Physics Commons^™

Quantum Phases And Phase Transitions In Designer Spin Models, Nisheeta Desai

Theses and Dissertations--Physics and Astronomy

This work focuses on numerical studies of quantum spin systems. These simple models are known to exhibit a variety of phases, some of which have no classical counterpart. Phase transitions between them are driven by quantum fluctuations and the unconventional nature of some such transitions make them a fascinating avenue of study.

Quantum Monte Carlo (QMC) is an indispensable tool in the study of these phases and phase transitions in two and higher dimensions. Nevertheless, we are limited by our inability to simulate models that suffer from the infamous sign problem. While the case of S=1/2 has been studied …

The Pion Form Factor And Momentum And Angular Momentum Fractions Of The Proton In Lattice Qcd, Gen Wang

Theses and Dissertations--Physics and Astronomy

Lattice Quantum Chromodynamics (QCD) provides a way to have a precise calculation and a new way of understanding the hadrons from first principles. From this perspective, this dissertation focuses first on a precise calculation of the pion form factor using overlap fermions on six ensembles of 2+1-flavor domain-wall configurations generated by the RBC/UKQCD collaboration with pion masses varying from 137 to 339 MeV. Taking advantage of the fast Fourier transform, low-mode substitution (LMS) and the multi-mass algorithm to access many combinations of source and sink momenta, we have done a simulation with various valence quark masses and with a range …

Theoretical Studies Of C And Cp Violation In $\Eta \To \Pi^+ \Pi^- \Pi^0$ Decay, Jun Shi

Theses and Dissertations--Physics and Astronomy

A violation of mirror symmetry in the $\eta\to\pi^+\pi^-\pi^0$ Dalitz plot has long been recognized as a signal of C and CP violation. In this thesis, we show how the isospin of the underlying C- and CP-violating structures can be reconstructed from their kinematic representation in the Dalitz plot. Our analysis of the most recent experimental data reveals, for the first time, that the C- and CP-violating amplitude with total isospin I = 2 is much more severely suppressed than that with total isospin I = 0.

In searching for C- and CP-violating sources beyond the SM, we enumerate the leading-dimension, …

Effects Of Aperiodicity And Frustration On The Magnetic Properties Of Artificial Quasicrystals, Barry Farmer

Theses and Dissertations--Physics and Astronomy

Quasicrystals have been shown to exhibit physical properties that are dramatically different from their periodic counterparts. A limited number of magnetic quasicrystals have been fabricated and measured, and they do not exhibit long-range magnetic order, which is in direct conflict with simulations that indicate such a state should be accessible. This dissertation adopts a metamaterials approach in which artificial quasicrystals are fabricated and studied with the specific goal of identifying how aperiodicity affects magnetic long-range order. Electron beam lithography techniques were used to pattern magnetic thin films into two types of aperiodic tilings, the Penrose P2, and Ammann-Beenker tilings. SQUID …

Graphene In A Uniform Magnetic Field, Ankur Das

Theses and Dissertations--Physics and Astronomy

We study monolayer graphene in a uniform magnetic field in the absence and presence of interactions. In the non-interacting limit, for p/q flux quanta per unit cell (p, q are coprime integer), the central two bands have 2q Dirac points in the Brillouin zone (BZ) in the nearest-neighbor model. These touchings and their locations are guaranteed by chiral symmetry and the lattice symmetries of the honeycomb structure. If we add a staggered potential and a next-nearest-neighbor hopping we find that their competition leads to a topological phase transition. We also study the stability of the Dirac touchings to one-body perturbations …

Sensitivity Of Electron-Proton Coincidence Asymmetries In Neutron Beta-Decay To Scalar And Tensor Interactions, Subash C. Nepal

Theses and Dissertations--Physics and Astronomy

We study the combined sensitivity of measurements of electron-proton coincidence asymmetries in polarized neutron beta-decay together with a measurement of the electron energy spectrum in unpolarized neutron beta-decay to beyond Standard Model (BSM) scalar and tensor interactions, via the appearance of such BSM physics in the Fierz interference terms b and b_v. Whereas measurements of the electron energy spectrum directly probe b, both the proton and neutrino asymmetries for which experimental results exist are not sensitive to b_v, but effectively to b - b_v. This results in reduced sensitivity to BSM scalar and …

