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Physics Faculty Research & Creative Works

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Articles 1171 - 1200 of 1959

Full-Text Articles in Physical Sciences and Mathematics

Approximately Self-Similar Critical Collapse In 2+1 Dimensions, Marco Cavaglia, Gerard Clement, Alessandro Fabbri Aug 2004

Approximately Self-Similar Critical Collapse In 2+1 Dimensions, Marco Cavaglia, Gerard Clement, Alessandro Fabbri

Physics Faculty Research & Creative Works

Critical collapse of a self-gravitating scalar field in a (2+1)-dimensional spacetime with negative cosmological constant seems to be dominated by a continuously self-similar solution of the field equations without cosmological constant. However, previous studies of linear perturbations in this background were inconclusive. We extend the continuously self-similar solutions to solutions of the field equations with negative cosmological constant, and analyze their linear perturbations. The extended solutions are characterized by a continuous parameter. A suitable choice of this parameter seems to improve the agreement with the numerical results. We also study the dynamics of the apparent horizon in the extended background.

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Instantons In Quantum Mechanics And Resurgent Expansions, Ulrich D. Jentschura, Jean Zinn-Justin Aug 2004

Instantons In Quantum Mechanics And Resurgent Expansions, Ulrich D. Jentschura, Jean Zinn-Justin

Physics Faculty Research & Creative Works

Certain quantum mechanical potentials give rise to a vanishing perturbation series for at least one energy level (which as we here assume is the ground state), but the true ground-state energy is positive. We show here that in a typical case, the eigenvalue may be expressed in terms of a generalized perturbative expansion (resurgent expansion). Modified Bohr-Sommerfeld quantization conditions lead to generalized perturbative expansions which may be expressed in terms of nonanalytic factors of the form exp(-a/g), where a>0 is the instanton action, and power series in the coupling g, as well as logarithmic factors. The ground-state energy, for …

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Hopping Versus Bulk Conductivity In Transparent Oxides: 12cao - 7al₂O₃, Julia E. Medvedeva, Arthur J. Freeman Aug 2004

Hopping Versus Bulk Conductivity In Transparent Oxides: 12cao - 7al₂O₃, Julia E. Medvedeva, Arthur J. Freeman

Physics Faculty Research & Creative Works

First-principles calculations of the mayenite-based oxide, [Ca12Al14O32]2+(2e-), reveal the mechanism responsible for its high conductivity. A detailed comparison of the electronic and optical properties of this material with those of the recently discovered transparent conducting oxide, H-doped UV-activated Ca12Al14O33, allowed us to conclude that the enhanced conductivity in [Ca12Al14O32]2+(2e-) is achieved by elimination of the Coulomb blockade of the charge carriers. This results in a transition from variable range-hopping behavior with a Coulomb gap in H-doped UV-irradiated Ca …

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Erratum: Test Of The Pluvinage Wave Function For The Helium Ground State (Physical Review A - Atomic, Molecular, And Optical Physics (2004) 70 (012712)), Darryl B. Jones, Joseph H. Macek, Don H. Madison Aug 2004

Erratum: Test Of The Pluvinage Wave Function For The Helium Ground State (Physical Review A - Atomic, Molecular, And Optical Physics (2004) 70 (012712)), Darryl B. Jones, Joseph H. Macek, Don H. Madison

Physics Faculty Research & Creative Works

No abstract provided.

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Test Of The Pluvinage Wave Function For The Helium Ground State, S. Jones, Joseph H. Macek, Don H. Madison Aug 2004

Test Of The Pluvinage Wave Function For The Helium Ground State, S. Jones, Joseph H. Macek, Don H. Madison

Physics Faculty Research & Creative Works

The accuracy of the Pluvinage wave function for the ground state of helium is investigated by considering a number of different physical processes including double ionization by photoabsorption, Compton scattering, and electron impact. In the high-energy limit of these processes, the accuracy of the initial ground state can be ascertained without reference to the final double-continuum state. In this limit, we find that a Hylleraas description is superior to the Pluvinage one. For intermediate energies, final-state correlation becomes important, so we employ a 3C description of the final state (the 3C wave function is the double-continuum analog of the Pluvinage …

