Physics Commons^™

Critical Dynamics Of Nonconserved Ising-Like Systems, K. E. Bassler, Beate Schmittmann

Beate Schmittmann

We show that the dynamical fixed point of Ising-like models, characterized by a single scalar, nonconserved ordering field, is stable near four dimensions with respect to all dynamic perturbations, including those of a nonequilibrium nature.

Kinetics Of Clustering In Traffic Flows, E. Ben-Naim, P. L. Krapivsky, S. Redner

Eli Ben-Naim

We study a simple aggregation model that mimics the clustering of traffic on a one-lane roadway. In this model, each ``car'' moves ballistically at its initial velocity until it overtakes the preceding car or cluster. After this encounter, the incident car assumes the velocity of the cluster which it has just joined. The properties of the initial distribution of velocities in the small velocity limit control the long-time properties of the aggregation process. For an initial velocity distribution with a power-law tail at small velocities, $\pvim$ as $v \to 0$, a simple scaling argument shows that the average cluster size …

The Spectral Center Of Gravity Effect And Auditory Filter Bandwidth, Marc Fagelson, Linda M. Thibodeau

Marc A. Fagelson

Low‐Temperature Phase Diagram Of Ybbipt, R. Movshovich, A. Lacerda, Paul C. Canfield, J. D. Thompson, Z. Fisk

Paul C. Canfield

Resistivity measurements are reported on the cubic heavy‐fermion compound YbBiPt at ambient and hydrostatic pressures to ≊19 kbar and in magnetic fields to 1 T. The phase transition at T c =0.4 K is identified by a sharp rise in resistivity. That feature is used to build low‐temperature H‐T and P‐Tphase diagrams. The phase boundary in the H‐T plane follows the weak‐coupling BCS expression remarkably well from T c to T c /4, while small hydrostatic pressure of ≊1 kbar suppresses the low‐temperature phase entirely. These effects of hydrostatic pressure and magnetic field on the phase transition are consistent with …

Electron Energy Loss In Oxygen Plasmas, G. A. Victor, John C. Raymond, Jane L. Fox

Jane L. Fox

The results of calculations of the energy deposition of energetic electrons in oxygen plasmas are given. In a pure oxygen plasma even with large fractional ionization, much of the electron energy results in the production of additional ionization and excited electronic states. Results are given for separate calculations using theoretical and experimental cross sections for the important O I excitations of ¹S and ¹D because the theoretical and experimental data are not in agreement. These results are useful for understanding the spectra of oxygen-rich supernova remnants.

Measurements Of The (E,E’Pπ-) Reaction On Nuclei In The Nucleon Resonance Region, L Elouadrhiri, R A. Miskimen, J Button-Shafer, P Degtyarenko,, Gerald Alvin Peterson, S Shuvalov, K Wang

Gerald Alvin Peterson

Nucleon resonances beyond the Δ(1232) have been observed in complex nuclei. Measurements of the (e,e’pπ-) reaction, at an incident energy of 14.5 GeV with 1.1 GeV

Measurements Of The Electric And Magnetic Form Factors Of The Proton From Q2=1.75 To 8.83 (Gev/C)2, L Andivahis, R S. Hicks, R A. Miskimen, Gerald Alvin Peterson, S Rokni

Gerald Alvin Peterson

The proton elastic form factors GEp(Q2) and GMp(Q2) have been extracted for Q2=1.75 to 8.83 (GeV/c)2 via a Rosenbluth separation to ep elastic cross section measurements in the angular range 13°≤θ≤90°. The Q2 range covered more than doubles that of the existing data. For Q2<4 >(GeV/c)2, where the data overlap with previous measurements, the total uncertainties have been reduced to < 14% in GEp and < 1.5% in GMp. Results for GEp(Q2) are consistent with the dipole fit GD(Q2)=(1+Q2/0.71)-2, while those for GMp(Q2)/μpGD(Q2) decrease smoothly from 1.05 to 0.92. Deviations from form factor scaling are observed up to 20%. The ratio Q2F2/F1 is observed to approach a constant value for Q2>3 (GeV/c)2. Comparisons are made to vector meson dominance, dimensional scaling, QCD sum rule, diquark, and constituent quark models, none of which fully characterize all the new data.

