Skyrmions And Biskyrmions In Magnetic Films, 2021 The Graduate Center, City University of New York

#### Skyrmions And Biskyrmions In Magnetic Films, Daniel Capic

*Dissertations, Theses, and Capstone Projects*

Skyrmions have garnered significant attention in condensed matter systems in recent years. In principle, they are topologically protected, so there is a large energy barrier preventing their annihilation. Furthermore, they can exist at the nanoscale, be manipulated with very small currents, and be created by a number of different methods. This makes them attractive for use in potential computing applications. This work studies ferromagnetic skyrmions. In particular, it highlights our small contributions to the field of skyrmions in condensed matter systems, specifically in thin-film ferromagnets.

Development Of A Diamond Defect Quantum Sensing Platform For Probing Novel Quantum Magnetic Phases, 2021 Washington University in St. Louis

#### Development Of A Diamond Defect Quantum Sensing Platform For Probing Novel Quantum Magnetic Phases, Jeffrey Ahlers

*Senior Honors Papers / Undergraduate Theses*

Nitrogen vacancy centers in diamond are highly effective quantum sensors due to their high spatial resolution and high magnetic field sensitivity. We present the construction of an optically detected magnetic resonance platform in order to facilitate the probing of magnetic phases in two-dimensional heterostructures. This includes the characterization of the required microwave voltage controlled oscillator and amplifier. In the presence of crystal strain, we measure ensemble nitrogen vacancy spin transitions with and without an applied magnetic field, and observe frequency shifts consistent with sample heating.

Spirals And Skyrmions In Antiferromagnetic Triangular Lattices, 2021 University of Nebraska - Lincoln

#### Spirals And Skyrmions In Antiferromagnetic Triangular Lattices, Wuzhang Fang, Aldo Raeliarijaona, Po-Hao Chang, Alexey Kovalev, Kirill Belashchenko

*Faculty Publications, Department of Physics and Astronomy*

We study realizations of spirals and skyrmions in two-dimensional antiferromagnets with a triangular lattice on an inversion-symmetry-breaking substrate. As a possible material realization, we investigate the adsorption of transition-metal atoms (Cr, Mn, Fe, or Co) on a monolayer of MoS2, WS2, or WSe2 and obtain the exchange, anisotropy, and Dzyaloshinskii-Moriya interaction parameters using first-principles calculations. Using energy minimization and parallel-tempering Monte Carlo simulations, we determine the magnetic phase diagrams for a wide range of interaction parameters. We find that skyrmion lattices can appear even with weak Dzyaloshinskii-Moriya interactions, but their stability is hindered by magnetic anisotropy. However, a weak easy ...

Topology Of The O(3) Non-Linear Sigma Model Under The Gradient Flow, 2021 William & Mary

#### Topology Of The O(3) Non-Linear Sigma Model Under The Gradient Flow, Stuart Thomas, Christopher Monahan

*Undergraduate Honors Theses*

Quantum field theory is an extraordinarily successful framework that describes phenomena in particle physics and condensed matter. The O(3) non-linear sigma model (NLSM) is a specific theory used in both of these fields, describing ferromagnets and acting as a prototype for the strong nuclear force. It features topologically stable configurations known as instantons which cannot continuously evolve to the ground state. The topological susceptibility is a parameter that describes this stability and is predicted to vanish in physical theories, however numerical simulations find that the topological susceptibility diverges in the continuum limit. This issue has motivated the application of ...

Pressure Effect On Band Inversion In Ae Cd2 As2 (Ae = Ca, Sr, Ba), 2021 Ames Laboratory and University of Florida

#### Pressure Effect On Band Inversion In Ae Cd2 As2 (Ae = Ca, Sr, Ba), Jonathan M. Destefano, Lin-Lin Wang

*Ames Laboratory Accepted Manuscripts*

Recent studies have predicted that magnetic EuCd2As2 can host several different topological states depending on its magnetic order, including a single pair of Weyl points. Here we report on the bulk properties and band inversion induced by pressure in the nonmagnetic analogs AECd2As2 (AE=Ca, Sr, Ba) as studied with density functional theory calculations. Under ambient pressure we find that these compounds are narrow band gap semiconductors, in agreement with experiment. The size of the band gap is dictated by both the increasing ionicity across the *AE* series which tends to increase the band gap, as well as the larger ...

