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1996

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Articles 1 - 30 of 327

Full-Text Articles in Physics

Temporal Characterization Of A Self-Modulated Laser Wakefield, S.P. Le Blanc, M.C. Downer, R. Wagner, Shouyuan Chen, Anatoly Maksimchuk, G. Mourou, Donald P. Umstadter Dec 1996

Temporal Characterization Of A Self-Modulated Laser Wakefield, S.P. Le Blanc, M.C. Downer, R. Wagner, Shouyuan Chen, Anatoly Maksimchuk, G. Mourou, Donald P. Umstadter

Donald Umstadter Publications

The temporal envelope of plasma density oscillations in the wake of an intense (l∼4×1018 W/cm2, λ = 1 μm) laser pulse (400 fs) is measured using forward Thomson scattering from a copropagating, frequency-doubled probe pulse. The wakefield oscillations in a fully ionized helium plasma (ne = 3×1019 cm-3) are observed to reach maximum amplitude ( δne/ne∼0.1) 300 fs after the pump pulse. The wakefield growth ( 3.5 ps-1) and decay ( 1.9 ps-1) rates are consistent with the forward Raman scattering instability and Landau damping, respectively.


Comparison Of The Temperature-Dependent Electronic Structure Of The Perovskites La0.65Ca0.35Mno3 (A=Ca, Ba), D.N. Mcilroy, C. Waldfried, Jiandi Zhang, Jaewu Choi, F. Foong, Sy_Hwang Liou, Peter A. Dowben Dec 1996

Comparison Of The Temperature-Dependent Electronic Structure Of The Perovskites La0.65Ca0.35Mno3 (A=Ca, Ba), D.N. Mcilroy, C. Waldfried, Jiandi Zhang, Jaewu Choi, F. Foong, Sy_Hwang Liou, Peter A. Dowben

Peter Dowben Publications

The electronic band structure and the local screening effects of the transition-metal perovskites La0.65A0.35MnO3 (A=Ba and Ca ) have been examined across the coupled magnetic-metallic phase transition using the techniques of angle-resolved photoemission, resonance photoemission, and inverse photoemission. Temperature-dependent band shifts of the eg and t2g bands of La0.65Ca0.35MnO3 have been observed. These changes in the observed electronic structure correlate with the phase transition, and are in qualitatively agreement with the predicted behavior associated with double exchange coupled with a dynamic Jahn-Teller distortion. Similar ...


Strain-Induced Distortion Of The Bulk Bands Of Gadolinium, Carlo Waldfried, D.N. Mcilroy, C.W. Hutchings, Peter A. Dowben Dec 1996

Strain-Induced Distortion Of The Bulk Bands Of Gadolinium, Carlo Waldfried, D.N. Mcilroy, C.W. Hutchings, Peter A. Dowben

Peter Dowben Publications

Thin films of gadolinium, approximately 8 ML thick, have been grown on the corrugated (112) surface of molybdenum and the electronic structure has been investigated with angle-resolved photoelectron spectroscopy. The unfavorable lattice match between the ‘‘steplike’’ Mo(112) substrate and the preferred hexagonally ordered Gd film results in an incommensurate Gd structure that appears to be ordered, but strained along the direction of the corrugations. The hexagonal Gd lattice is expanded by more than 20% along the ‘‘step’’ lines of the substrate, as determined from the reduced Brillouin-zone size along the ΓΣM high-symmetry line and low-energy electron diffraction. The induced ...


Thermally Induced Core-Electron Binding-Energy Shifts In Transition Metals: An Experimentalinvestigation Of Ta(100), D. Mark Riffe, B. Kim, W. Hale, J. L. Erskine Dec 1996

Thermally Induced Core-Electron Binding-Energy Shifts In Transition Metals: An Experimentalinvestigation Of Ta(100), D. Mark Riffe, B. Kim, W. Hale, J. L. Erskine

All Physics Faculty Publications

High-resolution photoemission spectra from the 4f7/2 levels of Ta(100) have been obtained between 77 K and room temperature. The data show an increase in both the surface and bulk core-level binding energies (BE’s) as the temperature is raised: between 77 and 293 K the bulk and surface BE’s increase by 31±3 and 13±2 meV, respectively. A model calculation of the bulk binding-energy increase, which is based upon the lattice expansion of the solid, is in good agreement with the experimental results and indicates that the shifts arise from both initial- and final-state effects ...


