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No description available.

This thesis reports on the design and construction of, and results from, an optical-dipole trapping apparatus developed to confine ultracold rubidium atoms in a conservative, large period, optical-dipole trap. An ultra-high vacuum system was designed and constructed to create a very low- background pressure. A new technique of viewport construction was developed, allowing for the fabrication of economical, high-quality windows for transmission of mid-infra-red laser radiation. The construction of a magneto-optical trap (MOT) and an optical molasses, and the subsequent characterisation, are discussed. A theory for the ac-Stark shift of atoms in a far-detuned laser field was developed. The nature of the scalar and tensorial light-shifts of the ground and first excited states of alkali atoms - the 5s(^2)S1/2 and 5p(^2)P3/2 states of Rb - has been examined. The effect of the differential light-shifts between these states on the operation of efficient laser cooling is discussed. A quasi-electrostatic dipole trap (QUEST) was formed from 50 W of CO2 laser power (٢ = 10.6 μm), focussed to <100 μm. The transfer of ultra-cold atoms from the MOT and optical molasses to the QUEST have been examined. Single beam and standing wave geometries of the QUEST have been implemented, with lifetimes of many seconds. The theory for the ac-Stark effect due to a single laser field has been further developed to consider orthogonally polarised fields with independent wavelengths. The use of an auxiliary laser field, a Nd:YAG laser at λ = 1.064 μm, to enhance the number and density of atoms loaded into the QUEST has been proposed and realised.

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Sodium Cobaltate (NaxCoO2) has emerged as a material of exceptional scientific and technological interest since it is among the best P-type thermoelectric materials. The superstructures in pure NaxCoO2 templates the Coulomb landscape on the Co layers and is found to control the physical properties. The combination of the high electrical conductivity in the Co layers with the low thermal conductivity due to the rattling of sodium ions in cages, are the precise conditions for thermoelectric materials with high figures of merit. Replacing Na by divalent ions was reported to lead to a dramatic improvement in thermoelectric performance. The superstructures of NaxCayCoO2 and NaxSryCoO2 have been determined by Laue diffraction using neutrons on SXD at ISIS and x-rays at Royal Holloway. Reverse Monte-Carlo methods were used to determine Na ion patterning including the locations of the divalent ions. Co and O displacements were also determined that show buckling following the Na structure. In the doped systems we find completely new multi-vacancy clusters. Di-vacancies form in Ca doped systems where the divalent ion sits at the central site. In the Sr doped systems two new superlattices are observed, explained by the clustering of separated Sr ions with associated vacancies. Multiple valence states have been detected by NMR, which is a local probe, but spatial charge ordering in the cobalt layer has not previously been observed. We report new measurements using Resonant X-ray Scattering on the Materials and Magnetism beamline I16 at Diamond. We find resonant x-ray scattering with the same periodicity as the sodium superstructure, directly demonstrating that the electronic ordering in these cobalt layers is controlled by the sodium ordering. We are able to reproduce the energy, polarisation and azimuthal dependencies of the resonant x-ray scattering in calculations using the FDMNES code.

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A study of self-diffusion in anthracene single crystals has been carried out using a radioactive tracer technique. A method was developed for the growth of large single crystals of anthracene from the melt, starting from the Bridgman technique. On to one surface of the crystals was evaporated a deposit of anthracene - 9 - carbon-14, prepared by a seven stage synthesis from barium 14-carbonate, and the crystals and their deposits were heated at constant temperatures in the range 150 -190 G for periods up to 230 hours. The crystals were then sectioned parallel to the initial active face, using a calibrated, hand operated, lathe. The quantity of radioactive material which had diffused into each section was determined by quantitative combustion of the sections to carbon dioxide in an "Empty Tube" rapid combustion apparatus, followed by the determination of the disintegration rate of this gas in a gas counting tube, using carbon disulphide as charge transfer agent. Penetration was found up to a depth of 4 x 10(^-2) cms. It was found that two concurrent diffusion processes were occurring in the crystal. One of these involved only a small proportion of the total diffusing activity and may represent diffusion along intersecting dislocations in the crystal. The second process involved most of the diffusing activity and was thought to represent bulk diffusion in the crystal The temperature dependance of the diffusion coefficient for the second process is given lay the equation. D = (1.31 ± 1.13) x l0(^ll)e (^–(42,400 ± 1,200)/RT) cm. (^2) per second. The pre-exponential factor and activation energy for this process are unusually high. This leads to the conclusion that a co-operative diffusion phenomenon is occurring in the crystal, in which diffusion occurs by way of vacant lattice sites, and that the loosening or premelting of from four to six molecules occurs in the neighbourhood of the vacancy during diffusion. This is one of the few studies of self-diffusion in molecular crystals.

