<p>The following dissertation shows the results of a series of inelastic neutron scattering experiments on the geometrically frustrated pyrochlore system Tb2Sn2-xTixO7 for x=0, 0.1, 0.2, 0.5, 1, 1.5 and 2. Inelastic neutron scattering measurements were performed on the SEQUOIA direct geometry time-of-flight spectrometer at the Spallation Neutron Source of Oak Ridge National Laboratory. For the two end members, x=0, Tb2Sn2O7 and x=2, Tb2Ti2O7 , they display related, but different exotic ground states, with Tb2Sn2O7 displaying “soft” spin ice order below T_N~0.87K, while Tb2Ti2O7 enters a glassy antiferromagnetic spin ice state below T_g~0.2K.</p> <p>The first two chapters give a brief introduction to the physics of geometrically frustrated magnetism and neutron scattering. Chapter 3 studies the two end members Tb2Ti2O7 and Tb2Sn2O7 experimentally and theoretically. Inelastic neutron scattering measurements and appropriate crystal field calculations together probe the crystal field states associated with the J=6 states of Tb^3+ within the appropriate Fd3m pyrochlore environment. These crystal field states determine the size and anisotropy of the Tb^3+ magnetic moment in each material’s ground state, information that is an essential starting point for any description of the low temperature phase behavior and spin dynamics in Tb2Ti2O7 and Tb2Sn2O7. Chapter 4 treats neutron scattering, as well as accompanying AC magnetic susceptibility and muSR measurements performed by our collaborators on a series of solid solutions Tb2Sn2-xTixO7 showing a novel, dynamic spin liquid state for all x other than the end members x=0 and x=2. This state is the result of disorder in the low lying Tb^3+ crystal field environments which de-stabilizes the mechanism by which quantum fluctuations contribute to ground state selection in Tb2Sn2-xTixO7.</p> / Master of Science (MSc)

<p> Atomically thin, semiconducting transition metal dichalogenides (TMDs), a special class of layered semiconductors, that can be shaped as a perfect two dimensional material, have garnered a lot of attention owing to their fascinating electronic properties which are achievable at the extreme nanoscale. In contrast to graphene, the most celebrated two-dimensional (2D) material thus far; TMDs exhibit a direct band gap in the monolayer regime. The presence of a non-zero bandgap along with the broken inversion symmetry in the monolayer limit brands semiconducting TMDs as the perfect candidate for future optoelectronic and valleytronics-based device application. These remarkable discoveries demand exploration of different materials that possess similar properties alike TMDs. Recently, III-VI layered semiconducting materials (example: InSe, GaSe etc.) have also emerged as potential materials for optical device based applications as, similar to TMDs, they can be shaped into a perfect two-dimensional form as well as possess a sizable band gap in their nano-regime. The perfect 2D character in layered materials cause enhancement of strong Coulomb interaction. As a result, excitons, a coulomb bound quasiparticle made of electron-hole pair, dominate the optical properties near the bandgap. The basis of development for future optoelectronic-based devices requires accurate characterization of the essential properties of excitons. Two fundamental parameters that characterize the quantum dynamics of excitons are: a) the dephasing rate, &gamma;, which represents the coherence loss due to the interaction of the excitons with their environment (for example- phonons, impurities, other excitons, etc.) and b) excited state population decay rate arising from radiative and non-radiative relaxation processes. The dephasing rate is representative of the time scale over which excitons can be coherently manipulated, therefore accurately probing the source of exciton decoherence is crucial for understanding the basic unexplored science as well as creating technological developments. The dephasing dynamics in semiconductors typically occur in the picosecond to femtosecond timescale, thus the use of ultrafast laser spectroscopy is a potential route to probe such excitonic responses. </p><p> The focus of this dissertation is two-fold: firstly, to develop the necessary instrumentation to accurately probe the aforementioned parameters and secondly, to explore the quantum dynamics and the underlying many-body interactions in different layered semiconducting materials. A custom-built multidimensional optical non-linear spectrometer was developed in order to perform two-dimensional spectroscopic (2DFT) measurements. The advantages of this technique are multifaceted compared to regular one-dimensional and non-linear incoherent techniques. 2DFT technique is based on an enhanced version of Four wave mixing experiments. This powerful tool is capable of identifying the resonant coupling, probing the coherent pathways, unambiguously extracting the homogeneous linewidth in the presence of inhomogeneity and decomposing a complex spectra into real and imaginary parts. It is not possible to uncover such crucial features by employing one dimensional non-linear technique. </p><p> Monolayers as well as bulk TMDs and group III-VI bulk layered materials are explored in this dissertation. The exciton quantum dynamics is explored with three pulse four-wave mixing whereas the phase sensitive measurements are obtained by employing two-dimensional Fourier transform spectroscopy. Temperature and excitation density dependent 2DFT experiments unfold the information associated with the many-body interactions in the layered semiconducting samples. </p>

