bluuebunny/crossref_metadata_2025 huggingface dataset:
Stray-field investigation on permalloy nanodisks
Authors: Hengstmann T. M. , Grundler D. , Heyn Ch. , Heitmann D.
Published: July 2002

The stray field of individual nanostructured permalloy (Py) disks has been measured by means of a sub-μm Hall magnetometer which incorporates a ballistic two-dimensional electron system. The circular nanomagnets have diameters of d=500, 600, and 700 nm and thickness of t=20 nm. The characteristic shape of the experimental Hall curves can be explained by the behavior expected from micromagnetic simulations. They indicate that near remanence the disks are in the vortex phase. For the largest Py nanomagnet, if saturated, an averaged value of the stray field of 36 mT is measured in our 300 nm wide Hall sensor.
A study of nonlinear properties of tearing modes
Authors: Jensen Torkil H.
Published: July 2002

The tool used for this study is a numerical code which treats the plasma as an incompressible fluid of low resistivity and which utilizes simplifying assumptions on geometry. For a given set of “almost ideal magnetohydrodynamic (MHD) constraints” [T. H. Jensen, A. W. Leonard, R. J. La Haye, and M. S. Chu, Phys. Fluids B 3, 1650 (1991)], the code can find equilibria which satisfy these constraints when boundary conditions and external currents are specified. Thus, given an initial MHD equilibrium (without an island), and thereby its constraints, the code can be used to find external currents needed for establishing an equilibrium with the same constraints and an island of specified width. The sign of these external currents determines whether the island would grow or shrink if the external currents were removed. Thus the code can provide information on nonlinear growth or decay, but not on the time scales involved. The two main points of the paper are (i) that the stability limit and saturated island width depend not only on the quantity Δ′ [H. P. Furth, J. Killeen, and M. N. Rosenbluth, Phys. Fluids 6, 459 (1963)] but also on the constraints imposed, and (ii) that a current density gradient at the singular surface can drive a tearing mode nonlinearly to a certain island width followed by a decrease until the island vanishes.
Experimental determination of the magnetic field spectrum in the Helically Symmetric Experiment using passing particle orbits
Authors: Talmadge J. N. , Sakaguchi V. , Anderson F. S. B. , Anderson D. T. , Almagri A. F.
Published: July 2002

The leading terms of the magnetic field spectrum for the Helically Symmetric Experiment [Fusion Technol. 27, 273 (1995)] at low magnetic field are determined by analyzing the orbits of passing particles. The images produced by the intersection of electron orbits with a fluorescent mesh are recorded with a charge coupled device and transformed into magnetic coordinates using a neural network. To obtain the spectral components, the transformed orbits are then fit to an analytic expression that models the drift orbits of the electrons. The results confirm for the first time that quasihelical stellarators have a large effective transform that results in small excursions of particles from a magnetic surface. The drift orbits are also consistent with a very small toroidal curvature component in the spectrum. An external magnetic perturbation, nearly resonant with the transform, is shown to induce a large excursion of the particle orbit off a flux surface.
Polarity dependence of defect generation in ultrathin SiO2/ZrO2 gate dielectric stacks
Authors: Houssa M. , Afanas’ev V. V. , Stesmans A. , Heyns M. M.
Published: July 2002

The generation of defects during the injection of charge carriers in metal–oxide–semiconductor capacitors with ultrathin SiON/ZrO2 gate stacks is investigated. A polarity dependence for the defect generation is revealed. It is shown that this polarity effect is inconsistent with the predictions of the anode–hole injection model, but can be explained by assuming the release of hydrogen close to the SiON/ZrO2 interface, followed by its transport and trapping in the gate dielectric, resulting in the generation of hydrogen-induced positive charge and bulk neutral traps.
Long-time stability of high-concentration copper complexes in silicon crystals
Authors: Nakamura Minoru
Published: July 2002

The stability of diffused Cu in silicon crystal was evaluated by measuring the changes of photoluminescence intensity of the 1.014 eV Cu center with long storage time. Although the solubility of Cu is known to be low (<0.1 atom/cm3), high concentrations (>1013 atom/cm3) of mobile Cu in various Cu complexes (including the Cu center) were always maintained during more than a two-year storage at room temperature without Cu precipitation for float-zone and Czochralski (Cz) crystals when carbon concentrations were not so high for the latter. The Cu center for Cz crystals decreased shortly after annealing due to oxygen-enhanced nucleation of Cu sinks. Different behaviors of the Cu center in different crystals were reasonably explained by analyzing the transformation of Cu atoms between the Cu complexes and sinks.
Reflection second-harmonic microscopy of individual semiconductor microstructures
Authors: Bozhevolnyi Sergey I. , Maidykovski Anton , Vohnsen Brian , Zwiller Valéry
Published: July 2002

Second-harmonic scanning optical microscopy in reflection is used to image at room temperature individual GaInP/GaAs microstructures formed on a GaAs (001) substrate. Second-harmonic images of individual microstructures are recorded along with first-harmonic images for four combinations of polarizations of the pump and second-harmonic radiation with different pump wavelengths in the range of 720–920 nm. We observe different second-harmonic images for different polarization configurations and their evolution when changing the pump wavelength. Comparing the dependencies of the second-harmonic signal from the bare sample surface with those related to the microstructures, we conclude that the second-harmonic radiation detected when illuminating the microstructures originates from the substrate at short wavelengths and directly from the microstructures at ∼900 nm of the pump wavelength. The appearance of the second-harmonic images for different polarization configurations is used to discuss the nonlinear properties of the investigated microstructures.
Formulation and implementation of the relativistic Fock-space coupled cluster method for molecules
Authors: Visscher Lucas , Eliav Ephraim , Kaldor Uzi
Published: July 2002

