Physical Sciences and Mathematics Commons^™

Molecular Dynamics Simulation Of The Order- Disorder Phase Transition In Solid Nano2, Wei-Guo Yin, Chun-Gang Duan, Wai-Ning Mei, Jianjun Liu, Robert W. Smith, John R. Hardy

Physics Faculty Publications

We present molecular dynamics simulations of solid NaNO2 using pair potentials with the rigid-ion model. The crystal potential surface is calculated by using an a priori method which integrates the ab initio calculations with the Gordon-Kim electron gas theory. This approach is carefully examined by using different population analysis methods and comparing the intermolecular interactions resulting from this approach with those from the ab initio Hartree-Fock calculations. Our numerics show that the ferroelectric-paraelectric phase transition in solid NaNO2 is triggered by rotation of the nitrite ions around the crystallographical c axis, in agreement with recent x-ray experiments @Gohda et al., …

Zero Magnetization States In Electrodeposited Co0.45fe0.55 Nanowire Arrays, Petru S. Fodor, Georgy M. Tsoi, Lowell E. Wenger

Physics Faculty Publications

Co0.45Fe0.55 alloy nanowires with 12 to 35 nm diameter and 12 μm length were fabricated by electrodeposition in porous anodic alumina templates. The initial magnetization curves reveal that the zero magnetization state is not unique and is determined by the field history (acdemagnetization process) leading to the zero average moment state. For acdemagnetization processes with the field applied parallel to the nanowire axis, the subsequent magnetization curves suggest that an individual nanowire behaves as a single domain with neighboring nanowires being antiparallel to each other in the zero magnetization state. However, for a demagnetization process with the field applied perpendicular …

Hydrogen Incorporation And Diffusivity In Plasma-Exposed Bulk Zno, K. Ip, M. E. Overberg, Y. W. Heo, D. P. Norton, S. J. Pearton, C. E. Stutz, B. Luo, F. Ren, David C. Look, J. M. Zavada

Physics Faculty Publications

Hydrogen incorporation depths of >25 μm were obtained in bulk, single-crystal ZnO during exposure to ²H plasmas for 0.5 h at 300 °C, producing an estimated diffusivity of ∼ 8×10⁻¹⁰ cm²/V⋅s at this temperature. The activation energy for diffusion was 0.17±0.12 eV, indicating an interstitial mechanism. Subsequent annealing at 500–600 °C was sufficient to evolve all of the hydrogen out of the ZnO, at least to the sensitivity of secondary ion mass spectrometry (<5×10¹⁵ cm⁻³). The thermal stability of hydrogen retention is slightly greater when the hydrogen is incorporated by direct implantation …

Measurement Of Correlated Multiple Light Scattering In Utracold Atomic 85Rb, P. Kulatunga, C. I. Sukenik, S. Balik, M. D. Havey, D. V. Kupriyanov, I. M. Sokolov

Physics Faculty Publications

We report an experimental study of correlated multiple light scattering in an ultracold gas of ⁸⁵Rb confined in a magneto-optic trap. Measurements are made of the polarization dependence of the spatial and spectral profile of light backscattered from the sample. The results show an interferometric enhancement sensitive to coherent multiple scattering in the atomic gas, and strong variations with the polarization of the incident and detected light. The spatial width and peak value of the enhancement are found to be dependent on the sample size. Comparison of all the measurements with realistic quantum Monte Carlo simulations yields a very …

Coherent Backscattering Of Light In Atomic Systems: Application To Weak Localization In An Ensemble Of Cold Alkali-Metal Atoms, D. V. Kupriyanov, I. M. Sokolov, P. Kulatunga, C. I. Sukenik, M. D. Havey

Physics Faculty Publications

Development of a theoretical treatment of multiple coherent light scattering in an ultracold atomic gas is reported. Specific application is made to coherent backscattering of a weak-radiation field from realistically modeled samples of ultracold atomic 85Rb. Comprehensive Monte Carlo simulations of the spatial, spectral, and polarization dependence of the backscattering line shape are made and compared with available experimental results.

