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Articles 30571 - 30600 of 36660

Full-Text Articles in Physical Sciences and Mathematics

Rapid Optical Variability In Radio-Quiet Quasars, Meenakshi Uppal Jan 1999

Rapid Optical Variability In Radio-Quiet Quasars, Meenakshi Uppal

Theses

Results from imaging and spectral surveys at radio wavelengths indicate that radio quiet quasars (RQQs) may have accretion-fuelled Super Massive Black Holes (SMBH) as their so-called central engines, similar to those hypothesised for radio loud quasars (RLQs). Recent evidence suggests that some RQQs may also harbour weak jets. Rapid optical variability is a common property of most RLQs with compact radio cores, flat radio spectra, and extended linear features emanating from the core and is linked to the presence of a powerful relativistically boosted jet. We present the results of a 3 year monitoring campaign of sixteen RQQs displaying evidence …

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Speed Range For Breakdown Waves, Mostafa Hemmati, Eric L. George, Frances Terry Jan 1999

Speed Range For Breakdown Waves, Mostafa Hemmati, Eric L. George, Frances Terry

Journal of the Arkansas Academy of Science

Considering the electrons as the main element in breakdown wave propagation and using a one-dimensional, steady-state, three-fluid, hydrodynamical model, previous investigations have resulted in the completion of a set of equations for conservation of mass, momentum, and energy. We will use the terms proforce and antiforce waves, depending on whether the applied electric field force on electrons is with or against the direction of wave propagation. In the case of antiforce waves, the electron gas temperature and therefore the electron fluid pressure is assumed to be large enough to sustain the wave propagation down the discharge tube. For strong discontinuity …

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Double Electron Removal And Fragmentation Model Of The [Formula Presented] Molecule By Highly Charged Ions, C. J. Wood, Ronald E. Olson Jan 1999

Double Electron Removal And Fragmentation Model Of The [Formula Presented] Molecule By Highly Charged Ions, C. J. Wood, Ronald E. Olson

Physics Faculty Research & Creative Works

A five-body classical trajectory Monte Carlo model has been developed to study double electron removal from H2 by collisions with highly charged ions at impact energies ranging from 1 eV/u to 1 GeV/u. The longitudinal and transverse final-state correlation between ejected electrons is calculated for double ionization of H2 by impact of Se28+ at 3.6 MeV/u and U92+ at 1 GeV/u; the electron-electron interaction is dynamically included during the collision when one of the electron’s total energy becomes positive. Relativistic corrections are incorporated to reflect the Lorentz contraction of the projectile’s electric field. The cross-section dependence …

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Five-Body Calculations Of [Formula Presented] Fragmentation By [Formula Presented] Impact, C. R. Feeler, Ronald E. Olson, Robert D. Dubois, T. Schlathölter, O. Hadjar, R. Hoekstra, R. Morgenstern Jan 1999

Five-Body Calculations Of [Formula Presented] Fragmentation By [Formula Presented] Impact, C. R. Feeler, Ronald E. Olson, Robert D. Dubois, T. Schlathölter, O. Hadjar, R. Hoekstra, R. Morgenstern

Physics Faculty Research & Creative Works

A five-body classical trajectory Monte Carlo model has been developed to study fragmentation of diatomic molecules after double electron removal by highly charged ion impact. A systematic study of the final-state deuteron energy and momentum spectra has been conducted for Xe19+ + D2 collisions at impact energies ranging from 1 eV/u to 100 keV/u. At the highest projectile energies, the fragment energies and momenta are determined by the Coulomb explosion of the doubly ionized molecule via the known Franck-Condon transition for the isolated molecule. The deuterons are emitted back-to-back with nearly equal energies. At the lowest projectile energies, …

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Synchrotron X-Ray Diffraction Of Layering Transitions Of Multilayer Nitrogen Physisorbed On Graphite, T. E. Burns, John R. Dennison, S. N. Ehrlich Jan 1999

