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Full-Text Articles in Physics

Neutron Diffraction And Mössbauer Effect Study Of Several Nd2Fe17-XAlX Solid Solutions, William B. Yelon, H. Xie, Gary J. Long, Oran Allan Pringle, Fernande Grandjean, K. H. J. Buschow Nov 1994

Neutron Diffraction And Mössbauer Effect Study Of Several Nd2Fe17-XAlX Solid Solutions, William B. Yelon, H. Xie, Gary J. Long, Oran Allan Pringle, Fernande Grandjean, K. H. J. Buschow

Physics Faculty Research & Creative Works

Aluminum-substituted Nd2Fe17-xAlx solid solutions with x=2.30, 4.44, 6.13, 8.00, and 9.40 have been studied by neutron diffraction and Mössbauer spectroscopy. A Rietveld analysis of the neutron scattering indicates that the aluminum atoms have a high initial affinity for the 18h site, show a high affinity for the 6c site at high aluminum concentrations, and are absent from the 9d site at all aluminum concentrations. The Mössbauer spectra show a maximum in both the maximum and weighted average hyperfine field for x≅2. This indicates that the lattice expansion which ...


Comment On "Critical Behavior Of The Coulomb Glass", Thomas Vojta, Michael Schreiber Nov 1994

Comment On "Critical Behavior Of The Coulomb Glass", Thomas Vojta, Michael Schreiber

Physics Faculty Research & Creative Works

No abstract provided.


Cosmological And Wormhole Solutions In Low-Energy Effective String Theory, Mariano Cadoni, Marco Cavaglia Nov 1994

Cosmological And Wormhole Solutions In Low-Energy Effective String Theory, Mariano Cadoni, Marco Cavaglia

Physics Faculty Research & Creative Works

We derive and study a class of cosmological and wormhole solutions of low-energy effective string field theory. We consider a general four-dimensional string effective action where moduli of the compactified manifold and the electromagnetic field are present. The cosmological solutions of the two-dimensional effective theory obtained by dimensional reduction of the former are discussed. In particular we demonstrate that the two-dimensional theory possesses a scale-factor duality invariance. Eucidean four-dimensional instantons describing the nucleation of the baby universes are found and the probability amplitude for the nucleation process given.


Quantum Electromagnetic Wormholes And Geometrical Description Of The Electric Charge, Marco Cavaglia Oct 1994

Quantum Electromagnetic Wormholes And Geometrical Description Of The Electric Charge, Marco Cavaglia

Physics Faculty Research & Creative Works

I present and discuss a class of solutions of the Wheeler-DeWitt equation describing wormholes generated by coupling of gravity to the electromagnetic field for Kantowski-Sachs and Bianchi type-I spacetimes. Since the electronic charge can be viewed as electric lines of force trapped in a finite region of spacetime, these solutions can be interpreted as the quantum corresponding to the Einstein-Rosen-Misner-Wheeler electromagnetic geon.


Neutron-Diffraction And Mössbauer Effect Study Of The Tb2Fe17-XAlX Solid Solutions, Gaya Kanishka Marasinghe, Sanjay R. Mishra, Oran Allan Pringle, Gary J. Long, Zhong Bo Hu, William B. Yelon, Fernande Grandjean, D. P. Middleton, K. H. J. Buschow Sep 1994

Neutron-Diffraction And Mössbauer Effect Study Of The Tb2Fe17-XAlX Solid Solutions, Gaya Kanishka Marasinghe, Sanjay R. Mishra, Oran Allan Pringle, Gary J. Long, Zhong Bo Hu, William B. Yelon, Fernande Grandjean, D. P. Middleton, K. H. J. Buschow

Physics Faculty Research & Creative Works

The magnetic properties of a series of Tb2Fe17-xAlx solid solutions, with nominal x compositions of 0, 2, 3, 4, 5, 6, 7, and 8, have been studied by neutron diffraction and Mössbauer spectroscopy. Neutron-diffraction data indicate that the compounds all crystallize with the Th2Zn17 structure and that the aluminum atoms are excluded from the 9d site and show a distinct preference for the 6c site only for an aluminum content greater than 6. The unit-cell volume increases by approximately 1% per aluminum atom substituted in the formula unit. The magnetic moment per ...


