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ItemNuclear structure of the N=88 isotones: the decay of 156Tm to 156Er(Georgia Institute of Technology, 2009-11-17) Dursun, SerkanThe N=88 nuclei lie in a transitional region of the nuclear chart. Collective structure and nuclear deformation for this region is commonly occurring. Previously, some of the nuclei having 88 neutrons have been well studied, e.g., 148Nd, 150Sm, 152Gd, 154Dy; however, 156Er has not. To be able to understand the nuclei in this region one needs to extract all the systematics of these deformed nuclei and the relations between them. The structure of 156Er is the main focus in this thesis and the work seeks understanding of the N=88 nuclei and systematics among them. To accomplish this task, 156Tm to 156Er beta decay was studied. Many new levels and gamma transitions have been added to the existing scheme. The significance for detailed decay scheme studies far from stability is also discussed.
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ItemElectric deflection measurements of sodium clusters in a molecular beam(Georgia Institute of Technology, 2009-11-10) Liang, AnthonyRotationally averaged polarizabilities and intrinsic electric dipole moments of sodium clusters are measured and reported. The experimental method is a molecular beam deflection. Our precision is the highest (<5%) and the range of the cluster sizes is the broadest to date (Na₁₀ ∼ Na₃₀₀). Compared to the earlier measurements, our data covers all sizes with no gaps up to the largest cluster. The fine structure in the polarizability curve is previously unobserved. We have carefully ruled out several possible explanations. And we find an earlier existing theory could explain the facts but will lead to magic numbers which were not seen in some previous experiments. A detailed theory is needed to understand the behaviors we see. Intrinsic electric dipole moments (EDM) of sodium clusters are probed to answer the intriguing question: Do metal clusters develop electric dipole moments like molecules? Some theories have predicted the existence of EDM in ground state sodium clusters and gave their magnitudes. We put upper bounds on the EDM of sodium clusters and find that they are orders of magnitude smaller than the predictions. This provokes an interesting question: how can one define metallicity in metal clusters? Our measurements are performed at cryogenic temperature 20 Kelvin. At this temperature the clusters are believed to be in their vibronic ground states.
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ItemDouble point contact single molecule absorption spectroscopy(Georgia Institute of Technology, 2009-08-24) Howard, John BrooksThe generation of high-frequency current oscillations when a constant voltage is applied across an insulating tunnel gap separating two superconductors was one of the fundamental theoretical predictions made by Brian Josephson, which earned him a share of the 1973 Nobel Prize in physics. Our primary objective is to utilize superconducting transport through microscopic objects to both excite and analyze the vibrational degrees of freedom of various molecules of a biological nature. The technique stems from a Josephson junction's ability to generate radiation that falls in the terahertz gap ( 10THz) and consequently can be used to excite vibrational modes of simple and complex molecules. Analysis of the change in IV characteristics coupled with the differential conductance ( ) allows determination of both the absorption spectra and the vibrational modes of biological molecules. Presented here are both the theoretical foundations of superconductivity relevant to our experimental technique and the fabrication process of our samples. Comparisons between our technique and that of other absorption spectroscopy techniques are included as a means of providing a reference upon which to judge the merits of our novel procedure. This technique is meant to improve not only our understanding of the vibrational degrees of freedom of useful biological molecules, but also these molecule's structural, electronic and mechanical properties.
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ItemGraphene nanoribbon array fabrication by oxygen gas plasma etching through sub-micrometer porous filters(Georgia Institute of Technology, 2009-08-07) Shaw, DavidWe investigate a novel method of fabricating a network of graphene nanoribbon structures. The process is a sharp departure from conventional nanolithographic techniques in both method and amount of time required. Epitaxial graphene prepared on single crystal 4H-SiC was etched with O2 plasma through 0.2 ìm porous filters adhered to the surface of the sample. Thickness measurements using ellipsometry and topological mappings using atomic force microscopy were conducted to ascertain the extent of graphene nanoribbon formation. Sheet resistance of the samples was measured using the four-point van der Pauw method to ensure the existence of electrical conductivity in the etched samples. Furthermore, the etch-rate of multilayer epitaxial graphene was determined.
