Physics Commons^™

Metal Modulation Epitaxy Growth For Extremely High Hole Concentrations Above 10(19) Cm(-3) In Gan, Gon Namkoong, Elaissa Trybus, Kyung Keun Lee, Michael Moseley, W. Alan Doolittle, David C. Look

Applied Research Center Publications

The free hole carriers in GaN have been limited to concentrations in the low 10¹⁸ cm⁻³ range due to the deep activation energy, lower solubility, and compensation from defects, therefore, limiting doping efficiency to about 1%. Herein, we report an enhanced doping efficiency up to ~10% in GaN by a periodic doping, metal modulation epitaxy growth technique. The hole concentrations grown by periodically modulating Ga atoms and Mg dopants were over ~1.5 x 10¹⁹ cm⁻³.

© 2008 American Institute of Physics.

Theory And Numerics Of Gravitational Waves From Preheating After Inflation, Jean-François Dufaux, Amanda Bergman, Gary Felder, Lev Kofman, Jean-Philippe Uzan

Physics: Faculty Publications

Preheating after inflation involves large, time-dependent field inhomogeneities, which act as a classical source of gravitational radiation. The resulting spectrum might be probed by direct detection experiments if inflation occurs at a low enough energy scale. In this paper, we develop a theory and algorithm to calculate, analytically and numerically, the spectrum of energy density in gravitational waves produced from an inhomogeneous background of stochastic scalar fields in an expanding universe. We derive some generic analytical results for the emission of gravity waves by stochastic media of random fields, which can test the validity/accuracy of numerical calculations. We contrast ...

Flux, Gaugino Condensation, And Antibranes In Heteroic M Theory, James Gray, André Lukas, Burt A. Ovrut

Department of Physics Papers

We present the potential energy due to flux and gaugino condensation in heterotic M theory compactifications with antibranes in the vacuum. For reasons which we explain in detail, the contributions to the potential due to flux are not modified from those in supersymmetric contexts. The discussion of gaugino condensation is, however, changed by the presence of antibranes. We show how a careful microscopic analysis of the system allows us to use standard results in supersymmetric gauge theory in describing such effects—despite the explicit supersymmetry breaking which is present. Not surprisingly, the significant effect of antibranes on the threshold corrections ...

An Explanation Of Srs Beam Cleanup In Graded-Index Fibers And The Absence Of Srs Beam Cleanup In Step-Index Fibers, Nathan B. Terry, Thomas G. Alley, Timothy H. Russell

Faculty Publications

Beam cleanup via stimulated Raman scattering in multimode fibers is modeled by numerically considering the competition between the Stokes modes of graded-index and step-index fibers. The relative gain of each Stokes mode is calculated by considering the overlap the various pump and Stokes modes of the fibers. Mode competition in a graded-index fiber favors the LP01 Stokes mode while mode competition in a step-index fiber does not favor the LP01 Stokes mode. This model explains why beam cleanup has only been reported for graded-index fibers and not for step-index fibers.

Observation Of P-Wave Threshold Behavior In Electron Attachment To F2 Molecules, M. Braun, M.-W. Ruf, Ilya I. Fabrikant, H. Hotop

Ilya Fabrikant Publications

Using the high resolution laser photoelectron attachment method, we demonstrate that the cross section for F^- formation due to electron capture by F² (X¹Σ_g⁺) molecules at very low energies exhibits p-wave threshold behavior. This finding confirms the theoretical expectation that low-energy attachment to F₂ proceeds through the F₂^-(² Σ_u⁺) p-wave shape resonance in contrast with previous experimental claims for s-wave threshold behavior.

Measurement Of The Pp̅ → Wz + X Cross Section At √S =1.96 Tev And Limits On Wwz Trilinear Gauge Couplings, V. M. Abazov, Kenneth A. Bloom, Gregory R. Snow, D0 Collaboration

Kenneth Bloom Publications

We present measurements of the process pp̅ → WZ + X → ℓ′νℓℓ&#; at √s = 1.96 TeV, where ℓ and ℓ′ are electrons or muons. Using 1 fb^-1 of data from the D0 experiment, we observe 13 candidates with an expected background of 4.5 ± 0.6 events and measure a cross section σ(WZ) = 2.7_-1.3^+1.7 pb. From the number of observed events and the Z boson transverse momentum distribution, we limit the trilinear WWZ gauge couplings to -0.17 ≤ λ_Z ≤ 0.21(Δκ_Z= 0) at the 95% C.L. for a form ...