Differential Double Excitation Cross Sections In Proton-Helium Collisions Studied By Energy-Loss Spectroscopy, W. T. Htwe, Tibor Vajnai, M. Barnhart, A. D. Gaus, Michael Schulz Sep 1994

Differential Double Excitation Cross Sections In Proton-Helium Collisions Studied By Energy-Loss Spectroscopy, W. T. Htwe, Tibor Vajnai, M. Barnhart, A. D. Gaus, Michael Schulz

Physics Faculty Research & Creative Works

We have measured ion energy-loss spectra for 150 keV proton-helium collisions as a function of the projectile scattering angle. From the data we obtained double excitation cross sections differential in the proton scattering angle as well as the ratios of both double excitation and single ionization to single excitation. In these ratios pronounced peak structures are observed at about 0.7 mrad. Two alternative interpretations of these peak structures are offered: They may be due to binary collisions between the projectile and the target electrons, or they could be a manifestation of an interference between different transition amplitudes leading to ...


Landau-Zener Treatment Of Intensity-Tuned Multiphoton Resonances Of Potassium, J. Greg Story, D. I. Duncan, Thomas F. Gallagher Aug 1994

Landau-Zener Treatment Of Intensity-Tuned Multiphoton Resonances Of Potassium, J. Greg Story, D. I. Duncan, Thomas F. Gallagher

Physics Faculty Research & Creative Works

When exposed to intense light of ~580 nm, the ground state of K shifts up in energy, passing through two photon resonances with Rydberg states, and finally crossing the two-photon ionization limit. We have used laser pulses of varying duration to study the nature of the population transfer from the ground state to the excited state due to the intensity-tuned resonances encountered during the rising edge of the pulse. A dynamic Floquet approach in which the resonances are treated as avoided crossings of the Floquet energy levels is used to model the population transfer and gives excellent agreement with the ...


Effect Of Carrier Gas Pressure On Condensation In A Supersonic Nozzle, Barbara Ellen Wyslouzil, Gerald Wilemski, M. G. Beals, Michael B. Frish Aug 1994

Effect Of Carrier Gas Pressure On Condensation In A Supersonic Nozzle, Barbara Ellen Wyslouzil, Gerald Wilemski, M. G. Beals, Michael B. Frish

Physics Faculty Research & Creative Works

Supersonic nozzle experiments were performed with a fixed water or ethanol vapor pressure and varying amounts of nitrogen to test the hypothesis that carrier gas pressure affects the onset of condensation. Such an effect might occur if nonisothermal nucleation were important under conditions of excess carrier gas in the atmospheric pressure range, as has been suggested by Ford and Clement [J. Phys. A 22, 4007 (1989)]. Although a small increase was observed in the condensation onset temperature as the stagnation pressure was reduced from 3 to 0.5 atm, these changes cannot be attributed to any nonisothermal effects. The pulsed ...


Neutron-Diffraction Study Of Γ-Fe At High Pressure, Oran Allan Pringle, Rafael Calvo, R. E. Parra Aug 1994

Neutron-Diffraction Study Of Γ-Fe At High Pressure, Oran Allan Pringle, Rafael Calvo, R. E. Parra

Physics Faculty Research & Creative Works

Face-centered-cubic iron (γ-Fe) coherently precipitated in a single crystal of Cu has been studied by neutron diffraction at high pressures up to 0.9 GPa and in the temperature range from about 5 to 110 K. It undergoes antiferromagnetic ordering at 67±2 K at ambient pressure. The ordering temperature TN monotonically decreases with increasing pressure following a relation TN(K)=67-12.8p - 11.1p2 - 6.17p3, where p is in units of GPa. The change of TN in γ-Fe is discussed in terms of the volume change caused by pressurization or by alloying.


