Condensed Matter Physics Commons^™

Hohenberg-Kohn Theorems In Electrostatic And Uniform Magnetostatic Fields, Xiao-Yin Pan, Viraht Sahni

Publications and Research

The Hohenberg-Kohn (HK) theorems of bijectivity between the external scalar potential and the gauge invariant nondegenerate ground state density, and the consequent Euler variational principle for the density, are proved for arbitrary electrostatic field and the constraint of fixed electron number. The HK theorems are generalized for spinless electrons to the added presence of an external uniform magnetostatic field by introducing the new constraint of fixed canonical orbital angular momentum. Thereby, a bijective relationship between the external scalar and vector potentials, and the gauge invariant nondegenerate ground state density and physical current density, is proved. A corresponding Euler variational principle …

Dependence Of Electrostatic Field Strength On Voltage Ramp Rates For Spacecraft Materials, Krysta Moser

Senior Theses and Projects

No abstract provided.

Effect Of Sm Content On Energy Product Of Rapidly Quenched And Oriented Smco5 Ribbons, Wenyong Zhang, Xingzhong Li, Shah R. Valloppilly

Nebraska Center for Materials and Nanoscience: Faculty Publications

The Sm-content dependence of phase composition, anisotropy, and other magnetic properties of Sm_1+δCo₅ (δ ≤ 0.12) ribbons melt spun at 10 m/s has been studied. The samples consist of hexagonal SmCo₅ grains whose c axes are preferentially aligned along the long direction of the ribbon. The lattice parameter a and the cell volume (V) increase with increasing Sm content δ, whereas c decreases. Sm addition appears to improve the degree of the preferred orientation of the c-axis and to increase the mean grain size, which weakens the effective intergranular exchange …

Force-Enhanced Atomic Refinement: Structural Modeling With Interatomic Forces In A Reverse Monte Carlo Approach Applied To Amorphous Si And Sio2, A. Pandey, Parthapratim Biswas, D. A. Drabold

Faculty Publications

We introduce a structural modeling technique, called force-enhanced atomic refinement (FEAR). The technique incorporates interatomic forces in reverse Monte Carlo (RMC) simulations for structural refinement by fitting experimental diffraction data using the conventional RMC algorithm, and minimizes the total energy and forces from an interatomic potential. We illustrate the usefulness of the approach by studying a−SiO₂ and a−Si. The structural and electronic properties of the FEAR models agree well with experimental neutron and x-ray diffraction data and the results obtained from previous molecular dynamics simulations of a−SiO₂ and a−Si. We have shown that the method is more efficient …

Pre-Breakdown Arcing As A Proxy For Dc Dielectric Breakdown Testing Of Polymeric Insulators, Jr Dennison, Allen Andersen

Presentations

No abstract provided.

Mixed Weibull Distribution Model Of Dc Dielectric Breakdowns With Dual Defect Modes, Allen Andersen, Jr Dennison

Journal Articles

This work provides physical insight into common statistical models for DC dielectric breakdown field strengths. Voltage step-up tests were performed on low density polyethylene films. The merits of generalizations to widely-used empirical Weibull models are discussed. The cumulative probability distributions of the breakdown fields were fit to standard two- and three-parameter Weibull distributions. Mixed two-parameter Weibull distributions, sometimes used in the literature to model multiple breakdown modes, were found to yield the best fits to the data. In addition, the same data were fit to a physically-motivated dual-defect mean field model incorporating both low- and high-energy defect modes with different …

Mixed Weibull Distribution Model Of Dc Dielectric Breakdowns With Dual Defect Modes, Allen Andersen, Jr Dennison

Presentations

Purpose/Aim

This work proposes an enhanced statistical model for DC dielectric breakdown properties, which incorporates both shallow and deep defect modes. By incorporating these physics-based concepts into a traditionally empirical model, its accuracy and utility can be extended.

Experimental/Modeling methods

Voltage step-up tests were performed for various polymeric films. The distributions of the breakdown fields are fit to standard two- and three-parameter Weibull distributions. These are extended for a dual defect model, with separate mean energies for shallow and deep level traps, using mixed Weibull distributions.

