoms, since their relative population is several order of magnitude smaller than the beam population. This thesis provides a method for increasing the intensity of metastable argon and neon beams effusing from a hot cathode, glow discharge by use of a longitudinal magnetic field. The argon and neon metastable atom intensities have been measured for a range of discharge pressure, voltage, and current for a magnetic field strengths from 0 to 31 mT. For both argon and neon, the metastable atom beam intensity rises to a maximum value about one order of magnitude above the zero field case. A qualitative discussion of the theory of this phenomenon is also presented.

Generalized atoms in molecules approach (GAIM approach)

This thesis involves two parts. The first is a new-proposed theoretical approach called generalized atoms in molecules (GAIM). The second is a computational study on the deamination reaction of adenine with OH⁻/nH₂O (n=0, 1, 2, 3) and 3H₂O. The GAIM approach aims to solve the energy of each atom variationally in the first step and then to build the energy of a molecule from each atom. Thus the energy of a diatomic molecule (A-B) is formulated as a sum of its atomic energies, EA and EB. Each of these atomic energies is expressed as, EA = Hᴬ + Vₑₑᴬᴬ + 1/2Vₑₑᴬ<>ᴮ EB = Hᴮ + Vₑₑᴮᴮ + 1/2Vₑₑᴬ<>ᴮ where; Hᴬ and Hᴮ are the kinetic and nuclear attraction energy of electrons of atoms A and B, respectively; Vₑₑᴬᴬ and Vₑₑᴮᴮ are the interaction energy between the electrons on atoms A and B, respectively; and Vₑₑᴬ<>ᴮ is the interaction energy between the electrons of atom A with the electrons of atom B. The energy of the molecule is then minimized subject to the following constraint, |ρA(r)dr + |ρB(r)dr = N where ρA(r) and ρB(r) are the electron densities of atoms A and B, respectively, and N is the number of electrons. The initial testing of the performance of GAIM was done through calculating dissociation curves for H₂, LiH, Li₂, BH, HF, HCl, N₂, F₂, and Cl₂. The numerical results show that GAIM performs very well with H₂, LiH, Li₂, BH, HF, and HCl. GAIM shows convergence problems with N₂, F₂, and Cl₂ due to difficulties in reordering the degenerate atomic orbitals Pₓ, Py, and Pz in N, F, and Cl atoms. Further work for the development of GAIM is required. Deamination of adenine results in one of several forms of premutagenic lesions occurring in DNA. In this thesis, mechanisms for the deamination reaction of adenine with OH⁻/nH₂O, (n = 0, 1, 2, 3) and 3H₂O were investigated. HF/6-31G(d), B3LYP/6-31G(d), MP2/6-31G(d), and B3LYP/6-31+G(d) levels of theory were employed to optimize all the geometries. Energies were calculated at the G3MP2B3 and CBS-QB3 levels of theory. The effect of solvent (water) was computed using the polarizable continuum model (PCM). Intrinsic reaction coordinate (IRC) calculations were performed for all transition states. Five pathways were investigated for the deamination reaction of adenine with OH⁻/nH₂O and 3H₂O. The first four pathways (A-D) begin with by deprotonation at the amino group of adenine by OH⁻, while pathway E is initiated by tautomerization of adenine. For all pathways, the next two steps involve the formation of a tetrahedral intermediate followed by dissociation to yield products via a 1,3-hydrogen shift. Deamination with a single OH⁻ has a high activation barrier (190 kJ mol⁻¹ using G3MP2B3 level) for the rate-determining step. Addition of one water molecule reduces this barrier by 68 kJ mol⁻¹ calculated at G3MP2B3 level. Adding more water molecules decreases the overall activation energy of the reaction, but the effect becomes smaller with each additional water molecule. The most plausible mechanism is pathway E, the deamination reaction of adenine with 3H₂O, which has an overall G3MP2B3 activation energy of 139 and 137 kJ mol⁻¹ in the gas phase and PCM, respectively. This barrier is lower than that for the deamination with OH⁻/3H₂O by 6 and 2 kJ mol⁻¹ in the gas phase and PCM, respectively.

