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

Receive updates for this collection## TF-μSR Sensitivity to Charge Density Wave Order in 2H-NbSe2

The sensitivity of transverse-field muon spin rotation (TF-μSR) to static charge-density wave (CDW) order in the bulk of 2H-NbSe2 is demonstrated. In the presence of CDW order, the quadrupolar interaction of the 93Nb nuclei with the local electric-field gradient is modified, and this in turn affects the magnetic dipolar coupling of the positive muon to these nuclei. For a weak magnetic field applied parallel to the c-axis, we observe a small enhancement of the muon depolarization rate at temperatures below the established CDW phase transition. Aligning the applied field perpendicular to the c-axis, we observe a sensitivity to static CDW order in regions of the sample extending up to a temperature nearly 3 times the CDW transition temperature T CDW. The results suggest that the muon is mobile over the temperature range explored above the superconducting transition temperature Tc, but becomes trapped in the vicinity of defects.

## Interactions and symmetries of electronic systems with quadratic band touching

In this essay we study two-dimensional electronic systems that exhibit quadratic band touching (QBT) in their low energy dispersion. A lattice realization is shown to introduce the emergence of a QBT in the framework of the tight binding approximation. We then make a continuum limit, which allows for a more general approach on the level of low energy Hamiltonians. The basic concepts and the applied methods are introduced via a simplified problem, a system of spinless fermions. We use a renormalization group (RG) approach to examine the effects of short-range electron-electron interactions. Once it is found that the system is unstable toward ordered phases, we use the mean field approximation to investigate how the phases compete energetically. We then introduce the spin degree of freedom of the electrons and follow analogous calculations using the RG approach. A number of ordered phases are identified, and the flow equations of the susceptibilities are calculated. We find a symmetry of the action that is larger than what we start with and emerges under some plausible conditions.

## Dynamics of Trapped Ions Near the Linear-Zigzag Structural Phase Transition

Laser-cooled ions held in a linear Paul trap with strong transverse confinement organize into a one-dimensional (1-D) linear crystal. If the transverse confinement is relaxed, the linear ion crystal undergoes a continuous, structural phase transition to a 2-D zigzag configuration. We study the dynamics near the critical point of the linear-zigzag transition. In the first part of this thesis, we study the spontaneous nucleation and dynamics of topological kink defects, formed as a result of a rapid quench across the linear-zigzag transition. The experimental results are compared to the Kibble-Zurek mechanism, which provides an intuitive model of defect formation and predicts a power-law scaling for the number of defectsformed as a function of transition quench rate. The second part of this thesis is focused on one of the key requirements for investigations of the near-transition dynamics in the quantum regime. To achieve an efficient ground state cooling of the zigzag vibrational mode, we demonstrate 3-D polarization-gradient cooling of strings of 1-4 trapped ions as an intermediate step between Doppler and sideband cooling, and study the polarization-gradient cooling rate and cooling limit as a function of the cooling beam intensity in and near the Lamb-Dicke regime. The results of this thesis pave the way towards our future experiments aimed at assessing the coherence time of a zigzag superposition state through measurements of tunneling oscillations.

## Spin pumping and spin transport in magnetic heterostructures

High quality, ultrathin magnetic films were prepared by means of molecular beam epitaxy (MBE). Magnetization dynamics and anisotropies were studied by means of ferromagnetic resonance (FMR) in GaAs|Fe|Au(001) structures as a function of the Fe layer thickness, allowing the determination of bulk and interface properties. Spin transport was studied in GaAs|Fe|Au|Pd structures, where two interesting results were found: (1) The spin pumping induced damping showed an oscillatory dependence on the Au spacer layer thickness when this layer's thickness was less than the electron mean free path. This effect is attributed to the formation of quantum well states in the Au layer. (2) The spin pumping induced damping was quickly suppressed with the addition of the Au spacer layer as compared to GaAs|Fe|Pd samples. It is experimentally shown that this reduction is not related to the removal of magnetic proximity effect induced damping at the Fe|Pd interface. It is shown that the Pd layer can neither be treated as an ideal spin sink nor as a simple normal metal (diffusive spin scatterer) with respect to spin currents and that the reduction in damping is due to a reflection of spin currents at the Au|Pd interface. Magnetization dynamics were investigated in ferrimagnetic insulator Yttrium Iron Garnet (YIG, Y3Fe5O12). Ferromagnetic resonance was used to determine the spin pumping induced damping in YIG and YIG|Au|Fe|Au structures. In the YIG|Au|Fe|Au structures, the YIG acts as a spin pump and the Fe as a spin sink when the YIG layer undergoes ferromagnetic resonance. Comparing the damping in the YIG and YIG|Au|Fe|Au structures allows one to determine the efficiency of spin pumping at the YIG|Au interface given by the spin mixing conductance. It is found that the spin mixing conductance of as grown YIG films is ~10% of that typically found at metallic FM|NM interfaces. Surface treatment of the YIG films by Ar+ etching is able to improve the spin pumping efficiency, approaching closely to that obtained by first principle electron band calculations.

