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Articles 1 - 25 of 25
Full-Text Articles in Physics
Effect Of Ammonia Flow Rate On Impurity Incorporation And Material Properties Of Si-Doped Gan Epitaxial Films Grown By Reactive Molecular Beam Epitaxy, Wook Kim, A. E. Botchkarev, H. Morkoç, Z-Q. Fang, David C. Look, David J. Smith
Effect Of Ammonia Flow Rate On Impurity Incorporation And Material Properties Of Si-Doped Gan Epitaxial Films Grown By Reactive Molecular Beam Epitaxy, Wook Kim, A. E. Botchkarev, H. Morkoç, Z-Q. Fang, David C. Look, David J. Smith
Physics Faculty Publications
Effect of ammonia flow rate on the impurity incorporation and material properties of Si-doped GaN films grown by reactive molecular beam epitaxy (RMBE) process is discussed. It appears that the ammonia flow rate has a marginal effect on the incorporation of impurities into the Si-doped GaN films except there was a little decrease in O and Si with increasing ammonia flow rate when the Si concentration in the film is higher than 1018 cm−3. Electron Hall mobility of Si-doped GaN films grown by RMBE varies with ammonia flow rate used during film growth. From deep level transient …
Excitation Efficiency Of A Morphology-Dependent Resonance By A Focused Gaussian Beam, James A. Lock
Excitation Efficiency Of A Morphology-Dependent Resonance By A Focused Gaussian Beam, James A. Lock
Physics Faculty Publications
The excitation efficiency of a morphology-dependent resonance (MDR) by an incident beam is defined as the fraction of the beam power channeled into the MDR. The efficiency is calculated for a focused Gaussian beam of arbitrary width incident on either a spherical particle or a cylindrical fiber located at an arbitrary position in the plane of the beam waist. In each case a simple formula for the efficiency is derived by use of the localized approximation for the beam-shape coefficients in the partial-wave expansion of the beam. The physical interpretation of the efficiency formulas is also discussed. (C) 1998 Optical …
Relaxational Mode Structure For Optical Probe Diffusion In High Molecular Weight Hydroxypropylcellulose, Kiril A. Streletzky, George D.J. Phillies
Relaxational Mode Structure For Optical Probe Diffusion In High Molecular Weight Hydroxypropylcellulose, Kiril A. Streletzky, George D.J. Phillies
Physics Faculty Publications
We studied translational diffusion of dilute monodisperse spheres (diameters 14 < d < 455 nm) in aqueous 1 MDa hydroxypropylcellulose (0 ≤ c ≤ 7 g/L) at 25°C using quasielastic light scattering. Spectra are highly bimodal. The two spectral modes (“slow,” “fast”) have different physical properties. Probe behavior differs between small (d < Rh) and large (d ≥ Rg) probes; Rh and Rg are the matrix polymer hydrodynamic radius and the radius of gyration, respectively. We examined the dependences of spectral lineshape parameters on d, c, scattering vector q, and viscosity η for all four probe-size and mode-type combinations. We find three time scale-separated modes: (1) a large-probe slow mode has properties characteristic of particle motion in a viscous medium; (2) a large-probe fast mode and small-probe slow modes share the same time scale, and have properties characteristic of probe motion coupled to internal chain dynamics; and (3) a small-probe fast mode has properties that can be attributed to the probe sampling local chain relaxations. In the analysis, we also attempted to apply the coupling/scaling (CS) model of Ngai and Phillies [Ngai, K. L., Phillies, G. D. J. J. Chem. Phys.,105, 8385 (1996)] to analyze our data. We find that the second mode is described by the coupling/scaling model for probe diffusion; the first and third modes do not follow the predictions of this model.
