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Physics Faculty Research & Creative Works

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Articles 931 - 960 of 1762

Full-Text Articles in Physical Sciences and Mathematics

Magnetization And Neutron Diffraction Studies On Dy5si2ge2, R. Nirmala, V. Sankaranarayanan, K. Sethupathi, A. V. Morozkin, Q. Cai, Z. Chu, J. B. Yang, William B. Yelon, S. K. Malik May 2005

Magnetization And Neutron Diffraction Studies On Dy5si2ge2, R. Nirmala, V. Sankaranarayanan, K. Sethupathi, A. V. Morozkin, Q. Cai, Z. Chu, J. B. Yang, William B. Yelon, S. K. Malik

Physics Faculty Research & Creative Works

The compound Dy5Si2Ge2 crystallizes in an orthorhombic structure (Sm5Ge4 type, space group Pnma). Magnetization measurements performed in the temperature range of 2-300 K in applied fields up to 7 T reveal that this compound orders antiferromagnetically at 56 K (TN) but with a positive paramagnetic Curie temperature θP. Magnetization-field isotherms, obtained at 5 K and 20 K, display a field-induced antiferromagnetic to ferromagnetic transition. the magnetization approaches saturation in a field of 6 T with a moment value of ~8μBDy3+. Neutron diffraction measurements, carried out at …

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Temperature Dependence Of Homogeneous Nucleation Rates For Water: Near Equivalence Of The Empirical Fit Of Wölk And Strey, And The Scaled Nucleation Model, Barbara N. Hale May 2005

Temperature Dependence Of Homogeneous Nucleation Rates For Water: Near Equivalence Of The Empirical Fit Of Wölk And Strey, And The Scaled Nucleation Model, Barbara N. Hale

Physics Faculty Research & Creative Works

It is pointed out that the temperature fitting function of Wölk and Strey [J. Phys. Chem. 105, 11683 (2001)], recently shown to convert the Becker-Döring [Ann. Phys. (Leipzig) 24, 719 (1935)] nucleation rate into an expression in agreement with much of the experimental water nucleation rate data, also converts the Becker-Döring rate into a form nearly equivalent with the scaled nucleation rate model, Jscaled=Joc exp[-16πΩ3(Tc/T-1)3/3(ln S)2]. In the latter expression Joc is the inverse thermal wavelength cubed/sec, evaluated at Tc.

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Doubly Differential Cross Sections For Single And Multiple Ionization Of Ne By Electron Impact, Antonio C. Santos, Ahmad Hasan, Robert D. Dubois Mar 2005

Doubly Differential Cross Sections For Single And Multiple Ionization Of Ne By Electron Impact, Antonio C. Santos, Ahmad Hasan, Robert D. Dubois

Physics Faculty Research & Creative Works

We present doubly differential cross sections for single and multiple ionization of the outer shell of neon by 750 eV electron impact. The distinction between single and multiple ionization was achieved by performing a charge state analysis of the recoil ions in coincidence with forward scattered, energy analyzed electrons. By a comparison to photon impact data, the contribution of the second-order double ionization mechanism is estimated and found to be neglible at this impact energy. Following a similar procedure adopted by J. A. R. Samson [Phys. Rev. Lett. 65, 2861 (1990)], the importance of the first-order TS-1 double ionization mechanism …

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Relic Gravitons On Kasner-Like Branes, Marco Cavaglia, G. De Risi, M. Gasperini Mar 2005

Relic Gravitons On Kasner-Like Branes, Marco Cavaglia, G. De Risi, M. Gasperini

Physics Faculty Research & Creative Works

We discuss the cosmological amplification of tensor perturbations in a simple example of brane-world scenario, in which massless gravitons are localized on a higher-dimensional Kasner-like brane embedded in a bulk AdS background. Particular attention is paid to the canonical normalization of the quadratic action describing the massless and massive vacuum quantum fluctuations, and to the exact mass-dependence of the amplitude of massive fluctuations on the brane. The perturbation equations can be separated. In contrast to de Sitter models of brane inflation, we find no mass gap in the spectrum and no enhancement for massless modes at high curvature. The massive …

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Positronium Formation In Positron-Li And Positron-Na Collisions At Low Energies, Anh-Thu Le, M. W. J. Bromley, C. D. Lin Mar 2005

