bluuebunny/crossref_metadata_2025 huggingface dataset:
A global simulation for laser-driven MeV electrons in 50-μm-diameter fast ignition targets
Authors: Ren C. , Tzoufras M. , Tonge J. , Mori W. B. , Tsung F. S. , Fiore M. , Fonseca R. A. , Silva L. O. , Adam J.-C. , Heron A.
Published: May 2006

The results from 2.5-dimensional particle-in-cell simulations for the interaction of a picosecond-long ignition laser pulse with a plasma pellet of 50-μm diameter and 40 critical density are presented. The high-density pellet is surrounded by an underdense corona and is isolated by a vacuum region from the simulation box boundary. The laser pulse is shown to filament and create density channels on the laser-plasma interface. The density channels increase the laser absorption efficiency and help generate an energetic electron distribution with a large angular spread. The combined distribution of the forward-going energetic electrons and the induced return electrons is marginally unstable to the current filament instability. The ions play an important role in neutralizing the space charges induced by the temperature disparity between different electron groups. No global coalescing of the current filaments resulted from the instability is observed, consistent with the observed large angular spread of the energetic electrons.
Cross-machine comparison of resonant field amplification and resistive wall mode stabilization by plasma rotation
Authors: Reimerdes H. , Hender T. C. , Sabbagh S. A. , Bialek J. M. , Chu M. S. , Garofalo A. M. , Gryaznevich M. P. , Howell D. F. , Jackson G. L. , La Haye R. J. , Liu Y. Q. , Menard J. E. , Navratil G. A. , Okabayashi M. , Pinches S. D. , Sontag A. C. , Strait E. J. , Zhu W. , Bigi M. , de Baar M. , de Vries P. , Gates D. A. , Gohil P. , Groebner R. J. , Mueller D. , Raman R. , Scoville J. T. , Solomon W. M. ,   the DIII-D Team,   JET-EFDA Contributors,   the NSTX Team
Published: May 2006

Dedicated experiments in the DIII-D tokamak [J. L. Luxon, Nucl. Fusion, 42, 614 (2002)], the Joint European Torus (JET) [P. H. Rebut, R. J. Bickerton, and B. E. Keen, Nucl. Fusion 25, 1011 (1985)], and the National Spherical Torus Experiment (NSTX) [M. Ono, S. M. Kaye, Y.-K. M. Peng et al., Nucl. Fusion 40, 557 (2000)] reveal the commonalities of resistive wall mode (RWM) stabilization by sufficiently fast toroidal plasma rotation in devices of different size and aspect ratio. In each device the weakly damped n=1 RWM manifests itself by resonant field amplification (RFA) of externally applied n=1 magnetic fields, which increases with the plasma pressure. Probing DIII-D and JET plasmas with similar ideal magnetohydrodynamic (MHD) stability properties with externally applied magnetic n=1 fields, shows that the resulting RFA is independent of the machine size. In each device the drag resulting from RFA slows the toroidal plasma rotation and can lead to the onset of an unstable RWM. The critical plasma rotation required for stable operation in the plasma center decreases with increasing q95, which is explained by the inward shift of q surfaces where the critical rotation remains constant. The quantitative agreement of the critical rotation normalized to the inverse Alfvén time at the q=2 surface in similar DIII-D and JET plasmas supports the independence of the RWM stabilization mechanism of machine size and indicates the importance of the q=2 surface. At low aspect ratio the required fraction of the Alfvén velocity increases significantly. The ratio of the critical rotation in similar NSTX and DIII-D plasmas can be explained by trapped particles not contributing to the RWM stabilization, which is consistent with stabilization mechanisms that are based on ion Landau damping. Alternatively, the ratio of the required rotation to the sound wave velocity remains independent of aspect ratio.
Collective fast ion instability-induced losses in National Spherical Tokamak Experiment
Authors: Fredrickson E. D. , Bell R. E. , Darrow D. S. , Fu G. Y. , Gorelenkov N. N. , LeBlanc B. P. , Medley S. S. , Menard J. E. , Park H. , Roquemore A. L. , Heidbrink W. W. , Sabbagh S. A. , Stutman D. , Tritz K. , Crocker N. A. , Kubota S. , Peebles W. , Lee K. C. , Levinton F. M.
Published: May 2006

