bluuebunny/crossref_metadata_2025 huggingface dataset:
A theory-based transport model with comprehensive physics
Authors: Staebler G. M. , Kinsey J. E. , Waltz R. E.
Published: May 2007

A new theory-based transport model with comprehensive physics (trapping, general toroidal geometry, fully electromagnetic, electron-ion collisions, impurity ions) has been developed. The core of the model is the new trapped-gyro-Landau-fluid (TGLF) equations, which provide a fast and accurate approximation to the linear eigenmodes for gyrokinetic drift-wave instabilities (trapped ion and electron modes, ion and electron temperature gradient modes, and kinetic ballooning modes). The new TGLF transport model is more accurate, and has an extended range of validity, compared to its predecessor GLF23. The TGLF model unifies trapped and passing particles in a single set of gyro-Landau-fluid equations. A model for the averaging of the Landau resonance by the trapped particles makes the equations work seamlessly over the whole drift-wave wave-number range from trapped ion modes to electron temperature gradient modes. A fast eigenmode solution method enables unrestricted magnetic geometry. The transport model uses the TGLF eigenmodes to compute quasilinear fluxes of energy and particles. A model for the saturated intensity of the turbulence completes the flux calculation. The intensity model is constructed to fit a large set of nonlinear gyrokinetic turbulence simulations with kinetic electrons. The TGLF model is valid in new physical regimes that GLF23 was not. These include the low aspect ratio spherical torus, which has both a high trapped fraction and strong shaping of magnetic flux surfaces. The TGLF model is also valid close to the magnetic separatrix so the transport physics of the H-mode pedestal region can be explored.
Intrinsic rotation in DIII-D
Authors: deGrassie J. S. , Rice J. E. , Burrell K. H. , Groebner R. J. , Solomon W. M.
Published: May 2007

In the absence of any auxiliary torque input, the DIII-D plasma consists of nonzero toroidal angular momentum, in other words, it rotates. This effect is commonly observed in tokamaks, being referred to as intrinsic rotation. Measurements of intrinsic rotation profiles have been made in DIII-D [J. Luxon, Nucl. Fusion 42, 614 (2002)] H-mode discharges, with both Ohmic heating (OH) and electron cyclotron heating (ECH) in which there is no auxiliary torque. Recently, the H-mode data set has been extended with the newly configured DIII-D simultaneous co- and counter-directed neutral beam injection (NBI) capability resulting in control of the local torque deposition, where co and counter refer to the direction relative to the toroidal plasma current. Understanding intrinsic rotation is important for projection toward burning plasma performance where any NBI torque will be relatively small. The toroidal velocity is recognizably important regarding issues of stability and confinement. In DIII-D ECH H-modes the rotation profile is hollow, co-directed at large minor radius and depressed, or actually counter-directed, nearer the magnetic axis. This profile varies with the ECH power deposition profile to some extent. In contrast, OH H-modes have a relatively flat co-directed rotation profile. There is a scaling of the DIII-D intrinsic toroidal velocity with W∕Ip, as seen in intrinsic rotation in Alcator C-Mod [J. Rice, Nucl. Fusion 39, 1175 (1999)], where W is the total plasma thermal energy and Ip is the magnitude of the toroidal plasma current. This common scaling resulted in a dimensionless similarity experiment between DIII-D and Alcator C-Mod on intrinsic rotation, obtaining a single spatial point match in the toroidal velocity normalized to the ion thermal velocity. The balanced NBI capability in DIII-D is a useful tool to push scaling studies to higher values of the plasma normalized energy, notwithstanding the details of torque deposition for co-NBI versus counter-NBI. There are theories which address intrinsic rotation, both extensions of neoclassical theory and related to turbulent transport. At this time, the comparisons with theory are qualitative.
Magnetic anisotropies in (210)-oriented bismuth substituted iron garnet thin films
Authors: Nistor I. , Holthaus C. , Tkachuk S. , Mayergoyz I. D. , Krafft C.
Published: May 2007

The liquid phase epitaxy growth and characterization of single crystal (210)-oriented thin garnet films with Bi substitution up to 1.5at.∕f.u. is reported. These epitaxial films exhibit an easy plane of magnetization which is inclined with respect to the film plane, making them uniquely suitable for garnet-based magneto-optic imagers (MOIs). In order to identify the optimal growth conditions to attain the highest sensitivity of such MOIs, the chemical composition of the films is discussed in relation with their magnetic and optic properties. It has been demonstrated that the increase in the amount of Pr tends to increase the in-plane orthorhombic anisotropy field HKi, while the rare-earth substitution by Bi has a strong effect on the canted orthorhombic anisotropy Kyz. The best MOI film had a saturation field of 130Oe and a sensitivity of 175deg∕A.
Magnetic anisotropy and crystalline texture in BaO(Fe2O3)6 thin films deposited on GaN∕Al2O3
Authors: Ohodnicki P. R. , Goh K. Y. , Hanlumyuang Y. , Ramos K. , McHenry M. E. , Cai Z. , Ziemer K. , Morkoc H. , Biyikli N. , Chen Z. , Vittoria C. , Harris V. G.
Published: May 2007

