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Full-Text Articles in Physical Sciences and Mathematics

Neutron-Diffraction And Mössbauer-Effect Studies Of Pr₂(Fe₁₋ₓMnₓ)₁₄B, Oran Allan Pringle, Jie Fu, Gary J. Long, William Joseph James, Decai Xie, William B. Yelon, Fernande Grandjean Dec 1993

Neutron-Diffraction And Mössbauer-Effect Studies Of Pr₂(Fe₁₋ₓMnₓ)₁₄B, Oran Allan Pringle, Jie Fu, Gary J. Long, William Joseph James, Decai Xie, William B. Yelon, Fernande Grandjean

Physics Faculty Research & Creative Works

A neutron-diffraction investigation of a series of Pr2(Fe 1-xMnx)14B samples, with x values of 0.00, 0.11, 0.22, 0.30, and 0.35, reveals a preference for the manganese to occupy the 8j2 transition-metal site,the transition-metal site with the largest Wigner-Seitz cell volume. Similar site occupancies have been reported previously for Er2(Fe1-xMnx) 14B and Y2(Fe1-xMnx) 14B. An analysis of the 295-K Mössbauer spectrum of Pr2(Fe0.89Mn0.11)14B indicates that the internal ...


Large-Cell Renormalization-Group Approach To Long-Range Hopping On Energetically Disordered Lattices, B. D. Bookout, Paul Ernest Parris Nov 1993

Large-Cell Renormalization-Group Approach To Long-Range Hopping On Energetically Disordered Lattices, B. D. Bookout, Paul Ernest Parris

Physics Faculty Research & Creative Works

We describe an approach for computing the conductivity associated with long-range hopping on energetically disordered lattices. Using a numerically exact supercell procedure we compute the distribution ρL(γ) of block conductances γL associated with conducting cubes of edge length L that are randomly chosen from the disordered system of interest. This distribution of block conductances is then used in a self-consistent numerical calculation to obtain the renormalized bulk conductivity. The approach displays a surprisingly fast approach to the infinite-system limit, allowing finite-size effects to be minimized. In this paper we use this approach to study transport in a series of ...


Neutron-Diffraction And Mössbauer Effect Study Of The Preferential Silicon Site Occupation And Magnetic Structure Of Nd₂Fe₁₄₋ₓSiₓB, Gaya Kanishka Marasinghe, Oran Allan Pringle, Gary J. Long, William Joseph James, Dianne J. Xiao, J. L. Li, William B. Yelon, Fernande Grandjean Nov 1993

Neutron-Diffraction And Mössbauer Effect Study Of The Preferential Silicon Site Occupation And Magnetic Structure Of Nd₂Fe₁₄₋ₓSiₓB, Gaya Kanishka Marasinghe, Oran Allan Pringle, Gary J. Long, William Joseph James, Dianne J. Xiao, J. L. Li, William B. Yelon, Fernande Grandjean

Physics Faculty Research & Creative Works

A neutron-diffraction study of Nd2Fe14-xSixB has shown that silicon preferentially occupies the 4c site in the transition-metal sublattice in Nd2Fe14B. Silicon also exhibits a moderate preference for the 8j1 site, is almost excluded from the 16k2 site, and avoids the 16k1, 8j2, and 4e sites. The silicon site occupancy is correlated with a preference for a silicon atom to have rare-earth atoms in its coordination environment. The Mössbauer spectra of Nd2Fe14-xSixB have been fit with a model which takes into ...


Spatially Resolved Transitions To Autoionizing States, J. Greg Story, D. I. Duncan, Thomas F. Gallagher Nov 1993

Spatially Resolved Transitions To Autoionizing States, J. Greg Story, D. I. Duncan, Thomas F. Gallagher

Physics Faculty Research & Creative Works

We have observed transitions to autoionizing states in Mg using two short optical pulses. Mg atoms are initially prepared in a high lying Rydberg wave packet with the first ps laser. A second ps laser is then used to excite the inner electron, producing an autoionizing state. The dependence of the transition probability on the delay between the two lasers shows that when the second laser is tuned away from the ionic resonance, the inner electron can make a transition only when the Rydberg wave packet is near the core.