Optical emission of strained direct-band-gap Ge quantum well embedded inside InGaAs alloy layers

We studied the optical properties of a strain-induced direct-band-gap Ge quantum well embedded in InGaAs. We showed that the band offsets depend on the electronegativity of the layer in contact with Ge, leading to different types of optical transitions in the heterostructure. When group-V atoms compose the interfaces, only electrons are confined in Ge, whereas both carriers are confined when the interface consists of group-III atoms. The different carrier confinement results in different emission dynamics behavior. This study provides a solution to obtain efficient light emission from Ge.

Organic functionalisation, doping and characterisation of semiconductor surfaces for future CMOS device applications

Organic Functionalisation, Doping and Characterisation of Semiconductor Surfaces for Future CMOS Device Applications Semiconductor materials have long been the driving force for the advancement of technology since their inception in the mid-20th century. Traditionally, micro-electronic devices based upon these materials have scaled down in size and doubled in transistor density in accordance with the well-known Moore’s law, enabling consumer products with outstanding computational power at lower costs and with smaller footprints. According to the International Technology Roadmap for Semiconductors (ITRS), the scaling of metal-oxide-semiconductor field-effect transistors (MOSFETs) is proceeding at a rapid pace and will reach sub-10 nm dimensions in the coming years. This scaling presents many challenges, not only in terms of metrology but also in terms of the material preparation especially with respect to doping, leading to the moniker “More-than-Moore”. Current transistor technologies are based on the use of semiconductor junctions formed by the introduction of dopant atoms into the material using various methodologies and at device sizes below 10 nm, high concentration gradients become a necessity. Doping, the controlled and purposeful addition of impurities to a semiconductor, is one of the most important steps in the material preparation with uniform and confined doping to form ultra-shallow junctions at source and drain extension regions being one of the key enablers for the continued scaling of devices. Monolayer doping has shown promise to satisfy the need to conformally dope at such small feature sizes. Monolayer doping (MLD) has been shown to satisfy the requirements for extended defect-free, conformal and controllable doping on many materials ranging from the traditional silicon and germanium devices to emerging replacement materials such as III-V compounds This thesis aims to investigate the potential of monolayer doping to complement or replace conventional doping technologies currently in use in CMOS fabrication facilities across the world.

Temperature measurement of cold atoms using transient absorption of a resonant probe through an optical nanofibre

Optical nanofibres are ultrathin optical fibres with a waist diameter typically less than the wavelength of light being guided through them. Cold atoms can couple to the evanescent field of the nanofibre-guided modes and such systems are emerging as promising technologies for the development of atom-photon hybrid quantum devices. Atoms within the evanescent field region of an optical nanofibre can be probed by sending near or on-resonant light through the fibre; however, the probe light can detrimentally affect the properties of the atoms. In this paper, we report on the modification of the local temperature of laser-cooled 87Rb atoms in a magneto-optical trap centred around an optical nanofibre when near-resonant probe light propagates through it. A transient absorption technique has been used to measure the temperature of the affected atoms and temperature variations from 160 μk to 850 μk, for a probe power ranging from 0 to 50 nW, have been observed. This effect could have implications in relation to using optical nanofibres for probing and manipulating cold or ultracold atoms.