Ba O ( Fe 2 O 3 ) 6 (BaM) thin films were deposited by pulsed laser deposition on GaN∕Al2O3 substrates. A pole figure obtained from the (006) reflection indicated that ∼81% of the film volume had the c axis tilted less than 5° from the film normal. A low anisotropy field was inferred from vector coil vibrating sample magnetometer (VVSM) measurements. The reduction in Ha from literature values and a two-step switching of the easy axis magnetization is postulated to result from interdiffusion and misalignment effects. To alleviate interdiffusion and to improve the c-axis alignment, experiments were repeated with lower deposition temperatures, thinner films, and MgO buffer layers. The features of the hysteresis loop due to two-step switching and the in-plane coercivity were reduced while the anisotropy field (Ha) was larger. Films deposited with MgO buffer layers are observed to have single-step switching of the easy axis magnetization, larger anisotropy fields, and sharp ferromagnetic resonance (FMR) peaks. Films with MgO buffer layers were determined to have anisotropy fields Ha=1.57T by FMR and Ha∼1.5–1.6T as determined from the difference in the saturation fields for the easy and hard axis loops.
Irreversible field induced magnetostriction at temperatures above and below the order-disorder transition in single crystal Tb5Si2.2Ge1.8
Authors: Ring A. P. , Lograsso T. , Schlagel D. , Snyder J. E. , Jiles D. C.
Published: May 2007

This paper reports on the behavior of single crystal Tb5Si2.2Ge1.8 in the vicinity of its order-disorder and order-order phase transition from a higher temperature paramagnetic∕monoclinic state to a lower temperature ferromagnetic∕orthorhombic state. Measurements have been made of thermal and field induced changes in strain along the crystallographic a axis. The material exhibits large strains of up to 1500ppm when a magnetic field is applied to it in its paramagnetic state but much smaller strains when a field is applied to it in its ferromagnetic state. These field induced strains are different from conventional magnetostriction because they result mostly from the change in crystal structure. As a result of this the field induced strain changes that accompany the phase transitions of this material are not fully reversible. The shape and slope of the strain versus magnetic field curves were distinctly different depending on whether the material started from above the Curie temperature (where the application of a magnetic field of sufficient strength induced a structural phase transformation) or started from below the Curie temperature (where the application of a field merely stabilized the existing magnetic order).
Response of multiscale turbulence to electron cyclotron heating in the DIII-D tokamak
Authors: Rhodes T. L. , Peebles W. A. , Van Zeeland M. A. , deGrassie J. S. , Bravenec R. V. , Burrell K. H. , DeBoo J. C. , Lohr J. , Petty C. C. , Nguyen X. V. , Doyle E. J. , Greenfield C. M. , Zeng L. , Wang G.
Published: May 2007

Small-scale density turbulence (k⊥ρi∼4–10) and electron thermal flux are both observed to increase during electron cyclotron heating (ECH) of a high-temperature tokamak plasma (k⊥ is the turbulent wavenumber and ρi the ion gyroradius). In contrast, large- (k⊥ρi≤1) and intermediate-scale (k⊥ρi∼1–3) turbulence ñ∕n levels and ion thermal transport remain effectively constant. This implies that the small-scale turbulence is not a remnant or tail of the ubiquitous, large-scale or intermediate-scale turbulence, and also indicates a potentially important role in determining anomalous electron thermal transport. Radial scans of small-scale turbulence during ECH indicate decreased fluctuations in the deep core compared with increased levels towards the edge. This trend is consistent with linear gyrokinetic growth rate predictions for electron temperature gradient driven instabilities.
Large positive magnetoresistance effect below Curie temperature in In1.90−xMn0.1SnxO3
Authors: Wang X. L. , Peleckis G. , Dou S. X. , Liu R. S. , Zhu J. G.
Published: May 2007

We report on the magnetoresistance (MR) and magnetic properties of In1.90−xMn0.1SnxO3 (0⩽x⩽0.06) oxide. All samples were found to be ferromagnetic below TC=46K. Sn doping changed In1.90Mn0.1O3 from an insulator to a highly conducting phase at 300K. A positive MR effect was observed over a wide temperature range just below TC. Calculated MR values reached a maximum of 20% at 5K. A change in the MR effect, from positive to negative, occurred under magnetic field H>4T at 5K. The results of x-ray absorption near-edge spectroscopy indicated that Mn ions are present both as Mn2+ and Mn4+.
Assessing the prospects for achieving double-shell ignition on the National Ignition Facility using vacuum hohlraums
Authors: Amendt Peter , Cerjan C. , Hamza A. , Hinkel D. E. , Milovich J. L. , Robey H. F.
Published: May 2007

The goal of demonstrating ignition on the National Ignition Facility [J. D. Lindl et al., Phys. Plasmas 11, 339 (2003)] has motivated a revisit of double-shell (DS) targets as a complementary path to the cryogenic baseline approach. Expected benefits of DS ignition targets include noncryogenic deuterium-tritium (DT) fuel preparation, minimal hohlraum-plasma-mediated laser backscatter, low threshold-ignition temperatures (≈4keV) for relaxed hohlraum x-ray flux asymmetry tolerances, and minimal (two-) shock timing requirements. On the other hand, DS ignition presents several formidable challenges, encompassing room-temperature containment of high-pressure DT (≈790atm) in the inner shell, strict concentricity requirements on the two shells (<3μm), development of nanoporous (<100nm cell size) low-density (<100mg∕cc) metallic foams for structural support of the inner shell and hydrodynamic instability mitigation, and effective control of hydrodynamic instabilities on the high-Atwood-number interface between the DT fuel and the high-Z inner shell. Recent progress in DS ignition designs and required materials science advances at the nanoscale are described herein. Two new ignition designs that use rugby-shaped vacuum hohlraums are presented that utilize either 1 or 2MJ of laser energy at 3ω. The capability of the National Ignition Facility to generate the requested 2 MJ reverse-ramp pulse shape for DS ignition is expected to be comparable to the planned high-contrast (≈100) pulse shape at 1.8MJ for the baseline cryogenic target. Nanocrystalline, high-strength, Au-Cu alloy inner shells are under development using electrochemical deposition over a glass mandrel, exhibiting tensile strengths well in excess of 790atm. Novel, low-density (85mg∕cc) copper foams have recently been demonstrated using 10mg∕cc SiO2 nanoporous aerogels with suspended Cu particles. A prototype demonstration of an ignition DS is planned for 2008, incorporating the needed novel nanomaterials science developments and the required fabrication tolerances for a realistic ignition attempt after 2010.
Electrical and thermal transport in metallic single-wall carbon nanotubes on insulating substrates
Authors: Pop Eric , Mann David A. , Goodson Kenneth E. , Dai Hongjie
Published: May 2007

