Condensed Matter Physics Commons^™

Hopping Conductivity Of Electron Glass, Mingzhu Cui

Graduate Masters Theses

Hopping conductance between the sites in disordered systems is mapped to a random resistor network named Miller-Abrahams network. The conductance between two sites is in an exponential form depending on two parameters, the space separation and energy separation between sites. Our effort is focused on the visualization of the random resistor network. We find all the realizations for one electron hop and plot out the according random resistor networks. By comparing the random resistor network structures, we find that Coulomb interaction plays an important role in calculating the hopping conductance between sites. Furthermore, we rank the realizations by total energy …

Three-Dimensional Nanomagnetism, Amalio Fernández-Pacheco, Robert Streubel, Olivier Fruchart, Riccardo Hertel, Peter Fischer, Russell P. Cowburn

Robert Streubel Papers

Magnetic nanostructures are being developed for use in many aspects of our daily life, spanning areas such as data storage, sensing and biomedicine. Whereas patterned nanomagnets are traditionally two-dimensional planar structures, recent work is expanding nanomagnetism into three dimensions; a move triggered by the advance of unconventional synthesis methods and the discovery of new magnetic effects. In three-dimensional nanomagnets more complex magnetic configurations become possible, many with unprecedented properties. Here we review the creation of these structures and their implications for the emergence of new physics, the development of instrumentation and computational methods, and exploitation in numerous applications.

Machine Learning Phases Of Strongly Correlated Fermions, Kelvin Ch'ng, Juan Carrasquilla, Roger Melko, Ehsan Khatami

Faculty Publications

Machine learning offers an unprecedented perspective for the problem of classifying phases in condensed matter physics. We employ neural network machine learning techniques to distinguish finite-temperature phases of the strongly-correlated fermions on cubic lattices. We show that a three-dimensional convolutional network trained on auxiliary field configurations produced by quantum Monte Carlo simulations of the Hubbard model can correctly predict the magnetic phase diagram of the model at the average density of one (half filling). We then use the network, trained at half filling, to explore the trend in the transition temperature as the system is doped away from half filling. …

Evidence For A Low-Temperature Magnetic Ground State In Double-Perovskite Iridates With Ir5+(5D4) Ions, Jasminka Terzic, H. Zheng, Feng Ye, H. D. Zhao, P. Schlottmann, Lance E. De Long, S. J. Yuan, Gang Cao

Physics and Astronomy Faculty Publications

We report an unusual magnetic ground state in single-crystal, double-perovskite Ba₂YIrO₆ and Sr-doped Ba₂YIrO₆ with Ir⁵⁺(5d⁴) ions. Long-range magnetic order below 1.7 K is confirmed by dc magnetization, ac magnetic susceptibility, and heat-capacity measurements. The observed magnetic order is extraordinarily delicate and cannot be explained in terms of either a low-spin S = 1 state, or a singlet J_eff = 0 state imposed by the spin-orbit interactions (SOI). Alternatively, the magnetic ground state appears consistent with a SOI that competes with comparable Hund's rule coupling and inherently large …

Nanosecond X-Ray Photon Correlation Spectroscopy On Magnetic Skyrmions, M. H. Seaberg, B. Holladay, J. C.T. Lee, M. Sikorski, A. H. Reid, S. A. Montoya, G. L. Dakovski, J. D. Koralek, G. Coslovich, S. Moeller, W. F. Schlotter, R. Streubel, S. D. Kevan, P. Fischer, E. E. Fullerton, J. L. Turner, F. J. Decker, S. K. Sinha, S. Roy, J. J. Turner

Robert Streubel Papers

We report an x-ray photon correlation spectroscopy method that exploits the recent development of the two-pulse mode at the Linac Coherent Light Source. By using coherent resonant x-ray magnetic scattering, we studied spontaneous fluctuations on nanosecond time scales in thin films of multilayered Fe/Gd that exhibit ordered stripe and Skyrmion lattice phases. The correlation time of the fluctuations was found to differ between the Skyrmion phase and near the stripe-Skyrmion boundary. This technique will enable a significant new area of research on the study of equilibrium fluctuations in condensed matter.

