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Articles 1 - 21 of 21
Full-Text Articles in Physical Sciences and Mathematics
First-Principles Investigation Of Sc-Iii/Iv Under High Pressure, Sheng-Cai Zhu, Xiao-Zhi Yan, Scott Fredericks, Yan-Ling Li, Qiang Zhu
First-Principles Investigation Of Sc-Iii/Iv Under High Pressure, Sheng-Cai Zhu, Xiao-Zhi Yan, Scott Fredericks, Yan-Ling Li, Qiang Zhu
Physics & Astronomy Faculty Research
Using an ab initio evolutionary structure prediction method in conjunction with density functional theory, we performed a systematic investigation of the structural transition of elemental scandium under pressure up to 250 GPa. Our prediction successfully reproduced several allotropes which have been reported in the literature, including the Sc-I, Sc-II, and Sc-V. Moreover, we observed a series of energetically degenerate and geometrically similar structures at 110–195 GPa, which can partly explain the experimental observations regarding the unsolved phases III and IV reported by Y. Akahama et al. [Phys. Rev. Lett. 94, 195503 (2005)]. A detailed comparison of the powder x-ray diffraction …
Studies Of Inner-Shell Chemistry Of Mercury Based Compounds At Extreme Conditions, Sarah Schyck
Studies Of Inner-Shell Chemistry Of Mercury Based Compounds At Extreme Conditions, Sarah Schyck
UNLV Theses, Dissertations, Professional Papers, and Capstones
It has been theoretically predicted that when mercury difluoride (HgF2) is pressurized to above 50 GPa in the presence of molecular fluorine, it will most likely transform into mercury tetrafluoride (HgF4), thus mercury will behave as a transition element at high pressure. However, there is no experimental evidence verifying this prediction yet. To begin with, the crystalline properties of pure HgF2 at high pressure were not experimentally established. In this thesis, the high pressure structural properties of HgF2 are investigated by means of synchrotron X-ray powder diffraction. Our results reveal that the predicted, ambient cubic structure of HgF2 with the …
Bayesian Model Comparisons In Planetary Science, John Henry Boisvert
Bayesian Model Comparisons In Planetary Science, John Henry Boisvert
UNLV Theses, Dissertations, Professional Papers, and Capstones
Model comparison in the modern era allows us to use statistical methods that were previously difficult with older machines. I present a state-of-the-art model comparison code that uses modern Bayesian statistics to measure the Bayes factor between two competing models. The Bayes factor is the ratio of the probability of the data given one model to the probability of the data given another model. My code was used to compare models in five problems in planetary science. The first three pertain to radial velocity exoplanet data. There is a degeneracy in the radial velocity exoplanet signal between a single planet …
Design, Layout, And Testing Of Sige Apds Fabricated In A Bicmos Process, Dane Laurence Gentry
Design, Layout, And Testing Of Sige Apds Fabricated In A Bicmos Process, Dane Laurence Gentry
UNLV Theses, Dissertations, Professional Papers, and Capstones
This Thesis is concerned with the design, layout, and testing of avalanche photodiodes (APDs). APDs are a type of photodetector and, thus, convert light signals into electrical signals (current or voltage). APDs can be fabricated using silicon (Si). In this Thesis, however, three integrated circuit (IC) chips containing various silicon-germanium (SiGe) APDs with different sizes, structures, and geometries were designed, laid out, and fabricated using the Austriamicrosystems (AMS) 0.35μm SiGe BiCMOS (S35) process. This was done in order to compare SiGe APDs to Si only APDs and investigate the hypothesis that SiGe APDs are capable of detecting longer wavelengths than …
Optimization Of Useful Hard X-Ray Photochemistry, David Lewis Goldberger
Optimization Of Useful Hard X-Ray Photochemistry, David Lewis Goldberger
UNLV Theses, Dissertations, Professional Papers, and Capstones