A Harmadik Világháború. Minden Héten Háromszor., Pal R. Molnar

Pal R. Molnar

Pesszimista jóslatok szerint közeledik a harmadik világháború, a világégés. Ezt a háborút valószínűleg atomfegyverekkel vívják, ami évezredekre megmérgezi a földet és elhozza az emberiség végét.

The World War Iii. Three Times In Every Week, Pal R. Molnar

Pal R. Molnar

The pessimistic prophecies say the third world war, the ‘world burning’ is approaching. This war probably will fight by nuclear weapons, it will poison the Earth for thousands of years, and it will bring the end of mankind.

Ultrashort Intense-Field Optical Vortices Produced With Laser-Etched Mirrors, James Strohaber, Timothy D. Scarborough, Cornelis J. Uiterwaal

C.J.G.J. Uiterwaal Publications

We introduce a simple and practical method to create ultrashort intense optical vortices for applications involving high-intensity lasers. Our method utilizes femtosecond laser pulses to laser etch grating lines into laser-quality gold mirrors. These grating lines holographically encode an optical vortex. We derive mathematical equations for each individual grating line to be etched, for any desired (integer) topological charge. We investigate the smoothness of the etched grooves. We show that they are smooth enough to produce optical vortices with an intensity that is only a few percent lower than in the ideal case. We demonstrate that the etched gratings can ...

Merged Ionization/Dissociation Fronts In Planetary Nebulae, William J. Henney, R. J. R. Williams, Gary J. Ferland, Gargi Shaw, C. R. O'Dell

Physics and Astronomy Faculty Publications

The hydrogen ionization and dissociation front around an ultraviolet radiation source should merge when the ratio of ionizing photon flux to gas density is sufficiently low and the spectrum is sufficiently hard. This regime is particularly relevant to the molecular knots that are commonly found in evolved planetary nebulae, such as the Helix Nebula, where traditional models of photodissociation regions have proved unable to explain the high observed luminosity in H₂ lines. In this paper we present results for the structure and steady state dynamics of such advection-dominated merged fronts, calculated using the Cloudy plasma/molecular physics code. We ...

Interfacial Coherency And Ferroelectricity Of Batio3/Srtio3 Superlattice Films, Y. L. Li, S. Y. Hu, Dmitri Tenne, A. Soukiassian, D. G. Schlom, X. X. Xi, K. J. Choi

Physics Faculty Publications and Presentations

We studied the phase transitions, domain morphologies, and polarizations in BaTiO₃/SrTiO₃ superlattices grown on SrTiO₃ substrates. Using the phase field approach, we discovered the remarkable influence of film/substrate interfacial coherency on the ferroelectricity of the SrTiO₃ layers within a superlattice: it is an orthorhombic ferroelectric for an incoherent interface while it exhibits only induced polarization by the adjacent BaTiO₃ layers for a coherent interface. We presented the domain morphologies within individual BaTiO₃ and SrTiO₃ layers which have different ferroelectric symmetries. The results are compared to ultraviolet Raman spectroscopy and variable temperature ...

From Twisted Supersymmetry To Orbifold Lattices, Simon Catterall

Physics

We show how to derive the supersymmetric orbifold lattices of Cohen et al. \cite{Cohen:2003xe,Cohen:2003qw} and Kaplan et al. \cite{Kaplan:2005ta} by direct discretization of an appropriate twisted supersymmetric Yang-Mills theory. We examine in detail the four supercharge two dimensional theory and the theory with sixteen supercharges in four dimensions. The continuum limit of the latter theory is the well known Marcus twist of {\cal N}=4 Yang-Mills. The lattice models are gauge invariant and possess one exact supersymmetry at non-zero lattice spacing.

