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Full-Text Articles in Physics

Slow Convergence Of The Born Approximation For Electron-Atom Ionization, Stephenie J. Jones, Don H. Madison Dec 2002

Slow Convergence Of The Born Approximation For Electron-Atom Ionization, Stephenie J. Jones, Don H. Madison

Physics Faculty Research & Creative Works

It is usually assumed that the first-Born approximation for electron-atom ionization becomes valid for the fully differential cross section at sufficiently high impact energies, at least for asymmetric collisions where the projectile suffers only a small energy loss and is scattered by a small angle. Here we investigate this assumption quantitatively for ionization of hydrogen atoms. We find that convergence of the Born approximation to the correct nonrelativistic result is generally achieved only at energies where relativistic effects start to become important. Consequently, the assumption that the Born approximation becomes valid for high energy is inaccurate, since by the time …


Two-Dimensional Ds/Cft Correspondence, Mariano Cadoni, P. Carta, Marco Cavaglia, S. Mignemi Sep 2002

Two-Dimensional Ds/Cft Correspondence, Mariano Cadoni, P. Carta, Marco Cavaglia, S. Mignemi

Physics Faculty Research & Creative Works

We investigate de Sitter/conformal field theory (dS/CFT) correspondence in two dimensions. We define the conserved mass of de Sitter spacetime and formulate the correspondence along the lines of the anti–de Sitter/conformal field theory duality. The asymptotic symmetry group, mass, and central charge of de Sitter spacetime are equal to those of anti–de Sitter spacetime. The entropy of two-dimensional de Sitter spacetime is evaluated by applying the Cardy formula. We calculate the boundary correlators induced by the propagation of the dilaton in two-dimensional de Sitter space. Although the dilaton is a tachyonic perturbation in the bulk, boundary conformal correlators have a …


Double-Logarithmic Two-Loop Self-Energy Corrections To The Lamb Shift, Ulrich D. Jentschura, I. Nandori Aug 2002

Double-Logarithmic Two-Loop Self-Energy Corrections To The Lamb Shift, Ulrich D. Jentschura, I. Nandori

Physics Faculty Research & Creative Works

Self-energy corrections involving logarithms of the parameter Z α can often be derived within a simplified approach, avoiding calculational difficulties typical of the problematic nonlogarithmic corrections (as customary in bound-state quantum electrodynamics, we denote by Z the nuclear charge number, and by α the fine-structure constant). For some logarithmic corrections, it is sufficient to consider internal properties of the electron characterized by form factors. We provide a detailed derivation of related self-energy “potentials” that give rise to the logarithmic corrections; these potentials are local in coordinate space. We focus on the double-logarithmic two-loop coefficient B62 for P states and …


A Problematic Set Of Two-Loop Self-Energy Corrections, Ulrich D. Jentschura, Jorg Evers, Christoph H. Keitel, Krzysztof Pachucki Jul 2002

A Problematic Set Of Two-Loop Self-Energy Corrections, Ulrich D. Jentschura, Jorg Evers, Christoph H. Keitel, Krzysztof Pachucki

Physics Faculty Research & Creative Works

We investigate a specific set of two-loop self-energy corrections involving squared decay rates and point out that their interpretation is highly problematic. The corrections cannot be interpreted as radiative energy shifts in the usual sense. Some of the problematic corrections find a natural interpretation as radiative nonresonant corrections to the natural line shape. They cannot uniquely be associated with one and only one atomic level. While the problematic corrections are rather tiny when expressed in units of frequency (a few hertz for hydrogenic P levels) and do not affect the reliability of quantum electrodynamics at the current level of experimental …


Coincident Fragment Detection In Strong Field Photoionization And Dissociation Of H₂, Horst Rottke, Christoph Trump, M. Wittmann, Georg Korn, Wolfgang Sandner, Robert Moshammer, Alexander Dorn, Claus Dieter Schroter, Daniel Fischer, Jose R. Crespo Lopez-Urrutia, Paul B. Neumayer, J. Deipenwisch, C. Hohr, Bernold Feuerstein, Joachim Hermann Ullrich Jul 2002

