bluuebunny/crossref_metadata_2025 huggingface dataset:
A consistent model for the equilibrium thermodynamic functions of partially ionized flibe plasma with Coulomb corrections
Authors: Zaghloul Mofreh R.
Published: January 2003

Flibe (2LiF–BeF2) is a molten salt that has been chosen as the coolant and breeding material in many design studies of the inertial confinement fusion (ICF) chamber. Flibe plasmas are to be generated in the ICF chamber in a wide range of temperatures and densities. These plasmas are more complex than the plasma of any single chemical species. Nevertheless, the composition and thermodynamic properties of the resulting flibe plasmas are needed for the gas dynamics calculations and the determination of other design parameters in the ICF chamber. In this paper, a simple consistent model for determining the detailed plasma composition and thermodynamic functions of high-temperature, fully dissociated and partially ionized flibe gas is presented and used to calculate different thermodynamic properties of interest to fusion applications. The computed properties include the average ionization state; kinetic pressure; internal energy; specific heats; adiabatic exponent, as well as the sound speed. The presented results are computed under the assumptions of local thermodynamic equilibrium (LTE) and electro-neutrality. A criterion for the validity of the LTE assumption is presented and applied to the computed results. Other attempts in the literature are assessed with their implied inaccuracies pointed out and discussed.
Low mass recyclable transmission lines for Z-pinch driven inertial fusion
Authors: Slutz S. A. , Olson C. L. , Peterson Per
Published: January 2003

Recyclable transmission lines (RTLs) are being studied as a means to repetitively drive Z pinches. Minimizing the mass of the RTL should also minimize the reprocessing costs. Low mass RTLs could also help reduce the cost of a single shot facility such as the proposed X-1 accelerator and make Z-pinch driven nuclear space propulsion feasible. Calculations are presented to determine the minimum electrode mass to provide sufficient inertia against the magnetic pressure produced by the large currents needed to drive the Z pinches. The results indicate an electrode thickness which is much smaller than the initial resistive skin depth. This suggests that the minimum electrode thickness may be not be solely determined by inertial effects, but also by the ability of the electrode to efficiently carry the current. A series of experiments have been performed to determine the ability of the electrodes to carry current as a function of the electrode thickness. The results indicate that electrodes much thinner than the initial resistive skin depth can efficiently carry large currents presumably due to the formation of a highly conducting plasma. This result implies that a transmission line with only a few tens of kilograms of material can carry the large Z-pinch currents needed for inertial fusion.
Representation of semigroups in rigged Hilbert spaces: Subsemigroups of the Weyl–Heisenberg group
Authors: Wickramasekara S. , Bohm A.
Published: January 2003

In this paper we study how differentiable representations of certain subsemigroups of the Weyl–Heisenberg group may be obtained in suitably constructed rigged Hilbert spaces. These semigroup representations are induced from a continuous unitary representation of the Weyl–Heisenberg group in a Hilbert space. Aspects of the rigged Hilbert space formulation of time asymmetric quantum mechanics are also investigated within the context of the results developed here.
Anisotropic magnetohydrostatic equilibria with arbitrary magnetic fields
Authors: Lortz D. , Spies G. O.
Published: January 2003

For any arbitrarily given magnetic field, an anisotropic pressure tensor of the form appearing in guiding center theories and drift theories exists such that the magnetohydrostatic pressure balance is met.
Analysis of the critical electron temperature gradient in Tore Supra
Authors: Hoang G. T. , Horton W. , Bourdelle C. , Hu B. , Garbet X. , Ottaviani M.
Published: January 2003

The Tore Supra [Equipe Tore Supra presented by R. Aymar in Plasma Physics and Controlled Nuclear Fusion Research, Proc. 12th Int. Conf., Nice, 1988 (IAEA, Vienna, 1989), Vol. 1, p. 9] database of fast wave electron heating discharges is analyzed with respect to the role of the critical electron temperature gradient. The experimental evidence for the critical gradient is presented from power balance thermal flux qe(∇Te) relation extrapolated to zero in the measured gradient ∇Te. The fluctuation spectra are also known to extrapolate to low levels versus ∇Te. The inferred critical gradient (∇Te)c is defined by the offset linear extrapolation of the thermal flux to zero values. Histograms of the anomalous fluxes before and after being normalized to the theoretical models are constructed. For a wide range of heating powers, the electron power balance radial heat flux is shown to be well described by Te3/2 (1/LTe−1/Lc), where Lc=(Te/∇Te)c is the gradient scale length from linear eigenmodes in a sheared magnetic field. The relative deviations of the normalized heat fluxes as a function of the excess of the temperature gradient beyond the critical gradient give one method of assessing the agreement between the models and the data. A second method is the predictive studies in which the transport codes are used to investigate the impact of the critical gradient at low and high radio frequency power levels with the model thermal diffusivities. The dominant dependence of the inferred critical temperature gradient is its proportionality to the magnetic shear as expected from the robust electron Landau damping on thermal electrons in electron temperature gradient turbulence. The evidence for the electrostatic density independent thermal diffusivity scaling versus the finite beta electromagnetic density dependent thermal diffusivity is shown in some detail. The results strongly favor the electromagnetic turbulent diffusivity formula.
Covariant phase difference observables in quantum mechanics
Authors: Heinonen Teiko , Lahti Pekka , Pellonpää Juha-Pekka
Published: January 2003

