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Articles 26401 - 26430 of 35391

Full-Text Articles in Physics

Microscopic Investigation Of Phonon Modes In Sige Alloy Nanocrystals, Shang-Fen Ren, Wei Cheng, Peter Y. Yu Jun 2004

Microscopic Investigation Of Phonon Modes In Sige Alloy Nanocrystals, Shang-Fen Ren, Wei Cheng, Peter Y. Yu

Faculty publications – Physics

Phonon modes in spherical silicon germanium alloy (SiGe) nanocrystals containing up to 1147 atoms (3.6 nm) have been investigated as a function of the Si concentration. Microscopic details of phonon modes, including phonon frequencies and vibrational amplitudes, phonon density-of-states are calculated directly from the dynamic matrices. In particular, the dependence of phonon frequency on the configuration (such as a different ratio of Si to Ge atoms), and location (surface or interior) of clusters of atoms in SiGe alloy nanocrystals have been investigated. Low frequency surface phonons that are related to the spheroidal and torsional modes of a continuum sphere are …

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Close-Coupling And Distorted-Wave Calculations For Electron-Impact Excitation Of The (5p⁵6p) States Of Xenon, K. Bartschat, Anand Dasgupta, Don H. Madison Jun 2004

Close-Coupling And Distorted-Wave Calculations For Electron-Impact Excitation Of The (5p⁵6p) States Of Xenon, K. Bartschat, Anand Dasgupta, Don H. Madison

Physics Faculty Research & Creative Works

We report on a series of calculations for electron-impact excitation of the (5p56p) states in xenon from the ground state (5p6)1S0 . As in previous calculations for other noble-gas targets, we find strong evidence of channel coupling for all incident energies considered (between threshold and 200 eV ). Although qualitative agreement with the experimental results of Fons and Lin [Phys. Rev. A 58, 4603 (1998)] is achieved, severe quantitative discrepancies of sometimes more than a factor of 2 remain.

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Classroom Demonstrations: Learning Tools Or Entertainment?, Catherine Hirshfeld Crouch, A. P. Fagen, J. P. Callan, E. Mazur Jun 2004

Classroom Demonstrations: Learning Tools Or Entertainment?, Catherine Hirshfeld Crouch, A. P. Fagen, J. P. Callan, E. Mazur

Physics & Astronomy Faculty Works

We compared student learning from different modes of presenting classroom demonstrations to determine how much students learn from traditionally presented demonstrations, and whether learning can be enhanced by simply changing the mode of presentation to increase student engagement. We find that students who passively observe demonstrations understand the underlying concepts no better than students who do not see the demonstration at all, in agreement with previous studies. Learning is enhanced, however, by increasing student engagement; students who predict the demonstration outcome before seeing it, however, display significantly greater understanding.

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Gravitational Energy Loss In High Energy Particle Collisions: Ultrarelativistic Plunge Into A Multidimensional Black Hole, Emanuele Berti, Marco Cavaglia, Leonardo Gualtieri Jun 2004

Gravitational Energy Loss In High Energy Particle Collisions: Ultrarelativistic Plunge Into A Multidimensional Black Hole, Emanuele Berti, Marco Cavaglia, Leonardo Gualtieri

Physics Faculty Research & Creative Works

We investigate the gravitational energy emission of an ultrarelativistic particle radially falling into a D-dimensional black hole. We numerically integrate the equations describing black hole gravitational perturbations and obtain the energy spectra, total energy, and angular distribution of the emitted gravitational radiation. The black hole quasinormal modes for scalar, vector, and tensor perturbations are computed in the WKB approximation. We discuss our results in the context of black hole production at the TeV scale.