An implementation of the relativistic multireference Fock-space coupled cluster method is presented which allows simultaneous calculation of potential surfaces for different oxidation states and electronic levels of a molecule, yielding values for spectroscopic constants and transition energies. The method is tested in pilot calculations on the I2 and HgH molecules, and is shown to give a good and balanced description of various electronic states and energies.
Response to “Comment on ‘Taylor dispersion of a solute in a microfluidic channel’ [J. Appl. Phys. 90, 6553 (2001)]”
Authors: Beard Daniel A.
Published: July 2002

Dorfman and Brenner uncover an error in a calculation presented in a recent article [J. Appl. Phys. 89, 4667 (2001)]. Unfortunately, beyond revealing this error, they have introduced a number of errors of their own. Dorfman and Brenner introduce an advection-diffusion equation which is clearly inappropriate. In addition, their proposed criterion for application of Taylor–Aris dispersion theory is shown to be incorrect based on a simple numerical experiment.
X-ray absorption fine structure and electron energy loss spectroscopy study of silicon nanowires at the Si L3,2 edge
Authors: Sun X.-H. , Tang Y.-H. , Zhang P. , Naftel S. J. , Sammynaiken R. , Sham T. K. , Peng H. Y. , Zhang Y.-F. , Wong N. B. , Lee S. T.
Published: July 2002

X-ray absorption fine structures (XAFS) and electron energy loss spectroscopy (EELS) at the Si L3,2 edge have been used to investigate a series of Si nanowires (as-prepared and HF refreshed). X-ray excited optical luminescence (XEOL) was also used to study the optical properties of these Si nanowires. Although no noticeable edge-jump blueshift (widened band gap) is observed in XAFS, a noticeable change in the edge jump (a less steep rise and the blurring of spectral features) is observed, indicating considerable degradation in the long-range order and size effects. However, EELS with a nanobeam exhibits a threshold blueshift and parabolic behavior for some selected wires indicating that there are grains smaller than the nominal diameter in these nanowires. Thus, XAFS probes the average of a distribution of wires of various sizes of which the majority is too large to exhibit detectable quantum confinement behavior (blueshift) observed and inferred in EELS and XEOL. The results and their implications are discussed.
The Fejér average and the mean value of a quantity in a quasiclassical wave packet
Authors: Liu Q. H. , Wang X. , Qi W. H. , Fu L. P. , Hu B.
Published: July 2002

The mean value of a quantity in an equally weighted wave packet was recently found in the classical limit to be the Fejér average of partial sums of Fourier series expansion of the classical quantity, and the number of stationary states in it is equal to that of partial sums. The incompleteness of the Fejér average in representing a classical quantity enables us to define a classical uncertainty relation which turns out to be the counterpart of the quantum one. In this paper, two typical quantum systems, a harmonic oscillator and a particle in an infinite square well, are used to illustrate the above-mentioned points.
Pseudo-Hermiticity versus PT symmetry: The necessary condition for the reality of the spectrum of a non-Hermitian Hamiltonian
Authors: Mostafazadeh Ali
Published: July 2002

We introduce the notion of pseudo-Hermiticity and show that every Hamiltonian with a real spectrum is pseudo-Hermitian. We point out that all the PT-symmetric non-Hermitian Hamiltonians studied in the literature belong to the class of pseudo-Hermitian Hamiltonians, and argue that the basic structure responsible for the particular spectral properties of these Hamiltonians is their pseudo-Hermiticity. We explore the basic properties of general pseudo-Hermitian Hamiltonians, develop pseudosupersymmetric quantum mechanics, and study some concrete examples, namely the Hamiltonian of the two-component Wheeler–DeWitt equation for the FRW-models coupled to a real massive scalar field and a class of pseudo-Hermitian Hamiltonians with a real spectrum.
Nonideal magnetohydrodynamic Kelvin–Helmholtz instability driven by the shear in the ion diamagnetic drift velocity in a high-β plasma
Authors: Miura Akira
Published: July 2002

A two-dimensional nonideal magnetohydrodynamic eigenmode equation for the most dangerous perturbation (k∥=0) is derived for a high-β plasma by making use of the generalized Ohm’s law and becomes identical to the equation describing the two-dimensional hydrodynamic stability. When the pressure is nonuniform, the density is uniform, and there is no unperturbed electric field or gravity, a Kelvin–Helmholtz instability, which is driven by the shear in the ion diamagnetic drift velocity, is found. When the unperturbed ion pressure is proportional to the unperturbed total pressure, a necessary condition for the instability is that d3B0/dx3 must change sign at least once between x=x1 and x=x2, where x is the direction of the nonuniformity perpendicular to the unperturbed magnetic field B0(x) and x=x1 and x2 are points where the x component of the velocity perturbation vanishes. An unstable B0(x) profile and the dispersion relation are obtained for a polygonal ion diamagnetic drift velocity profile.
Pd growth and subsequent Schottky barrier formation on chemical vapor cleaned p-type GaN surfaces
Authors: Hartlieb P. J. , Roskowski A. , Davis R. F. , Platow W. , Nemanich R. J.
Published: July 2002