Covariant phase difference observables are determined in two different ways, by a direct computation and by a group theoretical method. A characterization of phase difference observables which can be expressed as the difference of two phase observables is given. The classical limits of such phase difference observables are determined and the Pegg-Barnett phase difference distribution is obtained from the phase difference representation. The relation of Ban’s theory to the covariant phase theories is exhibited.
On certain geometric aspects of CPN harmonic maps
Authors: Grundland A. M. , Zakrzewski W. J.
Published: January 2003

A Weierstrass-type system of equations corresponding to CPN harmonic maps is presented. It constitutes a generalization of the previously constructed systems for CP1 and CP2 fields. From the linear spectral problem for the CPN model a set of conserved quantities is derived and used for a construction of a generalized Weierstrass representation for conformally parametrized surfaces immersed in multidimensional Euclidean spaces. Based on this representation a possible geometrical interpretation of CPN harmonic maps is discussed.
Algorithms and novel applications based on the isokinetic ensemble. I. Biophysical and path integral molecular dynamics
Authors: Minary Peter , Martyna Glenn J. , Tuckerman Mark E.
Published: January 2003

In this paper (Paper I) and a companion paper (Paper II), novel new algorithms and applications of the isokinetic ensemble as generated by Gauss’ principle of least constraint, pioneered for use with molecular dynamics 20 years ago, are presented for biophysical, path integral, and Car–Parrinello based ab initio molecular dynamics. In Paper I, a new “extended system” version of the isokinetic equations of motion that overcomes the ergodicity problems inherent in the standard approach, is developed using a new theory of non-Hamiltonian phase space analysis [M. E. Tuckerman et al., Europhys. Lett. 45, 149 (1999); J. Chem. Phys. 115, 1678 (2001)]. Reversible multiple time step integrations schemes for the isokinetic methods, first presented by Zhang [J. Chem. Phys. 106, 6102 (1997)] are reviewed. Next, holonomic constraints are incorporated into the isokinetic methodology for use in fast efficient biomolecular simulation studies. Model and realistic examples are presented in order to evaluate, critically, the performance of the new isokinetic molecular dynamic schemes. Comparisons are made to the, now standard, canonical dynamics method, Nosé–Hoover chain dynamics [G. J. Martyna et al., J. Chem. Phys. 97, 2635 (1992)]. The new isokinetic techniques are found to yield more efficient sampling than the Nosé–Hoover chain method in both path integral molecular dynamics and biophysical molecular dynamics calculations. In Paper II, the use of isokinetic methods in Car–Parrinello based ab initio molecular dynamics calculations is presented.
The effect of high intensity laser propagation instabilities on channel formation in underdense plasmas
Authors: Najmudin Z. , Krushelnick K. , Tatarakis M. , Clark E. L. , Danson C. N. , Malka V. , Neely D. , Santala M. I. K. , Dangor A. E.
Published: January 2003

Experiments have been performed using high power laser pulses (up to 50 TW) focused into underdense helium plasmas (ne⩽5×1019 cm−3). Using shadowgraphy, it is observed that the laser pulse can produce irregular density channels, which exhibit features such as long wavelength hosing and “sausage-like” self-focusing instabilities. This phenomenon is a high intensity effect and the characteristic period of oscillation of these instabilities is typically found to correspond to the time required for ions to move radially out of the region of highest intensity.
Permanent electric dipole moments and hyperfine interaction in ruthenium monocarbide, RuC
Authors: Steimle Timothy C. , Virgo Wilton L. , Brown John M.
Published: January 2003

The [18.1]1∏←X 1∑+ band system of a molecular beam sample of RuC has been recorded field free and in the presence of a static electric field using high resolution laser induced fluorescence spectroscopy. The Stark shifts of the optical features for the 102RuC isotopomer were analyzed to produce permanent electric dipole moments of 3.31(4) and 4.09(14) D for the [18.1]1∏ and X 1∑+ states, respectively. The hyperfine structure associated with spectral features for the 101RuC and 99RuC isotopomers was analyzed to produce electric quadrupole and magnetic hyperfine coupling parameters. The hyperfine structure and dipole moments are interpreted using a molecular orbital correlation model and compared with predictions from electronic structure calculations and isovalent iron monocarbide, FeC.
The favored cluster structures of model glass formers
Authors: Doye Jonathan P. K. , Wales David J. , Zetterling Fredrik H. M. , Dzugutov Mikhail
Published: January 2003

We examine the favored cluster structures for two new interatomic potentials, which both behave as monatomic model glass formers in bulk. We find that the oscillations in the potential lead to global minima that are noncompact arrangements of linked 13-atom icosahedra. The structural properties of the clusters correlate with the glass forming propensities of the potentials, and with the fragilities of the corresponding supercooled liquids.
Heat-transport suppression mechanism based on poloidal plasma rotation and mass-density fluctuation
Authors: Yoshizawa Akira
Published: January 2003

Turbulent heat transport is examined on the basis of a statistical turbulence theory based on the mass-weighted averaging that is suitable for investigating variable-density turbulence. The turbulent heat flux is shown to be composed of the usual gradient diffusion term and the term expressing the combined effects of the density fluctuation and the centripetal force due to poloidal rotation. This finding is discussed in the light of transport barriers in tokamaks.
Finite growth representations of infinite Lie conformal algebras
Authors: Boyallian Carina , Kac Victor G. , Liberati Jose I.
Published: January 2003