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Inelastic X-Ray Scattering Studies In Lithium And Metal Ammonia Solutions, Ayman Hasan Said Jun 2004

Inelastic X-Ray Scattering Studies In Lithium And Metal Ammonia Solutions, Ayman Hasan Said

Dissertations

In this work, inelastic x-ray scattering was used to measure the ionic collective excitations of a metal-ammonia system and the atomic form factor of lithium in a single crystal. For lithium and sodium ammonia systems, the measurements were carried out for different metal concentrations (lithium-ammonia with 2 0 , 16, and 13 mole percent metal [MPM] at T'=240K and sodium-ammonia with 18, 14, and 10 MPM at T=222K). These data were analyzed to determine the acoustic collective excitation dispersion relation and the linewidth. Deviations from the Bohm-Staver model for electron-ion coupling are discussed for the low electronic densities. For the …

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A Dunking Bird Of The Second Kind, Nadine Abraham, Peter Palffy-Muhoray May 2004

A Dunking Bird Of The Second Kind, Nadine Abraham, Peter Palffy-Muhoray

Peter Palffy-Muhoray

The conventional dunking bird is a heat engine that relies on the temperature difference between the head and the tail of the bird for its operation. We describe a new type of dunking bird that is not a heat engine, but one that directly uses the chemical potential difference between liquid water and its vapor.

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Simulations Of Dynamically Triangulated Gravity -- An Algorithm For Arbitrary Dimension, Simon Catterall May 2004

Simulations Of Dynamically Triangulated Gravity -- An Algorithm For Arbitrary Dimension, Simon Catterall

Physics - All Scholarship

Recent models for discrete euclidean quantum gravity incorporate a sum over simplicial triangulations. We describe an algorithm for simulating such models in general dimensions. As illustration we show results from simulations in four dimensions

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Characterization Of Ultrathin Gate Dielectrics And Multilayer Charge Injection Barriers, Edwin M. Dons May 2004

Characterization Of Ultrathin Gate Dielectrics And Multilayer Charge Injection Barriers, Edwin M. Dons

Dissertations

Since the invention of the first integrated circuit, the semiconductor industry has distinguished itself by a phenomenally rapid pace of improvements in device performance. This trend of ever smaller and faster devices is a result of the ability to exponentially reduce feature sizes of integrated circuits, a trend commonly known as "scaling". A reduction of overall feature sizes requires a simultaneous reduction in the thickness of the gate dielectric, SiO2, of a MOSFET. Gate oxides in the ultrathin regime (<35 A) feature a large direct tunneling leakage current. The presence of this leakage current requires a reevaluation of standard characterization techniques as well as a reevaluation of the continued usefulness of SiO2 as the gate dielectric of choice for future applications. On the other hand, a thorough understanding of …

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Understanding The Origin Of The Recovery Of Superconductivity In Halogenated Ybco Single Crystal : Atomic Structure Study, Lamine Mohamed Kollakoye Dieng May 2004

Understanding The Origin Of The Recovery Of Superconductivity In Halogenated Ybco Single Crystal : Atomic Structure Study, Lamine Mohamed Kollakoye Dieng

Dissertations

The recovery of superconductivity in underdoped YBa2Cu3O6+y (YBCO) by exposure to bromine, iodine, chlorine and fluorine is a long-standing problem which has not been clearly resolved. The key question concerns the role of these halogens in the lattice. In order to shed light to this problem, we have performed multiple-edge x-ray absorption fine structure (XAFS) measurements about the Y K, Ba L3, I L3, Cu K and Br K-edges at room temperature on brominated (non-brominated) and iodinated (non-iodinated) YBCO single crystals. Our XAFS results at the Br K (brominated) and I …

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Dynamics Of M-Theory Vacua, John Donoghue May 2004

Dynamics Of M-Theory Vacua, John Donoghue

John Donoghue

At very early times, the universe was not in a vacuum state. Under the assumtion that the deviation from equillibrium was large, in particular that it is higher than the scale of inflation, we analyse the conditions for local transitions between states that are related to different vacua. All pathways lead to an attractor solution of a description of the universe by eternal inflation with domains that have different low energy parameters. The generic case favors transitions between states that have significantly different parameters rather than jumps between nearby states in parameter space. I argue that the strong CP problem …

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Realization Of The Einstein-Podolsky-Rosen Paradox Using Momentum- And Position-Entangled Photons From Spontaneous Parametric Down Conversion, John C. Howell, Ryan S. Bennink, Sean J. Bentley, Robert W. Boyd May 2004

Realization Of The Einstein-Podolsky-Rosen Paradox Using Momentum- And Position-Entangled Photons From Spontaneous Parametric Down Conversion, John C. Howell, Ryan S. Bennink, Sean J. Bentley, Robert W. Boyd

Mathematics, Physics, and Computer Science Faculty Articles and Research

We report on a momentum-position realization of the EPR paradox using direct detection in the near and far fields of the photons emitted by collinear type-II phase-matched parametric down conversion. Using this approach we achieved a measured two-photon momentum-position variance product of 0.01ℏ2, which dramatically violates the bounds for the EPR and separability criteria.