Neutron Vibrational Spectroscopy And First-Principles Calculations Of The Ternary Hydrides Li4Si2H(D) And Li4Ge2H(D): Electronic Structure And Lattice Dynamics, Hui Wu, Wei Zhou, Terrence J. Udovic, John J. Rush, Tanner Yildirim, Michael R. Hartman, Robert C. Bowman Jr., John J. Vajo

Department of Physics Papers

Using combined neutron spectroscopy and first-principles calculations, we investigated the electronic structure and vibrational dynamics of the recently discovered class of ternary hydrides Li₄Tt₂H (Tt=Si and Ge). In these compounds, all hydrogen atoms are located in a single type of Li₆-defined octahedral site. The Tt atoms form long-range Tt-Tt chains sandwiched between each Li₆-octahedra layer. The Li-H interactions are strongly ionic, with bond lengths comparable to those in LiH. Our density functional theory calculations indicate that Li atoms transfer their electrons to both H and Tt atoms. Tt atoms within the ...

Study Of The Decay BS0 → DS(*)DS(*), V. M. Abazov, Kenneth A. Bloom, Gregory R. Snow, D0 Collaboration

Kenneth Bloom Publications

We report a study of the decay B_s⁰ → D_s^(*)D_s^(*) using a data sample corresponding to 1.3 fb^-1 of integrated luminosity collected by the D0 experiment in 2002–2006 during run II of the Fermilab Tevatron collider. One D_s^(*) meson was partially reconstructed in the decay D_s→ φμν, and the other D_s^(*) meson was identified using the decay D_s→ φπ where no attempt was made to distinguish D_s and D_s^(*) states. For the branching fraction Br(B_s⁰ → D_s^(*)D_s^(*)) we obtain a 90% C ...

Electron Beam Characterization Of Carbon Nanostructures, Eric Samuel Mandell

Dissertations

Atom-thick carbon nanostructures represent a class of novel materials that are of interest to those studying carbon's role in fossil fuel, hydrogen storage, scaled-down electronics, and other nanotechnology. Electron microscope images of "edge-on" graphene sheets show linear image features due to the projected potential of the sheets. Here, intensity profiles along these linear features can measure the curvature of the sheet, as well as the shape of the sheet (i.e. hexagonal, triangular). Also, electron diffraction powder profiles calculated for triangular graphene sheet shapes show a broadening of the low-frequency edge of diffraction rings, in comparison to those calculated ...

Deeply Virtual Compton Scattering Off The Neutron, M. Mazouz, Timothy Holmstrom, Jefferson Lab Hall A Collaboration

Chemistry and Physics Faculty Publications

The present experiment exploits the interference between the deeply virtual Compton scattering (DVCS) and the Bethe-Heitler processes to extract the imaginary part of DVCS amplitudes on the neutron and on the deuteron from the helicity-dependent D(e⃗,e^′γ)X cross section measured at Q²=1.9  GeV² and x_B=0.36. We extract a linear combination of generalized parton distributions (GPDs) particularly sensitive to E_q, the least constrained GPD. A model dependent constraint on the contribution of the up and down quarks to the nucleon spin is deduced.

Identification Of The Possible Defect States In Poly(3-Hexylthiophene) Thin Films, D.-Q. Feng, A. N. Caruso, Yaroslav B. Losovyj, D. L. Schulz, Peter A. Dowben

Peter Dowben Publications

The possible origins of a low density of defect states within the highest occupied molecular orbital to lowest unoccupied molecular orbital gap are suggested for regioregular poly(3- hexylthiophene). A number of chemical defects, impurities, and structural defects could contribute to features in photoemission for regioregular poly(3-hexylthiophene), observed within the highest occupied molecular orbital to lowest unoccupied molecular orbital gap of regioregular poly(3-hexylthiophene).

Spitzer Irs Observations Of Seyfert 1.8 And 1.9 Galaxies: A Comparison With Seyfert 1 And Seyfert 2, R. P. Deo, D. M. Crenshaw, S. B. Kraemer, M. Dietrich, Moshe Elitzur, H. Teplitz, T. J. Turner

Physics and Astronomy Faculty Publications

We present Spitzer mid-infrared spectra of 12 Seyfert 1.8 and 1.9 galaxies over the 5-38 μm region. We compare the spectral characteristics of this sample to those of 58 Seyfert 1 and 2 galaxies from the Spitzer archives. An analysis of the spectral shapes, the silicate 10 μm feature and the emission-line fluxes have enabled us to characterize the mid-IR properties of Seyfert 1.8/1.9s. We find that the EWs of the 10 μm silicate feature are generally weak in all Seyfert galaxies, as previously reported by several studies. The few Seyfert galaxies ...