## State-dependent diffusion of Brownian particles near a boundary wall

Brownian motion refers to the erratic random movement of microscopic particles suspended in a fluid. In a simple fluid, Brownian motion exhibits two key properties: the mean-squared displacement (MSD) increases linearly with time (the proportionality constant is the diffusivity D) and the displacement distribution is Gaussian. Although a linear MSD was initially assumed to always imply Gaussian displacements, recent experiments show that non-Gaussian displacements can coexist with a linear MSD in complex environments. Chubynsky et al. [PRL 113, 098302, 2014] have argued that such behavior arises when D has temporal and/or spatial fluctuations that are convolved together and form a non-Gaussian distribution. Experiments to date have been in complex settings where direct measurements of D(x, t) have not been possible. Here, we report experiments on a simple system where D(x, t) is known: the Brownian motion of a colloidal sphere near a boundary wall. By choosing the particle size carefully, we ensure that the bead explores a wide range of D. We observe a linear MSD curve and non-Gaussian displacements for vertical motion and directly confirm the proposed mechanism of Chubynsky et al. for such “diffusing diffusivity.”

## Radiative and pionic decays of heavy-light mesons using HISQ quarks

In this thesis we use the highly improved staggered quark (HISQ) formalism to study the radiative and pionic transitions of charmed mesons within the framework of lattice QCD. The HISQ action is one of the most accurate formulations of charm quarks and is a result of a perturbative Symanzik improvement program to reduce lattice discretization errors. Decay widths are calculated in numerical simulations on an ensemble of gauge field configurations with $N_f=2+1$ asqtad sea quarks generated by the MILC collaboration. In addition we study $H_s^\ast$ and charmonium radiative decays as well as meson electric form factors. Experimental measurements of the decay ratios of vector charmed $D^{\ast\pm0}$ and charmed strange $D_s^{\ast\pm}$ mesons, show a few curious features that are of great phenomenological interest in the study of low energy hadronic physics. Unlike most mesons, the strong hadronic decay modes of $D^{\ast0}$ and $D_s^{\ast\pm}$, are not dominant. However, while the neutral $D$ has a radiative mode that is competitive with its pionic mode, the charged $D$ meson's radiative decay is highly suppressed relative to that of the neutral. This suppression provides a detailed probe of strong interactions and is apparently due to an interesting near cancellation that takes place between the photon's coupling to the charm quark and to the down antiquark. The results are in agreement with all of the available experimental data, and in particular, we show that the HISQ action successfully accounts for the near cancellation of the charmed $D^{\ast\pm}$ radiative decay. The relative suppression is demonstrated in our result for the ratio of the radiative form factors of $D$ mesons $V^\pm(0)/V^0(0)=0.126(36)$ computed at heavier than physical $u/d$ quark masses. The quoted errors are purely statistical. Evidence from other lattice studies indicate small systematic errors in continuum and sea quark chiral extrapolations. Valence quark chiral extrapolation increases our errors by about 50\%. A rough extrapolation suggests an agreement with the measured radiative width within $2\sigma$.