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Electronic Structure And Light-Induced Conductivity Of A Transparent Refractory Oxide, Julia E. Medvedeva, Arthur J. Freeman, Mariana I. Bertoni, Thomas O. Mason Jul 2004

Electronic Structure And Light-Induced Conductivity Of A Transparent Refractory Oxide, Julia E. Medvedeva, Arthur J. Freeman, Mariana I. Bertoni, Thomas O. Mason

Physics Faculty Research & Creative Works

Combined first-principles and experimental investigations reveal the underlying mechanism responsible for a drastic change of the conductivity (by 10 orders of magnitude) following hydrogen annealing and UV irradiation in a transparent oxide, 12CaO · 7Al2O3, found by Hayashi et al. [Nature (London) 419, 462 (2002).] The charge transport associated with photoexcitation of an electron from H- occurs by electron hopping.We identify the atoms participating in the hops, determine the exact paths for the carrier migration, estimate the temperature behavior of the hopping transport, and predict a way to enhance the conductivity by specific doping.

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Charge Transfer In Slow Collisions Of O⁸⁺ And Ar⁸⁺ Ions With H(1s) Below 2 Kev/Amu, Teck-Ghee Lee, M. Hesse, Anh-Thu Le, C. D. Lin Jul 2004

Charge Transfer In Slow Collisions Of O⁸⁺ And Ar⁸⁺ Ions With H(1s) Below 2 Kev/Amu, Teck-Ghee Lee, M. Hesse, Anh-Thu Le, C. D. Lin

Physics Faculty Research & Creative Works

We calculated the charge-transfer cross sections for O⁸⁺ + H collisions for energies from 1eV/amu to 2keV/amu, using the recently developed hyperspherical close-coupling method. In particular, the discrepancy for electron capture to the n = 6 states of O⁷⁺ from the previous theoretical calculations is further analyzed. Our results indicate that at low energies (below 100eV∕amu) electron capture to the n=6 manifold of O7+ becomes dominant. The present results are used to resolve the long-standing discrepancies from the different elaborate semiclassical calculations near 100eV/amu. We have also performed the semiclassical atomic orbital close-coupling calculations with straight-line trajectories. We found the …

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Excitation Energies From Time-Dependent Density-Functional Theory Beyond The Adiabatic Approximation, Carsten A. Ullrich, Kieron Burke Jul 2004

Excitation Energies From Time-Dependent Density-Functional Theory Beyond The Adiabatic Approximation, Carsten A. Ullrich, Kieron Burke

Physics Faculty Research & Creative Works

The nonadiabatic effects for excitation energies from time-dependent density-functional theory (TDDFT) were analyzed. a general analytic expression for nonadiabatic corrections to excitation energies of finite systems was derived and singlet s→s and s→p excitations of closed-shell atoms were calculated. It is observed that as the Vignale, Ullrich and Conti (VUC) functional is based on frequency-dependent xc kernel of homogeneous electron gas so it produces excitation energies with small but finite imaginary parts. the results show that the VUC approach determines excitation energies for atomic and molecular systems as well as for plasmon-like excitations in solids.

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Comparison Of Hyperspherical Versus Common-Reaction-Coordinate Close-Coupling Methods For Ion-Atom Collisions At Low Energies, Anh-Thu Le, C. D. Lin, L. F. Errea, L. Mendez, A. Riera, B. Pons Jun 2004

Comparison Of Hyperspherical Versus Common-Reaction-Coordinate Close-Coupling Methods For Ion-Atom Collisions At Low Energies, Anh-Thu Le, C. D. Lin, L. F. Errea, L. Mendez, A. Riera, B. Pons

Physics Faculty Research & Creative Works

We present detailed comparisons between the two quantal approaches--hyperspherical close-coupling and common-reaction-coordinate close-coupling methods--on an exemplary case of He²⁺ + H(1s) collisions at center-of-mass energy from 20eV up to 1.6keV. It is shown that the partial-wave charge-transfer cross sections from the two approaches agree very well at low energy below 200eV down to 30eV. This good agreement is a strong indication of the validity of both methods. The small difference at very low energies and the convergence with respect to the number of channels in both approaches at higher energies are also discussed.