X-ray induced damage is generally considered a nuisance, but in the field of Useful Hard X-ray Photochemistry we harness the highly ionizing and penetrating properties of hard X-rays (> 7 keV) to initiate novel photochemical decomposition and synthesis at ambient and extreme conditions. Preliminary experiments suggest that the energy of irradiating photons and the sample pressure play roles in determining the nature of X-ray induced damage. Here, we present the X-ray energy dependence of damage induced in strontium oxalate, strontium nitrate, and barium nitrate, as well as the pressure dependence of X-ray induced damage of strontium oxalate. Our results indicate …
First Observation Of P-Odd Gamma Asymmetry In Polarized Neutron Capture On Hydrogen, D. Blyth, J. Fry, N. Fomin, R. Alarcon, L. Alonzi, E. Askanazi, S. Baeßler, S. Balascuta, L. Barrón-Palos, Alex Barzilov, J. D. Bowman, N. Birge, J. R. Calarco, T. E. Chupp, V. Cianciolo, C. E. Coppola, C. B. Crawford, K. Craycraft, D. Evans, C. Fieseler, E. Frlež, I. Garishvili, M. T. W. Gericke, R. C. Gillis, K. B. Grammer, G. L. Greene, J. Hall, J. Hamblen, C. Hayes, E. B. Iverson, M. L. Kabir
First Observation Of P-Odd Gamma Asymmetry In Polarized Neutron Capture On Hydrogen, D. Blyth, J. Fry, N. Fomin, R. Alarcon, L. Alonzi, E. Askanazi, S. Baeßler, S. Balascuta, L. Barrón-Palos, Alex Barzilov, J. D. Bowman, N. Birge, J. R. Calarco, T. E. Chupp, V. Cianciolo, C. E. Coppola, C. B. Crawford, K. Craycraft, D. Evans, C. Fieseler, E. Frlež, I. Garishvili, M. T. W. Gericke, R. C. Gillis, K. B. Grammer, G. L. Greene, J. Hall, J. Hamblen, C. Hayes, E. B. Iverson, M. L. Kabir
Mechanical Engineering Faculty Research
We report the first observation of the parity-violating gamma-ray asymmetry A(gamma)(np) in neutron-proton capture using polarized cold neutrons incident on a liquid parahydrogen target at the Spallation Neutron Source at Oak Ridge National Laboratory. A(gamma)(np) isolates the Delta I = 1, S-3(1)-> P-3(1) component of the weak nucleon-nucleon interaction, which is dominated by pion exchange and can be directly related to a single coupling constant in either the DDH meson exchange model or pionless effective field theory… See full text for full abstract.
Rare Helium-Bearing Compound Feo2he Stabilized At Deep-Earth Conditions, Jurong Zhang, Jian Lv, Hefei Li, Xiaolei Feng, Cheng Lu, Simon A. T. Redferm, Hanyu Liu, Changfeng Chen, Yanming Ma
Rare Helium-Bearing Compound Feo2he Stabilized At Deep-Earth Conditions, Jurong Zhang, Jian Lv, Hefei Li, Xiaolei Feng, Cheng Lu, Simon A. T. Redferm, Hanyu Liu, Changfeng Chen, Yanming Ma
Physics & Astronomy Faculty Research
There is compelling geochemical evidence for primordial helium trapped in Earth’s lower mantle, but the origin and nature of the helium source remain elusive due to scarce knowledge on viable helium-bearing compounds that are extremely rare. Here we explore materials physics underlying this prominent challenge. Our structure searches in conjunction with first-principles energetic and thermodynamic calculations uncover a remarkable helium-bearing compound FeO2He at high pressure-temperature conditions relevant to the core-mantle boundary. Calculated sound velocities consistent with seismic data validate FeO2He as a feasible constituent in ultralow velocity zones at the lowermost mantle. These mutually corroborating findings establish the first and …
Boron Oxides Under Pressure: Prediction Of The Hardest Oxides, Huafeng Dong, Artem R. Oganov, Vadim V. Brazhkin, Qinggao Wang, Jin Zhang, M. Mahdi Davari Esfahani, Xiang-Feng Zhou, Fugen Wu, Qiang Zhu
Boron Oxides Under Pressure: Prediction Of The Hardest Oxides, Huafeng Dong, Artem R. Oganov, Vadim V. Brazhkin, Qinggao Wang, Jin Zhang, M. Mahdi Davari Esfahani, Xiang-Feng Zhou, Fugen Wu, Qiang Zhu
Physics & Astronomy Faculty Research
We search for stable compounds of boron and oxygen at pressures from 0 to 500 GPa using the ab initio evolutionary algorithm uspex. Only two stable stoichiometries of boron oxides, namely, B6O and B2O3, are found to be stable, in good agreement with experiment. A hitherto unknown phase of B6O at ambient pressure, Cmcm−B6O, has recently been predicted by us and observed experimentally. For B2O3, we predict three previously unknown stable high-pressure phases—two of these (Cmc21 and P212121) are dynamically and mechanically stable at ambient pressure, and should be quenchable to ambient conditions. Their predicted hardnesses, reaching 33–35 GPa, make …
