Physics Commons^™

Fast Neutron Assay Of Elemental Content Of Bulk Samples, Calder Emerson Lane

UNLV Theses, Dissertations, Professional Papers, and Capstones

The purpose of this research was to analyze the capabilities of fast neutrons in the detection and analysis of various isotopes in bulk samples. The deuterium-tritium (DT) fusion reaction generates highly penetrating, high-energy (14.1-MeV) neutrons which induce nuclear reactions in irradiated targets. Neutrons and gamma rays are generated in these reactions. Emitted gamma rays are characteristic of the emitter; the gamma spectrum enables stoichiometric identification of the assayed samples. Neutron backscattering can also be used for identification of the elemental composition of the sample.

This work had three objectives. The first objective was to develop a computational technique to model …

Constraining Viscous Protoplanetary Disk Models Using Alma Kinematic Observations, Ian Rabago

UNLV Theses, Dissertations, Professional Papers, and Capstones

Recent molecular line observations by ALMA have revealed the 3D velocity structure of protoplanetary disks, providing new insight on the mechanisms of disk accretion and ring/gap formation. Although the constant α viscous disk model has been widely used to describe the disk surface density evolution, it may not adequately describe the gas velocity at different disk heights. By carrying out viscous hydrodynamic simulations, we confirm that the vertical velocity structure varies considerably between these models. On the other hand, the surface density evolution of the disk is only dependent on the vertically integrated stress, and does not depend on a …

Synthesis, X-Ray Absorption, And X-Ray Diffraction Spectroscopy Studies Of Uranium Based Heavy Fermion Systems, Vahe Mkrtchyan

UNLV Theses, Dissertations, Professional Papers, and Capstones

Heavy fermion systems have attracted a great deal of interest since the discovery of the phenomenon in the 1970s. Over the past 35 years, the research in this field revealed a variety of unprecedented properties that the conventional theoretical framework considered conflicting or impossible. The subset of intermetallic compounds containing rare earth and actinide elements is of special interest, as these materials exhibit a range of exotic properties associated with 4f or 5f electrons. A thorough examination of the reports in the literature indicates a delicate interplay between the competing ground states, giving rise to anomalous properties including non-Fermi liquid …

Vibration Overtone Hyperpolarizability Measured For H2 Using Electric Field Induced Second Harmonic Generation, Rachel Ellis

UNLV Theses, Dissertations, Professional Papers, and Capstones

The vibrational contribution to the second hyperpolarizability (γ) of gas phase H2 was measured using electric field induced second harmonic generation. Measurements were made for the pure vibrational 3-0 Q(J) overtone transitions (v=0,J → v’=3,J for J = 0,1,2,3). Measured intensities were found to be 4-14% larger than ab initio calculations. The first chapter of this thesis provides an introduction to nonlinear optics, second harmonic generation, and the tensor nature of hyperpolarizabilities. The second chapter provides theoretical expressions for γ and relevant static and Raman polarizabilities. The third chapter covers the experimental design and electric field induced second harmonic generation. …

Response Of The Mode Grüneisen Parameters With Anisotropic Compression: A Pressure And Temperature Dependent Raman Study Of Β-Sn, Jasmine K. Hinton, Christian Childs, Dean Smith, Paul B. Ellison, Keith V. Lawler, Ashkan Salamat

Physics & Astronomy Faculty Research

The lattice dynamic response of body-centered tetragonal β−Sn (I41/amd) under high pressure and -temperature conditions is determined using experimental optical vibration modes. Raman scattering is used to map the phase stability region of β−Sn to perform mode Grüneisen analysis, and we demonstrate the necessity of an optical intensity calibration for Raman thermometry. The Grüneisen tensor is evaluated along a set of isotherms to address shortcomings of single-mode Grüneisen parameters with respect to anisotropic deformations of this tetragonal structured soft metal. The changes observed here in the Grüneisen tensor as a function of temperature are related to anharmonicity and denote potential …

Xenon Iron Oxides Predicted As Potential Xe Hosts In Earth’S Lower Mantle, Feng Peng, Xianqi Song, Chang Liu, Quan Li, Maosheng Miao, Changfeng Chen, Yanming Ma

Physics & Astronomy Faculty Research

An enduring geological mystery concerns the missing xenon problem, referring to the abnormally low concentration of xenon compared to other noble gases in Earth’s atmosphere. Identifying mantle minerals that can capture and stabilize xenon has been a great challenge in materials physics and xenon chemistry. Here, using an advanced crystal structure search algorithm in conjunction with first-principles calculations we find reactions of xenon with recently discovered iron peroxide FeO2, forming robust xenon-iron oxides Xe2FeO2 and XeFe3O6 with significant Xe-O bonding in a wide range of pressure-temperature conditions corresponding to vast regions in Earth’s lower mantle. Calculated mass density and sound …

