Thesis (M.S.)--Montana State University--Bozeman, 2005. / Typescript. Chairperson, Graduate Committee: David Dickensheets. Includes bibliographical references (leaves 65-66).

Inaug.-dis.-Bern. / "Benutzte litteratur": p. 48.

Double perovskites with a single s = ½ magnetic ion and rocksalt order can show geometric frustration, due to arrangement of electron spins such that they cannot satisfy all nearest-neighbour antiferromagnetic interactions simultaneously. This can give rise to exotic magnetic states at low temperature. Compounds with unpaired 4d and 5d electrons have in this respect been studied far less than 3d compounds. Here, results of studies on the compounds Sr2CuWO6, Ba2YWO6 and LaSrMgWO6 are presented. The synthesis of the previously reported compound Ba2Y1WO6 (W5 +, 5d1) was attempted by many methods, but yttrium deficiencies were present in all samples. This led to the series Ba2YxWO6 with ⅔ ≤ x ≤ 0.8, dependent on the synthesis conditions. The x = ⅔ compound is known to exist as a metastable cubic phase and an 18H rhombohedral thermodynamic phase. The one-third vacant B sites on the x = ⅔ cubic material are doped with lithium, resulting in the Ba2Y2/3LixWO6 compound. This thesis focuses on the new x = 0.75 phase, and characterises its structural and magnetic properties. The Jahn-Teller distorted Sr2CuWO6, with the Cu2+ (3d9) magnetic ion, has an elongated c axis leading to separation of ab planes. This compound had been proposed as a pseudo-2D spin liquid model candidate, following a lack of evidence of transition to long-range order in SQUID magnetometry and heat capacity. However, recent μSR measurements did show a transition to long-range ordered state at 24K. This thesis details further bulk and local probe measurements which indicate that the low-temperature state is type-2 antiferromagnetic, and which point to a thermally activated spin-liquid-like state which occurs between 24K and ∼100K. La0.5Sr1.5MgWO6, has also been synthesised. The 1:1 compound LaSrMgWO6 had previously been reported as W5+ and described as “pseudo-cubic”: X-ray and neutron diffraction studies here characterise the low-temperature structure within the P2₁/n monoclinic space group, and suggest that no ordering of the atoms on the A site is observed.

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M.Ing. / This thesis, devoted to fibre optics, is primarily concerned with the utilization of fibre Bragg gratings and optical couplers to realize optical add-drop multiplexers (OADMs). A comparative study regarding various OADM configurations is undertaken on the basis of manufacturing and the performance in terms of insertion loss, channel isolation, tuning ranges, stability and cost. The heart of most of the OADMs is fibre Bragg gratings. The Runge-Kutta numerical integration method is used to solve the coupled-mode equations in order to simulate the spectral dependence of Bragg gratings numerically. Properties such as the grating strength, the grating length and the grating index profile governing the spectral dependence of Bragg gratings are investigated. In recent years, there has been an increasing interest in the dispersive properties of Bragg gratings. We investigate methods to limit the amount of dispersion induced by fibre Bragg gratings. The tuning of Bragg gratings for dynamic OADMs is also reviewed. High channel isolation Bragg gratings are theoretically and experimentally investigated. DC-apodized gratings were designed and manufactured by using the phase mask method through the use of a preconditioning technique. Bragg gratings with channel isolations of up to 24.61 dB have been realized by using this technique. The spectral dependence of DC-apodized gratings on the amount of preconditioning and the smoothness of the index envelope is simulated and in agreement with the experimental results. An athermal Bragg grating was designed and manufactured, exhibiting an average wavelength-temperature sensitivity of 2.76 pm/oC. An OADM comprising a DC-apodized Kaiser grating and an optical circulator was realized. The device showed an insertion loss of 1.84 dB and a channel isolation of 22.84 dB. The coupling mechanisms for different types of optical couplers are investigated. The distribution of power was established to be either by evanescent field coupling (etched, polished and weakly fused couplers) or due to the beating phenomenon (strongly fused couplers). The beating phenomenon of the HE11 and HE12 modes in the waist of the tapered-fused coupler is modelled and used to simulate different characteristics, such as wavelength, polarization and external refractive index dependence of tapered-fused couplers, in order to realize OADMs.