Pressure Effect On The Antiferromagnetic Compound Ce2ni3ge5, Jun Gouchi, Yuki Nakamura, Miho Nakashima, Tasushi Amako, Ravhi Kumar, Yoshiya Uwatoko
Pressure Effect On The Antiferromagnetic Compound Ce2ni3ge5, Jun Gouchi, Yuki Nakamura, Miho Nakashima, Tasushi Amako, Ravhi Kumar, Yoshiya Uwatoko
Physics & Astronomy Faculty Research
In this study, the electrical resistivity and magnetization of a single crystal of Ce2Ni3Ge5 heavy fermion compound were performed under pressure. The resistivity and magnetization showed two antiferromagnetic transitions at ambient pressure. On applying pressure, the transitions merged at 1 GPa. At higher pressures, the antiferromagnetic transition temperature decreases, and disappears. It is suggesting that the critical pressure of Ce2Ni3Ge5 was 4.1 GPa.
Computational Discovery Of A New Rhombohedral Diamond Phase, Zhen-Zhen Li, Jian-Tao Wang, Hiroshi Mizuseki, Changfeng Chen
Computational Discovery Of A New Rhombohedral Diamond Phase, Zhen-Zhen Li, Jian-Tao Wang, Hiroshi Mizuseki, Changfeng Chen
Physics & Astronomy Faculty Research
We identify by first-principles calculations a new diamond phase in R¯3c (D63d) symmetry, which has a 16-atom rhombohedral primitive cell, thus termed R16 carbon. This rhombohedral diamond comprises a characteristic all-sp3 six-membered-ring bonding network, and it is energetically more stable than previously identified diamondlike six-membered-ring bonded BC8 and BC12 carbon phases. A phonon mode analysis verifies the dynamic structural stability of R16 carbon, and electronic band calculations reveal that it is an insulator with a direct band gap of 4.45 eV. Simulated x-ray diffraction patterns provide an excellent match to recently reported distinct diffraction peaks found in milled fullerene soot, …
Unraveling The Stereodynamics Of Cold Controlled Hd−H2 Collisions, James F. E. Croft, Naduvalath Balakrishnan, Meng Huang, Hua Guo
Unraveling The Stereodynamics Of Cold Controlled Hd−H2 Collisions, James F. E. Croft, Naduvalath Balakrishnan, Meng Huang, Hua Guo
Chemistry and Biochemistry Faculty Research
Measuring inelastic rates with partial-wave resolution requires temperatures close to a Kelvin or below, even for the lightest molecule. In a recent experiment, Perreault, Mukherjee, and Zare [Nat. Chem. 10, 561 (2018).] studied collisional relaxation of excited HD molecules in the v=1, j=2 state by para- and ortho-H2 at a temperature of about 1 K, extracting the angular distribution of scattered HD in the v=1, j=0 state. By state preparation of the HD molecules, control of the angular distribution of scattered HD was demonstrated. Here, we report a first-principles simulation of that experiment which enables us to attribute the main …
Multiferroic And Ferroic Topological Order In Ligand-Functionalized Germanene And Arsenene, Liangzhi Kou, Yandong Ma, Ting Liao, Aijun Du, Changfeng Chen
Multiferroic And Ferroic Topological Order In Ligand-Functionalized Germanene And Arsenene, Liangzhi Kou, Yandong Ma, Ting Liao, Aijun Du, Changfeng Chen
Physics & Astronomy Faculty Research
Two-dimensional (2D) materials that exhibit ferroelectric, ferromagnetic, or topological order have been a major focal topic of nanomaterials research in recent years. The latest efforts in this field explore 2D quantum materials that host multiferroic or concurrent ferroic and topological order. We present a computational discovery of multiferroic state with coexisting ferroelectric and ferromagnetic order in recently synthesized CH2OCH3-functionalized germanene. We show that an electric-field-induced rotation of the ligand CH2OCH3 molecule can serve as the driving mechanism to switch the electric polarization of the ligand molecule, while unpassivated Ge p(z) orbits generate ferromagnetism. Our study also reveals coexisting ferroelectric and …
Unraveling The Structure And Bonding Evolution Of The Newly Discovered Iron Oxide Feo2, Cheng Lu, Maximilian Amsler, Changfeng Chen
