Digital Commons Network^™

Charge Conservation In The Incommensurate-Commensurate Transition Of Charge Density Waves, Jörg Kastrup

Masters Theses

W ith the complex continuation method, introduced by Gupta and Sutherland (1976), the grand partition function for the sine-Gordon model of charge density waves may be calculated without violation of charge conservation. Contrary to previous works by Okwamoto, Takayama and Shiba (1979) and Turkevich and Doniach (1982) the chemical potential becomes temperature dependent. By direct comparison of numerical results with those of the above mentioned authors it is shown that the incommensurate-commensurate transition does not occur in the previously predicted temperature range.

Localization Of Light In One Dimensional Randomly Disordered Dielectric Medium, K. Balaji

Masters Theses

The concept of localization of electrons in semiconductors has been known ever since the 1960's when P. W. Anderson and N. F. Mott made detail investigations on the transport properties of alloys. At the end of the last decade, however, physicist started looking for localization in other systems. Specifically, they looked at the localization of light and acoustic waves in disordered systems. Localization is a property which arises for all wavelike excitations in disordered media.

In this study we analyze the localization of light in one dimensional randomly disordered dielectric medium using computer simulation (numerical integration method) technique. The results …

Critical Behavior Of Charge Density Waves Below Threshold: Numerical And Scaling Analysis, Alan Middleton, Daniel S. Fisher

Physics - All Scholarship

The critical behavior of pinned charge density waves (CDW's) is studied as the threshold for sliding is approached. Using the Fukuyama-Lee-Rice Hamiltonian with relaxational dynamics, the polarization and linear response are calculated numerically. ... On the irreversible approach to threshold, the response due to avalanches triggered by local instabilities dominates the polarizability, which diverges in one and two dimensions. Characteristic diverging length scales are studied using finite-size scaling of the sample-to-sample variations of the threshold field in finite systems and finite-size effects in the linear polarizability and the irreversible polarization. A dominant diverging correlation length is found which controls the …

Planar H2O Masers In Star-Forming Regions, Moshe Elitzur, David J. Hollenbach, Christopher F. Mckee

Physics and Astronomy Faculty Publications

The planar geometry of shocked material is the key property in enabling the high brightness temperatures of H20 masars in star-forming regions. We solve for the brightness temperature, the beaming angle, and the maser spot size for thin, saturated planar masers under the assumption that the velocity change across the maser due to ordered motions is small compared with the thermal or microturbulent line width. For a given set of physical parameters, the brightness temperature is essentially fully determined by the length of the velocity-coherent region in the shocked plane along the line of sight. The geometry in the transverse …

Measurement Of The Λ→N+Γ Branching Ratio, William J. Fickinger, D. K. Robinson

Faculty Scholarship

The branching ratio for the Λ weak radiative decay has been measured to be B(Λ→n+γ)/(Λ→anything) =[1.78±0.24(stat)±0.160.14(syst)]×10-3. A low-energy kaon beam was used to produce the Λ hyperons via the reaction K-+p→Λ+π0 at rest. Photons from the signal channel and π0 decay were detected with a NaI(Tl) array. The final spectrum contains 287 events after background subtraction, an order of magnitude more events than from the only previous measurement.

Electron Mean-Free Paths In The Alkali Metals, G. K. Wertheim, D. Mark Riffe, N. V. Smith, P. H. Citrin

All Physics Faculty Publications

Photoemission data in which the signal from the first atomic layer is well resolved from that of the bulk are used to determine accurately the kinetic-energy dependence of the inelastic-electron mean free path in the alkali metals. At the higher kinetic energies, the data are in very good agreement with the theory of Penn. Below about 10 eV, the mean free path in the heavier alkali metals drops markedly below the theoretical values. This is attributed to electron decay processes involving the unoccupied d bands.

Flux Trapping In Superconducting Pellets, Andreas Straub

Dissertations and Theses

This research concerns the effects on samples of nominal composition Bi_1.8Pb_0.2Sr₂Ca₂Cu₃O_y which were exposed to hot, dense argon in a ballistic compressor.