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Gold Adatoms And Dimers On Relaxed Graphite Surfaces, Guan Ming Wang, Joseph J. Belbruno, Steven D. Kenny, Roger Smith May 2004

Gold Adatoms And Dimers On Relaxed Graphite Surfaces, Guan Ming Wang, Joseph J. Belbruno, Steven D. Kenny, Roger Smith

Dartmouth Scholarship

The interaction of deposited gold adatoms and dimers with multilayer relaxed graphite surfaces is investigated through a density functional approach with numerical orbitals and a relativistic core pseudopotential. The energy landscape for a gold adatom along [110] agrees with scanning tunneling microscopy observations including the preferred β binding site for adatoms and the mobility difference between silver and gold adatoms. Deposited particles are shown to induce surface deformation and polarization. Static relaxation and dynamic simulations indicate that the energetically preferred binding orientation for a gold dimer is normal rather than parallel to the graphite surface. The dimer response to a …

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Consequences Of Gravitational Radiation Recoil, David Merritt, Miloš Milosavljević, Marc Favata, Scott A. Hughes, Daniel E. Holz May 2004

Consequences Of Gravitational Radiation Recoil, David Merritt, Miloš Milosavljević, Marc Favata, Scott A. Hughes, Daniel E. Holz

Department of Physics and Astronomy Faculty Scholarship and Creative Works

Coalescing binary black holes experience an impulsive kick from anisotropic emission of gravitational waves. Recoil velocities are sufficient to eject most coalescing black holes from dwarf galaxies and globular clusters, which may explain the apparent absence of massive black holes in these systems. Ejection from giant elliptical galaxies would be rare, but coalescing black holes are displaced from the center and fall back on a timescale of order the half-mass crossing time. Displacement of the black holes transfers energy to the stars in the nucleus and can convert a steep density cusp into a core. Radiation recoil calls into question …

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How Black Holes Get Their Kicks: Gravitational Radiation Recoil Revisited, Marc Favata, Scott A. Hughes, Daniel E. Holz May 2004

How Black Holes Get Their Kicks: Gravitational Radiation Recoil Revisited, Marc Favata, Scott A. Hughes, Daniel E. Holz

Department of Physics and Astronomy Faculty Scholarship and Creative Works

Gravitational waves from the coalescence of binary black holes carry away linear momentum, causing center of mass recoil. This "radiation rocket" effect has important implications for systems with escape speeds of order the recoil velocity. We revisit this problem using black hole perturbation theory, treating the binary as a test mass spiraling into a spinning hole. For extreme ratios (q ≡ m1/m2 ≪ 1), we compute the recoil for the slow in-spiral epoch of binary coalescence very accurately; these results can be extrapolated to q ∼ 0.4 with modest accuracy. Although the recoil from the final plunge contributes significantly to …

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The Complex Neutral Gas Dynamics Of The Dwarf Starburst Galaxy Ngc 625, John M. Cannon, Naomi M. Mcclure-Griffiths, Evan D. Skillman, Stephanie Cote May 2004

The Complex Neutral Gas Dynamics Of The Dwarf Starburst Galaxy Ngc 625, John M. Cannon, Naomi M. Mcclure-Griffiths, Evan D. Skillman, Stephanie Cote

John Cannon

No abstract provided.