Coincident Fragment Detection In Strong Field Photoionization And Dissociation Of H₂, Horst Rottke, Christoph Trump, M. Wittmann, Georg Korn, Wolfgang Sandner, Robert Moshammer, Alexander Dorn, Claus Dieter Schroter, Daniel Fischer, Jose R. Crespo Lopez-Urrutia, Paul B. Neumayer, J. Deipenwisch, C. Hohr, Bernold Feuerstein, Joachim Hermann Ullrich

Physics Faculty Research & Creative Works

An investigation of the correlated electronic and nuclear motion in fragmentation of H2 in 4 x 1014 W/cm2, fs laser pulses at 795 nm was presented using the electron-ion momentum spectroscopy. The channel selective electron energy distribution indicated assumption of a new H2 ionization mechanism to interpret the differences found in the fragmentation channels. The results showed that the momentum distribution of H+ ions in the dissociation channels H(1s) + H+ + e- and 2H+ + 2e- was independent of the kinetic energy of photoelectrons.


Mechanism For Carrier Velocity Saturation In Pure Organic Crystals, Vasudev M. Kenkre, Paul Ernest Parris Jun 2002

Mechanism For Carrier Velocity Saturation In Pure Organic Crystals, Vasudev M. Kenkre, Paul Ernest Parris

Physics Faculty Research & Creative Works

We present a possible mechanism for the saturation of the velocity of photoinjected charge carriers in pure organic crystals, the analysis having been motivated by observations in pentacene. The mechanism is based on the strong wave vector dependence of the damping coefficient appearing in the appropriate effective mass equation. Our primary goal is to explore this mechanism in general terms rather than to apply it to a specific system such as pentacene. Our analysis is based on a Fokker-Planck equation treatment. We indicate how the damping coeffcient is related to relaxation times in usual Boltzmann equations and how it can …


Differential Cross Sections And Cross-Section Ratios For The Electron-Impact Excitation Of The Neon 2p⁵3s Configuration, Murtadha A. Khakoo, James M. Wrkich, Mary Lu Larsen, G. Kleiban, Isik Kanik, Sandor Trajmar, Michael J. Brunger, P. J O Teubner, Albert Crowe, Christopher J. Fontes, Robert E H Clark, Vlado Zeman, Klaus Bartschat, Don H. Madison, Rajesh C. Srivastava, Allan Daniel Stauffer Jun 2002

Differential Cross Sections And Cross-Section Ratios For The Electron-Impact Excitation Of The Neon 2p⁵3s Configuration, Murtadha A. Khakoo, James M. Wrkich, Mary Lu Larsen, G. Kleiban, Isik Kanik, Sandor Trajmar, Michael J. Brunger, P. J O Teubner, Albert Crowe, Christopher J. Fontes, Robert E H Clark, Vlado Zeman, Klaus Bartschat, Don H. Madison, Rajesh C. Srivastava, Allan Daniel Stauffer

Physics Faculty Research & Creative Works

Electron-impact differential cross-section measurements for the excitation of the 2p53s configuration of Ne are reported. The Ne cross sections are obtained using experimental differential cross sections for the electron-impact excitation of the n = 2 levels of atomic hydrogen [Khakoo et al., Phys. Rev. A 61, 012701-1 (1999)], and existing experimental helium differential cross-section measurements, as calibration standards. These calibration measurements were made using the method of gas mixtures (Ne and H followed by Ne and He), in which the gas beam profiles of the mixed gases are found to be the same within our experimental errors. We …


Magnetic And Neutron Diffraction Studies On Prmnsb₂, Satish K. Malik, Zili Chu, Amish G. Joshi, Jinbo Yang, William B. Yelon, Qingsheng Cai, William Joseph James, Kishore Kamaraju May 2002

Magnetic And Neutron Diffraction Studies On Prmnsb₂, Satish K. Malik, Zili Chu, Amish G. Joshi, Jinbo Yang, William B. Yelon, Qingsheng Cai, William Joseph James, Kishore Kamaraju

Physics Faculty Research & Creative Works

Magnetic and neutron diffraction studies have been carried out on PrMnSb2. Magnetization data in the temperature range of 5-400 K show two magnetic transitions: one at ∼175 K attributed to the antiferromagnetic ordering of the Mn moments, and the other at ∼35 K possibly due to the antiferromagnetic ordering of the Pr moments. The magnetization-field isotherms at various temperatures are consistent with the above. Neutron diffraction data obtained at various temperatures can be fitted with a nearly antiferromagnetically coupled Mn moment of ∼3μB at 70 K. At 10 K, moments both on Mn (∼3.5μB) and …


