A quartz crystal resonator has been used to excite shear waves at a frequency of 20.5 MHz in He films above and below the superfluid transition and just above the liquid-gas critical point. The wave has a viscous penetration depth 6-20 nm and the transverse acoustic impedance Z = R - iX of the film was found from changes in the quality factor and resonant frequency of the crystal. The thickness of a He I film was swept at constant temperature by creating a small temperature difference between the He film on the crystal and bulk liquid helium below it. Calculations of the impedance of a homogeneous film as a function of d/6 using transmission line theory show the film's thickness d could be swept from 1.5 to >60 nm . The impedance of six superfluid films of constant thickness in the range 14-23 nm has been measured for 0.4 T T . From the impedance in the ballistic limit, wI >> 1, the average probability of the quantum evaporation of a ⁴He atom by a roton incident upon the liquid-vapourinterface is estimated to be ~0.35 . A resonance, the temperature of which was dependent on film thickness, was observed in the superfluid film and has tentatively been identified with the resonance in the A/4 Kelvin mode of vortices pinned to the crystal surface. The transverse acoustic impedance Z of helium has also been measured 49 mK above the liquid-gas critical point for pressures up to 2000 torr. In the highly compressible critical region, the impedance shows the effects of the large density gradients that develop close to the crystal surface under van der Waals1 forces. At low pressures, the transition to non-hydrodynamic behaviour is observed, and it is estimated that a fraction 0.2 of He atoms incident upon the crystal are diffusely scattered from it.

This work investigates the use of the perovskite-type compound La1-xSrxCoO3 (x = 0.3, 0.5, 0.6, 0.7) as a catalyst for the reaction SO2 + 2CO → 1/x Sx + 2CO2. Adsorption of SO2 and CO separately on the degassed catalysts at several temperatures, was followed by TPD (temperature programmed desorption), with mass spectroscopic detection of the desorbed species. Trends in the mass changes on adsorption are explained in terms of the changes in reducibility of La1-xSrxCoO3 on changing x. Computation of the SO2 TPD results, which showed two and sometimes three adsorbed species, yielded desorption energies of the range 30-200 kJ mol⁻¹. Several SO2 and CO mixtures were allowed to flow over La1-xSrxCo03 at various temperatures and flow rates. The removal of SO2 and production of CO2 were followed using gas chromatography and sulphur was collected in a cold trap. LaO.7Srg 3COO3 at 550°C removed SO2 from a stoichiometric mixture of SO2 and CO very efficiently without producing COS. COS production, if it occurs, may be problematical in a commercial SO2 removal application. XRD (X-ray diffraction) revealed that the fresh compounds all had the perovskite-type structure, those exposed to either SO2 or CO retained this structure and those exposed to SO2 and CO together were no longer La, Srx CoO3 XRD of the used catalysts showed theI -x X 3)presence of SrSO4 on those exposed to only SO2 and the presence of sulphides of La, Sr and Co, on those exposed to SO2 and CO. XPS (X-ray photoelectron spectroscopy) of catalysts exposed to SO2 and CO indicated sulphidation, with some sulphate species also present. The results may be explained in terms of the relative oxidizibilitiesof the SO2, CO, SO2 reduced catalyst, 'CO reduced catalyst', 'reactionreduced catalyst' and La1-x Sr Co031x x 3The continuation of the operation of the material for the removal of SO2 suggests that in this instance the active catalyst is no longer a perovskite-type oxide, but possibly a metal sulphide.

A number of monomeric and oligomeric hydroxypyridone mono- and poly-methine bridged oxonol and merocyanine dyes have been prepared and their solution and solid-state properties have been investigated. One particular and commercially important feature of the solution chemistry of the oxonol dyes is the reaction with sulphite ion leading to bleaching of the dye. The mechanism of this process has not previously been investigated. A detailed kinetic and spectroscopic study supports the previously proposed mechanism in which the nucleophile adds to the methane bridge in a Michael-type fashion. The reaction intermediates have been detected spectroscopically for several nucleophiles but in general are too unstable to be isolated. Oxonol and merocyanine dyes readily undergo molecular aggregation in both solution and the solid-state. One previous X-ray diffraction analysis of an oxonol dye has been carried out which revealed that the anions formed planar molecular stacks. Such an arrangement is well suited to intermolecular charge transfer and hence electronic conduction. The main objective of the current work has been to investigate how this stacking might be influenced by variation of substituents and counter-ions with a view to modifying the conduction properties. X-ray structure studies reveal that variation of the cation leads to three different types of solid-state structure: herringbone; uniform; and dimeric. The electrical properties have been investigated in detail by d.c. conductivity and a.c. dielectric spectroscopy measurements. They suggest that in general, the simple hydroxypyridone trimethine oxonols are semi-insulating (10⁻⁸ < δ < 10⁻¹² Ω⁻¹ cm⁻¹) though some exceptions have been found. Any observable d.c. conduction is essentially ionic in nature. The merocyanines and pentamethine oxonols are semi-conducting (10⁻⁴ < δ < 10⁻⁸ Ω⁻¹ cm⁻¹) as a consequence of added electronic contributions to the conduction mechanism. The most highly conducting oxonol salt, containing the tetrathiafulvalene (TTF) radical cation has a d.c. conductivity, δ ≃ 10⁻³ Ω⁻¹ cm⁻¹ (compacted powder, room temperature). In this case electronic conduction dominates.