A wide variety of fast ion driven instabilities are excited during neutral beam injection (NBI) in the National Spherical Torus Experiment (NSTX) [Nucl. Fusion 40, 557 (2000)] due to the large ratio of fast ion velocity to Alfvén velocity, Vfast∕VAlfvén, and high fast ion beta. The ratio Vfast∕VAlfvén in ITER [Nucl. Fusion 39, 2137 (1999)] and NSTX is comparable. The modes can be divided into three categories: chirping energetic particle modes (EPM) in the frequency range 0 to 120kHz, the toroidal Alfvén eigenmodes (TAE) with a frequency range of 50kHz to 200kHz, and the compressional and global Alfvén eigenmodes (CAE and GAE, respectively) between 300kHz and the ion cyclotron frequency. Fast ion driven modes are of particular interest because of their potential to cause substantial fast ion losses. In all regimes of NBI heated operation we see transient neutron rate drops, correlated with bursts of TAE or fishbone-like EPMs. The fast ion loss events are predominantly correlated with the EPMs, although losses are also seen with bursts of multiple, large amplitude TAE. The latter is of particular significance for ITER; the transport of fast ions from the expected resonance overlap in phase space of a “sea” of large amplitude TAE is the kind of physics expected in ITER. The internal structure and amplitude of the TAE and EPMs has been measured with quadrature reflectometry and soft x-ray cameras. The TAE bursts have internal amplitudes of ñ∕n=1% and toroidal mode numbers 2<n<7. The EPMs are core localized, kink-like modes similar to the fishbones in conventional aspect ratio tokamaks. Unlike the fishbones, the EPMs can be present with q(0)>1 and can have a toroidal mode number n>1. The range of the frequency chirp can be quite large and the resonance can be through a fishbone-like precessional drift resonance, or through a bounce resonance.
Particle-in-cell Monte Carlo and fluid simulations of argon-oxygen plasma: Comparisons with experiments and validations
Authors: Lee S. H. , Iza F. , Lee J. K.
Published: May 2006

Particle-in-cell Monte Carlo collision (PIC-MCC) and fluid simulations of argon-oxygen plasmas in capacitively and inductively coupled plasma reactors are presented. Potential profiles and electron/ion kinetic information such as electron/ion energy distributions and temperatures are compared with experimental data as well as with other analytical and numerical results. One-dimensional PIC-MCC simulations compare favorably with experimental data obtained in capacitively coupled reactors over a wide range of pressure and power. Two-dimensional fluid simulations of capacitive discharges differs from the results of PIC-MCC simulations as nonlocal effects play an important role in these discharges. Fluid simulations as nonlocal inductively coupled plasmas, however, agree favorably with experimental observations.
Experimental test of the neoclassical theory of impurity poloidal rotation in tokamaks
Authors: Solomon W. M. , Burrell K. H. , Andre R. , Baylor L. R. , Budny R. , Gohil P. , Groebner R. J. , Holcomb C. T. , Houlberg W. A. , Wade M. R.
Published: May 2006

Despite the importance of rotation in fusion plasmas, our present understanding of momentum transport is inadequate. The lack of understanding is in part related to the difficulty of performing accurate rotation measurements, especially for poloidal rotation. Recently, measurements of poloidal rotation for impurity ions (Z>1) have been obtained in the core of DIII-D [J. L. Luxon, Nucl. Fusion 42, 6114 (2002)] plasmas using charge exchange recombination spectroscopy. The inferred poloidal rotation is based on careful consideration of the effective energy-dependent cross section and of the gyromotion of the ions. The rotation measurements are found to be consistent with the radial electric field determined independently from multiple impurity species as well as from motional Stark effect spectroscopic measurements. The poloidal rotation measurements have been compared with predictions based on the neoclassical theory of poloidal rotation from the code NCLASS [W. A. Houlberg et al., Phys. Plasmas 4, 3230 (1997)]. The comparison shows that the neoclassically predicted poloidal rotation is in general significantly smaller than the actual measurements.
Two fluid effects on three-dimensional reconnection in the Swarthmore Spheromak Experiment with comparisons to space data
Authors: Brown M. R. , Cothran C. D. , Fung J.
Published: May 2006