<p> Surface plasmons are collective oscillations of free charge carriers confined in interface between two dielectrics, where the real part of the dielectric changes sign (e.g a metal-insulator interface such as gold film and air). The study of surface plasmon has been a popular research theme with potential applications utilizing the fact that the wavelength of plasmons can be many order smaller than that of the incident lights. The potential applications include transfer of information in hundreds of terahertz instead of upper limit of gigahertz in traditional wires, photodetectors with frequency range from terahertz to mid-IR, and nano-imaging. In our experiment, we use an IR near-field microscopy with resolution as low as 10nm but energy scale of micron range. This is achieved by shinning an AFM tip with infrared laser on top of the sample and collecting the scattered light from the sample. The spatial resolution proportional to where a is the size of the tip and the resolution can reach 10nm. This technique beats the diffraction limit of near-IR (10um) by over 1000x. The wavelength and amplitude damping of plasmon greatly depends on the property of free carriers in the material. While metals such as gold had been widely studied and shown promising results, a better platform with longer propagation length and shorter wavelength is needed for application. Graphenes supreme electronic transport property makes it apiii pears to be an excellent candidate for plasmonic. Graphene plasmon across a p-n junction will be discussed. Oxygen doping of graphene with different dosage via UV ozone is studied. Oxygen doping has shown promising results for graphene plasmon guide. Plasmon fringes are developed in the interior breaking the limit of boundary condition. The UV ozone treatment can be fine controlled and without damaging the graphene sheet. One can, in theory, mask and selectively dope to create a robust graphene plasmon circuit that is stable in room temperature. </p>

<p> Exchange spring based superconducting heterostructures and Josephson junctions are studied to search for evidence of odd-triplet superconductivity. Cooper pairs from a superconductor can leak into a nonhomogeneous ferromagnet a much greater distance than they leak into a homogeneous ferromagnet. This is a result of a conversion of the superconducting condensate at the superconductor-nonhomogeneous ferromagnet interface from the singlet and triplet states to the odd-triplet state. The odd-triplet state is insensitive to the exchange field of the ferromagnet. </p><p> To generate the nonhomogeneous magnetic region, an exchange spring is used. The exchange spring consists of coupled hard and soft magnetic layers that are used to produce a nonhomogeneous magnetization. The system studied consists of superconducting Niobium (Nb) and a Samarium-Cobalt/Permalloy (SmCo/Py) exchange spring.</p><p> Initial samples of Niobium had a critical temperature lower than that obtainable in our laboratory (&lt; 1.8 K). Preliminary work was done to find the cause of the suppressed critical temperature of Nb and to increase it. This work resulted in obtaining Niobium thin films with critical temperatures as high as 6 K.</p><p> Indirect evidence of the odd-triplet component is searched for by looking at the critical temperature of superconductor/exchange spring bi-layers. As the nonhomogeneity of the magnetization is increased, it is expected that the critical temperature will decrease as the condensate leaks further into the exchange spring. In Nb/Py/SmCo systems, this behavior was observed, along with a modulation in the resistance that is attributed to the anisotropic magnetoresistance of the permalloy layer. A decrease in the critical temperature with increasing nonhomogeneity of the exchange spring was also observed in Nb/SmCo/Py layers, provided the SmCo layer is not too thick.</p><p> Direct evidence of the odd-triplet component is searched for by looking at the modulation of the critical current through exchange spring based Josephson junctions as exchange spring magnetization becomes more nonhomogeneous. As the nonhomogeneity of the magnetization increases, the critical current through the junction should increase as well. Fabrication of Josephson junctions with exchange spring interlayers was performed at Oak Ridge National Laboratory, and the procedure is presented here. The critical current through these junctions was observed to increase with increasing nonhomogeneity of the exchange spring magnetization, although more tests are needed to verify this is due to the odd-triplet component of the superconducting condensate.</p>

