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Full-Text Articles in Physics

The Paramagnetic Correlation Length Of Mn Thin Films, D.N. Mcilroy, Peter A. Dowben May 1995

The Paramagnetic Correlation Length Of Mn Thin Films, D.N. Mcilroy, Peter A. Dowben

Peter Dowben Publications

We have utilized Ginzberg-Landau mean field theory to analyze the branching ratio and soft X-ray magnetic circular dichroism measurements of the L3 and L2 adsorption edges of thin Mn overlayers on ferromagnetic fcc Co(001) at room temperature. We have determined a short range magnetic correlation of 4.25Å and that the long range magnetic order has a decay length of 1.29Å. These results indicate that the first monolayer of Mn ferromagnetically couple with the Co substrate. The paramagnetic Mn layer exhibits short range magnetic order, but lacks long range magnetic order. Due to the exponential decrease ...


Evidence Of Dilution-Induced Griffiths Instabilities In K2Cu1-XZnXF4 And Fe1-XZnXF2, Christian Binek, Wolfgang Kleeman May 1995

Evidence Of Dilution-Induced Griffiths Instabilities In K2Cu1-XZnXF4 And Fe1-XZnXF2, Christian Binek, Wolfgang Kleeman

Christian Binek Publications

Weak quasistatic singularities are evidenced within the Griffiths regime of temperatures Tc(x)≤T≤Tc(x=0) in the diluted antiferromagnet Fe1-xZnxF2, x=0.53, and in the diluted ferromagnet K2Cu1-xZnxF4, x=0.2, by measurements of the magnetic susceptibility, χ’ vs T, in zero external field. Significant deviations from classical (γ=1, for Fe0.47Zn0.53F2) and nonclassical (γ≳1, for K2Cu0.8Zn0.2F4) Curie-Weiss-type behavior, χ’∝[T-Tc(x)] -Y, are found ...


Microstructure Of The Cr Underlayer And Its Effect On Sm-Co//Cr Thin Films, Yi Liu, B. W. Robertson, Z.S. Shan, Sy_Hwang Liou, David J. Sellmyer Apr 1995

Microstructure Of The Cr Underlayer And Its Effect On Sm-Co//Cr Thin Films, Yi Liu, B. W. Robertson, Z.S. Shan, Sy_Hwang Liou, David J. Sellmyer

Si-Hwang Liou Publications

Sm-Co film is a potential candidate for the future high density recording media of 10 Gb/in2 which requires bit sizes of the order of 300 nm and grain sizes of about 10 nm. This article investigates the microstructure of the Cr underlayer in Sm-Co thin films and its effect on Sm-Co thin films prepared by the dc magnetron sputtering technique. The grain size of the Cr underlayer is found to be about 25 nm. Grains with small angle misorientation usually form local agglomerates. Studies by transmission electron microscopy (TEM) bright field images at different defocus settings and by ...


Resonantly Laser-Driven Plasma Waves For Electron Acceleration, Donald P. Umstadter, J. Kim, Eric Esarey, E. Dodd, T. Neubert Apr 1995

Resonantly Laser-Driven Plasma Waves For Electron Acceleration, Donald P. Umstadter, J. Kim, Eric Esarey, E. Dodd, T. Neubert

Donald Umstadter Publications

A method for generating large-amplitude nonlinear plasma waves, which utilizes an optimized train of independently adjustable, intense laser pulses, is analyzed in one dimension both theoretically and numerically (using both Maxwell-fluid and particle-in-cell codes). Optimal pulse widths and interpulse spacings are computed for pulses with either square or finite-rise-time sine shapes. A resonant region of the plasma wave phase space is found where the plasma wave is driven most efficiently by the laser pulses. The width of this region, and thus the optimal finite-rise-time laser pulse width, was found to decrease with increasing background plasma density and plasma wave amplitude ...