Positronium Formation In Positron-Li And Positron-Na Collisions At Low Energies, Anh-Thu Le, M. W. J. Bromley, C. D. Lin

Physics Faculty Research & Creative Works

We present the positronium formation cross sections for a positron colliding with lithium and sodium for the collision energies from 0.01 eV up to 20 eV by the hyperspherical close-coupling method. For Li, our results agree with the experimental data and with other calculations. Our results for Na remain in agreement with previous close-coupling calculations, but do not support the latest experimental data for Na below 1 eV. To validate our model potentials and method in the low-energy regime, the binding energies of positronic lithium and positronic sodium as well as the s-wave scattering lengths for positronium scattering from Li⁺ …

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Muon Transfer From Muonic Hydrogen To Atomic Oxygen And Nitrogen, Anh-Thu Le, C. D. Lin Feb 2005

Muon Transfer From Muonic Hydrogen To Atomic Oxygen And Nitrogen, Anh-Thu Le, C. D. Lin

Physics Faculty Research & Creative Works

The results of diabatic hyperspherical close-coupling calculations are presented for the charge exchange of a negative muon from muonic hydrogen to oxygen and nitrogen for collision energies from 10-3 to 103 eV. It is shown that converged results can be obtained using a much smaller number of channels than in the traditional adiabatic approach. For the energy range below 10 eV our results for nitrogen are in good agreements with the available experimental data and the recent calculations within hyperspherical elliptic coordinates. However, discrepancies were found in the case of oxygen, where a p -wave shape resonance is …

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Analysis Of High-Quality Modes In Open Chaotic Microcavities, W. Fang, Alexey Yamilov, Hui Cao Jan 2005

Analysis Of High-Quality Modes In Open Chaotic Microcavities, W. Fang, Alexey Yamilov, Hui Cao

Physics Faculty Research & Creative Works

We present a numerical study of the high-quality modes in two-dimensional dielectric stadium microcavities. Although the classical ray mechanics is fully chaotic in a stadium billiard, all of the high-quality modes show a ``strong scar'' around unstable periodic orbits. when the deformation (ratio of the length of the straight segments over the diameter of the half circles) is small, the high-quality modes correspond to whispering-gallery-type trajectories and their quality factors decrease monotonically with increasing deformation. At large deformation, each high-quality mode is associated with multiple unstable periodic orbits. Its quality factor changes nonmonotonically with the deformation, and there exists an …

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Half-Metallicity And Efficient Spin Injection In Aln/Gan:Cr (0001) Heterostructure, Julia E. Medvedeva, X. Y. Cui, C. Stampfl, N. Newman, Arthur J. Freeman Jan 2005

Half-Metallicity And Efficient Spin Injection In Aln/Gan:Cr (0001) Heterostructure, Julia E. Medvedeva, X. Y. Cui, C. Stampfl, N. Newman, Arthur J. Freeman

Physics Faculty Research & Creative Works

First-principles investigations of the structural, electronic, and magnetic properties of Cr-doped AlN/GaN (0001) heterostructures reveal the possibility of efficient spin injection from a ferromagnetic GaN:Cr electrode through an AlN tunnel barrier. We demonstrate that Cr atoms segregate into the GaN region and that these interfaces retain their half-metallic behavior leading to a complete, i.e., 100%, spin polarization of the conduction electrons. This property makes the wide band-gap nitrides doped with Cr to be excellent candidates for high-efficiency magnetoelectronic devices.

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Precollision And Postcollision Electron-Electron Correlation Effects For Intermediate-Energy Proton-Impact Ionization Of Helium, M. Foster, Jerry Peacher, Michael Schulz, Ahmad Hasan, Don H. Madison Jan 2005

Precollision And Postcollision Electron-Electron Correlation Effects For Intermediate-Energy Proton-Impact Ionization Of Helium, M. Foster, Jerry Peacher, Michael Schulz, Ahmad Hasan, Don H. Madison

