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Articles 12061 - 12090 of 14641

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Two-Photon Detachment Cross Sections And Dynamic Polarizability Of H- Using A Variationally Stable, Coupled-Channel Hyperspherical Approach, Mauro Masili, Anthony F. Starace Sep 2000

Two-Photon Detachment Cross Sections And Dynamic Polarizability Of H- Using A Variationally Stable, Coupled-Channel Hyperspherical Approach, Mauro Masili, Anthony F. Starace

Anthony F. Starace Publications

We present a generalization of the variationally stable method of Gao and Starace [B. Gao and A. F. Starace, Phys. Rev. Lett. 61, 404 (1988); Phys. Rev. A 39, 4550 (1989)] for two-electron atoms and ions that incorporates a coupled-channel adiabatic hyperspherical approach. Using this approach, we report results for two-photon detachment of H-, in which we have coupled one, two, three, and four adiabatic hyperspherical channels within each term level of the initial, intermediate, and final states. We present results also for the dynamic polarizability of H- as well as for the one-photon detachment cross section. …

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Reversible Dissociation Of Thiolate Ligands From Molybdenum In An Enzyme Of The Dimethyl Sulfoxide Reductase Family, Robert C. Bray, Benjamin Adams, Andrew T. Smith, Brian Bennett, Susan Bailey Sep 2000

Reversible Dissociation Of Thiolate Ligands From Molybdenum In An Enzyme Of The Dimethyl Sulfoxide Reductase Family, Robert C. Bray, Benjamin Adams, Andrew T. Smith, Brian Bennett, Susan Bailey

Physics Faculty Research and Publications

Much is unknown concerning the role of thiolate ligands of molybdenum in molybdopterin enzymes. It has been suggested that thiolate dissociation from molybdenum is part of the catalytic mechanism of bis-molybdopterin enzymes of the dimethyl sulfoxide reductase (DMSOR) family. For DMSOR from Rhodobacter capsulatus, thiolate dissociation has therefore been investigated crystallographically, by UV/visible spectroscopy, and by enzyme assays. When crystallized from sodium citrate, all four thiolates of DMSOR are within bonding distance of Mo, but after extended exposure to Na+-Hepes, a pair of thiolates dissociates, a mixture of structures being indicated after shorter exposures to this buffer. …

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Concept Of Local Polaritons And Optical Properties Of Mixed Polar Crystals, Lev I. Deych, Alexey Yamilov, Alexander A. Lisyansky Sep 2000

Concept Of Local Polaritons And Optical Properties Of Mixed Polar Crystals, Lev I. Deych, Alexey Yamilov, Alexander A. Lisyansky

Physics Faculty Research & Creative Works

The concept of local polaritons is used to describe the optical properties of mixed crystals in the frequency region of their restrahlen band. It is shown that this concept allows for a physically transparent explanation of the presence of weak features in the spectra of so-called one-mode crystals and for one-two mode behavior. The previous models Were able to explain these features only with the use of many fitting parameters. We show that under certain Conditions new impurity-induced polariton modes may arise within the restrahlen of the host crystals, and study their dispersion laws and density of states. Particularly, we …

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Algorithm Development For On-Line Control Of The Airborne Laser, Michael W. Oppenheimer Sep 2000

Algorithm Development For On-Line Control Of The Airborne Laser, Michael W. Oppenheimer

Theses and Dissertations

The use of adaptive optics entails the design of a controller. This requires the development of a model of the plant to be controlled, which, in this case, Consists of the atmosphere through which light is traveling. In optics, Zemike polynornials are used as a basis set for the expansion of wavefront phase distortions. Due to the turbulence induced stochastic nature of the underlying process involved, the spatial-temporal correlation functions of the Zemike polynomial phase expansion coefficients must be evaluated if a proper stochastic model of the plant is to be developed and adaptive optics is to be employed. In …

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Total Cross Sections For Dissociative Electron Attachment In Dichloroalkanes And Selected Polychloroalkanes: The Correlation With Vertical Attachment Energies, Kayvan Aflatooni, Paul Burrow Aug 2000

Total Cross Sections For Dissociative Electron Attachment In Dichloroalkanes And Selected Polychloroalkanes: The Correlation With Vertical Attachment Energies, Kayvan Aflatooni, Paul Burrow