We analyze transport in metallic single-wall carbon nanotubes (SWCNTs) on insulating substrates over the bias range up to electrical breakdown in air. To account for Joule self-heating, a temperature-dependent Landauer model for electrical transport is coupled with the heat conduction equation along the nanotube. The electrical breakdown voltage of SWCNTs in air is found to scale linearly with their length, approximately as 5V∕μm; we use this to deduce a thermal conductance between SWCNT and substrate g≈0.17±0.03WK−1m−1 per tube length, which appears limited by the SWCNT-substrate interface rather than the thermal properties of the substrate itself. We examine the phonon scattering mechanisms that limit electron transport, and find the strong temperature dependence of the optical phonon absorption rate to have a remarkable influence on the electrical resistance of micron-length nanotubes. Further analysis reveals that unlike in typical metals, electrons are responsible for less than 15% of the total thermal conductivity of metallic nanotubes around room temperature, and this contribution decreases at high bias or higher temperatures. For interconnect applications of metallic SWCNTs, significant self-heating may be avoided if power densities are limited below 5μW∕μm, or if the SWCNT-surrounding thermal interface is optimized.
Effects of finite poloidal gyroradius, shaping, and collisions on the zonal flow residual
Authors: Xiao Yong , Catto Peter J. , Dorland William
Published: May 2007

Zonal flow helps reduce and regulate the turbulent transport level in tokamaks. Rosenbluth and Hinton have shown that zonal flow damps to a nonvanishing residual level in collisionless [M. Rosenbluth and F. Hinton, Phys. Rev. Lett. 80, 724 (1998)] and collisional [F. Hinton and M. Rosenbluth, Plasma Phys. Control. Fusion 41, A653 (1999)] banana regime plasmas. Recent zonal flow advances are summarized including the evaluation of the effects on the zonal flow residual by plasma cross-section shaping, shorter wavelengths including those less than an electron gyroradius, and arbitrary ion collisionality relative to the zonal low frequency. In addition to giving a brief summary of these new developments, the analytic results are compared with GS2 numerical simulations [M. Kotschenreuther, G. Rewoldt, and W. Tang, Comput. Phys. Commun. 88, 128 (1991)] to demonstrate their value as benchmarks for turbulence codes.
Equation of state of aluminum-iron oxide-epoxy composite
Authors: Jordan Jennifer L. , Ferranti Louis , Austin Ryan A. , Dick Richard D. , Foley Jason R. , Thadhani Naresh N. , McDowell David L. , Benson David J.
Published: May 2007

We report on the measurements of the shock equation of state (Hugoniot) of an Al∕Fe2O3/epoxy composite, prepared by epoxy cast curing of powder mixtures. Explosive loading, with Baratol, trinitrotoluene (TNT), and Octol, was used for performing experiments at higher pressures, in which case shock velocities were measured in the samples and aluminum, copper, or polymethyl methacrylate (PMMA) donor material, using piezoelectric pins. The explosive loading of the metal donors (aluminum and copper) will be discussed. Gas gun experiments provide complementary lower pressure data in which piezoelectric polyvinylidene fluoride (PVDF) stress gauges were used to measure the input and propagated stress wave profiles in the sample and the corresponding shock propagation velocity. The results of the Hugoniot equation of state are compared with mesoscale finite-element simulations, which show good agreement.
Highly resolved measurements of defect evolution under heated-and-shocked conditions
Authors: Lanier N. E. , Workman J. , Holmes R. L. , Graham P. , Moore A.
Published: May 2007

One of the principal advantages of a double-shell capsule design is the potential for ignition without requiring cryogenic implosions. These designs compress deuterium fuel by transferring kinetic energy from a laser-ablated outer shell to an inner shell by means of a nearly elastic symmetric collision. However, prior to this collision the inner shell experiences varying levels of preheat such that any nonuniformities can evolve significantly. It is the condition of these perturbations at the time the collision-induced shock compresses the inner shell that ultimately dictates capsule performance. With this in mind, a series of experiments have been performed on the OMEGA laser facility [R. T. Boehly et al., Opt. Comm. 133, 495 (1997)] that produce highly resolved measurements of defect evolution under heated-and-shocked conditions. Tin L-shell radiation is used to heat a layered package of epoxy and foam. The epoxy can be engineered with a variety of surface perturbations or defects. As the system evolves, a strong shock can be introduced with the subsequent hydrodynamic behavior imaged on calibrated film via x-ray radiography. This technique allows density variations of the evolving system to be quantitatively measured. This paper summarizes the hydrodynamic behavior of rectangular gaps under heated conditions with detailed experimental measurements of their residual density perturbations. Moreover, the impact of these residual density perturbations on shock deformation and material flow is discussed.
Matrix-valued Berezin-Toeplitz quantization
Authors: Ali S. Twareque , Engliš M.
Published: May 2007