Results/discussion

As shown in Fig 1, such enhanced physics-based statistical models can result in …

Hohenberg-Kohn Theorems In Electrostatic And Uniform Magnetostatic Fields, Xiao-Yin Pan, Viraht Sahni

Publications and Research

The Hohenberg-Kohn (HK) theorems of bijectivity between the external scalar potential and the gauge invariant nondegenerate ground state density, and the consequent Euler variational principle for the density, are proved for arbitrary electrostatic field and the constraint of fixed electron number. The HK theorems are generalized for spinless electrons to the added presence of an external uniform magnetostatic field by introducing the new constraint of fixed canonical orbital angular momentum. Thereby a bijective relationship between the external scalar and vector potentials, and the gauge invariant nondegenerate ground state density and physical current density, is proved. A corresponding Euler variational principle …

The Space Survivability Test Chamber, Katie Gamaunt, Heather Tippets, Alex Souvall, Ben Russon, Jr Dennison

Posters

The Space Survivability Test chamber is a new ground-based research instrument being used for accelerated testing of environment-induced modifications of diverse samples. The chamber simulates space environment conditions, including neutral gas atmospheres and vacuum (<10^-5 Pa) environments, temperature (~100 K to >450 K), ionizing radiation, electron fluxes (<10 eV to ~2½ MeV), and vacuum ultraviolet through mid-infrared photon fluxes. This versatile test chamber is well-suited for cost-effective testing of complete systems up to the size (< 20 cm dia.) of a 1U CubeSat, smaller components or electronics, and individual material samples. Multiple in-flux or in-situ space survivability and radiation exposure tests can be performed simultaneously, as well as extensive before and after ex-situ tests. Currently the chamber is performing a series of radiation experiments using a Sr⁹⁰ beta radiation source which approximately mimics the geostationary high energy electron spectra at ~4-10X accelerated …

Comparing Experimental Apples And Oranges With Quantile-Quantile Plots, Allen Andersen, Jr Dennison

Presentations

An important question often encountered in experimental physics is, are two observables related or not? Quantile-quantile (q-q) analysis compares the cumulative distributions of two observations (or one set of observations and a theoretical curve) in a way that is both visually apparent and statistically quantifiable. If the two observables follow the same distribution, the q-q plot will be linear; if they are identical the plot will have unity slope. Deviations from a linear q-q plot indicate that the two observables do not follow the same distribution. We show that the q-q analysis method is applicable to a wide range of …

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

Posters

Previous tests done by the USU Materials Physics Group (MPG) using our electrostatic discharge (ESD) custom vacuum chamber have found that, for the polymeric materials polyimide and low density polyethylene (LDPE), the electrostatic field strength at breakdown depends on the voltage ramp rate applied across the materials. At ramp rates an order of magnitude lower than the maximum recommended rate of 500 V/s, the breakdown electrostatic field strength was also found to be significantly lower. The data from these tests were compared to a microscopic mean field theory for dielectric breakdown in highly disordered insulating materials. We present new ramp …

Enhanced Electron Yield Measurements Of Extremely Low-Conductivity High Yield Dielectrics, Justin Christensen, Jr Dennison

Presentations

An important question often encountered in experimental physics is, are two observables related or not? Quantile-quantile (q-q) analysis compares the cumulative distributions of two observations (or one set of observations and a theoretical curve) in a way that is both visually apparent and statistically quantifiable. If the two observables follow the same distribution, the q-q plot will be linear; if they are identical the plot will have unity slope. Deviations from a linear q-q plot indicate that the two observables do not follow the same distribution. We show that the q-q analysis method is applicable to a wide range of …

Properties Of Spacecraft Materials Exposed To Ionizing Radiation, Alex Souvall, Gregory Wilson, Katie Gamaunt, Ben Russon, Heather Tippets, Jr Dennison

Posters

The effects of ionizing radiation damage on the various properties of spacecraft materials resulting from exposure in the Space Survivability Testing chamber (SST) are being studied with both ex situ and in situ tests. The SST is a ground based test facility designed to mimic low earth orbit (LEO), and geosynchronous orbit to test potential environmental-induced modifications to small satellites , and materials. Tests described here expose spacecraft materials to a Sr⁹⁰ ionizing beta radiation source at room temperature and in high vacuum. Ex situ optical transmission/reflectivity measurements glass samples will monitor optical darkening. Properties of polymeric samples will …

Density Of State Models Of Steady-State Temperature Dependent Radiation Induced Conductivity, Jodie Corbridge Gillespie, Jr Dennison

Posters

Radiation induced conductivity (RIC) occurs when incident radiation deposits energy and excites electrons into the conduction band of insulators. The magnitude of the enhanced conductivity is dependent on a number of factors including temperature and the spatial- and energy-dependence and occupation of the material’s distribution of localized trap states within the band gap—or density of states (DOS). Expressions are developed for steady-state RIC over an extended temperature range, based on DOS models for highly disordered insulating materials. A general discussion of the DOS of disordered materials can be given using two simple distributions: one that monotonically decreases below the band …