Several new experimental results are reported from spheromak merging studies at the Swarthmore Spheromak Experiment [M. R. Brown, Phys. Plasmas 6, 1717 (1999)] with relevance to three-dimensional (3D) reconnection in laboratory and space plasmas. First, recent velocity measurements of impurity ions using ion Doppler spectroscopy are reported. Bidirectional outflow at nearly the Alfvén speed is clearly observed. Second, experimental measurements of the out-of-plane magnetic field in a reconnection volume showing a quadrupolar structure at the ion inertial scale are discussed. Third, a measurement of in-plane Hall electric field and nonideal terms of the generalized Ohm’s law in a reconnection volume of a weakly collisional laboratory plasma is presented. Time resolved vector magnetic field measurements on a 3D lattice [B(r,t)] enables evaluation of the various terms. Results show that the Hall electric field dominates everywhere (J×B∕ne) and also exhibits a quadrupolar structure at the ion inertial scale; resistive and electron inertia terms are small. Each of these is related to and compared with similar measurements in a solar or space context.
Dynamics of magnetic flux tubes in space and laboratory plasmas
Authors: Linton Mark G.
Published: May 2006

This paper gives a brief summary of topics discussed at a mini-conference on the dynamics of magnetic flux tubes in space and laboratory plasmas. This mini-conference was held as part of the Fall 2005 meeting of the American Physical Society Division of Plasma Physics.
Polar-direct-drive simulations and experiments
Authors: Marozas J. A. , Marshall F. J. , Craxton R. S. , Igumenshchev I. V. , Skupsky S. , Bonino M. J. , Collins T. J. B. , Epstein R. , Glebov V. Yu. , Jacobs-Perkins D. , Knauer J. P. , McCrory R. L. , McKenty P. W. , Meyerhofer D. D. , Noyes S. G. , Radha P. B. , Sangster T. C. , Seka W. , Smalyuk V. A.
Published: May 2006

Polar direct drive (PDD) [S. Skupsky et al., Phys. Plasmas 11, 2763 (2004)] will allow direct-drive ignition experiments on the National Ignition Facility (NIF) [J. Paisner et al., Laser Focus World 30, 75 (1994)] as it is configured for x-ray drive. Optimal drive uniformity is obtained via a combination of beam repointing, pulse shapes, spot shapes, and∕or target design. This article describes progress in the development of standard and “Saturn” [R. S. Craxton and D. W. Jacobs-Perkins, Phys. Rev. Lett. 94, 0952002 (2005)] PDD target designs. Initial evaluation of experiments on the OMEGA Laser System [T. R. Boehly et al., Rev. Sci. Instrum. 66, 508 (1995)] and simulations were carried out with the two-dimensional hydrodynamics code SAGE [R. S. Craxton et al., Phys. Plasmas 12, 056304 (2005)]. This article adds to this body of work by including fusion particle production and transport as well as radiation transport within the two-dimensional DRACO [P. B. Radha et al., Phys. Plasmas 12, 032702 (2005)] hydrodynamics simulations used to model experiments. Forty OMEGA beams arranged in six rings to emulate the NIF x-ray-drive configuration are used to perform direct-drive implosions of CH shells filled with D2 gas. Target performance was diagnosed with framed x-ray backlighting and by the measured fusion yield. Saturn target experiments have resulted in ∼75% of the yield from energy-equivalent, symmetrically irradiated implosions. The results of the two-dimensional PDD simulations performed with DRACO are in good agreement with experimental x-ray radiographs. DRACO is being used to further optimize standard PDD designs. In addition, DRACO simulations of NIF-scale PDD designs show ignition with a gain of 20 and the development of a 40μm radius, 10keV region with a neutron-averaged ρr of 1270mg∕cm2 near stagnation.
Production and study of high-beta plasma confined by a superconducting dipole magnet
Authors: Garnier D. T. , Hansen A. , Mauel M. E. , Ortiz E. , Boxer A. C. , Ellsworth J. , Karim I. , Kesner J. , Mahar S. , Roach A.
Published: May 2006

