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Physics Faculty Research & Creative Works

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Articles 1141 - 1170 of 1959

Full-Text Articles in Physical Sciences and Mathematics

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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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Two-Loop Bethe Logarithms For Higher Excited S Levels, Ulrich D. Jentschura Nov 2004

Two-Loop Bethe Logarithms For Higher Excited S Levels, Ulrich D. Jentschura

Physics Faculty Research & Creative Works

Processes mediated by two virtual low-energy photons contribute quite significantly to the energy of hydrogenic S states. The corresponding level shift is of the order of ( α / π )2 ( Z α )6 mec2 and may be ascribed to a two-loop generalization of the Bethe logarithm. For 1 S and 2 S states, the correction has recently been evaluated by Pachucki and Jentschura [Phys. Rev. Lett. 91, 113005 (2003)]. Here, we generalize the approach to higher excited S states, which in contrast to the 1 S and 2 S states can decay to …

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Nucleation Rates Of Water And Heavy Water Using Equations Of State, Abdalla Obeidat, Jin-Song Li, Gerald Wilemski Nov 2004

Nucleation Rates Of Water And Heavy Water Using Equations Of State, Abdalla Obeidat, Jin-Song Li, Gerald Wilemski

Physics Faculty Research & Creative Works

The original formula of Gibbs for the reversible work of critical nucleus formation is evaluated in three approximate ways for ordinary and heavy water. The least approximate way employs an equation of state to evaluate the pressure difference between the new and old phases. This form of the theory yields a temperature dependence for the nucleation rate close to that observed experimentally. This is a substantial improvement over the most commonly used (and most approximate) form of classical theory.©2004 American Institute of Physics.

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Hohenberg-Kohn Theory Including Spin Magnetism And Magnetic Fields, Walter Kohn, Andreas Savin, Carsten A. Ullrich Oct 2004

Hohenberg-Kohn Theory Including Spin Magnetism And Magnetic Fields, Walter Kohn, Andreas Savin, Carsten A. Ullrich

Physics Faculty Research & Creative Works

Beginning with work by U. von Earth and L. Hedin in 1972 and continuing with recent papers in 2001 by K. Capelle and G. Vignale and by H. Eschrig and W. E. Pickett, questions have been raised about the meaning and validity of Hohenberg-Kohn theory when spin magnetism and/or magnetic fields are present. This article offers clarifications of some of these questions. in particular, it also concludes that these questions do not affect spin-density functional theory for nondegenerate ground states as currently practiced. © 2004 Wiley Periodicals, Inc.

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Nonrelativistic Qed Approach To The Bound-Electron G Factor, Krzysztof Pachucki, Ulrich D. Jentschura, Vladimir A. Yerokhin Oct 2004

Nonrelativistic Qed Approach To The Bound-Electron G Factor, Krzysztof Pachucki, Ulrich D. Jentschura, Vladimir A. Yerokhin

Physics Faculty Research & Creative Works

Within a systematic approach based on nonrelativistic quantum electrodynamics, we derive the one-loop self-energy correction of order α ( Z α )4 to the bound-electron g factor. In combination with numerical data, this analytic result improves theoretical predictions for the self-energy correction for carbon and oxygen by an order of magnitude. Basing on one-loop calculations, we obtain the logarithmic two-loop contribution of order α2 ( Z α )4 ln [ ( Z α )- 2 ] and the dominant part of the corresponding constant term. The results obtained improve the accuracy of the theoretical predictions for …

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Fully Differential Three-Dimensional Angular Distributions Of Electrons Ionized In Ion-Atom Collisions, Michael Schulz, R. Moshammer, Daniel Fischer, Ahmad Hasan, N. V. Maydanyuk, J. D. Ullrich Oct 2004

Fully Differential Three-Dimensional Angular Distributions Of Electrons Ionized In Ion-Atom Collisions, Michael Schulz, R. Moshammer, Daniel Fischer, Ahmad Hasan, N. V. Maydanyuk, J. D. Ullrich

Physics Faculty Research & Creative Works

We compare experimental fully differential three-dimensional angular distributions of electrons ionized from He in collision with ionic projectiles covering a broad range of perturbations (0.065 to 4.4). Even at very small perturbations clear signatures of higher-order contributions are observable. At large perturbations, such contributions become even dominant, especially those involving the post-collision interaction between the outgoing projectile and the ionized electron. Our results show that single ionization is not nearly as well understood as was assumed previously.

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Nucleation Near The Spinodal: Limitations Of Mean Field Density Functional Theory, Gerald Wilemski, Jin-Song Li Oct 2004

Nucleation Near The Spinodal: Limitations Of Mean Field Density Functional Theory, Gerald Wilemski, Jin-Song Li

Physics Faculty Research & Creative Works

We investigate the diverging size of the critical nucleus near the spinodal using the gradient theory ~GT! of van der Waals and Cahn and Hilliard and mean field density functional theory ~MFDFT!. As is well known, GT predicts that at the spinodal the free energy barrier to nucleation vanishes while the radius of the critical fluctuation diverges. We show numerically that the scaling behavior found by Cahn and Hilliard for these quantities holds quantitatively for both GT and MFDFT. We also show that the excess number of molecules Dg satisfies Cahn-Hilliard scaling near the spinodal and is consistent with the …

