Physics Commons^™

Hydrodynamic Forces And Band Formation In Swimming Magnetotactic Bacteria, D. C. Guell, H. Brenners, Richard B. Frankel, H. Hartman

Physics

Dense suspensions of magnetotactic bacteria form long, thin, stable bands perpendicular to the common, net direction of motion of the cells in a magnetic field. A mechanism for this phenomenon is proposed in which the hydrodynamic coupling between two swimming cells is described in terms of an approximate, far-field solution for the flow around one swimming cell. The calculated hydrodynamic interaction force and torque based upon this analysis are of the correct orders of magnitude and in the right directions to provide a reasonable explanation for band formation and stability.

Effects Of Surfaces On Dynamic Percolation, H. K. Janssen, B. Schaub, Beate Schmittmann

Beate Schmittmann

The general epidemic process, which is a stochastic multiparticle process belonging to the universality class of dynamic percolation, is studied in a semi-infinite geometry. Critical exponents characterizing the fractal properties are calculated to O(ε) (ε=6-d, where d is the spatial dimension) with use of renormalization-group techniques.

Modified Broyden’S Method For Accelerating Convergence In Self-Consistent Calculations, Duane D. Johnson

Duane D. Johnson

A modification to Broyden’s method for obtaining stable and computationally efficient convergence in self-consistent calculations is developed and discussed. The method incorporates the advantages of two schemes proposed by Srivastava and by Vanderbilt and Louie without any increase in complexity. Its improvement over their methods is discussed. The present method is compared with two other widely used convergence methods, simple mixing and Anderson’s method, for the case of the disordered binary alloy Ni0.35Fe0.65 on the verge of a magnetic instability and is shown to be much improved in stability and rate of convergence.

Microstructure Of Epitaxial Ybba2Cu3O7 Superconducting Films Grown By A New Liquid-Gas-Solidification Technique, C.H. Chen, H.S. Chen, Sy_Hwang Liou

Sy-Hwang Liou Publications

We have studied the microstructures of epitaxial YbBa₂Cu₃O₇ superconducting films grown on SrTiO₃ [100] and [110] substrates by a new liquid-gas-solidification processing technique. Films grown on SrTiO₃ [100] and [110] substrates are epitaxially oriented with [001] and [110] axes normal to the film surface. Twinned domains of ~200 Å in size are observed in the [001] oriented films. Most defects observed can be attributed to the presence of stacking faults along the c axis. The presence of Cu-O bilayer defects is also observed. Rapid oxygen diffusion in the liquid phase is found to …

Nuclear-Spin Relaxation In Molecular Solids With Reorienting Methyl And T-Butyl Groups: The Spectral Density And The State Of The Solid, Peter A. Beckmann, Audrey I. Hill, Ellen B. Kohler, Hong Yu

Physics Faculty Research and Scholarship

There are two solid phases which either have a large hysteresis of at least 90 K, or are both stable below 200 K. The sample melts at 262 K. We interpret the high-temperature phase R-versus-T-1 data with three models. First, we adopt a one-correlation-time model using a Davidson-Cole spectral density which suggests that there is a distribution of correlation times, or, equivalently, a distribution of activation energies for t-butyl and methyl group reorientation. In this case, the methyl and t-butyl reorientation is characterized by a cutoff activation energy of 17±1 kJ/mol which is to be compared with 18±1 kJ/mol in …

Nd:Lna Laser Optical Pumping Of ⁴He: Application To Space Magnetometers, R. E. Slocum, Laird D. Schearer, P. Tin, R. Marquedant

Physics Faculty Research & Creative Works

We have observed Hanle signals and n=0, p=1 parametric resonances of ₂₃S¹ metastable helium atoms in a discharge cell by optically pumping the helium atoms with a tunable Nd:LNA laser. These resonances were used to construct a sensitive magnetometer for the measurement of very small magnetic fields. Since magnetometer sensitivity is proportional to the slope of the parametric resonance signal (signal amplitude divided by linewidth), the slopes for single-line laser pumping were compared with similar quantities obtained from conventional helium lamp pumping. Laser pumping yielded 45 times greater slopes with comparable power requirements, thus establishing the potential for …

On The Possibility Of Quasi-Static Convection In The Quiet Magnetotail, Margaret G. Kivelson, Harlan E. Spence