Spontaneous Structure Formation In Driven Systems With Two Species: Exact Solutions In A Mean-Field Theory, I. Vilfan, R. K. P. Zia, Beate Schmittmann

Beate Schmittmann

A stochastic lattice gas of particles, subject to an excluded volume constraint and to a uniform external driving field, is investigated. Using a mean-field theory for a system with equal number of oppositely charged particles, exact results are obtained. Focusing on the current-vs-density plot, we propose an explanation for the discontinuous transition found in earlier simulations. A critical value of the drive, below which this transition becomes continuous, is found. These results are supported by a bifurcation analysis, leading to an equation of motion for the amplitude of the soft mode.

X-Ray Studies Of Bond-Orientational Order In Liquid-Crystalline Orthogonal Hexatic-B Phase, E. Gorecka, Li Chen, Oleg Lavrentovich, W. Pyzuk

Oleg Lavrentovich

X-ray scattering measurements have been carried out for free-standing, thick . monodomain films of the orthogonal hexatic-B phase. From Fourier analysis of X-scans, 6n-fold-symmetry order parameters were determined and proved to fulfill the scaling relation C6n = C6n+lambdan(n-1). The temperature behavior of C6 and lambda was found nonuniversal when comparing homologues, PIR5 and PIR7, differing in their range of the hexatic phase. The hexatic order parameter varies with the critical exponents 0.25, 0.20 (+/- 0.03), respectively. For comparison results for compound RFL6, of other homologue series, are presented.

Nanometer Scale Patterning And Oxidation Of Silicon Surfaces With An Ultrahigh Vacuum Scanning Tunneling Microscope, J. W. Lyding, G. C. Abeln, T. -C. Shen, C. Wang, J. R. Tucker

T. -C. Shen

Nanoscale patterning of the Si(100)‐2×1 monohydride surface has been achieved by using an ultrahigh vacuum (UHV) scanning tunneling microscope(STM) to selectively desorb the hydrogen passivation. Hydrogen passivation on silicon represents one of the simplest possible resist systems for nanolithography experiments. After preparing high quality H‐passivated surfaces in the UHV chamber, patterning is achieved by operating the STM in field emission. The field emitted electrons stimulate the desorption of molecular hydrogen, restoring clean Si(100)‐2×1 in the patterned area. This depassivation mechanism seems to be related to the electron kinetic energy for patterning at higher voltages and the electron current for low …

Multiple Hadron Production By 14.5 Gev Electron And Positron Scattering From Nuclear Targets, P V. Degtyarenko, J Button-Shafer, L Elouadrhiri, R A. Miskimen, Gerald Alvin Peterson, K Wang

Gerald Alvin Peterson

Multiple proton and pion electroproduction from nuclei are studied. Final states including at least two protons produced by the interaction of 14.5 GeV electrons and positrons with light nuclei (mainly 12C and 16O) have been measured, and compared with analogous data from 40Ar. Scattered electrons and positrons were detected in the energy transfer range from 0.2 to 12.5 GeV, and four-momentum transfer squared range from 0.1 to 5.0 GeV2/c2. Phenomenological characteristics of the secondary hadron production cross sections such as temperature and velocity of the effective source of hadrons were found to be dependent on energy transfer to the nucleus …

Compositional Short-Range Ordering In Metallic Alloys: Band-Filling, Charge-Transfer, And Size Effects From A First-Principles All-Electron Landau-Type Theory, J. B. Staunton, Duane D. Johnson, F. J. Pinski

Duane D. Johnson

Using a mean-field statistical description, we derive a general formalism to investigate atomic short-range order in alloys based on a density-functional description of the finite-temperature, grand potential of the random alloy. This ‘‘first-principles,’’ Landau-type approach attempts to treat several contributions (electronic structure, Fermi surface, electrostatics, magnetism, etc.) to the electronic energy on an equal footing. An important ingredient for the statistical averaging is the replacement of the molecular mean fields (Weiss fields) with Onsager cavity fields, which forces the diagonal part of the fluctuation-dissipation theorem to be obeyed. To show its general applicability and usefulness, we apply the theory to …

First-Principles All-Electron Theory Of Atomic Short-Range Ordering In Metallic Alloys: D022- Versus L12-Like Correlations, Duane D. Johnson, J. B. Staunton, F. J. Pinski