The investigations were concentrated on two specimens which were exposed to hot, dense argon at about 1800 K (peak pressure 330 atm) and 1500 K (peak pressure 230 atm), respectively. Sample Bi #1 showed a completely melted surface structure after triple exposure in the ballistic compressor at 1800 K while the surface of sample Bi #7 was just partly melted after double exposure at 1500 K.

Changes …

Modification Of The Magnetic Flux-Line Interaction At A Superconductor's Surface, M. Cristina Marchetti

Physics - All Scholarship

The pair interaction between magnetic flux lines in a semi-infinite slab of an anisotropic type-II superconductor in an external field is derived in the London limit. The case where the applied field is normal to the superconductor/vacuum interface is considered. The presence of stray fields near the surface leads to an additional contribution to the repulsive interaction between flux lines that vanishes exponentially with the distance from the interface. The pair interaction is used to obtain the continuum elastic energy of a distorted semi-infinite flux-line array. The presence of the superconductor/vacuum interface yields surface contributions to the compressional and tilt …

Structures Of Selected Boranes And Carboranes, Sunwoo Lee, Peter A. Dowben, A.T. Wen, A.P. Hitchcock, John A. Glass Jr., James T. Spencer

Peter Dowben Publications

Nido-pentaborane(9) (B₅H₉), 1, nido-2,3-diethyl-2,3-dicarbahexaborane(8)[( C₂H₅)₂C₂B₄H₆], 2, nido-decaborane (14) (B₁₀H₁₄), 3, closo- 1,2-dicarbadodecaborane(12) (H₂C₂B₁₀H₁₀), 4, can be used as possible source compounds for boron and boron carbide thin film deposition. Inner shell electron energy-loss spectroscopy (ISEELS) studies of the boron Is and carbon Is core excitations of gas phase species have been undertaken so as to characterize these molecular precursors at solid surfaces. The near edge structure of ISEELS provides a good "fingerprint" …

Convective Instability Of Autocatalytic Reaction Fronts In Vertical Cylinders, D. A. Vasquez, J. W. Wilder, Boyd F. Edwards

All Physics Faculty Publications

Linear stability analysis predicts that the onset of convection for an ascending autocatalytic reaction front in a vertical cylinder corresponds to a nonaxisymmetric mode. This mode consists of a single convective roll confined to the region near the reaction front, with fluid rising in half of the cylinder and falling in the other half. Experiments show a flat front below the onset of convection and an axisymmetric front well above the onset of convection. New experiments are called for to closely examine the onset of convection in order to test this prediction.

Defect Models In Electron-Irradiated N-Type Gaas, B. Ziebro, Joseph W. Hemsky, David C. Look

Physics Faculty Publications

1 MeV electron irradiation has been performed in degenerate, n‐type (n≂2×10¹⁷ cm^-3), molecular beam epitaxial GaAs layers, and Hall effect measurements have been carried out during the irradiation in order to get accurate defect production data. The results have been fitted with statistical models, and are most consistent with the usual E1 (E_C-0.045 eV) and E2 (E_C-0.15 eV) levels being the (-/0) and (0/+) transitions of the As vacancy, respectively. Also, an acceptor well below EC-0.15 eV is produced at a much higher rate than that of E1 and E2.

Analysis Of Multifrequency Dispersive Optical Bistability And Switching In Nonlinear Ring Cavities With Large Medium-Response Times, Pawel Pliszka, Partha P. Banerjee

Electrical and Computer Engineering Faculty Publications

Using a simple model of a ring cavity comprising a cubically nonlinear medium, we analyze dispersive optical bistability in the presence of more than one spectral component. We show the phenomenon of so-called competition for resonance. In addition to presenting cavity characteristics for the cases of two and three different frequencies, we also discuss the general method for finding steady-state solutions and checking their stability. A simple and efficient algorithm, based on a relaxation method, is devised to find steady-state solutions satisfying appropriate boundary conditions. The relaxation dynamics is physically related to a finite response time of the medium.