Measurement Of Effects Of Long Term Ionizing Radiation On High Efficiency Solar Arrays, Ben Russon, Heather Tippets, Gregory Wilson, Katie Gamaunt, Alex Souvall, Jr Dennison

Posters

Degradation of power output efficiency for high-efficiency multilayer solar arrays due to ionizing radiation is measured using the Space Survivability Test chamber. Exposure to ionizing radiation disrupts the crystalline structure and can reduce solar array power output to the point that it no longer provides adequate output capacity. This can be a significant concern, particularly in the harsh environment of space where radiation dose rate is significantly higher and replacing components is often impossible. Ionizing radiation is simulated in a controlled environment to allow measurement and characterization of the power output of solar arrays, using a 100 mCi encapsulated Sr90 …

Oxygen Vacancies In Lialo2 Crystals, Maurio S. Holston, Ian P. Ferguson, John W. Mcclory, Nancy C. Giles, Larry E. Halliburton

Faculty Publications

Singly ionized oxygen vacancies are produced in LiAlO₂ crystals by direct displacement events during a neutron irradiation. These vacancies, with one trapped electron, are referred to as V+O centers. They are identified and characterized using electron paramagnetic resonance (EPR) and optical absorption. The EPR spectrum from the V+O centers is best monitored near 100 K with low microwave power. When the magnetic field is along the [001] direction, this spectrum has a g value of 2.0030 and well-resolved hyperfine interactions of 310 and 240 MHz with the two 27Al nuclei that are adjacent to the oxygen vacancy. A second …

Hybridized Criticality And Elementary Excitations In Lihof4, Haifu Ma

Dissertations, Theses, and Capstone Projects

In this dissertation, I study the magnetic properties of LiHoF₄. Quantum criticality in rare earth ferromagnet LiHoF₄ is complicated by the presence of strong crystal field and hyperfine interactions resulting, e.g., in incomplete mode softening reported by Rønnow et al. We construct a systematic framework for treating elementary excitations in this material across the phase diagram. These excitations interpolate between purely electronic, nuclear and lattice modes and exhibit two-types of quantum critical softening, both complete (as anticipated by elementary treatments, see e.g. Sachdev) but also incomplete, in close correspondence with nuclear scattering results.

Reconfigurable Large-Area Magnetic Vortex Circulation Patterns, Robert Streubel, Florian Kronast, Ulrich K. Rößler, Oliver G. Schmidt, Denys Makarov

Robert Streubel Papers

Magnetic vortices in nanodots own a switchable circulation sense. These nontrivial magnetization configurations can be arranged into extended and interacting patterns. We have experimentally created large arrays of magnetically reconfigurable vortex patterns in nonplanar honeycomb lattices using particle lithography. Optimizing height asymmetry of the vertices and applying an in-plane magnetic field provide means to switch between homocircular and staggered vortex patterns with a potentially high impact on magnonics and spintronics relying on chiral noncollinear spin textures. To this end, exchange coupling of extended vortex lattices with an out-of-plane magnetized layer allows one to realize artificial skyrmionic core textures with controllable …

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

Presentations

Previous tests done by the USU Materials Physics Group (MPG) using our electrostatic discharge (ESD) custom vacuum chamber have found that, for the polymeric materials polyimide and low density polyethylene (LDPE), the electrostatic field strength at breakdown depends on the voltage ramp rate applied across the materials. At ramp rates an order of magnitude lower than the maximum recommended rate of 500 V/s, the breakdown electrostatic field strength was also found to be significantly lower. The data from these tests were compared to a microscopic mean field theory for dielectric breakdown in highly disordered insulating materials. We present new ramp …

Pre-Breakdown Arcing As A Proxy For Dc Dielectric Breakdown Testing Of Polymeric Insulators, Allen Andersen, Jr Dennison

Journal Articles

Pre-breakdown arcing is proposed as a key indicator of DC breakdown properties of polymeric dielectric materials. Voltage step-up tests were performed for films of low-density polyethylene (LDPE) at ramp rates significantly lower than is common in most studies, allowing for the observation of both breakdown and transient pre-breakdown current spikes. The distributions of the breakdowns versus applied field were compared with the more frequent pre-breakdown arcs. A strong correlation was observed for LDPE between the distribution of breakdowns in step-up tests and the distribution of pre-breakdown arcing. Pre-breakdown arcing distributions are much easier to obtain than breakdown distributions and may …