Study Of Cell Charging Effects For The Neutron Electric Dipole Moment Experiment At Oak Ridge National Laboratory, Mark Broering

Theses and Dissertations--Physics and Astronomy

The neutron electric dipole moment (nEDM) collaboration at the Spallation Neutron Source plans to use ultra-cold neutrons in superfluid helium to improve the nEDM limit by about two orders of magnitude. In this apparatus, neutrons are stored in poly(methyl methacrylate), PMMA, cells located in a strong, stable electric field. This electric field is produced by high voltage electrodes located outside of the neutron cells. Several sources generate charged particles inside the neutron cells. The electric field pulls these charges farther apart, attracting each to the oppositely charged electrode. As the charges build up on the cells walls, they create an …

Transverse And Longitudinal Thermal Diffusivity Measurements Of Polymer And Small Molecule Organic Semiconductors With Different Techniques, Maryam Shahi

Theses and Dissertations--Physics and Astronomy

The main thrust of this research was to develop new probes to measure thermal conductivities (κ) of small-molecule crystals, as well as polymer blends of organic semiconductors, both to screen these for possible applications, e.g. as thermoelectric power generators, and to gain an understanding of thermal transport in them. Emphasis has been on the crystals of “TIPS pentacene” [TIPS = 6,13 bis(triisopropylsilylethynyl), and free-standing films of PEDOT:PSS [poly(3,4-ethylenedioxythiophene) polystyrene sulfonate] for different electrochemical and thermoelectric applications. Separate techniques were used for in-plane and transverse thermal conductivities in which 𝜅𝜅 is determined indirectly from measurements of the thermal diffusivity (D ≡ …

Large N Fields And Holography, Animik Ghosh

Theses and Dissertations--Physics and Astronomy

We study (nearly) AdS/CFT holography within the context of the Sachdev-Ye- Kitaev (SYK) model. We present a systematic procedure to extract the dynamics of the low energy Schwarzian mode in SYK type models with a single energy scale J and emergent reparametrization symmetry in the infrared within the framework of perturbation theory. We develop a systematic approach using Feynman diagrams in bilocal theory to obtain a formal expression for the enhanced and O(1) corrections to the bilocal propagator and apply this general technique to large q SYK. We show that the Schwarzian theory describes a sector of a general class …

A Theoretical And Experimental Study Of Charge Transport In Organic Thermoelectric Materials And Charge Transfer States In Organic Photovoltaics, Ashkan Abtahi

Theses and Dissertations--Physics and Astronomy

Applications of organic electronics have increased significantly over the past two decades. Organic semiconductors (OSC) can be used in mechanically flexible devices with potentially lower cost of fabrication than their inorganic counterparts, yet in many cases organic semiconductor-based devices suffer from lower performance and stability. Investigating the doping mechanism, charge transport, and charge transfer in such materials will allow us to address the parameters that limit performance and potentially resolve them. In this dissertation, organic materials are used in three different device structures to investigate charge transport and charge transfer. Chemically doped π-conjugated polymers are promising materials to be used …

A Detection And Data Acquisition System For Precision Beta Decay Spectroscopy, Aaron P. Jezghani

Theses and Dissertations--Physics and Astronomy

Free neutron and nuclear beta decay spectroscopy serves as a robust laboratory for investigations of the Standard Model of Particle Physics. Observables such as decay product angular correlations and energy spectra overconstrain the Standard Model and serve as a sensitive probe for Beyond the Standard Model physics. Improved measurement of these quantities is necessary to complement the TeV scale physics being conducted at the Large Hadron Collider. The UCNB, ⁴⁵Ca, and Nab experiments aim to improve upon existing measurements of free neutron decay angular correlations and set new limits in the search for exotic couplings in beta decay. To …

Effect Of Hydrogen Exposure On The Electronic And Optical Properties Of Insulating Titanates, John G. Connell

Theses and Dissertations--Physics and Astronomy

Hydrogen exposure of insulating d⁰-titanates, such as SrTiO₃ (STO), has displayed the formation of intriguing conducting states. These conducting states form through the use of forming gas (N₂/H₂) annealing or hydrogen plasma exposure, where hydrogen gas is exposed to high energy microwaves. The exposure of STO to hydrogen causes metallic conductivity due to the introduction of hydrogen cations on some of the oxygen sites. However, the optical properties of this hydrogen-exposed STO have not been well-studied. Further, Ba_0.5Sr_0.5TiO₃ (BST), an insulating dielectric, also shows changes in its conductivity …

