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.

Optimization Of Materials For Magnetic Refrigeration And Thermomagnetic Power Generation, 2021 The Graduate Center, City University of New York

#### Optimization Of Materials For Magnetic Refrigeration And Thermomagnetic Power Generation, Anthony N. Tantillo

*Dissertations, Theses, and Capstone Projects*

The magnetocaloric effect, by which a magnetic material experiences a change in temperature due to an applied magnetic field, can be used for refrigeration. The corollary to the magnetocaloric effect -- known as the pyromagnetic effect -- is the phenomenon by which a magnetic material experiences a thermally-induced change in magnetization that can be used to harvest thermal energy. This dissertation has two main parts: one focusing on novel materials for energy harvesting; and another focusing on methods of materials discovery for refrigeration purposes. Thermomagnetic power generation (TMG) is the process by which magnetic flux, which comes from a temperature-driven change of ...

Quantum Transport In Topological Magnets, 2021 The Graduate Center, City University of New York

#### Quantum Transport In Topological Magnets, Haiming Deng

*Dissertations, Theses, and Capstone Projects*

In the past several years, a new field of symmetry-protected topological materials has emerged in condensed matter physics, based on the wide range of consequences that result from the realization that certain properties of physical systems can be expressed as topological invariants, which are insensitive to local perturbations. This new class of materials hosts unique surface/edge states, such as the first known topological system – quantum Hall insulator with dissipationless chiral edge states, and massless spin-helical Dirac surface states in 3D topological insulators that are unlike any other known 1D or 2D electronic systems. In this thesis, to understand the ...

Solid State Synthesis Of Polar Magnetic Oxides, 2021 Kennesaw State University

#### Solid State Synthesis Of Polar Magnetic Oxides, Duy Pham

*Symposium of Student Scholars*

Non-centrosymmetric polar oxides are subjects of considerable interest due to varieties of important phenomena and associated functional properties. Magnetoelectric multiferroic oxides are one such system where the magnetic properties can be controlled by electric field or the electric properties can be controlled by the magnetic field. This cross tunability magnetic and electrical properties makes multiferroic materials ideal candidates for making actuators, field sensors and memory devices. Simultaneous presence of broken inversion symmetry (electric polarization) and magnetism are two key requirements for multiferroicity. Non-centrosymmetric polar magnetic oxides simultaneously offer both (polarization and magnetization) properties. Therefore, we are working toward synthesis and ...

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 ...

Multi-Scale Computational Modeling Of Metal/Ceramic Interfaces, 2021 Louisiana Tech University

#### Multi-Scale Computational Modeling Of Metal/Ceramic Interfaces, Abu Shama Mohammad Miraz

*Master's Theses*

Multi-scale atomistic calculations were carried out to understand the interfacial features that dictate the mechanical integrity of the metal/ceramic nanolaminates. As such, first principles density functional theory (DFT) calculations were performed to understand the electronic and atomistic factors governing adhesion and resistance to shear for simple metal/ceramic interfaces, whereas molecular dynamics (MD) simulations were performed to observe the impact of interfacial structures, such as misfit dislocation network geometries and orientation relationships, on interfacial mechanical properties.

For the DFT investigation, we choose metals with different crystal structures, namely - Cu (fcc), Cr (bcc) and Ti (hcp) along with a variety ...

Study Of Static And Dynamical Properties Of Complex Antiferroelectrics Materials, 2021 University of Arkansas, Fayetteville

#### Study Of Static And Dynamical Properties Of Complex Antiferroelectrics Materials, Kinnary Yogeshbhai Patel

*Theses and Dissertations*

The aim of this dissertation is the investigation of the static and dynamical properties of the complex antiferroelectric materials using Effective Hamiltonian method and First principles calculations. In chapter 3, a novel elemental interatomic coupling in perovskite materials which bilinearly couples the antiferroelectric displacements of cations with the rotations of the oxygen octahedra. This new coupling explains a very complex crystal structure of prototypical antiferroelectric PbZrO3. My explanation provides a unified description of many other complex antipolar crystal structures in variety of perovskite materials, including the occurrence of incommensurate phases in some of them. In chapter 4, results and analysis ...

Physics 516: Electromagnetic Phenomena (Spring 2020), 2021 University of Pennsylvania

#### Physics 516: Electromagnetic Phenomena (Spring 2020), Philip C. Nelson

*Department of Physics Papers*

These course notes are made publicly available in the hope that they will be useful. All reports of errata will be gratefully received. I will also be glad to hear from anyone who reads them, whether or not you find errors: pcn@upenn.edu.

Current Distributions By Moving Vortices In Superconductors, 2021 Ames Laboratory

#### Current Distributions By Moving Vortices In Superconductors, Vladimir G. Kogan, Norio Nakagawa

*Ames Laboratory Accepted Manuscripts*

We take account of normal currents that emerge when vortices move. Abrikosov vortices in the bulk and Pearl vortices in thin films are considered. Velocity-dependent distributions of both normal and persistent currents are studied in the frame of time-dependent London equations. In thin films near the Pearl vortex core, these distributions are intriguing in particular.

Data-Driven Approaches To Complex Materials: Applications To Amorphous Solids, 2021 The University of Southern Mississippi

#### Data-Driven Approaches To Complex Materials: Applications To Amorphous Solids, Dil Kumar Limbu

*Dissertations*

While conventional approaches to materials modeling made significant contributions and advanced our understanding of materials properties in the past decades, these approaches often cannot be applied to disordered materials (e.g., glasses) for which accurate total-energy functionals or forces are either not available or it is infeasible to employ due to computational complexities associated with modeling disordered solids in the absence of translational symmetry. In this dissertation, a number of information-driven probabilistic methods were developed for the structural determination of a range of materials including disordered solids to transition metal clusters. The ground-state structures of transition-metal clusters of iron, nickel ...

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 ...