Characterization of chemical vapor cleaned, Mg-doped, p-type GaN(0001) surfaces and Pd contacts sequentially deposited on these surfaces has been conducted using x-ray and ultraviolet photoelectron spectroscopies and low-energy electron diffraction. The band bending and the electron affinity at the cleaned p-GaN surface were 1.4±0.1 eV and 3.1±0.1 eV, respectively. A previously unidentified band of surface states was observed at ∼1.0 eV below the Fermi level on this surface. The Pd grew epitaxially on the cleaned surface in a layer-by-layer mode and formed an abrupt, unreacted metal–semiconductor interface. The induced Fermi level movement with Pd deposition has been attributed to a complex interaction between extrinsic and intrinsic surface states as well as metal induced gap states. The final Schottky barrier height at the Pd/p-GaN interface was 1.3±0.1 eV; the interface dipole contribution was 0.4±0.1 eV.
Infinite infrared regularization and a state space for the Heisenberg algebra
Authors: Schmidt Andreas U.
Published: July 2002

We present a method for the construction of a Krein space completion for spaces of test functions, equipped with an indefinite inner product induced by a kernel which is more singular than a distribution of finite order. This generalizes a regularization method for infrared singularities in quantum field theory, introduced by Morchio and Strocchi, to the case of singularities of infinite order. We give conditions for the possibility of this procedure in terms of local differential operators and the Gelfand–Shilov test function spaces, as well as an abstract sufficient condition. As a model case we construct a maximally positive definite state space for the Heisenberg algebra in the presence of an infinite infrared singularity.
Short distance asymptotics of Ising correlations
Authors: Palmer John
Published: July 2002

We prove that the short distance asymptotics for the even Ising model scaling functions from below Tc is given by the Luther–Peschel formula. Generalizations to the odd scaling functions and holonomic fields are given.
Effects of thermal annealing on the performance of polymer light emitting diodes
Authors: Liu Jie , Guo Tzung-Fang , Yang Yang
Published: July 2002

Thermal annealing plays an important role in controlling morphologies of polymer thin films and consequently the device performance, such as emission spectra, turn-on voltages, quantum efficiency of photoluminescence (PL) and electroluminescence (EL). In thermal annealing there is a tradeoff between hole-injection efficiency and PL efficiency. Annealing at a temperature higher than the glass transition temperature can improve the efficiency of hole injection at the expense of the PL efficiency, and vice versa. Optimizing the annealing conditions can improve the overall EL efficiency. The high efficiency of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene based polymer light-emitting diodes is demonstrated: 2.7 cd/A at a luminescence of 1000 cd/m2.
Theoretical and experimental analysis of the equilibrium contours of liquid bridges of arbitrary shape
Authors: Montanero J. M. , Cabezas G. , Acero J. , Perales J. M.
Published: July 2002

The equilibrium shape of the liquid bridge interface is analyzed theoretically and experimentally. Both axisymmetric and nonaxisymmetric perturbations are considered. The axisymmetric deviations are those related to volume effects, the difference between the radii of the disks, and the axial forces acting on the liquid bridge. The nonaxisymmetric deviations are those due to the eccentricity of the disk and the action of lateral forces. The theoretical study is performed using three different techniques: (i) an analytical expansion around the cylindrical solution, (ii) a finite difference scheme, and (iii) an approximate numerical approach valid only for slight nonaxisymmetric deviations. The results of the three methods are compared systematically. There is a very good agreement between the analytical and the numerical approaches for contours which are close to cylindrical, and the agreement extends to configurations with only moderate deviations from cylindrical. Experiments are performed using the so-called neutral buoyancy or plateau technique. Theoretical and experimental contours are compared considering a wide range of values for the parameters characterizing the perturbations. In general, the finite difference method provides reasonably accurate predictions even for large deviations of the liquid bridge contour from cylindrical.
Localization of carriers and polarization effects in quaternary AlInGaN multiple quantum wells
Authors: Kuokstis E. , Zhang J. , Ryu M.-Y. , Yang J. W. , Simin G. , Khan M. Asif , Gaska R. , Shur M. S.
Published: July 2002

We report on observing a long-wavelength band in low-temperature photoluminescence (PL) spectrum of quaternary Al0.22In0.02Ga0.76N/Al0.38In0.01Ga0.61N multiple quantum wells (MQWs), which were grown over sapphire substrates by a pulsed atomic-layer epitaxy technique. By comparing the excitation-power density and temperature dependence of the PL spectra of MQWs and bulk quaternary AlInGaN layers, we show this emission band to arise from the carrier and/or exciton localization at the quantum well interface disorders. PL data for other radiative transitions in MQWs indicate that excitation-dependent spectra position is determined by screening of the built-in electric field.
Enhancement of secondary electron emission by annealing and microwave hydrogen plasma treatment of ion-beam-damaged diamond films
Authors: Laikhtman A. , Hoffman A.
Published: July 2002