We classify all finite growth representations of all infinite rank subalgebras of the Lie conformal algebra gc1 that contain a Virasoro subalgebra.
Scattering on compact manifolds with infinitely thin horns
Authors: Brüning J. , Geyler V. A.
Published: January 2003

The quantum-mechanical scattering on a compact manifold with semi-axes attached to the manifold (“hedgehog-shaped manifold”) is considered. The complete description of the spectral structure of Schrödinger operators on such a manifold is done, the proof of existence and uniqueness of scattering states is presented, an explicit form for the scattering matrix is obtained and unitarity of this matrix is proven. It is shown that the positive part of the spectrum of the Schrödinger operator on the initial compact manifold as well as the spectrum of a point perturbation of such an operator may be recovered from the scattering amplitude for one attached half-line. Moreover, the positive part of the spectrum of the initial Schrödinger operator is fully determined by the conductance properties of an “electronic device” consisting of the initial manifold and two “wires” attached to it.
Numerical investigation of two-dimensional pure electron plasma equilibria on magnetic surfaces
Authors: Pedersen Thomas Sunn
Published: January 2003

Two-dimensional solutions to the equilibrium equation for finite temperature, low density pure electron plasmas confined on magnetic surfaces [T. S. Pedersen and A. H. Boozer, Phys. Rev. Lett. 88, 205002 (2002)] are presented for the first time. These equilibria are not maximum energy states, in contrast to Penning trap equilibria [J. Notte et al., Phys. Rev. Lett. 69, 3056 (1992)]. By varying the number of Debye lengths in the plasma, a/λD, from 0.1 to 10, we explore both relatively warm and relatively cold plasma equilibria. The effects of different boundary conditions and the implications for collisional transport rates are discussed.
Downstream instabilities of electronegative plasma discharges
Authors: Tuszewski M. , Gary S. Peter
Published: January 2003

Relaxation oscillations are observed in low-pressure inductive plasma discharges operated with argon and SF6 gas mixtures. The data suggest that instabilities may develop periodically at some downstream location. Linear fluid and kinetic calculations predict ion two-stream instabilities when the positive and negative ions develop sufficiently large drift velocities of opposite sign in the downstream region. The thresholds and the growth rates of the instabilities are calculated and are found in fair agreement with the observations. The instability models can explain several key experimental features.
The accumulation continua in ideal magnetohydrodynamics
Authors: Spies G. O. , Tataronis J. A.
Published: January 2003

The continuous spectrum formed by the accumulation points of eigenfrequencies of modes with large axial and/or azimuthal mode numbers (so-called “ballooning modes”) is studied in axially periodic circular cylinders with magnetic shear, assuming incompressibility. A singular layer analysis about the radius where the modes are localized leads to an eigenvalue problem for a second order ordinary differential equation in a stretched radial coordinate. This equation is the Fourier transform of the fourth order equation along magnetic field lines that arises from the so-called “ballooning representation.” Both equations therefore describe the radial variation of the perturbation rather than that along the field lines. The following results are readily obtained from the second order equation: The accumulation continua, if there are any, have no stable parts. They are traced out by the eigenvalues as the singular radius runs through the interval where Suydam’s criterion is violated. There is one such eigenvalue for each radial mode number.
Extra ion feature of Thomson scattered light in the interaction of a 600 ps laser with helium gas jet
Authors: Malka V. , De Wispelaere E. , Mounaix Ph. , Hüller S. , Amiranoff F. , Dorchies F. , Modena A.
Published: January 2003

The interaction of a 600 ps laser pulse at 0.53 μm wavelength with helium gas jet with an electron density of 8×1019 cm−3 has been studied. In this experiment the plasma parameters for density and temperature were well-defined via time-resolved Thomson scattering. Ion Thomson scattering measurements at 45° show two extra satellites which correspond to the rescattering of the Brillouin backscattered light off the thermal ion acoustic waves. Analysis of the relative amplitudes of these satellites gives a very high value of 65% of the instantaneous reflectivity at its maximum. Theoretical spectra are in good agreement with the experimental ones.
A new correlation functional based on a transcorrelated Hamiltonian
Authors: Imamura Yutaka , Scuseria Gustavo E.
Published: January 2003

We propose a new correlation functional based on a transcorrelated Hamiltonian that uses an exponential correlation factor. In our approach, electron–electron correlation effects are not calculated directly but are incorporated into an effective kinetic contribution. Our new functional reproduces accurate correlation energies for H–Ar atoms reasonably well. In order to investigate the behavior of this functional, we have also studied the correlation holes of He and Hooke atoms in detail.
Trajectory studies of SN2 nucleophilic substitution. IX. Microscopic reaction pathways and kinetics for Cl−+CH3Br
Authors: Wang Yanfei , Hase William L. , Wang Haobin
Published: January 2003