No description available.

This thesis presents the results of muon-spin relaxation (µ⁺<abbr>SR</abbr>) studies into magnetic materials, and demonstrates how these results can be exploited to quantify the materials’ low moments and reduced dimensionality. Dipole-field simulations, traditionally used to estimate likely muon sites within a crystal structure, are described. A novel Bayesian approach is introduced which allows bounds to be extracted on magnetic moment sizes and magnetic structures—previously very difficult using µ⁺<abbr>SR</abbr>—based on reasonable assumptions about positions in which muons are likely to stop. The simulations are introduced along with relevant theory, and MµCalc, a platform-independent program which I have developed for performing the calculations is described. The magnetic ground states of the isostructural double perovskites Ba₂NaOsO₆ and Ba₂LiOsO₆ are investigated with µ⁺<abbr>SR</abbr>. In Ba₂NaOsO₆ long-range magnetic order is detected via the onset of a spontaneous muon-spin precession signal below <var>T</var>_c = 7.2(2) K, while in Ba₂LiOsO₆ a static but spatially-disordered internal field is found below 8 K. Bayesian analysis is used to show that the magnetic ground state in Ba₂NaOsO₆ is most likely to be low-moment (˜ 0.2<var>µ</var>_B) ferromagnetism and not canted antiferromagnetism. Ba₂LiOsO₆ is antiferromagnetic and a spin-flop transition is found at 5.5 T. A reduced osmium moment is common to both compounds, probably arising from a combination of spin–orbit coupling and frustration. Results are also presented from µ⁺<abbr>SR</abbr> investigations concerning magnetic ordering in several families of layered, quasi–two-dimensional molecular antiferromagnets based on transition metal ions such as <var>S</var> = ½ Cu²⁺ bridged with organic ligands such as pyrazine. µ⁺<abbr>SR</abbr> allows us to identify ordering temperatures and study the critical behaviour close to <var>T</var>_N , which is difficult using conventional probes. Combining this with measurements of in-plane magnetic exchange <var>J</var> and predictions from quantum Monte Carlo simulations allows assessment of the degree of isolation of the 2D layers through estimates of the effective inter-layer exchange coupling and in-layer correlation lengths at <var>T</var>_N. Likely metal-ion moment sizes and muon stopping sites in these materials are identified, based on probabilistic analysis of dipole-fields and of muon–fluorine dipole–dipole coupling in fluorinated materials.

<p> Micro- and nano-mechanical resonators with low mass and high vibrational frequency are often studied for applications in mass and force detection where they can offer unparalleled precision. They are also excellent systems with which to study nonlinear phenomena and fundamental physics due to the numerous routes through which they can couple to each other or to external systems. </p><p> In this work we study the structural, thermal, and nonlinear properties of various micro-mechanical systems. First, we present a study of graphene-coated silicon nitride membranes; the resulting devices demonstrate the high quality factors of silicon nitride as well as the useful electrical and optical properties of graphene. We then study nonlinear mechanics in pure graphene membranes, where all vibrational eigenmodes are coupled to one another through the membrane tension. This effect enables coherent energy transfer from one mechanical mode to another, in effect creating a graphene mechanics-based frequency mixer. In another experiment, we measure the resonant frequency of a graphene membrane over a wide temperature range, 80K - 550K, to determine whether or not it demonstrates the negative thermal expansion coefficient predicted by prevailing theories; our results indicate that this coefficient is positive at low temperatures &ndash; possibly due to polymer contaminants on the graphene surface &ndash; and negative above room temperature. Lastly, we study optically-induced self-oscillation in metal-coated silicon nitride nanowires. These structures exhibit self-oscillation at extremely low laser powers (~1&mu;W incident on the nanowire), and we use this photo-thermal effect to counteract the viscous air-damping that normally inhibits micro-mechanical motion.</p>

