Physical Sciences and Mathematics Commons^™

Topological Hall Effect In Particulate Magnetic Nanostructure, Ahsan Ullah

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

Conduction electrons change their spin direction due to the exchange interaction with the lattice spins. Ideally, the spins of the conduction electrons follow the atomic spin adiabatically, so that spins like S1, S2, and S3 can be interpreted as time-ordered sequences t1 < t2 < t3. Such spin sequences yield a quantum-mechanical phase factor in the wave function,  →ei, where  is known as the Berry phase. The corresponding spin rotation translates into a Berry curvature and an emergent magnetic field and subsequently, Hall-effect contribution known as the topological Hall-effect. This dissertation explores topological Hall-effect in particulate magnets, where noncollinear spins are stabilized by competition between different magnetic interactions. The topologically non-trivial spin textures in these nanostructures are flower states, curling states, vortex, and magnetic bubbles, which give rise to topological Hall-effect and have finite spin chirality and Skyrmion number Q. Topological Hall-effect is investigated in noninteracting nanoparticles, exchanges coupled centrosymmetric nanoparticles, exchanges coupled non-centrosymmetric nanoparticles which possess Dzyaloshinskii-Moriya interaction (DMI), and exchanged coupled Hard and soft magnetic films. Micromagnetic modeling, simulations, analytical calculations, and experimental methods are used to determine topological Hall-effect. In very small noninteracting nanoparticles, the reverse magnetic fields enhance Q due to the flower state until the reversal occurs, whereas, for particles with a radius greater than coherence radius, the Q jumps to a larger value at the nucleation field representing the curling state. The comparisons of magnetization patterns between experimental and computed magnetic force microscopy (MFM) measurements show the presence of spin chirality. Magnetic and Hall-effect measurements identify topological Hall-effect in the exchange-coupled Co and CoSi-nanoparticle films. The origin of the topological Hall-effect namely, the chiral domains with domain-wall chirality quantified by an integer skyrmion number in Co and chiral spins with partial skyrmion number in CoSi. These spin structures are different from the Skyrmions due to DMI in B-20 crystals and multilayered thin films with Cnv symmetry. In these films THE caused by cooperative magnetization reversal in the exchange-coupled Co-nanoparticles and peripheral chiral spin textures in CoSi-nanoparticles.

Advisor: Xiaoshan Xu

Growth And Emergent Functionalities Of Oxide Thin Films Utilizing Interface Engineering, Detian Yang

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

Complex oxide interfaces have offered intriguing novel emergent phenomena and multiple functionalities through interfacial reconstructions of spin, orbital, charge, and lattice degrees of freedom. Interface engineering via manipulating interfacial interaction, defects and multiple interfacial quantum charges and orders constitutes the essential method and technique to achieve desired functionalities in oxide heterostructures. In this thesis, shown are two examples of utilizing interfacial reconstruction and interfacial strain engineering to achieve intrinsic exchange bias and realize epitaxial growth of mixed-valence hexagonal manganite thin films, respectively.

Firstly, we demonstrated intrinsic exchange bias induced by interfacial reconstruction in Ni_xCo_yFe_3-x-yO …

Diffractive Imaging Of Laser Induced Molecular Reactions With Kiloelectron-Volt Ultrafast Electron Diffraction, Yanwei Xiong

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

Capturing the structural changes during a molecular reaction with ultrafast electron diffraction (UED) requires a high spatiotemporal resolution and sufficiently high signal-to-noise to record the signals with high fidelity. In this dissertation, I have focused on the development of a tabletop gas phase keV-UED setup with a femtosecond temporal resolution. A DC electron gun was employed to generate electron pulses with a high repetition rate of 5 kHz. The space charge effect in the electron pulse was ameliorated by compressing the 90 keV electron pulse longitudinally with a time varying electric field in an RF cavity. The velocity mismatch between …

Investigation Of Transport Behavior In Two-Dimensional Ferroelectric Heterostructures, Pradeep Chaudhary

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

This dissertation summarizes an investigation of the polarization-related electronic transport behavior in the ferroelectric thin films and two-dimensional (2D) materials heterostructures using Scanning Probe Microscopy (SPM) techniques.