Perspectives On The Distributions Of Esd Breakdowns For Spacecraft Charging Applications, Allen Andersen, Krysta Moser, Jr Dennison

Journal Articles

Electrostatic discharge (ESD) continues to pose significant risks to space missions despite decades of intense study. Tabulated values of material breakdown strength used in spacecraft charging models are often based on cursory measurements that may not be fully relevant to a given mission. Materials physics offers insight into the pertinent variables that affect breakdown and how to address them experimentally for spacecraft applications. We present measured distributions of ESD data across several test configurations for three polymeric materials that, taken together, begin to provide an understanding of how to estimate the likelihood of ESD events over a spacecraft’s mission lifetime. …

Pea System Modeling And Signal Processing For Measurement Of Volume Charge Distributions In Thin Dielectric Films, Lee H. Pearson, Jr Dennison, Erick W. Griffiths, Anthony C. Pearson

Journal Articles

This paper discusses our effort to develop advanced pulsed electroacoustic (PEA) measurement system capabilities that incorporate (1) improved signal processing tools for increased signal/noise ratios; and (2) integrated PEA modeling tools. In addition, we emphasize state-of-the-art system electronic components, integrated environmental controls, and sensor improvements required to achieve high spatial resolution while maintaining reasonable temporal resolution for both ambient and in vacuo measurements of thin dielectrics charged using electron beam injection, which is most applicable for spacecraft charging tests. PEA measurement systems provide an important tool to investigate the spatial extent and dynamic evolution of embedded charge distributions in thin …

Measuring And Modeling The Conductivity Of Highly Insulating Materials, David King

Senior Theses and Projects

Satellites and other spacecraft must be able to withstand hazardous conditions in order to be viable in their expected operating environments. One specific hazard that occurs due to incident radiation is spacecraft charging. Insulating materials, frequently used in spacecraft and other electrical equipment, while very good at preventing charge flow, also store charge very well. This can create problems; specifically, “if the charge decay time exceeds the orbital period, not all charge will be dissipated before orbital conditions act again to further charge the satellite. As the insulator accumulates charge, the electric field will rise until the insulator breaks down” …

Dependence Of Electrostatic Field Strength On Voltage Ramp Rate For Spacecraft Materials, Krysta Moser, Allen Andersen, Jr Dennison

Journal Articles

This work investigated the dependence of electrostatic field strength for spacecraft materials on voltage ramp rate, by applying an increasing incremental electrostatic field until electrostatic breakdown occurred. Tests on Kapton E found that at ramp rates two or three orders of magnitude lower than the maximum recommended rate, the electrostatic breakdown field, FESD was lower by a factor of two or more. This suggests that tabulated values of FESD, which have been used by the spacecraft charging community, could substantially overestimate FESD in common slowly evolving spacecraft situations. This study expanded these ramp rate tests to include a wider range …

A Numerical Study Of The Interaction Between One Dimensional Carbyne Chain And Single Stranded Dna, Zeina Salman

Graduate Theses and Dissertations

resolution at the single nucleotide level when developing DNA sequencers. The purpose of this research was to numerically study the electrical properties associated with the interaction between 1D carbon chain, known as carbyne, and ssDNA. First, the electrical properties of the carbyne chain were calculated. Second, the electrical properties of the carbyne chain were calculated in the presence of different ssDNA bases. Analyzing the differences between the two cases led to determining the effects of these different bases on the electrical properties. The numerical simulation approach conducted in this research was based on the first-principle simulation. The first-principle simulation was …

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 …

Instrumentation And Measurement Of Thermoelectric And Structural Properties Of Binary Chalcogenides And Half-Heusler Alloys At Extreme Conditions Using A Paris-Edinburgh Press, Jason Lee Baker

UNLV Theses, Dissertations, Professional Papers, and Capstones

Understanding the high-pressure behavior of transport properties has been a driving force in the study of materials under extreme conditions for well over a century being pioneered by P.W. Bridgman in the early 20th century. Research dedicated to the study of these properties leads to a variety of important applications: exploration of insulator to semi-conductor to metal, structural, and electronic phase transitions, correlation of pressure-induced structural phase transitions and the electronic properties along phase boundaries, identifying electronic topological transitions, testing the validity of theoretical models and providing input parameters for calculations at high-pressure and high-temperature conditions, exploration of sample synthesis …