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Could Dark Energy Be Vector-Like?, Christian Armendariz-Picon May 2004

Could Dark Energy Be Vector-Like?, Christian Armendariz-Picon

Physics - All Scholarship

In this paper I explore whether a vector field can be the origin of the present stage of cosmic acceleration. In order to avoid violations of isotropy, the vector has be part of a ``cosmic triad'', that is, a set of three identical vectors pointing in mutually orthogonal spatial directions. A triad is indeed able to drive a stage of late accelerated expansion in the universe, and there exist tracking attractors that render cosmic evolution insensitive to initial conditions. However, as in most other models, the onset of cosmic acceleration is determined by a parameter that has to be tuned …

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Lattice Sigma Models With Exact Supersymmetry, Simon Catterall, Sofiane Ghadab May 2004

Lattice Sigma Models With Exact Supersymmetry, Simon Catterall, Sofiane Ghadab

Physics - All Scholarship

We show how to construct lattice sigma models in one, two and four dimensions which exhibit an exact fermionic symmetry. These models are discretized and {\it twisted} versions of conventional supersymmetric sigma models with N=2 supersymmetry. The fermionic symmetry corresponds to a scalar BRST charge built from the original supercharges. The lattice theories possess local actions and in many cases admit a Wilson term to suppress doubles. In the two and four dimensional theories we show that these lattice theories are invariant under additional discrete symmetries. We argue that the presence of these exact symmetries ensures that no fine tuning …

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Reaction Thresholds In Doubly Special Relativity, Daniel Heyman '03, F. Hinterleitner, Seth Major May 2004

Reaction Thresholds In Doubly Special Relativity, Daniel Heyman '03, F. Hinterleitner, Seth Major

Articles

Two theories of special relativity with an additional invariant scale, “doubly special relativity,” are tested with calculations of particle process kinematics. Using the Judes-Visser modified conservation laws, thresholds are studied in both theories. In contrast with some linear approximations, which allow for particle processes forbidden in special relativity, both the Amelino-Camelia and Magueijo-Smolin frameworks allow no additional processes. To first order, the Amelino-Camelia framework thresholds are lowered and the Magueijo-Smolin framework thresholds may be raised or lowered.

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Biconformal Supergravity And The Ads/Cftconjecture, L. B. Anderson, James Thomas Wheeler May 2004

Biconformal Supergravity And The Ads/Cftconjecture, L. B. Anderson, James Thomas Wheeler

All Physics Faculty Publications

Biconformal supergravity models provide a new gauging of the superconformal group relevant to the Maldacena conjecture. Using the group quotient method to biconformally gauge SU(2,2|N), we generate a 16-dim superspace. We write the most general even- and odd-parity actions linear in the curvatures, the bosonic sector of which is known to descend to general relativity on a 4-dim manifold.

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Investigation Of The Diffusion Processes In Self-Processing Acrylamide-Based Photopolymer System, Izabela Naydenova, Raghavendra Jallapuram, Robert Howard, Suzanne Martin, Vincent Toal May 2004

Investigation Of The Diffusion Processes In Self-Processing Acrylamide-Based Photopolymer System, Izabela Naydenova, Raghavendra Jallapuram, Robert Howard, Suzanne Martin, Vincent Toal

Articles

Results from the investigation of the diffusion processes in a dry acrylamide-based photopolymer system are presented. The investigation is carried out in the context of experimental work on optimization of the high spatial frequency response of the photopolymer. Tracing the transmission holographic grating dynamics at short times of exposure is utilized to measure diffusion coefficients. The results reveal that two different diffusion processes contribute with opposite sign to the refractive index modulation responsible for the diffraction grating build up. Monomer diffusion from dark to bright fringe areas increases the refractive index modulation. It is characterized with diffusion constant D0=1.6E-7 cm2/s. …

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Electrochemical Separation Of Curium And Americium, David W. Hatchett, Kenneth Czerwinski May 2004

Electrochemical Separation Of Curium And Americium, David W. Hatchett, Kenneth Czerwinski

Separations Campaign (TRP)

The objective of this project is to develop a method for the separation of Am from Cm based on electrochemical techniques. Electrochemical systems that allow the thermodynamics of actinide and lanthanide complexes to be systematically evaluated and tuned will be examined. The influence of complex formation on the ability to selectively isolate a given species electrochemically will be evaluated. Metal-ligand complex formation provides a useful derivation technique to increase solubility in solution environments that favor precipitation. In addition, the thermodynamic properties of a complex relative to the isolated species may be shifted to more suitably measurable electrochemical separation regimes. Electrochemical …

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Design Concepts And Process Analysis For Transmuter Fuel Manufacturing, Georg F. Mauer May 2004