Paul Burrow Publications

Electron attachment into the lowest unoccupied molecular orbital of a typical polychloroalkane in the gas phase forms a temporary negative ion in which the impinging electron resides on a combination of local C–Cl σ* orbitals. Because of the antibonding character of these orbitals, these anions may dissociate, producing Cl- with cross sections that vary enormously over the chloroalkane family. In this work, we present absolute total dissociative electron attachment (DEA) cross sections for 33 of these compounds, and we show that the peak values of the cross sections correlate strongly with the vertical attachment energies (VAEs) for formation of …

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Nondestructive Single-Photon Trigger, John C. Howell, John A. Yeazell Aug 2000

Nondestructive Single-Photon Trigger, John C. Howell, John A. Yeazell

Mathematics, Physics, and Computer Science Faculty Articles and Research

A triggering device sensitive to a single photon is discussed. It is based on a balanced quantum nondemolition (QND) measurement proposed by Chuang and Yamamoto [Phys. Rev. Lett. 76, 4281 (1996)]. The balanced measurement measures the total photon number and obtains no which-path/mode information. Hence, the timing of the photon can be determined without destroying its wave function or entangling the probe field. This could have extensive use in the realization of long-distance quantum communications systems.

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Measurement Of Exclusive Ω Electroproduction At Hera, J. Breitweg, S. Chekanov, M. Derrick, D. Krakauer, S. Magill, B. Musgrave, A. Pellegrino, J. Repond, R. Stanek, R. Yoshida, Margarita C. K. Mattingly, G. Abbiendi, F. Anselmo, P. Antonioli, G. Bari, M. Basile, L. Bellagamba, D. Boscherini, A. Bruni, G. Bruni, G. Cara Romeo, G. Castellini, L. Cifarelli, F. Cindolo, A. Contin, N. Coppola, M. Corradi, S. De Pasquale, P. Giusti, G. Iacobucci, G. Laurenti Aug 2000

Measurement Of Exclusive Ω Electroproduction At Hera, J. Breitweg, S. Chekanov, M. Derrick, D. Krakauer, S. Magill, B. Musgrave, A. Pellegrino, J. Repond, R. Stanek, R. Yoshida, Margarita C. K. Mattingly, G. Abbiendi, F. Anselmo, P. Antonioli, G. Bari, M. Basile, L. Bellagamba, D. Boscherini, A. Bruni, G. Bruni, G. Cara Romeo, G. Castellini, L. Cifarelli, F. Cindolo, A. Contin, N. Coppola, M. Corradi, S. De Pasquale, P. Giusti, G. Iacobucci, G. Laurenti

Faculty Publications

The exclusive electroproduction of ω mesons, ep → e ω p, has been studied in the kinematic range 3 < Q2 < 20 GeV2, 40 < W < 120 GeV and |t| < 0.6 GeV2 with the ZEUS detector at HERA using an integrated luminosity of 37.7 pb-1. The ω mesons were identified via the decay ω → π+ π- π0. The exclusive cross section in the above kinematic region is σ(ep) → e ω p = 0.108 ± 0.014 (stat.) ± 0.026(syst.) nb. The reaction ep → e φ p, φ → π+ π- π0, has also been measured. The cross sections, as well as the ratios σ γ·p → ωp/σ γ · p → p0p and σ γ · p → ωp/σγ·p → φp, are presented as a function of W and Q2. Thus, for the first time, the properties of ω electroproduction can be compared to those of p0,φ and J/ψ electroproduction at high W. (C) 2000 Elsevier Science B.V.

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Limits On Light Gravitino Production And New Processes With Large Missing Transverse Energy In Pp̅ Collisions At √S = 1.8 Tev, T. Affolder, Kenneth A. Bloom, Collider Detector At Fermilab Collaboration Aug 2000

Limits On Light Gravitino Production And New Processes With Large Missing Transverse Energy In Pp̅ Collisions At √S = 1.8 Tev, T. Affolder, Kenneth A. Bloom, Collider Detector At Fermilab Collaboration

Kenneth Bloom Publications

Events collected by the Collider Detector at Fermilab (CDF) with an energetic jet plus large missing transverse energy can be used to search for physics beyond the standard model. We see no deviations from the expected backgrounds and set upper limits on the production of new processes. We consider in addition the production of light gravitinos and set a limit at 95% confidence level on the breaking scale √F ≥ 217 GeV, which excludes gravitino masses smaller than 1.1 x 10-5 eV/c2.