We generalize some earlier results on a Berezin-Toeplitz type of quantization on Hilbert spaces built over certain matrix domains. In the present, wider setting, the theory could be applied to systems possessing several kinematic and internal degrees of freedom. Our analysis leads to an identification of those observables, in this general context, which admit a semi-classical limit and those for which no such limit exists. It turns out that the latter class of observables involves the internal degrees of freedom in an intrinsic way. Mathematically, the theory, being a generalization of the standard Berezin-Toeplitz quantization, points the way to applying such a quantization technique to possibly noncommutative spaces, to the extent that points in phase space are now replaced by N×N matrices.
Magnetically dominated jets inside collapsing stars as a model for gamma-ray bursts and supernova explosions
Authors: Uzdensky Dmitri A. , MacFadyen Andrew I.
Published: May 2007

It has been suggested that magnetic fields play a dynamically important role in core-collapse explosions of massive stars. In particular, they may be important in the collapsar scenario for gamma-ray bursts (GRB), where the central engine is a hyperaccreting black hole or a millisecond magnetar. The present paper is focused on the magnetar scenario, with a specific emphasis on the interaction of the magnetar magnetosphere with the infalling stellar envelope. First, the “pulsar-in-a-cavity” problem is introduced as a paradigm for a magnetar inside a collapsing star. The basic setup of this fundamental plasma-physics problem is described, outlining its main features, and simple estimates are derived for the evolution of the magnetic field. In the context of a collapsing star, it is proposed that, at first, the ram pressure of the infalling plasma acts to confine the magnetosphere, enabling a gradual buildup of the magnetic pressure. At some point, the growing magnetic pressure overtakes the (decreasing) ram pressure of the gas, resulting in a magnetically driven explosion. The explosion should be highly anisotropic, as the hoop stress of the toroidal field, confined by the surrounding stellar matter, collimates the magnetically dominated outflow into two beamed magnetic-tower jets. This creates a clean narrow channel for the escape of energy from the central engine through the star, as required for GRBs. In addition, the delayed onset of the collimated-explosion phase can explain the production of large quantities of nickel-56, as suggested by the GRB-supernova connection. Finally, the prospects for numerical simulations of this scenario are discussed.
Very-high-growth-factor planar ablative Rayleigh-Taylor experiments
Authors: Bradley D. K. , Braun D. G. , Glendinning S. G. , Edwards M. J. , Milovich J. L. , Sorce C. M. , Collins G. W. , Haan S. W. , Page R. H. , Wallace R. J. , Kaae J. L.
Published: May 2007

The Rayleigh-Taylor (RT) instability is an important factor in bounding the performance envelope of inertial confinement fusion targets. This paper describes an experiment for ablative RT instability that for the first time achieves growth factors close to those expected to occur in indirect-drive ignition targets at the National Ignition Facility (NIF) [J. A. Paisner, J. D. Boyes, S. A. Kumpan et al., Laser Focus World 30, 75 (1994)]. The large growth allows small-seed perturbations to be detected and can be used to place an upper bound on perturbation growth at the ablation front resulting from microstructure or surface roughness in the preferred Be ablator. The experiments were performed on the Omega laser [T. R. Boehly, D. L. Brown, R. S. Craxton et al., Optics Communications 133, 495 (1997)] using a two stepped x-ray pulse consisting of an early time section to emulate the NIF foot followed by a higher-radiation-temperature drive sustained over an additional 5–7ns. The trajectory of the ablator was measured using streaked backlit radiography, and the growth of a sinusoidal perturbation machined on the drive side of the ablator was measured using face-on radiography. The diagnostic view remained open until ∼11ns with maximum growth factors measured to be ∼200.
Modeling of dust-particle behavior for different materials in plasmas
Authors: Tanaka Y. , Pigarov A. Yu. , Smirnov R. D. , Krasheninnikov S. I. , Ohno N. , Uesugi Y.
Published: May 2007

The behavior of dust particles made of different fusion-related materials (Li, Be, B, C, Fe, Mo, or W) in tokamak plasmas is simulated using the dust transport code DUSTT [A. Pigarov et al., Phys. Plasmas 12, 122508 (2005)]. The dependencies of the characteristic lifetime of dust particles on plasma parameters are compared for the different dust materials. The dynamics of dust particles in the tokamak edge plasma is studied and the effects of dust material on the acceleration, heating, and evaporation/sublimation of particles are analyzed.
Gain and efficiency enhancement of a Cerenkov free electron laser by a prebunched electron beam in a dielectric loaded waveguide
Authors: Sharma Suresh C. , Bhasin Anuradha
Published: May 2007

A density prebunched electron beam on a Cerenkov free electron laser (CFEL) offers considerable enhancement in gain and efficiency in a dielectric loaded waveguide. The growth rate, efficiency, and gain were evaluated based on experimentally known parameters relevant to the CFEL. It was found that the growth rate, efficiency, and gain of the CFEL increase with the modulation index and has the largest value when the modulation index approaches unity in addition to the frequency and wave number of the prebunched beam are comparable to that of the radiation wave, i.e., when the prebunched beam velocity is comparable to the phase velocity of the radiation wave. The growth rate of the CFEL instability scales as one third power of the beam current. Moreover, the phase velocity of the radiation wave scales as minus one half power of the effective permittivity.
Analyses on the dispersion spectra of permeability and permittivity for NiZn spinel ferrites doped with SiO2
Authors: Han Mangui , Liang Difei , Deng Longjiang
Published: May 2007

