<p>We report low temperature specific heat and positive muon spin rotation measurements of both polycrystal and single crystal Yb2Ti2O7. Our zero field (ZF) measurements show little spin relaxation temperature dependence in the polycrystal Yb2Ti2O7, contrast to previously reported results. We observe no collinear ferromagnetic order, rather a hidden order ground state where spin fluctuations remain dynamic down to 16 mK. Single crystal Yb2Ti2O7 zero field measurements with the crystallographic [111] direction parallel to the initial muon polarization show small but measurable temperature dependence. In addition, our transverse field measurements show the spin susceptibility undergoes a distinct change at temperatures corresponding to the magnetic transition measured in the specific heat.</p> / Master of Science (MSc)

<p> This dissertation is an account of the first calorimetric detection of individual He*₂ excimers within a bath of superfluid ⁴He. When superfluid helium is subject to ionizing radiation, diatomic He molecules are created in both the singlet and triplet states. The singlet He molecules decay within nanoseconds, but due to a forbidden spin-flip the triplet molecules have a relatively long lifetime of 13 seconds in superfluid He. When He*₂ molecules decay, they emit a ~15 eV photon. Nearly all matter is opaque to these vacuum-UV photons, although they do propagate through liquid helium. The triplet state excimers propagate ballistically through the superfluid until they quench upon a surface; this process deposits a large amount of energy into the surface. The prospect of detecting both excimer states is the motivation for building a detector immersed directly in the superfluid bath.</p><p> The detector used in this work is a single superconducting titanium transition edge sensor (TES). The TES is mounted inside a hermetically sealed chamber at the baseplate of a dilution refrigerator. The chamber contains superfluid helium at 100 mK. Excimers are created during the relaxation of high-energy electrons, which are introduced into the superfluid bath either in situ via a sharp tungsten tip held above the field-emission voltage, or by using an external gamma-ray source to ionize He atoms. These excimers either propagate through the LHe bath and quench on a surface, or decay and emit vacuum-ultraviolet photons that can be collected by the detector.</p><p> This dissertation discusses the design, construction, and calibration of the TES-based excimer detecting instrument. It also presents the first spectra resulting from the direct detection of individual singlet and triplet helium excimers.</p>

<p> Engineered quantum systems are poised to revolutionize information science in the near future. A persistent challenge in applied quantum technology is creating controllable, quantum interactions while preventing information loss to the environment, decoherence. In this thesis, we realize mesoscopic superconducting circuits whose macroscopic collective degrees of freedom, such as voltages and currents, behave quantum mechanically. We couple these mesoscopic devices to microwave cavities forming a cavity quantum electrodynamics (QED) architecture comprised entirely of circuit elements. This application of cavity QED is dubbed Circuit QED and is an interdisciplinary field seated at the intersection of electrical engineering, superconductivity, quantum optics, and quantum information science. Two popular methods for taming active quantum systems in the presence of decoherence are discrete feedback conditioned on an ancillary system or quantum reservoir engineering. Quantum reservoir engineering maintains a desired subset of a Hilbert space through a combination of drives and designed entropy evacuation. Circuit QED provides a favorable platform for investigating quantum reservoir engineering proposals. A major advancement of this thesis is the development of a quantum reservoir engineering protocol which maintains the quantum state of a microwave cavity in the presence of decoherence. This thesis synthesizes strongly coupled, coherent devices whose solutions to its driven, dissipative Hamiltonian are predicted a <i>priori</i>. This work lays the foundation for future advancements in cavity centered quantum reservoir engineering protocols realizing hardware efficient circuit QED designs. </p>