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Measurement Of The Proton Structure Function F2 At Very Low Q2 At Hera, J. Breitweg, S. Chekanov, M. Derrick, D. Krakauer, S. Magill, A. Pellegrino, J. Repond, R. Stanek, R. Yoshida, Margarita C. K. Mattingly, G. Abbiendi, F. Anselmo, P. Antonioli, G. Bari, M. Basile, L. Bellagamba, D. Boscherini, A. Bruni, G. Bruni, G. Cara Romeo, G. Castellini, L. Cifarelli, F. Cindolo, A. Contin, N. Coppola, M. Corradi, S. De Pasquale, P. Giusti, G. Iacobucci, G. Laurenti, G. Levi Aug 2000

Measurement Of The Proton Structure Function F2 At Very Low Q2 At Hera, J. Breitweg, S. Chekanov, M. Derrick, D. Krakauer, S. Magill, A. Pellegrino, J. Repond, R. Stanek, R. Yoshida, Margarita C. K. Mattingly, G. Abbiendi, F. Anselmo, P. Antonioli, G. Bari, M. Basile, L. Bellagamba, D. Boscherini, A. Bruni, G. Bruni, G. Cara Romeo, G. Castellini, L. Cifarelli, F. Cindolo, A. Contin, N. Coppola, M. Corradi, S. De Pasquale, P. Giusti, G. Iacobucci, G. Laurenti, G. Levi

Faculty Publications

A measurement of the proton structure function F2(x, Q2) is presented in the kinematic range 0.045 GeV2 < Q2 < 0.65 GeV2 and 6 · 10-7 < x < 1 · 10-3. The results were obtained using a data sample corresponding to an integrated luminosity of 3.9pb-1 in e+ p reactions recorded with the ZEUS detector at HERA. Information from a silicon-strip tracking detector, installed in front of the small electromagnetic calorimeter used to measure the energy of the final-state positron at small scattering angles, together with an enhanced simulation of the hadronic final state, has permitted the extension of the kinematic range beyond that of previous measurements. The uncertainties in F2 are typically less than 4%. At the low Q2 values of the present measurement, the rise of F2 at low x is slower than observed in HERA data at higher Q2 and can be described by Regge theory with a constant logarithmic slope δlnF2/δln(1/x). The dependence of F2 on Q2 is stronger than at higher Q2 values, approaching, at the lowest Q2 values of this measurement, a region where F2 becomes nearly proportional to Q2. (C) 2000 Elsevier Science B.V.

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Inflation After Preheating, Gary Felder, Lev Kofman, Andrei Linde, Igor Tkachev Aug 2000

Inflation After Preheating, Gary Felder, Lev Kofman, Andrei Linde, Igor Tkachev

Physics: Faculty Publications

Preheating after inflation may lead to non-thermal phase transitions with symmetry restoration. These phase transitions may occur even if the total energy density of fluctuations produced during reheating is relatively small as compared with the vacuum energy in the state with restored symmetry. As a result, in some inflationary models one encounters a secondary, non-thermal stage of inflation due to symmetry restoration after preheating. We review the theory of non-thermal phase transitions and make a prediction about the expansion factor during the secondary inflationary stage. We then present the results of lattice simulations which verify these predictions, and discuss possible …

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Search For Antiproton Decay At The Fermilab Antiproton Accumulator, S. Geer, J. Marriner, M. Martins, R. E. Ray, J. Streets, W. Wester Aug 2000

Search For Antiproton Decay At The Fermilab Antiproton Accumulator, S. Geer, J. Marriner, M. Martins, R. E. Ray, J. Streets, W. Wester