A microprocessor control system for automatic data acquisition in a photon spectroscopy system was developed. The control process performed Doppler broadening measurements over the temperature range 4.2-600K with simultaneous recording when necessary of pressure. After work on the chalcogenide semiconductor (Pb,Ge)Te, Doppler broadening studies concentrated on the phenomenon of gas condensation on cooled graphite substrates. This class of adsorbent, of proven homogeneity was found ideal to explore the interaction of positrons at surfaces. By measurement of positronium, adlayer formation of methane, krypton, argon, nitrogen and oxygen was followed. All gases produced peaks in positronium emission with an investigation of the substrate further revealing an underlying thermal variation. Sound evidence in the form of accurate simultaneous pressure measurements and isotherms was found for a coverage of one half at the maximum in positronium. These studies of gas physisorption were conducted under a variety of temperatures and with pressures embracing atmospheric to under one Torr. Although results for the gases were of the same form each produced different maximum levels of positronium which appeared to relate to substrate-adsorbate binding energy. Maximum emission in the case of argon showed a temperature dependence however this requires verification. Oxygen was a special case as it yielded 12[percent] para-positronium at half-coverage. The coupling of this to a low 1.5[percent] ortho-positronium emission was attributed to chemical quenching. A convolution procedure based on the superposition of an additional para-positronium derived narrow gaussian to the usual gaussian and inverted parabola components was employed for an estimate of the positronium energy. This was used to attack the question of whether positronium is bound at the surface or is created flying.

Existing techniques have been reviewed for the production of thin electron-transparent films of alkali halides for study by electron microscopy and a new technique has been developed. A novel apparatus has been designed and built by means of which samples prepared from the vapour phase in high vacuum may be transported to, and examined in an electron microscope without exposure to the atmosphere. Special specimen holders and a couple of heaters have been designed so that the effects of varying temperatures on the growth and subsequent behaviour of alkali halide crystals may be studied. Other factors examined are the pressure during growth, the effects of atmospheric moisture after growth and the electron irradiation in the electron microscope. The surface of the alkali halide crystals have been examined by decoration with evaporated gold and subsequent examination in the electron microscope and their crystallographic structures investigated by electron diffraction. Thicknesses of the crystals have been measured by extinction contour technique. The principal materials studied are CsBr, CsCl, CsF and Csl, as over growth on electron-transparent crystals of Csl and on two different types of mica: muscovite and biotite. These are the first studies of alkali halides on Csl and on biotite. The main results of investigations are: 1. CsI may be prepared as a flat, electron-transparent single crystal suitable for electron microscopic studies in situ. 2. Small crystallites are produced by vapour deposition of alkali halides in the early stages and after exposure to the atmosphere large single crystals are formed. 3. CsBr, CsCl, CsF and CsI grow epitaxially on Csl at room temperature. 4. CsF may grow in bcc form in thin layers in certain circumstances, instead of the usual feeform in bulk. 5. No significant difference was observed in the growth of alkali halides on muscovite and biotite.

Self-propelled particles are a class of far-from-equilibrium systems which present many complex, emergent features that are not obvious from the microscopic dynamics. Simulations of well-chosen instances of such systems are a powerful yet tractable method of investigating many real-world phenomena. The frequently non-time-reversible interactions of many cases of self-propelled particles with surfaces means that the environment has an impact on large-scale behaviour in a way that would not be true for particles close to thermal equilibrium. This work investigates several examples of such systems, and compares them with experimental results for comparable systems: firstly, the spatial distribution of smooth-swimming mutants of Eschericia Coli within water-in-oil emulsion is investigated, and its dependence on inter-bacterial interactions and the size of water droplets. The nature of bacterial collisions is inferred through data analysis and simulation. Secondly, pattern formation by chemotactic run-and-tumble bacteria due to secretion of a chemoattractant by the bacteria themselves, demonstrating a range of approaches to control the formation of biofilms by bacteria. Finally the dependence of the bulk transport properties of chemotactic self-propelled particles in porous environments, on their detailed dynamics, is probed: how they interact with obstacles, their form of chemotactic response, their ability to actively enhance their rotational noise, and their method of sensing chemical gradients.