Physics Faculty Research & Creative Works

We report fully differential cross sections (FDCS) for the single ionization of helium by a 75 keV incident energy proton. Previous three-body distorted wave (3DW) calculations for this collision system are in poor agreement with the absolute magnitude of the experimental measurements. The 3DW approximation treats the four-body problem as an effective three-body problem in which the passive electron does not participate in the collision. We have developed a full four-body approach in which the passive electron fully participates in the collision. It will be shown that the FDCS is very sensitive to the treatment of the passive electron-ejected electron …

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Distorted Wave Born And Three-Body Distorted Wave Born Approximation Calculations Of The Fully Differential Cross Section For Electron-Impact Ionization Of Nitrogen Molecules, Junfang Gao, Don H. Madison, Jerry Peacher Jan 2005

Distorted Wave Born And Three-Body Distorted Wave Born Approximation Calculations Of The Fully Differential Cross Section For Electron-Impact Ionization Of Nitrogen Molecules, Junfang Gao, Don H. Madison, Jerry Peacher

Physics Faculty Research & Creative Works

Currently there are no reliable theoretical approaches for calculating fully differential cross sections (FDCSs) for low-energy electron-impact ionization of large molecules. We have recently proposed the orientation-averaged molecular orbital (OAMO) for calculating cross sections averaged over molecular orientations. In this paper, we use the OAMO to calculate distorted wave Born approximation (DWBA) and molecular three-body distorted wave (M3DW) Born approximation FDCS for electron-impact ionization of the nitrogen molecule. Both coplanar symmetric and asymmetric FDCSs are investigated in the energy range of 35.6-400 eV. by comparing with the experimental data, we found that the M3DW is reasonably accurate in this energy …

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Field And Intensity Correlations In Amplifying Random Media, Alexey Yamilov, Shih-Hui Chang, Alexander Burin, Allen Taflove, Hui Cao Jan 2005

Field And Intensity Correlations In Amplifying Random Media, Alexey Yamilov, Shih-Hui Chang, Alexander Burin, Allen Taflove, Hui Cao

Physics Faculty Research & Creative Works

We study local and nonlocal correlations of light transmitted through active random media. The conventional approach results in divergence of ensemble-averaged correlation functions due to the existence of lasing realizations. We introduce a conditional average for correlation functions by omitting the divergent realizations. Our numerical simulation reveals that amplification does not affect local spatial correlation. The nonlocal intensity correlations are strongly magnified due to selective enhancement of the contributions from long propagation paths. We also show that by increasing gain, the average mode linewidth can be made comparable to the average mode spacing. This implies that light transport through a …

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Near-Field Intensity Correlations In Semicontinuous Metal Films, K. Seal, H. Noh, Alexey Yamilov, A. K. Sarchev, D. A. Genov, V. M. Shalaev, Z. C. Ying, Hui Cao Jan 2005

Near-Field Intensity Correlations In Semicontinuous Metal Films, K. Seal, H. Noh, Alexey Yamilov, A. K. Sarchev, D. A. Genov, V. M. Shalaev, Z. C. Ying, Hui Cao

Physics Faculty Research & Creative Works

Spatial intensity correlation functions of semicontinuous metal-dielectric films of varying metal concentration, p, were obtained from near-field microscopy. The data shows a transition from propagation to localization and back to propagation of optical excitations with increase in p.

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Photonic Band Structure Of Zno Photonic Crystal Slab Laser, Alexey Yamilov, X. Wu, Hui Cao Jan 2005

Photonic Band Structure Of Zno Photonic Crystal Slab Laser, Alexey Yamilov, X. Wu, Hui Cao

Physics Faculty Research & Creative Works

We recently reported on the realization of ultraviolet photonic crystal laser based on zinc oxide [Appl. Phys. Lett. 85, 3657 (2004)]. Here we present the details of structural design and its optimization. We develop a computational supercell technique that allows a straightforward calculation of the photonic band structure of ZnO photonic crystal slab on sapphire substrate. We find that despite the small index contrast between the substrate and the photonic layer, the low-order eigenmodes have predominantly transverse-electric (TE) or transverse-magnetic polarization. Because emission from ZnO thin film shows a strong TE preference, we are able to limit our consideration to …

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Static Pairwise Annihilation In Complex Networks, M. F. Laguna, M. Aldana, H. Larralde, V. M. Kenkre, Paul Ernest Parris Jan 2005