The Levitated Dipole Experiment (LDX) [J. Kesner et al., in Fusion Energy 1998, 1165 (1999)] is a new research facility that is exploring the confinement and stability of plasma created within the dipole field produced by a strong superconducting magnet. Unlike other configurations in which stability depends on curvature and magnetic shear, magnetohydrodynamic stability of a dipole derives from plasma compressibility. Theoretically, the dipole magnetic geometry can stabilize a centrally peaked plasma pressure that exceeds the local magnetic pressure (β>1), and the absence of magnetic shear allows particle and energy confinement to decouple. In initial experiments, long-pulse, quasi-steady-state microwave discharges lasting more than 10s have been produced that are consistent with equilibria having peak beta values of 20%. Detailed measurements have been made of discharge evolution, plasma dynamics and instability, and the roles of gas fueling, microwave power deposition profiles, and plasma boundary shape. In these initial experiments, the high-field superconducting floating coil was supported by three thin supports. The plasma is created by multifrequency electron cyclotron resonance heating at 2.45 and 6.4GHz, and a population of energetic electrons, with mean energies above 50keV, dominates the plasma pressure. Creation of high-pressure, high-beta plasma is possible only when intense hot electron interchange instabilities are stabilized by sufficiently high background plasma density. A dramatic transition from a low-density, low-beta regime to a more quiescent, high-beta regime is observed when the plasma fueling rate and confinement time become sufficiently large.
Statisticodynamical approach of final state distributions in associative desorptions
Authors: Perrier A. , Bonnet L. , Rayez J.-C.
Published: May 2006

In associative desorptions governed by short-range forces, strong energy transfers between rotational, translational, and vibrational degrees of freedom of the nascent molecule take place beyond the transition state (TS). State or energy distributions are thus different at the TS and in the vacuum. Inspired by previous studies on unimolecular reactions, a statisticodynamical approach (SDA) is proposed which combines the transition state theory for the description of state distributions at the TS and a simple dynamical model to account for the effect of the post-TS dynamics on these distributions. The agreement found between the predictions of SDA, the much more demanding classical trajectory calculations, and experimental results in the case of H2 and D2 desorbing from Cu(111) is very satisfying. In addition to that, the formulation highlights some key factors of state distributions which would have been difficult to directly extract from a dynamical study. The notions of vibrational heating and rotational cooling are discussed within this context.
Cooperation of spike timing-dependent and heterosynaptic plasticities in neural networks: A Fokker-Planck approach
Authors: Zhu Liqiang , Lai Ying-Cheng , Hoppensteadt Frank C. , He Jiping
Published: May 2006

It is believed that both Hebbian and homeostatic mechanisms are essential in neural learning. While Hebbian plasticity selectively modifies synaptic connectivity according to activity experienced, homeostatic plasticity constrains this change so that neural activity is always within reasonable physiological limits. Recent experiments reveal spike timing-dependent plasticity (STDP) as a new type of Hebbian learning with high time precision and heterosynaptic plasticity (HSP) as a new homeostatic mechanism acting directly on synapses. Here, we study the effect of STDP and HSP on randomly connected neural networks. Despite the reported successes of STDP to account for neural activities at the single-cell level, we find that, surprisingly, at the network level, networks trained using STDP alone cannot seem to generate realistic neural activities. For instance, STDP would stipulate that past sensory experience be maintained forever if it is no longer activated. To overcome this difficulty, motivated by the fact that HSP can induce strong competition between sensory experiences, we propose a biophysically plausible learning rule by combining STDP and HSP. Based on the Fokker-Planck theory and extensive numerical computations, we demonstrate that HSP and STDP operated on different time scales can complement each other, resulting in more realistic network activities. Our finding may provide fresh insight into the learning mechanism of the brain.
Theoretical investigation of intramolecular vibrational energy redistribution in highly excited HFCO
Authors: Pasin Gauthier , Gatti Fabien , Iung Christophe , Meyer Hans-Dieter
Published: May 2006