A low temperature x-ray powder diffraction attachment for use on the vertical goniometer of a diffractometer is described. We have found that diffraction patterns obtained with the attachment mounted on the goniometer are of comparable quality to those obtained on the goniometer itself. Using this attachment, the lattice parameter discontinuities associated with a charge density wave phase transition in 1T-TaS₂ near T=200K are measured with an accuracy greater than that of any previous results. Electrochemically prepared samples of Li xTiS₂ near x=0.16 are studied at room temperature and below. Clear evidence for the formation of a stage two superlattice at low temperatures was not obtained. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Most solutions for electromagnetic wave diffraction by obstacles and apertures assume plane wave incidence or omnidirectional local sources. Solutions to diffraction problems for local directive sources are needed. The complex source point representation of directive beams together with uniform solutions to high frequency diffraction problems is a powerful combination for this. Here the method is applied to beam diffraction by planar structures with edges, such as the half-plane, slit, strip, wedge and circular aperture. Previously used restrictions to very narrow beams and paraxial regions, are removed here and the range of validity increased. Also it is shown that the complex source point method can give a better approximation to broad antenna beams than the Gaussian function. The solution derived for the half-plane problem is uniform, accurate and valid for all beam orientations. This solution can be used as a reference solution for other uniform or asymptotic solutions and is used to solve for the wide slit and complementary strip problems. Uniform solutions for omidirectional sources are developed and extended analytically to become solutions for directive beams. The uniform theory of diffraction is used to obtain uniform solutions where there are no simple exact solutions, such as for the wedge and circular aperture. Otherwise rigorously correct solutions at high frequencies for singly diffracted far fields are used, such as for the half-plane, slit and strip. The geometrical theory of diffraction and equivalent line currents are used to include interaction between edges. Extensive numerical results including the limiting cases; e.g. plane wave incidence, line and point sources are given. These solutions are compared with previous solutions, wherever possible and good agreement is evident Beam diffraction by a wedge with its edge on the beam axis is analysed. This solution completes a previous asymptotic solution which is infinite on the shadow boundaries and inaccurate in the transition regions. Finally, the diffraction by a circular aperture illuminated by normally incident acoustic beam, is derived and the singularity along the axial caustic is removed using Bessel functions and a closed form expression for multiple diffraction is derived. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

This thesis is involved with the use of low-energy electron diffraction (LEED) for determining the geometrical structures of well-characterized surfaces of single crystals. Specific applications are to surfaces of rhodium, both clean and when containing adsorbed species. A preliminary problem concerned discrepancies reported previously in the details of the geometrical structures for the clean (100) and (111) surfaces when using rhodium potentials from either a band structure calculation or from the linear superposition of charge density procedure for a metal cluster. A correction has now been made in the calculation of phase shifts for the band structure potential, and reinvestigations of the (100), (110) and (111) surface of rhodium with this potential resolve the discrepancies. These results now support the suggestion, as shown previously in this laboratory for Cu(lll), that the superposition potential provides a good approximation to a band structure potential for the purpose of LEED crystallography. In the structural determinations made here, the degree of correspondence between intensity versus energy curves for different beams from experiment and from multiple-scattering calculations were assessed with the reliability-index r[sub= r] proposed by Zanazzi and Jona. A new aspect considered involved the use of this index for determining the non-structural parameters required in the multiple-scattering calculations. Included in the latter for Rh(lll) are variations of the imaginary part of the constant potential (V[sub= oi]) between the muffin-tin spheres and the surface Debye temperature (θ[sub= D,surf]). Structural conclusions from r[sub r] are compared with visual analyses wherever possible, and this work generally supports the use of the Zanazzi-Jona index in LEED crystallography. The experimental part of this study involved the (100) and (110) surfaces of rhodium. A series of diffraction patterns were observed for the chemi-sorption of 0₂ and H₂S. Intensity versus energy curves were measured for the available diffracted beams for the surface structures designated Rh(100)-(3xl)-0, Rh(100)-p(2x2)-S and Rh(110)-c(2x2)-S. The latter two systems were analyzed by multiple-scattering calculations (using the renormalized forward scattering and layer-doubling methods) and surface structures determined. In each case S atoms adsorb on the centre sites; on Rh(100) S bonds to four neighbouring Rh atoms at a distance of 2.30 Å (very close to the Pauling single-bond value 2.29 Å), and on Rh(110) each S atom is 2.12 Å from the Rh atom directly below in the second layer and 2.45 Å from the four neighbouring Rh atoms in the top metallic layer. An investigation was also made for the use in LEED crystallography of the quasidynamical method recently proposed by Van Hove and Tong. This scheme includes interlayer multiple-scattering properly, but neglects multiple-scattering within individual layers, and has the potential for considerable savings in computing time and core storage. This method was investigated for the clean and sulphur-adsorbed (100) and (110) surfaces, and results compared with the more-complete multiple-scattering methods. The quasi-dynamical method appears to have some promise for making initial selections of the most significant trial structures prior to the more-detailed testing with full multiple-scattering calculations. / Science, Faculty of / Chemistry, Department of / Graduate

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