Development Of A Generalized Theoretical Model For The Response Of A Phase/Doppler Measurement System To Arbitrarily Oriented Fibers Illuminated By Gaussian Beams, Scott A. Schaub, James A. Lock, Amir A. Naqwi
Development Of A Generalized Theoretical Model For The Response Of A Phase/Doppler Measurement System To Arbitrarily Oriented Fibers Illuminated By Gaussian Beams, Scott A. Schaub, James A. Lock, Amir A. Naqwi
Physics Faculty Publications
We present a generalized theoretical model for the response of the phase/Doppler (P/D) measurement system to light scattered by cylindrical fibers. This theoretical model is valid for arbitrary fiber diameters and refractive indices, for Gaussian incident beams, and it accounts for arbitrary fiber orientations, fiber positions, and effects that are due to the two-dimensional receivers. The generalized P/D computer model (GPDCM) is the extension of an earlier study by the authors, combining past P/D simulation methodology with recent developments in modeling light scattering by tilted cylindrical fibers. A FORTRAN computer program that implements the GPDCM theoretical development was written and …
Comment On A Critique Of The Instantaneous Normal Mode (Inm) Approach To Diffusion, T. Keyes, Wu Xiong Li, Ulrich Zurcher
Comment On A Critique Of The Instantaneous Normal Mode (Inm) Approach To Diffusion, T. Keyes, Wu Xiong Li, Ulrich Zurcher
Physics Faculty Publications
A critique of the instantaneous normal mode (INM) theory of diffusion by Gezelter, Rabani, and Berne (GRB) [J. Chem. Phys. 107, 4618 (1997)] is analyzed. GRB assert that imaginary-frequency INM are corrupted with modes unrelated to barrier crossing, that proposals for removing such nondiffusive (ND) modes are inadequate, and thus that INM cannot be used to predict the self-diffusion constant, D. In rebuttal it is argued that Lennard-Jones, the system studied by GRB, is anamolously rich in ND modes. INM in molecular liquids are shown to behave as excellent indicators of barrier crossing. Even in LJ ND-INM, while plentiful, do …
Optical Implementation Of The Wavelet Transform By Using A Bacteriorhodopsin Film As An Optically Addressed Spatial Light Modulator, Joby Joseph, F. J. Aranda, D.V.G.L.N. Rao, B. S. Decristofano, B. R. Kimball, M. Nakashima
Optical Implementation Of The Wavelet Transform By Using A Bacteriorhodopsin Film As An Optically Addressed Spatial Light Modulator, Joby Joseph, F. J. Aranda, D.V.G.L.N. Rao, B. S. Decristofano, B. R. Kimball, M. Nakashima
Physics Faculty Publications
An optical system utilizing the photoinduced dichroism in a bacteriorhodopsin film has been demonstrated for the optical implementation of wavelet transforms. The dichroism, induced by the image of a wavelet filter on a bacteriorhodopsin film leads to polarization rotation of the Fourier components of an image. The polarization-rotated Fourier components of an input scene are analyzed with a polarizer to give the wavelet transform components. The dichroism is induced with beams whose profiles are determined by wavelet filters in order to perform the optical wavelet transform.
Spiral Morphology-Dependent Resonances In An Optical Fiber: Effects Of Fiber Tilt And Focused Gaussian Beam Illumination, Andrew W. Poon, Richard K. Chang, James A. Lock
Spiral Morphology-Dependent Resonances In An Optical Fiber: Effects Of Fiber Tilt And Focused Gaussian Beam Illumination, Andrew W. Poon, Richard K. Chang, James A. Lock
Physics Faculty Publications
Spiral morphology-dependent resonances have been observed in a tilted optical fiber. The polarization-preserving and the cross-polarized elastic-scattering spectra for plane-wave illumination show that the wavelengths of the resonances are blueshifted quadratically as the fiber tilt angle increases. When a focused Gaussian beam illuminates the fiber at its edge, the resonances are blueshifted and broadened as the detector is offset from the scattering plane with the maximum scattering intensity. The blueshift with focused beam illumination is also a consequence of the spiral resonances. (C) 1998 Optical Society of America.
Low Saturation Intensities In Two-Photon Ultracold Collisions, C. I. Sukenik, D. Hoffman, S. Bali, T. Walker
Low Saturation Intensities In Two-Photon Ultracold Collisions, C. I. Sukenik, D. Hoffman, S. Bali, T. Walker
Physics Faculty Publications
We have observed violet photon emission resulting from energy-pooling collisions between ultracold Rb atoms illuminated by two colors of near-resonant infrared laser light. We have used this emission as a probe of doubly excited state ultracold collision dynamics. We have observed the lowest saturation intensity for light-induced ultracold collisions seen to date which we identify as due to depletion of incoming ground state flux. We have also varied the detuning of the lasers which allows us to clearly identify the effect of spontaneous emission and optical shielding.