Lower And Upper Bounds On Internal-Wave Frequencies In Stratified Rotating Fluids, Benoit Cushman-Roisin Dec 1996

Lower And Upper Bounds On Internal-Wave Frequencies In Stratified Rotating Fluids, Benoit Cushman-Roisin

Open Dartmouth: Faculty Open Access Scholarship

According to classical theories, the frequencies of internal-gravity waves in stratified rotating fluids must lie between the Brunt-Väisälä frequency (a measure of the vertical density stratification) and the Coriolis frequency (equal to twice the rotation rate about the vertical axis). It is shown here that, in the case of the Earth's rotation where the pole-to-pole axis of rotation is almost everywhere not parallel to the local vertical, the range of realizable frequencies is broader. New formulas are derived for the lower and upper bounds of the frequencies.


Neutron Scattering Study Of Transverse Magnetism, Christian Binek, T. Kato, Wolfgang Kleemann, O. Petracic, D. Bertrand, F. Bourdarot, P. Burlet, H. Aruga Katori, K. Katsumata, K. Prokes, S. Welzel Dec 1996

Neutron Scattering Study Of Transverse Magnetism, Christian Binek, T. Kato, Wolfgang Kleemann, O. Petracic, D. Bertrand, F. Bourdarot, P. Burlet, H. Aruga Katori, K. Katsumata, K. Prokes, S. Welzel

Christian Binek Publications

In order to clarify the nature of the additional phase transition at H1 (T) < Hc (T) of the layered antiferromagnetic (AF) insulator FeBr2 as found by Aruga Katori et al. (1996) we measured the intensity of different Bragg-peaks in different scattering geometries. Transverse AF ordering is observed in both AF phases, AFI and AFII. Its order parameter exhibits a peak at T1 = T (H1) in temperature scans and does not vanish in zero field. Possible origins of the step-like increase of the transverse ferromagnetic ordering induced by a weak in-plane field component when entering AFI ...


Magnetic Levitation And Noncoalescence Of Liquid Helium, M. A. Weilert, Dwight L. Whitaker, H. J. Maris, G. M. Seidel Dec 1996

Magnetic Levitation And Noncoalescence Of Liquid Helium, M. A. Weilert, Dwight L. Whitaker, H. J. Maris, G. M. Seidel

Pomona Faculty Publications and Research

We describe experiments in which drops of liquid helium-4, as large as 2 cm in diameter, are magnetically levitated. We have found that, when two or more drops are levitated in the same magnetic trap, the drops often remain in a state of apparent contact without coalescing. It appears that this effect is caused by the slow evaporation of liquid from the drops.


Coarsening And Persistence In The Voter Model, E. Ben-Naim, L. Frachebourg, P.L. Krapivsky Dec 1996

Coarsening And Persistence In The Voter Model, E. Ben-Naim, L. Frachebourg, P.L. Krapivsky

Eli Ben-Naim

No abstract provided.


Constraints On The Minimal Supergravity Model From Non-Standard Vacua, Howard Baer, Michal Brhlik, Diego Castano Dec 1996

Constraints On The Minimal Supergravity Model From Non-Standard Vacua, Howard Baer, Michal Brhlik, Diego Castano

Chemistry and Physics Faculty Articles

We evaluate regions of parameter space in the minimal supergravity model where "unbounded from below" (UFB) or charge or color-breaking minima (CCB) occur. Our analysis includes the most important terms from the one-loop effective potential. We note a peculiar discontinuity of results depending on how renormalization group improvement is performed: One case leads to a UFB potential throughout the model parameter space, while the other typically agrees quite well with similar calculations performed using only the tree level potential. We compare our results with constraints from cosmology and naturalness and find a preferred region of parameter space which implies m ...