Interpretation Of The Θ+ As An Isotensor Pentaquark With Weakly Decaying Partners, Simon Capstick, Philip R. Page, Winston Roberts

Physics Faculty Publications

The Θ⁺(1540), recently observed at LEPS, DIANA and CLAS, is hypothesized to be an isotensor resonance. This implies the existence of a multiplet where the Θ⁺⁺, Θ⁺ and Θ⁰ have isospin-violating strong decays, and the Θ⁺⁺⁺ and Θ⁻ have weak decays and so are long-lived. Production mechanisms for the weakly-decaying states are discussed. The JP assignment of the Θ is most likely 1/2⁻ or 3/2⁻.

Coulomb Sum Rule For 4He, J. Carlson, J. Jourdan, Rocco Schiavilla, I. Sick

Physics Faculty Publications

We determine the Coulomb sum for ⁴He using the world data on ⁴He(e,e′) and compare the results to calculations based on realistic interactions and including two-body components in the nuclear charge operator. We find good agreement between theory and experiment when using free-nucleon form factors. The apparent reduction of the in-medium G_Ep implied by IA-interpretation of the L/T-ratios measured in ⁴He(e,e′p) and ⁴He(e→,e′p→) is not confirmed.

The Neutron Charge Form Factor And Target Analyzing Powers From 3He (E→, E 'N) Scattering, J. Bermuth, P. Merle, C. Carasco, D. Baumann, R. Böhm, D. Bosnar, M. Ding, M. O. Distler, J. Friedrich, A. Klein

Physics Faculty Publications

The charge form factor of the neutron has been determined from asymmetries measured in quasi-elastic ³He(e→, e 'n) at a momentum transfer of 0.67 (GeV/c)². In addition, the target analyzing power, Ayo has been measured to study effects of final state interactions and mespn exchange currents.

Final State Interaction Effects In 3He (E→, E' P), C. Carasco, J. Bermuth, P. Merle, P. Bartsch, D. Baumann, R. Böhm, D. Bosnar, M. Ding, M. O. Distler, A. Klein

Physics Faculty Publications

Asymmetries in quasi-elastic ³He(e→, e 'p) have been measured at a momentum transfer of 0.67 (GeV/c)² and are compared to a calculation which takes into account relativistic kinematics in the final state and a relativistic one-body current operator. With an exact solution of the Faddeev equation for the ³He-ground state and an approximate treatment offinal state interactions in the continuum good agreement is found with the experimental data.

Precision Measurement Of The Proton And Deuteron Spin Structure Functions G2 And Asymmetries A2, P. L. Anthony, R. G. Arnold, T. Averett, H. R. Band, N. Benmouna, W. Boeglin, H. Borel, P. E. Bosted, S. L. Bültmann, G. R. Court, C. E. Hyde-Wright, S. E. Kuhn, F. Sabatie, F. R. Wesselmann

Physics Faculty Publications

We have measured the spin struction functions g2(p) and g(2)(d) and the virtual photon asymmetries A(2)(p) and A(2)(d) over the kinetmatic range 0.02 less than or equal to x less than or equal to 0.8 and 0.7 less than or equal to Q² less than or equal to 20 GeV2 by scattering 29.1 and 32.3 GeV longitudinally polarized electrons from transversely polarized NH3 and (LiD)-Li-6 targets. Our measured g² approximately follows the twist-2 Wandzura-Wilczek calculation. The twist-3 reduced matrix elements d(2)(p) and d(2)(n) are less than two standard deviations from zero. The data are inconsistent with the Burkhardt-Cottingham …

Ep → Ep Π⁰ Reaction Studied In The Δ(1232) Mass Region Using Polarization Asymmetries, M. Bektasoglu, L. Ciciani, K.V. Dharmawardane, G. E. Dodge, T.A. Forest, C.E. Hyde-Wright, A. Klein, A. V. Klimenko, S. E. Kuhn, S. Stepanyan, L. B. Weinstein, Et Al., Clas Collaboration