<p><!-- Fragment document type declaration subset: ArborText, Inc., 1988-1999, v.4002 --> <formattedtext>The Role of TrkB and BDNF in Governing the Steroid-Mediated Regulation of Male Copulatory Behavior in the Syrian Hamster (<i>Mesocricetus auratus</i>)</formattedtext>

<p>Spin coherence is important for the fields of electron spin resonance (ESR), nuclear magnetic resonance (NMR), magnetic resonance imaging (MRI), and quantum devices. For donor spins in silicon, coherence both quantifies their potential as qubits and measures environmental processes. By understanding those processes, we can construct experiments which remove them to obtain longer coherence times. Silicon crystals are uniquely suited to this task, benefiting from decades of advancements in purification. The two most well-known decoherence mechanisms for donors in silicon are 29Si atoms and the donor spins themselves. Although well studied for electron spins, these mechanisms are less understood for nuclear spins. Using crystals with controlled concentrations of 29Si and 31P donors, I evaluate the limitations imposed on 31P nuclear spins. I find that nuclear spin echo decay times vary linearly with 29Si concentration. The non-exponential decays shown here establish a range of 29Si flip-flop rates, with some being fast compared to experiment timescales and others being slow. Furthermore, when compared to measurements of ionized nuclear spins, the echo decays here imply a "frozen core" picture where the donor electron spin protects the nuclear spin by detuning neighboring 29Si atoms. In studying spin coherence relative to 31P concentration, I find that nuclear spin echo experiments can measure donor electron spin flip-flop rates. A stochastic model reproduces the experiments by fitting a local Zeeman frequency linewidth. However, experiments in more lightly doped crystals (<10</p><p>15 P/cm</p><p>3) suggest that coherence is not limited by flip-flops. The source of decoherence in these crystals is unknown, but the experiments serve as an upper bound on electric field noise. Magnetic field fluctuations are known to obstruct spin coherence measurements. One popular method for removing that noise is dynamical decoupling via repeated pi rotations. However, these sequences also elongate echo decays for ensemble spins known to be decohered instead by instantaneous diffusion. This result suggested that cumulative rotation errors might artificially inflate decay times. After demonstrating that such effects were insignificant in practice, I find instead that pi rotation errors reduce the dipole-dipole coupling between spins.

abstract: Understanding the interplay between the electrical and mechanical properties of single molecules is of fundamental importance for molecular electronics. The sensitivity of charge transport to mechanical fluctuations is a key problem in developing long lasting molecular devices. Furthermore, harnessing this response to mechanical perturbation, molecular devices which can be mechanically gated can be developed. This thesis demonstrates three examples of the unique electromechanical properties of single molecules. First, the electromechanical properties of 1,4-benzenedithiol molecular junctions are investigate. Counterintuitively, the conductance of this molecule is found to increase by more than an order of magnitude when stretched. This conductance increase is found to be reversible when the molecular junction is compressed. The current-voltage, conductance-voltage and inelastic electron tunneling spectroscopy characteristics are used to attribute the conductance increase to a strain-induced shift in the frontier molecular orbital relative to the electrode Fermi level, leading to resonant enhancement in the conductance. Next, the effect of stretching-induced structural changes on charge transport in DNA molecules is studied. The conductance of single DNA molecules with lengths varying from 6 to 26 base pairs is measured and found to follow a hopping transport mechanism. The conductance of DNA molecules is highly sensitive to mechanical stretching, showing an abrupt decrease in conductance at surprisingly short stretching distances, with weak dependence on DNA length. This abrupt conductance decrease is attributed to force-induced breaking of hydrogen bonds in the base pairs at the end of the DNA sequence. Finally, the effect of small mechanical modulation of the base separation on DNA conductance is investigated. The sensitivity of conductance to mechanical modulation is studied for molecules of different sequence and length. Sequences with purine-purine stacking are found to be more responsive to modulation than purine-pyrimidine sequences. This sensitivity is attributed to the perturbation of &pi-&pi stacking interactions and resulting effects on the activation energy and electronic coupling for the end base pairs. / Dissertation/Thesis / Doctoral Dissertation Physics 2014

abstract: The identity and origin of dark matter is one of the more elusive mysteries in the fields of particle physics and cosmology. In the near future, direct dark matter detectors will offer a chance at observing dark matter non-gravitationally for the first time. In this thesis, formalisms are developed to analyze direct detection experiments and to quantify the extent to which properties of the dark matter can be determined. A range of non-standard assumptions about the dark matter are considered, including inelastic scattering, isospin violation and momentum dependent scattering. Bayesian inference is applied to realistic detector configurations to evaluate parameter estimation and model selection ability. A complete set of simplified models for spin-0, spin-1/2 and spin-1 dark matter candidates are formulated. The corresponding non-relativistic operators are found, and are used to derive observational signals for the simplified models. The ability to discern these simplified models with direct detection experiments is demonstrated. In the near future direct dark matter detectors will be sensitive to coherent neutrino scattering, which will limit the discovery potential of these experiments. It was found that eleven of the fourteen non-relativistic operators considered produce signals distinct from coherent scattering, and thus the neutrino background does not greatly affect the discovery potential in these cases. / Dissertation/Thesis / Doctoral Dissertation Physics 2016