A Magnetic, Neutron Diffraction, And Mössbauer Spectral Study Of Nd2Fe15Ga2 And The Tb2Fe17-XGaX Solid Solutions, Zhong Bo Hu, William B. Yelon, Sanjay R. Mishra, Gary J. Long, Oran Allan Pringle, D. P. Middleton, K. H. J. Buschow, Fernande Grandjean Jul 1994

A Magnetic, Neutron Diffraction, And Mössbauer Spectral Study Of Nd2Fe15Ga2 And The Tb2Fe17-XGaX Solid Solutions, Zhong Bo Hu, William B. Yelon, Sanjay R. Mishra, Gary J. Long, Oran Allan Pringle, D. P. Middleton, K. H. J. Buschow, Fernande Grandjean

Physics Faculty Research & Creative Works

An x-ray diffraction study of the substitution of gallium in Tb 2Fe17 to form the Tb2Fe17-xGax solid solutions indicates that the compounds adopt the rhombohedral Th2Zn17 structure. The unit cell volume and the a-axis lattice parameter increase linearly with increasing gallium content. The c-axis lattice parameter increases linearly from x=0 to 6 and then decreases between x=7 and 8. Magnetic studies show the Curie temperature increases by ~150° above that of Tb2Fe17 to reach a maximum between x=3 and 4, and then decreases with ...


Critical Correlations And Susceptibilities In The Random-Field Spherical Model, Thomas Vojta, Michael Schreiber Jul 1994

Critical Correlations And Susceptibilities In The Random-Field Spherical Model, Thomas Vojta, Michael Schreiber

Physics Faculty Research & Creative Works

We investigate the behavior of the correlation function and the susceptibility of the random-field spherical model at the critical point. In particular we calculate the critical exponents η and η- describing the divergences of the susceptibility and its disconnected part, respectively. In the case of short-range interactions we obtain η=η-=0. For power-law interactions Uij~rij-σ we find η=1/2η-=D+2-σ (for D<σ


Differential Transfer Ionization Cross Sections For 50175-Kev Proton-Helium Collisions, S. W. Bross, S. M. Bonham, A. D. Gaus, Jerry Peacher, Tibor Vajnai, Michael Schulz, Horst Schmidt-Bocking Jul 1994

Differential Transfer Ionization Cross Sections For 50175-Kev Proton-Helium Collisions, S. W. Bross, S. M. Bonham, A. D. Gaus, Jerry Peacher, Tibor Vajnai, Michael Schulz, Horst Schmidt-Bocking

Physics Faculty Research & Creative Works

We have measured coincidences between neutralized projectiles and He recoil ions for 50175-keV proton-helium collisions. From the data we obtained transfer ionization (TI) cross sections differential in the projectile scattering angle. Laboratory scattering angles range from 0 to 2.0 mrad. The experimental method allowed separation of the postcollision charge states of the target atoms. The ratio of the cross sections for TI to the sum of TI and single capture, F, is presented as a function of projectile scattering angle. Comparison is made to previous measurements of this ratio where data is available. The differential cross sections are compared ...


Anisotropic Wormhole: Tunneling In Time And Space, Marco Cavaglia, Vittorio De Alfaro, Fernado De Felice Jun 1994

Anisotropic Wormhole: Tunneling In Time And Space, Marco Cavaglia, Vittorio De Alfaro, Fernado De Felice

Physics Faculty Research & Creative Works

We discuss the structure of a gravitational Euclidean instanton obtained through coupling of gravity to electromagnetism. This Euclidean solution can be interpreted as a tunneling to a hyperbolic space (baby universe) or alternatively as a static wormhole that joins two asymptotically flat spaces of a Reissner-Nordström type solution.