Fabrication And Functionalization Of Carbon Nanocages For Use In Cfx Batteries., Nathan Smith

Electronic Theses and Dissertations

A relatively inexpensive method for obtaining high quality lithium-CF_x batteries is reported. Nickel acetate and citric acid are used to synthesize a precursor which is then annealed and subsequently etched to fabricate highly uniform 3 nm radius pristine CNCs. These CNCs are then functionalized with fluorine using an RF plasma system. The resulting fluorinated CNCs are then combined with a carbon binder and pressed onto a stainless steel mesh to form a CF_x cathode. This cathode is then tested in a coin cell primary battery with a lithium anode. Results indicate that a specific capacity of 660 mAh/g …

Model Of Orbital Populations For Voltage-Controlled Magnetic Anisotropy In Transition-Metal Thin Films, Jia Zhang, Pavel V. Lukashev, Sitaram S. Jaswal, Evgeny Y. Tsymbal

Evgeny Tsymbal Publications

Voltage-controlled magnetic anisotropy (VCMA) is an efficient way to manipulate the magnetization states in nanomagnets and is promising for low-power spintronic applications. The underlying physical mechanism for VCMA is known to involve a change in the d orbital occupation on the transition-metal interface atoms with an applied electric field. However, a simple qualitative picture of how this occupation controls the magnetocrystalline anisotropy (MCA) and even why in certain cases the MCA has the opposite sign remains elusive. In this paper, we exploit a simple model of orbital populations to elucidate a number of features typical for the interface MCA, and …

Suppression Of Magnetism In Ba5Alir2O11: Interplay Of Hund's Coupling, Molecular Orbitals, And Spin-Orbit Interaction, Sergey V. Streltsov, Gang Cao, Daniel I. Khomskii

Center for Advanced Materials Faculty Publications

The electronic and magnetic properties of Ba₅AlIr₂O₁₁ containing Ir-Ir dimers are investigated using the generalized gradient approximation (GGA) and GGA + spin-orbit coupling (SOC) calculations. We found that the strong suppression of the magnetic moment in this compound recently found by Terzic et al. [Phys. Rev. B 91, 235147 (2015)] is not due to charge ordering but is related to the joint effect of the spin-orbit interaction and strong covalency, resulting in the formation of metal-metal bonds. They conspire and act against the intraatomic Hund's rule exchange interaction to reduce total magnetic moment of the …

Spin-Polarized Two-Dimensional Electron Gas At Gdtio3/Srtio3 Interfaces: Insight From First-Principles Calculations, J. Betancourt, Tula R. Paudel, Evgeny Y. Tsymbal, J. P. Velev

Evgeny Tsymbal Publications

Two-dimensional electron gases (2DEGs) at oxide interfaces have been a topic of intensive research due to their high carrier mobility and strong confinement. Additionally, strong correlations in the oxide materials can give rise to new and interesting physics, such as magnetism and metal-insulator transitions at the interface. Using first-principles calculations based on density functional theory, we demonstrate the presence of a highly spin-polarized 2DEG at the interface between the Mott insulator GdTiO₃ and a band insulator SrTiO₃. The strong correlations in the dopant cause ferromagnetic alignment of the interface Ti atoms and result in a fully spin-polarized …

Simulation Of Alnico Coercivity, Liqin Ke, Ralph Skomski, Todd D. Hoffman, Lin Zhoue, Wei Tang, Duane D. Johnson, Matthew J. Kramer, Iver E. Anderson, C.Z. Wang

Nebraska Center for Materials and Nanoscience: Faculty Publications

Micromagnetic simulations of alnico show substantial deviations from Stoner-Wohlfarth behavior due to the unique size and spatial distribution of the rod-like Fe-Co phase formed during spinodal decomposition in an external magnetic field. The maximum coercivity is limited by single-rod effects, especially deviations from ellipsoidal shape, and by interactions between the rods. Both the exchange interaction between connected rods and magnetostatic interaction between rods are considered, and the results of our calculations show good agreement with recent experiments. Unlike systems dominated by magnetocrystalline anisotropy, coercivity in alnico is highly dependent on size, shape, and geometric distribution of the Fe-Co phase, all …

