Physical Sciences and Mathematics Commons^™

Critical Slowing Down Of Cluster Algorithms For Ising Models Coupled To 2-D Gravity, Mark Bowick, Marco Falcioni, Geoffrey Harris, Enzo Marinari

Physics - All Scholarship

We simulate single and multiple Ising models coupled to 2-d gravity using both the Swendsen-Wang and Wolff algorithms to update the spins. We study the integrated autocorrelation time and find that there is considerable critical slowing down, particularly in the magnetization. We argue that this is primarily due to the local nature of the dynamical triangulation algorithm and to the generation of a distribution of baby universes which inhibits cluster growth.

The Chemical Evolution Of Qsos And The Implications For Cosmology And Galaxy Formation, Fred Hamann, Gary J. Ferland

Physics and Astronomy Faculty Publications

We examine the chemical evolution of QSO broad-line gas by applying spectral synthesis and chemical enrichment models to the N V/C IV and N V/He II emission-line ratios. The models indicate that BLR metallicities are typically ~1 to perhaps ≳10 times solar. The enrichment must occur in ≲1 Gyr for sources where the redshift is ≳3 (if q₀ = ½). The higher metallicity QSOs require star formation favoring massive stars (compared to the Galactic disk). These results imply that extensive evolution usually occurs before the QSOs become observable. Our models of the evolution are equivalent to models proposed for …

Fe304 And Fe3S4 In A Bacterium, Dennis A. Bazylinski, Brigid R. Heywood, Stephen Mann, Richard B. Frankel

Physics

No abstract provided.

Response To "Comment On 'The Weakest Bond: Experimental Observation Of Helium Dimer'", Fei Luo, George C. Mcbane, Geunsik Kim, Clayton F. Giese, Ronald Gentry

Peer Reviewed Articles

No abstract provided.

Comparison Of Suppression Across Frequencies, Linda M. Thibodeau, Marc A. Fagelson

Marc A. Fagelson

Microphysical Approach To Nonequilibrium Dynamics Of Quantum Fields, Marcelo Gleiser, Rudnei O. Ramos

Dartmouth Scholarship

We examine the nonequilibrium dynamics of a self-interacting λφ4 scalar field theory. Using a real time formulation of finite temperature field theory we derive, up to two loops and O(λ2), the effective equation of motion describing the approach to equilibrium. We present a detailed analysis of the approxi- mations used in order to obtain a Langevin-like equation of motion, in which the noise and dissipation terms associated with quantum fluctuations obey a fluctuation-dissipation relation. We show that, in general, the noise is colored (time-dependent) and multiplicative (couples nonlinearly to the field), even though it is still Gaussian distributed. The noise …

Radial Excited States For Heavy Quark Systems In Nrqcd, Simon Catterall, F. R. Devlin, I. T. Drummond, R. R. Hogan

Physics - All Scholarship

Following the Non-Relativistic QCD approach we use a gauge invariant smearing method with factorization to measure the excitation energies for a heavy Q\bar{Q} system on a 24^3\times 48 lattice at \beta=6.2. The results come from averaging over an ensemble of 60 QCD configurations. In order to enhance the signal from each configuration we use wall sources for quark propagators. The quark Hamiltonian contains only the simplest non-relativistic kinetic energy term. The results are listed for a range of bare quark masses. The mass splittings are insensitive to this variable though there are a slight trends with increasing quark mass. For …

Modeling Of The Surface Plasmon Resonance (Spr) Effect For A Metal-Semiconductor (M-S) Junction At Elevated Temperatures, Erik De Jesus Sanchez

Dissertations and Theses

The effect of temperature increase on the optical excitation of Surface Plasmon Resonance (SPR) at an Ag-Si metal-semiconductor (M-S) junction at a wavelength of 1 . 1 52 pm is investigated theoretically using computer modeling in Fortran. In order to accurately quantify the SPR, the temperature dependent optical constants for Ag and Si are obtained theoretically or semiempirically , using a Drude model for Ag and previous experimentally determined equations for Si (the behavior of the optical constants for crystalline Si and doped Si are found to have very little deviation between each other for our case). An improvement in …

The Temperature Dependence Of Perpendicular Anisotropy In Co/Pt And Co/Au Multilayer Films, H.Y. Zhang, Y.J. Wang, G.G. Zheng, J.X. Shen, Z.S. Shan, David J. Sellmyer

David Sellmyer Publications

We report the temperature dependence of the effective perpendicular anisotropy and interface anisotropy in the temperature region of 80K to 290K for Co/Pt and Co/Au multilayers. Different temperature dependence of the interface anisotropy have been observed for these two types of multilayers. We speculate that the strain due to the lattice misfit may not be the main reason responsible for the perpendicular anisotropy in Co/Pt and Co/Au multilayers.