Static Pairwise Annihilation In Complex Networks, M. F. Laguna, M. Aldana, H. Larralde, V. M. Kenkre, Paul Ernest Parris

Physics Faculty Research & Creative Works

We study static annihilation on complex networks, in which pairs of connected particles annihilate at a constant rate during time. Through a mean-field formalism, we compute the temporal evolution of the distribution of surviving sites with an arbitrary number of connections. This general formalism, which is exact for disordered networks, is applied to Kronecker, Erdös-Rényi (i.e., Poisson), and scale-free networks. We compare our theoretical results with extensive numerical simulations obtaining excellent agreement. Although the mean-field approach applies in an exact way neither to ordered lattices nor to small-world networks, it qualitatively describes the annihilation dynamics in such structures. Our results …

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Critical Behavior And Griffiths Effects In The Disordered Contact Process, Thomas Vojta, Mark Dickison Jan 2005

Critical Behavior And Griffiths Effects In The Disordered Contact Process, Thomas Vojta, Mark Dickison

Physics Faculty Research & Creative Works

We study the nonequilibrium phase transition in the one-dimensional contact process with quenched spatial disorder by means of large-scale Monte Carlo simulations for times up to 109 and system sizes up to 107 sites. In agreement with recent predictions of an infinite-randomness fixed point, our simulations demonstrate activated (exponential) dynamical scaling at the critical point. The critical behavior turns out to be universal, even for weak disorder. However, the approach to this asymptotic behavior is extremely slow, with crossover times of the order of 104 or larger. In the Griffiths region between the clean and the dirty critical points, we …

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Interference Effects For Low-Energy Electron-Impact Ionization Of Nitrogen Molecules, Junfang Gao, Don H. Madison, Jerry Peacher Jan 2005

Interference Effects For Low-Energy Electron-Impact Ionization Of Nitrogen Molecules, Junfang Gao, Don H. Madison, Jerry Peacher

Physics Faculty Research & Creative Works

Young's double slit interference effects for low incident energy 75.6 eV electron impact ionization of N2 are investigated using the distorted-wave impulse approximation (DWIA) for both coplanar symmetric and asymmetric scattering. Although the DWIA does not predict observable interference effects for the coplanar symmetric case, it predicts some strong Young's double-slit-type interference effects for the highly asymmetric scattering case. These effects are strong enough that they should be experimentally observable if one can make measurements in the backscattering region.

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Near-Field Intensity Correlations In Semicontinuous Metal-Dielectric Films, K. Seal, A. K. Sarychev, H. Noh, D. A. Genov, Alexey Yamilov, V. M. Shalaev, Z. C. Ying, Hui Cao Jan 2005

Near-Field Intensity Correlations In Semicontinuous Metal-Dielectric Films, K. Seal, A. K. Sarychev, H. Noh, D. A. Genov, Alexey Yamilov, V. M. Shalaev, Z. C. Ying, Hui Cao

Physics Faculty Research & Creative Works

Spatial intensity correlation functions are obtained from near-field scanning optical microscope measurements of semicontinuous metal-dielectric films. The concentration of metal particles on a dielectric surface is varied over a wide range to control the scattering strength. At low and high metal coverages where scattering is weak, the intensity correlation functions exhibit oscillations in the direction of incident light due to excitation of propagating surface waves. In the intermediate regime of metal concentration, the oscillatory behavior is replaced by a monotonic decay as a result of strong scattering and anomalous absorption. Significant differences in the near-field intensity correlations between metallic and …

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Spin-System Radio-Frequency Superradiation: A Phenomenological Study And Comparison With Numeric Simulations, Carrie L. Davis, Victor K. Henner, Aleksandr V. Chernatynskiy, Ilia V. Kaganov Jan 2005

Spin-System Radio-Frequency Superradiation: A Phenomenological Study And Comparison With Numeric Simulations, Carrie L. Davis, Victor K. Henner, Aleksandr V. Chernatynskiy, Ilia V. Kaganov