Scanning Probe Microscopy Measurements On 2d Materials And Iridates, Armin Ansary

Theses and Dissertations--Physics and Astronomy

In the past two decades, there has been a quest to understand and utilize novel materials such as iridates and two-dimensional (2D) materials. These classes of materials show a lot of interesting properties both in theoretical predictions as well as experimental results. Physical properties of some of these materials have been investigated using scanning probe measurements, along with other techniques.

One-dimensional (1D) catalytic etching was investigated in few-layer hexagonal boron nitride (hBN) films. Etching of hBN was shown to share several similarities with that of graphitic films. As in graphitic films, etch tracks in hBN commenced at film edges and …

Magnetic Field Monitoring In The Sns Neutron Edm Experiment, Alina Aleksandrova

Theses and Dissertations--Physics and Astronomy

It is a well known fact that the visible universe is made almost entirely of baryonic matter. Yet, this is also one of the greatest puzzles that physicists are trying to solve: Where did all of this matter come from in the first place? The Standard Model (SM) of particle physics predicts a baryon asymmetry that is much smaller than what is observed in nature. In order to try and explain this discrepancy, Sakharov (1967) postulated three necessary conditions for baryogenesis in the early universe. One of these is the requirement that charge conjugation (C) and the product of C …

Prompt Fission Neutron Energy Spectrum Of N+235U, Jason M. Mcginnis

Theses and Dissertations--Physics and Astronomy

Despite nuclear fission prominence in nuclear physics, there are still several fundamental open questions about this process. One uncertainty is the energy distribution of neutrons emitted immediately after fission. In particular the relative energy distribution of neutrons above 8~MeV has been difficult to measure. This experiment measured the prompt neutron energy spectrum of n+²³⁵U from 3-10~MeV. The measurement took place at Los Alamos National Laboratory (LANL) and used a double time-of-flight technique to measure both the beam and fission neutron kinetic energies. Fission event timing was measured with a parallel plate avalanche counter. The fission neutron time-of-flight was …

Electronic Properties Of Atomically Thin Material Heterostructures, M. Javad Farrokhi

Theses and Dissertations--Physics and Astronomy

There is a movement in the electronic industry toward building electronic devices with dimensions smaller than is currently possible. Atomically thin 2D material, such as graphene, bilayer graphene, hBN and MoS2 are great candidate for this goal and they have a potential set of novel electronic properties compare to their bulk counterparts due to the exhibition of quantum confinement effects. To this goal, we have investigated the electric field screening of multilayer 2D materials due to the presence of impurity charge in the interface and vertical electric fifield from back gate. Our result shows a dramatic difference of screening behavior …

Determination Of The Neutron Beta-Decay Asymmetry Parameter A Using Polarized Ultracold Neutrons, Michael A.-P. Brown

Theses and Dissertations--Physics and Astronomy

The UCNA Experiment at the Los Alamos Neutron Science Center (LANSCE) is the first measurement of the β-decay asymmetry parameter A₀ using polarized ultracold neutrons (UCN). A₀ , which represents the parity-violating angular correlation between the direction of the initial neutron spin and the emitted decay electron’s momentum, determines λ = g_A /g_V , the ratio of the weak axial-vector and vector coupling constants. A high-precision determination of λ is important for weak interaction physics, and when combined with the neutron lifetime it permits an extraction of the CKM matrix element V_ud solely …

Η' Decay To Π+Π-Π+Π−, Ehsan Jafari

Theses and Dissertations--Physics and Astronomy

With the use of chiral theory of mesons [1], [2] we evaluate the decay rate of η′ → π⁺π⁻π⁺π⁻. Our theoretical study of this problem is different from the previous theo- retical study [3] and our predicted result is in a good agreement with the experiment. In this chiral theory we evaluate Feynman diagrams up to one loop and the decay rate is calculated with the use of triangle and box diagrams. The ρ⁰ meson includes in both type of diagrams as a resonance state. Divergent integrals in the loop calculations …