<p> The Moore-Read Pfaffian and anti-Pfaffian states have been under scrupulous review as candidates which describe the fractional quantum Hall effect at filling factor 5/2. Quantum states in the universality class of the Moore-Read Pfaffian/anti-Pfaffian have non-trivial intrinsic topological order and support low-energy non-Abelian excitations that have applications in fault-tolerant topological quantum computing schemes. Both states are exact ground states of three-body Hamiltonians that explicitly break particle-hole symmetry. We study the topological order of a competing ansatz state &PSgr;₂ that is the exact ground state of a two-body Hamiltonian that preserves particle-hole symmetry. In particular, we calculate the bipartite entanglement entropy and spectra in the lowest Landau level in the spherical geometry for &PSgr;₂. We perform such calculations for a finite number of electrons up to 14. We then extrapolate to the thermodynamic limit the topological entanglement entropy &gamma; as a measure of the topological order of the ansatz and compare to the known value of the Moore-Read Pfaffian/anti-Pfaffian state. We also study the orbital entanglement spectra for &PSgr;₂ and compare with the Moore-Read Pfaffian and two-body Coulomb ground states. We show that our extrapolation of &gamma; lies within the uncertainty of the known value of &gamma; for the Moore-Read Pfaffian state, and that the orbital entanglement spectra of &PSgr;₂ assumes a similar structure to that of the two-body Coulomb interaction.</p><p>

<p> Superconducting pairs are able to leak into non-superconducting materials when placed in close proximity. In the presence of ferromagnetism pair correlations are modified by the magnetization; singlet Cooper pairs transform into a mixture of singlet and triplet correlations. In this work we analyze how pair correlations are modified in a magnetic Josephson junction in the clean limit, and consider the effect of different magnetic configurations. We use a tight-binding Hamiltonian and the Bogoliubov-de Gennes(BdG) formalism, to describe the proximity system. Applying the Bogoliubov-Valatin transformation we generate the BdG equations in matrix form. We use an iterative process to diagonalize the matrix together with solving the self-consistency relation for the pair potential &Delta; numerically. From the solution we construct Gor'kov functions which are used to describe the pair amplitudes and Josephson current through the junction. Taking the simplest case first we apply our method to a normal metal Josephson junction and match our model to known results. We then apply it to a homogeneous magnetic Josephson junction and investigate how certain parameters such as magnetization and temperature affect the properties of the junction. Finally our methodology is applied to an inhomogeneous magnetic Josephson junction, to study the differing effects between gradual and abrupt changes in the magnetization on the pair correlations.</p>

<p>This thesis aims to build upon the previous work done on the magnetic relaxation of iron(II) phthalocyanine (FePc) thin films by exploring the dynamic aspects of coercive fields in order to determine whether FePc can be classified as a low-dimensional material known as a single chain magnet. In thin films, the chain length is controlled by deposition temperature and therefore systematic studies of the chain-length dependent properties can be made. Hysteresis loops of FePc thin-films with five different chain lengths ranging from approx. 30 nm to 300 nm were measured at a range of sweep speeds from 10.4 mT/s to 1.07 mT/s. Each measurement was repeated at 5 different temperatures in the interval from 2.5 K to 3.8 K, where hysteresis was observed. Significant reductions in coercivity with slower sweep speeds reveal the non-equilibrium behavior of the magnetic states. Mean-field theory based on one-dimensional chains within a Glauber-Ising model suggests a power law behavior of coercivity with sweep rate. Indeed all experimental data is consistent with that behavior. The critical exponent varies from 0.521 to 0.153 for short to long chains. Given the limited observational window, coercivity due to inter-chain coupling cannot fully be ruled out, yet a large dynamic response in the coercivity supports the notion of a single chain magnet.