Physics Commons^™

Application Of Global Search Methods To Materials Prediction And Design, Adam J. Payne

Graduate Theses, Dissertations, and Problem Reports

Due to increased availability and power of computational resources over the past few decades, prediction and design of novel materials using computational methods has become feasible. Simulation of material systems has become vital to the further realization of novel material systems. In order to ascertain physical properties, accurate determination and identification of stable crystalline structures is necessary. Additionally, further identification of novel properties, such as magnetic moments or orbital occupation, is necessary to further realize this goal. Global search methods provide a path to accurate prediction of these properties. In this dissertation, the Firefly algorithm and minima hopping methods are …

Noncollinear Spin Structure In Fe3+Xco3−Xti2 (X = 0, 2, 3) From Neutron Diffraction, Haohan Wang, Balamurugan Balamurugan, Rabindra Pahari, Ralph Skomski, Yaohua Liu, Ashfia Huq, D. J. Sellmyer, Xiaoshan Xu

Xiaoshan Xu Papers

Neutron powder diffraction has been used to investigate the spin structure of the hard-magnetic alloy Fe_3+xCo_3−xTi₂ (x = 0, 2, 3). The materials are produced by rapid quenching from the melt, they possess a hexagonal crystal structure, and they are nanocrystalline with crystallite sizes D of the order of 40 nm. Projections of the magnetic moment onto both the crystalline c axis and the basal plane were observed. The corresponding misalignment angle exhibits a nonlinear decrease with x, which we explain as a micromagnetic effect caused by Fe-Co site disorder. The underlying physics is a …

Nonvolatile Voltage Controlled Molecular Spin State Switching, G. Hao, A. Mosey, X. Jiang, A. J. Yost, K. R. Sapkota, G. T. Wang, X. Zhang, J. Zhang, A. T. N'Diaye, R. Cheng, X. Xu, P. A. Dowben

Xiaoshan Xu Papers

Voltage-controlled room temperature isothermal reversible spin crossover switching of [Fe{H₂B(pz)₂}₂(bipy)] thin films is demonstrated. This isothermal switching is evident in thin film bilayer structures where the molecular spin crossover film is adjacent to a molecular ferroelectric. The adjacent molecular ferroelectric, either polyvinylidene fluoride hexafluoropropylene or croconic acid (C₅H₂O₅), appears to lock the spin crossover [Fe{H₂B(pz)₂}₂(bipy)] molecular complex largely in the low or high spin state depending on the direction of ferroelectric polarization. In both a planar two terminal diode structure and a …

Development Of Nonlocal Green-Kubo Formalism With Applications To Coupled Heat And Mass Transport, Kevin Fernando

Honors Undergraduate Theses

Nonlocal equations for coupled heat and mass transport are developed within the Green-Kubo formalism. Nonlocal thermal transport in Lennard-Jones solids is computed to establish the existence of semi-ballistic transport. Deviations from the diffusive theory are shown by comparing the Fourier transform of the response function from the nonlocal theory to that of the diffusive one. It is shown that the deviations from the local theory correspond to acoustic phonons, whose frequency dependence gives rise to the observed deviations from the local theory.

Dimensionality Of Magnetism In Trirutile Cota2o6 And Its Derivatives, Raju Baral

Open Access Theses & Dissertations

In this thesis, we addressed the question of low dimensionality of trirutile compound CoTa2O6 and studied how the low dimensionality evolved with doping of Mg on Co-site. In order to study low dimensionality in CoTa2O6 and its derivative compounds Co1-xMgxTa2O6 (x = 0.1, 0.3, 0.5, 0.7, and 1), we used different techniques: X-ray diffraction, magnetic susceptibility, magnetization, specific heat and elastic neutron diffraction. We have addressed the question of low dimensional magnetism of CoTa2O6 by preparing phase-pure samples of the compound. In CoTa2O6 a broad feature is observed in magnetic susceptibility at 10 K and an antiferromagnetic phase transition is …

Proximitized Materials, Igor Žutić, Alex Matos-Abiague, Benedikt Scharf, Hanan Dery, Kirill Belashchenko

