Physical Sciences and Mathematics Commons^™

Scaling Behavior Of Fluctuations In Systems With Continuous Symmetry, Ulrich Zürcher

Physics Faculty Publications

In nematic liquid crystals, director fluctuations correpond to the dynamical mode that is critical for all temperatures in zero external fields. The Hurst exponent characterizes the temporal behavior of the mean square displacement of director fluctuations, 〈[δn(r,t)-δn(r,0)]2〉∼t2H. We find H=1/2 in finite fields and H=3/4 in zero field. This result differs from that of Zhang et al. [Phys. Rev. Lett. 70, 1834 (1993)] who find a Hurst exponent that varies continuously from H≃1 in zero field to H≃1/2 in moderate fields.

The Cataclysmic Variable Hx Pegasi = Pg 2337 + 123: Caught On The Rise To Maximum, F. A. Ringwald

Dartmouth Scholarship

A radial velocity study of the Hα emission line of the cataclysmic variable HX Peg = PG 2337 + 123 shows that its orbital period is 0.2008 ± 0.0005 d (4.82 h). Magnitudes from Harvard College Observatory archive plates suggest that it is either a dwarf nova or a VY Scl star. The quiescent spectrum shows KIλλ 7665, 7699 Å in absorption, consistent with the presence of an unusual subdwarf-K mass-losing star.

At the beginning of the second night of the run, O Iλ 7773 Å was in absorption. Near midnight, the continuum rose rapidly into maximum, …

Ac Response Of The Flux-Line Liquid In High-Tc Superconductors, Lee-Wen Chen, M. Cristina Marchetti

Physics - All Scholarship

We use a hydrodynamics theory to discuss the response of a viscous flux-line liquid to an ac perturbation applied at the surface of the sample. The theory incorporates viscoelastic effects and describes the crossover between liquid-like and solid-like response of the vortex array as the frequency of the perturbation increases. A large viscosity from flux-line interactions and entanglement leads to viscous screening of surface fields. As a result, two frequency-dependent length scales are needed to describe the penetration of an ac field. For large viscosities the imaginary part of the ac permeability can exihibit, in addition to the well-know peak …

Magnetic And Magneto-Optical Properties Of Mn5(Ge1-XMX)3 Alloys With M=Sn, Pb, Y. Zhang, A.P. Runge, Z.S. Shan, David J. Sellmyer

David Sellmyer Publications

We report on the magnetic and magneto-optical properties of the intermetallic alloys Mn₅(Ge_1-xM_x)₃, where M=Sn, Pb. Generally, these alloys were ordered ferromagnetically at T≥300 K for x<0.3. The structure of Mn₅(Ge_1-xSn_x)₃ remains in the hexagonal Mn₅Si₃ (hP16) phase up to x=0.25 and then changes to Ni₂In (hP6) as x approaches 1. The magnetization and Curie temperature of these alloys is largest for small x and reach a minimum value near x=0.5. The room temperature Polar Kerr rotation, however, …

Electronic Structure And Curie Temperature Of Yfe12-XMoXNY Compounds, A.S. Fernando, J.P. Woods, Sitaram Jaswal, D. Welipitiya, B.M. Patterson, David J. Sellmyer

David Sellmyer Publications

The electronic structures of YFe_12-xMo_xN_y , where x=1.0, 2.0 and y=0, 0.7, have been studied with photoemission and spin-polarized calculations. The peak near the Fermi level in the energy distribution curves (EDC) becomes successively broader with larger Mo concentration. The features in the calculated density of state at 1.3 and 2.7 eV are not readily seen in the EDC, and this may be due to lifetime effects in these compounds. Finally, changes in Curie temperature (T_c) with the change of N or Mo concentration are compared with prediction of the …

Magnetic Properties Of Nd(Fe,Ti) 12 And Nd(Fe,Ti) 12NX Films Of Perpendicular Texture, D. Wang, David J. Sellmyer, I. Panagiotopoulos, D. Niarchos