Nanotechnology in the food industry I: applications

Nanotechnology is a multidisciplinary science that is having a boom today, providing new products with attractive physicochemical properties for many applications. In agri/feed/food sector, nanotechnology offers great opportunities for obtaining products and innovative applications for agriculture and livestock, water treatment and the production, processing, storage and packaging of food. To this end, a wide variety of nanomaterials, ranging from metals and inorganic metal oxides to organic nanomaterials carrying bioactive ingredients are applied. This review shows an overview of current and future applications of nanotechnology in the food industry. Food additives and materials in contact with food are now the main applications, while it is expected that in the future are in the field of nano-encapsulated and nanocomposites in applications as novel foods, additives, biocides, pesticides and materials food contact.

van der Waals coefficients for positronium interactions with atoms

The random-phase approximation with exchange (RPAE) is used with a B-spline basis to compute dynamic dipole polarizabilities of noble-gas atoms and several other closed-shell atoms (Be, Mg, Ca, Zn, Sr, Cd, and Ba). From these, values of the van der Waals C₆ constants for positronium interactions with these atoms are determined and compared with existing data. After correcting the RPAE polarizabilities to fit the most accurate static polarizability data, our best predictions of C₆ for Ps–noble-gas pairs are expected to be accurate to within 1%, and to within a few percent for the alkaline-earth metals. We also used accurate dynamic dipole polarizabilities from the literature to compute the C₆ coefficients for the alkali-metal atoms. Implications of increased C₆ values for Ps scattering from more polarizable atoms are discussed.

Positronium collisions with rare-gas atoms

We calculate elastic scattering of positronium (Ps) by the Xe atom using the recently developed pseudopotential method (Fabrikant and Gribakin 2014 Phys. Rev. A 90 052717) and review general features of Ps scattering from heavier rare-gas atoms: Ar, Kr and Xe. The total scattering cross section is dominated by two contributions: elastic scattering and Ps ionization (break-up). To calculate the Ps ionization cross sections we use the binary-encounter method for Ps collisions with an atomic target. Our results for the ionization cross section agree well with previous calculations carried out in the impulse approximation. Our total Ps–Xe cross section, when plotted as a function of the projectile velocity, exhibits similarity with the electron-Xe cross section for the collision velocities higher than 0.8 a.u., and agrees very well with the measurements at Ps velocities above 0.5 a.u.

Formation of positron-atom bound states in collisions between Rydberg Ps and neutral atoms

Predicted 20 years ago, positron binding to neutral atoms has not yet been observed experimentally. A scheme is proposed to detect positron-atom bound states by colliding Rydberg positronium (Ps) with neutral atoms. Estimates of the charge-transfer reaction cross section are obtained using the first Born approximation for a selection of neutral atom targets and a wide range of incident Ps energies and principal quantum numbers. We also estimate the corresponding Ps ionization cross section. The accuracy of the calculations is tested by comparison with earlier predictions for charge transfer in Ps collisions with hydrogen and antihydrogen. We describe an existing Rydberg Ps beam suitable for producing positron-atom bound states and estimate signal rates based on the calculated cross sections and realistic experimental parameters. We conclude that the proposed methodology is capable of producing such states and of testing theoretical predictions of their binding energies.

Ionization of Atoms by Slow Heavy Particles, Including Dark Matter

Atoms and molecules can become ionized during the scattering of a slow, heavy particle off a bound electron. Such an interaction involving leptophilic weakly interacting massive particles (WIMPs) is a promising possible explanation for the anomalous 9σ annual modulation in the DAMA dark matter direct detection experiment [R. Bernabei et al., Eur. Phys. J. C 73, 2648 (2013)]. We demonstrate the applicability of the Born approximation for such an interaction by showing its equivalence to the semiclassical adiabatic treatment of atomic ionization by slow-moving WIMPs. Conventional wisdom has it that the ionization probability for such a process should be exponentially small. We show, however, that due to nonanalytic, cusplike behavior of Coulomb functions close to the nucleus this suppression is removed, leading to an effective atomic structure enhancement. We also show that electron relativistic effects actually give the dominant contribution to such a process, enhancing the differential cross section by up to 1000 times.