A classical trajectory simulation performed on the PES1(Br) analytic potential energy surface is used to study the atomic-level dynamics of the Cl−+CH3Br→ClCH3+Br−SN2 nucleophilic substitution reaction. At low reactant relative translational energies Erel of less than 5 kcal/mol, the reaction is dominated by an indirect mechanism in which the Cl−⋯CH3Br complex or both the Cl−⋯CH3Br and ClCH3⋯Br− complexes are formed. For Erel>10 kcal/mol a direct reaction mechanism dominates without the formation of either complex. For Erel of 5–10 kcal/mol there is a minimum in the SN2 rate constant which, for a CH3Br vibrational/rotational temperature Tvr of 300 K, is ∼400 times smaller than the rate at Erel of 0.1 kcal/mol. The dependence of the trajectory SN2 rate constants on Erel, Tv, and Tr is significantly different than the prediction of a statistical theoretical model. For Erel⩽10 kcal/mol there is a much more pronounced decrease in the trajectory SN2 rate constant as Erel is increased as compared to the statistical model, which arises from the inadequacy of the ion–molecule capture component of the statistical model. As Erel is increased the trajectory Cl−+CH3Br association rate constant becomes much smaller than that predicted by the ion–molecule capture model. Increasing the CH3Br rotational temperature from 300 to 600 K decreases the trajectory SN2 rate constant more than the prediction of the statistical model. At low Erel, where the reaction occurs by an indirect mechanism, the product energy is preferentially partitioned to CH3Cl vibration. For the direct mechanism, which dominates at high Erel, the majority of the energy is partitioned to product relative translation.
The adsorption of xenon on crystalline ice surfaces grown on Pt(111) studied with helium atom scattering
Authors: Graham Andrew P. , Toennies J. Peter
Published: January 2003

The adsorption of xenon on a thick crystalline ice layer grown in situ on a Pt(111) substrate has been investigated with high resolution helium atom scattering. The dependence of the specular reflectivity during xenon adsorption and desorption reveals a phase that desorbs at 50.5 K, which is thought to correspond to a single monolayer, and a lower coverage 0.25 monolayer phase with a desorption temperature of 53 K. Both phases exhibit (1×1) helium diffraction peaks. Time-of-flight spectra reveal perpendicular vibrational frequencies of ℏω=2.5 meV for both phases and an additional 2.0 meV mode for the monolayer phase.
Modes of energy transfer from the solar wind to the inner magnetosphere
Authors: Vassiliadis D. , Weigel R. S. , Klimas A. J. , Kanekal S. G. , Mewaldt R. A.
Published: January 2003

Energy transport from the interplanetary plasma to Earth’s inner magnetosphere occurs in a range of time scales and efficiencies. It is often hypothesized that this range is smoothly varying with radial geocentric distance, indicating the transport involves many processes, whose ranges overlap. Here we report evidence from observations, and time series analysis, and other data-based modeling which indicates that the coupling of magnetospheric relativistic electron fluxes to solar wind variables occurs in specific ranges of radial distance (L shell). These findings probably have important consequences for the understanding of physical mechanisms responsible for the acceleration in each region. We identify three distinct regions: P0 at approximately 3<L<4 RE, P1 at 4<L<7 RE, and P2 at L>7 RE. Each one responds to a different combination of solar wind variables, and couples to the main driver variable, the solar wind speed VSW, in a different way. Mode P1 is the prototypical response of the inner magnetosphere. The electron flux responds more slowly than the other two regions to VSW (2–3 days): high-speed streams are the most geoeffective structures for that region. Mode P0 responds significantly faster (<1 day) and seems to be more affected by the negative Bz component of the interplanetary field (probably through magnetic reconnection) and the magnitude of the field, rather than by variations in solar wind plasma variables. Region P2 contains much lower fluxes of trapped particles than the other two, and responds rapidly (∼1 day) to positive Bz and to lower solar wind speed. The interpretation is that these regions are representative of different modes of energy transfer from the interplanetary medium to the inner magnetosphere with implications for very different particle acceleration mechanisms.
Collective and quantum effects on elastic electron–ion collisions in strongly coupled semiclassical plasmas
Authors: Jung Young-Dae
Published: January 2003

Collective plasma screening and quantum-mechanical effects on elastic electron–ion collisions in strongly coupled semiclassical plasmas are investigated using the second-order eikonal method. The effective screened potential model taking into account both the plasma screening and quantum effects is applied to describe the elastic electron–ion interactions in strongly coupled semiclassical plasmas. The impact-parameter trajectory method is applied to the path of the projectile electron in order to investigate the variation of the eikonal cross section as a function of the impact parameter, thermal de Broglie wavelength, Debye length, and collision energy. It is found that the collective plasma screening and quantum effects reduce the elastic electron–ion scattering cross section for small impact parameters. However, the quantum effects are found to be less important than the plasma screening effects for large impact parameters.
Universality of intermittent convective transport in the scrape-off layer of magnetically confined devices
Authors: Antar Ghassan Y. , Counsell Glenn , Yu Yang , Labombard Brian , Devynck Pascal
Published: January 2003

The nature of intermittency, long observed in magnetic fusion devices, was revisited lately [G. Antar et al., Phys. Rev. Lett. 87, 065001 (2001)]. It was shown that intermittency is caused by large-scale events with high radial velocity reaching about 1/10th of the sound speed. These type of structures were named “avaloids.” In the present article, the universality of convective turbulence in magnetically confined plasmas is investigated. Turbulence properties in the scrape-off layer of four different magnetic fusion devices are compared. Namely, the Tore Supra tokamak [Tore Supra Team, Nuclear Fusion, 40, 1047 (2000)] with circular cross-section limiter-bounded plasma, the Alcator C-Mod tokamak [B. LaBombard et al., Phys. Plasmas 8, 2107 (2001)] which is a divertor device, the Mega-Ampere Spherical Tokamak (MAST) [A. Sykes et al., Phys. Plasmas 8, 2101 (2001)] with vacuum chamber walls far from the plasma last closed flux surface and the PISCES linear plasma device [D. Geobel et al., Rev. Sci. Istrum. 56, 1717 (1985)]. The statistical properties of the turbulent signals in the four devices are found to be identical allowing us to conclude that intermittent convective transport by avaloids is universal in the sense that it occurs and has the same properties in many confinement devices with different configurations.
Screening and quantum effects on the bremsstrahlung process in strongly coupled semiclassical plasmas
Authors: Jung Young-Dae
Published: January 2003