In this work we investigate the influence of annealing and microwave (MW) hydrogen plasma exposure of ion-beam-irradiated diamond film surfaces. In particular, we are interested in the recovery of secondary electron emission (SEE) and negative electron affinity (NEA) by removal of the damaged layer. To this aim, we correlate the SEE of variously treated Xe+ ion-damaged diamond films with their bonding structure in the near-surface region, as identified by near-edge x-ray absorption fine structure (NEXAFS) spectroscopy and x-ray photoelectron spectroscopy. The 30 keV Xe+ ion bombardment of hydrogenated polycrystalline diamond films to a dose of 2×1015 cm−2 results in the transformation of the near-surface region of a diamond film to sp2-bonded amorphous carbon, increased oxygen adsorption, shift of the electron affinity from negative to positive, and strong degradation of its electron emission properties, although it does not induce a pronounced depletion of hydrogen. Exposure of the ion-bombarded films to MW hydrogen plasma treatment for 30 min produces NEA diamond surfaces, but only partially regenerates SEE properties, retains some imperfection in the near-surface atomic layers, as determined by NEXAFS, and the concentration of oxygen remains relatively high. Subsequent annealing to 610 °C produces oxygen-free diamond films and somewhat increases their SEE. Annealing to 1000 °C results in desorption of the surface hydrogen, formation of positive electron affinity surfaces, and drastically degrades their electron emission properties. Prolonged (up to three hours) MW hydrogen plasma treatment of as-implanted diamond films gradually improves their crystal quality and results in a further increase of SEE intensity. The SEE intensity after three hours MW hydrogen plasma exposure of the ion-beam-irradiated films was found to be ∼50% above the value obtained for the as-deposited diamond films. This treatment does not, however, substantially reduce the concentration of oxygen in the previously damaged diamond, indicating its bulk diffusion during or after ion bombardment. Our results show that removal of damage from a highly disordered diamond surface and recovery of its electron emission properties are possible by MW hydrogen plasma. However, it is a slow process. This is most likely due to the very low etching rate of the low-level damage at the end of the ion beam range.
Elliptic blending model: A new near-wall Reynolds-stress turbulence closure
Authors: Manceau Rémi , Hanjalić Kemal
Published: July 2002

A new approach to modeling the effects of a solid wall in one-point second-moment (Reynolds-stress) turbulence closures is presented. The model is based on the relaxation of an inhomogeneous (near-wall) formulation of the pressure–strain tensor towards the chosen conventional homogeneous (far-from-a-wall) form using the blending function α, for which an elliptic equation is solved. The approach preserves the main features of Durbin’s Reynolds-stress model, but instead of six elliptic equations (for each stress component), it involves only one, scalar elliptic equation. The model, called “the elliptic blending model,” offers significant simplification, while still complying with the basic physical rationale for the elliptic relaxation concept. In addition to model validation against direct numerical simulation in a plane channel for Reτ=590, the model was applied in the computation of the channel flow at a “real-life” Reynolds number of 106, showing a good prediction of the logarithmic profile of the mean velocity.
Microlensed vertical-cavity surface-emitting laser for stable single fundamental mode operation
Authors: Park Si-Hyun , Park Yeonsang , Kim Hyejin , Jeon Heonsu , Hwang Seong Mo , Lee Jeong Kwan , Nam Seung Ho , Koh Byeong Cheon , Sohn J. Y. , Kim D. S.
Published: July 2002

We propose and demonstrate a vertical-cavity surface-emitting laser structure that operates predominantly in the single fundamental transverse mode over a wide operation current range. In this laser structure, a microlens is integrated on top of an otherwise ordinary surface-emitting laser so that a small portion of laser output is fed back into the cavity, forcing the lasing to occur in the fundamental mode. Model calculations reproduce the observed mode selection effect.
Light propagation in periodic microcavities
Authors: Furukawa Hiromitsu , Tenjimbayashi Koji
Published: July 2002

We demonstrated light propagation in a type of optical waveguide, which consisted of periodically arrayed dielectric particles. Although the form is similar to a photonic crystal waveguide, our method used resonance frequencies instead of band-gap frequencies. Advantages provided by this waveguide are that dynamic optical interconnection can be realized and that a multichannel waveguide is easily formed. To fabricate the system of periodic microcavities, we used polystyrene-latex particles that have a uniform size of 15.6 μm. Some discrete frequencies of fluorescence were selected to agree with the phase matching condition and they were propagated through the microspheres. The assembled microspheres can be taken as a multichannel waveguide.
Phase transitions and dielectric properties in A-site ion substituted (Na1/2Bi1/2)TiO3 ceramics (A=Pb and Sr)
Authors: Lee Jung-Kun , Hong Kug Sun , Kim Chang Kyung , Park Seung-Eek
Published: July 2002

The dielectric permittivity of A-site ion substituted (Na1/2Bi1/2)TiO3(A=Pb,Sr) solid solutions has been investigated with the aid of structural analysis. As the addition of PT increased, the first order characteristics of the phase transition appeared. In contrast, additions of ST made the solid solution exhibit relaxor ferroelectrics behavior. Microscopic structural analysis revealed that the dielectric properties were determined by cation ordering which controlled the translational symmetry and domain configuration. In the NBT–PT solid solution, A-site cations were randomly redistributed and macrodomains formed. However, the addition of ST distributed A-site cations with local ordering and divided the domains of NBT into micropolar regions. The contrasting effects of the addition of the Pb and Sr cations on the dielectric properties of NBT are discussed in terms of the polarizability and the tolerance factors as they relate to the perovskite structure.
Characterization of a high-density plasma produced by electrothermal capillary discharge
Authors: Kim J. U. , Suk H.
Published: July 2002

The characteristics of a pulsed plasma jet originating from an electrothermal capillary discharge are reported from optical emission spectroscopy as well as from laser-induced fluorescence (LIF) measurement. The results show that the plasma temperature and electron density jump across the Mach disk. Their values are approximately 25 000 and 14 000 K downstream and upstream of the Mach disk, and the corresponding electron densities are about 1018 and 1017 cm−3, respectively. By applying the LIF imaging method in the highly luminous underexpanded region of the plasma jet, a spatially well-resolved flow field was achieved.
Fourth-order quantum master equation and its Markovian bath limit
Authors: Jang Seogjoo , Cao Jianshu , Silbey Robert J.
Published: July 2002