Three-dimensional simulations of gravitationally confined detonations compared to observations of SN 1991T

The gravitationally confined detonation (GCD) model has been proposed as a possible explosion mechanism for Type Ia supernovae in the single-degenerate evolution channel. It starts with ignition of a deflagration in a single off-centre bubble in a near-Chandrasekhar-mass white dwarf. Driven by buoyancy, the deflagration flame rises in a narrow cone towards the surface. For the most part, the main component of the flow of the expanding ashes remains radial, but upon reaching the outer, low-pressure layers of the white dwarf, an additional lateral component develops. This causes the deflagration ashes to converge again at the opposite side, where the compression heats fuel and a detonation may be launched. We first performed five three-dimensional hydrodynamic simulations of the deflagration phase in 1.4 M⊙ carbon/oxygen white dwarfs at intermediate-resolution (2563computational zones). We confirm that the closer the initial deflagration is ignited to the centre, the slower the buoyant rise and the longer the deflagration ashes takes to break out and close in on the opposite pole to collide. To test the GCD explosion model, we then performed a high-resolution (5123 computational zones) simulation for a model with an ignition spot offset near the upper limit of what is still justifiable, 200 km. This high-resolution simulation met our deliberately optimistic detonation criteria, and we initiated a detonation. The detonation burned through the white dwarf and led to its complete disruption. For this model, we determined detailed nucleosynthetic yields by post-processing 106 tracer particles with a 384 nuclide reaction network, and we present multi-band light curves and time-dependent optical spectra. We find that our synthetic observables show a prominent viewing-angle sensitivity in ultraviolet and blue wavelength bands, which contradicts observed SNe Ia. The strong dependence on the viewing angle is caused by the asymmetric distribution of the deflagration ashes in the outer ejecta layers. Finally, we compared our model to SN 1991T. The overall flux level of the model is slightly too low, and the model predicts pre-maximum light spectral features due to Ca, S, and Si that are too strong. Furthermore, the model chemical abundance stratification qualitatively disagrees with recent abundance tomography results in two key areas: our model lacks low-velocity stable Fe and instead has copious amounts of high-velocity 56Ni and stable Fe. We therefore do not find good agreement of the model with SN 1991T.

Efficient Blue Electroluminescence Using Quantum-Confined Two-Dimensional Perovskites

Solution-processed hybrid organic–inorganic lead halide perovskites are emerging as one of the most promising candidates for low-cost light-emitting diodes (LEDs). However, due to a small exciton binding energy, it is not yet possible to achieve an efficient electroluminescence within the blue wavelength region at room temperature, as is necessary for full-spectrum light sources. Here, we demonstrate efficient blue LEDs based on the colloidal, quantum-confined 2D perovskites, with precisely controlled stacking down to one-unit-cell thickness (n = 1). A variety of low-k organic host compounds are used to disperse the 2D perovskites, effectively creating a matrix of the dielectric quantum wells, which significantly boosts the exciton binding energy by the dielectric confinement effect. Through the Förster resonance energy transfer, the excitons down-convert and recombine radiatively in the 2D perovskites. We report room-temperature pure green (n = 7–10), sky blue (n = 5), pure blue (n = 3), and deep blue (n = 1) electroluminescence, with record-high external quantum efficiencies in the green-to-blue wavelength region.

Comprendre et maîtriser le passage de type I à type II de puits quantiques d'In(x)Ga(1-x)As(y)Sb(1-y) sur substrat de GaSb

