<p> Magnetic nanostructures have been subject of intense research as a result of their unique magnetic properties such as superparamagnetism, enhanced magnetic moment, and high magnetic density storage due to shape anisotropy. A highly reproducible and affordable method to fabricate cobalt nanostructures on silicon substrates was devised in this thesis using nanosphere lithography. The surface morphology and magnetic properties of the nanopatterned cobalt thin film were characterized using an optical microscope, scanning electron microscope, atomic force microscope, and a magneto optical Kerr effect (MOKE) magnetometer. Modification to the surface of cobalt thin film was found to extensively alter its magnetic coercivity. Continuous cobalt thin film at 10 nm thick has a coercivity of 102&plusmn;2 Oe, whereas nanostructured cobalt thin films at the same thickness have a coercivity of 167&plusmn;16 Oe. The magnetic coercivity increases by 65&plusmn;18 Oe for an array of close packed cobalt nanostructures using nanosphere templates with diameters ranging from 203 nm to 600 nm. The cobalt nanostructured sample using a 930 nm diameter nanosphere template has a coercivity comparable to a continuous cobalt thin film at 108&plusmn;7 Oe. In addition, these nanostructures exhibit unusual magnetic properties such as multistep behavior and pinching/crossing-over of the magnetization curves with regards to the MOKE signal. These features become more prominent as the diameter of the nanosphere template decreases.</p>

Far infrared transmission properties of metallic and insulating granular free standing $\rm YBa\sb2Cu\sb3O\sb{x}$ thin films were measured. Since no substrate was present, we were able to cover a 10-650 cm$\sp{-1}$ frequency range, wider than in any previous transmission measurements on these materials. The films, prepared by a novel technique, were 0.1-1 $\mu$m thick and about 100 mm$\sp2$ in area. This is compared to the 1 mm$\sp2$ area of free standing films made by previous methods. The metallic films showed the superconducting transition below 85 K. All of the films had the stoichiometric composition and the expected polycrystalline lines in the x-ray spectra. / The transmission spectra of the metallic films were measured both in the normal and the superconducting states. These films behaved as a mixture of conducting and insulating small grains in the vicinity of the percolation threshold. The data were modeled by the two-dimensional effective-medium approximation. The calculated frequency-dependent absorption coefficients and the effective conductivities agreed with some previous theoretical studies. This experiment is the first investigation of percolative phenomena in an insulator-high-T$\sb{\rm c}$-superconductor mixture. / We also measured the reflectance spectra of modified Heusler alloys, UNiSn and ThNiSn, at temperatures 10-300 K in the frequency range 10-700 cm$\sp{-1},$ and at room temperature in the range 10-44800 cm$\sp{-1}.$ These are the first measurements on these materials in this frequency range. The reflectance spectra are consistent with the semiconducting behavior of ThNiSn, and the observed metal-semiconducting transition in the UNiSn system at 43 K. Below 42 K UNiSn orders antiferromagnetically, while the isostructural ThNiSn shows no such ordering. Two phonon lines, observed in the UNiSn spectra, split below 42 K, indicating a lattice change. A single phonon line was present in ThNiSn spectra at all temperatures. The dielectric functions and conductivities were calculated from the reflectance spectra using the Kramers-Kronig analysis. Below 42 K we observed an antiferromagnetic mode at 20 cm$\sp{-1}$ in the UNiSn sample. / Source: Dissertation Abstracts International, Volume: 53-07, Section: B, page: 3567. / Major Professor: Hon-Kie Ng. / Thesis (Ph.D.)--The Florida State University, 1992.