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 Nd₂Fe_14-xSi_xB 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 Nd₂Fe_14-xSi_xB have been fit with a model which …

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 in order to …

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 binary …

Structural And Magnetic Properties Of Lafe13-XSiX Nitrides, Z.X. Tang, X.H. Deng, George C. Hadjipanayis, V. Papaefthymiou, David J. Sellmyer

David Sellmyer Publications

LaFe_13-xSi_x compounds were found to have two types of crystal structure; the face-centered cubic Na_13--type (x ≤2.4) and the body-centered tetragonal Ce₂Ni₁₇Si₉-type (3.5≤x≤5.0). Nitrogenation has a strong effect on the cubic phase, including a large lattice expansion (6.2 to 10.8 %), an increase of Curie temperature (52 to 80 K) and spontaneous magnetization ( ~10 %). The tetragonal phase, however, was not significantly affected by the nitrogenation.

Nucleation Field And Energy Product Of Aligned Two-Phase Magnets - Progress Towards The '1 Mj/M3' Magnet, Ralph Skomski, J.M.D. Coey

Ralph Skomski Publications

Exchange hardening of nanostructured two-phase systems composed of an aligned hard phase and a soft phase with high magnetization is investigated using a micromagnetic approach which accounts for interactions between the soft regions. For Sm₂Fe₁₇N₃ (2.5 nm)/Fe₆₅Co₃₅ (9 nm) multilayers an energy product as high as 1 MJ/m³ (120 MCOe) is predicted, witb a rare-earth content of only 5 wt %. Giant energy products may also be achieved in suitable cellular and disordered structures.

Cation Distribution In Natural Ferrites, H. A. Maia, F. F. T. De Araújo, M. A. B. De Araújo, J. Danon, Richard B. Frankel

Physics

Mössbauer spectra of jacobsite have been recorded at temperatures of 4.2, 125, 180, 240 and 300 K and with an external magnetic field of 60 kOe applied parallel to the incident radiation at 4.2 K. The results show the presence of two different magnetic hyperfine interactions associated with the iron nuclei. Spectra of samples in external magnetic fields indicate the same sign for the magnetic hyperfine interactions, implying that both iron ions occupy only the octahedral sublattice. These results are compared with data from synthetic stoichiometry manganese ferrite, and a model of cation distribution, assuming the existence of vacancies, is …

Coherent Versus Incoherent Ladar Detection At 2.09 Μm, Jay A. Overbeck, Martin B. Mark, Scott H. Mccraken, Paul F. Mcmanamon, Bradley D. Duncan

Electro-Optics and Photonics Faculty Publications

A 2.09-μm ladar system is built to compare coherent to incoherent detection. The 2.09-μm wavelength is of interest because of its high atmospheric transmission and because it is eyesafe. The 2.09-μm system presented is capable of either a coherent or incoherent operational mode, is tunable in a small region around 2.09 μm, and is being used to look at the statistical nature of the ladar return pulses for typical glint and speckle targets. To compare coherent to incoherent detection the probability of detection is investigated as the primary performance criterion of interest. The probability of detection is dependent on both …

Phase Diagrams For The Blue Phases Of Highly Chiral Liquid Crystals, M. B. Bowling, Peter J. Collings, C. J. Booth, J. W. Goodby