Structure And Magnetic Properties Of The Mnbi Low Temperature Phase, Jinbo Yang, William B. Yelon, William Joseph James, Qingsheng Cai, Samit Roy, Naushad Ali May 2002

Structure And Magnetic Properties Of The Mnbi Low Temperature Phase, Jinbo Yang, William B. Yelon, William Joseph James, Qingsheng Cai, Samit Roy, Naushad Ali

Physics Faculty Research & Creative Works

High purity MnBi low temperature phase has been prepared and analyzed using magnetic measurements and neutron diffraction. The low-temperature phase of the MnBi alloy has a coercivity μ0iHc of 2.0 T at 400 K, and exhibits a positive temperature coefficient from 0 to at least 400 K. The neutron data refinement indicated that the Mn atom changes its spin direction from c axis above room temperature to nearly perpendicular to the c axis at 50 K. A canted magnetic structure has been observed below 200 K. The anisotropy field increases with increasing temperature which gives rise to …


Observation Of Novel Disordered Rhombohedral R₂Fe₁₇ (R=Rare Earth) Based Compounds, William B. Yelon, William Joseph James, Jinbo Yang, Kishore Kamaraju, Qingsheng Cai, Zili Chu, Satish K. Malik May 2002

Observation Of Novel Disordered Rhombohedral R₂Fe₁₇ (R=Rare Earth) Based Compounds, William B. Yelon, William Joseph James, Jinbo Yang, Kishore Kamaraju, Qingsheng Cai, Zili Chu, Satish K. Malik

Physics Faculty Research & Creative Works

A recent study of (SmGd)Fe 14Si3 showed an anomalous x-ray diffraction pattern [P. C. Ezekwenna, G. K. Marasinghe, J.-H. Nam, W. J. James, W. B. Yelon, M. Ellouze, and Ph. I'Héritier, J. Appl. Phys. 87, 6716 (2000)]. Although all observed peaks could be indexed to the rhombohedral 2:17 phase (R-3m) many lines were strongly reduced in intensity. Subsequently, the same phenomenon was seen in neutron diffraction patterns from Nd2-xDysFe17-ySiy alloys and conventional refinement using the rhombohedral phase left significant residuals. Fourier mapping revealed additional density along the c-axis chains of Fe dumbbells …


Magnetic And Mössbauer Studies On Oxygen Deficient Perovskite, La₀.₆Sr₀.₄Feo3-Δ, Jinbo Yang, William B. Yelon, William Joseph James, X.-D. Zhou, Yixiang Xie, Harlan U. Anderson, Zili Chu May 2002

Magnetic And Mössbauer Studies On Oxygen Deficient Perovskite, La₀.₆Sr₀.₄Feo3-Δ, Jinbo Yang, William B. Yelon, William Joseph James, X.-D. Zhou, Yixiang Xie, Harlan U. Anderson, Zili Chu

Physics Faculty Research & Creative Works

Samples of La0.6Sr0.4Fe3-δ with varying oxygen vacancy contents were prepared by heating them in different gas flows. Magnetization measurement showed that samples with low oxygen vacancies have a magnetic ordering temperature in the range of 300-325 K while those with 9%-12% oxygen vacancies have a magnetic ordering temperature of 800 K and higher. Mössbauer spectra at 300 K exhibit paramagnetic or weak magnetic characteristics for the N2, O2, and air-quenched samples, whereas an average hyperfine field of 52 T is found for the CO/CO2 reduced samples. The heat treatment in …


Magnetic Properties Of Iron-Rich Mixed Rare-Earth Sm₂-ₓtbₓfe17-YSiY Compounds, Kishore Kamaraju, Jinbo Yang, William B. Yelon, Oran Allan Pringle, William Joseph James, Ph. I'Héritier May 2002

Magnetic Properties Of Iron-Rich Mixed Rare-Earth Sm₂-ₓtbₓfe17-YSiY Compounds, Kishore Kamaraju, Jinbo Yang, William B. Yelon, Oran Allan Pringle, William Joseph James, Ph. I'Héritier