Physics & Astronomy

Abstract

The magnetotail is known to serve as a reservoir of energy transferred into the terrestrial magnetosphere from the solar wind. In principle, the stored energy can be dissipated impulsively, as in a substorm, or steadily through the process of steady adiabatic plasma convection. However, some theoretical arguments have suggested that quasi-static adiabatic convection cannot occur throughout the magnetotail because of the structure of the magnetic field. Here we reexamine the question. We show that in a magnetotail of finite width, downtail pressure gradients depend strongly on the ratio of the potential across half the tail to the ion temperature …

Quasidegenerate Variational Perturbation Theory And The Calculation Of First‐Order Properties From Variational Perturbation Theory Wave Functions, Robert J. Cave, Ernest R. Davidson

All HMC Faculty Publications and Research

In previous work on the treatment of correlation in molecular systems we have applied a multireference version of second‐order Hylleraas variational perturbation theory. The choice made for the partitioning of H treated the interactions between the correlating functions to infinite order and gave the corrections to the wave function to first order. The method was shown to be accurate in many cases, but became less so when near degeneracies occurred between the reference energy and other eigenvalues of H0. In this article we introduce an effective Hamiltonian method that is analogous to variational perturbation theory, but which is significantly more …

Tunneling Characteristics Of Internal Josephson Junctions In Yba2Cu3O7-Δ Thin Films, W.S. Brocklesby, Don Monroe, M. Hong, Sy_Hwang Liou, J. Kwo, G.J. Fisanick, P.M. Mankiewich, R.E. Howard

Sy-Hwang Liou Publications

We have measured detailed I-V characteristics of small areas of YBa₂Cu₃O_7-δ films, which exhibit structure associated with individual Josephson junctions within the film. Both hysteretic and nonhysteretic junctions are seen. A simple model relating the voltage steps to the junction properties is developed. The temperature dependence of l_c for the hysteretic junctions fits that predicted for quasiparticle tunneling junctions between conventional superconductors.

Cooperative Effects Among Partial Waves In Mie Scattering, James A. Lock

Physics Faculty Publications

Observations of an illuminated water droplet at a close distance are described mathematically by the Fourier transform of the Mie-scattering amplitude convolved with the aperture function of the observer’s eye. Most of the sharp enhancements found in the Fourier transform correspond to geometrical rays associated with the various terms in the Debye-series expansion of the Mie amplitude. However, there are some enhancements that cannot be ascribed to any individual Debye-series term. Instead, they arise from a constructive interference cooperation of the phase of a scattering resonance in a single partial wave with the region of the stationary phase corresponding to …

Spin-Polarized ³He Nuclear Targets And Metastable 4he Atoms By Optical Pumping With A Tunable, Nd:Yap Laser, C. L. Bohler, Laird D. Schearer, M. Leduc, P. J. Nacher, L. Zachorowski, R. G. Milner, R. D. Mckeown, C. E. Woodward

Physics Faculty Research & Creative Works

Several Nd:YAP lasers were constructed which could be broadly tuned in the 1083-nm region which includes the helium ²³S-²³P transition, using a Lyot filter and thin, uncoated etalons within the laser cavity. 1 W of power could be extracted at 1083 nm through a 1% transmitting output coupler. This laser beam was used to optically pump metastable ⁴He and ³He ²³S helium atoms in a weak discharge cell, spin polarizing the metastable ensemble. In a ³He cell the polarization is transferred to the nuclear spin system. A ³He target cell at 0.3 …

Superconducting Tl-Ba-Ca-Cu-O Films By Sputtering, M. Hong, Sy_Hwang Liou, D.D. Bacon, G.S. Grader, J. Kwo, A.R. Kortan, B.A. Davidson

Sy-Hwang Liou Publications

Superconducting Tl-Ba-Ca-Cu-O thin films 0.2–1.0 µm thick have been prepared on MgO(100) and SrTiO₃ (100) substrates by dc diode sputtering using a single oxide target. Films containing primarily the Tl₂Ba₂Ca₁Cu₂O₈ phase were obtained with a T_c (R =0) at 102 K and a transport J_c of 10⁴ A/cm² at 90 K. For the nearly pure phase Tl₂Ba₂Ca₂Cu₃O₁₀ films, the T_c's (R =0) are higher at 116 K and the transport J_c's …

A Current Algebra For Some Gauge Theory Amplitudes, V. Parameswaran Nair

Publications and Research

The classical amplitude for scattering of two positive helicity and n-2 negative helicity (or the other way) gauge bosons is shown to be generated by a Wess-Zumino-Witten (WZW) model for N= 4 supersymmetric gauge theory; i.e. the current algebra of the WZW model (with central charge k= 1 ) gives a Kac-Moody algebra as the symmetry behind these amplitudes.