Physics & Astronomy Faculty Works

Polarizing microscopy and optical-activity measurements are used to determine the phase diagram for the blue phases of chiral-racemic mixtures of terephthaloyloxy-bis-4-(2'-methylbutyl) benzoate. Contrary to an earlier report, it is the second blue phase (BP II) rather than the first blue phase (BP 1) that is not stable relative to the other blue phases at high chirality. With this development, all phase diagrams for the blue phases reported to date have the same topology. Using similar data for two other highly chiral systems, it is found that a simple scaling of the temperature and chiral-fraction axes produces phase diagrams in quantitative …

Sensitivity Improvement Of A 1-Μm Ladar System Incorporating An Active Optical Fiber Preamplifier, Michael S. Salisbury, Paul F. Mcmanamon, Bradley D. Duncan

Electro-Optics and Photonics Faculty Publications

In an effort to increase the SNR of a continuous wave, 1-μm all solid state ladar system, a rare-earth-doped optical fiber amplifier is investigated as a preamplifier for ladar return signals. The experimental system is detailed and a theoretical analysis of the fiber amplifier's effect on both heterodyne and direct detection schemes is provided. Beginning with the optical powers incident on the detector, the signal and noises are analyzed, through the detector electronics, to predict the SNR. The SNR is then plotted as a function of the return signal power, and a SNR threshold is defined to determine a minimum …

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.

Photodetachment Of The 2P2 (3PE) State Of H-, Ning-Yi Du, Anthony F. Starace, Min-Qi Bao

Anthony F. Starace Publications

Results of semiempirical, adiabatic hyperspherical calculations are presented for the photodetachment cross section of the process H^- 2p² (³P^e)+γ→H(n=2)+ e^- at photon energies in the range from threshold to 0.125 a.u. above. We find the detachment cross section to have a maximum value of 482 Mb at 25.7 meV above threshold, in good agreement with results of close-coupling calculations by Jacobs, Bhatia, and Temkin [Astrophys. J. 242, 1278 (1980)]. In contrast to the close-coupling results, however, we find that the detachment cross section rises by a factor of more than …

Far Infrared Fourier Transform Spectroscopy Using Ring Interferometer, Changqing Qiu

Theses

A circular mirror interferometer named ring interferometer, which was designed by Dr. K.D. Moller, was built and set up for far-infrared Fourier transform spectroscopy. The ring interferometer has ten circular mirrors with the same area. By applying Fraunhofer diffraction theory, the calculation shows that the ring interferometer will have large diffraction angle range compared with lamellar grating interferometer which has rectangular mirrors with same width. Therefore the ring interferometer will be more efficient than lamellar grating in far-infrared spectroscopy. The experimental results show that this ring interferometer will work as expected.

In addition, a "home made" helium cooled detector was …

Triply Differential (E,2e) Cross Sections For He From Threshold To 50 Ev Above, C. Pan, Anthony F. Starace

Anthony F. Starace Publications

Distorted wave calculations of the triply differential cross sections for electron-impact ionization of He are presented for electrons sharing ≤ 50 eV excess energy (E_x) and having the coplanar, θ₁₂ = π geometry. The results are in good agreement with absolute experimental measurements of T. Rösel et al. [Phys. Rev. A 46 (1992) 2539-2552] and of A. J. Murray, M. B. Woolf, and F. H. Read [J. Phys. B 25 (1992) 3021-3036]. For final state electrons sharing 2 eV excess energy equally, many-body perturbation theory corrections to the authors' first order (e,2e) triply differential cross …

Inclusion Of Charge Correlations In Calculations Of The Energetics And Electronic Structure For Random Substitutional Alloys, Duane D. Johnson, F. J. Pinski

Duane D. Johnson

Currently, the coherent-potential approximation (CPA) implemented via the multiple-scattering theory of Korringa, Kohn, and Rostoker (KKR) gives the best first-principles description of the electronic structure for random substitutional alloys. However, the total energy has an important component of electrostatic energy missing, namely, that arising from the correlation of charges with varying atomic environments. We develop a ‘‘charge-correlated’’ CPA method (cc-CPA) which includes (some) local environmental charge correlations within the KKR-CPA method. We investigate the cc-CPA energetics for several alloys and show that the formation energies are in better agreement with experimental results. These calculations show that the excess charge on …

Differential Cross Sections For Single And Double Ionization Of Helium By Protons And Antiprotons, L. Meng, Ronald E. Olson, R. Dömer, J. Ullrich, H. Schmidt-Bocking