Fourth-order quantum master equations (FQMEs) are derived in both time nonlocal and local forms for a general system Hamiltonian, with new detailed expressions for the fourth-order kernel, where the bath correlation functions are explicitly decoupled from the system superoperators. Further simplifications can be made for the model of linearly coupled harmonic oscillator bath. Consideration of the high temperature Ohmic bath limit leads to a general Markovian FQME with compact forms of time independent superoperators. Two examples of this equation are then considered. For the system of a quantum particle in a continuous potential field, the equation reduces to a known form of the quantum Fokker–Planck equation, except for a fourth-order potential renormalization term that can be neglected only in the weak system-bath interaction regime. For a two-level system with off-diagonal coupling to the bath, fourth-order corrections do not alter the relaxation characteristics of the second-order equation and introduce additional coherence terms in the equations for the off-diagonal elements.
Efficient use of the correlation consistent basis sets in resolution of the identity MP2 calculations
Authors: Weigend Florian , Köhn Andreas , Hättig Christof
Published: July 2002

The convergence of the second-order Møller–Plesset perturbation theory (MP2) correlation energy with the cardinal number X is investigated for the correlation consistent basis-set series cc-pVXZ and cc-pV(X+d)Z. For the aug-cc-pVXZ and aug-cc-pV(X+d)Z series the convergence of the MP2 correlation contribution to the dipole moment is studied. It is found that, when d-shell electrons cannot be frozen, the cc-pVXZ and aug-cc-pVXZ basis sets converge much slower for third-row elements then they do for first- and second-row elements. Based on the results of these studies criteria are deduced for the accuracy of auxiliary basis sets used in the resolution of the identity (RI) approximation for electron repulsion integrals. Optimized auxiliary basis sets for RI-MP2 calculations fulfilling these criteria are reported for the sets cc-pVXZ, cc-pV(X+d)Z, aug-cc-pVXZ, and aug-cc-pV(X+d)Z with X=D, T, and Q. For all basis sets the RI error in the MP2 correlation energy is more than two orders of magnitude smaller than the usual basis-set error. For the auxiliary aug-cc-pVXZ and aug-cc-pV(X+d)Z sets the RI error in the MP2 correlation contribution to the dipole moment is one order of magnitude smaller than the usual basis set error. Therefore extrapolations towards the basis-set limit are possible within the RI approximation for both energies and properties. The reduction in CPU time obtained with the RI approximation increases rapidly with basis set size. For the cc-pVQZ basis an acceleration by a factor of up to 170 is observed.
The Hamiltonian structure of a two-dimensional rigid circular cylinder interacting dynamically with N point vortices
Authors: Shashikanth Banavara N. , Marsden Jerrold E. , Burdick Joel W. , Kelly Scott D.
Published: July 2002

This paper studies the dynamical fluid plus rigid-body system consisting of a two-dimensional rigid cylinder of general cross-sectional shape interacting with N point vortices. We derive the equations of motion for this system and show that, in particular, if the vortex strengths sum to zero and the rigid-body has a circular shape, the equations are Hamiltonian with respect to a Poisson bracket structure that is the sum of the rigid body Lie–Poisson bracket on se(2)*, the dual of the Lie algebra of the Euclidean group on the plane, and the canonical Poisson bracket for the dynamics of N point vortices in an unbounded plane. We then use this Hamiltonian structure to study the linear and nonlinear stability of the moving Föppl equilibrium solutions using the energy-Casimir method.
The physics of radiation transport in dense plasmas
Authors: Apruzese J. P. , Davis J. , Whitney K. G. , Thornhill J. W. , Kepple P. C. , Clark R. W. , Deeney C. , Coverdale C. A. , Sanford T. W. L.
Published: July 2002

Radiation transport redistributes energy within a medium through the emission and reabsorption of photons. These processes also have a pronounced effect on the spectrum of radiation that escapes the medium. As the deliverable energies of plasma drivers such as lasers and pulsed-power generators steadily increase, denser and/or more massive plasmas can be created. Such plasmas are more absorptive to their own emitted radiation, with portions of the line spectrum frequently being highly opaque. Thus, radiation transport becomes more important, along with the need to consider its impact on the design of experiments and their diagnosis. This tutorial paper covers the basic theory and equations describing radiation transport, its physical effects, experimental examples of transport phenomena, and current challenges and issues. Among the specific topics discussed are requirements for local thermodynamic equilibrium (LTE), conditions for diffusion and the use of the diffusion approximation, the formation of emission and absorption lines, the approach of an emitted spectrum to the Planck limit, and diagnostic applications of transport effects.
Euclidean Friedman–Robertson–Walker cosmology in Brans–Dicke-like theories of gravity
Authors: Chauvineau Bertrand
Published: July 2002

An explicit and tractable form of the complete solution of Brans–Dicke Euclidean Friedman–Robertson–Walker cosmology, in the matter dominated area, is proposed. In this derivation, the (generally imposed) constraint on the Brans–Dicke parameter ω>−3/2 is relaxed. An isolated power-law solution is exhibited, which turns out to describe an accelerated expansion of the universe, without recourse to any cosmological constant or quintessence matter. It appears that this solution can be generalized for all Brans–Dicke-like theories of gravity.
Patterned ferro-, pyro- and piezoelectricity in poly(vinylidene fluoride) by means of a laser-induced irreversible β→α phase transformation
Authors: Wegener M. , Hesse J. , Wegener T. , Gerhard-Multhaupt R.
Published: July 2002