Translational Diffusion Of Small And Large Mesoscopic Probes In Hydroxypropylcellulose-Water In The Solutionlike Regime, Kiril A. Streletzky, George D.J. Phillies
Translational Diffusion Of Small And Large Mesoscopic Probes In Hydroxypropylcellulose-Water In The Solutionlike Regime, Kiril A. Streletzky, George D.J. Phillies
Physics Faculty Publications
Quasi-elastic light scattering spectroscopy was used to study the translational diffusion of monodisperse spheres in aqueous 1 MDa hydroxypropylcellulose (HPC) at 25 °C. Probe diameters d spanned 14–455 nm; HPC concentrations were 0⩽c⩽7g/L. Light scattering spectroscopy consistently found spectra having the form g(1)(t)=(1−Af)exp(−θtβ)+Af exp(−θftβf). Here θf and βf refer to the “fast” mode; θ and β describe the “slow” mode. We examine the dependence of θ, β, θf, βf, and Af on d, c, scattering vector q, and viscosity η. β=1 for large probes; elsewise, β and βf are ∈(0,1). The slow mode, with short-lived memory function, is diffusive; for …
Known Source Detection Predictions For Higher Order Correlators, Lisa A. Pflug, George E. Ioup, Juliette W. Ioup
Known Source Detection Predictions For Higher Order Correlators, Lisa A. Pflug, George E. Ioup, Juliette W. Ioup
Physics Faculty Publications
The problem addressed in this paper is whether higher order correlation detectors can perform better in white noise than the cross correlation detector for the detection of a known transient source signal, if additional receiver information is included in the higher order correlations. While the cross correlation is the optimal linear detector for white noise, additional receiver information in the higher order correlations makes them nonlinear. In this paper, formulas that predict the performance of higher order correlation detectors of energy signals are derived for a known source signal. Given the first through fourth order signal moments and the noise …
Deep Centers In N-Gan Grown By Reactive Molecular Beam Epitaxy, Z-Q. Fang, David C. Look, W. Kim, Z. Fan, A. Botchkarev, H. Morkoç
Deep Centers In N-Gan Grown By Reactive Molecular Beam Epitaxy, Z-Q. Fang, David C. Look, W. Kim, Z. Fan, A. Botchkarev, H. Morkoç
Physics Faculty Publications
Deep centers in Si-doped n-GaN layers grown by reactive molecular beam epitaxy have been studied by deep-level transient spectroscopy as a function of growth conditions. Si-doped GaN samples grown on a Si-doped n+-GaN contact layer at 800 °C show a dominant trap C1 with activation energy ET = 0.44 eV and capture cross-section σT = 1.3×10−15 cm−2, while samples grown at 750 °C on an undoped semi-insulating GaN buffer show prominent traps D1 and E1, with ET = 0.20 eV and σT = 8.4×10−17 cm …
High Mobility Algan/Gan Heterostructures Grown By Gas-Source Molecular Beam Epitaxy, L. K. Li, J. Alperin, W. I. Wang, David C. Look, Donald C. Reynolds
High Mobility Algan/Gan Heterostructures Grown By Gas-Source Molecular Beam Epitaxy, L. K. Li, J. Alperin, W. I. Wang, David C. Look, Donald C. Reynolds
Physics Faculty Publications
We report on the growth of high electron mobility AlGaN/GaN heterostructures on sapphire substrates by gas-source molecular beam epitaxy (GSMBE) using ammonia as the nitrogen source. Improvements in structural, electrical, and optical properties of GaN and AlGaN layers have been made to achieve this goal. For the growth of AlGaN layers, the reflection high-energy electron diffraction revealed a twofold surface reconstruction, indicative of atomic smoothness of the film surface. High mobility two-dimensional electron gas has been achieved in both unintentionally doped (by piezoelectric effect induced by lattice mismatch strain) and modulation doped AlGaN/GaN heterostructures. The modulation-doped n+-Al0.2Ga …
Neutral-Donor-Bound-Exciton Complexes In Zno Crystals, D. C. Reynolds, David C. Look, B. Jogai, C. W. Litton, T. C. Collins, W. C. Harsch, G. Cantwell
Neutral-Donor-Bound-Exciton Complexes In Zno Crystals, D. C. Reynolds, David C. Look, B. Jogai, C. W. Litton, T. C. Collins, W. C. Harsch, G. Cantwell
Physics Faculty Publications
Neutral-donor–bound-exciton transitions have been observed in ZnO. The isolated neutral donors are made up of defect pair complexes. The neutral-donor nature of these pair complexes was determined from magneticfield measurements and from two-electron transitions. Excited states of the neutral-donor bound excitons were observed in the form of rotator states analogous to rotational states of the H2 molecule.