Kirill Belashchenko Publications

Advances in scaling down heterostructures and having an improved interface quality together with atomically thin two-dimensional materials suggest a novel approach to systematically design materials. A given material can be transformed through proximity effects whereby it acquires properties of its neighbors, for example, becoming superconducting, magnetic, topologically nontrivial, or with an enhanced spin–orbit coupling. Such proximity effects not only complement the conventional methods of designing materials by doping or functionalization but also can overcome their various limitations. In proximitized materials, it is possible to realize properties that are not present in any constituent region of the considered heterostructure. While the …

Coupling Of Light's Orbital Angular Momentum To A Quantum Dot Ensemble, Alaa A. Bahamran

Electronic Theses and Dissertations

We theoretically and experimentally investigate the transfer of orbital angular momentum from light to an ensemble of semiconductor-based nanostructures composed of lead sulfide quantum dots. Using an ensemble of quantum dots offers a higher cross-section and more absorption of twisted light fields compared to experimentally challenging single-nanostructure measurements. However, each quantum dot (except for on-center) sees a displaced light beam parallel to its own axis of symmetry. The transition matrix elements for the light-matter interaction are calculated by expressing the displaced light beam in terms of the appropriate light field centered on the nanoparticles. The resulting transition rate induced by …

Unusual Perpendicular Anisotropy In Co2Tisi Films, Yunlong Jin, Shah R. Valloppilly, Parashu R. Kharel, Rohit Pathak, Arti Kashyap, Ralph Skomski, David J. Sellmyer

David Sellmyer Publications

Thin films of Co₂TiSi on MgO are investigated experimentally and theoretically. The films were produced by magnetron sputtering on MgO(001) and have a thickness of about 100 nm. As bulk Co₂TiSi, they crystallize in the normal cubic Heusler (L2₁) structure, but the films are slightly distorted (c/a = 1.0014) and contain some antisite disorder. The films exhibit a robust perpendicular anisotropy of 0.5 MJ m⁻³. This result is surprising for several reasons. First, surface and interface anisotropies are too small to explain perpendicular anisotropy in such rather thick …

Spin Filtering In Cri3 Tunnel Junctions, T. R. Paudel And E. Y. Tsymbal

Evgeny Tsymbal Publications

No abstract provided.

Anisotropic Spin-Orbit Torque Generation In Epitaxial Sriro3 By Symmetry Design, T. Nan, T. J. Anderson, J. Gibbons, K. Hwang, N. Campbell, H. Zhou, Y. Q. Dong, G. Y. Kim, D. F. Shao, T. R. Paudel, N. Reynolds, X. J. Wang, N. X. Sun, E. Y. Tsymbal, S. Y. Choi, M. S. Rzchowski, Y. B. Kim, D. C. Ralph And C. B. Eom

Evgeny Tsymbal Publications

No abstract provided.

Spin-Dependent Transport In Van Der Waals Magnetic Tunnel Junctions With Fe3gete2 Electrodes, X. L. Li, J.-T. Lü, J. Zhang, Y. R. Su, And E. Y. Tsymbal

Evgeny Tsymbal Publications

No abstract provided.

Freestanding Crystalline Oxide Perovskites Down To The Monolayer Limit, D. X. Ji, S. H. Cai, T. R. Paudel, H. Y. Sun, C. C. Zhang, Y. F. Wei, Y. P. Zang, M. Gu, L. Han, Y. Zhang, W. P. Gao, H. X. Huyan, W. Guo, D. Wu, Z. B. Gu, E. Y. Tsymbal, P. Wang, Y. F. Nie, And X. Q. Pan

Evgeny Tsymbal Publications

No abstract provided.

Polarization-Dependent Electric Potential Distribution Across Nanoscale Ferroelectric Hf0.5zr0.5o2 In Functional Memory Capacitors, Y. Matveyev, V. Mikheev, D. Negrov, S. Zarubin, A. Kumar, E. D. Grimley, J. M. Lebeau, A. Gloskovskii, E. Y. Tsymbal, And A. Zenkevich

Evgeny Tsymbal Publications

No abstract provided.

Grating Coupler For Surface Waves Based On Electrical Displacement Currents, Jonathan R. Brescia

Honors Undergraduate Theses

Bound electromagnetic surface waves can be excited by free-space waves on a corrugated conduction surface. These electromagnetic surface waves, called surface plasmon polaritons (SPPs), are coupled to a plasma of free charges, which travel together with the wave. We investigated the effect of separating metal corrugations from the smooth metal ground plane with a thin dielectric layer and show that SPPs can be excited via displacement currents. However, the SPP excitation resonances broaden and disappear as the dielectric thickness approaches 1% of the wavelength.