Superfluid Swimmers, 2021 CUNY New York City College of Technology

#### Superfluid Swimmers, German Kolmakov, Igor S. Aranson

*Publications and Research*

The propulsion of living microorganisms ultimately relies on viscous drag for body-fluid interactions. The self-locomotion in superfluids such as ^{4}He is deemed impossible due to the apparent lack of viscous resistance. Here, we investigate the self-propulsion of a Janus (two-face) light-absorbing particle suspended in superfluid helium ^{4}He (He-II). The particle is energized by the heat flux due to the absorption of light from an external source. We show that a quantum mechanical propulsion force originates due to the transformation of the superfluid to a normal fluid on the heated particle face. The theoretical analysis is supported by the ...

Anisotropic Superconductivity In The Spin-Vortex Antiferromagnetic Superconductor Cak(Fe0.95ni0.05)(4)As-4, 2021 Universidad Autónoma de Madrid

#### Anisotropic Superconductivity In The Spin-Vortex Antiferromagnetic Superconductor Cak(Fe0.95ni0.05)(4)As-4, José Benito Llorens, Edwin Herrera, Víctor Barrena, Beilun Wu, Niclas Heinsdorf, Vladislav Borisov, Roser Valentí, William R. Meier, Sergey L. Bud’Ko, Paul C. Canfield, Isabel Guillamón, Hermann Suderow

*Ames Laboratory Accepted Manuscripts*

High critical temperature superconductivity often occurs in systems where an antiferromagnetic order is brought near T=0 K by slightly modifying pressure or doping. CaKFe4As4 is a superconducting, stoichiometric iron-pnictide compound showing optimal superconducting critical temperature with Tc as large as 35 K. Doping with Ni induces a decrease in Tc and the onset of spin-vortex crystal (SVC) antiferromagnetic order, which consists of spins pointing inwards to or outwards from alternating As sites on the diagonals of the in-plane square Fe lattice. Here we study the band structure of CaK(Fe0.95Ni0.05)4As4 (Tc=10 K, TSVC=50 K ...

Spin Superfluidity In Noncollinear Antiferromagnets, 2021 University of Nebraska - Lincoln

#### Spin Superfluidity In Noncollinear Antiferromagnets, Bo Li, Alexey Kovalev

*Faculty Publications, Department of Physics and Astronomy*

We explore the spin superfluid transport in exchange interaction dominated three-sublattice antiferromagnets. The system in the long-wavelength regime is described by an $SO(3)$ invariant field theory. Additional corrections from Dzyaloshinskii-Moriya interactions or anisotropies can break the symmetry; however, the system still approximately holds a $U(1)$-rotation symmetry. Thus, the power-law spatial decay signature of spin superfluidity is identified in a nonlocal-measurement setup where the spin injection is described by the generalized spin-mixing conductance. We suggest iron jarosites as promising material candidates for realizing our proposal.

Magnetic Crystalline-Symmetry-Protected Axion Electrodynamics And Field-Tunable Unpinned Dirac Cones In Euin2as2, 2021 Ames Laboratory

#### Magnetic Crystalline-Symmetry-Protected Axion Electrodynamics And Field-Tunable Unpinned Dirac Cones In Euin2as2, Simon X. M. Riberolles, Thais V. Trevisan, Brinda Kuthanazhi, T. W. Heitman, F. Ye, David C. Johnston, Sergey L. Bud’Ko, D. H. Ryan, Paul C. Canfield, Andreas Kreyssig, A. Vishwanath, Robert J. Mcqueeney, Lin-Lin Wang, Peter P. Orth, Benjamin G. Ueland