The Behlen Observatory Variable Star Survey, Iv, Edward G. Schmidt, Anil Seth Dec 1996

The Behlen Observatory Variable Star Survey, Iv, Edward G. Schmidt, Anil Seth

Edward Schmidt Publications

Finding charts, accurate coordinates, and light curves are presented for 106 variable stars including 6 which are newly discovered. Parameters descriptive of the light curves are tabulated including periods for 16 stars which lacked them. The periods from the General Catalog of Variable Stars for ten stars were found to be seriously in error. The classification of the stars is discussed. Revisions or refinements of the classifications from the General Catalogue of Variable Stars are suggested for 20 stars and classifications are given for 7 which were previously unclassified. 15%-23% of the Bailey type ab RR Lyrae stars show ...


Stars Classified As Constant In The General Catalogue Of Variable Stars, Ii, Edward G. Schmidt Dec 1996

Stars Classified As Constant In The General Catalogue Of Variable Stars, Ii, Edward G. Schmidt

Edward Schmidt Publications

Photometric observations have been made of 36 stars classified as CST or CST: in the General Catalogue of Variable Stars. Six of these stars are found to vary and three others were deemed possible variables. Of the certain variables, two (MR Her and TY Sge) are red pulsators, one (V1585 Cyg) is a rapid irregular variable, one is apparently a short period Cepheid-strip star (V432 Oph), one is an eclipsing binary (AQ Boo), and one is of uncertain type (V351 Cyg). The stars we have observed as MR Her and TY Sge are the same ones which were identified on ...


An Inline Optical Electron Polarimeter, K. W. Trantham, Timothy J. Gay, R. J. Vandiver Dec 1996

An Inline Optical Electron Polarimeter, K. W. Trantham, Timothy J. Gay, R. J. Vandiver

Timothy J. Gay Publications

The design and operation of a simple inline optical electron polarimeter is presented. It is based on exchange excitation of ground state neon atoms. The electron polarization is determined from the degree of circular polarization of the subsequent 2p53p 3D3→2p53s 3P2 (6402 Å) fluorescence. This device can characterize both longitudinally and transversely polarized electron beams in a nondestructive fashion, and is inexpensive and easily constructed.


Optical Properties And Electronic Structures Of Equiatomic Xti (X=Fe,Co,Andni) Alloys, Joo Yull Rhee, Bruce N. Harmon, David W. Lynch Dec 1996

Optical Properties And Electronic Structures Of Equiatomic Xti (X=Fe,Co,Andni) Alloys, Joo Yull Rhee, Bruce N. Harmon, David W. Lynch

Physics and Astronomy Publications

The dielectric functions of equiatomic XTi (X=Fe,Co,andNi) alloys in the B2 phase (cubic CsCl structure) were measured by spectroscopic ellipsometry in the energy range of 1.5-5.4 eV. The optical conductivity spectra show close resemblance to each other with a peak at 1.9-2.3 eV and another at 3.1-3.4 eV. Fine structures observed in previously reported measurements were not seen. The band structures and the optical conductivity spectra were calculated in the B2 phase using the linearized-augmented-plane-wave method with the local density approximation. The agreement between the measured and calculated spectra is markedly ...


Auroral Electrojet Irregularity Theory And Experiment: A Criticalreview Of Present Understanding And Future Directions, J. D. Sahr, Bela G. Fejer Dec 1996

Auroral Electrojet Irregularity Theory And Experiment: A Criticalreview Of Present Understanding And Future Directions, J. D. Sahr, Bela G. Fejer

Bela G. Fejer

We review the experimental observations of meter scale plasma irregularities in the auroral E region and the status of their theoretical understanding. Most of the experimental data is derived from VHF radar scatter experiments, but sounding rockets also provide crucial information not obtainable from radars. Linear theories correctly predict the altitude of occurence, strong magnetic aspect sensitivity, marginal instability, and typical phase velocities. Subsequent nonlinear theories have been developed to account for other observed features but with less satisfying application. Further understanding of auroral electrojet irregularities is impeded by precision limitations of existing instruments, by radar data which may seem ...