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Breakup Of H₂ In Singly Ionizing Collisions With Fast Protons: Channel-Selective Low-Energy Electron Spectra, Christina Dimopoulou, Robert Moshammer, Daniel Fischer, C. Hohr, Alexander Dorn, Pablo Daniel Fainstein, Jose R. Crespo Lopez-Urrutia, Claus Dieter Schroter, Holger Kollmus, Rido Mann, Siegbert Hagmann, Joachim Hermann Ullrich Sep 2004

Breakup Of H₂ In Singly Ionizing Collisions With Fast Protons: Channel-Selective Low-Energy Electron Spectra, Christina Dimopoulou, Robert Moshammer, Daniel Fischer, C. Hohr, Alexander Dorn, Pablo Daniel Fainstein, Jose R. Crespo Lopez-Urrutia, Claus Dieter Schroter, Holger Kollmus, Rido Mann, Siegbert Hagmann, Joachim Hermann Ullrich

Physics Faculty Research & Creative Works

The dissociation of H2 in singly ionizing collisions with fast protons was analyzed using channel-selective low-energy electron spectra. Dissociative and nondissociative single ionization of H2 by 6MeV proton impact was described in a kinematically by determining momentum vectors of electron and H+ fragment of H2+ target ion, respectively. The electron spectra exhibited role of autoionization of doubly and singly excited states of H2. The doubly and singly excited states of H2 involve coupling between electronic and nuclear motion of molecule.

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Electronic Structure Properties And Bcs Superconductivity In Ss-Pyrochlore Oxides: KoSS₂O₆, Rolando Saniz, Julia E. Medvedeva, Lin Hui Ye, Tatsuya Shishidou, Arthur J. Freeman Sep 2004

Electronic Structure Properties And Bcs Superconductivity In Ss-Pyrochlore Oxides: KoSS₂O₆, Rolando Saniz, Julia E. Medvedeva, Lin Hui Ye, Tatsuya Shishidou, Arthur J. Freeman

Physics Faculty Research & Creative Works

We report a first-principles density-functional calculation of the electronic structure and properties of the recently discovered superconducting β-pyrochlore oxide KOs2O6. We find that the electronic structure near the Fermi energy EF is dominated by strongly hybridized Os 5d and O 2p states. A van Hove singularity very close to Ef leads to a relatively large density of states at EF, and the Fermi surface exhibits strong nesting along several directions. These features could provide the scattering processes leading to the observed anomalous temperature dependence of the resistivity and to the rather large …

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Electron Loss From 1.4-Mev / U U4,6,10+ Ions Colliding With Ne, N₂, And Ar Targets, Robert D. Dubois, Antonio C. Santos, Th. Stohlker, F. Bosch, A. Brauning-Demian, A. Gumberidze, S. Hagmann, C. Kozhuharov, R. Mann, A. Orsic Muthig, U. Spillman, S. Tachenov, W. Bart, L. Dahl, B. Franzke, J. Glatz, L. Groning, S. Richter, D. Wilms, K. Ullmann, O. Jagutzki Sep 2004

Electron Loss From 1.4-Mev / U U4,6,10+ Ions Colliding With Ne, N₂, And Ar Targets, Robert D. Dubois, Antonio C. Santos, Th. Stohlker, F. Bosch, A. Brauning-Demian, A. Gumberidze, S. Hagmann, C. Kozhuharov, R. Mann, A. Orsic Muthig, U. Spillman, S. Tachenov, W. Bart, L. Dahl, B. Franzke, J. Glatz, L. Groning, S. Richter, D. Wilms, K. Ullmann, O. Jagutzki

Physics Faculty Research & Creative Works

Absolute, total, single- and multiple-electron-loss cross sections are measured for 1.4-MeV / u U4,6,10+ ions colliding with neon and argon atoms and nitrogen molecules. It is found that the cross sections all have the same dependence on the number of electrons lost and that multiplying the cross sections by the initial number of electrons in the 6s, 6p, and 5f shells yields good agreement between the different projectiles. By combining the present data with previous measurements made at the same velocity, it is shown that the scaled cross sections slowly decrease in magnitude for …

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Exotic Versus Conventional Scaling And Universality In A Disordered Bilayer Quantum Heisenberg Antiferromagnet, Rastko Sknepnek, Thomas Vojta, Matthias Vojta Aug 2004

Exotic Versus Conventional Scaling And Universality In A Disordered Bilayer Quantum Heisenberg Antiferromagnet, Rastko Sknepnek, Thomas Vojta, Matthias Vojta

Physics Faculty Research & Creative Works

We present Monte Carlo simulations of a two-dimensional bilayer quantum Heisenberg antiferromagnet with random dimer dilution. In contrast with exotic scaling scenarios found in other random quantum systems, the quantum phase transition in this system is characterized by a finite-disorder fixed point with power-law scaling. After accounting for corrections to scaling, with a leading irrelevant exponent of ω ≈ 0.48, we find universal critical exponents z = 1.310(6) and ν = 1.16(3). We discuss the consequences of these findings and suggest new experiments.

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Broadening Of A Nonequilibrium Phase Transition By Extended Structural Defects, Thomas Vojta Aug 2004

Broadening Of A Nonequilibrium Phase Transition By Extended Structural Defects, Thomas Vojta

Physics Faculty Research & Creative Works

We study the effects of quenched extended impurities on nonequilibrium phase transitions in the directed percolation universality class. We show that these impurities have a dramatic effect: they completely destroy the sharp phase transition by smearing. This is caused by rare strongly coupled spatial regions which can undergo the phase transition independently from the bulk system. We use extremal statistics to determine the stationary state as well as the dynamics in the tail of the smeared transition, and we illustrate the results by computer simulations.

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