Energy And Angular Distributions Of Electrons From Ion Impact On Atomic And Molecular Hydrogen. I. 20- 114-Kev H+ + H2, M. W. Gealy, G. W. Kerby, Iii, Y.-Y. Hsu, M. Eugene Rudd Mar 1995

Energy And Angular Distributions Of Electrons From Ion Impact On Atomic And Molecular Hydrogen. I. 20- 114-Kev H+ + H2, M. W. Gealy, G. W. Kerby, Iii, Y.-Y. Hsu, M. Eugene Rudd

M. Eugene Rudd Publications

Apparatus and procedures are described for the measurement of absolute cross sections, differential in ejected electron energy and angle, for ionization of atomic and molecular hydrogen by ion impact. A hemispherical electrostatic energy analyzer, rotatable from 15° to 165° with respect to the direction of the incident ion beam, was used to measure energy spectra of secondary electrons from 1.5 to 300 eV. Cross sections at ten angles (nine at some energies) and five incident-ion energies from 20 to 114 keV for H+ +H2 collisions are given. The doubly differential cross sections were integrated over angle and electron ...


Electron-Impact Ionization Of Na, A.R. Johnston, Paul Burrow Mar 1995

Electron-Impact Ionization Of Na, A.R. Johnston, Paul Burrow

Paul Burrow Publications

Recent convergent close-coupling calculations of the ionization cross section of Na by Bray [Phys. Rev. Lett. 73, 1088 (1994)] are in poor agreement with the experimental determinations, most of which were carried out in the 1960s. We report here a measurement of the ratio of the peak ionization cross section to the cross section for excitation of the 32P state near threshold. By normalizing the latter to the theoretical cross section computed by Moores and Norcross [J. Phys. B 5, 1482 (1972)] and more recently by Trail et al. [Phys. Rev. A 49, 3620 (1994)], we find an ...


Energy And Angular Distributions Of Electrons From Ion Impact On Atomic And Molecular Hydrogen. Ii. 20- 114-Kev H+ + H, G. W. Kerby, Iii, M. W. Gealy, Y.-Y. Hsu, M. Eugene Rudd, D. R. Schultz, C. O. Reinhold Mar 1995

Energy And Angular Distributions Of Electrons From Ion Impact On Atomic And Molecular Hydrogen. Ii. 20- 114-Kev H+ + H, G. W. Kerby, Iii, M. W. Gealy, Y.-Y. Hsu, M. Eugene Rudd, D. R. Schultz, C. O. Reinhold

M. Eugene Rudd Publications

Results of crossed-beam measurements of cross sections differential in ejected electron energy and angle for ionization of atomic hydrogen by 20-1 14-keV protons are reported. Secondary electrons were measured over an energy range of 1.5-300 eV and an angular range of 15°-165°. Atomic-hydrogen targets were produced in a radio-frequency discharge source with a dissociation fraction of about 74%. Ratios of cross sections for H targets to those for Hz targets were obtained from measurements on the mixed target. From these ratios, the measured dissociation fractions, and the absolute cross sections measured for H, targets, the cross sections for ...


Dissociative Attachment In Hot Ch3Cl: Experiment And Theory, D. M. Pearl, Paul Burrow, Ilya I. Fabrikant, Gordon A. Gallup Feb 1995

Dissociative Attachment In Hot Ch3Cl: Experiment And Theory, D. M. Pearl, Paul Burrow, Ilya I. Fabrikant, Gordon A. Gallup

Paul Burrow Publications

The dissociative attachment (DA) cross section of hot CH3Cl has been measured in a crossed electron–molecule beam apparatus at temperatures up to 750 K and electron energies from 0–0.5 eV. The results are compared to cross sections computed using a mixed ab initio-semiempirical approach, treating CH3Cl as a quasidiatomic molecule. The theoretical treatment requires an anion potential curve in the stable region as a portion of the input data. Computations with three different basis sets show the results to be sensitive to the size of basis set from which the potential is ...


Method For Generating A Plasma Wave To Accelerate Electrons, Donald P. Umstadter, Eric Esarey, Joon K. Kim Feb 1995

Method For Generating A Plasma Wave To Accelerate Electrons, Donald P. Umstadter, Eric Esarey, Joon K. Kim

Donald Umstadter Publications

The invention provides a method and apparatus for generating large amplitude nonlinear plasma waves, driven by an optimized train of independently adjustable, intense laser pulses. In the method, optimal pulse widths, interpulse spacing, and intensity profiles of each pulse are determined for each pulse in a series of pulses. A resonant region of the plasma wave phase space is found where the plasma wave is driven most efficiently by the laser pulses. The accelerator system of the invention comprises several parts: the laser system, also called beam source, which preferably comprises photo cathode electron source and RF-LINAC accelerator; electron photo-cathode ...