*Physics and Astronomy Publications*

Knowledge of magnetic symmetry is vital for exploiting nontrivial surface states of magnetic topological materials. EuIn2As2 is an excellent example, as it is predicted to have collinear antiferromagnetic order where the magnetic moment direction determines either a topological-crystalline-insulator phase supporting axion electrodynamics or a higher-order-topological-insulator phase with chiral hinge states. Here, we use neutron diffraction, symmetry analysis, and density functional theory results to demonstrate that EuIn2As2 actually exhibits low-symmetry helical antiferromagnetic order which makes it a stoichiometric magnetic topological-crystalline axion insulator protected by the combination of a 180∘ rotation and time-reversal symmetries: C2×T=2′. Surfaces protected by 2′ are ...

Avoided Ferromagnetic Quantum Critical Point In Pressurized La5 Co2 Ge3, 2021 Iowa State University and Ames Laboratory

#### Avoided Ferromagnetic Quantum Critical Point In Pressurized La5 Co2 Ge3, Li Xiang, Elena Gati, Sergey L. Bud’Ko, Scott M. Saunders, Paul C. Canfield

*Ames Laboratory Accepted Manuscripts*

We present the pressure-temperature phase diagram La5Co2Ge3 up to ∼5 GPa, which was constructed from magnetization, resistivity, and specific heat measurements. At ambient pressure, La5Co2Ge3 is an itinerant ferromagnet with a Curie temperature TC∼ 4 K. Upon increasing pressure up to ∼1.7 GPa, TC is suppressed down to ∼3 K. Upon further increasing pressure, our results suggest that La5Co2Ge3 enters a different low-temperature ground state. The corresponding transition temperature T∗ has a nonmonotonic pressure dependence up to ∼5 GPa. Our results demonstrate that the ferromagnetic quantum critical point in La5Co2Ge3 is avoided by the appearance of a different, likely ...

Quasi-One-Dimensional Magnetism In The Spin-1/2 Antiferromagnet Bana2 Cu (Vo4)2, 2021 Indian Institute of Science Education and Research

#### Quasi-One-Dimensional Magnetism In The Spin-1/2 Antiferromagnet Bana2 Cu (Vo4)2, Sebin J. Sebastian, K. Somesh, M. Nandi, N. Ahmed, P. Bag, M. Baenitz, B. Koo, J. Sichelschmidt, A. A. Tsirlin, Yuji Furukawa, R. Nath

*Ames Laboratory Accepted Manuscripts*

We report synthesis and magnetic properties of quasi-one-dimensional spin-12 Heisenberg antiferromagnetic chain compound BaNa2Cu(VO4)2. This orthovanadate has a centrosymmetric crystal structure, C2/c, where the magnetic Cu2+ ions form spin chains. These chains are arranged in layers, with the chain direction changing by 62∘ between the two successive layers. Alternatively, the spin lattice can be viewed as anisotropic triangular layers upon taking the interchain interactions into consideration. Despite this potential structural complexity, temperature-dependent magnetic susceptibility, heat capacity, electron spin resonance intensity, and nuclear magnetic resonance (NMR) shift agree well with the uniform spin-1/2 Heisenberg chain model with ...

Temperature Dependence Of London Penetration Depth Anisotropy In Superconductors With Anisotropic Order Parameters, 2021 Ames Laboratory

#### Temperature Dependence Of London Penetration Depth Anisotropy In Superconductors With Anisotropic Order Parameters, Vladimir G. Kogan, Ruslan Prozorov

*Ames Laboratory Accepted Manuscripts*

We study the effects of anisotropic order parameters on the temperature dependence of London penetration depth anisotropy γ λ ( T ) . After MgB 2 , this dependence is commonly attributed to distinct gaps on multiband Fermi surfaces in superconductors. We have found, however, that the anisotropy parameter may depend on temperature also in one-band materials with anisotropic order parameters Δ ( T , k F ) ; a few such examples are given. We have also found that for different order parameters, the temperature dependence of Δ ( T ) / Δ ( 0 ) can be represented with good accuracy by the interpolation suggested by Einzel [J. Low Temp. Phys ...