This work represents an investigation of the structure and dynamics of a new self-assembled system arising from the interaction of alkyl halides on alkali halides surfaces. Monolayer films were prepared in solution and their thickness verified by X-ray Photoelectron Spectroscopy. Furthermore, the molecules were found to have the halogen atom close to the substrate while the rest of the chain extends away from it. However, the main thrust of this study was the deposition in vacuum of bromohexane monolayer films on KBr (001) and their investigation using Helium Atom Scattering. Long range order of the films is temperature dependent: They exhibit an order-disorder phase transition around 166 K and they reorder into (2 X 1) and (1 X 2) domains prior to desorption at 180 K. Results of the bromohexane film surface multiphonon dynamics are also reported here. / Source: Dissertation Abstracts International, Volume: 57-04, Section: B, page: 2585. / Major Professor: James G. Skofronick. / Thesis (Ph.D.)--The Florida State University, 1996.

This dissertation is comprised of three projects associated with cholesteric liquid crystals. Each project was motivated by observations in liquid crystalline deoxyribonucleic acid (lcDNA). The first project is a Monte Carlo study of defects in planar cholesterics. A microscopic model is used to obtain defects and defect energies. The second project is a microscopic model for the cholesteric to hexagonal transition observed in lcDNA. The model is solved in the mean-field and forms for the twist wave vector and its associated elastic modulus are obtained. The model is also used to obtain expressions for the elastic constants. The third project is a computational study of cholesteric liquid crystals under high magnetic fields. Differential equations obtained from the free energy of defects are solved numerically. The solutions obtained provide theoretical justification for structure observed in lcDNA under 9.4 Tesla fields. / Source: Dissertation Abstracts International, Volume: 55-11, Section: B, page: 4916. / Major Professor: David H. Van Winkle. / Thesis (Ph.D.)--The Florida State University, 1994.

No description available.

Computational proposal for locating local defects in superconducting tapes

<p> It is known that for a topologically ordered state the area law for the entanglement entropy shows a negative universal additive constant contribution, &ndash;&gamma;, called the topological entanglement entropy. We theoretically study the entanglement entropy of the two-dimensional Shastry-Sutherland quantum antiferromagnet using exact diagonalization on clusters of 16 and 24 spins. By utilizing the Kitaev-Preskill construction, we extract a finite topological term, &ndash;&gamma; , in the region of bond-strength parameter space corresponding to high geometrical frustration. Thus, we provide strong evidence for the existence of an exotic topologically ordered state and shed light on the nature of this model's strongly frustrated, and long controversial, intermediate phase.</p>

Near normal incidence multilayer mirrors are optical elements that are suitable for the extreme ultraviolet wavelength (EUV) region where applications include lithography, astronomy, and microscopy. Multilayer mirrors are made from alternating layers of two materials, called the "absorber" and "spacer," with the thickness of the layers designed such that reflections from each interface add in phase resulting in a large overall reflectivity. The criteria I used for the selection of six new material pairs included achieving the maximum theoretical reflectivity while taking into consideration the possible structural properties of the interfaces based on binary phase diagrams. The pairs were: C-Cu, B₄C-Ag, B₄C-Sn, Y-Pd, Be-Mo, and Be-Y. My experimental results on sputtered C-Cu and B₄C-Ag multilayers showed that they are not suitable as mirror materials due to the formation of discontinuous layers of Cu and Ag for small bilayer periods Λ. I also found it not possible to sputter tin films with small enough interfacial roughness values that would result in useful B₄C-Sn mirrors. My analysis of Y-Pd multilayers showed asymmetric alloying at the interfaces with an approximately 40 Å thick alloy region at the Y on Pd interface which would result in negligible mirror reflectivity. I used one of our molecular beam epitaxy (MBE) machines to attempt to grow single crystal Be-Mo mirrors. Although my attempts were unsuccessful to date, this combination cannot be excluded due to various problems with the MBE sample manipulator during the growth study. Finally I used the same MBE machine to grow Be-Y mirrors with up to 40 bilayers. These multilayers had extremely smooth interfaces (σ = 3.5-4.5 A) with a predicted mirror reflectivity larger than 65%. I found the stability of the Be-Y interfaces to be excellent under atmospheric long term storage. An X-ray photoelectron spectroscopy (XPS) annealing study I conducted also showed stable interfaces for temperatures of up to 200°C. Be-Y mirrors should be suitable for a variety of applications including EUV-lithography.