Physics Faculty Publications

Measurements of the angular distributions of target and double-spin asymmetries for the Δ⁺(1232) in the exclusive channel ^→ p(^→e,e'p)π⁰ obtained at the Jefferson Lab in the Q² range from 0.5 to 1.5 GeV²/c² are presented. Results of the asymmetries are compared with the unitary isobar model [D. Drechsel , Nucl. Phys. A645, 145 (1999)], dynamical models [T. Sato and T. S. Lee, Phys. Rev. C 54, 2660 (1996); S. S. Kamalov , Phys. Lett. B 27, 522 (2001)], and the effective Lagrangian theory [R. M. Davidson , Phys. Rev. …

First Measurement Of Transferred Polarization In The Exclusive →E P → E'K+ →Λ, M. Bektasoglu, K. V. Dharmawardane, G. E. Dodge, T. A. Forest, C. E. Hyde-Wright, A. V. Klimenko, S. E. Kuhn, R. A. Niyazov, L. M. Qin, L. B. Weinstein, J. Yun

Physics Faculty Publications

The first measurements of the transferred polarization for the exclusive ^→ep → e'K+ ^→Λ over right arrow reaction have been performed at Jefferson Laboratory using the CLAS spectrometer. A 2.567 GeV beam was used to measure the hyperon polarization over Q² from 0.3 to 1.5 (GeV/c)², W from 1.6 to 2.15 GeV, and over the full K⁺ center-of-mass angular range. Comparison with predictions of hadrodynamic models indicates strong sensitivity to the underlying resonance contributions. A nonrelativistic quark-model interpretation of our data suggests that the s ^-s quark pair is produced with spins predominantly …

Kinematically Complete Measurement Of The Proton Structure Function F₂ In The Resonance Region And Evaluation Of Its Moments, M. Bektasoglu, L. Ciciani, K. V. Dharmawardane, G. E. Dodge, C. E. Hyde-Wright, A. Klein, A. V. Klimenko, S. E. Kuhn, F. Sabatié, S. Stepanyan, L. B. Weinstein, J. Yun, Et Al., Clas Collaboration

Physics Faculty Publications

We measured the inclusive electron-proton cross section in the nucleon resonance region (W <2.5 GeV) at momentum transfers Q² below 4.5 (GeV/c)² with the CLAS detector. The large acceptance of CLAS allowed the measurement of the cross section in a large, contiguous two-dimensional range of Q² and x, making it possible to perform an integration of the data at fixed Q² over the significant x interval. From these data we extracted the structure function F₂ and, by including other world data, we studied the Q² evolution of its moments, M_n(Q²), in order to estimate higher twist …

Measurement Of Ep → E ' P Π⁺Π⁻ And Baryon Resonance Analysis, M. Bektasoglu, L. Ciciani, K. V. Dharmawardane, G. E. Dodge, T. A. Forest, C. E. Hyde-Wright, A. Klein, A. V. Klimenko, S. E. Kuhn, R. A. Niyazov, L. M. Qin, F. Sabatié, S. Stepanyan, L. B. Weinstein, J. Yun, Et Al., Clas Collaboration

Physics Faculty Publications

The cross section for the reaction ep→ e'pπ⁺π^- was measured in the resonance region for 1.4 2 < 1.5 GeV²/c²using the CLAS detector at Jefferson Laboratory. The data show resonant structures not visible in previous experiments. The comparison of our data to a phenomenological prediction using available information on N* and Δ states shows an evident discrepancy. A better description of the data is obtained either by a sizable change of the properties of the P₁₃(1720) resonance or by introducing a new baryon state, not reported in published analyses.