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Gravitational Energy Loss In High Energy Particle Collisions: Ultrarelativistic Plunge Into A Multidimensional Black Hole, Emanuele Berti, Marco Cavaglia, Leonardo Gualtieri Jun 2004

Gravitational Energy Loss In High Energy Particle Collisions: Ultrarelativistic Plunge Into A Multidimensional Black Hole, Emanuele Berti, Marco Cavaglia, Leonardo Gualtieri

Physics Faculty Research & Creative Works

We investigate the gravitational energy emission of an ultrarelativistic particle radially falling into a D-dimensional black hole. We numerically integrate the equations describing black hole gravitational perturbations and obtain the energy spectra, total energy, and angular distribution of the emitted gravitational radiation. The black hole quasinormal modes for scalar, vector, and tensor perturbations are computed in the WKB approximation. We discuss our results in the context of black hole production at the TeV scale.

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New Limits On The Drift Of Fundamental Constants From Laboratory Measurements, Marc P. Fischer, Nikolai N. Kolachevsky, Marcus Zimmermann, Ronald Holzwarth, Th H. Udem, Theodor Wolfgang Hansch, Michel Abgrall, Jan Grunert, Ivan Maksimovic, Sebastien Bize, Harold Marion, Franck Pereira M Dos Santos, Pierre Lemonde, Giorgio Santarelli, Ph Laurent, Andre Clairon, Christophe Salomon, Martin K. Haas, Ulrich D. Jentschura, Christoph H. Keitel Jun 2004

New Limits On The Drift Of Fundamental Constants From Laboratory Measurements, Marc P. Fischer, Nikolai N. Kolachevsky, Marcus Zimmermann, Ronald Holzwarth, Th H. Udem, Theodor Wolfgang Hansch, Michel Abgrall, Jan Grunert, Ivan Maksimovic, Sebastien Bize, Harold Marion, Franck Pereira M Dos Santos, Pierre Lemonde, Giorgio Santarelli, Ph Laurent, Andre Clairon, Christophe Salomon, Martin K. Haas, Ulrich D. Jentschura, Christoph H. Keitel

Physics Faculty Research & Creative Works

We have remeasured the absolute 1S-2S transition frequency νH in atomic hydrogen. A comparison with the result of the previous measurement performed in 1999 sets a limit of (-29 ± 57) Hz for the drift of νH with respect to the ground state hyperfine splitting νCs in 133Cs. Combining this result with the recently published optical transition frequency in 199Hg+ against νCs and a microwave 87Rb and 133Cs clock comparison, we deduce separate limits on α˙ / α = (-0.9 &plusm; 2.9) x 10-15 yr-1 and the fractional time …

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Electron Self-Energy For Higher Excited S Levels, Ulrich D. Jentschura, Peter J. Mohr Jun 2004

Electron Self-Energy For Higher Excited S Levels, Ulrich D. Jentschura, Peter J. Mohr

Physics Faculty Research & Creative Works

A nonperturbative numerical evaluation of the one-photon electron self-energy for the 3S and 4S states with charge numbers Z=1 to 5 is described. The numerical results are in agreement with known terms in the expansion of the self-energy in powers of Zα.

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Close-Coupling And Distorted-Wave Calculations For Electron-Impact Excitation Of The (5p⁵6p) States Of Xenon, K. Bartschat, Anand Dasgupta, Don H. Madison Jun 2004

Close-Coupling And Distorted-Wave Calculations For Electron-Impact Excitation Of The (5p⁵6p) States Of Xenon, K. Bartschat, Anand Dasgupta, Don H. Madison

Physics Faculty Research & Creative Works

We report on a series of calculations for electron-impact excitation of the (5p56p) states in xenon from the ground state (5p6)1S0 . As in previous calculations for other noble-gas targets, we find strong evidence of channel coupling for all incident energies considered (between threshold and 200 eV ). Although qualitative agreement with the experimental results of Fons and Lin [Phys. Rev. A 58, 4603 (1998)] is achieved, severe quantitative discrepancies of sometimes more than a factor of 2 remain.