Duane D. Johnson

We use a ‘‘first-principles’’ concentration-wave approach based on a finite-temperature, electronic density-functional, mean-field, grand potential of the random alloy to investigate the high-temperature atomic short-range order (ASRO) in Ni75V25 and Pd75V25 solid solutions. Experimentally, these similar alloys both develop D022-type long-range order at low temperatures but different ASRO at high temperatures. Our calculations describe the measured ASRO well. We compare these results with those found for a hypothetical Co75Ti25 solid solution. Since this alloy orders directly from the melt into the L12 phase, it should exhibit strong L12-like ASRO, as we find in our calculations. We analyze the features in …

Surface-Induced Ordering In Asymmetric Block Copolymers, Y. Liu, W. Zhao, X. Zheng, Alexander H. King, A. Sing, M. H. Rafailovich, J. Sokolov, K. H. Dai, E. J. Kramer, S. A. Schwarz, O. Gebizlioglu, S. K. Sinha

Alexander H. King

The surface-induced ordering in thin films of asymmetric deuterated polystyrene (dPS)- poly(viny1pyridine) (PVP) diblock and triblock copolymers of comparable polymerization index and PVP volume fraction - 0.25) was studied using transmission electron microscopy, atomic force microscopy, secondary ion massspectrometry, and neutron reflectivity. The morphology of both di- and triblock copolymer films was found to be cylindrical except for the layer adjacent to the silicon oxide surface, which due to the strong interaction of silica with PVP, was lamellar. The spacing between adjacent cylindrical layers was found to be consistent with mean field theory predictions. In the triblock copolymer films the …

More On Twinkling, Terry L. Smith, Jay S. Huebner

Terry L. Smith

Addendum to Why Magnification Works , The Physics Teacher, 1994, 32, 102.

Renormalization-Group Study Of A Hybrid Driven Diffusive System, K. E. Bassler, Beate Schmittmann

Beate Schmittmann

We consider a d-dimensional stochastic lattice gas of interacting particles, diffusing under the influence of a short-ranged, attractive Ising Hamiltonian and a ‘‘hybrid’’ external field which is a superposition of a uniform and an annealed random drive, acting in orthogonal subspaces of dimensions one and m, respectively. Driven into a nonequilibrium steady state, the half-filled system phase segregates via a continuous transition at a field-dependent critical temperature. Using renormalization-group techniques, we show that its critical behavior falls into a new universality class with upper critical dimension dc=5-m, characterized by two distinct anisotropy exponents, which, like all other indices, are computed …

Why Magnification Works, Terry L. Smith, Jay S. Huebner

Terry L. Smith

The simplest way to magnify the view of a small object is to bring the object closer to the eye, and of course science teachers know about magnifying glasses, telescopes, and microscopes.But why magnification works seems intuitive and is not usually explained to our students. We present here a few ideas on magnification that we use in our classroom and some general information on vision that we hope will be helpful to other teachers.

Why Magnification Works, Terry L. Smith, Jay S. Huebner

Jay S Huebner

The simplest way to magnify the view of a small object is to bring the object closer to the eye, and of course science teachers know about magnifying glasses, telescopes, and microscopes.But why magnification works seems intuitive and is not usually explained to our students. We present here a few ideas on magnification that we use in our classroom and some general information on vision that we hope will be helpful to other teachers.

Biaxiality In A Cyclic Thermotropic Nematic Liquid-Crystal, Jian-Feng Li, V. Percec, Ch. Rosenblatt, Oleg Lavrentovich

Oleg Lavrentovich

Birefringence and textural data are presented for a ring-shaped trimeric liquid crystal. The material apparently undergoes a uniaxial-to-biaxial nematic phase transition as a function of temperature.

Click‐Evoked Otoacoustic Emissions In Normal‐Hearing Children, Jacek Smurzynski

Jacek Smurzynski

Magnetic And Crystallographic Order In Α‐Manganese, A. C. Lawson, Allen C. Larson, M. C. Aronson, S. Johnson, Z. Fisk, Paul C. Canfield, J. D. Thompson, R. B. Von Dreele

Paul C. Canfield

We have made time‐of‐flight neutron diffractionmeasurements on α‐manganese metal. Powder diffraction measurements were made at 14 temperatures between 15 and 305 K, and single crystalmeasurements were made at 15 and 300 K. We found that the crystal structure of α‐Mn is tetragonal below its Néel point of 100 K, with crystal symmetryI4̄2m and magnetic (Shubnikov) symmetry P I 4̄21 c. In agreement with the earlier results of Yamada et al., there are six independent magnetic atoms, and we found that their moments are weakly temperature dependent. The onset of magnetic order causes slight changes in the atomic positions and in …