Straightening Of Thermal Fluctuations In Semi-Flexible Polymers By Applied Tension, Udo Seifert, Wolfgang Wintz, Philip C. Nelson Dec 1996

Straightening Of Thermal Fluctuations In Semi-Flexible Polymers By Applied Tension, Udo Seifert, Wolfgang Wintz, Philip C. Nelson

Department of Physics Papers

We investigate the propagation of a suddenly applied tension along a thermally excited semiflexible polymer using analytical approximations, scaling arguments, and numerical simulation. This problem is inherently nonlinear. We find subdiffusive propagation with a dynamical exponent of 1/4. By generalizing the internal elasticity, we show that tense strings exhibit qualitatively different tension profiles and propagation with an exponent of 1/2.


Role Of Bilayer Tilt Difference In Equlibrium Membrane Shapes, Udo Seifert, Julian Shillcock, Philip C. Nelson Dec 1996

Role Of Bilayer Tilt Difference In Equlibrium Membrane Shapes, Udo Seifert, Julian Shillcock, Philip C. Nelson

Department of Physics Papers

Lipid bilayer membranes below their main transition have two tilt order parameters, corresponding to the two monolayers. These two tilts may be strongly coupled to membrane shape but only weakly coupled to each other.We discuss some implications of this observation for rippled and saddle phases, spontaneous breaking bilayer tubules, and bicontinuous phases. Tilt difference introduces a lengthscale into the elastic theory of tilted fluid membranes. It can drive an instability of the flat phase; it also provides a simple spontaneous breaking of inversion symmetry seen in some recent experiments.


L-Shell Resonant Electron Transfer And Excitation In Cu18-19+ + H2 Collisions, Jeffrey A. Wroblewski Dec 1996

L-Shell Resonant Electron Transfer And Excitation In Cu18-19+ + H2 Collisions, Jeffrey A. Wroblewski

Master's Theses

Measurements of L-shell resonant electron transfer and excitation (RTE) were conducted for 45-72 MeV Cu18-19+ (Na-like and Ne-like, respectively) ions colliding with H2 using the Western Michigan University EN tandem Van de Graaff accelerator. The RTE process involves an electron-electron interaction between the projectile and the target where electron capture occurs simultaneously with projectile excitation. In previous measurements involving RTE with subsequent L x-ray relaxation for Nbq+ (q=28-31) ions, the magnitudes of the RTE cross sections were found to be nearly a factor of two smaller than theory. In the analogous electron-ion process of dielectronic recombination ...


Exact Island-Size Distributions For Submonolayer Deposition: Influence Of Correlations Between Island Size And Separation, M. C. Bartelt, James W. Evans Dec 1996

Exact Island-Size Distributions For Submonolayer Deposition: Influence Of Correlations Between Island Size And Separation, M. C. Bartelt, James W. Evans

Mathematics Publications

We determine the exact scaling form of the size distribution of islands created via homogeneous nucleation and growth during submonolayer deposition. This scaling form is shown to be controlled by the dependence on size of the propensity for islands to capture diffusing adatoms. This size dependence is determined directly from simulations. It is distinct from mean-field predictions, reflecting strong correlations between island size and separation.


Spheromak Experiment Using Separate Guns For Formation And Sustainment, Michael R. Brown, A. Martin Dec 1996

Spheromak Experiment Using Separate Guns For Formation And Sustainment, Michael R. Brown, A. Martin

Physics & Astronomy Faculty Works

An experiment is described that incorporates the use of separate magnetized plasma guns for formation and sustainment of a spheromak. It is shown that energy coupling efficiency approaches unity if the gun and spheromak are of comparable size. A large gun should be able to operate at lower current and therefore lower voltage. In addition, it is expected that a gun matched to the size of the spheromak will cause less perturbation to the equilibrium. It is proposed to use a smaller gun for spheromak formation and a large, efficient gun for sustainment. The theoretical basis for the experiment is ...


Building A Competition Buzzer Set, Jr Dennison, Richard Davis Dec 1996

Building A Competition Buzzer Set, Jr Dennison, Richard Davis

All Physics Faculty Publications

No abstract provided.