The permeability spectra of NiZn spinel ferrites with different amounts of SiO2 (0,0.3,0.5wt%) aggregated at the grain boundaries have been resolved into two contribution components: domain wall resonance and spin rotation relaxation. The fitting results of permeability dispersion show that the spin rotation relaxation mechanism makes major contributions to the dispersion. With increasing the contents of SiO2, the fitting results of permittivity dispersion spectra show a wider distribution of relaxation time and a longer relaxation time.
Eigenvalue problems for Beltrami fields arising in a three-dimensional toroidal magnetohydrodynamic equilibrium problem
Authors: Hudson S. R. , Hole M. J. , Dewar R. L.
Published: May 2007

A generalized energy principle for finite-pressure, toroidal magnetohydrodynamic (MHD) equilibria in general three-dimensional configurations is proposed. The full set of ideal-MHD constraints is applied only on a discrete set of toroidal magnetic surfaces (invariant tori), which act as barriers against leakage of magnetic flux, helicity, and pressure through chaotic field-line transport. It is argued that a necessary condition for such invariant tori to exist is that they have fixed, irrational rotational transforms. In the toroidal domains bounded by these surfaces, full Taylor relaxation is assumed, thus leading to Beltrami fields ∇×B=λB, where λ is constant within each domain. Two distinct eigenvalue problems for λ arise in this formulation, depending on whether fluxes and helicity are fixed, or boundary rotational transforms. These are studied in cylindrical geometry and in a three-dimensional toroidal region of annular cross section. In the latter case, an application of a residue criterion is used to determine the threshold for connected chaos.
Collisionless electrostatic shock generation and ion acceleration by ultraintense laser pulses in overdense plasmas
Authors: Chen Min , Sheng Zheng-Ming , Dong Quan-Li , He Min-Qing , Li Yu-Tong , Bari Muhammad Abbas , Zhang Jie
Published: May 2007

Collisionless electrostatic shock (CES) generation and subsequent ion acceleration in laser plasma interaction are studied numerically by particle-in-cell simulations. Usually a CES is composed of a high ion density spike surrounded by a bipolar electric field. Ions in front of it can be either submerged or reflected by the shock front. The submerged ions experience few oscillations before becoming part of the shock itself, while the reflected ions are accelerated to twice the shock speed. The effects of the target thickness, density, ion mass, preplasma conditions, as well as the laser intensity on the shock generation are examined. Simulations show that such shocks can be formed in a wide range of laser and target conditions. The characteristic of the shock propagation through a plane interface between two targets with different properties is also investigated. These results are useful for future experimental studies of shock generation and acceleration.
Photoelectron studies of neutral Ag3 in helium droplets
Authors: Przystawik Andreas , Radcliffe Paul , Diederich Thomas , Döppner Tilo , Tiggesbäumker Josef , Meiwes-Broer Karl-Heinz
Published: May 2007

Photoelectron spectra of neutral silver trimers, grown in ultracold helium nanodroplets, are recorded after ionization with laser pulses via a strong optical resonance of this species. Varying the photon energy reveals that direct vertical two-photon ionization is hindered by a rapid relaxation into the lower edge of a long-living excited state manifold. An analysis of the ionization threshold of the embedded trimer yields an ionization potential of 5.74±0.09eV consistent with the value found in the gas phase. The asymmetrical form of the electron energy spectrum, which is broadened toward lower kinetic energies, is attributed to the influence of the matrix on the photoionization process. The lifetime of the excited state was measured in a two-color pump-probe experiment to be 5.7±0.6ns.
Large-eddy simulation of the flow in a lid-driven cubical cavity
Authors: Bouffanais Roland , Deville Michel O. , Leriche Emmanuel
Published: May 2007

Large-eddy simulations of the turbulent flow in a lid-driven cubical cavity have been carried out at a Reynolds number of 12000 using spectral element methods. Two distinct subgrid-scales models, namely a dynamic Smagorinsky model and a dynamic mixed model, have been both implemented and used to perform long-lasting simulations required by the relevant time scales of the flow. All filtering levels make use of explicit filters applied in the physical space (on an element-by-element approach) and spectral (modal) spaces. The two subgrid-scales models are validated and compared to available experimental and numerical reference results, showing very good agreement. Specific features of lid-driven cavity flow in the turbulent regime, such as inhomogeneity of turbulence, turbulence production near the downstream corner eddy, small-scales localization and helical properties are investigated and discussed in the large-eddy simulation framework. Time histories of quantities such as the total energy, total turbulent kinetic energy or helicity exhibit different evolutions but only after a relatively long transient period. However, the average values remain extremely close.
Formation of HO2 radicals from the photodissociation of H2O2 at 248nm
Authors: Thiebaud Jérôme , Aluculesei Alina , Fittschen Christa
Published: May 2007

The quantum yield of HO2 formation after 248nm photodissociation of H2O2 has been directly observed by laser photolysis/cw-cavity ring-down spectroscopy. HO2 profiles have been measured in the absence and presence of O2. A quantum yield of <0.01 has been found for direct HO2 formation and <0.02 for the formation of O(D1).
Effective charge of a small absorbing body in highly collisional plasma subject to an external electric field
Authors: Chaudhuri M. , Khrapak S. A. , Morfill G. E.
Published: May 2007