Structure Formation Inside Triaxial Dark Matter Halos: Galactic Disks, Bulges, And Bars, Clayton Heller, Isaac Shlosman, Evangelie Athanassoula

Physics and Astronomy Faculty Publications

We investigate formation and evolution of galactic disks immersed in assembling live DM halos. Models have been evolved from cosmological initial conditions and represent the collapse of an isolated density perturbation. The baryons include gas participating in star formation (SF) and stars with the energy feedback onto the ISM. We find that (1) the triaxial halo figure tumbling is insignificant and the angular momentum (J) is channeled into the internal circulation, while the baryonic collapse is stopped by the centrifugal barrier; (2) density response of the (disk) baryons is out of phase with DM, thus washing out the inner halo ...

New Ekpyrotic Cosmology, Evgeny I. Buchbinder, Justin Khoury, Burt A. Ovrut

Department of Physics Papers

In this paper, we present a new scenario of the early universe that contains a pre-big bang ekpyrotic phase. By combining this with a ghost condensate, the theory explicitly violates the null energy condition without developing any ghostlike instabilities. Thus the contracting universe goes through a nonsingular bounce and evolves smoothly into the expanding post-big bang phase. The curvature perturbation acquires a scale-invariant spectrum well before the bounce in this scenario. It is sourced by the scale-invariant entropy perturbation engendered by two ekpyrotic scalar fields, a mechanism recently proposed by Lehners et al. Since the background geometry is nonsingular at ...

Solid Phase Crystallization Of Vanadium Dioxide Thin Films And Characterization Through Scanning Electron Microscopy, Felipe Rivera

Theses and Dissertations

Crystalline films of vanadium dioxide were obtained through thermal annealing of amorphous vanadium dioxide thin films sputtered on silicon dioxide. An annealing process was found that yielded polycrystalline vanadium dioxide thin films, semi-continuous thin films, and films of isolated single-crystal particles. Orientation Imaging Microscopy (OIM) was used to characterize and study the phase and the orientation of the vanadium dioxide crystals obtained, as well as to diferentiate them from other vanadium oxide stoichiometries that may have formed during the annealing process. There was no evidence of any other vanadium oxides present in the prepared samples. Indexing of the crystals for ...

Charge Effects In Bilayer Electric Transport, Christopher Czech

All Theses

The mathematical isomorphism between the layer index in a bilayer system and the spin index in a spin-polarized system is used to draw an analogy between phenomena that occur in the two systems as a result of the electron-electron (e-e) interaction. First, the classical case of the Coulomb drag, a phenomenon resulting from momentum transfer through e-e collisions, is reviewed for both bilayer systems and spin-polarized systems. Then, the spin-backflow phenomenon, a result of the interaction-induced changes in the local quasiparticle energy that affects the charge transport in spin-polarized systems is presented. Finally, the iso-spin correlation is exploited to establish ...

Characterization Of Metalferroelectric-Insulator-Semiconductor Structures Based On Ferroelectric Langmuir-Blodgett Polyvinylidene Fluoride Copolymer Films For Nondestructive Random Access Memory Applications, Timothy J. Reece

Theses, Dissertations, and Student Research: Department of Physics and Astronomy

Ferroelectric field effect transistors (FeFETs) have attracted much attention recently because of their ability to combine high speed, low power consumption, and fast nondestructive readout with the potential for high density nonvolatile memory. The polarization of the ferroelectric is used to switch the channel at the silicon surface between states of high and low conductance.

Among the ferroelectric thin films used in FET devices; the ferroelectric copolymer of Polyvinylidene fluoride, PVDF (C₂H₂F₂), with trifluoroethylene, TrFE (C₂HF₃), has distinct advantages, including low dielectric constant, low processing temperature, low cost and compatibility with organic ...

How To Hunt A Wild Neutrino, Janet Conrad

IMSA Great Minds Program ®

Of all of the particles which make up matter, the neutrino is the most mysterious. Remnants of the big bang, neutrinos are everywhere -- up in your attic, out in the cornfields, deep in the farthest galaxies. Catching them has always been a problem. But recently we have discovered one of their tricks. They can change their spots -- from one type into another! This talk will discuss neutrinos, how to catch them, and in the context of the research at Fermilab.