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Coherent Control Of Intersubband Optical Bistability In Quantum Wells, H. O. Wijewardane, Carsten A. Ullrich May 2004

Coherent Control Of Intersubband Optical Bistability In Quantum Wells, H. O. Wijewardane, Carsten A. Ullrich

Physics Faculty Research & Creative Works

The nonlinear intersubband (ISB) response of conduction electrons in quantum well were investigated. the coherent control of ISB optical biostability was analyzed using terahertz (THz) control pulses to induce picosecond switching between the bistable states. the switching speed was also determined by the ISB relaxation and decoherence times, T 1 and T 2. the results show that the shortest down-switching times of 50-6- ps are achieved by control pulses with intensity not far from the driving field, 30 W/cm 2.

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Self-Energy Correction To The Two-Photon Decay Width In Hydrogenlike Atoms, Ulrich D. Jentschura May 2004

Self-Energy Correction To The Two-Photon Decay Width In Hydrogenlike Atoms, Ulrich D. Jentschura

Physics Faculty Research & Creative Works

We investigate the guage invariance of the leading logarithmic radiative correction to the two-photon decay width in hydrogenlike atoms, was investigated. The effective treatment of the correction using a Lamb-shift led to the equivalent results in both the length and velocity gages. The relevant radiative corrections were found to be related to the energies that entered into the propagator denominators, to the Hamiltonian, to the wave functions, and to the energy conservation condition, that holds between two photons. The results show that the dominant radiative correction to the 2S two-photon decay width is about -2.020 536(α/π)(Zα)2 1n[(Zα)-2] …

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Protonium Formation In The P̅-H Collision At Low Energies By A Diabatic Approach, M. Hesse, Anh-Thu Le, C. D. Lin May 2004

Protonium Formation In The P̅-H Collision At Low Energies By A Diabatic Approach, M. Hesse, Anh-Thu Le, C. D. Lin

Physics Faculty Research & Creative Works

We present a diabatization technique in combination with the recently developed hyperspherical close coupling (HSCC) method. In contrast to the strict diabatization, our simple diabatization procedure transforms only sharp avoided crossings in the adiabatic hyperspherical potential curves into real crossings. With this approach, the weak collision channels can be removed from the close-coupling calculations. This method is used to study the antiproton-hydrogen collision at low energies. In the case of a scaled down (anti)proton mass, we show that a 10-channel calculation is enough to obtain converged cross sections at low energies. The results also indicate that protonium formation occurs mostly …

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Comment On "Role Of The Ground State In Electron-Atom Double Ionization" [1] (Multiple Letters), Jamal Berakdar, Stephenie J. Jones, Don H. Madison Apr 2004

Comment On "Role Of The Ground State In Electron-Atom Double Ionization" [1] (Multiple Letters), Jamal Berakdar, Stephenie J. Jones, Don H. Madison

Physics Faculty Research & Creative Works

No abstract provided.

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Scaling Laws For Single And Multiple Electron Loss From Projectiles In Collisions With A Many-Electron Target, Antonio C. Santos, Robert D. Dubois Apr 2004

Scaling Laws For Single And Multiple Electron Loss From Projectiles In Collisions With A Many-Electron Target, Antonio C. Santos, Robert D. Dubois

Physics Faculty Research & Creative Works

Using measured cross sections, empirical scaling laws are extracted for projectile stripping induced by collisions with a many-electron target. by scaling both the cross sections and the impact velocities, it is shown that a single universal curve can be used to fit data for single and multiple electron loss from negative ions, neutral particles, and singly or multiply charged positive ions. The scaling applies to projectiles ranging from hydrogen to uranium and collision energies ranging from below keV∕u to hundreds of MeV∕u . At high energies, existing data are consistent with a v−1 impact velocity dependence for scaled velocities less …