Gregory Snow Publications

A search for antiproton decay has been made at the Fermilab Antiproton Accumulator. Limits are placed on 13 antiproton decay modes. The results include the first explicit experimental limits on the muonic decay modes of the antiproton, and the first limits on the decay modes ℯ-γγ and ℯ-ω. The most stringent limit is for the decay modep̅→ ℯ-γ. At 90% C.L. we find that τ p̅¯ / B ( p̅→ ℯ-γ)>7x105 yr. The most stringent limit for decay modes with a muon in the final state …

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A Lattice Path Integral For Supersymmetric Quantum Mechanics, Simon Catterall, Eric B. Gregory Aug 2000

A Lattice Path Integral For Supersymmetric Quantum Mechanics, Simon Catterall, Eric B. Gregory

Physics - All Scholarship

We report on a study of the supersymmetric anharmonic oscillator computed using a euclidean lattice path integral. Our numerical work utilizes a Fourier accelerated hybrid Monte Carlo scheme to sample the path integral. Using this we are able to measure massgaps and check Ward identities to a precision of better than one percent. We work with a non-standard lattice action which we show has an {\it exact} supersymmetry for arbitrary lattice spacing in the limit of zero interaction coupling. For the interacting model we show that supersymmetry is restored in the continuum limit without fine tuning. This is contrasted with …

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Magnetic Localization In Transition-Metal Nanowires, Ralph Skomski, H. Zeng, M. Zheng, David J. Sellmyer Aug 2000

Magnetic Localization In Transition-Metal Nanowires, Ralph Skomski, H. Zeng, M. Zheng, David J. Sellmyer

David Sellmyer Publications

Magnetization reversal in transition-metal nanowires is investigated. Model calculations explain why magnetization reversal is localized, as opposed to the sometimes assumed delocalized coherent-rotation and curling modes. The localization is a quite general phenomenon caused by morphological inhomogenities and occurring in both polycrystalline and single-crystalline wires. In the polycrystalline limit, the competition between interatomic exchange and anisotropy gives rise to a variety of random-anisotropy effects, whereas nearly single-crystalline wires exhibit a weak localization of the nucleation mode. Model predictions are used to explain the coercive and magnetic-viscosity behavior of Co (and Ni) nanowires electrodeposited in self-assembled alumina pores.

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The Narrow-Line Region In The Seyfert 2 Galaxy Ngc 3393, Andrew J. Cooke, J. A. Baldwin, Gary J. Ferland, Hagai Netzer, Andrew S. Wilson Aug 2000

The Narrow-Line Region In The Seyfert 2 Galaxy Ngc 3393, Andrew J. Cooke, J. A. Baldwin, Gary J. Ferland, Hagai Netzer, Andrew S. Wilson

Physics and Astronomy Faculty Publications

The narrow-line region (NLR) of the Seyfert 2 galaxy NGC 3393 is dominated by a symmetric structure which appears as S-shaped arms in Hubble Space Telescope (HST) images. These arms, which occupy the central few arcseconds of the nucleus, border a linear, triple-lobed radio source. We use HST imaging and spectra, ground-based optical images, long-slit spectra, Fabry-Perot imaging spectroscopy, and VLA radio data to perform a detailed investigation of the kinematics and ionization of the line-emitting gas in NGC 3393 and of its relationship with the relativistic gas responsible for the radio emission. The excitation map [O III] …

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Effect Of Disorder On Perpendicular Magnetotransport In Co/Cu Multilayers, Evgeny Y. Tsymbal Aug 2000

Effect Of Disorder On Perpendicular Magnetotransport In Co/Cu Multilayers, Evgeny Y. Tsymbal

Evgeny Tsymbal Publications

We investigate the spin-dependent conductance of the Co/Cu multilayers in the current-perpendicular-tothe- plane (CPP) geometry. Using a realistic tight-binding model for the electronic band structure of the multilayer, and introducing disorder in the on-site atomic energies we calculate the conductance and giant magnetoresistance (GMR) within the quantum-mechanical linear response theory by performing averaging over random disorder configurations. By varying the thickness and the number of individual layers and the degree of disorder in the multilayers, we analyze factors influencing the CPP GMR. In particular, we show the importance of the thickness-dependent interface resistance, which depends on the mean free path …

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Limits On Anomalous Ww Γ And Wwz Couplings From Ww/Wz→Eνjj Production, B. Abbott, Gregory Snow, D0 Collaboration Aug 2000