The polarization-related resistive switching in hafnium oxide thin films-based ferroelectric tunnel junction has been demonstrated by employing semiconducting MoS₂ as a top electrode. We explored a coupling between the semiconducting properties of MoS₂ and the polarization of Hf₀.₅Zr₀.₅O₂ resulted in an enhanced tunneling electroresistance effect of up to 3 orders of magnitude. These results provide a possible pathway for the fabrication of …

Fermion-Induced Electroweak Symmetry Non-Restoration Via Temperature-Dependent Masses, Yu Hang Ng

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

Standard Model (SM) and many extensions of SM predict that the electroweak (EW) symmetry was restored in the early universe when the temperature was around 160 GeV. However, recent studies showed that the interactions between some new scalars and SU(2)_L Higgs doublet(s) can cause the EW symmetry to remain broken at temperatures well above the EW scale in certain renormalizable extensions of SM. In this study, we found that new fermions from renormalizable models can also induce this EW symmetry non-restoration effect, provided that they have the appropriate temperature-dependent masses. These masses can arise naturally from the interactions between the …

Nanoscale Studies Of The Ferroelectric And Electromechanical Properties Of Hafnia-Based Capacitors, Pratyush Buragohain

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

The work presented in this dissertation aims to provide nanoscopic insights into the electrical and electromechanical behavior of the recently discovered ferroelectric HfO₂ or hafnia-based capacitors. Hafnia-based ferroelectrics are highly promising for technological applications due to compatibility with the existing Si technology. To realize the full potential of hafnia, however, requires comprehensive understanding of its properties. In this regard, this dissertation hopes to bridge a gap between an understanding of the nanoscopic and macroscopic properties of hafnia by performing combined high-resolution piezoresponse force microscopy (PFM) and pulse switching studies.

More specifically, the dynamics of domain nucleation and wall motion …

Magnetism Of Novel Rare-Earth-Free Intermetallic Compounds, Haohan Wang

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

Rare-earth-free magnets have drawn lots of interest because of their low cost, and the production is not limited by the shortage of rare-earth elements. This dissertation focuses on three rare-earth-free materials, Fe-Co-Ti alloys, Fe-Ni-B alloys, and Co-Si. All of them are synthesized by arc melting followed by melt-spinning. Fe_3+xCo_3−xTi₂ (x = 0, 2, 3) alloys exhibit hexagonal crystal structures and show non-collinear spin structures according to neutron diffraction. The magnetic moments have projections on both the c-axis and basal plane, and the corresponding misalignment angle exhibits a nonlinear decrease with …

Spin-Dependent Electronic Transport In Noncollinear Antiferromagnetic Antiperovskites, Gautam Gurung

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

Spin-dependent properties are the heart of spintronic devices. Spintronics exploits electron’s spin, in addition to charge, to process and store the information. Recently, antiferromagnetic (AFM) spintronics has emerged as a subfield of spintronics, where an AFM order parameter (the Néel vector) is exploited to control spin-dependent transport properties. Due to being robust against magnetic perturbations, producing no stray fields, and exhibiting ultrafast dynamics, antiferromagnets can serve as promising functional materials for spintronic applications.

Among antiferromagnets, high Néel temperature noncollinear antiperovskites ANMn₃ (A = Ga, Ni, Sn, and Pt) are interesting due to their magnetic group symmetry supporting non-trivial spin-dependent …

Manipulation Of Spin Crossover Phenomenon In An Fe (Ii) Molecular Complex And Application To Molecular Spintronics, Guanhua Hao

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

Molecules with a large local magnetic moment have attracted considerable attention for application in spintronic devices. One candidate of a suitable device goes to the spin crossover molecule, where these 3d transition metal compounds are able to exhibit a robust spin state transition between distinct states. By proper design, the spintronic devices fabricated via spin crossover molecular thin films could achieve novel functionality while retaining flexibility and other traits based on its “organic” nature.

Controlling the spin state transition is a key factor of these possibilities. This thesis work investigates the manipulation of the spin state transition in [Fe{H …

The Design, Creation, And Cognitive Evaluation Of Ranking Tasks In Introductory Astronomy, Emily A. Welch

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

Ranking tasks are a type of interactive formative assessment. They allow students to explore a concept by ranking similar situations for a specified variable, preferably without computation of that variable. I created two sets of introductory astronomy ranking tasks: the first connects the Hertzsprung–Russell (HR) diagram and the Stephan-Boltzmann luminosity equation; the second uses the transit method (TM) to rank exoplanets by comparing the depth, duration, and frequency of transits.