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Doubly Differential Multiple Ionization Of Krypton By Positron And Electron Impact, Kay Dubois, Antonio C. Santos, Ahmad Hasan Mar 2004

Doubly Differential Multiple Ionization Of Krypton By Positron And Electron Impact, Kay Dubois, Antonio C. Santos, Ahmad Hasan

Physics Faculty Research & Creative Works

Measurements of doubly differential single and multiple ionization of krypton atoms have been performed for 750 eV positron and electron impact. Data were measured as a function of projectile energy-loss and scattering angle. For electrons, the energy-loss range was 0-85% of the initial projectile energy and scattering angles were between ±22°. Following the procedure adopted previously for argon, the electron impact data were placed on an absolute scale by normalizing to total ionization cross sections available in the literature. The present results for krypton show differences between positron and electron impact that are less pronounced than was found for argon. …

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Laser Writing Of Semiconductor Nanoparticles And Quantum Dots, Massimo F. Bertino, Raghuveer Reddy Gadipalli, J. Greg Story, C. G. Williams, Guo-Hui Zhang, Chariklia Sotiriou-Leventis, Akira Tokuhiro, Suchi Guha, Nicholas Leventis Jan 2004

Laser Writing Of Semiconductor Nanoparticles And Quantum Dots, Massimo F. Bertino, Raghuveer Reddy Gadipalli, J. Greg Story, C. G. Williams, Guo-Hui Zhang, Chariklia Sotiriou-Leventis, Akira Tokuhiro, Suchi Guha, Nicholas Leventis

Physics Faculty Research & Creative Works

Silica aerogels were patterned with CdS using a photolithographic technique based on local heating with infrared (IR) light. The solvent of silica hydrogels was exchanged with an aqueous solution of the precursors CdNO3 and NH4 OH, all precooled to a temperature of 5°C. Half of the bathing solution was then replaced by a thiourea solution. After thiourea diffused into the hydrogels, the samples were exposed to a focused IR beam from a continuous wave, Nd-YAG laser. The precursors reacted in the spots heated by the IR beam to form CdS nanoparticles. We lithographed features with a diameter of …

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Progress At The Heidelberg Ebit, Jose R. Crespo Lopez-Urrutia, Jean Pierre Braun, Gunter Brenner, Hjalmar Bruhns, Christina Dimopoulou, I. N. Draganic, Daniel Fischer, Antonio J. Gonzalez, A. Lapierre, V. Mironov, Robert Moshammer, R. Soria Orts, Hiroyuki Tawara, M. Trinczek, Joachim Hermann Ullrich Jan 2004

Progress At The Heidelberg Ebit, Jose R. Crespo Lopez-Urrutia, Jean Pierre Braun, Gunter Brenner, Hjalmar Bruhns, Christina Dimopoulou, I. N. Draganic, Daniel Fischer, Antonio J. Gonzalez, A. Lapierre, V. Mironov, Robert Moshammer, R. Soria Orts, Hiroyuki Tawara, M. Trinczek, Joachim Hermann Ullrich

Physics Faculty Research & Creative Works

Two years after the relocation of the Heidelberg EBIT, several experiments are already in operation. Spectroscopic measurements in the optical region have delivered the most precise reported wavelengths for highly charged ions, in the case of the forbidden transitions of Ar XIV and Ar XV. The lifetimes of the metastable levels involved in those transitions has been determined with an error of less than 0.2%. A new, fully automatized x-ray crystal spectrometer allows systematic measurements with very high precision and reproducibility. Absolute measurements of the Lyman series of H-like ions are currently underway. Dielectronic recombination studies have yielded information on …

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Renormalization-Group Analysis Of The Generalized Sine-Gordon Model And Of The Coulomb Gas For D > 3 Dimensions, I. Nandori, Ulrich D. Jentschura, Kornel Sailer, Gerhard Soff Jan 2004