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ItemNonlinear approach to electronic beam steering(Georgia Institute of Technology, 2009-07-14) Wiesenfeld, Kurt
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ItemMeasurement of complex ultrashort laser pulses using frequency-resolved optical gating(Georgia Institute of Technology, 2009-07-06) Xu, LinaThis thesis contains three components of research: a detailed study of the performance of Frequency-Resolved Optical Gating (FROG) for measuring complex ultrashort laser pulses, a new method for measuring the arbitrary polarization state of an ultrashort laser pulse using Tomographic Ultrafast Retrieval of Transverse Light E-fields (TURTLE) technique, and new approach for measuring two complex pulses simultaneously using PG blind FROG. In this thesis, we compare the performance of three versions of FROG to measure complex ultrashort laser pulses: second-harmonic-generation (SHG) FROG, polarization-gate (PG) FROG, and cross-correlation FROG (XFROG). We found that the XFROG algorithm achieves 100% convergence, while PG FROG and SHG FROG GP algorithm achieve 100% convergence after doing the noise deduction and increasing the sampling range. The second part of this thesis describes a method for measuring the intensity, phase and the complete polarization state of a laser pulse having a time-dependent polarization state (i.e. a polarization shaped pulse). This technique is called tomographic ultrafast retrieval of transverse light E-fields (TURTLE). TURTLE typically involves making three FROG measurements: one of the intensity and phase of the pulse's horizontal polarization component, one of its vertical component, and another of the 45o component. Performing a simple minimization using these three FROG measurements, the time-dependent polarization state of the ultrashort pulse can be determined. The third part of this thesis introduces a method for measuring two complex pulses simultaneously using a single FROG device. This technique is based on Polarization-gate (PG) FROG and it is called PG blind FROG. It involves two measurements: One of them is a PG FROG trace using the intensity of pulse 1 to gate pulse 2 and other one is the PG FROG trace using the intensity of pulse 2 to gate pulse 1. An iterative phase retrieval algorithm based on generalized projection (GP) is used to reconstruct the intensity and phase of these two pulses. This approach is an elegant way to measure complex and/or very spectrally broad pulses such as those due to super continuum.
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ItemCold elastic collisions of sodium and rubidium(Georgia Institute of Technology, 2009-07-01) Breuer, JohnIn this thesis we numerically compute the scattering lengths and bound states for sodium-rubidium collisions at low energy. This work was motivated by experiments which aim to produce Bose-Einstein condensates (BEC) mixtures of sodium-rubidium. Elastic collision properties are important for the rethermalization of the atoms during the evaporative cooling process. Inelastic processes, which we also discuss to some extent, cause trap losses in those experiments. In order to reach the required temperature and density the elastic collision rates should be sufficiently large compared to the inelastic rates. The scattering lengths, which completely specify the elastic collision parameters at low energy, determine the miscibility and phase diagram of the sodium-rubidium condensate mixture. We calculate the scattering lengths approximately and find agreement with previous calculations indicating that miscible phases of sodium and rubidium condensates do not appear to be feasible in the absence of external fields.