Unraveling The Structure And Bonding Evolution Of The Newly Discovered Iron Oxide Feo2, Cheng Lu, Maximilian Amsler, Changfeng Chen
Physics & Astronomy Faculty Research
Recently reported synthesis of FeO2 at high pressure has stimulated great interest in exploring this new iron oxide and elucidating its properties. Here, we present a systematic computational study of crystal structure, chemical bonding, and sound velocity of FeO2 in a wide range of pressure. Our results establish thermodynamic stability of the experimentally observed pyrite phase (P-phase) of FeO2 at pressures above 74 GPa and unveil two metastable FeO2 phases in Pbcn and P4(2)/mnm symmetry at lower pressures. Simulated x-ray diffraction (XRD) spectra of Pbcn and P4(2)/mnm FeO2 match well with measured XRD data of the decompression products of P-phase …
Conformal Mapping Improvement Of The Boundary Element Method Solution For Underground Water Flow In A Domain With A Very Singular Boundary, Megan Romero
UNLV Theses, Dissertations, Professional Papers, and Capstones
Numerical solutions using a Boundary Element Method (BEM) for a confined flow in a very singular finite domain are developed. Typically, in scientific journal publications, authors avoid domains with many and more malignant singularities due to the extremely big and difficult to estimate errors in the numerical calculations. Using exact Conformal Mapping solutions for simplified domains with the same singularity as in the original domain, this problem can be solved numerically with improvements introduced by Conformal Mapping solutions. Firstly, to experiment with improving the BEM solution by Conformal Mapping, a domain inside a rectangle is considered. The exact solution inside …
Crystallographic Exploration Of Fundamental Technetium Species At Nonambient Conditions, Daniel S. Mast
Crystallographic Exploration Of Fundamental Technetium Species At Nonambient Conditions, Daniel S. Mast
UNLV Theses, Dissertations, Professional Papers, and Capstones
The study of mater under extreme conditions has yielded fascinating insight into the physical and chemical nature of materials that under more modest conditions are considered mundane but have quite unusual properties when under pressure. Examples of this are the lightest of elements found in the first two rows and in the heaviest elements of the lanthanide and actinide series. Pure elements, though the simplest possible chemical systems, have proven to be among the most complex and interesting. Technetium metal is a simple HCP transition metal that, due to its half-filled d-shell, has a rich chemistry and an important place …
Topological Nodal Line Semimetal In An Orthorhombic Graphene Network Structure, Jian-Tao Wang, Changfeng Chen, Yoshiyuki Kawazoe
Topological Nodal Line Semimetal In An Orthorhombic Graphene Network Structure, Jian-Tao Wang, Changfeng Chen, Yoshiyuki Kawazoe
Physics & Astronomy Faculty Research
Topological semimetals are a fascinating class of quantum materials that possess extraordinary electronic and transport properties. These materials have attracted great interest in recent years for their fundamental significance and potential device applications. Currently a major focus in this research field is to theoretically explore and predict and experimentally verify and realize material systems that exhibit a rich variety of topological semimetallic behavior, which would allow a comprehensive characterization of the intriguing properties and a full understanding of the underlying mechanisms. In this paper, we report on ab initio calculations that identify a carbon allotrope with simple orthorhombic crystal structure …
Evaluation Of A Continuous Air Monitoring System On An Unmanned Aerial Vehicle For Measurement Of Airborne Radioactive Material, Adam Pfannenstein
Evaluation Of A Continuous Air Monitoring System On An Unmanned Aerial Vehicle For Measurement Of Airborne Radioactive Material, Adam Pfannenstein
UNLV Theses, Dissertations, Professional Papers, and Capstones