Physics Faculty Research & Creative Works

We discuss the coherent behavior of a polarized, nuclear or electron, spin system for which the magnetic dipole radiation emitted in the radio-frequency region, has approximately quadratic dependence on the number of spins. An effective method of describing these phenomena is provided by computer simulation of a microscopic model of the spin system. Important aspects of this numeric simulation are described, together with a comparison with the theoretical predictions. The behavior of the transverse component of the magnetic moment, M+ (t), in super-radiant conditions is studied. In addition, the role of dipole-dipole interactions in super-radiation phenomena is investigated in …

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Quantum Griffiths Effects In Itinerant Heisenberg Magnets, Thomas Vojta, Jörg Schmalian Jan 2005

Quantum Griffiths Effects In Itinerant Heisenberg Magnets, Thomas Vojta, Jörg Schmalian

Physics Faculty Research & Creative Works

We study the influence of quenched disorder on quantum phase transitions in itinerant magnets with Heisenberg spin symmetry, paying particular attention to rare disorder fluctuations. In contrast to the Ising case where the Landau damping of the spin fluctuations suppresses the tunneling of the rare regions, the Heisenberg system displays strong power-law quantum Griffiths singularities in the vicinity of the quantum critical point. We discuss these phenomena based on general scaling arguments, and we illustrate them by an explicit calculation for O(N) spin symmetry in the large-N limit. We also discuss broad implications for the classification of quantum phase transitions …

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The Effect Of Cu-Doping On The Magnetic And Transport Properties Of La₀.₇Sr₀.₃Mno₃, M. S. Kim, Jinbo Yang, Qingsheng Cai, X.-D. Zhou, William B. Yelon, Paul Ernest Parris, William Joseph James Jan 2005

The Effect Of Cu-Doping On The Magnetic And Transport Properties Of La₀.₇Sr₀.₃Mno₃, M. S. Kim, Jinbo Yang, Qingsheng Cai, X.-D. Zhou, William B. Yelon, Paul Ernest Parris, William Joseph James

Physics Faculty Research & Creative Works

The effects of Cu-doping on the structural, magnetic, and transport properties of La0.7Sr0.3Mn1xCuxO3 (0<=x<=0.20) have been studied using neutron diffraction, magnetization, and magnetoresistance (MR) measurements. All samples show the rhombohedral structure with the R[overline 3]c space-group from 10 K to room temperature (RT). Neutron diffraction data suggest that some of the Cu ions have a Cu3+ state in these compounds. The substitution of Mn by Cu affects the MnO bond length and Mn-O-Mn bond angle resulting from the minimization of the distortion of the MnO6 octahedron. Resistivity measurements show that a metal to insulator transition occurs for the x>=0.15 samples. The x=0.15 sample shows the highest MR([approximate]80%), which might result from the co-existence of Cu3-Cu2+ and the dilution effect of Cu-doping on the double exchange interaction

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Combining High Conductivity With Complete Optical Transparency: A Band Structure Approach, Julia E. Medvedeva, Arthur J. Freeman Jan 2005

Combining High Conductivity With Complete Optical Transparency: A Band Structure Approach, Julia E. Medvedeva, Arthur J. Freeman

Physics Faculty Research & Creative Works

A comparison of the structural, optical and electronic properties of the recently discovered transparent conducting oxide (TCO), nanoporous Ca12Al14O33, with those of the conventional TCOs (such as Sc-doped CdO) indicates that this material belongs conceptually to a new class of transparent conductors. For this class of materials, we formulate criteria for the successful combination of high electrical conductivity with complete transparency in the visible range and emphasize the significant correlation between their structural characteristics and electronic and optical properties. Our analysis suggests that this set of requirements can be met for a group of novel materials called electrides which may …

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Fabrication Of Inverted Opal Zno Photonic Crystals By Atomic Layer Deposition, Michael Scharrer, Xiaohua Wu, Alexey Yamilov, Hui Cao, Robert P. H. Chang Jan 2005

Fabrication Of Inverted Opal Zno Photonic Crystals By Atomic Layer Deposition, Michael Scharrer, Xiaohua Wu, Alexey Yamilov, Hui Cao, Robert P. H. Chang

Physics Faculty Research & Creative Works

We have fabricated three-dimensional optically active ZnO photonic crystals by infiltrating polystyrene opal templates using a low-temperature atomic layer deposition process. The polystyrene is removed by firing the samples at elevated temperatures, and reactive ion etching is used to remove the top layer of ZnO and expose the (111) photonic crystal surface. The resulting structures have high filling fractions, possess photonic band gaps in the near-UV to visible spectrum, and exhibit efficient photoluminescence.