David Sellmyer Publications

Nd(Fe,Ti)₁₂ and Nd(Fe,Ti)₁₂N_x films textured with c axis perpendicular to the film plane were synthesized by sputtering. For the Nd(Fe,Ti)₁₂ films a spin reorientation took place at about 170 K. For the Nd(Fe,Ti)₁₂N_x films, the anisotropy field is about 85 kOe at room temperature and 130 kOe at 5 K, corresponding to a coercivity of 2.7 and 12 kOe, respectively. Initial magnetization curves show a behavior characteristic of nucleation type of magnetization reversal mechanism. Attempts were made to fit the experimental data with Kronmüller's magnetization reversal model. The results suggest that nucleation …

Magneto-Optical And Structural Properties Of Bialdylg/Fe Multilayers, J. X. Shen, Kurt W. Wierman, Y. B. Zhang, Roger D. Kirby, John A. Woollam, David J. Sellmyer

Roger Kirby Publications

Bi- and Al-doped DyIG(y Å)/Fe(x Å) (y = 50 to 130 Å and x = 5 to 15 Å ) multilayer thin films were made by magnetron sputtering onto Si(111) substrates. Rapid thermal annealing was used to crystallize these garnet multilayers. The crystallized multilayer samples possess well-defined interfaces. Atomic force microscopy showed that the annealed samples had smooth surfaces, with small grain sizes (~30 to 80 nm). All of the samples had perpendicular magnetic anisotropy with very square hysteresis loops. The coercivity was found to vary from 600 Oe to 2 kOe, depending on Fe thickness and annealing …

Temperature-Dependent Interface Magnetism And Magnetization Reversal In Co/Pt Multilayers, Z. S. Shan, J. X. Shen, Roger D. Kirby, David J. Sellmyer, Y. J. Wang

Roger Kirby Publications

We report on the temperature dependence of the magnetic properties and interface magnetism of Co/Pt multilayers. The magnetic properties including magnetization and anisotropy change substantially as the temperature varies from 300 to 10 K for samples with Co layer thickness in the range from 3 to 7 Å. The interface anisotropy of about 0.38 erg/cm² is nearly independent of temperature. The magnetization reversal is dominated by domain wall motion for the thinner Co layers and dominated by nucleation for the thicker Co layers.

Blocking Of Logarithmic Temporal Relaxation Of Magnetic Remanence By Piezomagnetically Induced Domains In Fe1-XZnXF2, J. Kushauer, Christian Binek, Wolfgang Kleemann

Christian Binek Publications

Faraday rotation and superconducting quantum interference device magnetometry were used to investigate the relaxation of the weak-field induced remanence, µ, of the random-field Ising model system Fe_1-xZn_xF₂. The stretched logarithmic decay law, as predicted by Nattermann and Vilfan for the case of compact domains with fractal surfaces, was confirmed for freezing fields of 3≤B≤5 T. Virtually constant remanence, µ, was found at low fields, 0.0002≤B≤1.5 T. This ferrimagnetic moment is due to the piezomagnetic effect acting on random-stress induced immobile domains. Chemical etching, which removes surface stress originating from the sample preparation, proves to …

Magnetic And Magneto-Optic Properties Of Sputtered Co/Ni Multilayers, Y. B. Zhang, P. He, John A. Woollam, J. X. Shen, Roger D. Kirby, David J. Sellmyer

Roger Kirby Publications

We have investigated the magnetic and magneto-optic properties of Co/Ni multilayers deposited on Ag and Au buffer layers. The samples with Au buffer layers show perpendicular magnetic anisotropy, but those with Ag buffer layers do not. The. structure and degree of crystalline alignment of the buffer layer are evidently crucial to development of perpendicular magnetic anisotropy. We also present the results of polar Kerr rotation measurements as a function of wavelength and layer thickness of the multilayers.