Collective plasma screening and quantum mechanical effects on the low-energy electron–ion Coulomb bremsstrahlung process are investigated for both the soft and hard photon radiations in strongly coupled semiclassical plasmas. The effective screened pseudopotential taking into account both the screening and quantum effects is applied to describe the electron–ion interactions in strongly coupled semiclassical plasmas. The impact-parameter trajectory method is applied to the motion of the projectile electron in order to investigate the variation of the bremsstrahlung cross section as a function of the impact parameter, Debye length, thermal de Broglie wavelength, projectile energy, and radiation photon energy. The results show that the collective plasma screening and quantum mechanical effects reduce the electron–ion bremsstrahlung cross section, especially, for small impact parameters. It is also found that the quantum mechanical effects are found to be more important than the collective plasma screening effects in strongly coupled semiclassical plasmas. The collective plasma screening and quantum mechanical effects on the bremsstrahlung cross section for soft photon radiations are found to be more significant than those for hard photon radiations.
Magnetic reconnection in a force-free plasma: Simulations of micro- and macroinstabilities
Authors: Nishimura Kazumi , Gary S. Peter , Li Hui , Colgate Stirling A.
Published: January 2003

Two-and-one-half-dimensional particle-in-cell plasma simulations are used to study the stability of a force-free sheet pinch equilibrium, in which currents flow parallel to the sheared magnetic field and the plasma is uniform. The magnetic shear drives a long wavelength, electromagnetic tearing instability, whereas a sufficiently large current may excite a shorter wavelength electrostatic electron/ion instability. This latter instability heats electrons locally, which causes the formation of a new configuration with inhomogeneous electron density and temperature which, in turn, enhances the growth of the tearing mode. Both instabilities lead to conversion of magnetic energy into plasma particle energy, but only the tearing mode leads to significant reconnection.
Structural reconstruction of hexagonal to cubic ZnO films on Pt/Ti/SiO2/Si substrate by annealing
Authors: Kim Sung-Kyu , Jeong Se-Young , Cho Chae-Ryong
Published: January 2003

It is well known that ZnO generally has the wurtzite structure. We report the growth of both hexagonal and cubic ZnO on Pt(111)/Ti/SiO2/Si substrate by a solution deposition. The wurtzite structure of ZnO film is enhanced up to the annealing temperature of 600 °C, and disappeared for annealing temperatures above 700 °C. The Pt(111)/Ti/SiO2/Si substrate is reoriented to hexagonal-Pt3Ti(004)/Ti/SiO2/Si when annealed at 700 °C and above due to the Ti out-diffusion and the ZnO thin film grown on the substrate has a cubic structure. The diffusion of Ti was evidenced by Auger electron spectroscopy measurements. From the photoluminescence measurement, the band gap of the wurtzite structure of ZnO film grown by the annealing below 600 °C was 3.37 eV, as is already known, but the cubic ZnO had a band gap of 3.28 eV, which suggests a zinc blende structure. The stability of the zinc blende structure on Zn2TiO4 layer was checked by the calculation of the lattice mismatch using the extended atomic distance mismatch model. Additionally the formation of the zinc blende ZnO could be prevented by using the Pt(111)/TiO2/SiO2/Si substrate.
Probability density distribution of random line segments inside a convex body: Application to random media
Authors: Mazzolo Alain
Published: January 2003

We study the probability density distribution of random line segments entirely contained inside a convex body of Rn. Relations between this distribution function and other distribution functions of importance in geometric probability are shown. The expression for the probability density distribution of random line segments is given for several simple geometric bodies in R2, R3, and Rn, application to random media is then presented.
Proof of a mass singularity free property in high temperature QCD
Authors: Grandou T.
Published: January 2003

It is shown that three series of diagrams entering the calculation of some hot QCD process, are mass (or collinear) singularity free, indeed. This generalizes a result which was recently established up to the third nontrivial order of (thermal) perturbation theory.
Bias effect on the luminescent properties of rectangular and trapezoidal quantum-well structures
Authors: Cheong M. G. , Choi R. J. , Suh E.-K. , Lee H. J.
Published: January 2003

We have investigated the properties of two types of InGaN/GaN quantum-well (QW) structures. Photoluminescence (PL) measurements were carried out by varying the external bias voltage. The magnitude of the variation in PL peak position and intensity of trapezoid QWs (TQWs) is much smaller than that of rectangular QWs (RQWs). According to transmission electron microscopy measurements, quantum dots are more densely and uniformly distributed in TQWs than in RQWs. The electroluminescence image of a light-emitting diode fabricated using TQWs as active layers (TQW-LED) is more uniform than that of a light-emitting diode fabricated using RQWs as active layers (RQW-LED). Optical output power of a TQW-LED is larger than that of a RQW-LED. These results show that the origin of strong emission from InGaN/GaN QWs is attributed to exciton localization quantum dots, and InGaN/GaN TQWs are considered as active materials in order to increase performance in optoelectronic device.
Computer simulations on three-dimensional magnetic loop dynamics by the spontaneous fast reconnection model
Authors: Ugai M. , Kondoh K. , Shimizu T.
Published: January 2003