Simulation Of Bulk Silicon Crystals And Si(111) Surfaces With Application To A Study Of Fluorine Coverage Of The Surfaces, Chen K. Lutrus, T. Oshiro, Donald E. Hagen, Sung-Ho Suck Salk Nov 1993

Simulation Of Bulk Silicon Crystals And Si(111) Surfaces With Application To A Study Of Fluorine Coverage Of The Surfaces, Chen K. Lutrus, T. Oshiro, Donald E. Hagen, Sung-Ho Suck Salk

Physics Faculty Research & Creative Works

Computational efficiency for the simulation of bulk crystals and surfaces is highly desirable. In an effort to study semiconductor crystals, we present a self-consistent treatment for the simulation of silicon crystals and surfaces based on the combination of a siligen model and a semiempirical Hamiltonian method. An artificial atom called siligen is introduced for the application of the semiempirical method to finite-size silicon clusters. The calculated average bond energies for the saturated silicon clusters are between 2.045 and 2.568 eV, compared to the measured value of 2.31 eV. A simulated bulk silicon surface using siligens is introduced ...


Energy-Projected Effective-Medium Theory Of Long-Range Hopping On Energetically Disordered Lattices, Paul Ernest Parris, B. D. Bookout Oct 1993

Energy-Projected Effective-Medium Theory Of Long-Range Hopping On Energetically Disordered Lattices, Paul Ernest Parris, B. D. Bookout

Physics Faculty Research & Creative Works

We introduce energy-projected equations of motion to treat the diffusive transport of charge carriers that undergo long-range (i.e., greater than nearest-neighbor) hopping among the sites of an energetically disordered lattice. This approach leads naturally to an energy-projected effective-medium theory for treating such systems. Exact expressions for the diffusion constant associated with the energy-projected effective medium theory are obtained. Using the formalism in conjunction with what is normally a rather poor approximation, i.e., the virtual-crystal approximation, we are able to obtain the exact diffusion constant for the long-range symmetric-random-well problem. Effective-medium calculations and numerical simulations are presented for nearest-neighbor ...


Temperature And Supersaturation Dependent Nucleation Rates Of Heterogeneous Water By Molecular Cluster Model Calculation, Chen K. Lutrus, Donald E. Hagen, Sung-Ho Suck Salk Aug 1993

Temperature And Supersaturation Dependent Nucleation Rates Of Heterogeneous Water By Molecular Cluster Model Calculation, Chen K. Lutrus, Donald E. Hagen, Sung-Ho Suck Salk

Physics Faculty Research & Creative Works

A statistical mechanical method to evaluate the energy of formation of water clusters attached to a foreign particle surface is described, with the binding energy being evaluated on a molecular level, using semiempirical modified neglect of diatomic overlap (MNDO) theory. The model is applied to water nucleation on a silicon oxide surface. The binding energy contribution, which represents the energy of formation at T=0 K, is found to slightly (but not negligibly in the thermal sense) increase with the number of hydrogen bonds between the water cluster and the condensation nucleus whose surface is made of silicon oxide. An ...


Three-Body Distorted-Wave Born Approximation For Electron-Atom Ionization, Stephenie J. Jones, Don H. Madison, Achim Franz, P. L. Altick Jul 1993

Three-Body Distorted-Wave Born Approximation For Electron-Atom Ionization, Stephenie J. Jones, Don H. Madison, Achim Franz, P. L. Altick

Physics Faculty Research & Creative Works

We report a theoretical calculation for electron-impact ionization of atoms that includes short-range electron-atom effects for wave functions that satisfy the exact asymptotic boundary conditions. Results of this theory, which we label as the three-body distorted-wave Born approximation, are compared with experiment and other theories for ionization of hydrogen and helium for incident-electron energies from 4 to 18 times the ionization energy.


Long-Range Random Walks On Energetically Disordered Lattices, B. D. Bookout, Paul Ernest Parris Jul 1993

Long-Range Random Walks On Energetically Disordered Lattices, B. D. Bookout, Paul Ernest Parris

Physics Faculty Research & Creative Works

Although the master equation describing long-range random walks on an energetically disordered lattice is governed by a nonsymmetric transition matrix W, it may be mapped through a similarity transform onto an imaginary-time Schrödinger equation governed by a Hermitian (Hamiltonian) operator H0 having a nondegenerate ground state. Under this mapping the diffusion constant D can be expressed in terms of the exact ground state energy of operators that are infinitesimally perturbed from H0.


Connection Between Superelastic And Inelastic Electron-Atom Collisions Involving Polarized Collision Partners, Klaus Bartschat, Don H. Madison Jul 1993

Connection Between Superelastic And Inelastic Electron-Atom Collisions Involving Polarized Collision Partners, Klaus Bartschat, Don H. Madison

Physics Faculty Research & Creative Works

It is shown how the results of a recent experiment by Jiang, Zuo, Vuković, and Bederson [Phys. Rev. Lett. 68, 915 (1992)], who investigated low-energy electron scattering from laser-excited polarized sodium atoms in the initial (3p) 23/2 (F=3, MF=3) state, can be related to the inelastic 3S→3P transition involving initially unpolarized electron and atom beams. Hence, this method can provide an independent check of the traditional electron-scattering experiment with unpolarized beams.


