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# Physics - Theses, Dissertations, and other Required Graduate Degree Essays

Receive updates for this collection## THz measurements of semiconductor nanostructures

This thesis describes measurements made using terahertz (THz) spectroscopy applied to semiconductor nanostructures. Our samples take the form of either thin continuous films or nanowire fields. We fit the Drude model to our results on thin continuous films to obtain their carrier density and mobility, and compare these to DC transport measurements. We compare our measurements of nanowire fields to expectations based on the Bruggeman effective medium theory.

## f(R) Gravity and Spherical Collapse

This thesis studies f(R) model building, cosmological dynamics, Solar System tests of f(R) gravity, and spherical collapse in f(R) gravity. We apply the running coupling idea to gravity. We describe several well-known f(R) models in a simple way in terms of infrared renormalization group flow. We explore two logarithmic models, produced by the flows. These two models generate a large hierarchy between the Planck scale and the cosmological constant scale. We study the cosmological dynamics of a range of f(R) models, presenting generic features of phase-space dynamics in f(R) cosmology. New techniques to explore phase space dynamics are developed. These techniques are very general and can be applied to other similar dynamical systems. We investigate the Solar System tests of f(R) gravity. The metric is rederived by directly focusing on the equations of motion. The chameleon mechanism in the Jordan frame is considered. These approaches provide a more intuitive understanding of the Solar System tests of f(R) gravity. We explore spherical scalar collapse in f(R) gravity numerically. We study the dynamics throughout the collapse. Mesh refinement and asymptotic analysis are implemented in the vicinity of the singularity of the formed black hole. The Kasner solution for spherical scalar collapse in f(R) gravity is obtained. These results support the Belinskii-Khalatnikov-Lifshitz conjecture well in the context of black hole physics.

## Optical NMR Study of 31P Donor Spins in Isotopically Enriched 28Si

A quantum computer requires a quantum system that is isolated from its environment, but can be integrated into devices, and whose states can be measured with high accuracy. Nuclear spin qubits using shallow neutral donors in semiconductors have been studied extensively as quantum memories due to their promise of long coherence lifetimes. However, the nuclear spins of neutral donors are not only difficult to initialize into known states and detect with high sensitivity, they are limited to use at cryogenic temperatures. The nuclear spins of ionized donors, on the other hand, have the potential for high-temperature operation. In this thesis, I show how the distinctive optical properties of enriched 28Si enable the use of hyperfine-resolved optical transitions of donor bound excitons, as previously applied to great effect for isolated atoms and ions in vacuum. Together with efficient Auger photoionization, these optical transitions permit rapid nuclear hyperpolarization and electrical spin readout. These techniques are combined to detect nuclear magnetic resonance from dilute 31P in an isotopically purified 28Si sample, at concentrations inaccessible to conventional NMR techniques. Dynamical decoupling is used to measure cryogenic coherence times of over 180 seconds and 3 hours for an ensemble of neutral and ionized 31P nuclear spins in 28Si, respectively. A room-temperature coherence time of over 39 minutes is demonstrated in the latter system, which is more than an order of magnitude longer than the previous solid-state coherence time record. I further show that a coherent spin superposition can be cycled from 4.2 Kelvin to room temperature and back.

## Evidence for the Standard Model Higgs Boson Decaying to One Semi-hadronically Decaying Tau and One Leptonically Decaying Tau at ATLAS

This thesis reports the search for the Higgs boson decaying to a pair of tau leptons, where one tau decays leptonically and the other decays semi-hadronically. The search was conducted by the ATLAS experiment on the proton-proton collisions supplied by the LHC in 2012. It is complicated by background from a mix of known processes from the Standard Model of Particle Physics. It makes use of boosted decision trees to separate the Higgs signal from the background. The gluon-gluon fusion and the vector boson fusion production mechanisms are searched for separately. The statistical significance of the excess observed over the background-only hypothesis is 3.3 standard deviations, and reaches 4.1 standard deviations when combined with other ditau channels. The observed signal appears fully compatible with the Higgs boson of the Standard Model.