Limits On Anomalous Ww Γ And Wwz Couplings From Ww/Wz→Eνjj Production, B. Abbott, Gregory Snow, D0 Collaboration

Gregory Snow Publications

Limits on anomalous WW γ and WWZ couplings are presented from a study of WW/WZ→eνjj events produced in pp̅ collisions at √s = 1.8 TeV. Results from the analysis of data collected using the DØ detector during the 1993–1995 Tevatron collider run at Fermilab are combined with those of an earlier study from the 1992–1993 run. A fit to the transverse momentum spectrum of the W boson yields direct limits on anomalous WW γ and WWZ couplings. With the assumption that the WW γ and WWZ couplings are equal, we obtain -0.34 < λ < 0.36 (with Δk=0) and -0.43< Δk<0.59 (with λ =0) at the 95% confidence level for a form-factor scale Λ = 2.0 TeV.

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42nd Rocky Mountain Conference On Analytical Chemistry Jul 2000

42nd Rocky Mountain Conference On Analytical Chemistry

Rocky Mountain Conference on Magnetic Resonance

Abstracts from the 42nd annual meeting of the Rocky Mountain Conference on Analytical Chemistry, co-sponsored by the Colorado Section of the American Chemical Society and the Rocky Mountain Section of the Society for Applied Spectroscopy. Held in Broomfield, Colorado, July 30 - August 3, 2000.

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Energetics And Geometry Of Excitations In Random Systems, Alan Middleton Jul 2000

Energetics And Geometry Of Excitations In Random Systems, Alan Middleton

Physics - All Scholarship

Methods for studying droplets in models with quenched disorder are critically examined. Low energy excitations in two dimensional models are investigated by finding minimal energy interior excitations and by computing the effect of bulk perturbations. The numerical data support the assumptions of compact droplets and a single exponent for droplet energy scaling. Analytic calculations show how strong corrections to power laws can result when samples and droplets are averaged over. Such corrections can explain apparent discrepancies in several previous numerical results for spin glasses

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Thermolectric Materials Based On Intercalated Layered Metallic Systems, Peter C. Eklund, Leonid Grigorian, Keith A. Williams, Gamini U. Sumanasekera Jul 2000

Thermolectric Materials Based On Intercalated Layered Metallic Systems, Peter C. Eklund, Leonid Grigorian, Keith A. Williams, Gamini U. Sumanasekera

Physics and Astronomy Faculty Patents

A novel thermoelectric material in the form of a unit cell including a first reagent intercalated as a semiconducting layer into a metallic layered host and a method for producing the thermoelectric material are disclosed. The unit cell is characterized by a Seebeck coefficient S of 80-140 μV/K° and an electrical conductivity σ of 103 -104 (Ωcm)-1, as well as a figure of merit Z of about 2×10-3 K-1 at 100° K.

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Plasma Oscillations And Expansion Of An Ultracold Neutral Plasma, Scott D. Bergeson, S. Kulin, T. C. Killian, S. L. Rolston Jul 2000

Plasma Oscillations And Expansion Of An Ultracold Neutral Plasma, Scott D. Bergeson, S. Kulin, T. C. Killian, S. L. Rolston

Faculty Publications

We report the observation of plasma oscillations in an ultracold neutral plasma. With this collective mode we probe the electron density distribution and study the expansion of the plasma as a function of time. For classical plasma conditions, i.e., weak Coulomb coupling, the expansion is dominated by the pressure of the electron gas and is described by a hydrodynamic model. Discrepancies between the model and observations at low temperature and high density may be due to strong coupling of the electrons.

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Kinetic Energy Release In Electron-Induced Decay Reactions Of Molecular Ions: C3h8+ And C3h7+, S. Matt, Olof E. Echt, A. Stamatovic, T. D. Mark Jul 2000

Kinetic Energy Release In Electron-Induced Decay Reactions Of Molecular Ions: C3h8+ And C3h7+, S. Matt, Olof E. Echt, A. Stamatovic, T. D. Mark