Renormalization-Group Analysis Of The Generalized Sine-Gordon Model And Of The Coulomb Gas For D > 3 Dimensions, I. Nandori, Ulrich D. Jentschura, Kornel Sailer, Gerhard Soff

Physics Faculty Research & Creative Works

Renormalization-group (RG) flow equations have been derived for the generalized sine-Gordon model (GSGM) and the Coulomb gas (CG) in d ≥ 3 of dimensions by means of the Wegner-Houghton method, and by way of the real-space RG approach. The UV scaling laws determined by the leading-order terms of the flow equations are in qualitative agreement for all dimensions d ≥ 3, independent of the dimensionality, and in sharp contrast to the special case d=2. For the 4-dimensional GSGM it is demonstrated explicitly (by numerical calculations) that the blocked potential tends to a constant effective potential in the infrared limit, satisfying …

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Effects Of Localization And Amplification On Intensity Distribution Of Light Transmitted Through Random Media, Alexey Yamilov, Hui Cao Jan 2004

Effects Of Localization And Amplification On Intensity Distribution Of Light Transmitted Through Random Media, Alexey Yamilov, Hui Cao

Physics Faculty Research & Creative Works

We numerically study the statistical distribution of intensity of light transmitted through quasi-one-dimensional random media by varying the dimensionless conductance g and the amount of absorption or gain. A markedly non-Rayleigh distribution is found to be well fitted by the analytical formula of Nieuwenhuizen et al. [Phys. Rev. Lett. 74, 2674 (1995)] with a single parameter g′ . We show that in the passive random system g′ is uniquely related to g , while in amplifying or absorbing random media g′ also depends on the gain or absorption coefficient.

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Highest-Quality Modes In Disordered Photonic Crystals, Alexey Yamilov, Hui Cao Jan 2004

Highest-Quality Modes In Disordered Photonic Crystals, Alexey Yamilov, Hui Cao

Physics Faculty Research & Creative Works

We studied the modes of the highest-quality factor Qm in disordered photonic crystals. by varying the strength of disorder, we identified five different scaling regimes of the ensemble averaged (Qm) with the system size. For sufficiently large systems, (Qm) reaches the maximum at some finite degree of disorder, where its value is comparable to the quality factor of an intentionally introduced single defect at the center of a photonic band gap. Near this optimal degree of disorder, we predict a superexponential increase of (Qm) with the system size, due to migration of the frequencies of the highest-quality modes toward the …

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Kinetics Of Binary Nucleation Of Vapors In Size And Composition Space, Sergey P. Fisenko, Gerald Wilemski Jan 2004

Kinetics Of Binary Nucleation Of Vapors In Size And Composition Space, Sergey P. Fisenko, Gerald Wilemski

Physics Faculty Research & Creative Works

We reformulate the kinetic description of binary nucleation in the gas phase using two natural independent variables: the total number of molecules g and the molar composition x of the cluster. The resulting kinetic equation can be viewed as a two-dimensional Fokker-Planck equation describing the simultaneous Brownian motion of the clusters in size and composition space. Explicit expressions for the Brownian diffusion coefficients in cluster size and composition space are obtained. For characterization of binary nucleation in gases three criteria are established. These criteria establish the relative importance of the rate processes in cluster size and composition space for different …

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Order-Parameter Symmetry And Mode-Coupling Effects At Dirty Superconducting Quantum Phase Transitions, Rastko Sknepnek, Thomas Vojta, Rajesh Narayanan Jan 2004

Order-Parameter Symmetry And Mode-Coupling Effects At Dirty Superconducting Quantum Phase Transitions, Rastko Sknepnek, Thomas Vojta, Rajesh Narayanan

Physics Faculty Research & Creative Works

We derive an order-parameter field theory for a quantum phase transition between a disordered metal and an exotic (non- s -wave) superconductor. Mode coupling effects between the order parameter and other fermionic soft modes lead to an effective long-range interaction between the anomalous density fluctuations which is reflected in singularities in the free energy functional. However, this long-range interaction is not strong enough to suppress disorder fluctuations. The asymptotic critical region is characterized by run-away flow to large disorder. For weak coupling, this asymptotic region is very narrow. It is preempted by a wide crossover regime with mean-field critical behavior …