Avalanche Statistics And Time-Resolved Grain Dynamics For A Driven Heap, Adam Ross Abate, Hiroaki Katsuragi, Douglas J. Durian

Department of Physics Papers

We probe the dynamics of intermittent avalanches caused by steady addition of grains to a quasi-twodimensional heap. To characterize the time-dependent average avalanche flow speed v(t), we image the top free surface. To characterize the grain fluctuation speed δv(t), we use speckle-visibility spectroscopy. During an avalanche, we find that the fluctuation speed is approximately one-tenth the average flow speed, δv ≈ 0.1v, and that these speeds are largest near the beginning of an event. We also find that the distribution of event durations is peaked, and that event sizes are correlated with the time interval ...

New Multiwavelength Variability And Optical Microvariability Investigations Of X-Ray And Radio Selected Blazars, Margaret Angela Osterman

Physics and Astronomy Dissertations

The extreme AGN known as blazars can be classified based on their spectral properties into X-ray and radio selected objects, known as XBLs and RBLs, respectively. In this work, the results of new multiwavelength campaigns are presented for two XBLs and two RBLs. Each campaign contains simultaneous observations in the radio, optical, and X-ray regimes. A campaign on a third RBL was completed using near-simultaneous archival radio, optical/IR, and gamma-ray data. The simultaneous multiwavelength behavior exhibited in each campaign was analyzed by examining the multiwavelength variability and using spectral analysis. Observations of prominent optical microvariability were quantitatively analyzed. Previously ...

Hard Disks On The Hyperbolic Plane, Carl D. Modes, Randall D. Kamien

Department of Physics Papers

We examine a simple hard disk fluid with no long range interactions on the two-dimensional space of constant negative Gaussian curvature, the hyperbolic plane. This geometry provides a natural mechanism by which global crystalline order is frustrated, allowing us to construct a tractable model of disordered monodisperse hard disks. We extend free-area theory and the virial expansion to this regime, deriving the equation of state for the system, and compare its predictions with simulation near an isostatic packing in the curved space.

Slowmo: A Search For Nearby Stars, Misty Adana Brown

Physics and Astronomy Theses

I report on suspected nearby stars with proper motions 1.0 arcsec > μ ≥ 0.5 arcsec/yr in the southern sky (DEC = −90° to 00°). This sample of slow-motion (SLOWMO) stars complements the work of Jao (2004), who reported on faster moving stars with μ ≥ 1.0 arcsec/yr in the entire sky for his doctoral dissertation, and the work of Finch (2007), who uncovered stars moving slower than 0.5 arcsec/yr between declinations −90° and −47°. Characterizations of SLOWMO systems include trigonometric parallaxes, optical and infrared photometry. For stars without trigonometric parallaxes, colors and apparent magnitudes are used ...

). Size Dependency Of The Elastic Modulus Of Zno Nanowires: Surface Stress Effect, Guofeng Wang, Xiaodong Li

Faculty Publications

Relation between the elastic modulus and the diameter (D) of ZnOnanowires was elucidated using a model with the calculated ZnOsurface stresses as input. We predict for ZnOnanowires due to surface stress effect: (1) when D>20nm, the elastic modulus would be lower than the bulk modulus and decrease with the decreasing diameter, (2) when 20nm>D>2nm, the nanowires with a longer length and a wurtzite crystal structure could be mechanically unstable, and (3) when D<2nm, the elastic modulus would be higher than that of the bulk value and increase with a decrease in nanowire diameter.

Construction And Analysis Of An Ozone Profile Climatology Over Houston, Texas, Gary A. Morris, Anne M. Thompson, Ryan Perna, John Yorks, Bernhard Rappengluek, Greg Ostermann, Barry Lefer, Renee Boudreaux, Aaron Chow, Bonnie Ford, Elizabeth Thompson, Scott Hersey, Brittni Emery

Gary A. Morris

Since the summer of 2004, over 200 ozonesondes have been launched from the campuses of Rice University or the University of Houston (29.7 N, 95.3 W), each about 3 miles from downtown Houston. These sounding launches have been sponsored by NASA, the Shell Center for Sustainability of Rice University, and the Texas Commissions for Environmental Quality as part of a large effort to understand Houston’s ozone problem. Data from these soundings have provided valuable insight into the seasonal and diurnal variations of the vertical ozone distribution and their relationship to changes in atmospheric conditions. In this presentation ...