Physics & Astronomy

We have measured the kinetic energy release (KER) distributions for electron-induced dissociation of mass-selected molecular parent and fragment ions of propane. They are compared with distributions determined for spontaneous (metastable) dissociation. The average KER for induced dissociation of C3H8+ into C3H7+ is 13.2 +/- 1.2 meV, about 42% larger than for the spontaneous reaction. This large difference is attributed to the dramatically reduced time at which the induced reaction can be sampled. In contrast, the KER for dissociation of C3H7+ into C3H5+, which is characterized by a large reverse activation energy, is hardly affected by the experimental time scale. (C) …

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Static Quark Potential In Three Flavor Qcd, James E. Hetrick, C. Bernard Jul 2000

Static Quark Potential In Three Flavor Qcd, James E. Hetrick, C. Bernard

All Faculty Articles - School of Engineering and Computer Science

We study the effects of dynamical quarks on the static quark potential at distances shorter than those where string breaking is expected. Quenched calculations and calculations with three flavors of dynamical quarks are done on sets of lattices with the lattice spacings matched within about one percent. The effect of the sea quarks on the shape of the potential is clearly visible. We investigate the consequences of these effects in a very crude model, namely solving Schrödinger’s equation in the resulting potential.

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Effective Surface Debye Temperature For Nimnsb.100. Epitaxial Films, C.N. Borca, Takashi Komesu, Hae-Kyung Jeong, Peter A. Dowben, D. Ristoiu, Ch. Hordequin, J. Pierre, J. P. Nozie`Res Jul 2000

Effective Surface Debye Temperature For Nimnsb.100. Epitaxial Films, C.N. Borca, Takashi Komesu, Hae-Kyung Jeong, Peter A. Dowben, D. Ristoiu, Ch. Hordequin, J. Pierre, J. P. Nozie`Res

Peter Dowben Publications

The surface Debye temperature of the NiMnSb (100) epitaxial films has been obtained using low energy electron diffraction, inverse photoemission, and core-level photoemission. The normal dynamic motion of the (100) surface results in a value for the effective surface Debye temperature of 145613 K. This is far smaller than the bulk Debye temperature of 31265 K obtained from wave vector dependent inelastic neutron scattering. The large difference between these measures of surface and bulk dynamic motion indicates a soft and compositionally different (100) surface. © 2000 American Institute of Physics. [S0003-6951(00)05127-5]

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Search For The Charged Higgs Boson In The Decays Of Top Quark Pairs In The EΤ And Μτ Channels At √S =1.8 Tev, T. Affolder, Kenneth A. Bloom, Collider Detector At Fermilab Collaboration Jul 2000

Search For The Charged Higgs Boson In The Decays Of Top Quark Pairs In The EΤ And Μτ Channels At √S =1.8 Tev, T. Affolder, Kenneth A. Bloom, Collider Detector At Fermilab Collaboration

Kenneth Bloom Publications

Top quark production offers the unique opportunity to search for a charged Higgs boson (H±), as the contribution from t→H+b→ τ + νb can be large in extensions of the standard model. We use results from a search for top quark pair production by the Collider Detector at Fermilab (CDF) in the e τ +ET+jets and μ τ+ET+jets signatures to set an upper limit on the branching ratio of Ɓ (t→H+b) in 106 pb-1 of data. The upper …

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Thermal Lattice Boltzmann Simulation For Multispecies Fluid Equilibration, Linda L. Vahala, Darren Wah, George Vahala, Jonathan Carter, Pavol Pavlo Jul 2000

Thermal Lattice Boltzmann Simulation For Multispecies Fluid Equilibration, Linda L. Vahala, Darren Wah, George Vahala, Jonathan Carter, Pavol Pavlo

Electrical & Computer Engineering Faculty Publications

The equilibration rate for multispecies fluids is examined using thermal lattice Boltzmann simulations. Two-dimensional free-decay simulations are performed for effects of velocity shear layer turbulence on sharp temperature profiles. In particular, parameters are so chosen that the lighter species is turbulent while the heavier species is laminar-and so its vorticity layers would simply decay and diffuse in time. With species coupling, however, there is velocity equilibration followed by the final relaxation to one large co- and one large counter-rotating vortex. The temperature equilibration proceeds on a slower time scale and is in good agreement with the theoretical order of magnitude …

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Electroweak Structure Of Three-And Four-Body Nuclei, Laura Elisa Marcucci Jul 2000