Recoil Effects And Cp Violation In Neutron Scattering, Vladimir Gudkov

Faculty Publications

The problem of the imitation of CP violation in neutron scattering is discussed. The thermal motion of nuclei cannot contribute to symmetry-violating effects. The influence of the nuclear depolarization due to neutron scattering is estimated.

Multi-Ion Ambipolar Diffusion, Thomas E. Gist

Theses and Dissertations

The continuity and momentum equations for electrons and multiple ion species are solved analytically and numerically using a new model for ambipolar diffusion. The general form of the model is valid for any quasi-neutral plasma for which the diffusion approximation is appropriate, including positive or negative ions, arbitrary geometries, and time dependence. The model provides criteria for determining when single ion diffusion theory is appropriate for describing multi-ion discharges, when a multi-ion proportional model is appropriate, and when a multi-ion nonproportional model is necessary. An expression is developed which provides a necessary condition for proportionality and provides the proportionality constant …

Electronic Structure Of Borane Cage Molecules Adsorbed On Si(111), Sunwoo Lee, Dongqi Li, Sophie M. Cendrowski-Guillaume, Peter A. Dowben, F. Keith Perkins, S.P. Frigo, R.A. Rosenberg

Peter Dowben Publications

We compare the valence band photoemission resulting from molecular adsorption of nido-decaborane(14) (B₁₀H₁₄) and nido-2,3-diethyl-2,3-dicarbahexaborane on Si(111) at 100 K. The relative shift of the apparent valence band edge with increasing molecular coverage is consistent with the relative energy difference in the highest occupied molecular orbital for these two molecules as inferred from modified neglect of differential overlap calculations. The results suggest that the screening is very similar, despite the very different molecular structures of these two nido cage molecules.

Photoluminescence Study Of Si1-XGeX/Si And Si/Ge Strained Layer Superlattices, Todd D. Steiner

Theses and Dissertations

In recent years, there has been interest in obtaining efficient photoluminescence (PL) and electroluminescence from Si_1-xGe_x/Si or Si/Ge superlattices. In this dissertation, longperiod c/Si superlattices grown by molecular beam epitaxy at 500°C and annealed post-growth using rapid thermal annealing were investigated using photoluminescence (PL). The as-grown samples have broad PL bands from isoelectronic centers related to Ge complexes as well as sharp near-edge bound exciton lines. The broad PL band was found to be 120 meV below the band gap, after accounting for the effects of confinement of holes. Post-growth annealing resulted in a reduction in …

Experimental Investigation Of Crossflow Instability Waves In Rotating Disk Flow, Alan E. Blanchard

Mechanical & Aerospace Engineering Theses & Dissertations

For the past 40 years, the crossflow (or inflectional) instability associated with rotating disk flow has been generally assumed to be a pure vortex that is fixed with respect to the disk; however, the terms "wave" and "vortex" have also been used interchangeably throughout the computational and experimental literature to describe this instability. Through the use of both hot-wire surveys and smoke-wire flow visualizations in the present investigation, the existence of crossflow instability waves prior to vortex formation has been shown. The results of the hot-wire surveys and smoke-wire flow visualizations for both a "clean" disk case and a disk …

Investigation Of Alignment Dynamics In Mg-Ne And Mg-Ar Two-Photon Fractional Collisions, David Allen Olsgaard

Physics Theses & Dissertations

A new technique is used to observe the dynamics of electronic alignment in a dissociating collision complex. The technique requires the absorption of two optical photons during a single binary collision. Two exciting light fields are tuned into the wings of the 3s¹s₀-3p₁ and 3p¹p₁-5s¹s₀ resonances of Mg where strong collisions with rare gas atoms induce transitions between corresponding molecular states. The alignment produced in the intermediate state by the first excitation is probed by the second laser as a function of relative linear laser polarization and laser …