Physics Faculty Research & Creative Works

A series of Sm2-xTbxFe17-ySiy solid solutions with x = 0, 1, and 1.5 and y = 1, 2, and 3 were prepared by induction melting stoichiometric amounts of high purity elements. The x-ray diffraction data confirm that the postannealed samples are 2:17 intermetallics of the R3 space group. The lattice parameters and the unit cell volumes were calculated using a modified Rietveld program. The fitted intensities showed behavior related to a disordered rhombohedral structure as inferred from neutron data. It was observed that for a particular y, the unit cell volume decreased almost …


Possible Ordering Of Ru And Cu In The Charge-Reservoir Of Magneto-Superconductor Rusr₂Gdcu₂O₈ (Ru-1212): Magnetic, Transport, And Tem Microstructural Studies, V. P. S. Awana, S. Ichihara, J. Nakamura, M. Karppinen, H. Yamauchi, Jinbo Yang, William B. Yelon, William Joseph James, Satish K. Malik May 2002

Possible Ordering Of Ru And Cu In The Charge-Reservoir Of Magneto-Superconductor Rusr₂Gdcu₂O₈ (Ru-1212): Magnetic, Transport, And Tem Microstructural Studies, V. P. S. Awana, S. Ichihara, J. Nakamura, M. Karppinen, H. Yamauchi, Jinbo Yang, William B. Yelon, William Joseph James, Satish K. Malik

Physics Faculty Research & Creative Works

Magnetization vs temperature behavior of RuSr2GdCu2O8-δ (Ru-1212) measured in an field of 5 Oe, shows a clear branching of zero-field-cooled (ZFC) and field-cooled (FC) curves around 140 K, a cusp at 135 K, and a diamagnetic transition around 20 K (in the ZFC branch). The cusp at 135 K is due to the antiferromagnetic ordering of the Ru moments. The magnetization-field isotherms, below 50 K, show a nonlinear contribution from a ferromagnetic component. The resistance vs temperature behavior of the compound, in applied fields of 0, 3, and 7 T, confirms that the sample is …


Saturation Of Charge Carrier Velocity With Increasing Electric Fields: Theoretical Investigations For Pure Organic Crystals, Vasudev M. Kenkre, Paul Ernest Parris May 2002

Saturation Of Charge Carrier Velocity With Increasing Electric Fields: Theoretical Investigations For Pure Organic Crystals, Vasudev M. Kenkre, Paul Ernest Parris

Physics Faculty Research & Creative Works

Should one expect injected charge carrier velocities to saturate as the applied electric field is increased simply because of the nonparabolicity of bands? Does the apparent saturation observed in recent experiments in hydrocarbon crystals signify, as supposed in some current interpretations of the data, that the carrier motion is coherent or bandlike and that the disappearance of the saturation at higher temperatures is indicative of a crossover from coherent to incoherent motion? These questions are addressed with the help of general theoretical investigations involving Drude-like considerations, quantum kinetic equations, and a Fokker-Planck analysis.


Orbital Ordering In Paramagnetic Lamno₃ And Kcuf₃, Julia E. Medvedeva, Michael A. Korotin, Vladimir I. Anisimov, Arthur J. Freeman May 2002

Orbital Ordering In Paramagnetic Lamno₃ And Kcuf₃, Julia E. Medvedeva, Michael A. Korotin, Vladimir I. Anisimov, Arthur J. Freeman

Physics Faculty Research & Creative Works

Ab initio studies of the stability of orbital ordering, its coupling to magnetic structure and its possible origins (electron-phonon and/or electron-electron interactions) are reported for two perovskite systems, LaMnO3 and KCuF3. We present an average spin state calculational scheme that allowed us to treat a paramagnetic state and to succesfully describe the experimental magnetic or orbital phase diagram of both LaMnO3 and KCuF3 in crystal structures when the Jahn-Teller distortions are neglected. Hence, we conclude that the orbital ordering in both compounds is purely electronic in origin.