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Smeared Phase Transition In A Three-Dimensional Ising Model With Planar Defects: Monte Carlo Simulations, Rastko Sknepnek, Thomas Vojta Jan 2004

Smeared Phase Transition In A Three-Dimensional Ising Model With Planar Defects: Monte Carlo Simulations, Rastko Sknepnek, Thomas Vojta

Physics Faculty Research & Creative Works

We present results of large-scale Monte Carlo simulations for a three-dimensional Ising model with short-range interactions and planar defects, i.e., disorder perfectly correlated in two dimensions. We show that the phase transition in this system is smeared, i.e., there is no single critical temperature, but different parts of the system order at different temperatures. This is caused by effects similar to but stronger than Griffiths phenomena. In an infinite-size sample there is an exponentially small but finite probability to find an arbitrary large region devoid of impurities. Such a rare region can develop true long-range order while the bulk system …

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Study Of Structural And Magnetic Properties Of Iron-Rich Mixed Rare-Earth Nddyfe (17-Y-X)Coxsiy Compounds, Kishore Kamaraju, Jinbo Yang, William B. Yelon, Oran Allan Pringle, M. S. Kim, Qingsheng Cai, William Joseph James Jan 2004

Study Of Structural And Magnetic Properties Of Iron-Rich Mixed Rare-Earth Nddyfe (17-Y-X)Coxsiy Compounds, Kishore Kamaraju, Jinbo Yang, William B. Yelon, Oran Allan Pringle, M. S. Kim, Qingsheng Cai, William Joseph James

Physics Faculty Research & Creative Works

A series of NdDyFe(17-y-x)CoxSiy solid solutions with = 2 and 3 and = 0.5 1.0 and 1.5 were prepared by induction melting stoichiometric amounts of high-purity elements. The postannealed samples consist of two phases belonging to the space groups R3 m and P63 mmc . The lattice parameters and the unit cell volumes were calculated from the refinements of the magnetic and structural unit cells using the FULLPROF version of the Rietveld program. For a fixed content of Co, the maximum Curie temperatures (305 C to 405 C) were observed in samples with = 1 and having two phases, a …

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Traversal Times For Random Walks On Small-World Networks, V. M. Kenkre, Paul Ernest Parris Jan 2004

Traversal Times For Random Walks On Small-World Networks, V. M. Kenkre, Paul Ernest Parris

Physics Faculty Research & Creative Works

We study the mean traversal time tau for a class of random walks on Newman-Watts small-world networks, in which steps around the edge of the network occur with a transition rate F that is different from the rate f for steps across small-world connections. when f>>F, the mean time tau to traverse the network exhibits a transition associated with percolation of the random graph (i.e., small-world) part of the network, and a collapse of the data onto a universal curve. This transition was not observed in earlier studies in which equal transition rates were assumed for all allowed steps. …

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Ultraviolet Photonic Crystal Laser, Alexey Yamilov, Hui Cao, Robert P. H. Chang, Xiaohua Wu, Xang Liu, Shuyou Li, Vinayak P. Dravid Jan 2004

Ultraviolet Photonic Crystal Laser, Alexey Yamilov, Hui Cao, Robert P. H. Chang, Xiaohua Wu, Xang Liu, Shuyou Li, Vinayak P. Dravid

Physics Faculty Research & Creative Works

We fabricated two-dimensional photonic crystal structures in zinc oxide films with focused-ion-beam etching. Lasing is realized in the near-ultraviolet frequency at room temperature under optical pumping. From the measurement of lasing frequency and spatial profile of the lasing modes, as well as the photonic band structure calculation, we conclude that lasing occurs in the strongly localized defect modes near the edges of photonic band gap. These defect modes originate from the structure disorder unintentionally introduced during the fabrication process. ©2004 American Institute of Physics

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