Electroweak Structure Of Three-And Four-Body Nuclei, Laura Elisa Marcucci

Physics Theses & Dissertations

This work reports results for (i) the elastic electromagnetic form factors of the trinucleons; (ii) the nuclear response functions of interest in [e → e'] experiments, at excitation energies below the deuteron breakup threshold; (iii) the astrophysical S-factor for proton weak capture on 3He (the hep reaction). The initial and final state wave functions are calculated using the correlated hyperspherical harmonics method, from a realistic Hamiltonian consisting of the Argonne v18 two-nucleon and Urbana IX three-nucleon interactions. The nuclear electroweak charge and current operators include one- and many-body components. The predicted magnetic form factor of …

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Essentials Of K-Essence, Christian Armendariz-Picon, V. Mukhanov, Paul J. Steinhardt Jun 2000

Essentials Of K-Essence, Christian Armendariz-Picon, V. Mukhanov, Paul J. Steinhardt

Physics - All Scholarship

We recently introduced the concept of "k-essence" as a dynamical solution for explaining naturally why the universe has entered an epoch of accelerated expansion at a late stage of its evolution. The solution avoids fine-tuning of parameters and anthropic arguments. Instead, k-essence is based on the idea of a dynamical attractor solution which causes it to act as a cosmological constant only at the onset of matter-domination. Consequently, k-essence overtakes the matter density and induces cosmic acceleration at about the present epoch. In this paper, we present the basic theory of k-essence and dynamical attractors based on evolving scalar fields …

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The Primordial Helium Abundance: Toward Understanding And Removing The Cosmic Scatter In The Dy/Dz Relation, D. R. Ballantyne, Gary J. Ferland, P. G. Martin Jun 2000

The Primordial Helium Abundance: Toward Understanding And Removing The Cosmic Scatter In The Dy/Dz Relation, D. R. Ballantyne, Gary J. Ferland, P. G. Martin

Physics and Astronomy Faculty Publications

We present results from photoionization models of low-metallicity H II regions. These nebulae form the basis for measuring the primordial helium abundance. Our models show that the helium ionization correction factor (ICF) can be nonnegligible for nebulae excited by stars with effective temperatures larger than 40,000 K. Furthermore, we find that when the effective temperature rises to above 45,000 K, the ICF can be significantly negative. This result is independent of the choice of stellar atmosphere. However, if an H II region has an [O III] λ5007/[O I] λ6300 ratio greater than 300, then our models show that, regardless of …

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Electronic Structure Of Bas And Boride Iii-V Alloys, Gus L. W. Hart, Alex Zunger Jun 2000

Electronic Structure Of Bas And Boride Iii-V Alloys, Gus L. W. Hart, Alex Zunger

Faculty Publications

Boron arsenide, the typically ignored member of the Group-III–V arsenide series BAs-AlAs-GaAs-InAs is found to resemble silicon electronically: its Γ conduction-band minimum is p-like (Γ15), not s-like (Γ1c), it has an X1c-like indirect band gap, and its bond charge is distributed almost equally on the two atoms in the unit cell, exhibiting nearly perfect covalency. The reasons for these are tracked down to the anomalously low atomic p orbital energy in the boron and to the unusually strong s–s repulsion in BAs relative to most other Group-III–V compounds. We find unexpected valence-band offsets of BAs with respect to GaAs and …

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State-Selective Cross Section Scalings For Electron Capture Collisions, K. R. Cornelius, K. Wojtkowski, Ronald E. Olson Jun 2000

State-Selective Cross Section Scalings For Electron Capture Collisions, K. R. Cornelius, K. Wojtkowski, Ronald E. Olson

Physics Faculty Research & Creative Works

The classical-trajectory Monte Carlo method is used to calculate total and final nf state-selective electron capture cross sections for various fully stripped ions, ranging from He2+ to Ne10+, incident on H(ni = 1-4) and Li(2s and 2p) targets over the collision energies of 1-100 keV u-1. Theoretical derivations of two scaling laws for state-selective electron capture cross sections as a function of collision energy, initial and final bound electron states and projectile charge are presented. The new scaling rules reduce the state-selective capture cross sections to very distinct universal curves. Curve fits …

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