Changes In Electron Localization And Density Of States Near EF Across The Nonmetal-Metal Transition In Mg Overlayers, Jiandi Zhang, D.N. Mcilroy, Peter A. Dowben

Peter Dowben Publications

A nonmetal-to-metal transition in Mg monolayers on Mo(112) has been indicated by photoemission and resonant photoemission. The dramatic changes of the density of states, the dispersion of bands near E_F, and screening are observed across the nonmetal-to-metal transition. The changes of the resonance photon energy and the intensity of Mg 2p→εd excitation, with different coverages, indicate that there exists a correlation between the electronic structure (particularly final-state screening effects) and the overlayer structure. The commensurate-to-incommensurate transition beyond 0.5 monolayer of coverage corresponds to the overlayer nonmetal-to-metal transition, which is due to the hybridization of Mg s and p …

Low-Temperature Characterization Of The Magnetic Properties Of Mnbial Thin Films, Kurt W. Wierman, J. X. Shen, Roger D. Kirby, David J. Sellmyer

Roger Kirby Publications

The magnetic properties of thin-film samples of MnBi_0.8Al_x with aluminum concentrations of x = 0.0, 0.4, 0.6, and 0.8 were systematically studied over a temperature range of 20 to 300 K. The as-deposited films are amorphous and nonmagnetic, but highly textured polycrystalline films that are ferromagnetic are formed by annealing at 350 °C. Our measurements show that the coercivity of such films rapidly decreases, then approaches a constant value (4 kOe for x = 0.4) with increasing annealing time. Magnetic measurements show that both anisotropy and coercivity decrease with decreasing temperature. Unlike bulk MnBi, our MnBi_{0.8 …}

Magnetization Reversal In Compositionally Modulated Tb/Fe Multilayers (Abstract), Roger D. Kirby, J. X. Shen, David J. Sellmyer

Roger Kirby Publications

We have prepared compositionally modulated Tb(X Å) thin film multilayers with X ranging from 3.5 to 7 Å by magnetron sputtering on glass substrates. Magnetization measurements using SQUID and alternating gradient force magnetometers show that these samples have uniaxial anisotropies (K_u) up to 10⁷ erg/cm³ and saturation magnetizations (M_s) between 50 and 350 emu/cm³. Constant field magnetization reversal measurements at room temperature show that these samples reverse by slow nucleation followed by relatively rapid domain wall motion. The samples with larger uniaxial anisotropies and smaller saturation magnetizations reverse completely …

Be Stars In Young Clusters, Shaukat N. Goderya, Edward G. Schmidt

Edward Schmidt Publications

Photometric Hχ and Hβ line indices for 71 B stars in seven galactic clusters are presented. Some stars in three of the clusters were observed at two epochs. Using these data nine Be stars were identified. Three of them were previously detected as emission-line stars, while six do not appear to have been previously recognized as such. Of the 62 stars for which we find no indication of emission, three were previously found to exhibit emission. We find that emission-line objects are absent among the most luminous early B giants.

Femtosecond Free-Electron Laser By Chirped Pulse Amplification, L.H. Yu, E. Johnson, D. Li, Donald P. Umstadter

Donald Umstadter Publications

In this work we combine elements of chirped pulse amplification techniques, now familiar in solid-state lasers, with an amplifier based upon a seeded free-electron laser (FEL). The resulting device would produce amplified pulses of unprecedented brevity at wavelengths shorter than can be currently obtained by any tunable laser system. We use a subharmonically seeded FEL to illustrate the concept. Radiation from a Ti:sapphire laser is frequency tripled and stretched optically to provide a coherent seed pulse for the FEL. When coupled to an electron beam inside a magnetic wiggler, the seed radiation introduces an additional energy modulation on the electron …

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 …

Renormalization Group Study Of The Standard Model And Its Extensions: The Minimal Supersymmetric Standard Model, Diego Castano, E. J. Piard, P. Raymond

Chemistry and Physics Faculty Articles

In this paper we summarize the minimal supersymmetric standard model as well as the renormalization group equations of its parameters. We proceed to examine the feasibility of the model when the breaking of supersymmetry is parametrized by the soft terms suggested by supergravity theories. In such models, the electroweak symmetry is exact at the tree level and is broken spontaneously at one-loop order. We make the additional assumption that the GUT-inspired relation m_b=m_τ be valid at the scale where the gauge coupling constants unify, which constrains the value of the top quark mass. For all types of …

Low-Temperature Growth Of High Resistivity Gaas By Photoassisted Metalorganic Chemical Vapor Deposition, J. C. Roberts, K. S. Boutros, S. M. Bedair, David C. Look

Physics Faculty Publications

We report the photoassisted low‐temperature (LT) metalorganic chemical vapor deposition (MOCVD) of high resistivity GaAs. The undoped as‐grown GaAs exhibits a resistivity of ∼10⁶ Ω cm, which is the highest reported for undoped material grown in the MOCVD environment. Photoassisted growth of doped and undoped device quality GaAs has been achieved at a substrate temperature of 400 °C in a modified atmospheric pressure MOCVD reactor. By using silane as a dopant gas, the LT photoassisted doped films have high levels of doping and electron mobilities comparable to those achieved by MOCVD for growth temperatures, T_g≳600 °C.