Les antimoniures sont des semi-conducteurs III-V prometteurs pour le développement de dispositifs optoélectroniques puisqu'ils ont une grande mobilité d'électrons, une large gamme spectrale d'émission ou de détection et offrent la possibilité de former des hétérostructures confinées dont la recombinaison est de type I, II ou III. Bien qu'il existe plusieurs publications sur la fabrication de dispositifs utilisant un alliage d'In(x)Ga(1-x)As(y)Sb(1-y) qui émet ou détecte à une certaine longueur d'onde, les détails, à savoir comment sont déterminés les compositions et surtout les alignements de bande, sont rarement explicites. Très peu d'études fondamentales sur l'incorporation d'indium et d'arsenic sous forme de tétramères lors de l'épitaxie par jets moléculaires existent, et les méthodes afin de déterminer l'alignement des bandes des binaires qui composent ces alliages donnent des résultats variables. Un modèle a été construit et a permis de prédire l'alignement des bandes énergétiques des alliages d'In(x)Ga(1-x)As(y)Sb(1-y) avec celles du GaSb pour l'ensemble des compositions possibles. Ce modèle tient compte des effets thermiques, des contraintes élastiques et peut aussi inclure le confinement pour des puits quantiques. De cette manière, il est possible de prédire la transition de type de recombinaison en fonction de la composition. Il est aussi montré que l'indium ségrègue en surface lors de la croissance par épitaxie par jets moléculaires d'In(x)Ga(1-x)Sb sur GaSb, ce qui avait déjà été observé pour ce type de matériau. Il est possible d'éliminer le gradient de composition à cette interface en mouillant la surface d'indium avant la croissance de l'alliage. L'épaisseur d'indium en surface dépend de la température et peut être évaluée par un modèle simple simulant la ségrégation. Dans le cas d'un puits quantique, il y aura une seconde interface GaSb sur In(x)Ga(1-x)Sb où l'indium de surface ira s'incorporer. La croissance de quelques monocouches de GaSb à basse température immédiatement après la croissance de l'alliage permet d'incorporer rapidement ces atomes d'indium et de garder la seconde interface abrupte. Lorsque la composition d'indium ne change plus dans la couche, cette composition correspond au rapport de flux d'atomes d'indium sur celui des éléments III. L'arsenic, dont la source fournit principalement des tétramères, ne s'incorpore pas de la même manière. Les tétramères occupent deux sites en surface et doivent interagir par paire afin de créer des dimères d'arsenic. Ces derniers pourront alors être incorporés dans l'alliage. Un modèle de cinétique de surface a été élaboré afin de rendre compte de la diminution d'incorporation d'arsenic en augmentant le rapport V/III pour une composition nominale d'arsenic fixe dans l'In(x)Ga(1-x)As(y)Sb(1-y). Ce résultat s'explique par le fait que les réactions de deuxième ordre dans la décomposition des tétramères d'arsenic ralentissent considérablement la réaction d'incorporation et permettent à l'antimoine d'occuper majoritairement la surface. Cette observation montre qu'il est préférable d'utiliser une source de dimères d'arsenic, plutôt que de tétramères, afin de mieux contrôler la composition d'arsenic dans la couche. Des puits quantiques d'In(x)Ga(1-x)As(y)Sb(1-y) sur GaSb ont été fabriqués et caractérisés optiquement afin d'observer le passage de recombinaison de type I à type II. Cependant, celui-ci n'a pas pu être observé puisque les spectres étaient dominés par un niveau énergétique dans le GaSb dont la source n'a pu être identifiée. Un problème dans la source de gallium pourrait être à l'origine de ce défaut et la résolution de ce problème est essentielle à la continuité de ces travaux.

The Inverse Jacobian Control Method Applied to a Four Degree of Freedom Confined Space Robot

Thesis (Master's)--University of Washington, 2016-06

Position-dependent spin-orbit coupling for ultracold atoms

We theoretically explore atomic Bose-Einstein condensates (BECs) subject to position-dependent spin-orbit coupling (SOC). This SOC can be produced by cyclically laser coupling four internal atomic ground (or metastable) states in an environment where the detuning from resonance depends on position. The resulting spin-orbit coupled BEC (SOBEC) phase separates into domains, each of which contain density modulations-stripes-aligned either along the x or y direction. In each domain, the stripe orientation is determined by the sign of the local detuning. When these stripes have mismatched spatial periods along domain boundaries, non-trivial topological spin textures form at the interface, including skyrmions-like spin vortices and anti-vortices. In contrast to vortices present in conventional rotating BECs, these spin-vortices are stable topological defects that are not present in the corresponding homogenous stripe-phase SOBECs.