The total force which is the resultant of the electric, ion, and electron drag forces has been calculated for a small absorbing spherical grain immersed in a highly collisional, weakly ionized plasma subject to a weak external electric field. Linear dielectric response formalism has been used and both ion and electron absorption on the grain have been taken into account. It is shown that the total force is always directed along the direction of the electric force. The “effective” charge of the grain which can be defined as the ratio of the total force to the strength of the electric field is calculated. It is shown that its magnitude is comparable to the magnitude of the actual grain’s charge.
Transport and Helfand moments in the Lennard-Jones fluid. I. Shear viscosity
Authors: Viscardy S. , Servantie J. , Gaspard P.
Published: May 2007

The authors propose a new method, the Helfand-moment method, to compute the shear viscosity by equilibrium molecular dynamics in periodic systems. In this method, the shear viscosity is written as an Einstein-type relation in terms of the variance of the so-called Helfand moment. This quantity is modified in order to satisfy systems with periodic boundary conditions usually considered in molecular dynamics. They calculate the shear viscosity in the Lennard-Jones fluid near the triple point thanks to this new technique. They show that the results of the Helfand-moment method are in excellent agreement with the results of the standard Green-Kubo method.
Phase-space approach to dynamical density functional theory
Authors: Marconi Umberto Marini Bettolo , Melchionna Simone
Published: May 2007

The authors consider a system of interacting particles subjected to Langevin inertial dynamics and derive the governing time-dependent equation for the one-body density. They show that, after suitable truncations of the Bogoliubov-Born-Green-Kirkwood-Yvon hierarchy, and a multiple time scale analysis, they obtain a self-consistent equation involving only the one-body density. This study extends to arbitrary dimensions previous work on a one-dimensional fluid and highlights the subtleties of kinetic theory in the derivation of dynamical density functional theory.
Circuit for precision simulation of a capacitive Josephson junction
Authors: Blackburn James A.
Published: May 2007

A circuit is described which functions as a precision electronic analog of a resistively shunted, capacitive Josephson junction. This design offers significant improvements over earlier simulators, particularly because no analog switches are required, and also because high performance op-amps have been matched to the demands of the circuit. The junction analog is used to generate I-V curves, and to model the dynamics when an abrupt bias pulse is applied. The simulator is shown to be very accurate when tested against numerical solutions for the same systems.
Stress anisotropy of the R-line luminescence lifetime in single crystal Cr-doped sapphire (ruby)
Authors: Margueron Samuel H. , Clarke David R.
Published: May 2007

The effect of uniaxial stress applied along the principal crystallographic directions (a, m, and c axes) of sapphire on the lifetime of the R-line luminescence doublet is described. All the measurements were performed at room temperature on samples cut from the same sapphire single crystal containing 250ppm Cr3+ and the luminescence excited with a laser at 532nm. The luminescence decays were all single exponentials with lifetimes that increased linearly with compression and decreased linearly in tension. The luminescence lifetimes in compression were crystallographically dependent with the variation in the basal plane being an order of magnitude greater than that along the c axis. The measured lifetimes were independent of the polarization of the luminescence emission. The stress anisotropy of the lifetimes is shown to be dominated by the σ emission and is consistent with estimates based on the anisotropy of coupling of T24 with the T1u distortion. The sum of the lifetime variation measured along the three principal crystallographic directions is equal to that reported for hydrostatic stresses. This is in agreement with calculations of the orientation dependence of the stress variation in dipole oscillator strength based on crystal field analysis.
Concentration dependent microstructure and transport properties of the magnetic semiconductor Gd-Si
Authors: Helgren E. , Queen D. , Hellman F. , Zeng L. , Islam R. , Smith David J.
Published: May 2007

The transport properties and microstructure of amorphous GdxSi1−x alloys are presented. The conductivity increases from x=0 through the metal-insulator transition (x=14 at.%), up to a dopant concentration of 25 at.%. A sharp cusp in the magnitude of the conductivity is then observed and the flattening of the conductivity versus temperature curve occurs at higher concentrations. These transport results are explained in terms of high-resolution electron micrographs which demonstrate the formation of nano-crystallites at x≥25 at.%. The flattening of the conductivity versus the temperature curve is identical to the results for annealing of a-GdxSi1−x alloys with a low Gd concentration.
Deterministic control of plasma-assembled self-organized Ge∕Si quantum dot arrays
Authors: Ho J. C. , Levchenko I , Ostrikov K.
Published: May 2007