Design Concepts And Process Analysis For Transmuter Fuel Manufacturing, Georg F. Mauer

Fuels Campaign (TRP)

This proposal addresses the subject heading ‘Transmutation Fuel Development’ in the 2004 research topic list of the UNLV Transmutation Research Program (TRP) and DOE Advanced Fuel Cycle Initiative (AFCI). The large-scale deployment of remote fabrication and refabrication processes (with a capacity of approx. 100 metric tons of Minor Actinides (MA) annually) will be required for all transmutation scenarios. The objective of this project is the design, analysis, and evaluation of manufacturing processes for transmuter fuel fabrication. Fabrication processes for different fuel types differ in terms of equipment types, throughput, and cost. The evaluation of the fabrication processes will create a …

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Growth And Transport Properties Of Complementary Germanium Nanowire Field Effect Transistors, Andrew B. Greytak, Lincoln J. Lauhon, Mark S. Gudiksen, Charles M. Lieber May 2004

Growth And Transport Properties Of Complementary Germanium Nanowire Field Effect Transistors, Andrew B. Greytak, Lincoln J. Lauhon, Mark S. Gudiksen, Charles M. Lieber

Faculty Publications

n- and p-type Ge nanowires were synthesized by a multistep process in which axial elongation, via vapor–liquid–solid (VLS) growth, and doping were accomplished in separate chemical vapor deposition steps. Intrinsic, single-crystal, Ge nanowires prepared by Au nanocluster-mediated VLS growth were surface-doped in situ using diborane or phosphine, and then radial growth of an epitaxial Ge shell was used to cap the dopant layer. Field-effect transistors prepared from these Ge nanowires exhibited on currents and transconductances up to 850 µA/µm and 4.9 µA/V, respectively, with device yields of >85%.

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Multistrange Baryon Production In Au-Au Collisions At √ SNn = 130 Gev, Star Collaboration, T.D. Gutierrez May 2004

Multistrange Baryon Production In Au-Au Collisions At √ SNn = 130 Gev, Star Collaboration, T.D. Gutierrez

Physics

The transverse mass spectra and midrapidity yields for Ξs and Ωs are presented. For the 10% most central collisions, the ¯Ξ¯+/h− ratio increases from the Super Proton Synchrotron to the Relativistic Heavy Ion Collider energies while the Ξ−/h−stays approximately constant. A hydrodynamically inspired model fit to the Ξ spectra, which assumes a thermalized source, seems to indicate that these multistrange particles experience a significant transverse flow effect, but are emitted when the system is hotter and the flow is smaller than values obtained from a combined fit to π, K, p, and Λs.

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Plasmaspheric Plumes: Crres Observations Of Enhanced Density Beyond The Plasmapause, Jeff Sanny, M. B. Moldwin, J. Howard, J. D. Bocchicchio, H. K. Rassoul, R. R. Anderson May 2004

Plasmaspheric Plumes: Crres Observations Of Enhanced Density Beyond The Plasmapause, Jeff Sanny, M. B. Moldwin, J. Howard, J. D. Bocchicchio, H. K. Rassoul, R. R. Anderson

Physics Faculty Works

CRRES plasma wave receiver density data were used to study the distribution and properties of dense plasmaspheric-like plasma observed outside the plasmapause. Our study indicates that outer plasmaspheric structure, often called plasmaspheric plumes, blobs, tails, or detached plasma regions, can exist at all local times under all levels of geomagnetic activity. Of the 558 CRRES orbits that had at least one clearly defined plasmapause, 169 (or 30%) had plasmaspheric-like density structures at higher L shells than the plasmapause. Most of the occurrences of plasmaspheric-like plasma observed by CRRES were in the noon-to-dusk sector in the aftermath of enhanced geomagnetic activity …

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Moduli Stabilization With The String Higgs Effect, Scott Watson May 2004

Moduli Stabilization With The String Higgs Effect, Scott Watson

Physics - All Scholarship

We review the notion of the Higgs effect in the context of string theory. We find that by including this effect in time dependent backgrounds, one is led to a natural mechanism for stabilizing moduli at points of enhanced gauge symmetry. We consider this mechanism for the case of the radion (size of the extra dimensions) and find that as decompactification of the large spatial dimensions takes place the radion will remain stabilized at the self dual radius. We discuss how this mechanism can be incorporated into models of string cosmology and brane inflation to resolve some outstanding problems. We …