Temporal Effects In Multiphoton Ionization Of Lithium, J. Greg Story, Thomas F. Gallagher Jun 1993

Temporal Effects In Multiphoton Ionization Of Lithium, J. Greg Story, Thomas F. Gallagher

Physics Faculty Research & Creative Works

The temporal effects of multiphoton ionization are investigated using a two-color method which determines both the time and intensity at which the process occurs. We show that the total ionization efficiency depends strongly on the time at which an atom makes a transition to an excited state during an intense laser pulse. This result clearly shows that the ac-Stark-shifted bound-state resonances not only enhance the cross section for photoionization, but that the excited bound states provide temporary storage states for the atomic population during the photoionization process.


Resonant Inhibition Of Multiphoton Ionization, J. Greg Story, D. I. Duncan, Thomas F. Gallagher May 1993

Resonant Inhibition Of Multiphoton Ionization, 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. Using laser pulses of 0.5 to 13 ps duration, we have shown experimentally that ionization occurs for short pulses, but for long pulses the population is diverted into the Rydberg states where some population survives the peak intensity of the pulse, in excellent agreement with a dynamic Floquet model.


A Neutron Diffraction And Mössbauer Effect Study Of The Magnetic Structure Of Y₂(Fe₁₋ₓMnₓ)₁₄B, Gaya Kanishka Marasinghe, Oran Allan Pringle, Gary J. Long, William Joseph James, William B. Yelon, Fernande Grandjean May 1993

A Neutron Diffraction And Mössbauer Effect Study Of The Magnetic Structure Of Y₂(Fe₁₋ₓMnₓ)₁₄B, Gaya Kanishka Marasinghe, Oran Allan Pringle, Gary J. Long, William Joseph James, William B. Yelon, Fernande Grandjean

Physics Faculty Research & Creative Works

The magnetic structures of a series of Y2(Fe 1-xMnx)14B samples, with x equal to 0.03, 0.10, 0.25, and 0.37, have been studied by powder neutron diffraction and Mössbauer spectroscopy. Y2(Fe0.97Mn 0.03)14B and Y2(Fe0.9Mn 0.1)14B are ferromagnetic at both 295 and 85 K, Y 2(Fe0.63Mn0.37)14B is paramagnetic at both 295 and 85 K, whereas Y2(Fe0.75Mn 0.25)14B is paramagnetic at ...


Monte Carlo Simulations Of Water-Ice Layers On A Model Silver Iodide Substrate: A Comparison With Bulk Ice Systems, James H. Taylor, Barbara N. Hale Apr 1993

Monte Carlo Simulations Of Water-Ice Layers On A Model Silver Iodide Substrate: A Comparison With Bulk Ice Systems, James H. Taylor, Barbara N. Hale

Physics Faculty Research & Creative Works

Two water layers adsorbed on a model silver iodide basal face are simulated at nine temperatures from 150 to 425 K using Monte Carlo methods. The periodic unit cell of 96 internally rigid water molecules (interacting via the revised central-force potentials) couples to the rigid-substrate atoms via effective pair potentials with Lennard-Jones short-range and Coulomb long-range terms. The distribution of molecules perpendicular to the substrate exhibits layering, and individual layer structure factors, dipole moments, and "pseudodiffusion" coefficients are calculated. A complex temperature dependence with the two layers taking on different solidlike, quasiliquid, or liquid properties at the same T is ...


Residual-Ion Orientation After Autoionization, G. T. Xu, Xiao Wang, J. Greg Story, William E. Cooke Apr 1993

Residual-Ion Orientation After Autoionization, G. T. Xu, Xiao Wang, J. Greg Story, William E. Cooke

Physics Faculty Research & Creative Works

We have measured the relative population in the two mj states of barium 6p1/2 ions produced by the autoionization of (6p3/2nd)J=MJ=3 states by detecting the helicity of the emitted fluorescence. These populations can be related to the relative branching ratio into the 6p1/2εd5/2 and 6p1/2εg7/2 continuum channels. Rydberg states with n=16-23 have similar branching ratios, producing approximately six times as much 6p1/2εd5/2 population as 6p1/2εg7/2 population.