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ItemTheory of phase transitions in disordered crystal solids(Georgia Institute of Technology, 2009-06-29) Li, HuamingSolid-state amorphization of a crystalline solid to an amorphous phase is extensively studied as a first order phase transition at low temperature for almost thirty years. In this dissertation, we report the recent progress on phenomenological models employed for thermodynamic description of macroscopic systems and fluctuations and nucleation of mesoscopic inhomogeneous systems in binary solid solutions under polymorphic constraints with no long-range diffusion involved. Based on our understanding on atomic picture of solid-state amorphization in binary solid solutions, we propose a Landau free energy to describe amorphization as the first order phase transition. The order parameter is defined which represents the loss of long-range translational order. The elastic strain field induced by composition disorder plays the important role through the bilinear coupling with the order parameter. Elastic softening and amorphization happen simultaneously. From the similarity between the melting and amorphization, we use the temperature and composition as two external variables and treat solid-state amorphization as low temperature melting under polymorphic constraints. For homogeneous system, the phase diagrams for endothermic melting and exothermic melting are built separately and the corresponding thermodynamic quantities are presented. A microscopic homogeneous nucleation mechanism is proposed conceptually in binary solid solutions under polymorphic constraints. The formation of an amorphous embryo is initiated from the composition modulation in the crystal state and a subsequent polymorphous nucleation within the as-formed heterophase fluctuation. This homogeneous nucleation path is thought to be associated with the nonlinear energy localization mechanism connected with the localized large-amplitude excitations of atoms, which are induced by nonlinear and disorder. A Landau-Ginzburg free energy is constructed to describe the critical nucleus and the growth of the new phase in one-dimensional systems. Analytical and numerical methods contribute to the understanding the fluctuations and nucleation processes. Size-dependent melting and amorphization in nanosolids are investigated. Two models are proposed for nanocrystalline solid solutions to glass transformations. Based on the thin film model with finite thickness, we build one-dimensional Landau-Ginzburg approach, which includes surface contribution and size dependence, and numerical results do show similarity with experimentsâ results qualitatively.
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ItemAggregation in the Schelling model and inverted biomass pyramids in ecosystems(Georgia Institute of Technology, 2009-06-25) Singh, AbhinavThomas Schelling proposed a simple spatial model to illustrate how, even with relatively mild assumptions on each individual's nearest neighbor preferences, an integrated city would likely unravel to a segregated city, even if all individuals prefer integration. Many authors assumed that the segregation which Schelling observed in simulations on very small cities persists for larger, realistic size cities. We describe how different measures can be used to quantify the segregation and unlock its dependence on city size, disparate neighbor comfortability threshold, and population density. We develop highly efficient simulation algorithms and quantify aggregation in large cities based on thousands of trials. In particular, we show that for the values of disparate neighbor comfortability threshold used by Schelling, the striking global aggregation Schelling observed is strictly a small city phenomenon. Along the way we prove that in the Schelling model, in the process of evolution, the total perimeter of the interface between the different agents always decreases, which provides a useful analytical tool to study the evolution. At the isolated reef Kingman, it was recently discovered that apex predators constitute 85% of the total fish biomass. This is in sharp contrast to most reefs, where the prey biomass substantially dominates the total fish biomass. The recent study at the two pristine reefs, Kingman and Palmyra also indicates that the predator:prey fish biomass ratio is an increasing function of reef cover. Based on these field observations, we model the fish biomass structure at a pristine coral reef. We introduce a new refuge based mechanism for predator-prey interaction with an explicit dependence on refuge size. Our refuge based model does not assume mass action interaction between predators and prey and may provide a new mechanism in ecology to produce inverted biomass pyramids. Our model yields both the inverted biomass pyramid and the increasing dependence of the predator:prey biomass ratio on reef cover. We add various forms of fishing to our model, and show that sufficiently high fishing pressure with quite general types of fishing transforms the inverted biomass pyramid to be bottom heavy.
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ItemInelastic collision and three-body recombination(Georgia Institute of Technology, 2009-05-19) Li, BoThe quantum impulse approximation theory has been extended to the inelastic collision. The total inelastic cross sections for the degenerated states with different angular momenta was calculated. It was proved that summing over the transitions from nl to n' and from nl to n'l' would give us the total cross section of transition from n to n'. Rate coefficients were calculated for the common gases in the atmosphere being the third particle. The resonant effect of the rate coefficients had been observed. Recombination coefficients were then calculated in terms of rate coefficients. Previous calculations were carried out in compare with the net rate flow through certain excited levels, which were found to be more stable and reflected a clearer picture of the whole process. Results have been compared with the elastic collision. A dramatic decreasing of rates when temperature increased was also observed. More thermal energy increases the probability of electrons for being re-ionized. Similar calculations had been carried out for the upper atmosphere gases, such as N₂, O₂, CO, CO₂, and H₂O. The recombination coefficients for electron combining with metallic ion Na+ were also calculated.