Angle-Resolved X-Ray Circular And Magnetic Circular Dichroisms: Definitions And Applications, Sy Y. Tong, X. Q. Guo, James G. Tobin, George Daniel Waddill Dec 1996

Angle-Resolved X-Ray Circular And Magnetic Circular Dichroisms: Definitions And Applications, Sy Y. Tong, X. Q. Guo, James G. Tobin, George Daniel Waddill

Physics Faculty Research & Creative Works

We introduce definitions of angle-resolved x-ray circular dichroism (ARXCD) and magnetic x-ray circular dichroism (ARMXCD). As defined, the much larger effect of circular dichroism (ARXCD) is separated from the smaller magnetic (ARMXCD) effect. In all materials, ARXCD is zero along mirror planes while nonzero elsewhere. ARMXCD is nonzero only in magnetic materials. The measurement and analysis of ARMXCD allow element specific surface magnetism and surface structure as well as their inter-relationship to be studied as functions of the outgoing electron's direction.


Selective Excitation And Structure In The Continuum, C. E. Carroll, Foek T. Hioe Dec 1996

Selective Excitation And Structure In The Continuum, C. E. Carroll, Foek T. Hioe

Physics Faculty/Staff Publications

We show that efficient transfer of molecules or atoms from one bound state to another is possible via the continuum in some cases, using two overlapping laser pulses. The structure of the continuum determines the pulse delay, laser frequencies, and laser intensities that should be used. We present simple formulas that relate the optimum values of pulse intensities, pulse separation, and detuning; these estimates can be used as a guide. More detailed calculations give quantitative results for two specific potentials.


Fabrication Of N-Type Nickel Doped B5 C1+Δ Homojunction And Heterojunction Diodes, Seong-Don Hwang, Ken Yang, Peter A. Dowben, Ahmad A. Ahmad, N.J. Ianno, J.Z. Li, J.Y. Lin, H.X. Jiang, D.N. Mcilroy Nov 1996

Fabrication Of N-Type Nickel Doped B5 C1+Δ Homojunction And Heterojunction Diodes, Seong-Don Hwang, Ken Yang, Peter A. Dowben, Ahmad A. Ahmad, N.J. Ianno, J.Z. Li, J.Y. Lin, H.X. Jiang, D.N. Mcilroy

Peter Dowben Publications

We have successfully nickel doped a boron carbide (B5C) alloy film. The nickel doped boron-carbide (Ni-B5C1+δ) thin films were fabricated from a single source carborane cage molecule and nickelocene [Ni(C5H5)2] using plasma enhanced chemical vapor deposition. Nickel doping transforms the highly resistive undoped film from a p-type material to an n-type material. This has been verified from the characteristics of diodes constructed of Ni-B5C1+δ on both n-type silicon and p-type B5C. The homojunction diodes exhibit excellent rectifying properties over a wide range of temperatures. ©1997 ...


Far-Field Scattering Of A Non-Gaussian Off-Axis Axisymmetric Laser Beam By A Spherical Particle, James A. Lock, Joseph T. Hodges Nov 1996

Far-Field Scattering Of A Non-Gaussian Off-Axis Axisymmetric Laser Beam By A Spherical Particle, James A. Lock, Joseph T. Hodges

Physics Faculty Publications

Experimental laser beam profiles often deviate somewhat from the ideal Gaussian shape of the axisymmetric TEM(00) laser mode. To take these deviations into account when calculating light scattering of an off-axis beam by a spherical particle, we use our phase-modeling method to approximate the beam-shape coefficients in the partial wave expansion of an experimental laser beam. We then use these beam-shape coefficients to compute the near-forward direction scattering of the off-axis beam by the particle. Our results are compared with laboratory data, and we give a physical interpretation of the various features observed in the angular scattering patterns. (C ...


Nanoscale Investigation Of Fatigue Effects In Pb(Zr,Ti)O3 Films, Alexei Gruverman, O. Auciello, H. Tokumoto Nov 1996

Nanoscale Investigation Of Fatigue Effects In Pb(Zr,Ti)O3 Films, Alexei Gruverman, O. Auciello, H. Tokumoto

Alexei Gruverman Publications

Scanning force microscopy has been used to perform a comparative nanoscale study of domain structures and switching behavior of Pb(ZrxTi1-x)O3 (PZT) thin films integrated into heterostructures with different electrodes. The study revealed a significant difference between polarization state of as-deposited PZT films on RuO2 and Pt electrodes. The PZT/RuO2 films exhibit polydomain crystallites and show almost symmetric switching behavior, while the PZT/Pt films are mainly in a single polarity state and exhibit highly asymmetric piezoelectric hysteresis loops. Formation of unswitchable polarization within the grains of submicron size as a result ...