Electrochemical Capacitance-Voltage Analysis Of Delta-Doped Pseudomorphic High-Electron-Mobility Transistor Material, C. E. Stutz, B. Jogai, David C. Look, J. M. Ballingall, T. J. Rogers

Physics Faculty Publications

This work shows how electrochemical capacitance‐voltage (EC‐V) measurements can be used to evaluate delta‐doped pseudomorphic high electron mobility transistor material. These EC‐V measurements are compared with magnetic‐field‐dependent Hall effect (M‐Hall) measurements and a self‐consistent Poisson/k⋅p calculation of the band structure and electron concentration. The EC‐V technique can clearly delineate the cap layer, the delta‐doped layer, and the In_xGa_1−xAs channel layer, whereas the M‐Hall method characterizes only the cap and In_xGa_1−xAs channel layers. The amount of electron charge seen by …

Relaxation Technique For The Fitting Of Numerical Data, Michael J. Lehmann

Opportunities for Undergraduate Research Experience Program (OURE)

Data, in particular that generated through successive computer approximations, may not always follow the smooth curve it is suppose to. Approximations in input data, round off error, and estimations made in the mathematical formulas, are just some of the things that could cause this problem. In any respect, this data must somehow be smoothed or fitted for publication. One way of fitting such data sets is the relaxation method. This method does not rely on the data conforming to a specific mathematical shape, as opposed to other programs that require the data to be fitable by some mathematically generated curve. …

Ccd Photometry At The Umr Observatory, Christopher P. Smith

Opportunities for Undergraduate Research Experience Program (OURE)

During the summer of 1993 a number of advances at the University of Missouri-Rolla’s observatory have taken place. These advances range from cleaning the mirrors and polar aligning the 16-inch Cassegrain telescope to the acquisition of needed supporting material. A program for observing variable stars, implementing the newly acquired CCD1R image reduction program, searching galaxies for supernova, and the possibility of adding low resolution stellar spectroscopy to our research capabilities will be discussed. This manual will discuss how to implement the new CCDIR program.

A Study Of Magnetic Activity Effects On The Thermospheric Winds In The Low Latitude Ionosphere, Ricardo Cruz Davila

All Graduate Theses and Dissertations, Spring 1920 to Summer 2023

The purpose of this thesis is to examine the effects of magnetic activity on the low latitude F-region thermospheric winds. The F-region (120-1600 km) is a partially ionized medium where O⁺ and O are the major ion and neutral species, respectively. The thermospheric winds at these altitudes are driven primarily by pressure gradient forces resulting from the solar heating during the day and cooling at night. For this study, we use measured Fabry-Perot Interferometer (FPD winds at Arequipa (16.5°S, 71.5°W) and measured FPI and Incoherent Scatter Radar (ISR) winds at Arecibo (18.6°N, 66.8°W).

Previous wind studies at Arequipa and …

A Monte Carlo Simulation Of Coulomb Collisions And Wave-Particle Interactions In Space Plasma At High Latitudes, Imad Ahmad Barghouthi

All Graduate Theses and Dissertations, Spring 1920 to Summer 2023

Four studies were considered to simulate the ion behavior in the auroral region and the polar wind.