Measurement Of Inclusive Spin Structure Functions Of The Deuteron, J. Yun, S. E. Kuhn, G. E. Dodge, T. A. Forest, M. Bektasoglu, L. Ciciani, C. E. Hyde-Wright, A. Klein, A. V. Klimenko, L. M. Qin, S. Stepanyan, L. B. Weinstein, Et Al., Clas Collaboration

Physics Faculty Publications

We report the results of a new measurement of spin structure functions of the deuteron in the region of moderate momentum transfer [Q² = 0.27-1.3 (GeV/c)²] and final hadronic state mass in the nucleon resonance region (W=1.08-2.0 GeV). We scattered a 2.5 GeV polarized continuous electron beam at Jefferson Lab off a dynamically polarized cryogenic solid state target (¹⁵ND₃) and detected the scattered electrons with the CEBAF large acceptance spectrometer. From our data, we extract the longitudinal double spin asymmetry A∥ and the spin structure function g₁^d. Our data are …

Kerr-Resonance-Condition-Coupled Enhancement In Magneto-Optic Media, A. De, A. Puri

Physics Faculty Publications

We derive an expression for cyclotron frequency ω_c, which sets Re[ϵ₊ϵ₋]=1 in a magneto-optic (MO) substrate, at any incident photon energy. Thereby, at any desired part of the optical spectrum, large Kerr effects can be obtained, which are generally known to occur either at the free-charge-carrier-plasma-resonance frequency ω_p, or at frequencies where active electronic transitions take place. Under these conditions, ω_p is seen to play a very different role; it is seen that for any ω, the magnitude of the Kerr resonance in a single MO (InSb here) substrate increases …

Ultrafine Nife2o4 Powder Fabricated From Reverse Microemulsion Process, Jiye Fang, Narayan Shama, Le Duc Tung, Eun Young Shin, Charles J. O'Connor, Kevin L. Stokes, Gabriel Caruntu, John B. Wiley, Leonard Spinu, Jinke Tang

Physics Faculty Publications

NiFe2O4 ultrafine powder with high crystallinity has been prepared through a reverse microemulsion route. The composition in starting solution was optimized, and the resulting NiFe2O4 was formed at temperature of around 550–600 °C, which is much lower than that observed from the solid-state reaction. Magnetic investigation indicates that samples are soft-magnetic materials with low coercivity and with the saturation magnetization close to the bulk value of Ni ferrite.

Comparison Of The Theoretical And Experimental Band Structure Of Poly (Vinylidene Fluoride) Crystal, Chun-Gang Duan, Wai-Ning Mei, John R. Hardy, Stephen Ducharme, Jaewu Choi, Peter A. Dowben

Physics Faculty Publications

The electronic structure and band symmetries of the ferroelectric phase of poly (vinylidene fluoride) (PVDF) are studied by using the first-principles density-functional method. Our calculated results agree well with the angle-resolved photoemission experiments. We find that the PVDF crystal has an energy gap of about 6 eV at the Γ-point. Large dispersion of the valence band is found only along the chain direction, which is consistent with the quasi-one-dimensional nature of PVDF. In particular, the band symmetries of the valence states deduced theoretically are in good agreement with experiment. Finally, further investigations on the electronic structure of poly(trifluoroethylene) (PTrFE) …

Spectral Functions Of The Falicov-Kimball Model With Electronic Ferroelectricity, Wei-Guo Yin, Wai-Ning Mei, Chun-Gang Duan, Hai-Qing Lin, John R. Hardy

Physics Faculty Publications

We calculate the angular resolved photoemission spectrum of the Falicov-Kimball model with electronic ferroelectricity where d- and f-electrons have different hoppings. In mix-valence regimes, the presence of strong scattering processes between d – f excitons and a hole, created by emission of an electron, leads to the formation of pseudospin polarons and novel electronic structures with bandwidth scaling with that of d – f excitons. Especially, in the two-dimensional case, we find that flat regions exist near the bottom of the quasiparticle band in a wide range of the d- and f-level energy difference.

Self-Assembly Of Fept Nanoparticles Into Nanorings, Weilie L. Zhou, Jibao He, Jiye Fang, Tuyet-Anh Huynh, Trevor J. Kennedy, Kevin L. Stokes, Charles J. O'Connor

Physics Faculty Publications

The application of nanoparticles as quantum dots in nanoelectronics demands their arrangement in ordered arrays. Shape controlled self-assembly is a challenge due to the difficulties of obtaining proper self-assembling parameters, such as solvent concentration, organic ligands, and nanoparticle size. In this article, hard magnetic FePt nanoparticles were synthesized using a combination approach of reduction and thermal decomposition. The nanoparticles are about 4.5 nm and appeared as truncated octahedral enclosed by the