Synchrotron X-Ray Diffraction Of Layering Transitions Of Multilayer Nitrogen Physisorbed On Graphite, T. E. Burns, John R. Dennison, S. N. Ehrlich

All Physics Faculty Publications

We use synchrotron x-ray diffraction for structural analysis of the behavior of multilayer nitrogen films physisorbed on graphite foam. We provide structural information and concentrations of 2D and 3D solid phases at a coverage of Θ = 8 ML (Θ / 1 ML for a %3 x %3 structure) for temperatures from below the bulk α-ß transition temperature [Tα-ß = 34 " 0.5 K] to above the bulk triple point [Ttp = 63 K]. Our data indicate layering begins near Tα-ß, with subsequent layering occurring as the temperature is raised; all bulk nitrogen forms disordered film …

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Null Geodesics In The Alcubierre Warp Drive Spacetime: The View From The Bridge, Chad Clark, William A. Hiscock, Shane L. Larson Jan 1999

Null Geodesics In The Alcubierre Warp Drive Spacetime: The View From The Bridge, Chad Clark, William A. Hiscock, Shane L. Larson

All Physics Faculty Publications

The null geodesic equations in the Alcubierre warp-drive spacetime are numerically integrated to determine the angular deflection and redshift of photons which propagate through the distortion of the `warp-drive' bubble to reach an observer at the origin of the warp effect. We find that for a starship with an effective warp speed exceeding the speed of light, stars in the forward hemisphere will appear closer to the direction of motion than they would to an observer at rest. This aberration is qualitatively similar to that caused by special relativity. Behind the starship, a conical region forms from within which no …

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Midisuperspace Models Of Canonical Quantum Gravity, Charles G. Torre Jan 1999

Midisuperspace Models Of Canonical Quantum Gravity, Charles G. Torre

All Physics Faculty Publications

A midisuperspace model is a field theory obtained by symmetry reduction of a parent gravitational theory. Such models have proven useful for exploring the classical and quantum dynamics of the gravitational field. I present three recent classes of results pertinent to canonical quantization of vacuum general relativity in the context of midisuperspace models. (1) I give necessary and sufficient conditions such that a given symmetry reduction can be performed at the level of the Lagrangian or Hamiltonian.(2) I discuss the Hamiltonian formulation of models based upon cylindrical and toroidal symmetry. In particular, I explain how these models can be identified …

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The Conductor-Dielectric Junctions In A Low Density Plasma, B. V. Vayner, J. T. Galofaro, D. C. Ferguson, Win De Groot, Clint Thomson, John R. Dennison, Robert Davies Jan 1999

The Conductor-Dielectric Junctions In A Low Density Plasma, B. V. Vayner, J. T. Galofaro, D. C. Ferguson, Win De Groot, Clint Thomson, John R. Dennison, Robert Davies

All Physics Faculty Publications

A conductor-dielectric junction exposed to the space environment is a frequent spacecraft design feature. Due to spacecraft charging and/or solar array operation, the conductor can acquire a high potential with respect to the surrounding plasma. If this potential is positive the insulators adjacent to exposed conductors can collect current as if they were conductors themselves. This phenomenon, called snapover, results in a substantial increase in current collection, and may even result in a glow discharge if the potential is high enough. If a conductor has a negative potential, arcing can occur at the site of a junction. Both of these …

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Constructing A Portable Sundial, Michelle B. Larson Jan 1999

Constructing A Portable Sundial, Michelle B. Larson

All Physics Faculty Publications

Throughout history people have used shadows and sundials to measure the passage of time. This article describes how to construct a personal sundial that can be worn as a necklace or carried as a key chain. The activity is easily adaptable for grade levels elementary through high school. The instructions can be scaled for a schoolyard project. We’ve found sundial construction to be a terrific project that not only encourages students to investigate movement of the Sun in the sky, but also allows them to practice their math skills.