Self-assembly of size-uniform and spatially ordered quantum dot (QD) arrays is one of the major challenges in the development of the new generation of semiconducting nanoelectronic and photonic devices. Assembly of Ge QD (in the ∼5−20 nm size range) arrays from randomly generated position and size-nonuniform nanodot patterns on plasma-exposed Si(100) surfaces is studied using hybrid multiscale numerical simulations. It is shown, by properly manipulating the incoming ion/neutral flux from the plasma and the surface temperature, the uniformity of the nanodot size within the array can be improved by 34%−53%, with the best improvement achieved at low surface temperatures and high external incoming fluxes, which are intrinsic to plasma-aided processes. Using a plasma-based process also leads to an improvement (∼22% at 700 K surface temperature and 0.1 ML∕s incoming flux from the plasma) of the spatial order of a randomly sampled nanodot ensemble, which self-organizes to position the dots equidistantly to their neighbors within the array. Remarkable improvements in QD ordering and size uniformity can be achieved at high growth rates (a few nm∕s) and a surface temperature as low as 600 K, which broadens the range of suitable substrates to temperature-sensitive ultrathin nanofilms and polymers. The results of this study are generic, can also be applied to nonplasma-based techniques, and as such contributes to the development of deterministic strategies of nanoassembly of self-ordered arrays of size-uniform QDs, in the size range where nanodot ordering cannot be achieved by presently available pattern delineation techniques.
GaN hybrid microcavities in the strong coupling regime grown by metal-organic chemical vapor deposition on sapphire substrates
Authors: Alyamani A. , Sanvitto D. , Khalifa A. A. , Skolnick M. S. , Wang T. , Ranalli F. , Parbrook P. J. , Tahraoui A. , Airey R.
Published: May 2007

We observe exciton-photon strong coupling at low and room temperature in the ultraviolet spectral region in a GaN-based one wavelength bulk microcavity. The hybrid cavity is composed of 25 pairs of epitaxially grown Al0.2Ga0.8N∕Al0.6Ga0.4N distributed Bragg reflectors (DBRs) on the lower side of the cavity and 9 pairs of Si3N4∕SiO2 as the upper mirror, to obtain cavity Q values up to 160. Anticrossing is observed between the cavity mode and the bulk GaN exciton, showing the formation of polariton modes with normal mode splitting of 43 meV. The lower polariton dispersion is observed in both reflectivity and photoluminescence, with good agreement between the two obtained over a large tuning range of 40 meV. Good fits are obtained to the spectra of the Bragg mirrors. From simulations it is found that the reflectivity spectra of the Bragg mirrors are not significantly limited by absorption in the Al0.2Ga0.8N layers.
Reversal time of the magnetization of single-domain ferromagnetic particles with cubic anisotropy in the presence of a uniform magnetic field
Authors: Kalmykov Yuri P.
Published: May 2007

The reversal time of the magnetization of a single-domain ferromagnetic particle with a cubic anisotropy in the presence of a uniform magnetic field is evaluated using the Kramers escape rate theory for intermediate-to-high damping as extended by Brown [W. F. Brown, Jr., IEEE Trans. Magn. 15, 1196 (1979)] to the analogous magnetic spin problem. It is shown that this formula is in complete agreement with the relaxation time calculated numerically from the solution of the stochastic Landau-Lifshitz-Gilbert equation describing the dynamics of the magnetization of the particles. It is also shown that knowledge of the reversal time τ allows one to predict accurately the spectrum of the longitudinal magnetic susceptibility in the low-frequency range.
Kepler problem with time-dependent and resonant perturbations
Authors: Vrbik Jan
Published: May 2007

Perturbations of a satellite’s orbit due to harmonic perturbing forces are computed using a new technique. Special attention is paid to resonances resulting from the satellite’s orbital period being commeasurable with the period of the perturbing force. The results are tested, numerically, using perturbations due to sectorial and tesseral harmonics of a rotating primary. This is the first article explicitely demonstrating the time-dependent version of the technique.
Molecular flexibility effects upon liquid dynamics
Authors: Heffernan Julieanne V. , Budzien Joanne , Wilson Aaron T. , Baca Robert J. , Aston Victoria J. , Avila Francisco , McCoy John D. , Adolf Douglas B.
Published: May 2007

Simulation results for the diffusive behavior of polymer chain/penetrant systems are analyzed. The attractive range and flexibility of simple chain molecules were varied in order to gauge the effect on dynamics. In all cases, the dimensionless diffusion coefficient, D*, is found to be a smooth, single-valued function of the packing fraction, η. The functions D*(η) are found to be power laws with exponents that are sensitive to both chain stiffness and particle type. For a specific system type, the D*’s for both penetrant and chain-center-of-mass extrapolate to zero at the same packing fraction, η0. This limiting packing fraction is interpreted to be the location of the glass transition, and (η0−η), the distance to the glass transition.
Resonant processes and Coulomb interactions in (C59N)2
Authors: Schulte K. , Wang L. , Moriarty P. J. , Prassides K. , Tagmatarchis N.
Published: May 2007

We have determined the on-site molecular Coulomb interaction energy U of a (C59N)2 bulk film and find values ranging from 1.10±0.10eV for the highest occupied molecular orbital to 1.35±0.10eV for the deeper lying orbitals, comparable to values found in C60. The on-site Coulomb interaction between a carbon core hole and valence electrons, Uc, is, however, substantially lower than in C60 at 1.35±0.07eV. Resonant photoemission (RESPES) results show a weakened participator decay channel, especially around the N 1s threshold, where resonance of the highest occupied molecular orbital shoulder is absent. Near-edge x-ray absorption fine structure and constant initial state measurements, taken in parallel with the RESPES data, indicate, however, that matrix element effects cannot be ruled out.
Interpolating moving least-squares methods for fitting potential energy surfaces: Computing high-density potential energy surface data from low-densityab initiodata points
Authors: Dawes Richard , Thompson Donald L. , Guo Yin , Wagner Albert F. , Minkoff Michael
Published: May 2007