Valence Intersubband Lasers With Inverted Light-Hole Effective Mass, Greg Sun, Y. Lu, Jacob B. Khurgin
Valence Intersubband Lasers With Inverted Light-Hole Effective Mass, Greg Sun, Y. Lu, Jacob B. Khurgin
Physics Faculty Publications
We propose a novel intersubband laser based on transition between the ground-state heavy-hole subband (HH1) and light-hole subband (LH1) in a k-space region where the light-hole effective mass is inverted. The laser structure can be electrically pumped with a simple quantum cascade scheme. Our calculation shows that with only a small fraction of the carrier population in the upper subband (LH1), it is possible to achieve population inversion between the two subbands locally in K space where the light-hole effective mass is inverted. Optical gain in excess of 150/cm can be achieved with a pumping current density on the …
Ray Theory Analysis Of The Shadow Blister Effect, James A. Lock
Ray Theory Analysis Of The Shadow Blister Effect, James A. Lock
Physics Faculty Publications
When an extended light source such as the Sun illuminates two objects so that their shadows lie close to each other, the shadow of one of the objects occasionally appears to bulge out toward the shadow of the other. This effect is caused by the overlap of the penumbras of the shadows and is analyzed here with ray theory. A laboratory demonstration of this phenomenon is performed and compared with theoretical predictions. (C) 1998 Optical Society of America.
Rainbow Scattering By A Cylinder With A Nearly Elliptical Cross Section, Charles L. Adler, James A. Lock, Bradley R. Stone
Rainbow Scattering By A Cylinder With A Nearly Elliptical Cross Section, Charles L. Adler, James A. Lock, Bradley R. Stone
Physics Faculty Publications
We both theoretically and experimentally examine the behavior of the first-and the second-order rainbows produced by a normally illuminated glass rod, which has a nearly elliptical cross section, as it is rotated about its major axis. We decompose the measured rainbow angle, taken as a function of the rod's rotation angle, into a Fourier series and find that the rod's refractive index, average ellipticity, and deviation from ellipticity are encoded primarily in the m = 0, 2, 3 Fourier coefficients, respectively. We determine these parameters for our glass rod and, where possible, compare them with independent measurements. We find that …
Amplification Of High-Order Rainbows Of A Cylinder With An Elliptical Cross Section, James A. Lock, Charles L. Adler, Bradley R. Stone, Patrick D. Zajak
Amplification Of High-Order Rainbows Of A Cylinder With An Elliptical Cross Section, James A. Lock, Charles L. Adler, Bradley R. Stone, Patrick D. Zajak
Physics Faculty Publications
The intensity of high-order rainbows for normally incident light and certain rotation angles of a cylinder with an elliptical cross section is greatly amplified with respect to the intensity for a circular cross-sectional cylinder. The amplification is due to a number of the internal reflections occurring past the critical angle for total internal reflection, and the effect is especially strong for odd-order rainbows, beginning with the third order. Experimentally, the fourth-and the fifth-order rainbows of a nearly elliptical cross-sectional glass rod were observed and analyzed. (C) 1998 Optical Society of America.
In Situ Measurement Of Radiation Induced Conductivity In Oxide Insulators During Neutron Irradiation, D. P. White, L. L. Snead, S. J. Zinkle, W. S. Earthley
In Situ Measurement Of Radiation Induced Conductivity In Oxide Insulators During Neutron Irradiation, D. P. White, L. L. Snead, S. J. Zinkle, W. S. Earthley
Physics Faculty Publications
No abstract provided.
Electron-Irradiation-Induced Deep Level In N-Type Gan, Z-Q. Fang, Joseph W. Hemsky, David C. Look, M. P. Mack
Electron-Irradiation-Induced Deep Level In N-Type Gan, Z-Q. Fang, Joseph W. Hemsky, David C. Look, M. P. Mack
Physics Faculty Publications
Deep-level transient spectroscopy measurements of n-type GaN epitaxial layers irradiated with 1-MeV electrons reveal an irradiation-induced electron trap at EC−0.18 eV. The production rate is approximately 0.2 cm−1, lower than the rate of 1 cm−1 found for the N vacancy by Hall-effect studies. The defect trap cannot be firmly identified at this time. ©1998 American Institute of Physics.