Electronic And Optical Properties Of Metastable Epitaxial Thin Films Of Layered Iridates, Maryam Souri

Theses and Dissertations--Physics and Astronomy

The layered iridates such as Sr₂IrO₄ and Sr₃Ir₂O₇, have attracted substantial attention due to their novel electronic states originating from strong spin-orbit coupling and electron-correlation. Recent studies have revealed the possibilities of novel phases such as topological insulators, Weyl semimetals, and even a potential high-T_c superconducting state with a d-wave gap. However, there are still controversial issues regarding the fundamental electronic structure of these systems: the origin of the insulating gap is disputed as arising either from an antiferromagnetic ordering, i.e. Slater scheme or electron-correlation, i.e. Mott scheme. …

Probing The Low-X Gluon Helicity Distribution With Dijet Double Spin Asymmetries In Polarized Proton Collisions At √S = 510 Gev, Suvarna Ramachandran

Theses and Dissertations--Physics and Astronomy

The proton is a complex subatomic particle consisting of quarks and gluons, and one of the key questions in nuclear physics is how the spin of the proton is distributed amongst its constituents. Polarized deep inelastic scattering experiments with leptons and protons estimate that the quark spin contribution is approximately 30%. The limited kinematic reach of these experiments, combined with the fact that they are only indirectly sensitive to the electrically neutral gluon, means they can provide very little information about the gluon contribution to the spin of the proton. In contrast, hadronic probes, such as polarized proton collisions provide …

Studies Of Magnetically Induced Faraday Rotation By Polarized Helium-3 Atoms, Joshua Abney

Theses and Dissertations--Physics and Astronomy

Gyromagnetic Faraday rotation offers a new method to probe limits on properties of simple spin systems such as the possible magnetic moment of asymmetric dark matter or as a polarization monitor for polarized targets. Theoretical calculations predict the expected rotations of linearly polarized light due to the magnetization of spin-1/2 particles are close to or beyond the limit of what can currently be measured experimentally (10⁻⁹ rad). So far, this effect has not been verified. Nuclear spin polarized ³He provides an ideal test system due to its simple structure and ability to achieve high nuclear spin polarization via …

Nanoscale Devices Consisting Of Heterostructures Of Carbon Nanotubes And Two-Dimensional Layered Materials, Mohsen Nasseri

Theses and Dissertations--Physics and Astronomy

One dimensional carbon nanotubes (CNTs) and two-dimensional layered materials like graphene, MoS₂, hexagonal boron nitride (hBN), etc. with different electrical and mechanical properties are great candidates for many applications in the future. In this study the synthesis and growth of carbon nanotubes on both conducting graphene and graphite substrates as well as insulating hBN substrate with precise crystallographic orientation is achieved. We show that the nanotubes have a clear preference to align to specific crystal directions of the underlying graphene or hBN substrate. On thicker flakes of graphite, the edges of these 2D materials can control the orientation …

Structural, Transport, And Topological Properties Induced At Complex-Oxide Hetero-Interfaces, Justin K. Thompson

Theses and Dissertations--Physics and Astronomy

Complex-oxides have seen an enormous amount of attention in the realm of Condensed Matter Physics and Materials Science/Engineering over the last several decades. Their ability to host a wide variety of novel physical properties has even caused them to be exploited commercially as dielectric, metallic and magnetic materials. Indeed, since the discovery of high temperature superconductivity in the “Cuprates” in the late 1980’s there has been an explosion of activity involving complex-oxides. Further, as the experimental techniques and equipment for fabricating thin films and heterostructures of these materials has improved over the last several decades, the search for new and …

Magnetic Field Design To Reduce Systematic Effects In Neutron Electric Dipole Moment Measurements, James Ryan Dadisman

Theses and Dissertations--Physics and Astronomy

Charge-Conjugation (C) and Charge-Conjugation-Parity (CP) Violation is one of the three Sakharov conditions to explain via baryogenesis the observed baryon asymmetry of the universe (BAU). The Standard Model of particle physics (SM) contains sources of CP violation, but cannot explain the BAU. This motivates searches for new physics beyond the standard model (BSM) which address the Sakharov criteria, including high-precision searches for new sources of CPV in systems for which the SM contribution is small, but larger effects may be present in BSM theories. A promising example is the search for the electric dipole moment of the neutron (nEDM), which …

Neutron-Antineutron Transitions: Exploring B – L Violation With Quarks, Xinshuai Yan

Theses and Dissertations--Physics and Astronomy

In the Standard Model (SM), the quantity baryon number (B) − lepton number (L), B − L, is perfectly conserved. Therefore, the observation of B − L violation would reveal the existence of physics beyond the SM. Traditionally, given the severe experimental constraints on |∆B| = 1 processes, B − L violation with baryons is probed via neutron-antineutron (n − n̄) oscillations, although this process suffers from quenching in the presence of external fields or matter.