Three-dimensional (3D) dynamics of a large-scale magnetic loop is studied by precise magnetohydrodynamic simulations on the basis of the spontaneous fast reconnection model. Once a (current-driven) anomalous resistivity is ignited, the fast reconnection mechanism drastically evolves by the positive feedback between the (3D) global reconnection flow and the anomalous resistivity; on the nonlinear saturation phase, the global reconnection flow has grown so that the reconnection (diffusion) region shrinks to a small extent, and the fast reconnection mechanism involving a pair of standing slow shocks is established in the finite extent. When the 3D plasmoid, formed ahead of the fast reconnection jet, collides with the mirror plane boundary, the reconnected field lines are piled up, leading to formation of a large-scale 3D magnetic loop. Since the resulting 3D fast reconnection jet becomes supersonic, a definite fast shock builds up at the interface between the magnetic loop top and the fast reconnection jet. The 3D fast reconnection jet is limited in a narrow channel between the pair of slow shocks, so that the resulting fast shock is also limited to a small extent ahead of the magnetic loop top. On the other hand, for the uniform resistivity model the 3D fast reconnection mechanism cannot be realized without any vital positive feedback between the reconnection flow and the local magnetic diffusion; hence, such an effective resistivity that can be self-consistently enhanced locally at the X reconnection point by the global reconnection flow is essential for the fast reconnection mechanism to be realized in actual systems.
Modulational instability of cylindrical and spherical dust ion-acoustic waves
Authors: Jukui Xue , He Lang
Published: January 2003

Using the reductive perturbation method, a cylindrically and spherically modified nonlinear Schrödinger equation describing the slow modulation of the dust ion-acoustic wave amplitude in an unmagnetized dusty plasma is obtained for the first time. The modulational instability of the dust ion-acoustic waves due to the cylindrical and spherical geometry effect is investigated. It is shown that the properties of the modulational instability of dust ion-acoustic wave in a cylindrical and spherical geometry differ from those in a planar one-dimensional geometry.
Hot electron generation in laser cluster plasma
Authors: Smirnov M. B. , Krainov V. P.
Published: January 2003

A new mechanism is proposed that leads to an efficient hot electron generation in a laser cluster plasma. The mechanism is similar to the usual induced inverse bremsstrahlung. Under the action of the laser field, electrons are escaped from the clusters, then they scatter on the positively charged ionized clusters and gain energy. An analytical expression has been obtained for the transfer cross section at the elastic scattering of free electrons on the ionized clusters. This expression is exploited to calculate electron energy absorption by the electrons during the laser pulse. It was found that the inelastic scattering is negligible. Examples are considered for heating of electrons ejected from Xe clusters irradiated by a 50 fs laser pulse with a peak intensity of 1018 W/cm2. Expansion of clusters is derived within the frame of the Coulomb explosion model. The decrease of the cluster expansion has been taken into account because of the surrounding cloud of free escaped electrons.
Neutron radiation effects on the nonlinear current–voltage characteristics of ilmenite-hematite ceramics
Authors: Padmini P. , Pulikkathara M. , Wilkins R. , Pandey R. K.
Published: January 2003

Ilmenite-hematite (IH) is a wide-band gap semiconducting material with a potential for low-voltage varistor applications. The nonlinear characteristics of this material were investigated before and after exposure to high-energy neutron radiation. The typical current–voltage characteristics of a varistor device are retained in all our samples subjected to neutron radiation. However, the effect is more pronounced on the device parameters like the nonlinear coefficient (α) and switching field (Es). The crystallinity of the ceramic remains unaffected after irradiation. Our observations confirm that IH-based varistor devices can perform satisfactorily even in radiation dominant environments such as in space and nuclear reactors.
Harmonic Langmuir waves. I. Nonlinear dispersion relation
Authors: Yoon P. H. , Gaelzer R. , Umeda T. , Omura Y. , Matsumoto H.
Published: January 2003

Generation of electrostatic multiple harmonic Langmuir modes during beam–plasma interaction process has been observed in laboratory and spaceborne active experiments, as well as in computer simulation experiments. Despite earlier efforts, such a phenomenon has not been completely characterized both theoretically and in terms of numerical simulations. This paper is a first in a series of three papers in which analytic expressions for harmonic Langmuir mode dispersion relations are derived and compared against the numerical simulation result.
Harmonic Langmuir waves. II. Turbulence spectrum
Authors: Gaelzer R. , Yoon P. H. , Umeda T. , Omura Y. , Matsumoto H.
Published: January 2003

The Langmuir wave turbulence generated by a beam–plasma interaction has been studied since the early days of plasma physics research. In particular, mechanisms which lead to the quasi-power-law spectrum for Langmuir waves have been investigated, since such a spectrum defines the turbulence characteristics. Meanwhile, the generation of harmonic Langmuir modes during the beam–plasma interaction has been known for quite some time, and yet has not been satisfactorily accounted for thus far. In paper I of this series, nonlinear dispersion relations for these harmonics have been derived. In this paper (paper II), generalized weak turbulence theory which includes multiharmonic Langmuir modes is formulated and the self-consistent particle and wave kinetic equations are solved. The result shows that harmonic Langmuir mode spectra can indeed exhibit a quasi-power-law feature, implying multiscale structure in both frequency and wave number space spanning several orders of magnitude.
Harmonic Langmuir waves. III. Vlasov simulation
Authors: Umeda T. , Omura Y. , Yoon P. H. , Gaelzer R. , Matsumoto H.
Published: January 2003