Suppression Of Switchable Polarization In Kdp By Ionizing Radiation, Timothy D. Usher
Suppression Of Switchable Polarization In Kdp By Ionizing Radiation, Timothy D. Usher
Physics Faculty Publications
Switching curves were obtained from KH2PO4 single crystals exposed to x-ray radiation for various time intervals, up to 8 h. The applied electric field was varied between 370 and 740 V/cm, as well. The temperature was held constant at 99 K. The switching curves were fit to a three-parameter nucleation and growth model based on the original works by Johnson and Mehl, and independently by Avrami. The two dynamic parameters, characteristic time tc, and effective domain wall dimensionality n, produced values consistent with unirradiated studies, however, they did not show any clear dependancy on …
Novel Electronic And Magnetic Properties Of Ultrathin Chromium Oxide Films Grown On Pt(111), P S. Robbert, H Geisler, C A. Ventrice Jr, J Van Ek, S Chaturvedi, J A. Rodriguez, M Kuhn, U Diebold
Novel Electronic And Magnetic Properties Of Ultrathin Chromium Oxide Films Grown On Pt(111), P S. Robbert, H Geisler, C A. Ventrice Jr, J Van Ek, S Chaturvedi, J A. Rodriguez, M Kuhn, U Diebold
Physics Faculty Publications
The growth of epitaxial metal–oxide films on lattice-mismatched metal substrates often results in the formation of unique overlayer structures. In particular, epitaxial chromium oxide films grown on Pt(111) exhibit a p(2×2) symmetry through the first two monolayers of growth which is followed by a (√3×√3)R30° phase that is attributed to the growth of a Cr2O3(0001) overlayer. Ultraviolet photoelectron spectroscopy measurements have been performed on the CrOx/Pt(111) system. The electronic structures of CrO2, Cr2O3, and Cr3O4 were calculated using the linear muffin-tin orbital method in …
Solar Cycle Variability Of Hot Oxygen Atoms At Mars, Jhoon Kim, Andrew F. Nagy, Jane L. Fox, Thomas E. Cravens
Solar Cycle Variability Of Hot Oxygen Atoms At Mars, Jhoon Kim, Andrew F. Nagy, Jane L. Fox, Thomas E. Cravens
Physics Faculty Publications
The population of hot oxygen atoms in the Martian exosphere is reexamined using newly calculated hot O production rates for both low and high solar cycle conditions. The hot oxygen production rates are assumed to result from the dissociative recombination of O2+ ions. These calculations take into account the calculated vibrational distribution of O2+ and the new measured branching ratios. Furthermore, these calculations also consider the variation of the dissociative recombination cross section with the relative speed of the participating ions and electrons, the rotational energy of the O2+ ions, and the spread of …
Effect Of High-Temperature Annealing On Electrical And Optical Properties Of Undoped Semi-Insulating Gaas, Z-Q. Fang, D. C. Reynolds, David C. Look, N. G. Paraskevopoulos, T. E. Anderson, R. L. Jones
Effect Of High-Temperature Annealing On Electrical And Optical Properties Of Undoped Semi-Insulating Gaas, Z-Q. Fang, D. C. Reynolds, David C. Look, N. G. Paraskevopoulos, T. E. Anderson, R. L. Jones
Physics Faculty Publications
A comprehensive characterization, including room temperature Hall effect, near infrared absorption, temperature dependent dark current and photocurrent (using 1.13 eV light), normalized thermally stimulated current (NTSC), photoluminescence at 4.2 K in both near band edge and deep level regions, and selective pair photoluminescence (SPL) at 2 K, has been carried out on undoped semi-insulating GaAs samples, cut from four wafers which were grown by the low pressure liquid encapsulated Czochralski technique and annealed by three different schedules: a 1100 °C anneal with either fast or slow cooling, or a 1000 °C standard anneal. The 1100 °C anneal clearly introduces higher …
Factorization For High-Energy Scattering, Ian Balitsky
Factorization For High-Energy Scattering, Ian Balitsky
Physics Faculty Publications
I demonstrate that the amplitude for the high-energy scattering can be factorized into a product of two independent functional integrals over “fast” and “slow” fields which interact by means of Wilson-line operators—gauge factors ordered along the straight lines.
Factorization And Effective Action For High-Energy Scattering In Qcd, Ian Balitsky
Factorization And Effective Action For High-Energy Scattering In Qcd, Ian Balitsky
Physics Faculty Publications
The author demonstrates that the amplitude of the high-energy scattering can be factorized in a convolution of the contributions due to fast and slow fields. The fast and slow fields interact by means of Wilson-line operators -- infinite gauge factors ordered along the straight line. The resulting factorization formula gives a starting point for a new approach to the effective action for high-energy scattering.