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Projectile-Residual-Target-Ion Scattering After Single Ionization Of Helium By Slow Proton Impact, N. V. Maydanyuk, Ahmad Hasan, M. Foster, B. Tooke, Don H. Madison, Michael Schulz, Emilio Nanni Jan 2005

Projectile-Residual-Target-Ion Scattering After Single Ionization Of Helium By Slow Proton Impact, N. V. Maydanyuk, Ahmad Hasan, M. Foster, B. Tooke, Don H. Madison, Michael Schulz, Emilio Nanni

Physics Faculty Research & Creative Works

We have measured fully differential single ionization cross sections for 75 keV p+He collisions. At this relatively small projectile velocity, signatures of the projectile-residual-target-ion interaction, which are not observable for fast projectiles and for electron impact, are revealed rather sensitively. In fact, this interaction appears to be more important than the postcollision interaction, which so far was assumed to be the most important factor in higher-order effects for slow ion impact. These features are not well reproduced by our three-distorted-wave calculations.

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How Generic Scale Invariance Influences Quantum And Classical Phase Transitions, D. Belitz, T. Kirkpatrick, Thomas Vojta Jan 2005

How Generic Scale Invariance Influences Quantum And Classical Phase Transitions, D. Belitz, T. Kirkpatrick, Thomas Vojta

Physics Faculty Research & Creative Works

This review discusses a paradigm that has become of increasing importance in the theory of quantum phase transitions, namely, the coupling of the order-parameter fluctuations to other soft modes and the resulting impossibility of constructing a simple Landau-Ginzburg-Wilson theory in terms of the order parameter only. The soft modes in question are manifestations of generic scale invariance, i.e., the appearance of long-range order in whole regions in the phase diagram. The concept of generic scale invariance and its influence on critical behavior is explained using various examples, both classical and quantum mechanical. The peculiarities of quantum phase transitions are discussed, …

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Tunable Conductivity And Conduction Mechanism In An Ultraviolet Light Activated Electronic Conductor, Mariana I. Bertoni, Thomas O. Mason, Julia E. Medvedeva, Arthur J. Freeman, Kenneth R. Poeppelmeier, B. Delley Jan 2005

Tunable Conductivity And Conduction Mechanism In An Ultraviolet Light Activated Electronic Conductor, Mariana I. Bertoni, Thomas O. Mason, Julia E. Medvedeva, Arthur J. Freeman, Kenneth R. Poeppelmeier, B. Delley

Physics Faculty Research & Creative Works

A tunable conductivity has been achieved by controllable substitution of an ultraviolet light activated electronic conductor. The transparent conducting oxide system H-doped Ca12-xMgxAl14O33 (x=0,0.1,0.3,0.5,0.8,1.0) presents a conductivity that is strongly dependent on the substitution level and temperature. Four-point dc-conductivity decreases with x from 0.26 S/cm (x=0) to 0.106 S/cm (x=1) at room temperature. At each composition the conductivity increases (reversibly with temperature) until a decomposition temperature is reached; above this value, the conductivity drops dramatically due to hydrogen recombination and loss. The observed conductivity behavior is consistent with the predictions of our first principles density functional calculations for the Mg-substituted …

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An Elementary Method For Calculating Orientation-Averaged Fully Differential Electron-Impact Ionization Cross Sections For Molecules, Junfang Gao, Jerry Peacher, Don H. Madison Jan 2005

An Elementary Method For Calculating Orientation-Averaged Fully Differential Electron-Impact Ionization Cross Sections For Molecules, Junfang Gao, Jerry Peacher, Don H. Madison

Physics Faculty Research & Creative Works

Currently there are no reliable theoretical approaches for calculating fully differential cross sections (FDCS) for low-energy electron-impact ionization of large molecules. We have recently introduced the distorted-wave impulse approximation as a first step in developing improved theoretical approaches. One significant obstacle to evaluating improved theoretical approaches which require significant computer resources lies in the fact that the existing experimental data require taking averages over all molecular orientations. To circumvent this problem, it has been proposed to approximate the orientation-average by using an orientation-averaged molecular orbital in the calculation of the FDCS. The theoretical justification and expected range of validity for …