Channel Closing In Multiphoton Ionization Of Mg, J. Greg Story, D. I. Duncan, Thomas F. Gallagher May 1994

Channel Closing In Multiphoton Ionization Of Mg, J. Greg Story, D. I. Duncan, Thomas F. Gallagher

Physics Faculty Research & Creative Works

Experimental data are presented showing the channel closing of four-photon ionization of Mg. It is shown that, for circularly polarized light, the ionization versus intensity spectra exhibit sharp breaks from the normal I4 intensity dependence at the critical intensity where the channel closing occurs. Above the critical intensity, the population of Rydberg states which survives the laser pulse is observed. The residual Rydberg population is found to be greatly reduced for linearly polarized light due to the relatively large probability of ionization of the low-angular-momentum Rydberg states. The data are in good agreement with a model which includes averaging ...


Spin-Resolved (E,2e) Coincidences For Heavy Rare-Gas Targets, Stephenie J. Jones, Don H. Madison, G. Friedrich Hanne Apr 1994

Spin-Resolved (E,2e) Coincidences For Heavy Rare-Gas Targets, Stephenie J. Jones, Don H. Madison, G. Friedrich Hanne

Physics Faculty Research & Creative Works

It has been well established that the Coulomb force alone can produce spin-dependent effects for electron-impact excitation of heavy rare-gas atoms if the incident electrons are spin polarized and the final J state of the atom is resolved. This effect has become known as the fine-structure effect. Here we demonstrate that the same type of effect may be expected for electron-impact ionization.


Correlated Two-Electron Wave Functions Of Any Symmetry, Christopher Bottcher, David R. Schultz, Don H. Madison Mar 1994

Correlated Two-Electron Wave Functions Of Any Symmetry, Christopher Bottcher, David R. Schultz, Don H. Madison

Physics Faculty Research & Creative Works

Using a procedure originally due to Hylleraas, a convenient expansion in coupled spherical harmonics which terminates in a very small number of terms is applied to the treatment of fully correlated two-electron wave functions of any symmetry (total angular momentum, parity, and spin). Coupled equations satisfied by these wave functions are derived which are well adapted to computation and which we discretize on a numerical lattice utilizing the basis-spline collocation method. Use of this method which relies on very flexible basis functions is intended to facilitate subsequently the consideration of time-dependent rearrangments such as autoionization, photoionization, and electron-impact excitation and ...


Generalized Coulomb Gap In The Spherical Version Of A Lattice Model Of Disordered And Correlated Localized Particles, Thomas Vojta, Michael Schreiber Mar 1994

Generalized Coulomb Gap In The Spherical Version Of A Lattice Model Of Disordered And Correlated Localized Particles, Thomas Vojta, Michael Schreiber

Physics Faculty Research & Creative Works

We study the spherical version of a model of localized particles in a random potential which are subject to a power-law interaction Uij ~ rij. In the case of a repulsive interaction with σ = 1 the model is identical to the spherical version of the Coulomb-glass model of disordered localized electrons. The use of continuous variables instead of discrete occupation numbers of the sites renders the model exactly solvable. Analytic results are obtained for the free energy and for the single-particle density of states (DOS) as an example for single-particle properties. The zero-temperature DOS shows a hard gap ...


The Spatial Evolution Of Particles Diffusing In The Presence Of Randomly Placed Traps, David H. Dunlap, Randall A. Laviolette, Paul Ernest Parris Feb 1994

The Spatial Evolution Of Particles Diffusing In The Presence Of Randomly Placed Traps, David H. Dunlap, Randall A. Laviolette, Paul Ernest Parris

Physics Faculty Research & Creative Works

The evolution of a particle undergoing a continuous-time random walk in the presence of randomly placed imperfectly absorbing traps is studied. At long times, the spatial probability distribution becomes strongly localized in a sequence of trap-free regions. The subsequent intermittent transfer of the survival probability from small trap-free regions to larger trap-free regions is described as a time-directed variable range hopping among localized eigenstates in the Lifshitz tail. An asymptotic expression for the configurational average of the spatial distribution of surviving particles is obtained based on this description. The distribution is an exponential function of distance which expands superdiffusively, with ...