These tasks are designed within the constructivist pedagogical framework. They require students to call upon their own relevant schema to establish an assessment rule by which to rank the tasks. …

Voltage-Controlled Magnetization In Chromia-Based Magnetic Heterostructures, William Echtenkamp

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

Spintronics promises a new generation of low-power, high-speed, non-volatile memory and logic devices. Heterostructures based on magnetoelectric chromia enable the direct manipulation of magnetization by applied electric fields and emerged as a promising building block for spintronic devices. In this dissertation, several interesting emergent magnetic properties arising in these device-enabling building blocks are examined. In some cases, exchange coupling at the interface between the magnetoelectric antiferromagnet and an adjacent ferromagnet stabilizes the interfacial antiferromagnetic domain state against the electrically induced rotation of the bulk spin structure. Upon magnetically cycling the ferromagnet, the magnetoelectric antiferromagnet relaxes towards a commensurate spin structure …

Band Structure Topology And Spin Transport In Magnon Systems, Bo Li

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

As the spin excitation quanta in magnetic materials, the magnon is at the heart of the spintronics research because it plays a key role in magnetic dynamics, energy and spin transport, and even determining the ground state of magnetic systems. In this thesis, we will study the band-structure topology and transport properties of magnons in both collinear and noncollinear magnets. Inspired by the great success of topological insulators, exploring magnon topology can unveil the topological nature of bosonic particles and widen the zoo of topological materials. We propose a three-dimensional magnon topological insulator model protected by sublattice chiral symmetries, which …

The Electronic Properties Of The Quasi-One-Dimensional Transition Metal Trichalcogenides: Tis3 And Zrs3, Simeon Gilbert

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

The transition metal trichalcogenides are a class of materials formed by 1D chains of covalently bound MX₃ (M=Ti, Zr, Hf, Ta, Nb; X=S, Se, Te) trigonal prisms which are held together by weak van der Waals forces to form 2D sheets. Because of their superior edge termination, these materials suppress edge scattering effects that plague other two-dimensional materials thus enabling devices scaling to widths below 10 nm. Furthermore, this quasi-one-dimensional structure results in highly anisotropic electronic and optical properties which were examined through angle resolved photoemission spectroscopy and scanning photocurrent microscopy. These measurements show that the hole carrier masses …

Symmetry And Interface Considerations For Interactions On Mos2, Prescott E. Evans

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

The critical role of symmetry, in adsorbate-MoS₂ interactions, has been demonstrated through a variety of electronic structure, topology, and catalytic studies of MoS₂ and MoS₂ composites.A combination of density functional theory and experiment exhibiting diiodobenzene isomer dependent adsorption rates highlight frontier orbital symmetry as key to adsorption on MoS₂. It is clear that the geometry and symmetry of MoS₂ influences the creation and stability of surface defects, that in turn affect catalytic activity and a myriad of other applications. We have shown that surface reactions such the methanol to methoxy reaction can create defects …

Growth And Characterization Of Organic Ferroelectric And Magnetic Thin Films, Xuanyuan Jiang

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

Compared to inorganic materials, organic materials are environmentally friendly, flexible, and often with low cost. Inspired by these advantages, organic materials-based electronics have been intensively studied for comparable or better functionalities to inorganic electronics.

This dissertation mainly focuses on the growth and characterizations of organic ferroelectrics and magnetic thin films. For organic ferroelectrics, we investigate the growth and ferroelectric measurements of thin film croconic acid (CA), a proton-transfer molecular ferroelectric (FE) material with a large spontaneous polarization and a small coercive field, as well as the origin of ferroelectricity in CA in terms of the photostriction effect, including the discovery …

Improvement For Generating High-Order Harmonics And Attosecond Pulses With Ultrashort Laser Fields, Dian Peng

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

Nonlinear processes of high-order harmonic generation (HHG) produced by ultrashort few-cycle laser pulses possess interesting features which HHG produced by long pulses of many cycles may not have. First, HHG spectra produced by ultrashort pulses are extremely sensitive to the driving pulse waveform, which can be controlled by laser parameters such as carrier-envelope phases (CEPs), time delays or frequency chirps. Second, HHG spectra produced by ultrashort pulses can exhibit broad uneven peaks which are different from usual odd-ordered harmonic peaks that long pulses produce.

