DRUM Collection: Physics Theses and Dissertations
http://hdl.handle.net/1903/2800
2015-02-27T08:07:15ZStructural Changes and the Nature of Superconductivity in Rare-earth Doped CaFe2As2
http://hdl.handle.net/1903/16255
Title: Structural Changes and the Nature of Superconductivity in Rare-earth Doped CaFe2As2
Authors: Drye, Tyler Brunson
Abstract: Chemical substitution into iron-pnictide parent compounds (e.g. AFe<sub>2<\sub>As<sub>2<\sub> where A=Ba, Sr, or Ca) has proven to be an effective means to induce bulk high-temperature superconductivity in these systems. By doping CaFe<sub>2<\sub>As<sub>2<\sub> with rare-earth lanthanides (La, Ce, Pr, and Nd), we have observed a 47 K superconducting phase coexisting with a lattice distorting “collapse” transition. Both of these effects have important ramifications: the collapse transition occurs when interlayer As atoms form a bond, shrinking the <italic>c-axis<\italic> lattice constant and simultaneously quenching the iron magnetic moment. This transition is further explored in context of a similar system, Sr-doped BaNi<sub>2<\sub>As<sub>2<\sub>. The superconducting phase, given the right combination of conditions, appears with a critical temperature as high as 49 K, but always in a very small volume of the sample (as determined by shielding effects). This has led to interesting theories about the nature of this superconductivity. A recently posited idea of “interfacial superconductivity” has been ruled out by our tests. Additionally, increasing the concentration of rare-earth atoms does not increase the superconducting volume fraction, but, in fact lowers the transition temperature, excluding the hypothesis that rare-earth defects are responsible for the minority superconducting phase. New pressure measurements have shown that the superconducting phase is stabilized when antiferromagnetic order is fully suppressed.2014-01-01T00:00:00ZSearch for Pair Production of Third-Generation Scalar Leptoquarks and R-Parity Violating Top Squarks in Proton-Proton Collisions at sqrt(s) = 8 TeV
http://hdl.handle.net/1903/16252
Title: Search for Pair Production of Third-Generation Scalar Leptoquarks and R-Parity Violating Top Squarks in Proton-Proton Collisions at sqrt(s) = 8 TeV
Authors: Pedro, Kevin
Abstract: This dissertation presents a search for pair production of third-generation scalar leptoquarks or top squarks in R-parity violating supersymmetry decaying through the coupling λ′<sub>333</sub>, with the new scalar particle decaying to a tau lepton and a bottom quark. The data used in this search were collected by the Compact Muon Solenoid experiment and comprise 19.7 fb<sup>-1</sup> of proton-proton collision data from the Large Hadron Collider at a center-of-mass energy of √(s) = 8 TeV. No excess of events above the standard model background prediction is observed. The existence of third-generation scalar leptoquarks or top squarks with masses below 740 GeV is excluded at the 95% confidence level, assuming the branching fraction for the decay to a tau lepton and a bottom quark is 100%. This is currently the strongest limit on this kind of leptoquark or top squark. The search is extended to address a chargino-mediated decay of the top squark involving the R-parity violating coupling λ′<sub>3jk</sub>, producing a tau lepton, a bottom quark, and two light quarks. The existence of top squarks in this model with masses below 580 GeV is excluded at the 95% confidence level. This is the first search for the pair production of top squarks with the decay involving the R-parity violating coupling λ′<sub>3jk</sub>.2014-01-01T00:00:00ZCharacterization of Nonlinear Waveguides for All-Optical Signal Processing
http://hdl.handle.net/1903/16250
Title: Characterization of Nonlinear Waveguides for All-Optical Signal Processing
Authors: Wathen, Jeremiah
Abstract: This dissertation documents research of the optical properties of chip-based nonlinear waveguides to assess their suitability for all-optical processing in the telecommunications spectrum. Three related topics are presented: (1) enhanced continuous-wave four-wave mixing in AlGaAs waveguides; (2) a new method to characterize nonlinear refraction and loss in waveguides; and (3) the non-instantaneous optical nonlinearity of a hydrogenated amorphous silicon waveguide.
For topic (1), enhancements of the continuous-wave four-wave-mixing efficiency and bandwidth are accomplished by engineering the growth and fabrication of AlGaAs waveguides. The bandgap of the core-layer alloy is tailored to suppress nonlinear absorption and the fabrication is optimized to produce small-mode-area, dispersion-engineered waveguides with sub-dB/cm propagation losses. A four-wave-mixing conversion efficiency of -6.8 dB is observed in an AlGaAs waveguide that exhibits a propagation loss of 0.56 dB/cm. This conversion efficiency is among the highest reported for any passive semiconductor or glass waveguide. Another low-loss AlGaAs waveguide is featured, which is dispersion-engineered to produce a measured conversion-bandwidth of 63.8 nm with a pump wavelength of 1550 nm.