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Effects Of Magnetospheric Precipitation And Ionospheric Conductivity On The Ground Magnetic Signatures Of Traveling Convection Vortices, Lie Zhu, Robert W. Schunk, Jan Josef Sojka Jan 1999

Effects Of Magnetospheric Precipitation And Ionospheric Conductivity On The Ground Magnetic Signatures Of Traveling Convection Vortices, Lie Zhu, Robert W. Schunk, Jan Josef Sojka

All Physics Faculty Publications

By using an improved TCV model (Zhu et al., 1997), a quantitative study of the effects of magnetospheric precipitation and ionospheric background conductivity on the ground magnetic signatures of traveling convection vortices (TCVs) has been conducted. In this study the localized conductivity enhancement associated with the TCVs is present and the ratio of the Hall and Pedersen conductances vary both spatially and temporally according to the hardness of the TCV precipitation. It is found that a strong conductivity enhancement associated with hard TCV precipitation can significantly distort the TCV current closure in the ionosphere and lead to ground magnetic disturbance …

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Functional Evolution Of Free Quantum Fields, Charles G. Torre, Madhavan Varadarajan Jan 1999

Functional Evolution Of Free Quantum Fields, Charles G. Torre, Madhavan Varadarajan

All Physics Faculty Publications

We consider the problem of evolving a quantum field between any two (in general, curved) Cauchy surfaces. Classically, this dynamical evolution is represented by a canonical transformation on the phase space for the field theory. We show that this canonical transformation cannot, in general, be unitarily implemented on the Fock space for free quantum fields on flat spacetimes of dimension greater than 2. We do this by considering time evolution of a free Klein-Gordon field on a flat spacetime (with toroidal Cauchy surfaces) starting from a flat initial surface and ending on a generic final surface. The associated Bogolubov transformation …

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Effects Of Spacecraft Potential On Secondary Electron Yields In Geosynchronous Orbit, Neal E. Nickles Jan 1999

Effects Of Spacecraft Potential On Secondary Electron Yields In Geosynchronous Orbit, Neal E. Nickles

All Physics Faculty Publications

Surface charging due to interactions with the earth=s plasma is a hazard for orbiting spacecraft. Secondary electron (SE) emission is an important physical process in spacecraft charging. Current spacecraft charging models do not consider the SE energy or angular distributions and their implications for estimating the return of SE to the spacecraft. Comprehensive work on the application of SE energy and angular distributions to spacecraft charging has been published [Nickles et al., 1999] and part of that work is summarized here. The application of SE energy distributions to the case of positive charging in geosynchronous orbit is discussed and shown …

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Spectroscopy Of Brown Dwarf Candidates In The Ρ Ophiuchi Molecular Core, Bruce Wilking, Thomas Greene, Michael Meyer Jan 1999

Spectroscopy Of Brown Dwarf Candidates In The Ρ Ophiuchi Molecular Core, Bruce Wilking, Thomas Greene, Michael Meyer

Physics Faculty Works

We present an analysis of low-resolution infrared spectra for 20 brown dwarf candidates in the core of the ρ Ophiuchi molecular cloud. Fifteen of the sources display absorption-line spectra characteristic of late-type stars. By comparing the depths of water vapor absorption bands in our candidate objects with a grid of M dwarf standards, we derive spectral types that are independent of reddening. Optical spectroscopy of one brown dwarf candidate confirms the spectral type derived from the water bands. Combining their spectral types with published near-infrared photometry, effective temperatures and bolometric stellar luminosities are derived, enabling us to place our sample …

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Evolution Of The Electronic Structure And Properties Of Neutral And Charged Aluminum Clusters: A Comprehensive Analysis, B. K. Rao, P. Jena Jan 1999

Evolution Of The Electronic Structure And Properties Of Neutral And Charged Aluminum Clusters: A Comprehensive Analysis, B. K. Rao, P. Jena