Coherent Control Of H- Photodetachment In Parallel Static Electric And Magnetic Fields, Qiaoling Wang, Anthony F. Starace Feb 1995

Coherent Control Of H- Photodetachment In Parallel Static Electric And Magnetic Fields, Qiaoling Wang, Anthony F. Starace

Anthony F. Starace Publications

Theoretical calculations of quantum interference effects occurring in photodetachement of H- with short-pulse lasers in the presence of a uniform static electric field [Q. Wang and A. F. Starace, Phys. Rev. A 48, R1741 (1993)] are examined in more detail and extended to the case of parallel static electric and magnetic fields. We show that modulation factors characterizing near-threshold cross sections resulting from detachment by relatively long laser pulses may be employed to set the parameters that are most effective for control of detachment cross-section magnitudes with short coherent laser pulses. Use of a static magnetic field to control detached ...


Heavy Ion Track-Structure Calculations For Radial Dose In Arbitrary Materials, Francis A. Cucinotta, Robert Katz, John W. Wilson, Rajendra R. Dubey Feb 1995

Heavy Ion Track-Structure Calculations For Radial Dose In Arbitrary Materials, Francis A. Cucinotta, Robert Katz, John W. Wilson, Rajendra R. Dubey

Robert Katz Publications

The δ-ray theory of track structure is compared with experimental data for the radial dose from heavy ion irradiation. The effects of electron transmission and the angular dependence of secondary electron ejection are included in the calculations. Several empirical formulas for electron range and energy are compared in a wide variety of materials in order to extend the application of the track-structure theory. The model of Rudd for the secondary electron spectrum in proton collisions, which is based on a modified classical kinematics binary encounter model at high energies and a molecular promotion model at low energies, is employed. For ...


Far-Ultraviolet Stellar Photometry: Fields In Sagittarius And Scorpius, Edward G. Schmidt, George R. Carruthers Feb 1995

Far-Ultraviolet Stellar Photometry: Fields In Sagittarius And Scorpius, Edward G. Schmidt, George R. Carruthers

Edward Schmidt Publications

Far-ultraviolet photometry for 741 objects in a field in Sagittarius centered near M8 and 54 1 objects in a field centered near ζ Scorpii is presented. These data were extracted from electrographic images obtained with two cameras during a shuttle flight in 1991 April/May. The cameras provided band passes with λeff= 1375 Å and λeff= 1781 Å. Synthetic colors show that these bands are sensitive to effective temperature for hot stars. Our measurements were placed on a quantitative far-ultraviolet magnitude scale by convolving the spectra of stars observed by IUE with our cameras' spectral response functions. Fifty-eight ...


State-Selected Ion-Molecule Reactions: Charge Transfer And Atomic Rearrangement Processes In Thermal Energy Collisions Of H2+ (X;V)+N2 And Of N2+ (X,A;V) + H2, Cornelis J. Uiterwaal, J. Van Eck, A. Niehaus Jan 1995

State-Selected Ion-Molecule Reactions: Charge Transfer And Atomic Rearrangement Processes In Thermal Energy Collisions Of H2+ (X;V)+N2 And Of N2+ (X,A;V) + H2, Cornelis J. Uiterwaal, J. Van Eck, A. Niehaus

C.J.G.J. Uiterwaal Publications

Using the photo-electron-product-ion-coincidence method (PEPICO) we have measured state-selective cross sections for the following processes: (A) N2+ (X,A;v)+ H2→N2H+ + H, (B) H2+ (X;v) + N2→N2H+ + H, (C) N2+ (X,A;v)+H2→H2+ + N2, and (D) H2+ (X;v)+N2→N2+ + H2 . The measurements were performed at thermal velocities (Ec.m. ≈ 40 meV).We have found that the charge transfer processes (C) and (D) have cross sections that are at least an order of magnitude smaller than the cross ...