For topic (2), a new technique is described for accurately measuring the ratio between the imaginary part and the real part of a waveguide's third-order nonlinearity. Unlike most other methods, it does not depend on precise knowledge of the coupling efficiencies, the optical propagation loss, or the optical pulse shape. The method is applied to measure the nonlinear loss tangents of a GaAs waveguide, two bandgap-engineered AlGaAs waveguides, a large-area crystalline silicon waveguide, a crystalline silicon nanowire waveguide, and a hydrogenated amorphous silicon nanowire waveguide.
For topic (3), pump-probe spectroscopy and continuous-wave cross-phase- and cross-amplitude-modulation measurements are used to study the optical nonlinearity of a hydrogenated amorphous silicon nanowire waveguide. The results are compared to those of a crystalline silicon waveguide of similar dimensions. The hydrogenated amorphous silicon nanowire shows essentially zero instantaneous two-photon absorption, but it displays a strong, long-lived non-instantaneous nonlinearity that is both absorptive and refractive. Power-scaling measurements show that the non-instantaneous nonlinearity of the hydrogenated amorphous silicon scales as a third-order nonlinearity. Phase-transient measurements show that the refractive component possesses the opposite sign to that expected for free-carrier dispersion.2014-01-01T00:00:00ZAPPLICATION OF FLUCTUATION ANALYSIS TO CHARACTERIZE MULTI-SCALE NATURE AND PREDICTABILITY OF COMPLEX SYSTEMS
http://hdl.handle.net/1903/16211
Title: APPLICATION OF FLUCTUATION ANALYSIS TO CHARACTERIZE MULTI-SCALE NATURE AND PREDICTABILITY OF COMPLEX SYSTEMS
Authors: Setty, Venkat Anurag
Abstract: Complexity is a result of interactions among individual components of a distributed
system, each with their own dynamical time scale. Statistical techniques
such as fluctuation analysis are used to quantify extent of long range correlations
within time series data by estimating a scaling exponent commonly known as Hurst
exponent. Data from magnetospheric dynamics, physiology and finance are known
to show multi-exponent nature (two exponents in particular) in their fluctuation
analysis with a crossover between the power laws. This correlation crossover can be seen due to the statistical approach taken in the analysis of a range of systems with differing dynamical time scales, particularly due to the nature of interactions with one another. We refer to this property as multi-scale nature in the time series data from complex systems. The main contributions of the thesis are as follows:
Study of crossover in fluctuation analysis of data from magnetosphere, physiology and finance:
We propose an innovative regression scheme, whose mathematical model well describes two exponent nature with an intermediate crossover regime seen in fluctuation analysis - the Hyperbolic regression. Slopes of the asymptotes of the hyperbola are the Hurst exponents, and, center of the resulting hyperbolic fit is an estimate of the correlation crossover time. It is key to note that in this regression, there are no assumptions made about the crossover time, unlike previous approach to crossover fitting. Different data sets corresponding to different
physical processes demonstrate multi-scale nature. However, each data presents
a unique challenge to be addressed, as a result of characterization of its scaling
crossover. Application of hyperbolic regression on the crossover seen in fluctuation
analysis of auroral electrojet index data from magnetosphere resulted in estimation
of Hurst exponents before and after the crossover. Also, the correlation crossover
time scale is now measured by improved modeling of such data using a stochastic
model that demonstrates crossover in fluctuation functions - the OU-Langevin
model. Characterization of nature of crossover seen in fluctuation analysis of generalized volatility within financial index data has shown differing nature of financial markets. Such a study would help characterize individual markets utilizing features which were not used previously. Heart rate variability data from healthy patients and patients with congestive heart failure demonstrate differing extent of crossover within the crossover seen in fluctuation functions. This resulted in proposal of a parameter i.e., the extent of crossover parameter that can be used to distinguish patients with the congestive heart failure ailment from normal cases.
Quantifying predictability of complex systems using Hurst exponents:
Predictability of complex systems suffers due to noise in the data. Long range correlations in noise are seen to cause extreme events or build ups leading to
extreme events in such data. The increased probability of extreme event occurrence makes prediction of resulting time series data difficult. Tail exponent is an exponent resulting out of power laws seen in heavy tailed distributions and is used as a measure of the probability of extreme events in such data. The Hurst exponent which measures the extent of long range correlations using fluctuation analysis has a known relationship with the tail exponent through Taqqu's theorem. Fluctuation analysis of time series data is oftentimes complicated due to existence of trends in the time series data. Dynamical features in data commonly reflect as trends, and, as a result nonlinear dynamical time series prediction techniques can be used to measure these trends. We refer to this method as Fluctuation Analysis after Trend Elimination (FATE) and apply it to data from space weather and finance. Hurst exponent estimated from FATE and its relationship with tail exponent measured by a commonly used Hill estimator technique are shown. Insufficient data sizes limit our ability to robustly estimate the tail exponent from observational data as extreme events are usually rare. This forms the motivation to use Hurst exponent obtained from FATE as a measure of predictability of complex systems demonstrated by application on auroral electrojet index data. Conversely, Hurst exponent can be used to quantify the ability of a technique to predict time series data as demonstrated by application of FATE.2014-01-01T00:00:00Z