Materials with patterned pyro- or piezoelectricity are useful for a range of applications such as sensor arrays with reduced cross talk between individual elements, piezoelectric gratings for direction-sensitive acoustic-wave detection/emission, or motion-sensitive pyroelectric sensors. Here, the successful patterning of pyro- and piezoelectric poly(vinylidene fluoride) (PVDF) films, by controlled scanning of a focused laser beam across their top electrodes, is reported. The resulting patterns are based on the differences between the ferroelectric polar β phase and the paraelectric nonpolar α phase of PVDF. Nonpolar α-phase areas are generated in a poled β-PVDF film through a local heating process that yields a β→α phase transition throughout the thickness of the film. An important feature of this process is the fact that the PVDF film remains intact and can still be used even in applications that require free-standing films. Infrared spectroscopy and pyroelectrical depth profiling are employed to prove the β→α phase transition and the accompanying pyroelectricity pattern, respectively.
Growth and electrical characterization of GdSi1.7 epilayers formed by channeled ion beam synthesis
Authors: Hogg S. M. , Vantomme A. , Wu M. F.
Published: July 2002

Epitaxial GdSi1.7 layers have been produced by channeled ion beam synthesis. The crystalline quality improves with implant dose and substrate temperature. The latter determines the extent of dynamic annealing whereas both factors are instrumental in driving precipitate growth. This, in turn, promotes the formation of a continuous silicide layer following annealing. The information obtained in this study allows the production of high quality layers with a room temperature resistivity of 86 μΩ cm. Evidence of magnetic ordering is apparent below 44 K.
A hydrodynamic theory for solutions of nonhomogeneous nematic liquid crystalline polymers of different configurations
Authors: Wang Qi
Published: July 2002

A hydrodynamic theory is developed for solutions of nonhomogeneous nematic liquid crystalline polymers (LCPs) of a variety of molecular configurations in proximity of spheroids, extending the Doi kinetic theory for rodlike molecules. The new theory accounts for the molecular aspect ratio as well as the finite range molecular interaction so that it is applicable to liquid crystals ranging from the rodlike liquid crystal at large aspect ratios to the discotic one at small aspect ratios. It also exhibits enhanced shape effects in the viscous stress and warrants a positive entropy production, thereby, the second law of thermodynamics. When restricted to uniaxial symmetry in the weak flow limit, the theory recovers the director equation of the Leslie–Eriksen (LE) theory, but the stress tensor contains excessive gradient terms in addition to the LE stress tensor. The theory predicts that the elastic moduli K1, K2, and K3 obey the ordering K3<K1<K2 for stable, discotic (oblate spheroidal) LCPs in the weak flow limit. The ordering is reversed for rodlike (prolate spheroidal) LCPs. It yields a positive Leslie viscosity α2 (α3) in the flow-aligning (tumbling) regime for oblate (prolate) spheroidal LCPs. Moment averaged, approximate, mesoscopic theories for complex flow simulations are obtained via closure approximations.
Investigation of the effect of laser treatment on the longitudinal and transverse velocity of an aluminum alloy
Authors: Zhang Y. K. , Zhang X. R. , Zhang S. Y. , Zhou M. , Fu Y. H. , Yang Y. J. , Cai L.
Published: July 2002

The velocities of longitudinal and transverse waves for aluminum alloy 2024-T62 shocked by a high-power Nd: Glass laser are investigated by a laser ultrasonics method. The results show that the velocity of the longitudinal wave measured after the laser-shock processing increases at the center and decreases at the edge of the laser-shocked area (LSA). The maximum relative increase is 28% and the maximum decrease up to 10%, when compared with that measured before the alloy was shocked by laser processing. The velocity of the transverse wave decreases at any position within LSA, the maximum relative decrease being 13%. The decrease at the edge of LSA is larger than that at the center.
Associated Lamé and various other new classes of elliptic potentials from sl(2,R) and related orthogonal polynomials
Authors: Ganguly Asish
Published: July 2002

Using representations of sl(2,↛) generators which yield associated Lamé Hamiltonians we obtain new classes of elliptic potentials. We explicitly calculate eigenstates and spectra for these potentials and construct the associated orthogonal polynomials. We show that in the proper limit these potentials reduce to well-known exactly solvable potentials.
Ultradense phosphorous delta layers grown into silicon from PH3 molecular precursors
Authors: Shen T.-C. , Ji J.-Y. , Zudov M. A. , Du R.-R. , Kline J. S. , Tucker J. R.
Published: July 2002

Phosphorous δ-doping layers were fabricated in silicon by PH3 deposition at room temperature, followed by low-temperature Si epitaxy. Scanning tunneling microscope images indicate large H coverage, and regions of c(2×2) structure. Hall data imply full carrier activation with mobility <40 cm2/V s when the surface coverage is ≲0.2 ML. Conductivity measurements show a ln(T) behavior at low temperatures, characteristic of a high-density two-dimensional conductor. Possible future applications to atom-scale electronics and quantum computation are briefly discussed.
In situ x-ray scattering study of PbTiO3 chemical-vapor deposition
Authors: Murty M. V. Ramana , Streiffer S. K. , Stephenson G. B. , Eastman J. A. , Bai G.-R. , Munkholm A. , Auciello O. , Thompson Carol
Published: July 2002