This work is an attempt to determine the origin and. behaviour of the defect centres in the forbidden gap of cadmium sulphide. The methods of measurement used were thermally stimulated current and infra-red luminescence techniques. After introductory chapters on semiconductor theory and the material, cadmium sulphide, the published results on T.S.C. and infra-red luminescence measurements are surveyed and analysed. Results are then given for a series of samples with varying degrees of sulphur doping. During the work, the need for a more accurate method of T.S.C. curve analysis arose, and the curve fitting technique was developed for this purpose. It was found to have many advantages over existing techniques. The combination of T.S.C. and infra-red techniques led to the identification of the centres involved in the infrared luminescence. Important traps at O.48, 0.62 and O.84 eV below the conduction band are identified, and their photochemical reactions with the luminescent centres described. It is shown that the luminescent centres are identical with the sensitising centres.

The aim of the investigation was the study of the influence of crystal fine structure on conductivity and cation self diffusion in single crystals of sodium chloride. Crystals were grown by the Kyropoulos technique and subjected to thermal treatment likely to alter the crystal fine structure. Both annealing and chilling of single crystals produced changes in the crystals conductivity and in sodium-22 self-diffusion rate at temperatures below 520ºC. In the investigation of conductivity a study was made of electrode materials and for the diffusion measurements a sensitive absorption method was used. The changes in diffusion rates and conductivity might have been the result of changes in internal surfaces and grain boundaries. The possibility of ion movement in grain boundaries was investigated by autoradiography of sections of crystal into which sodium-22 had penetrated. The rte of movement of sodium-22 on the surface of sodium chloride was also investigate and a limit set for the surface diffusion coefficient. It was concluded that cation movement in sodium chloride single crystals does not take place more rapidly in the grain boundaries or on internal surfaces than it does through the bulk of the crystal. An explanation of the changes in diffusion rates and conductivity on thermal treatment of the crystal is offered in terms of vacancies, rain boundaries and impurity ions.

A greatly improved apparatus for the detection and measurement of micro-quantities of helium and neon was constructed and used to investigate the behaviour of helium in lithium/aluminium alloy, gold, beryllium and meteoritic iron. The work described involved neutron transmutation techniques for the production of helium in lithium/aluminium alloy and beryllium, and took advantage of the fact that helium is produced in meteoritic iron when bombarded with cosmic rays. The work on gold utilised the alpha- decay scheme of radon as a source of helium. The diffusion of helium in lithium/aluminium alloy appears to takeplace by a substitutional mechanism according to the equation. D = 42. exp. (formula) where D is the diffusion coefficient expressed in square centimetres per second. The conditions under which this equation is obeyed, however, are shown to be very restricted and highly dependent on the physical state of the metal and its previous annealing history. Accumulation of the gas into bubbles is believed to be the cause of the marked decrease in evolution of helium observed when annealing times longer than a few hours are used. The values obtained for the diffusivity of helium in gold are believed to be invalid on account of the presence of voids along the line of the cold- weld used to sandwich helium-rich layers of this metal together. The validity of this particular technique for gag/metal systems is reviewed in some detail. No values are presented for the diffusivity of helium in beryllium or meteoritic iron but the conclusion is that bubble-formation is a significant factor in diminishing the evolution of helium from these metals.