## Variance in Initiation Factors Does Not Strongly Affect the Replication Profile of Budding Yeast DNA

DNA replication starts at many sites (origins) throughout eukaryotic DNA. To fully understand the replication program in higher organisms, one needs to understand the behaviour of these origins. In Saccharomyces cerevisiae (budding yeast), the spatial organization of the origins is simple: The origins are confined to known specific sites on the genome. The temporal behaviour of origins in budding yeast is more complex: They fire stochastically with a broad distribution of firing times. Several key proteins take part in the DNA replication process. The MCM2-7 hexamer in particular forms a helicase that unwinds DNA locally, allowing access for other proteins to replicate the separated DNA. Past analysis of the budding yeast replication program suggested the Multiple Initiator Model (MIM), which hypothesizes that the number of these MCMs loaded at an origin predicts the firing time of that origin. Part of the MIM formalism assumes that the number of loaded MCMs is large; in this case the relative fluctuations between cells will be small and are ignored. However, a recent experiment measuring the number of loaded MCMs has revealed that the number is low, and thus, cell-to-cell fluctuations may be larger than expected. The purpose of this thesis is to investigate the impact of large relative fluctuations in the number of MCMs on the MIM. To measure this effect, we built the "MIM simulator," a modular program that simulates the replication process. Although a naive argument suggests that the MIM should fail when the number of MCMs is low, the impact of these fluctuations is mitigated by the contributions from neighbouring origins. We conclude that inferences made with the MIM remain accurate in the case that the number of MCMs is lower than first assumed.

## Improvements to Modelling of Raman Scattering Intensity for Molybdenum Disulfide

Single layer molybdenum disulfide (MoS2) has attracted significant interest as a semiconducting two-dimensional material. In this work, ultrathin layers of MoS2 were exfoliated on geland wax substrates. Raman studies of the ultrathin layers of MoS2 were carried out to characterize the thickness. During Raman measurement, an anomaly occurs: the strongest Raman intensity appears at a finite thickness of the MoS2, a phenomenon also seen in graphite. A previous work [Y. Y. Wang et al., Appl. Phys. Lett. 92, 043121 (2008)] theoretically explained this unexpected phenomenon but with some questionable details. To improve their model, the complex index of refraction was properly used in the optical equations, and the assumption of optical interference of Raman signals from an infinitesimal source was added. This modified model successfully predicted the experimental results when applied to graphite and was then applied to MoS2. The simulations indicated that a constant complex index of refraction for MoS2 of 6.5-1.7i provided the best fit to published experimental results for MoS2 on SiO2/Si. This helped to estimate the thickness of ultrathin layers on gel. Seven layers MoS2 on gel showed the strongest Raman intensity according to this simulation.

## Switchable resonant hyperpolarization transfer from phosphorus-31 nuclei to silicon-29 nuclei in natural silicon

Silicon has been the backbone of the microelectronics industry for decades. As spin-based technologies continue their rapid development, silicon is emerging as a material of primary interest for a number of these applications. There are several techniques that currently exist for polarizing the spin-1/2 silicon-29 nuclei, which account for 4.7% of the isotopic makeup of natural silicon (the other two stable isotopes, silicon-28 and silicon-30, have zero nuclear spin). Polarized silicon-29 nuclei may find use in quantum computing (QC) implementations and magnetic resonance (MR) imaging. Both of these applications benefit from the extremely long T1 and T2 of the silicon-29 nuclear spins. However, the lack of interactions between the silicon-29 nuclei and their surroundings that allow for these long relaxation times also means that it is difficult to find a source of spin-polarization that effectively couples to the silicon-29 spin ensemble. We identify and exploit a field-dependent, frequency-matched resonant transfer process between phosphorus-31 donor and silicon-29 nuclear spins in natural silicon to efficiently hyperpolarize the bulk silicon-29 to over 6%. This all-optical technique requires no microwave irradiation, and the coupling can be switched off to recover the ultra-long nuclear spin relaxation lifetimes of silicon-29. This switchable hyperpolarization technique significantly enhances the usefulness of silicon-29 spins in QC and MR imaging applications.

## Cosmological constraints on new scalar gravitational interactions

Scalar-Tensor theory is a framework for modified gravity that encompasses many well-studied alternatives to General Relativity. Of particular interests are theories which possess a screening mechanism, which allows for the satisfaction of the bounds of solar system and laboratory scale tests of gravity, while giving rise to novel effects on cosmological scales. Among these are Large-Curvature f(R) as well as Chameleon, Symmetron and Dilaton gravity. All of these models can be described in terms of a recently introduced parametrization of Scalar-Tensor Theory, which involves two free functions of time alone. These models have been implemented by modifying the existing code MGCAMB. In this thesis, we discuss our implementation of these models. We present the results of Fisher forecasts for the constraints on the parameters of the 4 aforementioned models, taking the Large Synoptic Survey Telescope (LSST) and Planck as representative surveys. We also use the Principal Components Analysis approach, forecasting constraints for bins of the two functions upon discretization.