Toughening A Superstrong Carbon Crystal: Sequential Bond-Breaking Mechanisms, Hui Liang, Hefei Li, Quan Li, Changfeng Chen

Physics & Astronomy Faculty Research

A complex orthorhombic carbon allotrope in Pbam symmetry with 32 atoms in its unit cell, thus termed Pbam-32 carbon, was recently predicted [C. Y. He et al., Phys. Rev. Lett. 121, 175701 (2018)]. Its crystal structure comprises alternating fivefold, sixfold, and sevenfold carbon rings and exhibits reduced bonding anisotropy compared to diamond, raising the prospects of finding a superstrong material with distinct and favorable mechanical properties. Here we report findings from first-principles calculations that reveal peculiar stress-strain relations in Pbam-32 carbon. The obtained stress responses under various tensile and shear strains display outstanding characteristics contrasting those of traditional superhard materials …

Pressure-Stabilized Divalent Ozonide Cao3 And Its Impact On Earth’S Oxygen Cycles, Yanchao Wang, Meiling Xu, Liuxiang Yang, Bingmin Yan, Qin Qin, Xuecheng Shao, Yunwei Zhang, Dajian Huang, Xiaohuan Lin, Jian Lv, Dongzhou Zhang, Huiyang Gou, Ho-Kwang Mao, Changfeng Chen, Yanming Ma

Physics & Astronomy Faculty Research

High pressure can drastically alter chemical bonding and produce exotic compounds that defy conventional wisdom. Especially significant are compounds pertaining to oxygen cycles inside Earth, which hold key to understanding major geological events that impact the environment essential to life on Earth. Here we report the discovery of pressure-stabilized divalent ozonide CaO3 crystal that exhibits intriguing bonding and oxidation states with profound geological implications. Our computational study identifies a crystalline phase of CaO3 by reaction of CaO and O2 at high pressure and high temperature conditions; ensuing experiments synthesize this rare compound under compression in a diamond anvil cell with …

Machine Learning Corrected Quantum Dynamics Calculations, A. Jasinski, J. Montaner, R. C. Forrey, B. H. Yang, P. C. Stancil, Naduvalath Balakrishnan, J. Dai, A. Vargas-Hernandez, R. V. Krems

Chemistry and Biochemistry Faculty Research

Quantum scattering calculations for all but low-dimensional systems at low energies must rely on approximations. All approximations introduce errors. The impact of these errors is often difficult to assess because they depend on the Hamiltonian parameters and the particular observable under study. Here, we illustrate a general, system- and approximation-independent, approach to improve the accuracy of quantum dynamics approximations. The method is based on a Bayesian machine learning (BML) algorithm that is trained by a small number of exact results and a large number of approximate calculations, resulting in ML models that can generalize exact quantum results to different dynamical …

High Pressure Synchrotron Xrd, Nfs, And Nrixs Studies Of Fesb2, Melanie Alden White

UNLV Theses, Dissertations, Professional Papers, and Capstones

This work combines high pressure synchrotron X-ray diffraction (XRD), nuclear forward scattering (NFS), and nuclear resonant inelastic X-ray scattering (NRIXS) experiments on iron diantimonide (FeSb2), a promising thermoelectric material. NRIXS uses the interaction between excited nuclear and lattice states to directly measure the phonon density of states (PDOS) for samples containing certain Mössbauer isotopes. For our experiments, we synthesized FeSb2 enriched with the isotope 57Fe and measured the partial PDOS for 57Fe sites in the FeSb2 lattice. NFS, also known as synchrotron Mössbauer spectroscopy, is a sister technique to NRIXS that studies the hyperfine structure of the excited Mössbauer nuclei. …

Non-Adiabatic Quantum Dynamics Of The Ultracold Li+Lina→ Li2+Na Chemical Reaction, B. K. Kendrick, M. Li, H. Li, S. Kotochigova, J. F.E. Croft, Balakrishnan Naduvalath

Chemistry and Biochemistry Faculty Research

We report non-adiabatic dynamics of the Li+LiNa→Li2+Na chemical reaction at cold and ultracold temperatures employing accurate ab initio electronic potential energy surfaces in a quantum dynamics formulation employing a diabatic representation. Results are compared against those from a single adiabatic ground state potential energy surface and a universal model based on the long-range interaction potential. We discuss signatures of non-universal behavior in the total rate coefficients as well as strong non-adiabatic effects in the state-to-state rotationally resolved rate coefficients.