Magnetic And Structural Properties Of Melt-Spun Rare-Earth Transition-Metal Intermetallics With Thmn12 Structure, E.W. Singleton, J. Strzeszewski, George C. Hadjipanayis, David J. Sellmyer

David Sellmyer Publications

The magnetic and structural properties of melt-spun R₈Fe₈₄Ti₈ with R=Nd, Sm, Dy, Gd and R₈ (Fe,Co) ₇₆ [V(Mo)] ₁₆ with R=Nd, Sm have been studied with x-ray diffraction, transmission electron microscopy, and magnetic measurements. The tetragonal ThMn₁₂-type structure was found in all alloys after annealing at about 700 °C. The Curie temperature (T_c) of the as-spun amorphous samples was in the range of 40–100 °C. Crystallized Nd₈ (Fe_1-xCo_x)₈₄Ti₈ samples with the 1:12 structure have a T_c around 250 °C for …

Structure And Magnetic Properties Of Nanostructured Dy/Transition-Metal Multilayered Films, Z.S. Shan, David J. Sellmyer

David Sellmyer Publications

We report the results of magnetic and microstructural studies for T/Dy (T=Fe, Co, Ni) compositionally modulated films prepared in a multiple-gun sputtering system. The perpendicular anisotropy and magnetization were measured systematically for X-Å Fe/Y-Å Dy and X-Å Co/Y-Å Dy films. The layer-thickness dependence of the magnetization for Co/Dy and Fe/Dy was interpreted in terms of the antiparallel coupling between transition-metal and Dy magnetic moments. For Co/Dy films the ranges of X and Y required for perpendicular anisotropy were determined. A comparision of the structural and magnetic properties of Ni/Dy, Co/Dy, and Fe/Dy is given and the origin of the perpendicular …

Magnetic And Structural Properties Of Nd2Fe14B Permanent-Magnet Films And Multilayers With Fe And Ag, K.D. Aylesworth, Z.R. Zhao, David J. Sellmyer, George C. Hadjipanayis

David Sellmyer Publications

The effects of sputtering conditions on the structure and magnetic properties of Nd₂ (Fe_0.9Co_0.1)₁₄B films and multilayers are reported. The samples were sputtered onto temperature-controlled mica and Ta substrates, at temperatures between 20 and 620 °C. X-ray diffraction results indicate that films deposited directly onto 500–600 °C substrates with Fe buffer layers are nearly single-phase materials with the c axis of the grains largely perpendicular to the film plane. Room-temperature magnetization measurements to 17.4 kOe indicate that these films have strong perpendicular anisotropy with coercivities in excess of 10 kOe and magnetizations greater …

Electronic Structure And Surface Reactivity Of Nd2Fe14B And Related Compounds, Sitaram Jaswal, Marjorie Langell, Y.G. Ren, M.A. Engelhardt, David J. Sellmyer

David Sellmyer Publications

Studies of electronic and magnetic structure and surface reactivity are reported for R₂T₁₄B compounds where R=Y, Nd, Gd, and T=Fe, Co. The calculations employed the self-consistent, spin-polarized linear-muffin-tin orbital method. Experiments included ultraviolet, x-ray, and Auger electron spectroscopy. Comparisons are made between the calculated and measured electronic densities of states, and the reactions of the surfaces with O₂, H₂, and CO gases are studied. The effects of heat treatment on surface segregation are also reported. Journal of Applied Physics is copyrighted by The American Institute of Physics.