## Models for protein localization in bacteria and chromatin structure in eukaryotes

In the first part of this thesis, we focus on the problem of how proteins localize within the cytoplasm of bacteria. Experimentally, it is found that proteins that have attractive interactions are able to localize to one or both of the poles in cylindrical bacteria. We put forward a model that only relies on the aggregating tendency of proteins and the occlusion by the bacterial nucleoid. Monte-Carlo simulations enabled us to find the stable and metastable localization patterns, allowing us to explore the phase space of parameters in our model. Our findings explains the different patterns observed for PopZ localization in Escherichia coli and Caulobacter crescentus as well as misfolded proteins. We find the kinetics of expressing proteins has a crucial role: unipolar patterning is an energetically favorable state while other polar patterns can be achieved at higher rates of protein expression. Using sets of different GFP tagged aggregating proteins we are able to experimentally test this prediction and alter the localization from unipolar to bipolar simply by increasing the rate of expression. In the second part, we consider the structuring of the DNA within the eukaryotic nucleous and its associated proteins. A new high-throughput experimental technique, Hi-C, is able to measure the looping frequencies between all parts of the genome. We transform the measured data and are able to extract a distance independent free energy by subtracting out the background free energy of interactions due to the polymer nature of DNA. Our mean-field model quantities the interaction strengths between chromatin factors and loops along the chromosomes in a protein pairwise interaction matrix J. Since the Hi-C data carries dierent biases, using our approach we are able to assess the best sets of corrections that lead to the free energy having the most mutual information with the underlying chromatin profiles. Further to this, we use Principal Component Analysis (PCA) to identify the frequent modes of genome wide looping. Hence, we are able to correlate these with known domain structures such as boundaries between active and silent regions of the genome.

## Material Composition Analysis Using Time-Domain Terahertz Spectroscopy

In this work, we apply terahertz time-domain spectroscopy to measure the thickness and composition of dielectric samples consisting of multiple constituents. We accomplish this in two stages. First, a two-component model is introduced that allows us to estimate the volume fractions of two constituents in addition to the sample thickness. Here the sample is assumed to be a homogeneous mixture, with its permittivity given by the Bruggeman effective medium theory (EMT). We apply the procedure to paper samples to extract thickness and moisture content. We perform Monte-Carlo simulations with a realistic noise model to understand the parameter correlations and predict the expected uncertainties. The simulation results are consistent with the observed repeatability measurements performed on paper samples. We show that our measurements can compete with existing thickness and moisture sensors currently deployed in the industry. Second, a three-component model is presented to determine the volume fractions of three constituents, along with the thickness. We also develop two different scattering models to account for the effects of scattering from inhomogeneities. We validate our model by measuring pulp samples compressed to different thicknesses thus having variable densities. Again, our measurements match well with the independently measured parameter values. Monte-Carlo simulations suggest avenues of improvement of the performance of the system.

## Microwave Spectroscopy of Magnetically Trapped Atomic Antihydrogen

We have every reason to believe that equal amounts of matter and antimatter were produced in the early universe. Moreover, theory predicts that the laws of physics make no distinction between the two. In this light, the fact that the observable universe is overwhelmingly dominated by matter is inexplicable. ALPHA is an international project located at CERN involving approximately 40 physicists from 15 different institutions in 7 countries. The primary goal of the collaboration is to study the antihydrogen atom at the highest level of precision possible, and thereby enable comparisons between hydrogen and antihydrogen. Through these comparisons it hopes to improve our understanding of the distinction between matter and antimatter, and perhaps shed some light on the puzzle of why we live in a matter dominated universe. The hyperfine energy intervals of ground-state hydrogen and antihydrogen represent an opportunity for a precision comparison. A discrepancy between the energy levels of these two atomic systems would indicate a major revolution in physics, and in our understanding of the universe. This thesis describes and interprets the first proof-of-principle spectroscopic measurements performed on magnetically trapped antihydrogen atoms. The experiments were performed by the ALPHA collaboration using microwave radiation tuned to induce transitions between hyperfine levels of ground state antihydrogen atoms. Our observations confirm that positron spin resonance transitions between hyperfine levels of ground state antihydrogen are consistent with expectations for hydrogen to within 4 parts in 10^3. The hyperfine splitting of ground state antihydrogen atoms is also constrained to 1420 +/- 85 MHz.