Evolution Of Atomic Motion In An Intense Standing Wave, Jian Chen, J. Greg Story, Randall G. Hulet Mar 1993

Evolution Of Atomic Motion In An Intense Standing Wave, Jian Chen, J. Greg Story, Randall G. Hulet

Physics Faculty Research & Creative Works

We have investigated the effect of the dipole force and its fluctuation on the motion of Li atoms in an intense, one-dimensional, near-resonant standing light wave. The duration of the interaction of the atoms with the standing wave was varied from several tens of spontaneous-emission lifetimes to several hundreds. For a standing-wave frequency blue detuned from resonance, diffusive heating can dominate the time-averaged dissipative dipole force so that there is no steady-state momentum distribution. However, for sufficiently large blue detunings, the rate of diffusion is so slow that the resulting distribution approaches a quasisteady state. For red detunings, the diffusion ...


The Light Ion Lmf And Its Relevance To Ife, Ronald E. Olson, G. O. Allshouse, D. L. Cook, T. R. Lockner, M. G. Mazarakis, C. L. Olson, D. L. Smith Jan 1993

The Light Ion Lmf And Its Relevance To Ife, Ronald E. Olson, G. O. Allshouse, D. L. Cook, T. R. Lockner, M. G. Mazarakis, C. L. Olson, D. L. Smith

Physics Faculty Research & Creative Works

The inertial confinement fusion (ICF) program at Sandia National Laboratories (SNL) is directed toward validating light ions as an efficient driver for ICF defense and energy applications. The light ion laboratory microfusion facility (LMF) is envisioned as a facility in which high gain ICF targets could be developed and utilized in defense-related experiments. The relevance of LMF technology to eventual inertial fusion energy (IFE) applications is assessed via a comparison of LMF technologies with those projected in the Light Ion Beam Reactor Assessment (LIBRA) conceptual reactor design study


Trapping-To-Percolation Transition In The Hopping Diffusion Of Substitutionally Disordered Solids With A Binary Energy Distribution, Paul Ernest Parris, B. D. Bookout Jan 1993

Trapping-To-Percolation Transition In The Hopping Diffusion Of Substitutionally Disordered Solids With A Binary Energy Distribution, Paul Ernest Parris, B. D. Bookout

Physics Faculty Research & Creative Works

We consider charge carriers that undergo nearest-neighbor hopping among the sites of a binary random lattice, each site of which is associated with one of two possible energies E1 or E2. A general and recently observed feature of this problem not predicted by previous treatments of disordered hopping models is a crossover between trap-limited conduction and percolation. We introduce new energy-projected equations of motion whose solutions reveal the deep conductivity minimum associated with this phenomenon, and compare the results predicted to numerical simulations.


The Light-Ion Pulsed Power Induction Accelerator For The Laboratory Microfusion Facility (Lmf), Ronald E. Olson, J. Boyes, M. G. Mazarakis, D. L. Smith, L. F. Bennett, T. R. Lockner, J. W. Poukey Jan 1993

The Light-Ion Pulsed Power Induction Accelerator For The Laboratory Microfusion Facility (Lmf), Ronald E. Olson, J. Boyes, M. G. Mazarakis, D. L. Smith, L. F. Bennett, T. R. Lockner, J. W. Poukey

Physics Faculty Research & Creative Works

In order to initiate ignition and substantial energy yield from an inertial confinement fusion target (ICF), a light-ion pulse of ~700 TW peak power and 15-20 ns duration is required. The preconceptual design presented provides this power. The HERMES-III technology of linear inductive voltage addition in a self-magnetically insulated transmission line (MITL) is utilized to generate the 25-36 MV peak voltage needed for lithium ion beams. The 15-20 MA ion current is achieved by utilizing many accelerating modules in parallel. The lithium ion beams are produced in two-stage extraction diodes. To provide the two separate voltage pulses required by the ...


A Pulsed Power Design For The Linear Inductive Accelerator Modules For The Laboratory Microfusion Fa, Ronald E. Olson, D. L. Smith, M. G. Mazarakis, L. F. Bennett, T. R. Lockner Jan 1993

A Pulsed Power Design For The Linear Inductive Accelerator Modules For The Laboratory Microfusion Fa, Ronald E. Olson, D. L. Smith, M. G. Mazarakis, L. F. Bennett, T. R. Lockner

Physics Faculty Research & Creative Works

No abstract provided.


The Lmf Triaxial Mitl Voltage Adder System, Ronald E. Olson, M. G. Mazarakis, D. L. Smith, L. F. Bennett, T. R. Lockner, J. W. Poukey Jan 1993

The Lmf Triaxial Mitl Voltage Adder System, Ronald E. Olson, M. G. Mazarakis, D. L. Smith, L. F. Bennett, T. R. Lockner, J. W. Poukey

Physics Faculty Research & Creative Works

No abstract provided.