We present in situ surface x-ray scattering measurements of PbTiO3 epitaxy by metal–organic chemical-vapor deposition. Oscillations in crystal truncation rod intensity corresponding to layer-by-layer growth are observed under a variety of growth conditions. At lower PbO overpressures, we observe a transition to step-flow growth and an increased rate of recovery after growth, indicating a higher surface mobility.
Inference of mix in direct-drive implosions on OMEGA
Authors: Radha P. B. , Delettrez J. , Epstein R. , Yu Glebov V. , Keck R. , McCrory R. L. , McKenty P. , Meyerhofer D. D. , Marshall F. , Regan S. P. , Roberts S. , Sangster T. C. , Seka W. , Skupsky S. , Smalyuk V. , Sorce C. , Stoeckl C. , Soures J. , Town R. P. J. , Yaakobi B. , Frenje J. , Li C. K. , Petrasso R. , Seguin F. , Fletcher K. , Padalino S. , Freeman C. , Izumi N. , Lerche R. , Phillips T. W.
Published: July 2002

Direct-drive implosions on the OMEGA laser [T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 (1997)] have been diagnosed using a wide range of techniques based on neutrons, charged particles, and x rays. These implosions use full single-beam smoothing (distributed phase plates, 1-THz-bandwidth smoothing by spectral dispersion and polarization smoothing). The beam-to-beam power imbalance is ⩽5%. Fuel areal densities close to those in one-dimensional (1-D) simulations are inferred for implosions with calculated convergence ratios ∼15. The experimental neutron yields are ∼35% of 1-D yields. The complementary nature of the experimental observables is exploited to infer fuel shell mix in these implosions. Data suggest that this mix occurs at relatively small scales. Analysis of the experimental observables results in a picture of the core and mix region indicating that nearly 70% of the compressed fuel areal density is unmixed, and about 20% of the compressed shell areal density is in the mixed region. Comparisons of this model with inferred core conditions from argon-doped implosions are also presented.
Microstructural and optical properties of InAs/GaAs quantum dots embedded in modulation-doped AlxGa1−xAs/GaAs heterostructures
Authors: Lee H. S. , Lee J. Y. , Kim T. W. , Lee D. U. , Choo D. C. , Jung M. , Kim M. D.
Published: July 2002

The microstructural and the optical properties of InAs/GaAs quantum-dot (QD) arrays inserted into undoped GaAs barriers embedded in an AlxGa1−xAs/GaAs were investigated by using transmission electron microscopy (TEM) and photoluminescence (PL) measurements. The TEM images and the selected-area electron diffraction patterns showed that vertically stacked InAs QD self-assembled arrays were embedded in the GaAs barriers. The temperature-dependent PL spectra showed that the peak corresponding to the interband transitions from the ground electronic subband to the ground heavy-hole band of the InAs QDs shifted to lower energy with increasing temperature. The PL intensity of the InAs dots was significantly enhanced by the modulation-doped AlxGa1−xAs/GaAs heterostructure, and the thermal activation energy of the InAs dots was decreased by the addition of the modulation-doped AlxGa1−xAs/GaAs heterostructure. The present results can help to improve the understanding of the microstructural and the optical properties in InAs QD arrays inserted into GaAs barriers embedded in a modulation-doped AlxGa1−xAs/GaAs heterostructure.
Reversing quantum dynamics with near-optimal quantum and classical fidelity
Authors: Barnum H. , Knill E.
Published: July 2002

We consider the problem of reversing quantum dynamics, with the goal of preserving an initial state’s quantum entanglement or classical correlation with a reference system. We exhibit an approximate reversal operation, adapted to the initial density operator and the “noise” dynamics to be reversed. We show that its error in preserving either quantum or classical information is no more than twice that of the optimal reversal operation. Applications to quantum algorithms and information transmission are discussed.
Nonequilibrium dynamics and fluctuation-dissipation relation in a sheared fluid
Authors: Berthier Ludovic , Barrat Jean-Louis
Published: July 2002

The nonequilibrium dynamics of a binary Lennard-Jones mixture in a simple shear flow is investigated by means of molecular dynamics simulations. The range of temperature T investigated covers both the liquid, supercooled, and glassy states, while the shear rate γ covers both the linear and nonlinear regimes of rheology. The results can be interpreted in the context of a nonequilibrium, schematic mode-coupling theory developed recently, which makes the theory applicable to a wide range of soft glassy materials. The behavior of the viscosity η(T,γ) is first investigated. In the nonlinear regime, strong shear-thinning is obtained, η∼γ−α(T), with α(T)≃23 in the supercooled regime. Scaling properties of the intermediate scattering functions are studied. Standard “mode-coupling properties” of factorization and time superposition hold in this nonequilibrium situation. The fluctuation-dissipation relation is violated in the shear flow in a way very similar to that predicted theoretically, allowing for the definition of an effective temperature Teff for the slow modes of the fluid. Temperature and shear rate dependencies of Teff are studied using density fluctuations as an observable. The observable dependence of Teff is also investigated. Many different observables are found to lead to the same value of Teff, suggesting several experimental procedures to access Teff. It is proposed that a tracer particle of large mass mtr may play the role of an “effective thermometer.” When the Einstein frequency of the tracers becomes smaller than the inverse relaxation time of the fluid, a nonequilibrium equipartition theorem holds with 〈mtrvz2〉=kBTeff, where vz is the velocity in the direction transverse to the flow. This last result gives strong support to the thermodynamic interpretation of Teff and makes it experimentally accessible in a very direct way.
A computer simulation study of multiphase squeezing flows
Authors: Lee Jonghoon , Ladd Anthony J. C.
Published: July 2002