In study I, a Monte Carlo simulation was used to investigate the behavior of O⁺ ions that are E x B-drifting through a background of neutral O, with the effect of O⁺(Coulomb) self-collisions included. Wide ranges of the ion-to-neutral density ratio n_i / n_n and electrostatic field E were considered in order to investigate the change of ion behavior with respect to the solar cycle and altitude. For low altitudes and/or solar minimum (n_i / …

Tabletop X-Ray Lasers, D. C. Eder, P. Amendt, L. B. Dasilva, R. A. London, B. J. Macgowan, D. L. Matthews, B. M. Penetrante, M. D. Rosen, S. C. Silks, Thomas D. Donnelly, R. W. Falcone, G. L. Strobel

All HMC Faculty Publications and Research

Details of schemes for two tabletop size x‐ray lasers that require a high‐intensity short‐pulse driving laser are discussed. The first is based on rapid recombination following optical‐field ionization. Analytical and numerical calculations of the output properties are presented. Propagation in the confocal geometry is discussed and a solution for x‐ray lasing in Li‐like N at 247 Å is described. Since the calculated gain coefficient depends strongly on the electron temperature, the methods of calculating electron heating following field ionization are discussed. Recent experiments aimed at demonstrating lasing in H‐like Li at 135 Å are discussed along with modeling results. The …

Comparative Mössbauer Effect Study Of Several R₂Fe₁₇ And R₂Fe₁₇Nₓ Compounds, Gary J. Long, Sanjay R. Mishra, Oran Allan Pringle, Fernande Grandjean, K. H. Buschow

Chemistry Faculty Research & Creative Works

The Mössbauer spectra of Sm₂Fe₁₇ and Ho ₂Fe₁₇ and their nitrides have been measured between 295 and 85 K and analyzed with a model which is consistent with our earlier work on R₂Fe₁₇ and R₂Fe₁₇N_x compounds, where R is Pr, Nd, and Th. This model is completely consistent throughout these rare-earth compounds and is in agreement with the crystallographic changes occurring upon nitrogenation and with the prediction of band structure calculations. The dramatic increase in Curie temperature in the nitrides results from the expansion of the crystallographic …

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 …

Structure And Phase Transition Of The 6,5-Annulene Isomer Of C_{61}H_{2}, Andrea N. Lommen, Paul A. Heiney, Gavin B. M. Vaughan, Peter W. Stephens, Dengfa Liu, Dayin Li, Allan L. Smith, Andrew R. Mcghie, Robert M. Strongin, Laurent Brard, Amos B. Smith Iii

Chemistry Faculty Publications and Presentations

We have used differential scanning calorimetry and x-ray diffraction to study the crystalline phases of the 6,5-annulene isomer of C₆₁H₂. At 308 K the methylenated fullerenes are orientationally disordered in a face-centered cubic lattice with lattice parameter 14.19±0.02 Å. Below Tc=290 K the symmetry is lowered to Pa3¯, a simple-cubic structure. At 20 K the lattice parameter is 14.06±0.02 Å, with substantial statistical disorder remaining. The transition temperature in C₆₁H₂ is slightly higher than in C₆₀O and higher still than in C₆₀, but the three compounds exhibit orientationally …

Nebular Properties And The Ionizing Radiation Field In The Galactic Center, Joseph C. Shields, Gary J. Ferland

Physics and Astronomy Faculty Publications

Nebulosity in the central parsec of the Milky Way exhibits a low-ionization spectrum that has led previous analyses of this region to conclude that this material is photoionized by a relatively soft continuum. We have reanalyzed the infrared emission-line spectrum of te Galactic center in order to consider whether the data could actually be explained with photoionization by a relatively hard, yet dilute continuum, and additionally to constrain the properties of the reaiating plasma. We conclude that the composite infrared spectrum does not place strong restrictions on the nature of the ionizing continuum and that much of the ionized gas …

Hot Gas And The Origin Of The Nebular Continuum In Novae, Pedro Saizar, Gary J. Ferland

Physics and Astronomy Faculty Publications

A featureless blue continuum (with constant f_ν) is a defining feature of declining classical novae. The fact that f_ν is constant into the infrared, and the absence of a Balmer jump, suggests that this continuum originates in hot tenous gas. The electron temperature and density of the hot gas phase of classical nova QU Vul 1984 are estimated from ground-based optical and IUE ultraviolet observations. This region has a temperature of 10⁵ - 10⁶, and a density in the range of 2 x 10⁴ to 3 x 10⁵cm^-3. …