Hdr Brachytherapy For Skin Cancer Using H.A.M. Applicator, Timothy Richard Thatcher

UNLV Theses, Dissertations, Professional Papers, and Capstones

Skin cancer is the most common type of malignancy affecting both men and women in the United States. A majority of these cases are nonmelanoma skin cancers (NMSC) such as basal cell carcinoma (BCC) and squamous cell carcinoma (SCC). The goal of any skin cancer treatment is to cure the lesion itself while also preserving function and optimizing superficial cosmesis. High dose rate (HDR) brachytherapy provides an alternative to superficial x-ray or electron beam for skin cancer treatment. Utilizing an Iridium 192 source soldered to the end of a wire, surface dose can be administered at predetermined positions with sharper …

Investigations Of Technetium Metal And The Synthesis Of Binary Technetium Nitrides Under Extreme Conditions, Emily Siska

UNLV Theses, Dissertations, Professional Papers, and Capstones

Technetium (Tc) is the lightest of the radioactive elements and has no stable isotopes. Significant quantities of Tc are not naturally occurring on earth. However, technetium is found in high fission yield in nuclear reactors and produced for medical imaging. With its long half life, and high mobility in the environment make it of particular interest. To that end, the fundamental chemistry of Tc and Tc compounds is not as well understood compared to neighboring elements on the periodic table. Therefore, fundamental studies designed to better understand this transition metal, coupled with more targeted investigation at high temperature and pressure …

Development Of Co2 Laser-Heating For The Study Of Wide Band Gap Oxide Materials, Christian Matthew Childs

UNLV Theses, Dissertations, Professional Papers, and Capstones

The ability to access a vast region of the pressure-temperature landscape using energy density tuning enables exotic states of matter to be probed. A well documented method for such exploration, under static conditions, is the use of the laser-heated diamond anvil cell (LH-DAC), accessing a combination of high pressure (>300 GPa) and high temperature (>5000 K). This thesis presents our development of direct CO2 laser heating techniques to study a series of wide band gap insulators, La2Sn2O7, ZrO2, and CeO2, under high pressure conditions. The lasing frequency of CO2 lasers is such that these wide band gap materials …

Infiltration Into Water-Repellent Sand: The Role Of Sorptivity, Rose Marie Shillito

UNLV Theses, Dissertations, Professional Papers, and Capstones

Soil water repellency (SWR) is believed to contribute to the increased potential for post-fire runoff and erosion. Currently, no quantitative method exists to account for SWR in physically-based runoff models. A physically-based model incorporating SWR (through the contact angle) in the infiltration parameter, sorptivity, was developed. The model was tested and verified with controlled laboratory tests on a fully characterized sand treated to create various degrees of SWR. The ability to create the wetting water characteristic curve for untreated and treated sand, appropriate for the infiltration process, was presented. Additionally, the ability to measure sorptivity with widely used infiltration tests …

On-Chip Terahertz Modulation And Emission With Integrated Graphene Junctions, Joshua O. Island, Peter Kissin, Jacob Schalch, Xiaomeng Cui, Sheikh Rubaiat Ui Haque, Alex Potts, Takashi Taniguchi, Kanji Watanabe, Richard D. Averitt, Andrea F. Young

Physics & Astronomy Faculty Research

The efficient modulation and control of ultrafast signals on-chip is of central importance in terahertz (THz) communications and a promis- ing route toward sub-diffraction limit THz spectroscopy. Two-dimensional (2D) materials may provide a platform for these endeavors. We explore this potential, integrating high-quality graphene p–n junctions within two types of planar transmission line circuits to modulate and emit picosecond pulses. In a coplanar strip line geometry, we demonstrate the electrical modulation of THz signal transmission by 95%. In a Goubau waveguide geometry, we achieve complete gate-tunable control over THz emission from a photoexcited graphene junction. These studies inform the development …

Vibration Overtone Hyperpolarizability Measured For H2, Rachel M. Ellis, David P. Shelton

Physics & Astronomy Faculty Research

The second hyperpolarizability (γ) of the H2 molecule was measured by gas-phase electric field induced second harmonic generation at the frequencies of the one-photon resonance for the 3–0 Q(J) overtone transitions (v, J = 0, J → 3, J for J = 0, 1, 2, and 3). The magnitude of the resonant contribution to γ was measured with 2% accuracy using the previously determined non-resonant γ for calibration. Pressure broadening and frequency shift for the transitions were also measured. A theoretical expression for the resonant vibrational γ contribution in terms of transition polarizabilities is compared to the observations. The measured …