Physics Publications

Density-functional theory with generalized gradient approximation for the exchange-correlation potential has been used to calculate the global equilibrium geometries and electronic structure of neutral, cationic, and anionic aluminumclusters containing up to 15 atoms. The total energies of these clusters are then used to study the evolution of their binding energy, relative stability, fragmentation channels, ionization potential, and vertical and adiabatic electron affinities as a function of size. The geometries are found to undergo a structural change from two dimensional to three dimensional when the cluster contains 6 atoms. An interior atom emerges only when clusters contain 11 or more atoms. …

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Search For “Quadrupole-Bound” Anions. I, Gennady L. Gutsev, Puru Jena, Rodney J. Bartlett Jan 1999

Search For “Quadrupole-Bound” Anions. I, Gennady L. Gutsev, Puru Jena, Rodney J. Bartlett

Physics Publications

In a classical model, some anions exist due to the attraction between an electron and a molecule’s dipole moment. When the dipole moment is sufficiently large (μcrit>2.5 D), an electron can be trapped. Can a sufficiently large quadrupole moment produce the same effect? To help answer this question, we can search for molecules with a large quadrupole moment and use predictive, ab initio, correlated quantum chemistry methods to assess whether an anion forms and, if it does, to discover its nature. For this purpose, coupled-cluster calculations are reported for the structure and properties of KnClm and KnCl−m (n,m=0–2). …

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Reality And Theory In A Collision, Norman Derby, Robert Fuller Jan 1999

Reality And Theory In A Collision, Norman Derby, Robert Fuller

Robert G. Fuller Publications

Billiard-ball collisions are frequently cited in introductory physics textbooks, usually as examples of elastic collisions. Many articles describing such collisions have appeared in this journal and elsewhere, but comparisons between theoretical results and actual collisions are rare, and most of the theoretical analyses have simply assumed friction to be negligible during the collision time. Students trying to simulate billiard-ball collisions on a popular simulation program such as Interactive Physics® may encounter interesting collisions, such as ones in which the cue ball is thrown up into the air when the frictional coefficients are set at even modest levels. We decided …

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Magnetic And Crystallographic Properties Of Lani₅₋ₓfex, C. Tan, Oran Allan Pringle, Mingxing Chen, William B. Yelon, J. Gebhardt, Naushad Ali, C. Y. Tai, G. K. Marasinghe, George Daniel Waddill, William Joseph James Jan 1999

Magnetic And Crystallographic Properties Of Lani₅₋ₓfex, C. Tan, Oran Allan Pringle, Mingxing Chen, William B. Yelon, J. Gebhardt, Naushad Ali, C. Y. Tai, G. K. Marasinghe, George Daniel Waddill, William Joseph James

Physics Faculty Research & Creative Works

No abstract provided.

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Spin Dependent Tunneling, Peter M. Levy, Shufeng Zhang Jan 1999

Spin Dependent Tunneling, Peter M. Levy, Shufeng Zhang

Physics Faculty Research & Creative Works

During the past four years, researchers made significant advances in fabricating magnetic tunnel junctions with reproducible magnetic and magneto transport properties. Important developments include optimization of oxidation processes, discovery of new class of magnetic tunnel junctions, combination of spin dependent tunneling with the Coulomb blockade effect, and a better theoretical understanding of the I-V characteristics of magnetic tunnel junctions. These developments make them promising candidates for magnetic random-access memories. © Elsevier Science Ltd.

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History Of The Forum On Physics And Society, David W. Hafemeister Jan 1999

History Of The Forum On Physics And Society, David W. Hafemeister

Physics

Physics is a major component of many of society's difficult issues: nuclear arms and their proliferation, energy shortages and energy impacts, climate change and technical innovation. Because physics principles underlie so many of these societal issues and because physics offers a way to quantify some aspects of them, members of the American Physical Society (APS) should be encouraged to understand, analyze and debate them. That's precisely why APS members formed the Forum on Physics and Society (FPS). To those of us who have been long involved in FPS affairs, it seems but yesterday that we attended the organizing meeting at …