Comparison Between Calculatedand Observed F-Region Density Profiles At Jicamarca, Peru, A. J. Preble, D. N. Anderson, Bela G. Fejer, P. H. Doherty

Bela G. Fejer

Electron density profiles and isodensity contours derived from Jicamarca incoherent scatter radar observations in Peru for October 1–2, 1970, are compared in detail with results from the Phillips Laboratory global theoretical ionospheric model. This model solves the ion continuity equation for O+ concentration through production, loss, and transport of ionization. The primary factor controlling the peak plasma density at Jicamarca is the vertical E×B drift, which drives the ionization upward during the day and downward at night. When we use the measured drift in the model, we achieve excellent results with the measured electron density profiles. We illustrate the sensitivity …

Nanoscale Patterning And Oxidation Of H-Passivated Si(100)-2x1 Surfaces With An Ultrahigh Vacuum Scanning Tunneling Microscope, J. W. Lyding, T. -C. Shen, J. S. Hubaceck, J. R. Tucker, G. C. Abeln

T. -C. Shen

Nanoscale patterning of the hydrogen terminated Si(100)‐2×1 surface has been achieved with an ultrahigh vacuum scanning tunneling microscope.Patterning occurs when electrons field emitted from the probe locally desorb hydrogen, converting the surface into clean silicon. Linewidths of 1 nm on a 3 nm pitch are achieved by this technique. Local chemistry is also demonstrated by the selective oxidation of the patterned areas. During oxidation, the linewidth is preserved and the surrounding H‐passivated regions remain unaffected, indicating the potential use of this technique in multistep lithography processes.

Direct Simulation Monte Carlo For Thin Film Bearings, Alejandro Garcia, B. Alder, F. J. Alexander

Alejandro Garcia

The direct simulation Monte Carlo (DSMC) scheme is used to study the gas flow under a read/write head positioned nanometers above a moving disk drive platter (the slider bearing problem). In most cases, impressive agreement is found between the particle-based simulation and numerical solutions of the continuum hydrodynamic Reynolds equation which has been corrected for slip. However, at very high platter speeds the gas is far from equilibrium, and the load capacity for the slider bearing cannot be accurately computed from the hydrodynamic pressure.

Microscopic Simulation Of Dilute Gases With Adjustable Transport Coefficients, Alejandro Garcia, F. Baras, M. Malek Mansour

Alejandro Garcia

The Bird algorithm is a computationally efficient method for simulating dilute gas flows. However, due to the relatively large transport coefficients at low densities, high Rayleigh or Reynolds numbers are difficult to achieve by this technique. We present a modified version of the Bird algorithm in which the relaxation processes are enhanced and the transport coefficients reduced, while preserving the correct equilibrium and nonequilibrium fluid properties. The present algorithm is found to be two to three orders of magnitude faster than molecular dynamics for simulating complex hydrodynamical flows.

Efficient Timestamp Input And Output, Curtis Dyreson, Richard T. Snodgrass

Curtis Dyreson

In this paper we provide efficient algorithms for converting between the internal form of a timestamp, and various external forms, principally character strings specifying Gregorian dates. We give several algorithms that explore a range of time and space tradeoffs. Unlike previous algorithms that explore a range of time and space tradeoffs. Unlike previous algorithms, those discussed here have a constant time cost over a greatly extended range of timestamp values. These algorithms are especially useful in operating systems and in database management systems.

Thermodynamics Of A Free Q Fermion Gas, Asim Gangopadhyaya

Asim Gangopadhyaya

Hexagons And Squares In A Passive Nonlinear Optical System, John Geddes, R.A. Indik, J.V. Moloney, Willie Firth

John B. Geddes

Pattern formation is analyzed and simulated in a nonlinear optical system involving all three space dimensions as well as time in an essential way. This system, counterpropagation in a Kerr medium, is shown to lose stability, for sufficient pump intensity, to a nonuniform spatial pattern. We observe hexagonal patterns in a self-focusing medium, and squares in a self-defocusing one, in good agreement with analysis based on symmetry and asymptotic expansions.

Determination Of Plasma Parameters In A Dc Arcjet Diamond Reactor: Part 1, Scott Reeve, Wayne Weimer

Scott Reeve

No abstract provided.