Ferromagnetism In Metastable 304 Stainless Steel With Bcc Structure, J. Childress, Sy_Hwang Liou, C.L. Chien

Sy-Hwang Liou Publications

We have studied the consequence of structure on the magnetic properties of 304 stainless steel in two distinct crystalline states. Ordinary 304 stainless steel has an fcc structure and is nonmagnetic at room temperature. By using a vapor quenching method, we have fabricated single-phase metastable bcc 304 stainless steel which is strongly ferromagnetic. Films a few µm thick have been made by high-rate sputter deposition onto substrates at room temperature or liquid-nitrogen temperature. Vibrating sample magnetometry and ⁵⁷Fe Mössbauer spectroscopy reveal that the bcc phase has a magnetization of 130 emu/g, due largely to the Fe moment. The Curie …

Transport Critical Current And Magnetization Measurements Of Melt-Processed Yba2cu3o7-X, A. H. Hermann, Z. Z. Sheng, W. Kiehl, D. Marsh, A. Elali, Paul D. Hambourger, C. Almasan, J. Estrada, T. Datta

Physics Faculty Publications

We report magnetic field dependence of the transport critical current and dc magnetic susceptibility measurements on YBa2Cu3O7−x superconductors formed by melt-solid reactions at 950 °C between Ba-Cu-O (or Tb-Ba-Cu-O) and solid nonstoichiometric Y-Ba-Cu-oxide. Four-probe dc critical current measurements at 77, 64, and 4.2 K show strong depression of the critical current density with increasing magnetic field in agreement with a model of weakly linked superconducting regions. Diamagnetic shielding and Meissner flux expulsion measurements in the temperature range 10–300 K show about one third volume fraction of perfect superconductivity. Both shielding and flux expulsion were observed to be approximately temperature independent …

Resonant Transfer And Excitation In Li-Like F Colliding With H₂, Michael Schulz, Reinhold S. Schuch, Sheldon Datz, Edson L B Justiniano, Philip D. Miller, Harald Schone

Physics Faculty Research & Creative Works

We have measured coincidences between x rays and projectiles that have captured one electron in F⁶⁺ + H₂ collisions at projectile energies between 15 and 33 MeV. The cross sections for capture and simultaneous x-ray emission as a function of projectile energy show clear structures. Indications of an unexpectedly high population of high-n states predominantly formed by resonant transfer and excitation (RTE) were found. Above the Kln (n>1) RTE resonance energies another maximum was observed.

Roughness-Induced Resonance For Molecular Fluorescence Near A Corrugated Metallic Surface, P.T. Leung, Young Sik Kim, Thomas F. George

Physics Faculty Publications and Presentations

Based on a dynamical energy-transfer theory for the molecular decay rate near a rough metallic surface, a new resonance structure is predicted which is mediated by the roughness and which is absent in the static image theory. The implication of this new resonance structure to other phenomena, such as electron-energy-loss spectrum and cross coupling in thin films, is pointed out.

Magnetism And Superconductivity In (Er₀.₁₆Ho₀.₈₄)Rh₄B₄, Oran Allan Pringle, Herbert A. Mook, Steven E. Lambert, M. Brian Maple

Physics Faculty Research & Creative Works

The superconducting and ferromagnetic phase boundaries in the (Er_1-xHox)Rh₄B₄ mixed ternary alloy system meet in a multicritical point at x_cr ≈ 0.9. For xcr, the compounds first become superconducting as the temperature is lowered, and then lose superconductivity in a transition to ferromagnetism. The coexistence of superconductivity and ferromagnetism for alloys near the erbium-rich end of the phase diagram is well established. It has also been suggested that ferromagnetism and superconductivity coexist in alloys with x just below x_cr. We have carried out neutron-diffraction, ac magnetic susceptibility, and heat-capacity …

A Neutron Diffraction And Mössbauer Spectral Study Of The Structure And Magnetic Properties Of The Y₂Fe₁₄₋ₓsiₓb Solid Solutions, Gaya Kanishka Marasinghe, Oran Allan Pringle, Gary J. Long, William B. Yelon, Fernande Grandjean

Physics Faculty Research & Creative Works

A neutron diffraction and Mössbauer spectral study of Y₂Fe_14-xSi_xB shows that silicon preferentially occupies the 4c, and to a lesser extent, the 8j₁ sites in Y₂Fe_14-xSi_xB. The trend in the site occupancy is the same as in Nd₂Fe_14-xSi_xB. The Curie temperature of Y₂Fe_14-xSi_xB increases with increasing silicon content. Neutron diffraction data show that the increase in Curie temperature is accompanied by a contraction of the unit cell. Wigner-Seitz cell calculations, using the Y₂Fe …

Excitation Of The Lowest Autoionizing Levels In Lithiumlike Ions By Electron Impact, Rajesh C. Srivastava, Don H. Madison

Physics Faculty Research & Creative Works

We present theoretical, differential, and total cross sections for electron impact excitation of the lowest autoionizing levels of various lithiumlike ions (viz., Be⁺, B²⁺, C³⁺, O⁵⁺, and Ne⁷⁺). For these ions, the autoionizing level of interest results from excitation of an inner-shell electron. A distorted-wave Born approximation (with exchange) is used for the calculation. The present results are compared with previous theoretical calculations and it is concluded that the Coulomb-Born approach is unreliable, particularly near threshold.