## Lateral spin injection and detection via electrodeposited Fe/GaAs contacts

Experimental studies of lateral spin injection and detection through electrodeposited Fe/GaAs tunnel contacts are reported in this thesis. An enhanced spin valve voltage is demonstrated via non-local lateral spin transport measurements compared to their vacuum-deposited counterparts. We have proposed a simple theoretical model to explain this result. Combined with experimental evidence for interfacial oxygen from atom probe tomography, we speculate that the enhancements occur due to a magnetic iron oxide layer forming at the Fe/GaAs interface during the electrodeposition. This layer acts as a tunnel barrier with a spin-dependent height. This discovery of greatly enhanced spin injection into GaAs via electrodeposited contacts introduces a promising new direction for the development of practical semiconductor spintronic devices. This thesis addresses three major challenges: i) The electrodeposition of Fe onto an epitaxial n-GaAs layer on a semi-insulating substrate to fabricate the tunnel contacts and lower-doped channel required for lateral spin injection. ii) Demonstration of spin accumulation and transport using patterned contacts in lateral configurations. iii) Understanding magnetic in-homogeneities and defects in the thin Fe film and correlating these to the observed enhanced spin injection. Continuous Fe film coverage was achieved over a desired area of the epitaxial GaAs by creating a uniform potential at the back of the sample. Nucleation and growth of Fe was observed within a range of applied current densities from 0.05 to 0.20 mA/mm^2, with the best Fe epitaxy occurring at 0.15 mA/mm^2. Modelling via a micromagnetic simulator showed that magnetic hysteresis curves from the electrodeposited Fe did not follow the standard behavior of a thin Fe film (single or polycrystalline). Instead, these Fe films demonstrated inhomogeneous magnetization controlled by strong local uniaxial anisotropies along both the <100> and <110> crystallographic directions. The presence of defects and coalescence boundaries responsible for these in-homogeneities were detected by transmission electron microscopy. Spin valve and Hanle measurements showed evidence of a local magnetostatic field, possibly originating from magnetic impurities at the electrodeposited Fe/epitaxial GaAs interface. We suggest that these magnetic impurities enhanced the tunneling probability and the spin accumulation within the GaAs channel while reducing the spin lifetime.

## Physics of Birefringent Fermions

There has been much recent interest in Dirac fermions due to their physical realizationas low energy excitations in graphene. In this thesis we introduce birefringent relativisticfermions, for which the chiral symmetry usually present for Dirac fermions is broken, andthere can be more than one Fermi velocity. We first introduce a lattice model of spinlessfermions that can arise from a scheme to introduce an artificial magnetic field for coldatoms. This model has an unusual Hofstadter-like spectrum as a function of the flux perplaquette. When there is an average of half a flux quantum per plaquette, the model hasDirac points in its spectrum and exhibits low energy excitations with two different “speedsof light”, i.e. birefringent fermions. We investigate the effects of several perturbations onthe spectrum such as staggered potentials and topological defects and we study the orderedphases that can arise from interactions. We find that sufficiently strong nearest neighbourinteractions lead to a charge density wave phase but that next-nearest neighbour interactionsallow the possibility of other phases. We also study the response of birefringent fermionsto a magnetic field and discuss how both Landau levels and the Integer Quantum Hall effectfor regular Dirac fermions are modified for birefringent fermions.

## Terahertz Conductivity Measurements of MnSi

This thesis presents measurements of the dynamical conductivity of a MnSi lm via terahertz time domain spectroscopy. We determine the Drude scattering rate and plasma frequency at low temperatures, and compare these to theoretical predictions. From a comparison of the plasma frequency measurement with band theory, we determine a mass renormalization of m*/m~5.5. Above the critical temperature, ts to the Drude model yield negative values for the scattering lifetime, indicating the existence of a pseudogap. At low temperatures and low frequencies, the resistivity has the well-known Fermi liquid form with the ratio of the temperature dependent term to the frequency dependent term, b>1. At the lowest temperature we estimate b ~ 4, with a large systematic uncertainty that we characterize for later improvement. This result is consistent with Fermi liquid theory predictions for electron-electron scattering.