Physics Faculty Research & Creative Works

Theoretical and experimental cross sections are presented for single and double ionization of He differential in the projectile scattering angle and the transverse energy of the outgoing recoil ion. A dynamical four-body classical trajectory Monte Carlo method that includes radial electron correlation is utilized to calculate total and differential cross sections for 300 and 500 keV protons and antiprotons in impact collisions with He. The azimuthal emission characteristics of electrons, recoil ion and projectile relative to each other are analyzed to provide detailed information on the dynamical differences between single and double ionization and between proton and antiproton impact. © …

Linear Dichroism In Photoemission From Oriented Molecules, N. A. Cherepkov, G. Schönhense

Department of Physics and Astronomy: Faculty Publications

It is shown that oriented molecules reveal both Linear Dichroism (LD) and Linear Dichroism in the Angular Distribution (LDAD) of photoelectrons, which means a difference between photoelectron currents ejected at a definite angle by linearly polarized light of two mutually perpendicular polarizations. Measurements of LD and LDAD enable one to investigate the orientation of molecules and to make an essential step towards the complete quantum-mechanical experiment. As an example, LDAD in a model case of oriented heteronuclear diatomic molecules is presented.

Spaceborne Radar Sensing Of Precipitation Above An Ocean Surface: Polarization Contrast Study, Alexander Kostinski, John Kwiatkowski, A. R. Jameson

Department of Physics Publications

This feasibility study explores the potential benefits of polarization adjustment for spaceborne radar sensing of precipitation. More specifically, the role of the wave polarization in separating or “distinguishing” ocean surface return from the hydrometeor echoes of a “chirped” signal is examined.

To that end, experimental as well as computational data for the polarization scattering matrices of hydrometeors and ocean surfaces are obtained and used to calculate ocean and precipitation “response” to the transmitted pulse for various rain rates and incidence angles. The analysis is restricted to X and C bands, but simulations are performed for several signal-to-noise ratios, rain rates, …

Low-Field Magnetoresistance In Granular Pb Films Near The Insulator-Superconductor Transition, Richard P. Barber Jr., Robert C. Dynes

Physics

We have studied the insulator-superconductor transition in quench-condensed granular Pb films from 0.1 to 10 K. Resistance measurements were made in zero magnetic field and low noise conditions. Magnetoresistance measurements were also performed for low magnetic fields (less than 100 G). The magnetoresistance results on the superconducting side of the transition suggest that we are directly probing the finite length scales associated with the range of phase coherence in these granular materials.

New Exactly Solvable Hamiltonians - Shape Invariance And Self-Similarity, David T. Barclay, Ranabir Dutt, Asim Gangopadhyaya, Avinash Khare, A. Pagnamenta, Uday P. Sukhatne

Physics: Faculty Publications and Other Works

We discuss in some detail the self-similar potentials of Shabat and Spiridonov which are reflectionless and have an infinite number of bound states. We demonstrate that these self-similar potentials are in fact shape invariant potentials within the formalism of supersymmetric quantum mechanics. In particular, using a scaling ansatz for the change of parameters, we obtain a large class of new, reflectionless, shape invariant potentials of which the Shabat-Spiridonov ones are a special case. These new potentials can be viewed as q-deformations of the single soliton solution corresponding to the Rosen-Morse potential. Explicit expressions for the energy eigenvalues, eigenfunctions and transmission …

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 and long-range …

Electric-Field Effects On H- Photodetachment With Excitation Of H(N=2), Ning-Yi Du, Ilya I. Fabrikant, Anthony F. Starace

Anthony F. Starace Publications

A theory of the photodetachment of hydrogen negative ions in an external static electric field with the production of hydrogen in excited states is developed. We use a combination of the frame-transformation theory and the Green’s-function method. The cross section in nonzero fields is expressed in terms of the photodetachment matrix elements for zero field. The latter are calculated using the adiabatic hyperspherical representation. For energies close to the threshold we use a smooth extrapolation of the zero-field photodetachment matrix elements down to threshold taking care to enforce the proper threshold behavior of cross sections for nonzero fields. The ripples …