<p>Transition metal oxides (TMOs) with face centered cubic arrangement of magnetic ions are composed of triangular sub-lattices. When antiferromagnetic (AFM) interactions of the same strengths between all three pathways in triangular settings are in place, spin constraints cannot be fulfilled simultaneously and the system undergoes geometric magnetic frustration (GMF). The purpose of the work presented in this thesis is to better understand the criteria for a system to undergo GMF. To achieve this, the novel B-site ordered double perovskites Ca2ScOsO6 and Ca2.2Mg0.8OsO6 were synthesized in polycrystalline form utilizing the conventional solid-state method. The crystal structure of these compounds were characterized through X-ray diffraction, and magnetic properties were explored through magnetic susceptibility measurements. Employing the spin-dimer analysis method, relative magnetic exchange interactions were calculated and modeled. These novel osmium-based B-site ordered double perovskites were then compared to isostructural compounds to study the effects of the osmium oxidation state on crystal structure and the exhibited properties.

<p>This thesis presents a first principle approach to model helium diffusionand retention in molybdenum. Results from electron structure calculations within the framework of density functional theory are used to assess parameters in a rate theory model. The model is used to reproduce experimental desorption spectra, which, to a large degree of accuracy, coincide with experimental data in temperature regions relevant for nuclear fuel applications. The models indicate that produced helium will diffuse out into the fuel pin during operation. However, some helium will be trapped in molybdenum vacancies. The amount of trapped helium will largely depend on the fuel operational temperature.</p><p>Data presented in the thesis is a first step towards a self consistent dataset of first principle data on helium diffusion in CERMET fuel, one candidate fuel suggested for transmutation of nuclear waste. To realise the use of CERMET fuel, modelling of fuel performance is essential, and to accomplish this, the understanding of helium diffusion and retention in molybdenum is one important aspect.</p><p> </p>

No description available.

The synthesis of type-enriched semiconducting or metallic single-walled carbon nanotubes (sc/m-SWCNTs) with high quality by means of catalytic chemical vapor deposition (CCVD) are essential prerequisites for implementing of SWCNTs into nanodevices. In particular, the Fe−Co bimetallic catalysts system is promising due to its ability to grow SWCNTs by CCVD. However, there is still a gap in understanding how to adjust catalyst composition aiming further improvements in SWCNTs properties with respect to the electronic type. In particular, formation of well-defined nanoalloy of bimetallic catalysts during catalyst conditioning as well as its impact on SWCNTs growth are not clearly understood. Here we present a systematic investigation on effects of catalyst composition based on different Molar ratios of Fe:Co catalysts on the properties of CCVD grown SWCNTs using Raman spectroscopy. After CVD growth of SWCNTs on different molar ratios of Fe:Co, We find that by using molar ratio of Fe:Co=1:1.5, growth of SWNTs that are strongly dominated by two types of semiconducting tubes, the (7,5) and (12,1) tubes, can be achieved.

This thesis consists of two distinct parts. The first part, based on the first two accompanied papers, is in the field of topological phases of matter and the second part, based on the third accompanied paper, looks at a problem in the field of quantum graphs, a rapidly growing field of mathematical physics. First, we investigate the entanglement property of the Laughlin state by looking at the rank of the reduced density operator when particles are divided into two groups. We show that the problem of determining this rank translates itself into a  question about symmetric polynomials, namely, one has to determine the lower bound for the degree in each variable of the symmetric polynomials that vanish under a transformation that clusters the particles into groups of equal size and then brings the particles in each group together. Although we were not able to prove this, but we were able to determine the lower bound for the total degree of symmetric polynomials that vanish under the  transformation described. Moreover, we were able to characterize all symmetric polynomials that vanish under this transformation. In the second paper, we introduce a one-dimensional model of interacting su(2)k anyons. The specific feature of this model is that, through pairing terms present in the Hamiltonian,  the number of anyons of the chain can fluctuate. We also take into account the possibility that anyons hop to empty neighboring sites. We investigate the model in five different points of the parameter space. At one of these points, the Hamiltonian of the model becomes a sum of projectors and we determine the explicit form of all the zero-energy ground states for odd values of k. At the other four points, the system is integrable and we determine the behavior of the model at these integrable points. In particular, we show that the system is critical and determine the CFT describing the system at these points. It is known that there are non-Hermitian Hamiltonians whose spectra are entirely real. This property can be understood in terms of a certain symmetry of these Hamiltonians, known as PT-symmetry. It is also known that the spectrum of a non-Hermitian PT-symmetric Hamiltonian has reflection symmetry with respect to the real axis. We then ask the reverse question whether or not the reflection symmetry of a non-Hermitian Hamiltonian necessarily implies that the Hamiltonian is PT-symmetric. In the context of quantum graphs, we introduce a model for which the answer to this question is positive.