Superconductivity In Compression-Shear Deformed Diamond, Chang Liu, Xianqi Song, Quan Li, Yanming Ma, Changfeng Chen

Physics & Astronomy Faculty Research

Diamond is a prototypical ultrawide band gap semiconductor, but turns into a superconductor with a critical temperature Tc≈4 K near 3% boron doping [E. A. Ekimov et al., Nature (London) 428, 542 (2004)]. Here we unveil a surprising new route to superconductivity in undoped diamond by compression-shear deformation that induces increasing metallization and lattice softening with rising strain, producing phonon mediated Tc up to 2.4–12.4 K for a wide range of Coulomb pseudopotential μ∗=0.15–0.05. This finding raises intriguing prospects of generating robust superconductivity in strained diamond crystal, showcasing a distinct and hitherto little explored approach to driving materials into superconducting …

The Breakup Of A Helium Cluster After Removing Attractive Interaction Among A Significant Number Of Atoms In The Cluster, Tao Pang

Physics & Astronomy Faculty Research

The breakup of a quantum liquid droplet is examined through a 4He cluster by removing the attractive tail in the interaction between some of the atoms in the system with the diffusion quantum Monte Carlo simulation. The ground-state energy, kinetic energy, cluster size, and density profile of the cluster are evaluated against the percentage of the atoms without the attractive tail. The condition for the cluster to lose its ability to form a quantum liquid droplet at zero temperature is found and analyzed. The cluster is no longer able to form a quantum liquid droplet when about two-thirds of pairs …

High Dielectric Ternary Oxides From Crystal Structure Prediction And High-Throughput Screening, Jingyu Qu, David Zagaceta, Weiwei Zhang, Qiang Zhu

Physics & Astronomy Faculty Research

The development of new high dielectric materials is essential for advancement in modern electronics. Oxides are generally regarded as the most promising class of high dielectric materials for industrial applications as they possess both high dielectric constants and large band gaps. Most previous researches on high dielectrics were limited to already known materials. In this study, we conducted an extensive search for high dielectrics over a set of ternary oxides by combining crystal structure prediction and density functional perturbation theory calculations. From this search, we adopted multiple stage screening to identify 441 new low-energy high dielectric materials. Among these materials, …

Pair-A-Dice Lost: Experiments In Dice Control, Robert H. Scott Iii, Donald R. Smith

UNLV Gaming Research & Review Journal

This paper presents our findings from experiments designed to test whether we could use a custom-made dice throwing machine applying common dice control methods to produce dice rolls that differ from random. In earlier research we calculated the percentages of control a craps player needs to break even or beat the house (Smith and Scott, 2018). Using the most common practices of dice control in craps, we established how dice should be configured (i.e., set) and thrown to achieve certain outcomes such as not rolling a seven in the point cycle. We decided to run experiments to see if a …

Author Correction: Closing The Nuclear Fuel Cycle With A Simplified Minor Actinide Lanthanide Separation Process (Alsep) And Additive Manufacturing, Artem V. Gelis, Peter Kozak, Andrew T. Breshears, M. Alex Brown, Cari Launiere, Emily L. Campbell, Gabriel B. Hall, Tatiana G. Levitskaia, Vanessa E. Holfeltz, Gregg J. Lumetta

Chemistry and Biochemistry Faculty Research

No abstract provided.

Buckling Of Blue Phosphorus Nanotubes Under Axial Compression: Insights From Molecular Dynamics Simulations, Shiping Jiang, Huiling Wu, Liangzhi Kou, Chun Tang, Chengyuan Wang, Changfeng Chen

Physics & Astronomy Faculty Research

We report on mechanical properties of blue phosphorus nanotubes (BluePNTs) from systematic molecular dynamics simulations, adopting a Stillinger-Weber potential with parameters determined by fitting to energetic and structural data from first-principles calculations. Our results corroborate the previously reported bending poison effect and size-dependent buckling behaviors. Under axial compression, current simulations predict a shell-to-column buckling mode transition for BluePNTs with increasing aspect ratios; further compression of BluePNTs with large aspect ratios results in a column-to-shell buckling mode transition. Associated critical buckling strains can be described by the continuum mechanics theory. We also simulated buckling behavior of black phosphorus nanotubes (BlackPNTs) and …