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Q-Calculus And Irreversible Dynamics On A Hierarchical Lattice, Ayşe Erzan, Ayşe Gorbon Jan 1999

Q-Calculus And Irreversible Dynamics On A Hierarchical Lattice, Ayşe Erzan, Ayşe Gorbon

Turkish Journal of Physics

On a lattice with equal spacing on the logarithmic scale, a momentum operator that respects the asymmetry of this nonlinear lattice yields a kinetics that can be understood in terms of diffusion on an underlying ultrametric space, if one also identifies the cannonical commutator with the time dilation operator. The motion to which this non-conventional kinetics corresponds is irreversible, with an explicit violation of time reversal symmetry resulting from the spreading with time of a probability distribution over a larger and larger volume of the phase space.

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A New Formalism For Nonextensive Physical Systems: Tsallis Thermostatistics, Uğur Tirnakli, Fevzi Büyükkiliç, Doğan Demi̇rhan Jan 1999

A New Formalism For Nonextensive Physical Systems: Tsallis Thermostatistics, Uğur Tirnakli, Fevzi Büyükkiliç, Doğan Demi̇rhan

Turkish Journal of Physics

Although Boltzmann-Gibbs (BG) statistics provides a suitable tool which enables us to handle a large number of physical systems satisfactorily, it has some basic restrictions : (i) the range of the microscopic interactions must be small compared to the linear size of the macroscopic systems (short-range interactions) , (ii) the time range of the microscopic memory must be small compared to the observation time (Marcovian processes) and (iii) the system must evolve in an Euclidean-like space-time. In the case of a breakdown in one and/or the others of these restrictions, BG statistics fails. More precisely, the situation could be classified …

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Neural Classifiers For Learning Higher-Order Correlations, Marifi Güler Jan 1999

Neural Classifiers For Learning Higher-Order Correlations, Marifi Güler

Turkish Journal of Physics

Studies by various authors suggest that higher-order networks can be more powerful and are biologically more plausible with respect to the more traditional multilayer networks. These architectures make explicit use of nonlinear interactions between input variables in the form of higher-order units or product units. If it is known a priori that the problem to be implemented possesses a given set of invariances like in the translation, rotation, and scale invariant pattern recognition problems, those invariances can be encoded, thus eliminating all higher-order terms which are incompatible with the invariances. In general, however, it is a serious set-back that the …

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The Renormalization-Group Microscope: The Local Statistical Mechanics Of Heterogeneous Systems, Dicle Yeşi̇lleten, A. Nihat Berker Jan 1999

The Renormalization-Group Microscope: The Local Statistical Mechanics Of Heterogeneous Systems, Dicle Yeşi̇lleten, A. Nihat Berker

Turkish Journal of Physics

Renormalization-group theory is developed to yield all local microscopic thermodynamic densities in heterogeneous systems. Local energy densities and local magnetizations are thus obtained for random-bond systems, random-field systems, and spin-glasses, in two and three dimensions. Different order-disorder mechanisms in these diverse systems, such as chaotic ordering and domain-wall melting, become quantitatively evident.

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Assessment Of The Degree Of Mix Of Powder Mixtures, Orhan Vai̇zoğlu Jan 1999

Assessment Of The Degree Of Mix Of Powder Mixtures, Orhan Vai̇zoğlu

Turkish Journal of Physics

The older methods for the determination of the degree of a random mix was solely based on the calculation of a simple standard deviation. Physical characteristics like, density, average particle weight and size of each component was not considered. Therefore, the evaluation, when the state of a random mix is reached, was somewhat superficial. Poole, Taylor, and Wall [1] have derived equations, where the physical characteristics of the components were taken into consideration. By this means, it is possible to calculate, in advance, the theoretical coefficient of variation and the corresponding confidence limits. One can then experimentally follow under which …

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Studies On Extremal Segments In Random Sequences, Deniz Ertaş, Yacov Kantor Jan 1999