A Mössbauer Effect Study Of Y₂Fe₁₄B And Its Aluminum Solid Solutions, Dwayne E. Tharp, Gary J. Long, Oran Allan Pringle, Gaya Kanishka Marasinghe, William Joseph James, Fernande Grandjean

Chemistry Faculty Research & Creative Works

The Mössbauer spectra of Y₂Fe₁₄B have been measured from 85 to 296 K. Analysis of the spectra indicates that the near-neighbor rare-earth atoms directly influence the orientation of the principal axis of the electric field gradient. The hyperfine parameters are very similar to those of Nd₂Fe₁₄B, but the internal fields are somewhat smaller at room temperature in the yttrium compound. The Mösssbauer spectra of Y₂(Fe_1-xAl_x) ₁₄B, where x equals 0.00, 0.02, 0.04, 0.06, and 0.08, have been measured at 85 K. The average internal hyperfine …

Redox Reactivity Of Bacterial And Mammalian Ferritin: Is Reductant Entry Into The Ferritin Interior A Necessary Step For Iron Release?, G. D. Watt, D. Jacobs, Richard B. Frankel

Physics

Both mammalian and bacterial ferritin undergo rapid reaction with small-molecule reductants, in the absence of Fe²⁺ chelators, to form ferritins with reduced (Fe²⁺) mineral cores. Large, low-potential reductants (flavoproteins and ferredoxins) similarly react anaerobically with both ferritin types to quantitatively produce Fe²⁺ in the ferritin cores. The oxidation of Fe²⁺ ferritin by large protein oxidants [cytochrome c and Cu(II) proteins] also occurs readily, yielding reduced heme and Cu(I) proteins and ferritins with Fe³⁺ in their cores. These latter oxidants also convert enthetically added Fe²⁺, bound in mammalian or bacterial apo- or holo-ferritin, …

Testimony On The Verification And Compliance Of Nuclear Testing Treaties, David W. Hafemeister

Physics

No abstract provided.

Slip, Twinning, And Fracture At A Grain Boundary In The L12 Ordered Structure—Aς = 9 Tilt Boundary, Alexander H. King, M. H. Yoo

Alexander H. King

The role of interaction between slip dislocations and a Σ = 9 tilt boundary in localized microplastic deformation, cleavage, or intergranular fracture in the Li2 ordered structure has been analyzed by using the anisotropic elasticity theory of dislocations and fracture. Screw superpartials cross slip easily at the boundary onto the (1$\overline 1$1) and the (001) planes at low and high temperatures, respectively. Transmission of primary slip dislocations onto the conjugate slip system occurs with a certain degree of difficulty, which is eased by localized disordering. When the transmission is impeded, cleavage fracture on the ($\overline 1$11) plane is predicted to …

Differential Cross Sections For Secondary Electron Production By 1.5-Kev Electrons In Water Vapor, K. W. Hollman, G. W. Kerby, Iii, M. Eugene Rudd, J. H. Miller, S. T. Manson

M. Eugene Rudd Publications

Discrepancies between previous experimental values of differential cross sections for electron-impact ionization of water vapor and recent model calculations have been largely resolved. A new measurement with improved suppression of spurious electrons has removed most of the discrepancy in the midrange of detected electron energies. A second discrepancy at secondary energies just below the primary energy has been explained by a more accurate accounting for electrons scattered at angles between zero and the minimum angle of the experimental apparatus. The improved data show more clearly the oxygen K-shell edge in the spectra at small angles and the Bethe ridge at …

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

Physics Faculty Research & Creative Works

Using a microphysical approach to nucleation, we present an extensive study of water nucleation rates for wide ranges of both temperature and supersaturation ratio. Based on the fundamental molecular properties of clusters instead of bulk properties, the microphysical approach is demonstrated to predict good agreement with measured nucleation rates over this broad range of conditions. Predicted critical sizes for nucleation are found to be relatively small, and are in the molecular cluster size regime rather than in a size regime that should be characterized by bulk values. Estimated sticking coefficient values cover the range of ~0.9 to ~0.2 for the …