Effects Of Track Structure And Cell Inactivation On The Calculation Of Heavy Ion Mutation Rates In Mammalian Cells, Francis Cucinotta, J. W. Wilson, M.R. Shavers, Robert Katz Jan 1995

Effects Of Track Structure And Cell Inactivation On The Calculation Of Heavy Ion Mutation Rates In Mammalian Cells, Francis Cucinotta, J. W. Wilson, M.R. Shavers, Robert Katz

Robert Katz Publications

It has long been suggested that inactivation severely effects the probability of mutation by heavy ions in mammalian cells. Heavy ions have observed cross sections of inactivation that approach and sometimes exceed the geometric size of the cell nucleus in mammalian cells. In the track structure model of Katz the inactivation cross section is found by summing an inactivation probability over all impact parameters from the ion to the sensitive sites within the cell nucleus. The inactivation probability is evaluated using the dose-response of the system to γ-rays and the radial dose of the ions and may be equal to ...


Effects Of Track Structure And Cell Inactivation On The Calculation Of Heavy Ion Mutation Rates In Mammalian Cells, Francis A. Cucinotta, J. W. Wilson, M. R. Shavers, Robert Katz Jan 1995

Effects Of Track Structure And Cell Inactivation On The Calculation Of Heavy Ion Mutation Rates In Mammalian Cells, Francis A. Cucinotta, J. W. Wilson, M. R. Shavers, Robert Katz

Robert Katz Publications

It has long been suggested that inactivation severely effects the probability of mutation by heavy ions in mammalian cells. Heavy ions have observed cross sections of inactivation that approach and sometimes exceed the geometric size of the cell nucleus in mammalian cells. In the track structure model of Katz the inactivation cross section is found by summing an inactivation probability over all impact parameters from the ion to the sensitive sites within the cell nucleus. The inactivation probability is evaluated using the dose-response of the system to γ-rays and the radial dose of the ions and may be equal to ...


Nonlinear Temporal Diffraction And Frequency Shifts Resulting From Pulse Shaping In Chirped-Pulse Amplification Systems, X Liu, R Wagner, Anatoly Maksimchuk, E Goodman, J Workman, Donald P. Umstadter, A Migus Jan 1995

Nonlinear Temporal Diffraction And Frequency Shifts Resulting From Pulse Shaping In Chirped-Pulse Amplification Systems, X Liu, R Wagner, Anatoly Maksimchuk, E Goodman, J Workman, Donald P. Umstadter, A Migus

Donald Umstadter Publications

We present experimental results of the amplif ication of strongly amplitude-modulated chirped pulses resulting from the coherent addition of two delayed short pulses. The nonlinearities in the amplifier chain induce a temporal diffraction resulting in prepulses and postpulses, in addition to the two main pulses when compressed. Simultaneously, temporal-resolved and spectral-resolved output pulses show that the prepulses and postpulses are blue shifted and red shifted, respectively, explaining the causality of the system.


Failure To Observe Electron Circular Dichroism In Camphor, K. W. Trantham, M. E. Johnston, Timothy J. Gay Jan 1995

Failure To Observe Electron Circular Dichroism In Camphor, K. W. Trantham, M. E. Johnston, Timothy J. Gay

Timothy J. Gay Publications

We have searched for evidence of electron circular dichroism, the preferential
transmission of longirudinally polarized electrons lhrough a chiral medium, using stereoisomers of camphor. Such an effect was seen by Campbell and Farago in 1987 for 5 eV elecvons. Within our detection limit (~0.02%) we have not observed asymmehic transmission at either an incident electron energy of 1 eV or 5 eV. ln preparation for this inquiry, we also measured the total scattering cross section for camphor in the energy range 0.5-5 eV.