Doubly Differential Electron-Emission Spectra In Single And Multiple Ionization Of Noble-Gas Atoms By Fast Highly-Charged-Ion Impact, Tom Kirchner, Laszlo Gulyas, Robert Moshammer, Michael Schulz, Joachim Hermann Ullrich Apr 2002

Doubly Differential Electron-Emission Spectra In Single And Multiple Ionization Of Noble-Gas Atoms By Fast Highly-Charged-Ion Impact, Tom Kirchner, Laszlo Gulyas, Robert Moshammer, Michael Schulz, Joachim Hermann Ullrich

Physics Faculty Research & Creative Works

Low-energy electron emission spectra are studied in collisions of 3.6 MeV/amu Au53+ ions with neon and argon atoms for well-defined degrees of target ionization. We calculate doubly differential cross sections as functions of the recoil-ion charge state in the continuum-distorted-wave with eikonal initial-state approximation using a binomial analysis of the total and differential ionization probabilities, and compare them with the present and with previously published experimental data. Very good agreement is found for the single-ionization spectra and for double ionization of neon, while some discrepancies are observed in the spectra for double and triple ionization of argon.


Local Versus Nonlocal Order-Parameter Field Theories For Quantum Phase Transitions, Dietrich Belitz, Theodore R. Kirkpatrick, Thomas Vojta Apr 2002

Local Versus Nonlocal Order-Parameter Field Theories For Quantum Phase Transitions, Dietrich Belitz, Theodore R. Kirkpatrick, Thomas Vojta

Physics Faculty Research & Creative Works

General conditions are formulated that allow us to determine which quantum phase transitions in itinerant electron systems can be described by a local Landau-Ginzburg-Wilson (LGW) theory solely in terms of the order parameter. A crucial question is the degree to which the order parameter fluctuations couple to other soft modes. Three general classes of zero-wave-number order parameters, in the particle-hole spin-singlet and spin-triplet channels and in the particle-particle channel, respectively, are considered. It is shown that the particle-hole spin-singlet class does allow for a local LGW theory, while the other two classes do not. The implications of this result for …


Electron-Impact Excitation From The (4p⁵5s) Metastable States Of Krypton, Arati K. Dasgupta, Klaus Bartschat, D. Vaid, Alexei N. Grum-Grzhimailo, Don H. Madison, Milan Blaha, John L. Giuliani Apr 2002

Electron-Impact Excitation From The (4p⁵5s) Metastable States Of Krypton, Arati K. Dasgupta, Klaus Bartschat, D. Vaid, Alexei N. Grum-Grzhimailo, Don H. Madison, Milan Blaha, John L. Giuliani

Physics Faculty Research & Creative Works

Theoretical results from multistate semirelativistic Breit-Pauli R-matrix calculations and two first-order distorted-wave calculations are presented for electron-impact excitation of krypton from the (4p55s) J = 0,2 metastable states to the (4p55s) and (4p55p) manifolds. Except for a few cases, in which the method to account for relativistic effects becomes surprisingly critical, fair overall agreement between the predictions from the various theoretical models is achieved for intermediate and high energies. However, significant discrepancies remain with the few available experimental data.


Simultaneous Projectile-Target Ionization: A Novel Approach To (E, 2e) Experiments On Ions, Holger Kollmus, Robert Moshammer, Ronald E. Olson, Siegbert Hagmann, Michael Schulz, Joachim Hermann Ullrich Feb 2002

Simultaneous Projectile-Target Ionization: A Novel Approach To (E, 2e) Experiments On Ions, Holger Kollmus, Robert Moshammer, Ronald E. Olson, Siegbert Hagmann, Michael Schulz, Joachim Hermann Ullrich

Physics Faculty Research & Creative Works

A kinematically complete experiment for simultaneous ionization of a projectile and target has been performed for 3.6 MeV/u C2+ on He collisions measuring the final vector momenta of the He1+ recoil ion and of two electrons (projectile, target) in coincidence with the emerging C3+ projectile. The feasibility of an event-by-event separation of the various reaction channels, among them the ionization of C2+ by the interaction with a quasifree target electron, is demonstrated in agreement with six-body classical trajectory Monte Carlo calculations, paving the way to kinematically complete electron-ion scattering experiments.