Formation Of Short-Range Magnetic Order And Avoided Ferromagnetic Quantum Criticality In Pressurized Lacrge3, 2021 Iowa State University and Ames Laboratory

#### Formation Of Short-Range Magnetic Order And Avoided Ferromagnetic Quantum Criticality In Pressurized Lacrge3, Elena Gati, John M. Wilde, Rustem Khasanov, Li Xiang, Sachith Dissanayake, Ritu Gupta, Masaaki Matsuda, Feng Ye, Bianca Haberl, Udhara Kaluarachchi, Robert J. Mcqueeney, Andreas Kreyssig, Sergey L. Bud’Ko, Paul C. Canfield

*Ames Laboratory Accepted Manuscripts*

LaCrGe3 has attracted attention as a paradigm example of the avoidance of ferromagnetic (FM) quantum criticality in an itinerant magnet. Here, we combined thermodynamic (specific heat and thermal expansion), transport, x-ray, and neutron scattering as well as μSR measurements to obtain insights on the temperature-pressure phase diagram of LaCrGe3. Consistent with previous studies of the phase diagram by transport measurements, our thermodynamic data shows clearly that the FM transition at TFM changes its character from second order to first order when it is suppressed to low temperatures by pressure. In addition, previous studies demonstrated that for high pressures a new ...

Manipulation Of Electronic Property Of Epitaxial Graphene On Sic Substrate By Pb Intercalation, 2021 Fudan University, Ames Laboratory, and Iowa State University

#### Manipulation Of Electronic Property Of Epitaxial Graphene On Sic Substrate By Pb Intercalation, Jinjin Wang, Minsung Kim, Liangyao Chen, Kai-Ming Ho, Michael C. Tringides, Cai-Zhuang Wang, Songyou Wang

*Ames Laboratory Accepted Manuscripts*

Manipulating the electronic properties of graphene has been a subject of great interest since it can aid material design to extend the applications of graphene to many different areas. In this paper, we systematically investigate the effect of lead (Pb) intercalation on the structural and electronic properties of epitaxial graphene on the SiC(0001) substrate. We show that the band structure of Pb-intercalated few-layer graphene can be effectively tuned through changing intercalation conditions, such as coverage, location of Pb, and the initial number of graphene layers. Lead intercalation at the interface between the buffer layer (BL) and the SiC substrate ...

Plasmonic Waveguides To Enhance Quantum Electrodynamic Phenomena At The Nanoscale, 2021 Nanjing University of Information Science and Technology

#### Plasmonic Waveguides To Enhance Quantum Electrodynamic Phenomena At The Nanoscale, Ying Li, Christos Argyropoulos

*Faculty Publications from the Department of Electrical and Computer Engineering*

The emerging field of plasmonics can lead to enhanced light-matter interactions at extremely nanoscale regions. Plasmonic (metallic) devices promise to efficiently control both classical and quantum properties of light. Plasmonic waveguides are usually used to excite confined electromagnetic modes at the nanoscale that can strongly interact with matter. The analysis of these nanowaveguides exhibits similarities with their low frequency microwave counterparts. In this article, we review ways to study plasmonic nanostructures coupled to quantum optical emitters from a classical electromagnetic perspective. These quantum emitters are mainly used to generate single-photon quantum light that can be employed as a quantum bit ...

Expansive Open Fermi Arcs And Connectivity Changes Induced By Infrared Phonons In Zrte5, 2021 Ames Laboratory

#### Expansive Open Fermi Arcs And Connectivity Changes Induced By Infrared Phonons In Zrte5, Lin-Lin Wang

*Ames Laboratory Accepted Manuscripts*

Expansive open Fermi arcs covering most of the surface Brillouin zone (SBZ) are desirable for detection and control of many topological phenomena, but they have generally been reported for Kramers-Weyl points, or unconventional chiral fermions, pinned at time-reversal invariant momentum in chiral materials. Here using first-principles band structure calculations, we show that for conventional Weyl points in ZrTe5 with the chirality of +1/−1 near the BZ center at general momentum induced by one of the infrared phonons—the second lowest B1u mode for breaking inversion symmetry—they can also form expansive open Fermi arcs across the SBZ boundary to ...