Cathodoluminescence Studies Of Defects In Coated Boron Nitride, Kévin Guerch, Justin Dekany, Jr Dennison, Thierry Paulmier, Sophie Guillemet-Fritsch, Pascal Lenormand

Journal Articles

Optical emission properties of Boron Nitride (BN) substrates, BN with alumina (Al2O3) coating, and thermally-annealed alumina-coated boron nitride (an-BN/Al2O3) were investigated under electron irradiation using cathodoluminescence (CL) measurements. Tests were performed temperatures ranging from ~100 K to ~300 K, with monoenergetic beams from 5 keV to 30 keV, and electron flux densities from 1 nA.cm-2 to 500 nA.cm-2. These experiments were conducted to identify the effects of coating and thermal annealing on the nature and occupation of defect states in different samples with BN substrates. Previous studies have shown that these treatments can limit the charging of BN substrates. Consequently, …

Reversible Spin Texture In Ferroelectric Hfo2, L. L. Tao, Tula R. Paudel, Alexey Kovalev, Evgeny Y. Tsymbal

Evgeny Tsymbal Publications

Spin-orbit coupling effects occurring in noncentrosymmetric materials are known to be responsible for nontrivial spin configurations and a number of emergent physical phenomena. Ferroelectric materials may be especially interesting in this regard due to reversible spontaneous polarization making possible a nonvolatile electrical control of the spin degrees of freedom. Here, we explore a technologically relevant oxide material, HfO₂, which has been shown to exhibit robust ferroelectricity in a noncentrosymmetric orthorhombic phase. Using theoretical modelling based on density-functional theory, we investigate the spin-dependent electronic structure of the ferroelectric HfO₂ and demonstrate the appearance of chiral spin textures driven …

Vortex Circulation Patterns In Planar Microdisk Arrays, Sven Velten, Robert Streubel, Alan Farhan, Noah Kent, Mi Young Im, Andreas Scholl, Scott Dhuey, Carolin Behncke, Guido Meier, Peter Fischer

Robert Streubel Papers

We report a magnetic X-ray microscopy study of the pattern formation of circulation in arrays of magnetic vortices ordered in a hexagonal and a honeycomb lattice. In the honeycomb lattice, we observe at remanence an ordered phase of alternating circulations, whereas in the hexagonal lattice, small regions of alternating lines form. A variation in the edge-to-edge distance shows that the size of those regions scales with the magnetostatic interaction. Micromagnetic simulations reveal that the patterns result from the formation of flux closure states during the nucleation process.

Electronic Structure And Direct Observation Of Ferrimagnetism In Multiferroic Hexagonal Ybfeo3, Shi Cao, Kishan Sinha, Xin Zhang, Xiaozhe Zhang, Xiao Wang, Yuewei Yin, Alpha T. N’Diaye, Jian Wang, David J. Keavney, Tula R. Paudel, Yaohua Liu, Xuemei Cheng, Evgeny Y. Tsymbal, Peter A. Dowben, Xiaoshan Xu

Evgeny Tsymbal Publications

The magnetic interactions between rare-earth and Fe ions in hexagonal rare-earth ferrites (h-RFeO₃), may amplify the weak ferromagnetic moment on Fe, making these materials more appealing as multiferroics. To elucidate the interaction strength between the rare-earth and Fe ions as well as the magnetic moment of the rare-earth ions, element-specific magnetic characterization is needed. Using x-ray magnetic circular dichroism, we have studied the ferrimagnetism in h-YbFeO₃ by measuring the magnetization of Fe and Yb separately. The results directly show antialignment of magnetization of Yb and Fe ions in h-YbFeO₃ at low temperature, …

Temperature Dependence Of Electron Yield In Low-Density Polyethylene, Jordan Lee

Senior Theses and Projects

No abstract provided.