A highly accurate and efficient method for molecular global potential energy surface (PES) construction and fitting is demonstrated. An interpolating-moving-least-squares (IMLS)-based method is developed using low-density ab initio Hessian values to compute high-density PES parameters suitable for accurate and efficient PES representation. The method is automated and flexible so that a PES can be optimally generated for classical trajectories, spectroscopy, or other applications. Two important bottlenecks for fitting PESs are addressed. First, high accuracy is obtained using a minimal density of ab initio points, thus overcoming the bottleneck of ab initio point generation faced in applications of modified-Shepard-based methods. Second, high efficiency is also possible (suitable when a huge number of potential energy and gradient evaluations are required during a trajectory calculation). This overcomes the bottleneck in high-order IMLS-based methods, i.e., the high cost/accuracy ratio for potential energy evaluations. The result is a set of hybrid IMLS methods in which high-order IMLS is used with low-density ab initio Hessian data to compute a dense grid of points at which the energy, Hessian, or even high-order IMLS fitting parameters are stored. A series of hybrid methods is then possible as these data can be used for neural network fitting, modified-Shepard interpolation, or approximate IMLS. Results that are indicative of the accuracy, efficiency, and scalability are presented for one-dimensional model potentials as well as for three-dimensional (HCN) and six-dimensional (HOOH) molecular PESs.
Combined vacuum ultraviolet laser and synchrotron pulsed field ionization study of CH2BrCl
Authors: Li Juan , Yang Jie , Mo Yuxiang , Lau K. C. , Qian X. M. , Song Y. , Liu Jianbo , Ng C. Y.
Published: May 2007

The pulsed field ionization-photoelectron (PFI-PE) spectrum of bromochloromethane (CH2BrCl) in the region of 85320–88200cm−1 has been measured using vacuum ultraviolet laser. The vibrational structure resolved in the PFI-PE spectrum was assigned based on ab initio quantum chemical calculations and Franck-Condon factor predictions. At energies 0–1400cm−1 above the adiabatic ionization energy (IE) of CH2BrCl, the Br–C–Cl bending vibration progression (ν1+=0–8) of CH2BrCl+ is well resolved and constitutes the major structure in the PFI-PE spectrum, whereas the spectrum at energies 1400–2600cm−1 above the IE(CH2BrCl) is found to exhibit complex vibrational features, suggesting perturbation by the low lying excited CH2BrCl+(ÃA″2) state. The assignment of the PFI-PE vibrational bands gives the IE(CH2BrCl)=85612.4±2.0cm−1 (10.6146±0.0003eV) and the bending frequencies ν1+(a1′)=209.7±2.0cm−1 for CH2BrCl+(X̃A′2). We have also examined the dissociative photoionization process, CH2BrCl+hν→CH2Cl++Br+e−, in the energy range of 11.36–11.57eV using the synchrotron based PFI-PE-photoion coincidence method, yielding the 0K threshold or appearance energy AE(CH2Cl+)=11.509±0.002eV. Combining the 0K AE(CH2Cl+) and IE(CH2BrCl) values obtained in this study, together with the known IE(CH2Cl), we have determined the 0K bond dissociation energies (D0) for CH2Cl+–Br (0.894±0.002eV) and CH2Cl–Br (2.76±0.01eV). We have also performed CCSD(T, full)/complete basis set (CBS) calculations with high-level corrections for the predictions of the IE(CH2BrCl), AE(CH2Cl+), IE(CH2Cl), D0(CH2Cl+–Br), and D0(CH2Cl–Br). The comparison between the theoretical predictions and experimental determinations indicates that the CCSD(T, full)/CBS calculations with high-level corrections are highly reliable with estimated error limits of <17meV.
The generalized added mass revised
Authors: De Wilde Juray
Published: May 2007

The reformulation of the generalized or apparent added mass presented by De Wilde [Phys. Fluids 17, 113304 (2005)] neglects the presence of a drag-type force in the gas and solid phase momentum equations. Reformulating the generalized added mass accounting for the presence of a drag-type force, an apparent drag force appears next to the apparent distribution of the filtered gas phase pressure gradient over the phases already found by De Wilde in the above-cited reference. The reformulation of the generalized added mass and the evaluation of a linear wave propagation speed test then suggest a generalized added mass type closure approach to completely describe filtered gas-solid momentum transfer, that is, including both the filtered drag force and the correlation between the solid volume fraction and the gas phase pressure gradient.
Ultraviolet photoelectron spectroscopy study of the thermochromic phase transition in urethane-substituted polydiacetylenes
Authors: Wang Xiaoyu , Whitten James E. , Sandman Daniel J.
Published: May 2007

Threshold solid-state ionization energies determined from ultraviolet photoelectron spectra are reported for the thermochromic polydiacetylenes (PDAs) from the bis-ethyl- and bis-n-propyl urethanes of 5,7-dodecadiyn-1,12-diol (ETCD and PUDO, respectively) and the nonthermochromic 1,6-bis-p-toluenesulfonate of 2,4-hexadiyne-1,6-diol (PTS) at temperatures above and below the thermochromic phase transition. PDA-PTS has an ionization energy of 5.66eV which does not change significantly as the temperature is raised above 140°C. At 25°C, PDA-ETCD and PDA-PUDO have threshold ionization energies of 5.65 and 5.51eV, respectively. The ionization energies of these PDAs increase by approximately 0.34eV as temperature is raised above 140°C and returns to the lower values as temperature is reduced to 25°C. The magnitude of the increase in ionization energy on heating to temperatures above the thermochromic transition is very close to the shift in energy of the electronic spectrum over the same temperature range. These observations suggest that the structural changes that take place in the course of the thermochromic transition are primarily associated with the valence band and are consistent with partial relief of mechanical strains.