{100} and {111}

crystal facets of fcc structure. The nanoparticles are of hexagonal close packing and orient randomly in the self-assembly nanoarrays.  By diluting the solution for large-area self-assembly, …

Mim And Nonlinear Least-Squares Inversions Of Aem Data In Barataria Basin, Louisiana, Melissa Whitten Bryan, Kenneth W. Holladay, Clyde J. Bergeron Jr., Juliette W. Ioup, George E. Ioup

Physics Faculty Publications

An airborne electromagnetic survey was performed over the marsh and estuarine waters of the Barataria basin of Louisiana. Two inversion methods were applied to the measured data to calculate layer thicknesses and conductivities: the modified image method (MIM) and a nonlinear least-squares method of inversion using two two-layer forward models and one three-layer forward model, with results generally in good agreement. Uniform horizontal water layers in the near-shore Gulf of Mexico with the fresher (less saline, less conductive) water above the saltier (more saline, more conductive) water can be seen clearly. More complex near-surface layering showing decreasing salinity/conductivity with depth …

Coulomb Sum Rule For ⁴He, J. Carlson, J. Jourdan, R. Schiavilla, I. Sick

Physics Faculty Publications

We determine the Coulomb sum for ⁴He using the world data on ⁴He(e,e′) and compare the results to calculations based on realistic interactions and including two-body components in the nuclear charge operator. We find good agreement between theory and experiment when using free-nucleon form factors. The apparent reduction of the in-medium G_Ep implied by IA-interpretation of the L/T-ratios measured in ⁴He(e,e′p) and ⁴He(e→,e′p→) is not confirmed.

Si Doping Of High-Al-Mole Fraction Alxga1-Xn Alloys With Rf Plasma-Induced Molecular-Beam-Epitaxy, Jeonghyun Hwang, William J. Schaff, Lester F. Eastman, Shawn T. Bradley, Leonard J. Brillson, David C. Look, J. Wu, Wladek Walukiewicz, Madalina Furis, Alexander N. Cartwright

Physics Faculty Publications

Very high levels of n-type doping of Al_xGa_1−xN alloys were recently achieved by rf plasma-induced molecular-beam epitaxy on sapphire substrates and Si as a dopant. Electron concentrations were obtained up to 1.25×10²⁰ cm⁻³ when the Al mole fraction was 50%, and 8.5×10¹⁹ cm⁻³ electrons were measured even when the Al mole fraction was 80%. Other material properties were determined by optical absorption, photoluminescence, cathodoluminescence, x-ray diffraction, and atomic force microscopy measurements and high optical and morphological qualities were shown.

Recombination Of Excitons Bound To Oxygen And Silicon Donors In Freestanding Gan, A. Wysmolek, K. P. Korona, R. Stepniewski, J. M. Baranowsky, J. Bloniarz, M. Potemski, R. L. Jones, David C. Look, J. Kuhl, S. S. Park, S. K. Lee

Physics Faculty Publications

The neutral donor bound exciton recombination processes in freestanding GaN have been studied. The photoluminescence spectrum shows emission lines related to silicon and oxygen donors. Time-resolved luminescence allows us to correlate the principal donor bound exciton lines with their two-electron satellites. The magnetic field splitting of the two-electron lines is well described by the theory of the hydrogen atom in a magnetic field. For the oxygen donor a 1.5 meV chemical shift and a 30.8 meV effective Rydberg have been evaluated. Two-electron satellites involving excitations to the 2p and 2s donor states are separated by an energy of 1.0 and …

Enhancement Of Photoinduced Anisotropy And All-Optical Switching In Bacteriorhodopsin Films, Pengfei Wu, D.V.G.L.N. Rao, B. R. Kimball, M. Nakashima, B. S. Decristofano

Physics Faculty Publications

Large enhancement of photoanisotropic effects is demonstrated in thin films of the biomaterial Bacteriorhodopsin by using two exciting beams of orthogonal polarization. The mechanism of the enhancement originates from optimization of direction-selected photoisomerization of the biomaterial controlled by the polarized exciting beams. The technique is applied for achieving an all-optical switch with the additional feature of output sign control.