Optical Signatures Of Spin-Orbit Exciton In Bandwidth-Controlled Sr2Iro4 Epitaxial Films Via High-Concentration Ca And Ba Doping, Maryam Souri, B. H. Kim, John H. Gruenewald, John G. Connell, J. Thompson, J. Nichols, Jsaminka Terzic, B. I. Min, Gang Cao, Joseph W. Brill, Sung S. Ambrose Seo

Physics and Astronomy Faculty Publications

We have investigated the electronic and optical properties of (Sr_1−xCa_x)₂IrO₄ (x = 0–0.375) and (Sr_1−yBa_y)₂IrO₄ (y = 0–0.375) epitaxial thin films, in which the bandwidth is systematically tuned via chemical substitutions of Sr ions by Ca and Ba. Transport measurements indicate that the thin-film series exhibits insulating behavior, similar to the J_eff = 1/2 spin-orbit Mott insulator Sr₂IrO₄. As the average A-site ionic radius increases from (Sr_1−xCa_x)₂IrO₄ to (Sr …

Wave Propagation Inside Random Media, Xiaojun Cheng

Dissertations, Theses, and Capstone Projects

This thesis presents results of studies of wave scattering within and transmission through random and periodic systems. The main focus is on energy profiles inside quasi-1D and 1D random media.

The connection between transport and the states of the medium is manifested in the equivalence of the dimensionless conductance, g, and the Thouless number which is the ratio of the average linewidth and spacing of energy levels. This equivalence and theories regarding the energy profiles inside random media are based on the assumption that LDOS is uniform throughout the samples. We have conducted microwave measurements of the longitudinal energy profiles …

Usu Space Environment Effects Materials Test Facility Flyer, Jr Dennison

Posters

No abstract provided.

Reversible Spin Texture In Ferroelectric Hfo2, L. L. Tao, Tula R. Paudel, Alexey Kovalev, Evgeny Tsymbal

Department of Physics and Astronomy: Faculty Publications

Spin-orbit coupling effects occurring in noncentrosymmetric materials are known to be responsible for nontrivial spin configurations and a number of emergent physical phenomena. Ferroelectric materials may be especially interesting in this regard due to reversible spontaneous polarization making possible a nonvolatile electrical control of the spin degrees of freedom. Here, we explore a technologically relevant oxide material, HfO2, which has been shown to exhibit robust ferroelectricity in a noncentrosymmetric orthorhombic phase. Using theoretical modelling based on density-functional theory, we investigate the spin-dependent electronic structure of the ferroelectric HfO2 and demonstrate the appearance of chiral spin textures driven by spin-orbit coupling. …

Ferroelectric-Domain-Patterning-Controlled Schottky Junction State In Monolayer Mos2, Zhiyong Xiao, Jingfeng Song, David K. Ferry, Stephen Ducharme, Xia Hong

Stephen Ducharme Publications

We exploit scanning-probe-controlled domain patterning in a ferroelectric top layer to induce nonvolatile modulation of the conduction characteristic of monolayer MoS₂ between a transistor and a junction state. In the presence of a domain wall, MoS₂ exhibits rectified I-V characteristics that are well described by the thermionic emission model. The induced Schottky barrier height Φ^eff_B varies from 0.38 to 0.57 eV and is tunable by a SiO₂ global back gate, while the tuning range of Φ^eff_B depends sensitively on the conduction-band-tail trapping states. Our work points to a new route to achieving programmable …

Effects Of Temperature And Radiation Dose On Radiation Induced Conductivity, Jr Dennison

Presentations

Radiation induced conductivity (RIC) is an important conduction mechanism in highly disordered insulating materials exposed to ionizing radiation. Measurements of RIC as a function of dose rate and exposure time of polymeric and glassy insulators will be presented. RIC results from excitation of carriers into conduction states by the ionizing radiation. The measurements will be discussed in terms of models for the distribution of localized disordered states. The effects of temperature and radiation damage from ionizing radiation on the density and occupation of trap states, and how this affects RIC, will also be discussed.

Effects Of Temperature And Radiation Dose On Radiation Induced Conductivity, Jr Dennison

Presentations

No abstract provided.

Enhanced Test Facility For Survivability And Characterization Of Evolving Multiscale Materials In Extreme Plasma Environments, Jr Dennison

Posters

No abstract provided.