Generation of harmonic Langmuir modes during beam–plasma interaction is studied by means of nonlinear theoretical calculations and computer simulations. The present Vlasov simulation of multiple harmonic Langmuir modes (up to 12th harmonics), generalizes the previously available simulations which were restricted to the second harmonic only. The frequency-wave-number spectrum obtained by taking the Fourier transformation of simulated electric field both in time and space shows an excellent agreement with the theoretical nonlinear dispersion relations for harmonic Langmuir waves. The saturated wave amplitude features a quasi-power-law spectrum which reveals that the harmonic generation process may be an integral part of the Langmuir turbulence.
Deformation of semiflexible chains
Authors: Winkler Roland G.
Published: January 2003

The force-extension relation and the end-to-end distribution function are calculated in the constant force and constant extension ensemble, respectively, for a semiflexible chain of Gaussian segments. Qualitative differences are found for these quantities when the persistence length is on the order of the chain length. In particular, beyond a certain persistence length, the free energy assumes two extreme values in the constant extension ensemble corresponding to zero force at zero and at a finite extension. The comparison of the force-extension relation with experimental results on DNA exhibits excellent agreement. The approach provides a simple expression for the end-to-end distribution function which is in excellent agreement with Monte Carlo simulations of the Kratky–Porod semiflexible chain model.
Microfluidic actuation by modulation of surface stresses
Authors: Darhuber Anton A. , Valentino Joseph P. , Davis Jeffrey M. , Troian Sandra M. , Wagner Sigurd
Published: January 2003

We demonstrate the active manipulation of nanoliter liquid samples on the surface of a glass or silicon substrate by combining chemical surface patterning with electronically addressable microheater arrays. Hydrophilic lanes designate the possible routes for liquid migration while activation of specific heater elements determine the trajectories. The induced temperature fields spatially modulate the liquid surface tension thereby providing electronic control over the direction, timing, and flow rate of continuous streams or discrete drops. Temperature maps can be programed to move, split, trap, and mix ultrasmall volumes without mechanically moving parts and with low operating voltages of 2–3 V. This method of fluidic actuation allows direct accessibility to liquid samples for handling and diagnostic purposes and provides an attractive platform for palm-sized and battery-powered analysis and synthesis.
Electrical properties of metal contacts on laser-irradiated n-type GaN
Authors: Jang Ho Won , Kim Jong Kyu , Lee Jong-Lam , Schroeder J. , Sands T.
Published: January 2003

The electrical properties of metal contacts on laser-irradiated n-type GaN were investigated using synchrotron radiation photoemission spectroscopy. A KrF excimer laser pulse of 600 mJ/cm2 onto GaN led to a decrease in the Ni Schottky barrier height from 0.91 to 0.47 eV, resulting in the formation of a nonalloyed Ohmic contact with a specific contact resistivity of 1.7×10−6 Ω cm2. Metallic Ga decomposed from GaN by laser irradiation was transformed into GaOx, playing a role in promoting outdiffusion of N atoms. A large number of N vacancies were produced, forming a degenerated GaN layer near the surface, resulting in the good Ohmic contact.
Oblique propagation of surface waves in an ideal Hall-magnetohydrodynamic finite β plasma slab
Authors: Zhelyazkov Ivan , Mann Gottfried
Published: January 2003

The paper considers the most general case of oblique propagation of sausage and kink magnetohydrodynamic (MHD) surface waves in an ideal finite β magnetized plasma slab by taking into account the Hall term in the generalized Ohm’s law. It is found that, like the cases of incompressible (β→∞) and cold (β→0) plasmas, the combining action of the Hall effect and the oblique wave propagation makes possible, for a given wave vector k making an angle θ with respect to the ambient magnetic field B0, the existence of multivalued solutions to the dispersion relations of both kinds of MHD surface waves. Like in unbounded Hall-MHD plasmas, in the low-frequency limit (the wave frequency ω smaller than the ion-cyclotron frequency ωci), there is generally observed three type of waves, notably fast, intermediate (or Alfvén) and slow modes. In view of possible solar-wind applications, here, is considered only Alfvén and slow surface waves. A peculiarity of sausage and kink surface waves is that their structure (in the direction perpendicular to the ambient magnetic field B0) is determined by four attenuation coefficients (two pairs inside and outside the layer, respectively) being real or imaginary quantities. This complex structure of Hall-MHD surface waves make them akin (however, not equivalent) to the Rayleigh-type waves in solids and geophysics.
Enhanced electroluminescence from light-emitting devices based on poly(9,9-dihexadecylfluorene-2,7-diyl) and polysilane blends
Authors: Cimrová Věra , Výprachtický Drahomı́r
Published: January 2003