Based on the high sensitivity on pulse waveform of HHG spectra produced by ultrashort pulses, we …

Measurement Of The Production Cross Section Of Four Top Quarks In Proton-Proton Collisions At 13 Tev, Caleb Fangmeier

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

The field of particle physics involves not only searches for new particles and measurements of their interactions, but also the design and construction of advanced particle detectors. This thesis presents the measurement of the production cross section of four top quarks in proton-proton collisions at a center-of-mass energy of 13 TeV using 137 fb$^{-1}$ of integrated luminosity recorded by the CMS experiment at the LHC. This analysis considers events in the final state of a same-sign pair of leptons, notable for being a final state with relatively few Standard Model background events. A boosted decision tree is utilized to discriminate …

Search For Resonant Double Higgs Production With Bbzz Decays In The Bbℓℓνν Final State In Pp Collisions At √S = 13 Tev, Rami Kamalieddin

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

Since the discovery of the Higgs boson in 2012 by the ATLAS and CMS experiments, most of the quantum mechanical properties that describe the long-awaited Higgs boson have been measured. Due to the outstanding work of the LHC, over a hundred of fb⁻¹ of proton collisions data have been delivered to both experiments. Finally, it became sensible for analyses teams to start working with a very low cross section processes involving the Higgs boson, e.g., a recent success in observing ttH and VHbb processes. One of the main remaining untouched topics is a double Higgs boson production. However, an …

Free Electron Sources And Diffraction In Time, Eric R. Jones

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

The quantum revolution of the last century advanced synergistically with technology, for example, with control of the temporal and spatial coherence, and the polarization state of light. Indeed, experimental confirmation of the quirks of quantum theory, as originally highlighted by Einstein, Podolsky, and Rosen, through Bohm, and then Bell, have been performed with photons, i.e., electromagnetic wave packets prepared in the same quantum states. Experimental tests of quantum mechanics with matter wave packets have been limited due to challenges in preparing all of the packets with similar quantum states. While great strides have been made for trapped atoms and Bose-Einstein …

Perturbative Generalization Of Nonparaxial Ultrashort Tightly-Focused Elegant Laguerre-Gaussian Beams, Andrew M. Vikartofsky

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

An analytical method for calculating the electromagnetic fields of a nonparaxial elegant Laguerre-Gaussian (eLG) vortex beam is presented for arbitrary pulse duration, spot size, and LG mode. This perturbative approach provides a numerically tractable model for the calculation of arbitrarily high radial and azimuthal LG modes in the nonparaxial regime, without requiring integral representations of the fields. A key feature of this perturbative model is its use of a Poisson-like frequency spectrum, which allows for the proper description of pulses of arbitrarily short duration. The time-domain representation of this model is presented as a non-recursive closed-form expression to any order …

Search For Production Of A Higgs Boson And A Single Top Quark In Multilepton Final States In Pp Collisions At √S = 13 Tev, Jose Andres Monroy Montanez

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

The exciting work in high energy physics includes not only the analysis of the data taken by the experiment but also the development of detection systems. In this thesis, the results of a search for the production of a Higgs boson in association with a single top quark (tH) are presented. This process is of particular interest due to its sensitivity to the relative sign of the top-Higgs coupling and the vector bosons-Higgs coupling. The focus is on leptonic signatures provided by the H → W W , H → τ τ , and H → ZZ decay modes.

The …

Investigations Of Novel Sources Of Spin-Polarized Electrons, Evan Brunkow

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

Using a pulsed laser, we investigated the spin-polarization of electrons emitted from bulk GaAs, Ti and Pd chiral nanostructures, and electro-chemically thinned GaAs. Standard sources of spin-polarized electrons from GaAs can have polarizations of approximately 30%, while state-of-the-art spin-polarized electron sources using GaAs cathodes can have as high as 85% spin polarization. Drawbacks for these sources are that they require constant upkeep, have strict vacuum requirements, and are very difficult to learn how to use. For these reasons, we investigated new methods through a different emission process and different materials to see if we could measure a spin-polarization from these …

Angle-Resolved Observation Of X-Ray Second Harmonic Generation In Diamond, Björn Senfftleben

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

This thesis reports angularly-resolved observation of X-ray second harmonic generation (XSHG) in diamond at several phase-matching geometries. The XSHG signal was produced by ultra-short, highly intense X-ray pulses with a photon energy of 9.831 keV generated by a free-electron laser. In some geometries for high pulse energies more than 10 second harmonic photons per pulse were generated.