Photoionization Of Atoms, Anthony F. Starace Nov 1996

Photoionization Of Atoms, Anthony F. Starace

Anthony F. Starace Publications

This chapter outlines the theory of atomic photoionization, and the dynamics of the photon-atom collision process. Those kinds of electron correlation that are most important in photoionization are emphasized, although many qualitative features can be understood within a central field model. The particle-hole type of electron correlations are discussed, as they are by far the most important for describing the single photoionization of atoms near ionization thresholds. Detailed reviews of atomic photoionization are presented in Refs. [I] and 121. Current activities and interests are well-described in two recent books [3,4]


The Electromagnetic Mass Differences Of Pions And Kaons, John F. Donoghue, Antonio F. Pérez Nov 1996

The Electromagnetic Mass Differences Of Pions And Kaons, John F. Donoghue, Antonio F. Pérez

Physics Department Faculty Publication Series

We use the Cottingham method to calculate the pion and kaon electromagnetic mass differences with as few model dependent inputs as possible. The constraints of chiral symmetry at low energy, QCD at high energy and experimental data in between are used in the dispersion relation. We find excellent agreement with experiment for the pion mass difference. The kaon mass difference exhibits a strong violation of the lowest order prediction of Dashen’s theorem, in qualitative agreement with several other recent calculations.


Geometrical Folding Transitions Of The Triangular Lattice In The Face-Centred Cubic Lattice, Mark Bowick, Oliver Golinelli, Emmanuel Guitter, S. Mori Nov 1996

Geometrical Folding Transitions Of The Triangular Lattice In The Face-Centred Cubic Lattice, Mark Bowick, Oliver Golinelli, Emmanuel Guitter, S. Mori

Physics

We study the folding of the regular two-dimensional triangular lattice embedded in the regular three-dimensional Face-Centred Cubic lattice, a discrete model for the crumpling of membranes. Possible folds are complete planar folds, folds with the angle of a regular tetrahedron (71 degrees) or with that of a regular octahedron (109 degrees). We study this model in the presence of a negative bending rigidity K, which favours the folding process. We use both a cluster variation method (CVM) approximation and a transfer matrix approach. The system is shown to undergo two separate geometrical transitions with increasing |K|: a first discontinuous transition ...


Potentials With Two Shifted Sets Of Equally Spaced Eigenvalues And Their Calogero Spectrum, Asim Gangopadhyaya, Uday P. Sukhatme Nov 1996

Potentials With Two Shifted Sets Of Equally Spaced Eigenvalues And Their Calogero Spectrum, Asim Gangopadhyaya, Uday P. Sukhatme

Physics: Faculty Publications and Other Works

Motivated by the concept of shape invariance in supersymmetric quantum mechanics, we obtain potentials whose spectrum consists of two shifted sets of equally spaced energy levels. These potentials are similar to the Calogero-Sutherland model except the singular term αx−2 always falls in the transition region -1/4 < a < 3/4 and there is a δ-function singularity at x=0.


Asymptotic Conservation Laws In Field Theory, Ian M. Anderson, Charles G. Torre Nov 1996

Asymptotic Conservation Laws In Field Theory, Ian M. Anderson, Charles G. Torre

All Physics Faculty Publications

A new, general, field theoretic approach to the derivation of asymptotic conservation laws is presented. In this approach asymptotic conservation laws are constructed directly from the field equations according to a universal prescription which does not rely upon the existence of Noether identities or any Lagrangian or Hamiltonian formalisms. The resulting general expressions of the conservation laws enjoy important invariance properties and synthesize all known asymptotic conservation laws, such as the Arnowitt-Deser-Misner energy in general relativity.