<p> This work contributes to the study of magnetic interactions in the low-dimensional antiferromagnets <i>M</i>(Sb,Ta)₂O₆, where <i> M</i> is a transition metal. By virtue of the trirutile structure, <i> M-O-O-M</i> chains propagate along [110] at <i>z</i> = 0 and [1<span style="text-decoration:overline">1</span>0] at <i>z</i> = 1/2 of the unit cell. These chains are separated along [001] by sheets of weakly-interacting diamagnetic ions. The spin-exchange coupling perpendicular to the chains is weak, permitting the low-dimensional classification. Single crystals have been grown using chemical vapor deposition and the floating zone method. Magnetization, in-field heat capacity, and high-resolution thermal expansion measurements have been performed along various axes, revealing significant anisotropy due to the peculiar magnetic structures and low dimensionality.</p><p> The Neel temperature, <i>T_N,</i> at which long-range order occurs is found to be unstable against the application of magnetic field above 2 T. Large fields tend to lower <i>T_N</i> of the set of moments with projections along the applied field. Moments which are aligned perpendicular to the field are significantly less affected. This can lead to the formation of a secondary peak in heat capacity when magnetic field is along either [110] or [1<span style="text-decoration:overline">1</span>0]. The change in heat capacity at the location of the newly formed peak means there is a change in entropy, which depends upon the direction of applied field with respect to the magnetic moments. Consequently, an anisotropic magnetocaloric effect arises due to the unique magnetic structure. The anisotropic nature of this effect has potential applications in magnetic refrigeration.</p>

This thesis reports magnetotransport measurements in graphene Hall bar devices. Graphene samples fabricated from different techniques (epitaxial growth on silicon carbide, exfoliation, and CVD) are measured and compared. Measurements are taken primarily using a 21T magnet, at liquid Helium 4 temperatures. The first three chapters present the background for the work. Chapter One details the motivation for the thesis, and gives a general background to carbon and the state of carbon research. Chapter Two covers the theoretical background of graphene, including the anomalous quantum Hall effect and weak localization. Chapter Three covers the synthesis of graphene and a typical procedure undertaken for device fabrication. The next three chapters report experimental results. Chapter Four presents measurements of the energy loss rates in exfoliated graphene. The mechanism of carrier energy loss is investigated, and compared to theory. Further, the breakdown of the quantum Hall effect in the device is investigated, demonstrating peak current densities far in excess of those found in the literature for exfoliated graphene. Chapter Five shows measurements comparing the carrier energy loss rates in graphene derived from the epitaxial, exfoliated and CVD fabrication techniques. An unconventional method for measuring the energy loss rate based on measuring the weak localization peak is developed, and trends in the energy loss rates with carrier density are investigated for a wide range of devices. Chapter Six reports a comparison of the decomposed weak localization scattering lengths from graphene devices derived from the epitaxial and CVD methods, and compares these to measurements from the literature. Further, a previously reported saturation of the weak localization in graphene is investigated, and demonstrated to be an experimental artefact. This thesis provides a development of the understanding, and an experimental verification, of several aspects of heat transfer in graphene. An understanding of heat transfer is of critical importance to proposed high-density nano-electronics, and bolometry applications. The high breakdown currents and observed trends in carrier density are also of significant assistance in the design of low-cost resistance metrology devices based on graphene.