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Fully Differential Cross Sections For Low-Energy Electron-Impact Ionization Of Nitrogen Molecules, Junfang Gao, Don H. Madison, Jerry Peacher Jan 2005

Fully Differential Cross Sections For Low-Energy Electron-Impact Ionization Of Nitrogen Molecules, Junfang Gao, Don H. Madison, Jerry Peacher

Physics Faculty Research & Creative Works

Currently there is no accurate theoretical approach available for treating fully differential cross sections (FDCS) for low-energy electron-impact ionization of large molecules. For high incident energies, the plane wave impulse approximation (PWIA) generally yields good agreement with experimental data. In this paper, the distorted wave impulse approximation (DWIA) is used to calculate FDCS for low-energy electron-impact ionization of N2. To perform the necessary average over all molecular orientations, we propose an orientation average (OA) approximation. Although the DWIA results represent an improvement over the PWIA for intermediate energies, an improved theory is necessary for lower energies. However, the OA approximation …

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Role Of Embedded Clustering In Dilute Magnetic Semiconductors: Cr Doped Gan, Julia E. Medvedeva, B. Delley, N. Newman, C. Stampfl, X. Y. Cui, Arthur J. Freeman Jan 2005

Role Of Embedded Clustering In Dilute Magnetic Semiconductors: Cr Doped Gan, Julia E. Medvedeva, B. Delley, N. Newman, C. Stampfl, X. Y. Cui, Arthur J. Freeman

Physics Faculty Research & Creative Works

Results of extensive density-functional studies provide direct evidence that Cr atoms in Cr:GaN have a strong tendency to form embedded clusters, occupying Ga sites. Significantly, for larger than 2-Cr-atom clusters, states containing antiferromagnetic coupling with net spin in the range 0.06-1.47 µB/Cr are favored. We propose a picture where various configurations coexist and the statistical distribution and associated magnetism will depend sensitively on the growth details. Such a view may elucidate many puzzling observations related to the structural and magnetic properties of III-N and other dilute semiconductors.

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Multiple Ionization Of Ne By H2⁺ Ions With Correlated Projectile Electron Loss, M. M. Sant'anna, H. Luna, Antonio C. Santos, C. Mcgrath, M. B. Shah, E. G. Cavalcanti, G. M. Sigaud, E. C. Montenegro Dec 2004

Multiple Ionization Of Ne By H2⁺ Ions With Correlated Projectile Electron Loss, M. M. Sant'anna, H. Luna, Antonio C. Santos, C. Mcgrath, M. B. Shah, E. G. Cavalcanti, G. M. Sigaud, E. C. Montenegro

Physics Faculty Research & Creative Works

We consider multiple ionization of Ne by fast H2+, with and without projectile electron loss. Measurements of the total cross section for the direct ionization channel support the similarity of ionization of Ne by an H2+ projectile that does not break and Ne ionization by an equi-velocity proton. the multiple target ionization with projectile electron loss is analyzed within a proposed model, adapted from the independent particle model, taking into account post-collisional decay channels following the ionization of Ne 2s electrons. We point out that, for the H2+ + Ne collision system, the post-collisional decay channels …

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Relativistic And Radiative Corrections To The Mollow Spectrum, Jorg Evers, Ulrich D. Jentschura, Christoph H. Keitel Dec 2004

Relativistic And Radiative Corrections To The Mollow Spectrum, Jorg Evers, Ulrich D. Jentschura, Christoph H. Keitel

Physics Faculty Research & Creative Works

The incoherent inelastic part of the resonance fluorescence spectrum of a laser-driven atom is known as the Mollow spectrum [B. R. Mollow, Phys. Rev. 188, 1969 (1969)]. Starting from this level of description, we discuss theoretical foundations of high-precision spectroscopy using the resonance fluorescence light of strongly laser-driven atoms. Specifically, we evaluate the leading relativistic and radiative corrections to the Mollow spectrum, up to the relative orders of ( Z α )2 and α ( Z α )2 , respectively, and Bloch-Siegert shifts as well as stimulated radiative corrections involving off-resonant virtual states. Complete results are provided for …

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