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Alpha And Gamma-Ray Spectroscopic Studies Of Au, Pt, And Ir Nuclei Near The Proton Dripline, Tuck-Meng Goon May 2004

Alpha And Gamma-Ray Spectroscopic Studies Of Au, Pt, And Ir Nuclei Near The Proton Dripline, Tuck-Meng Goon

Doctoral Dissertations

This dissertation reports the observation of α-decay and in-beam level structures in the neutron-deficient nuclei 176,174Au, 173,174Pt, and 175Ir from an experiment which was performed at Argonne National Laboratory (ANL). A beam of 84Sr projectiles was used to bombard 92,94,96Mo targets. From these reactions, the 176,178,180Hg compound nuclei were made which eventually decayed into more than 20 product nuclei. The experiment was performed with the Atlas accelerator and utilized the Gammasphere array at the target position in conjunction with the Fragment Mass Analyzer (FMA) for mass identification and a Double-Sided Silicon Strip Detector (DSSD), …

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Spectral Characteristics Of Water Megamaser Galaxies. Ii. Eso103-G035, Txs 2226-184, And Ic1481, N. Bennert, H. Schulz, C. Henkel May 2004

Spectral Characteristics Of Water Megamaser Galaxies. Ii. Eso103-G035, Txs 2226-184, And Ic1481, N. Bennert, H. Schulz, C. Henkel

Physics

Long-slit optical emission-line spectra of the H2O megamaser galaxies ESO 103-G035, TXS 2226-184, and IC 1481 are evaluated in order to look for characteristics typical for water-megamaser galaxies. We present rotation curves, line ratios, electron densities, temperatures, and H luminosities. The successful line-profile decompositions rest on d-Lorentzians with an additional parameter d to adjust the wings, rather than Gaussians or Lorentzians as basic functions. No significant velocity gradient is found along the major axis in the innermost 2 kpc of TXS 2226-184. IC 1481 reveals a spectrum suggestive of a vigorous starburst in the …

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Interaction Of Carbon Nanotubes With Saccharides: A Possible Route To Biocompatible Composites, Alan Casey, Jonathan Moghal, Gerald Farrell, Hugh Byrne, Gordon Chambers May 2004

Interaction Of Carbon Nanotubes With Saccharides: A Possible Route To Biocompatible Composites, Alan Casey, Jonathan Moghal, Gerald Farrell, Hugh Byrne, Gordon Chambers

Conference papers

Inclusion complexes of saccharides and HiPco Carbon Nanotubes (HCNT) were formed, resulting in water soluble complexes. These complexes were then characterised using a variety of spectroscopic techniques. Raman spectroscopy was used to probe changes in the vibrational character of both the host and guest species and also to assess if any selectivity of tube diameter occurred. UV-Vis-NIR and circular dichroic spectroscopies were used to assess the effect of inclusion on the electronic properties of the tubes. Spectroscopic analysis of these species showed clear evidence of an intermolecular interaction between the host material and the HCNT.

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Anomalous Capture And Emission From Internal Surfaces Of Semiconductor Voids: Nanopores In Sic, David C. Look, Z-Q. Fang, S. Soloviev, T. S. Sudarshan, J. J. Boeckl May 2004

Anomalous Capture And Emission From Internal Surfaces Of Semiconductor Voids: Nanopores In Sic, David C. Look, Z-Q. Fang, S. Soloviev, T. S. Sudarshan, J. J. Boeckl

Physics Faculty Publications

Deep level transient spectroscopy in nanoporous, n-type SiC reveals a new type of deep (˜0.8 eV) trap that can hold more than 100 electrons and that has anomalous capture and emission behavior. Here we quantitatively explain these effects with a new, general formalism that treats both emission and capture in the presence of dynamic energy barriers, resulting from the charging and discharging of states on the internal surfaces of voids, such as pores or nanopipes. The capture kinetics display a logarithmic time dependence over a certain filling range, as has often been observed in connection with dislocation-related trapping.

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