Electric Field Gradients In S-, P-, And D-Metal Diborides And The Effect Of Pressure On The Band Structure And TC In Mgb₂, Nadezhda I. Medvedeva, Alexander L. Ivanovskii, Julia E. Medvedeva, Arthur J. Freeman, Dmitrii L. Novikov Feb 2002

Electric Field Gradients In S-, P-, And D-Metal Diborides And The Effect Of Pressure On The Band Structure And TC In Mgb₂, Nadezhda I. Medvedeva, Alexander L. Ivanovskii, Julia E. Medvedeva, Arthur J. Freeman, Dmitrii L. Novikov

Physics Faculty Research & Creative Works

Results of full-potential linear muffin-tin orbital generalized gradient approximation calculations of the band structure and boron electric field gradients (EFG's) for the new medium-Tc superconductor MgB2 and related diborides MB2, M = Be, Al, Sc, Ti, V, Cr, Mo, and Ta are reported. The boron EFG variations are found to be related to specific features of their band structure and particularly to the M-B hybridization. The strong charge anisotropy at the B site in MgB2 is completely defined by the valence electrons - a property which sets MgB2 apart from other diborides. The boron …


Comment On "A Convergent Series For The Qed Effective Action", Darrell R. Lamm, Sree Ram Valluri, Ulrich D. Jentschura, Ernst Joachim Weniger Feb 2002

Comment On "A Convergent Series For The Qed Effective Action", Darrell R. Lamm, Sree Ram Valluri, Ulrich D. Jentschura, Ernst Joachim Weniger

Physics Faculty Research & Creative Works

No abstract provided.


Effect Of Oxygen Vacancies On The Magnetic Structure Of The La₀.₆Sr₀.₄Feo3-Δ Perovskite: A Neutron Diffraction Study, Zili Chu, William B. Yelon, Jinbo Yang, William Joseph James, Harlan U. Anderson, Yixiang Xie, Satish K. Malik Jan 2002

Effect Of Oxygen Vacancies On The Magnetic Structure Of The La₀.₆Sr₀.₄Feo3-Δ Perovskite: A Neutron Diffraction Study, Zili Chu, William B. Yelon, Jinbo Yang, William Joseph James, Harlan U. Anderson, Yixiang Xie, Satish K. Malik

Physics Faculty Research & Creative Works

Magnetic interactions in perovskite compounds of the type La1-xSrxMO3-δ (M=3d transition such as Mn and Fe) are presumed to arise through a super exchange between 3d electrons of the magnetic ions via oxygen orbitals. The magnetic structure of La0.6Sr0.4FeO3-δ has been studied with neutron diffraction. Oxygen vacancies were created by annealing samples under various gases including N2, air and mixtures of CO/CO2. All La0.6Sr0.4FeO3-δ compounds maintain the rhombohedral structure (space group R3̄c). The air- or oxygen-annealed samples have almost no oxygen …


Quantum Gravitational Corrections To Black Hole Geometries, Marco Cavaglia, A. Fabbri Jan 2002

Quantum Gravitational Corrections To Black Hole Geometries, Marco Cavaglia, A. Fabbri

Physics Faculty Research & Creative Works

We calculate perturbative quantum gravity corrections to eternal two-dimensional black holes. We estimate the leading corrections to the AdS2 black hole entropy and determine the quantum modification of N-dimensional Schwarzschild spacetime.


Large Enhancement Of Spontaneous Emission Rates Of Inas Quantum Dots In Gaas Microdisks, W. Fang, Alexey Yamilov, Yinfa Ma, J. Y. Xu, S. T. Ho, G. S. Solomon, Hui Cao Jan 2002

Large Enhancement Of Spontaneous Emission Rates Of Inas Quantum Dots In Gaas Microdisks, W. Fang, Alexey Yamilov, Yinfa Ma, J. Y. Xu, S. T. Ho, G. S. Solomon, Hui Cao

Physics Faculty Research & Creative Works

Control of spontaneous emission in a microcavity has many important applications, e.g. improvement of the efficiency of light emitting devices. InAs quantum dots (QDs) embedded in microdisks are ideal systems for spontaneous emission control. The whispering gallery (WG) modes of microdisks have low volume and high quality factor. The homogeneous linewidth of InAs quantum dots is smaller than the spectral width of WG modes. Thus, a large enhancement of the spontaneous emission rates should be expected for QDs coupled to WG modes. However, large inhomogeneous broadening of the QD energy levels and random spatial distribution of the QDs in a …