Microcathodoluminescence And Electron Beam Induced Current Observation Of Dislocations In Freestanding Thick N-Gan Sample Grown By Hydride Vapor Phase Epitaxy, A. Y. Polyakov, A. V. Govorkov, N. B. Smirnov, Z-Q. Fang, David C. Look, Seong-Ju S. Park, J. H. Han

Physics Faculty Publications

Microcathodolumunescence (MCL) spectra measurements, MCL and electron beam induced current (EBIC) imaging of the freestanding n-GaN samples grown by hydride vapor phase epitaxy were made. Dark-spot defects in plan-view EBIC and MCL images and dark line defects in MCL images taken on the cleaved surface of the samples could be associated with dislocations. MCL spectra measurements in the vicinity of dislocations and in the matrix do not reveal specific luminescence bands that could be attributed to dislocations but rather suggest that dislocation regions have higher density of deep nonradiative traps.

Deep Electron And Hole Traps In Freestanding N-Gan Grown By Hydride Vapor Phase Epitaxy, A. Y. Polyakov, N. B. Smirnov, A. V. Govorkov, Z-Q. Fang, David C. Look, Seong-Ju S. Park, J. H. Han

Physics Faculty Publications

Deep level electron and hole traps were studied by means of deep level transient spectroscopy with electrical and optical injection on a freestanding thick n-GaN sample with low dislocation density. It is shown that at both the upper and the lower surface of the sample there exists a thin, ∼0.5 μm layer of damaged material with lowered concentration of electrons and enhanced density of deep centers. Deep in the bulk of the film the densities of the majority of the electron and hole traps are shown to be very low, but measurably higher on the lower face (N face), …

Simultaneous Trapping Of Rubidium And Metastable Argon In A Magneto-Optical Trap, C. I. Sukenik, H. C. Busch

Physics Faculty Publications

We have simultaneously confined rubidium and metastable argon in a dual-species magneto-optical trap (MOT). Here a binary mixture of atomic species from different groups of the periodic table have been optically confined at ultracold temperatures. We describe the apparatus and characterize the individual, single species MOTs and the dual-species MOT. Both fluorescence and ion production are monitored. With both species trapped, we observe ~5106 85Rb atoms and ~2106 40Ar* atoms. Realization of the dual-species trap opens the way for detailed studies of Penning and associative ionization, photoassociative spectroscopy, and eventually for the production of bound, ultracold RbAr molecules.

H2o–D2o Condensation In A Supersonic Nozzle, Christopher H. Heath, Kiril A. Streletzky, Barbara E. Wyslouzil, Judith Woelk, Reinhard Strey

Physics Faculty Publications

We examined the condensation of H2O, D2O, and four intermediate mixtures (20, 40, 60, and 80 mol % D2O) in a supersonic nozzle. Because the physical and chemical properties of protonated and deuterated water are so similar, this system is ideal for studying the change in condensation behavior as a function of condensible composition. In our experiments dilute mixtures of condensible vapor in N2 are expanded from three different stagnation temperatures resulting in a broad range of onset temperatures (190–238 K) and pressures (27–787 kPa). For a fixed stagnation temperature, the partial pressure required to maintain the onset of condensation …

Thermoelectric Effect Spectroscopy Of Deep Levels In Semi-Insulating Gan, U. V. Desnica, M. Pavlovic, Z-Q. Fang, David C. Look

Physics Faculty Publications

The report of thermoelectric effect spectroscopy (TEES) applied on semi-insulating GaN was presented. The type of TEES setup, especially suitable for film-on-substrate samples, was devised. TEES enabled determination of sign of observed deep traps. Using TEES and thermally stimulated current spectroscopy measurements in combination with the simultaneous multiple peak analysis formalism all important trap parameters were determined. The shallowest identified electron and hole traps had activation energies E_c−0.09 eV and E_v+0.167 eV, respectively. Results indicate that both these traps, oppositely charged are present in the studied material in relatively high concentrations causing the electrical compensation and …