The Structural Information Filtered Features Potential For Machine Learning Calculations Of Energies And Forces Of Atomic Systems., Jorge Arturo Hernandez Zeledon

Graduate Theses, Dissertations, and Problem Reports

In the last ten years, machine learning potentials have been successfully applied to the study of crystals, and molecules. However, more complex materials like clusters, macro-molecules, and glasses are out reach of current methods. The input of any machine learning system is a tensor of features (the most universal type are rank 1 tensors or vectors of features), the quality of any machine learning system is directly related to how well the feature space describes the original physical system. So far, the feature engineering process for machine learning potentials can not describe complex material. The current methods are highly inefficient …

Two-Dimensional Bloch Electrons In Electric And Magnetic Fields, Edward Aris Diaz Fajardo

Graduate Research Theses & Dissertations

Due to their extraordinary mechanical, and fascinating electronic properties, atomically flat two-dimensional materials are attractive avenues in the search for new and interesting physical phenomena. Symmetry is a powerful tool in studying the dynamics of Bloch electrons in these crystalline solids. Here, using a tight binding description, a systematic scheme is developed to derive the symmetry labels, called irreducible representations (IRs), characterizing the Bloch eigenstates in a crystal, including an extensive discussion on the possibility that these IRs are not always unique. This theory is illustrated using monolayer MoS$_2$ and few-layer graphene as examples. Using this symmetry analysis in conjunction …

Magnetoelectric Control Of Topological Phases In Graphene, Hiroyuki Takenaka, Shane Sandhoefner, Alexey Kovalev, Evgeny Tsymbal

Department of Physics and Astronomy: Faculty Publications

Topological antiferromagnetic (AFM) spintronics is an emerging field of research, which involves the topological electronic states coupled to the AFM order parameter known as the Néel vector. The control of these states is envisioned through manipulation of the Néel vector by spin-orbit torques driven by electric currents. Here we propose a different approach favorable for low-power AFM spintronics, where the control of the topological states in a two-dimensional material, such as graphene, is performed via the proximity effect by the voltage induced switching of the Néel vector in an adjacent magnetoelectric AFM insulator, such as chromia. Mediated by the symmetry …

Depth Dependent Atomic Valence Determination In La0.7sr0.3mno3 Thin Films Using Synchrotron Techniques, Robbyn B. Trappen

Graduate Theses, Dissertations, and Problem Reports

The valence of atoms often has a strong effect on the properties of materials, such as magnetism, conductivity, and superconductivity. The atomic valence is often perturbed at the surface and/or interface and this deviation may play a strong role in many physical phenomena such as interfacial coupling and dead layers, both magnetic and electric. In this dissertation, I present a non-destructive approach of combining two X-ray absorption detection modes, electron yield and fluorescence, with very different probing depths in conjunction with theory to map out the layer-by-layer valence of a thin film.

The weighted average Mn atomic valence as measured …

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 …

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 …

Electron Transport In One And Two Dimensional Materials, Samuel William Lagasse

Legacy Theses & Dissertations (2009 - 2024)

This dissertation presents theoretical and experimental studies in carbon nanotubes (CNTs), graphene, and van der Waals heterostructures. The first half of the dissertation focuses on cutting edge tight-binding-based quantum transport models which are used to study proton irradiation-induced single-event effects in carbon nanotubes [1], total ionizing dose effects in graphene [2], quantum hall effect in graded graphene p-n junctions [3], and ballistic electron focusing in graphene p-n junctions [4]. In each study, tight-binding models are developed, with heavy emphasis on tying to experimental data. Once benchmarked against experiment, properties of each system which are difficult to access in the laboratory, …

Theoretical Studies Of The Structure And Stability Of Metal Chalcogenide Crntem (1≤N≤6, 1≤M≤8) Clusters, Fnu Sweta Prabha

Theses and Dissertations

In the presented work, first principle studies on electronic structure, stability, and magnetic properties of metal chalcogenide, Cr_nTe_m clusters have been carried out within a density functional framework using generalized gradient functions to incorporate the exchange and correlation effects. The energetic and electronic stability was investigated, and it was found that they are not always correlated as seen in the cluster Cr₆Te₈ which has smaller gap between its HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) and a high electron affinity of 3.39 eV indicating lower electronic stability whereas higher fragmentation …