The release of airborne radioactive material presents a health risk hazard to many individuals, emergency responders and public. It is necessary to characterize the unknown radioactive dangers produced in the event of these incidents. Advantages to utilizing unmanned aerial systems in this effort are personnel risk reduction and quick attainment of data points in a plume. By pairing a continuous air monitor and detector with a drone, radioactive material concentration can be quantified, and thus the extent of potential doses can be estimated. A small, low-flow air sampler with a Geiger-Mueller counter was characterized using measurements of sources representing a …
Design & Delivery Of Automated Winston-Lutz Test For Isocentric & Off-Axis Delivery Stability Utilizing Truebeam Developer Mode & Electronic Portal Imaging Device, Mahmoud Mohammad Yaqoub
Design & Delivery Of Automated Winston-Lutz Test For Isocentric & Off-Axis Delivery Stability Utilizing Truebeam Developer Mode & Electronic Portal Imaging Device, Mahmoud Mohammad Yaqoub
UNLV Theses, Dissertations, Professional Papers, and Capstones
The uncertainties in treatment delivery cannot be ignored in radiation therapy. Thus, the quality assurance QA tests are very important task of the medical physicist in clinical practice. Assuring the coincidence between the mechanical isocenter of the Linear Accelerator (LINAC) and its radiation beams isocenter is one of the most important qualities need to be tested, and the Winston Lust (WL) test is the
most popular technique to perform this task, especially for the treatment modalities which need high precision in beam delivery such as the stereotactic radiosurgery/stereotactic body radiotherapy (SRS/SBRT). The linear accelerator-based SRS/SBRT is a well-established method in …
Reply To 'Anisotropy Governs Strain Stiffening In Nanotwinned-Materials', Bing Li, Hong Sun, Changfeng Chen
Reply To 'Anisotropy Governs Strain Stiffening In Nanotwinned-Materials', Bing Li, Hong Sun, Changfeng Chen
Physics & Astronomy Faculty Research
No abstract provided.
Predicting The Ground-State Structure Of Sodium Boride, Xin-Ling He, Xiao Dong, Quansheng Wu, Zhisheng Zhao, Qiang Zhu, Artem R. Oganov, Yongjun Tian, Dongli Yu, Xiang-Feng Zhou, Hui-Tian Wang
Predicting The Ground-State Structure Of Sodium Boride, Xin-Ling He, Xiao Dong, Quansheng Wu, Zhisheng Zhao, Qiang Zhu, Artem R. Oganov, Yongjun Tian, Dongli Yu, Xiang-Feng Zhou, Hui-Tian Wang
Physics & Astronomy Faculty Research
Binary borides has been a subject of extensive research. However, the exact compositions and crystal structures of sodium borides remained controversial. Here, using the ab initio variable-composition evolutionary algorithm, a new stable Na2B30 with I212121 symmetry (I212121-Na2B30) is found, which is -7.38meV/atom lower in energy than the Imma-Na2B30 structure reported by experimentalists. Interestingly, the Imma-Na2B30 structure is predicted to be a topological nodal line semimetal, which may result in superior electronic transport. In contrast, I212121-Na2B30 is an ultrahard semiconductor with an unprecedented open-framework structure, whose interstitial helical boron sublattice enhances its hardness and energetic stability.
Two-Dimensional Ferroelectric Topological Insulators In Functionalized Atomically Thin Bismuth Layers, Liangzhi Kou, Huixia Fu, Yandong Ma, Binghai Yan, Ting Liao, Aijun Du, Changfeng F. Chen
Two-Dimensional Ferroelectric Topological Insulators In Functionalized Atomically Thin Bismuth Layers, Liangzhi Kou, Huixia Fu, Yandong Ma, Binghai Yan, Ting Liao, Aijun Du, Changfeng F. Chen
Physics & Astronomy Faculty Research
We introduce a class of two-dimensional (2D) materials that possess coexisting ferroelectric and topologically insulating orders. Such ferroelectric topological insulators (FETIs) occur in noncentrosymmetric atomic layer structures with strong spin-orbit coupling (SOC). We showcase a prototype 2D FETI in an atomically thin bismuth layer functionalized by CH2OH, which exhibits a large ferroelectric polarization that is switchable by a ligand molecule rotation mechanism and a strong SOC that drives a band inversion leading to the topologically insulating state. An external electric field that switches the ferroelectric polarization also tunes the spin texture in the underlying atomic lattice. Moreover, the functionalized bismuth …