Photoluminescence and electroluminescence (EL) of poly(9,9-dihexadecylfluorene-2,7-diyl) (PFC16) and its blends with hole-transporting poly[methyl(phenyl)silanediyl] modified with pyrene were studied. Efficient blue light-emitting devices were fabricated. Blending of PFC16 with modified polysilane led to a significant improvement in the EL efficiency and stability compared with the devices fabricated from neat PFC16. An increase in the EL efficiency of up to two orders of magnitude was achieved. The increase in the EL efficiency was attributed to the modification of the charge transport and recombination in the blend.
Near-field optical lithography of a conjugated polymer
Authors: Riehn Robert , Charas Ana , Morgado Jorge , Cacialli F.
Published: January 2003

We report the fabrication of poly(p-phenylene vinylene) nanostructures by direct scanning near-field lithography of its soluble precursor. The technique is based on the spatially selective inhibition of the precursor solubility by exposure to the ultraviolet optical field present at the apex of scanning near-field fiber probes with aperture diameters between 40 and 80 nm (±5 nm). After development in methanol and thermal conversion under vacuum we obtain features with a minimum dimension of 160 nm. We demonstrate the use of the technique for the direct writing of two-dimensional photonic crystals with intentional defects and a periodicity relevant to applications in the visible range. Using a Bethe–Bouwkamp model, we then discuss the influence of probe size, tip-sample distance, and film thickness on the resolution of the lithographic process. We also discuss limitations to the resolution that can arise from physical properties of the lithographic medium.
High-Al-content crack-free AlGaN/GaN Bragg mirrors grown by molecular-beam epitaxy
Authors: Natali F. , Byrne D. , Dussaigne A. , Grandjean N. , Massies J. , Damilano B.
Published: January 2003

We report on the growth by molecular-beam epitaxy on 2 in. sapphire substrates of crack-free AlxGa1−xN/GaN distributed Bragg reflectors (DBRs) with high-Al composition (x=0.5). This is achieved by introducing a thick AlN interlayer and strain mediating AlyGa1−yN layer between the substrate and DBR. The relatively larger refractive index ratio between Al0.5Ga0.5N and GaN permits one to obtain a quite large spectral stopband width (49 nm) and a high reflectance value (69%) for only eight mirror periods.
Hybrid electro-optic polymer and selectively buried sol-gel waveguides
Authors: Enami Y. , Meredith G. , Peyghambarian N. , Kawazu M. , Jen A. K.-Y.
Published: January 2003

An approach to utilizing electro-optically (EO) active polymers in hybrid waveguide structures is described and demonstrated. Buried sol-gel waveguides provide a robust basis for planar integrated circuitry, suitable for coupling with optical fibers. By selectively exposing the core of a buried channel guide, overcoating with an EO polymer, and using appropriately tapered transition zones, a guided mode can be made to undergo adiabatic transitions from sol-gel waveguide up into the EO polymer and back. Such a single-mode device was constructed. These transitions were proven via modulation of the emerging wave’s polarization ellipse by application of voltage to the EO polymer.
Three-color integration on rare-earth-doped GaN electroluminescent thin films
Authors: Wang Y. Q. , Steckl A. J.
Published: January 2003

We have realized full color integration on rare-earth-doped thin-film electroluminescent (EL) GaN using lateral integration. Tm, Er, and Eu dopants were in situ doped into GaN thin films during growth in order to obtain blue, green, and red emission, respectively. Three color pixel arrays have been fabricated using spin-on-glass films as the sacrificial layers for lift-off lithography. The pixel dimensions are 0.2×0.7 mm2, and the separation is 0.2 mm. dc EL devices were fabricated using indium tin oxide transparent electrodes. Typical applied voltage was 30–40 V. The blue emission from Tm-doped GaN has a peak at 477 nm, the green emission from Er-doped GaN has two peaks at 537 and 558 nm, while the red emission from Eu-doped GaN has a peak at 621 nm.
Blue-light emission from sputtered Si:SiO2 films without annealing
Authors: Hanaizumi Osamu , Ono Kazutaka , Ogawa Yuichi
Published: January 2003

We observed a photoluminescence (PL) spectrum at room temperature that had a peak with full width of half maximum (FWHM) of 0.38 eV near the band gap energy of 3.2 eV from a Si:SiO2 sputtered film without annealing. Blue-light emission could be seen by the naked eye. A low-intensity PL peak with FWHM of 0.20 eV was also observed at around 1.6 eV. We have already demonstrated that our method automatically forms Si clusters contributing to visible emission. Our results did not contradict the well-founded conjecture that there were two mechanisms of emission from Si clusters: emission at 1.6–1.7 eV due to the surface state of the oxidized Si nanocrystals and emission at the band gap energy originating from the quantum confinement effect.
Failure of nitrogen cluster states to emerge into the bandgap of GaAsN with application of pressure
Authors: Kent P. R. C. , Zunger Alex
Published: January 2003

The electronic structure of GaAsN alloys was previously described in terms of nitrogen “cluster states” (CS) that exist in the dilute alloy in the bandgap, and “perturbed host states” (PHS) inside the conduction band. As the nitrogen concentration increases, the PHS plunge down in energy overtaking the CS. We show theoretically that the CS respond to the application of pressure in two different ways: the highly localized deep CS emerge (or remain) in the gap, because their pressure coefficient is lower than that of the conduction band minimum. In contrast, the shallow CS (first to be overtaken) hybridize so strongly with the conduction band that their pressure coefficient becomes comparable to that of the conduction states. These states fail to emerge into the gap upon application of pressure because they move, with application of pressure, at a similar rate with conduction states.