New Perspectives On The Schrödinger-Pauli Theory Of Electrons: Part I, Viraht Sahni

Publications and Research

Schrödinger-Pauli (SP) theory is a description of electrons in the presence of a static electromagnetic field in which the interaction of the magnetic field with both the orbital and spin moments is explicitly considered. The theory is described from the new perspective of the individual electron via its equation of motion or ‘Quantal Newtonian’ first law. The law is in terms of ‘classical’ fields whose sources are quantum mechanical expectation values of Hermitian operators taken with respect to the system wave function. The law states that each electron experiences an external and an internal field, the sum of which vanish. …

Novel Computational Methods For Catalytic Applications, Gihan Uthpala Panapitiya

Graduate Theses, Dissertations, and Problem Reports

Thiolate protected nanoclusters gold nanoparticles are gaining interest of many researchers due to their promising applications in a variety of fields the development of synthesizing techniques capable of producing atomically precise nanoclusters with high purity. Au₂₅(SR)₁₈ is one of the widely studied nanoclsuters due its remarkable stability. In this first part of this study, we explore the structural, electronic and catalytic properties of bimetallic Au_25−xAg_x(SR)₁₈ (for x = 6, 7, 8). Due to the combinatorial enormity of the total number of possible alloyed isomers, we choose a randomly selected subset corresponding to …

Modelling Potential Fluctuations In Double Layer Graphene Systems As A Periodic Oscillation In Electron Density & Its Effect On Coulomb Drag, Ryan Bogucki

Williams Honors College, Honors Research Projects

An expression for the drag transresistivity in a graphene double layer system exhibiting potential fluctuations modelled as a periodic oscillation in electron density is derived. Our model starts from the Coulombic interaction and we derive the correlation between a sinusoidal fluctuation in electron density in the first layer and the induced electron density in the second layer. Previous models in the literature have employed an arbitrary correlation between each layer’s electron density, and the model presented is the first attempt in the literature to explicitly derive this correlation. Recent experiments have found that the drag transresistivity in graphene double layers …

Electronic And Geometric Structure Of Alnom And Alnom +, Albert R. Armstrong

Theses and Dissertations

Generally, the electronic stability of aluminum clusters is associated with either closed electronic shells of delocalized electrons, or aluminum in the +3 state. To explore alternative routes for electronic stability in aluminum oxide clusters, theoretical methods were used to examine the geometric and electronic structure of Al_nO_m (2≤n≤7; 1≤m≤10) clusters. Two types of electronically stable clusters with large HOMO-LUMO gaps were identified the first being Al_2nO_3m clusters with a +3 oxidation state on the aluminum, and the second being planar clusters such as Al₄O₄, Al₅O₃, Al …

Straintronic Nanomagnetic Devices For Non-Boolean Computing, Md Ahsanul Abeed

Theses and Dissertations

Nanomagnetic devices have been projected as an alternative to transistor-based switching devices due to their non-volatility and potentially superior energy-efficiency. The energy efficiency is enhanced by the use of straintronics which involves the application of a voltage to a piezoelectric layer to generate a strain which is ultimately transferred to an elastically coupled magnetostrictive nanomaget, causing magnetization rotation. The low energy dissipation and non-volatility characteristics make straintronic nanomagnets very attractive for both Boolean and non-Boolean computing applications. There was relatively little research on straintronic switching in devices built with real nanomagnets that invariably have defects and imperfections, or their adaptation …

The Structure And Stability Of Cationic Metal-Benzene Clusters, Daniel P. Rabayda

Theses and Dissertations

We have investigated the size-dependent stability and structure of benzene, aluminum-benzene, and vanadium-benzene clusters. Motivated by gas-phase experimental studies performed by an experimental collaborator, we have used first-principle electronic structure methods to identify the structure of Al⁺(Bz)_n, V⁺(Bz)_n, and Bz_n clusters. Our studies reveal that cationic aluminum-benzene clusters have a magic number of 13, and that its high stability may be understood by analyzing the structure of the cluster. We also investigate the structure of vanadium-benzene clusters which have a magic number of 2. Here I examine the benzene-cation and benzene-benzene …