Interactions Of Organic Fluorophores With Plasmonic Surface Lattice Resonances, 2021 The Graduate Center, City University of New York

#### Interactions Of Organic Fluorophores With Plasmonic Surface Lattice Resonances, Robert J. Collison

*Dissertations, Theses, and Capstone Projects*

It is common knowledge that metals, alloys and pure elements alike, are lustrous and reflective, the more so when a metal surface is flat, polished, and free from oxidation and surface fouling. However, some metals reflect visible light, in the 380 nm to 740 nm range of wavelengths, much more strongly than others. In particular, some metals reflect wavelengths in certain portions of the ultraviolet (UV), visible, and near-infrared (NIR) regime, let us say 200 nm to 2000 nm, while absorbing light strongly in other segments of this range. There are several factors that account for this difference between various ...

Moving Pearl Vortices In Thin-Film Superconductors, 2021 Iowa State University and Ames Laboratory

#### Moving Pearl Vortices In Thin-Film Superconductors, Vladimir G. Kogan, Norio Nakagawa

*Ames Laboratory Accepted Manuscripts*

The magnetic field hz of a moving Pearl vortex in a superconducting thin-film in (x,y) plane is studied with the help of the time-dependent London equation. It is found that for a vortex at the origin moving in +x direction, hz(x,y) is suppressed in front of the vortex, x>0, and enhanced behind (x<0). The distribution asymmetry is proportional to the velocity and to the conductivity of normal quasiparticles. The vortex self-energy and the interaction of two moving vortices are evaluated.

Fundamental Transport Properties In Silicon Quantum Structures, 2021 The University of Western Ontario

#### Fundamental Transport Properties In Silicon Quantum Structures, Nazban M. Darukhanawalla

*Electronic Thesis and Dissertation Repository*

In the field of silicon photonics, there is an effort to bridge the gap between electrical and optical signals on a single platform, creating a need for Si-based light sources. In this project, Si quantum structures – Si quantum wells and quantum dots in SiO2 were fabricated via solid state precipitation methods. Their properties were studied using X-ray photoelectron spectroscopy, photoluminescence and I-V measurements. Rutherford backscattering spectroscopy was used for depth analysis in monitoring the Si distribution. Different electrical transport mechanisms were explored to understand how an ensemble of silicon QD’s or a silicon quantum well behaves in an SiO2 ...

Multi-Atom Quasiparticle Scattering Interference For Superconductor Energy-Gap Symmetry Determination, 2021 Cornell University and University of Maryland

#### Multi-Atom Quasiparticle Scattering Interference For Superconductor Energy-Gap Symmetry Determination, Rahul Sharma, Andreas Kreisel, Miguel Antonio Sulangi, Jakob Böker, Andrey Kostin, Milan P. Allan, H. Eisaki, Anna E. Böhmer, Paul C. Canfield, Ilya Eremin, J. C. Séamus Davis, P. J. Hirschfeld, Peter O. Sprau

*Ames Laboratory Accepted Manuscripts*

Complete theoretical understanding of the most complex superconductors requires a detailed knowledge of the symmetry of the superconducting energy-gap Δαk, for all momenta k on the Fermi surface of every band α. While there are a variety of techniques for determining |Δαk|, no general method existed to measure the signed values of Δαk. Recently, however, a technique based on phase-resolved visualization of superconducting quasiparticle interference (QPI) patterns, centered on a single non-magnetic impurity atom, was introduced. In principle, energy-resolved and phase-resolved Fourier analysis of these images identifies wavevectors connecting all k-space regions where Δαk has the same or opposite sign ...