Quasi-Discretization Of The Electron Continuum Emitted In Collisions Of 0.6 Mev U−¹ Au11+ With Noble Gases, S. Hagmann, W. Wolff, J. L. Shinpaugh, H. E. Wolf, Ronald E. Olson, C. P. Bhalla, R. Shingal, C. Kelbch, R. Herrmann, O. Jagutzki, R. Dörner, R. Koch, J. Euler, U. Ramm

Physics Faculty Research & Creative Works

We have measured relative doubly differential cross sections for electron emission in collisions of 0.6 MeV u⁻¹ Au¹¹⁺ projectile ions with He, Ne and Ar targets for laboratory electron-detection angles between 17' and 80' and electron energies from 100 eV to well above the classical binary encounter region. The authors observe that, independent of the target Zt, the electron spectra display three characteristic peak-line structures whose energies are nearly invariant with observation angle. These structures are attributed to the diffraction of quasi-free target electrons in the potential of the projectile. © 1992 IOP Publishing Ltd.

Electron Emission From Foils Induced By Energetic Heavy-Ion Impact, R. A. Sparrow, Ronald E. Olson, D. Schneider

Physics Faculty Research & Creative Works

Doubly differential cross sections have been measured and calculated for the electron emission produced by collisions of 3.5 MeV u⁻¹ U³⁸⁺ on thin carbon foils. The electron emission is observed in both forward and backward directions. Calculations are in reasonable agreement with the experimental results in both shape and absolute magnitude. At intermediate foil thicknesses, a double binary peak structure is observed that is due to 0° binary electrons being elastically scattered to angle θ along with those that are produced at θ and suffer no large-angle scattering events in the foil. Absolute electron yields show that several …

Numerical Study Of C>1 Matter Coupled To Quantum Gravity, Simon Catterall, John B. Kogut, Ray L. Renken

Physics - All Scholarship

We present the results of a numerical simulation aimed at understanding the nature of the `c = 1 barrier' in two dimensional quantum gravity. We study multiple Ising models living on dynamical \phi^3 graphs and analyse the behaviour of moments of the graph loop distribution. We notice a universality at work as the average properties of typical graphs from the ensemble are determined only by the central charge. We further argue that the qualitative nature of these results can be understood from considering the effect of fluctuations about a mean field solution in the Ising sector.

Coherent Population Transfer Via The Continuum, C. E. Carroll, Foek T. Hioe

Physics Faculty/Staff Publications

We present a remarkable analytic result which suggests that a continuum can be used as an intermediary for a significant transfer of population from one discrete state to another discrete state in a stimulated Raman transition. the population transfer is accomplished by employing two laser pulses that overlap in time, arranged in the so-called counterintuitive order. That a continuum can be used for population transfer is shown here for the first time. It promises to open up more channels for selective coherent excitation of atoms or molecules.

Diffusion-Limited Reactions In Spherical Cavities, Amy Lisa Graves, Elizabeth L. Grossman , '92, Dane D. Morgan Iii, '92, B. J. Berne

Physics & Astronomy Faculty Works

We study a quenching reaction occurring at sinks within a spherical cavity and at the cavity surface. One may think of reactions at these two, distinct locations as two, coupled reactive channels. Reactions of the type D* + A-->D + A are studied in the limit of nondilute A, present at both locations, and dilute D, present within the cavity. We use a Monte Carlo algorithm to compute mean rates, pseudo-first-order rates and branching ratios, and compare with results obtained by assuming that the two reactive channels operate in parallel. The ratio of activities of the two channels are …

Self Organization And A Dynamical Transition In Traffic Flow Models, Alan Middleton, Ofer Biham, Dov Levine

Physics - All Scholarship

A simple model that describes traffic flow in two dimensions is studied. A sharp {\it jamming transition } is found that separates between the low density dynamical phase in which all cars move at maximal speed and the high density jammed phase in which they are all stuck. Self organization effects in both phases are studied and discussed.