Photoluminescence from direct-bandgap semiconductor quantum wells after non-resonant excitation is predominantly observed at energetic position of the 1s exciton resonance. The time evolution of the photoluminescence is generally interpreted as direct monitor of an excitonic population; a rise of the signal is interpreted as a buildup and the decrease as decay of the excitonic population. Recent microscopic calculations, however, have shown that even without an incoherent excitonic population, pure plasma decay yields photoluminescence peaked at the is exciton resonance. Experimental time-resolved photoluminescence spectra are taken across a large region of the parameter space of carrier density and lattice temperature. They are compared to the expected thermal equilibrium spectra, calculated from nonlinear absorption measurements taken under identical conditions. Under none of the experimentally explored parameters is the is emission as bright as expected for thermal equilibrium. To distinguish excitonic and plasma contributions, the deviations from thermal equilibrium at the is exciton resonance are then analyzed using a microscopic calculation. The dipole moment is adjusted to reproduce the excitonic binding energy and oscillator strength of the samples under investigation. The carrier densities and carrier temperatures are determined experimentally; no free fit parameters are necessary. The differences between experimental values and pure plasma calculation are explained with the presence of an incoherent excitonic population. Although at first the emission spectra under all conditions do not vary significantly, a more detailed analysis reveals that the sources of the photoluminescence can be either predominantly excitonic or plasma. For low temperatures and low densities the excitonic emission is extremely sensitive to even minute exciton populations making it possible to extract a phase diagram for incoherent excitonic populations. The maximum contribution of bright excitons is found at intermediate densities and low lattice temperatures; the absolute number of bright excitons is tiny, less than 0.04% of the total carrier density. However, it is not possible to determine the total number of bright and dark exciton by using photoluminescence.

As the device feature size shrinks, films of silicon oxide (SiO₂) will become unsuitable for MOSFET gate dielectric applications and have to be replaced by thicker films of a high-k dielectric material. Among the high-k materials, hafnium oxide (HfO₂) and zirconium oxide (ZrO₂) are the most promising candidates. Molecular contamination can affect the quality of the new gate dielectric films in a manner similar to ultrathin SiO2 films. Therefore, characterization of contaminant adsorption behavior of these high-k films should assist in deciding their potential for successful integration in silicon MOS technology. The interactions of moisture and organic (in particular IPA) contamination with ALCVD(TM) deposited 5-nm HfO₂ and ZrO₂ films were investigated using mass spectrometry. HfO₂ and ZrO₂ were found to have similar moisture adsorption loadings, but significantly higher than that of SiO₂. The new high-k materials also retained a higher portion of the adsorbed moisture after an isothermal nitrogen purge. Almost all the adsorbed moisture could be removed from SiO₂ and HfO₂ after a 300°C bake under nitrogen purge, whereas ZrO₂ surfaces retained significant amounts of the adsorbed moisture. Experiments with ppb-levels of IPA showed that the adsorption loading on the three surfaces had the following order: ZrO₂ > HfO₂ > SiO₂. The relatively slow desorption kinetics of H2O and IPA highlighted the difficulty in removal of these contaminants from HfO₂ and ZrO₂ surfaces. Presence of pre-adsorbed moisture increased IPA adsorption on SiO₂, but reduced adsorption on HfO₂ and ZrO₂. Isotope labeling studies with D₂O showed that IPA reacted with surface hydroxyl groups to form a chemisorbed alkoxy species on all oxides. A multilayer model for adsorption of water and IPA was developed to understand the mechanism of interactions of contaminants with these surfaces. Results indicated that ZrO₂ formed the strongest surface-hydroxyl bond and also physisorbed IPA stronger than HfO₂ and SiO₂. The practical application of the adsorption model is also demonstrated. The results of this work should aid in the selection of the most appropriate dielectric film and design of process/equipment so that it can be more readily integrated into silicon technology.