Studies On Extremal Segments In Random Sequences, Deniz Ertaş, Yacov Kantor

Turkish Journal of Physics

We review our main findings on the size distribution of the largest neutral segments in a sequence of N randomly charged monomers. Upon mapping to one--dimensional random walks (RWs), this corresponds to finding the probability distribution for the size L of the largest segment that returns to its starting position in an N--step RW. Using analytical, exact enumeration, and Monte Carlo methods, we reveal the complex structure of the probability distribution in the large N limit. In particular, the size of the longest neutral segment has a distribution with a square-root singularity at \ell\equiv L/N = 1, an essential singularity …

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Diverging Strains Near Threshold: Breakdown Of The Elastic Description Of A Charge Density Wave Model, Muhittin Mungan, Susan N. Coppersmith, Valeri M. Vinokur Jan 1999

Diverging Strains Near Threshold: Breakdown Of The Elastic Description Of A Charge Density Wave Model, Muhittin Mungan, Susan N. Coppersmith, Valeri M. Vinokur

Turkish Journal of Physics

We analyze the strains near threshold in 1-d charge density wave models at zero temperature and strong pinning. We show that in these models local strains diverge near the depinning threshold and characterize the scaling behavior of the phenomenon. This helps quantify when the underlying elastic description breaks down and plastic effects have to be included.

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Angular Momentum Evolution In Close Late-Type Binaries, Carla Maceroni Jan 1999

Angular Momentum Evolution In Close Late-Type Binaries, Carla Maceroni

Turkish Journal of Physics

Accurate monitoring of the orbital period of late-type close binaries can provide important information on their secular evolution and on the structural changes of their components on very short timescales (i.e. decades). The orbital period turns out to be a powerful tool of investigation: it can be measured to great accuracy even with simple equipments and its change produces an easily detectable cumulative effect over time. This paper focuses on two physical mechanisms that cause orbital periods changes in late type binaries with negligible mass loss from the system: the dynamical evolution due to magnetic braking in tidally locked systems, …

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Short-Period Rs Cvn And W Uma Binaries: How Are They Related?, Frans Van't Veer Jan 1999

Short-Period Rs Cvn And W Uma Binaries: How Are They Related?, Frans Van't Veer

Turkish Journal of Physics

Two groups of late-type very close main sequence binaries, the so-called short-period RS Canum Venaticorum (RS CVn) and W Ursae Majoris (W UMa) systems, are important for the understanding of the structure and evolutionary consequencies of angular momentum loss (AML) resulting from magnetic stellar winds of fast rotating solar type stars. The components of RS CVn stars are close to their Roche lobes, but still detached, whereas W UMa binaries are contact systems; both are rotating fast. One of the crucial problems is trying to answer the question if there is a dynamical relation between the two types of binaries …

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Three-Channel General-Purpose Astrophotometer, Romualdas Kalytis Jan 1999

Three-Channel General-Purpose Astrophotometer, Romualdas Kalytis

Turkish Journal of Physics

The high-speed three-channel photometer suitable for the time-series observations of the "Whole Earth Telescope" (WET) project as well as for multicolor photometry is described. The photometer, designed for the Capodimonte Astronomical Observatory, Italy, is the latest version of the WET photometers developed by the WET group of Vilnius University since 1992. The main optical and mechanical layouts of the optico-mechanical module as well as the block-diagram of electronics are presented. The main specifications of the photometer expected after its completion are given.

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The Light And Colour Variations Of Mm Her, Günay Taş Jan 1999

The Light And Colour Variations Of Mm Her, Günay Taş

Turkish Journal of Physics

RS CVn type eclipsing binary MM Herculis have been observed using B, V, R filters in 1997 and obtained light and colour curves were examined together with the earlier 12 light and colour curves obtained between 1976 and 1997. It was seen that the system is bluer, while it is fainter. Such finding probably reveals the evolutionary effect of the photospheric bright facular structures surrounding starspots.

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