In this dissertation, we discuss another possibility, n − n̄ conversion, in which the |∆ …

Deconfined Quantum Criticality In 2d Su(N) Magnets With Anisotropy, Jonathan D'Emidio

Theses and Dissertations--Physics and Astronomy

In this thesis I will outline various quantum phase transitions in 2D models of magnets that are amenable to simulation with quantum Monte Carlo techniques. The key player in this work is the theory of deconfined criticality, which generically allows for zero temperature quantum phase transitions between phases that break distinct global symmetries. I will describe models with different symmetries including SU(N), SO(N), and "easy-plane" SU(N) and I will demonstrate how the presence or absence of continuous transitions in these models fits together with the theory of deconfined criticality.

Thermal Conductivities Of Organic Semiconductors, Yulong Yao

Theses and Dissertations--Physics and Astronomy

Organic semiconductors have gained a lot of interest due to their ease of processing, low-cost and inherent mechanical flexibility. Although most of the research has been on their electronic and optical properties, knowledge of the thermal properties is important in the design of electronic devices as well. Our group has used ac-calorimetric techniques to measure both in-plane and transverse thermal conductivities of a variety of organic semiconductors including small-molecule crystals and polymer blends. For layered crystals composed of molecules with planar backbones and silylethynyl (or germylethynyl) sidegroups projecting between the layers, very high interplanar thermal conductivities have been observed, presumably …

Disconnected-Sea Quarks Contribution To Nucleon Electromagnetic Form Factors, Raza Sabbir Sufian

Theses and Dissertations--Physics and Astronomy

We present comprehensive analysis of the light and strange disconnected-sea quarks contribution to the nucleon electric and magnetic form factors. The lattice QCD estimates of strange quark magnetic moment G^s_M (0) = −0.064(14)(09) μ_N and the mean squared charge radius ⟨r²_s⟩_E = −0.0043(16)(14) fm² are more precise than any existing experimental measurements and other lattice calculations. The lattice QCD calculation includes ensembles across several lattice volumes and lattice spacings with one of the ensembles at the physical pion mass. We have performed a simultaneous chiral, infinite volume, and continuum extrapolation in …

A First Experimental Limit On The Relative Rates Of Muon Capture On Deuterium From The Quartet And Doublet Hyperfine Spin States Of The Μd Atom, Ray Kreswell Neely

Theses and Dissertations--Physics and Astronomy

The MuSun experiment will determine the muon capture rate on deuterium (µ− + d → n + n + ν_µ) from the doublet hyperfine spin state, Λ_d, of muonic deuterium to a precision of 1.5%. Muon capture can occur from either the quartet or doublet state of the 1S orbital of the µd atom; however, the V-A nature of the process strongly suppresses the rate of capture from the quartet state, Λ_q. Muons in ultrapure deuterium gas may also catalyze d+d → ³He+n fusion through the formation …

Studying Transverse Momentum Dependent Distributions In Polarized Proton Collisions Via Azimuthal Single Spin Asymmetries Of Charged Pions In Jets, James Kevin Adkins

Theses and Dissertations--Physics and Astronomy

A complete, fundamental understanding of the proton must include knowledge of the underlying spin structure. The transversity distribution, h₁(x), which describes the transverse spin structure of quarks inside of a transversely polarized proton, is only accessible through channels that couple h₁(x) to another chiral odd distribution, such as the Collins fragmentation function (Δ^N D_π/q^↑(z,j_T)). Significant Collins asymmetries of charged pions have been observed in semi-inclusive deep inelastic scattering (SIDIS) data. These SIDIS asymmetries combined with e⁺e^- process asymmetries have allowed for …