Lattice- And Molecular-Dynamics Studies Of Rbliso4, V. Katkanant, John R. Hardy Jan 1995

Lattice- And Molecular-Dynamics Studies Of Rbliso4, V. Katkanant, John R. Hardy

John R. Hardy Papers

Using our previously developed method for calculating parameter-free potential-energy surfaces for ionic molecular crystals, specifically sulfates, we study phase transitions in RbLiSO4 by means of lattice and molecular dynamics. We found that the high-temperature phase I (Pnam) is highly unstable and transforms to the observed lower temperature phase VI (P1121/n) at about 475–525 K. Compared with isomorphous CsLiSO4, there are more branches of unstable modes in the Pnam phase for RbLiSO4. The maximum instability of these modes occurs away from the zone center, q=(0.118a*,0,0), which implies that a high-order incommensurate ...


Characterization Of Piezoceramic Crosses With Large Range Scanning Capability And Applications For Low Temperature Scanning Tunneling Microscopy, J. A. Helfrich, Shireen Adenwalla, J. B. Ketterson, G. A. Zhitomirsky Jan 1995

Characterization Of Piezoceramic Crosses With Large Range Scanning Capability And Applications For Low Temperature Scanning Tunneling Microscopy, J. A. Helfrich, Shireen Adenwalla, J. B. Ketterson, G. A. Zhitomirsky

Shireen Adenwalla Papers

We have developed a large amplitude piezoceramic scanner which should have numerous applications. Scanning tunneling microscopy (STM) and other scanning probe microscopies predominantly use piezoceramics for the scanning elements. Similarly adaptive optics, high resolution lithography, and micromanipulators are other examples of research which regularly utilize piezoceramic scanners. We present a new geometry for a piezoceramic scanner which allows for both high resolution (~nanometers) and large amplitude (~400 µm) displacements. The cross-shaped geometry makes it possible to produce extremely long pieces with very high tolerances. We have shown its effectiveness by using it as the major component of a low temperature ...


Temporary Negative-Ion States Of Na, K, Rb, And Cs, A.R. Johnston, Paul Burrow Jan 1995

Temporary Negative-Ion States Of Na, K, Rb, And Cs, A.R. Johnston, Paul Burrow

Paul Burrow Publications

Electron-transmission spectroscopy is used to observe structure in the electron-scattering cross section associated with the formation of temporary negative ions and channel-opening effects. A strong ‘‘family’’ resemblance is apparent among the resonances of the alkali metals. From published calculations and angular scattering studies, the lower-lying features are known with some confidence. Based on these, plausible assignments for some of the higher-lying resonances are put forth. We propose that the absence of resonances near the Cs 5 2D and possibly the Na 4 2P and 5 2P thresholds is associated with the negative polarizabilities of these excited states.


Electronic Structures And Curie Temperatures Of Iron-Based Rare-Earth Permanent-Magnet Compounds , J.P. Woods , B.M. Patterson, A.S. Fernando, Sitaram Jaswal, D. Welipitiya, David J. Sellmyer Jan 1995

Electronic Structures And Curie Temperatures Of Iron-Based Rare-Earth Permanent-Magnet Compounds , J.P. Woods , B.M. Patterson, A.S. Fernando, Sitaram Jaswal, D. Welipitiya, David J. Sellmyer

David Sellmyer Publications

The modification of the electronic structures of Sm2Fe17-xAlxNy, NdFe11TiNy, and YFe12-xMox upon alloying and nitriding are examined with self-consistent spin-polarized calculations and soft-x-ray photoemission measurements between 18 and 135 eV. The changes in the Curie temperature Tc with substitutional modifications and nitrogen addition are modeled with self-consistent spin-polarized electronic structure calculations and the spin-fluctuation theory of Mohn and Wohlfarth which relates the electronic structure to Tc. The calculations show that the spin-summed density of states at the Fermi energy is related to Tc. The ...


A Simple Technique To Measure Stress In Ultrathin Films During Growth, D. Sander, Axel Enders, J. Kirschner Jan 1995

A Simple Technique To Measure Stress In Ultrathin Films During Growth, D. Sander, Axel Enders, J. Kirschner

Axel Enders Publications

We demonstrate an easy implementation of the cantilever bending beam approach to measure stress during film growth in ultrahigh vacuum. Using a simple and compact optical deflection technique, film stress with sub-monolayer sensitivity can be detected. A stress measurement during FeSi, formation on Si(l11) is presented.