The rheology of dense colloidal suspensions is sensitive to modifications in the surface properties of the particles. We are using lattice Boltzmann simulations to investigate the effects of polymer coatings on the squeezing flow between parallel plates. In this study, we used random arrays of stationary and mobile particles as the simplest models of the polymer coat. We have calculated the flow field in the gap and the force between the plates, and have compared the simulation results with analytic solutions based on the Stokes and Brinkman equations. Our models span the limiting characteristics of a polymer coated layer; namely a viscous suspension and a porous medium. We find good agreement between simulations and analytic solutions, even when the coat thickness is only a few Brinkman screening lengths.
Pseudo-Hermiticity versus PT-symmetry. II. A complete characterization of non-Hermitian Hamiltonians with a real spectrum
Authors: Mostafazadeh Ali
Published: July 2002

We give a necessary and sufficient condition for the reality of the spectrum of a non-Hermitian Hamiltonian admitting a complete set of biorthonormal eigenvectors.
Preparation of boron nitride nanocapsules by plasma-assisted pulsed laser deposition
Authors: Komatsu Shojiro , Shimizu Yoshiki , Moriyoshi Yusuke , Okada Katsuyuki , Mitomo Mamoru
Published: July 2002

Nanocapsules of crystalline boron nitride (BN), whose diameters ranged from 50 to 300 nm, have been synthesized by pulsed-laser-vaporization of BN, where the laser plume was controlled by the modulated plasma jet flow field. Their shapes varied from polyhedrons to cocoons and the interlayer spacings along the c axis were enlarged according to their size and shape (curvature). Without the synchronization of the laser pulses with the plasma modulation, soot-like BN was obtained. This soot-like BN represented the trace of the precursor state which should have been molded into the nanocapsules if it had been just hit by the modulated plasma jet at the appropriate speed.
Nitrogen-related local vibrational modes in ZnO:N
Authors: Kaschner A. , Haboeck U. , Strassburg Martin , Strassburg Matthias , Kaczmarczyk G. , Hoffmann A. , Thomsen C. , Zeuner A. , Alves H. R. , Hofmann D. M. , Meyer B. K.
Published: July 2002

We study the influence of nitrogen, a potential acceptor in ZnO, on the lattice dynamics of ZnO. A series of samples grown by chemical vapor deposition (CVD) containing different nitrogen concentrations, as determined by secondary ion mass spectroscopy (SIMS), was investigated. The Raman spectra revealed vibrational modes at 275, 510, 582, 643, and 856 cm−1 in addition to the host phonons of ZnO. The intensity of these additional modes correlates linearly with the nitrogen concentration and can be used as a quantitative measure of nitrogen in ZnO. These modes are interpreted as local vibrational modes. Furthermore, SIMS showed a correlation between the concentration of incorporated nitrogen and unintentional hydrogen, similar to the incorporation of the p-dopant magnesium and hydrogen in GaN during metalorganic CVD.
Technique for distinguishing and determining the origin of photon emissions from complex plasmas
Authors: Mostefaoui T. M. , Adams N. G. , Babcock L. M.
Published: July 2002

A technique has been developed to separately obtain emission spectra from individual ion–molecule and electron–ion recombination reactions. This involves using a flowing afterglow to create the reaction producing the emissions of interest and pulse modulating a gas into the afterglow that reacts with only the ion species causing that reaction in the plasma. In this way, photon emissions have been detected where other interfering emissions from the ionization source and from other reaction processes, are more than an order of magnitude larger. The technique has been tested with an He+/Ar+ plasma to which OCS had been added. In this case, with pulsed modulation of H2 into the plasma, emissions can be isolated from Ar+ reactions and from the electron recombination of its ion products, as well as from similar reactions involving H3+ (generated from the Ar+ reaction with H2). The technique is such that the majority of the data acquisition time is devoted to collecting data where there is significant signal with little time being used in wavelength regions where there is no significant signal. In these scans, photon counting times are varied to give all signal peaks the same signal to noise ratio independent of intensity and thus the same statistical significance. This is a considerable improvement over standard spectral scanning approaches. Critical features of the technique are evaluated and the means of optimizing the scan are discussed.
Intersubband carrier dynamics in a biased GaAs/AlGaAs quantum-well infrared photodetector
Authors: Schmidt S. R. , Seilmeier A. , Liu H. C.
Published: July 2002

We present time and spectral resolved pump and probe studies of the intersubband dynamics in a bound to quasibound GaAs/AlGaAs quantum-well infrared photodetector dependent on the external electric field. Without bias, the excited electrons are scattered back to the ground subband with a time constant of about 2 ps. Subsequent intrasubband cooling is observed in the ground state, which gives rise to a relaxation component on a 50 ps time scale. With an applied electrical field, 35%–45% of the excited electrons escape to continuum states and are accelerated along the electrical field. Within time constants between 5 and 8 ps, the extended electrons are recaptured by wells in the low field bulk domain close to the collector contact. The redistribution of the carriers in the picosecond time regime and the screening of the external field due to the separation between electrons and donors leads to a change of the absorption spectrum for a time period shorter than the intrinsic response time of the electrical circuit.