Different phase-matched geometries were used for XSHG to investigate the angular dependence of the efficiency of the process. Furthermore, for each phase-matching condition, the quadratic dependence for second harmonic generation at each geometry was verified and the crystal rocking curves were measured. The …

Design And Construction Of A High-Current Femtosecond Gas-Phase Electron Diffraction Setup, Omid Zandi

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

We designed and constructed a state-of-the-art high current ultrafast gas electron diffraction experimental setup, which resolved two main challenges that constraint temporal resolution in previous setups. These aforementioned bottlenecks were: the space charge effect due to the Coulomb expansion, and the velocity mismatch between the sub-relativistic electrons (probe) and the exciting laser pulse (pump). In our setup, the problem of space charge effect was ameliorated by compressing 90 keV photo-emitted electron pulses using a radio-frequency electric field. The compression allowed us to increase the beam current by almost two orders of magnitude higher than previously reported. We developed a laser-activated …

A Measurement Of The Wγ Cross Section At √S = 8 Tev In Pp Collisions With The Cms Detector, Ekaterina Avdeeva

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

A measurement of cross section of the Wγ → lνγ production in proton-proton collisions using 19.6 fb − 1 of LHC data collected by CMS detector at the center- √ of-mass collision energy of s = 8 TeV is reported. The W bosons are identified in their electron and muon decay modes. The process of Wγ production in the Standard Model (SM) involves a pure gauge boson coupling, a WWγ vertex, which allows one to test the electroweak sector of the SM in a unique way not achievable by studies of other processes. In addition to the total cross section, …

From Quantum To Classical Interactions Between A Free Electron And A Surface, Peter Beierle

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

Quantum theory is often cited as being one of the most empirically validated theories in terms of its predictive power and precision. These attributes have led to numerous scientific discoveries and technological advancements. However, the precise relationship between quantum and classical physics remains obscure. The prevailing description is known as decoherence theory, where classical physics emerges from a more general quantum theory through environmental interaction. Sometimes referred to as the decoherence program, it does not solve the quantum measurement problem. We believe experiments performed between the microscopic and macroscopic world may help finish the program. The following considers a free …

Surfaces And Interfaces Of Magnetoelectric Oxide Systems, Shi Cao

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

Magnetoelectric materials Cr2O3, hexagonal LuFeO3 and YbFeO3 are studied in this thesis. The surface of chromia (Cr2O3) has a surface electronic structure distinct from the bulk. Our work shows that placing a Cr2O3 single crystal into a single domain state will result in net Cr2O3 spin polarization at the boundary, even in the presence of a gold overlayer. From the Cr 2p3=2 X-ray magnetic circular dichroism signal, there is clear evidence of interface polarization with overlayers of both Pd and Pt on chromia. Cobalt thin films on Cr2O3(0001) show larger magnetic contrast in magnetic force microscopy indicating enhancement of perpendicular …

Novel Half-Metallic And Spin-Gapless Heusler Compounds, Yunlong Jin

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

This thesis is devoted to experimental studies of Heusler compounds CoFeCrAl, CoFeCrX (X = Si, Ge) and Mn₂PtSn. These Heusler alloys present an interesting class of ferromagnetic materials for spintronic applications since they are predicted to be spin gapless semiconductors and have half-metallic properties with 100 % spin polarization at the Fermi level. In this thesis, the structural, magnetic, spin-polarization and electron- transport properties of the fabricated alloys were studied. CoFeCrAl thin films deposited on MgO exhibit nearly perfect epitaxy and a high degree of L2₁ Heusler order. All considered types of chemical disorder destroy the spin-gapless …

Fabrication And Study Of The Structure And Magnetism Of Rare-Earth Free Nanoclusters, Bhaskar Das

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

No abstract provided.

The Organic Ferroelectric Vinylidene Fluoride Oligomer: Vacuum Deposition, Properties, And Interfaces, Keith Foreman

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

Organic ferroelectric materials combine the versatility and customizability afforded by organic synthesis with the functionality of ferroelectric materials. The model ferroelectric polymer, poly(vinylidene fluoride) (PVDF), is used in a wide variety of applications and is still the subject of fundamental research nearly 80 years after it was first polymerized. Unfortunately, PVDF suffers from thermal decomposition during thin film evaporation in vacuum. Since PVDF thin films cannot be deposited in the ferroelectric phase in vacuum conditions, its use in new, 21^st century technologies may be limited since the interface between the organic and adjacent metallic thin films is less than …