Magnetization And 57Fe Hyperfine Fields In Y2Fe17Z3- Δ (Z=H, C, Or N) Interstitial Compounds, Qi-Nian Qi, Hong Sun, Ralph Skomski, J.M.D. Coey

Ralph Skomski Publications

Measured magnetization and ⁵⁷Fe hyperfine fields at T∼0 K for Y₂Fe₁₇ and Y₂Fe₁₇Z_{3- δ} , with Z=H, C, or N and δ∼0.5, are analyzed to determine the influence of the interstitial atoms on the 3d magnetism. All are weak ferromagnets with a nearly-full 3d subband, although when Z=N, the magnetic moment of 38.1μ_B is very close to the fully saturated value of 39.4μ_B. On a local scale, the order of hyperfine fields B_hf^4f (=35 T)> B_hf^6g (=33 T)> B_hf^{12j …}

Native Donors And Acceptors In Molecular-Beam Epitaxial Gaas Grown At 200 Degrees C, David C. Look, D. C. Walters, M. Mier, C. E. Stutz, S. K. Brierley

Physics Faculty Publications

Absorption measurements at 1.1 and 1.2 μm were used along with the known electron and hole photoionization cross sections for EL2 to determine deep donor (EL2‐like) and acceptor concentrations N_D=9.9×10¹⁹ and N_A=7.9×10¹⁸ cm⁻³, respectively, in a 2‐μm‐thick molecular‐beam epitaxial GaAs layer grown at 200 °C on a 2‐in.‐diam semi‐insulating wafer. Both lateral and depth uniformities of N_D over the wafer were excellent as was also the case for the conductivity. Band conduction was negligible compared to hopping conduction at 296 K as evidenced by the lack of a measurable Hall coefficient.

Double Differential Cross Sections For State-Selective Electron Capture By Low-Energy Ar4+ And Ar5+ Ions From He And Ar, Serefettin Yaltkaya

Masters Theses

Doubly differential cross sections, in angle and energy, for state-selective single-electron capture from He and Ar by Ar⁴⁺ and Ar⁵⁺ ions have been studied experimentally at laboratory impact energies of 25q, 50q, and lOOq eV (q = 4 and 5, where q is the ion charge state) and at laboratory scattering angles between 0° and 5.5°, by means of translational energy spectroscopy. The translational energy-gain spectra show that only a few final states are selectively populated. Final-state transition probabilities were found to depend strongly on the charge state of the projectile ion and the ionization potential of the …

Mechanisms For Gaas Surface Passivation By A Molecular-Beam Epitaxial Cap Layer Grown At 200-Degrees-C, David C. Look, D. C. Walters, C. E. Stutz, K. R. Evans, J. R. Sizelove

Physics Faculty Publications

A thin, undoped, molecular beam epitaxial (MBE) GaAs cap layer grown on top of an n‐type conductive layer significantly reduces the free‐electron depletion from the latter. By analyzing electron transfer to surface, interface, and bulk acceptor states in the cap, as a function of cap thickness, we show that either (1) the usual E_C−0.7 eV surface states are absent, (2) a dense donor near E_C−0.4 eV exists or (3) a high donor interface charge (∼5×10¹² cm⁻²) is present. Any of these conclusions constitutes an important new aspect of low‐temperature MBE GaAs.

Three Dimensional Quantum Gravity Coupled To Ising Matter, Simon Catterall, Ray L. Renken, John B. Kogut

Physics - All Scholarship

We establish the phase diagram of three--dimensional quantum gravity coupled to Ising matter. We find that in the negative curvature phase of the quantum gravity there is no disordered phase for ferromagnetic Ising matter because the coordination number of the sites diverges. In the positive curvature phase of the quantum gravity there is evidence for two spin phases with a first order transition between them.

Direct Measurement Of The Van Der Waals Interaction Between An Atom And Its Images In A Micron-Sized Cavity, V. Sandoghdar, C. I. Sukenik, E. A. Hinds, S. Haroche

Physics Faculty Publications

The authors have measured by laser spectroscopy the energy of interaction between a sodium atom and its images in the walls of a micron-sized cavity. This cavity-QED study is the first direct quantitative test of the Lennard-Jones van der Waals interaction as a function of controlled atom-surface separation and mean-square electric dipole moment.