The present paper is devoted to the simulations of the intramolecular vibrational energy redistribution (IVR) in HFCO initiated by an excitation of the out-of-plane bending vibration [nν6=2,4,6,…,18,20]. Using a full six-dimensional ab initio potential energy, the multiconfiguration time-dependent Hartree (MCTDH) method was exploited to propagate the corresponding six-dimensional wave packets. This study emphasizes the stability of highly excited states of the out-of-plane bending mode which exist even above the dissociation threshold. More strikingly, the structure of the IVR during the first step of the dynamics is very stable for initial excitations ranging from 2ν6 to 20ν6. This latter result is consistent with the analysis of the eigenstates obtained, up to 10ν6, with the aid of the Davidson algorithm in a foregoing paper [Iung and Ribeiro, J. Chem. Phys. 121, 174105 (2005)]. The present study can be considered as complementary to this previous investigation. This paper also shows how MCTDH can be used to predict the dynamical behavior of a strongly excited system and to determine the energies of the corresponding highly excited states.
Chaotic synchronization through coupling strategies
Authors: Guan Shuguang , Li Kun , Lai C.-H.
Published: May 2006

Usually, complete synchronization (CS) is regarded as the form of synchronization proper of identical chaotic systems, while generalized synchronization (GS) extends CS in nonidentical systems. However, this generally accepted view ignores the role that the coupling plays in determining the type of synchronization. In this work, we show that by choosing appropriate coupling strategies, CS can be observed in coupled chaotic systems with parameter mismatch, and GS can also be achieved in coupled identical systems. Numerical examples are provided to demonstrate these findings. Moreover, experimental verification based on electronic circuits has been carried out to support the numerical results. Our work provides a method to obtain robust CS in synchronization-based chaos communications.
An atomic force microscope study of thermal behavior of phospholipid monolayers on mica
Authors: Luo M. F. , Yeh Y. L. , Chen P. L. , Nien C.-H. , Hsueh Y. W.
Published: May 2006

We observed by using atomic force microscope (AFM) phospholipid (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) monolayers on mica being annealed and cooled to a selection of temperatures through steps of 2–4°C∕min. The annealed phospholipid monolayers started to disappear at 45–50°C and disappeared completely above 60–63°C under AFM observation. The phospholipid monolayers reformed when the samples were cooled below 60°C and developed from fractal into compact monolayer films with decreasing temperatures. Simultaneously the height of the reformed phospholipid films also increased with decreasing temperatures from 0.4nm to the value before annealing. The observed thermal features are attributed to a phase-transition process that upon heating to above 45–50°C, the lipids condensed in the monolayers transform into a low-density expanded phase in which the lipids are invisible to AFM, and the transformation continues and completes at 60–63°C. The lipid densities of the expanded phase inferred from the dissociated area of the condensed phase are observed to be a function of the temperature. The behavior contrasts with a conventional first-order phase transition commonly seen in the Langmuir films. The temperature-dependent height and shape of the reformed phospholipid films during cooling are argued to arise from the adjustment of the packing and molecular tilting (with respect to the mica surface) of the phospholipids in order to accommodate more condensed phospholipids.
Using laser entrance hole shields to increase coupling efficiency in indirect drive ignition targets for the National Ignition Facility
Authors: Callahan D. A. , Amendt P. A. , Dewald E. L. , Haan S. W. , Hinkel D. E. , Izurni N. , Jones O. S. , Landen O. L. , Lindl J. D. , Pollaine S. M. , Suter L. J. , Tabak M. , Turner R. E.
Published: May 2006

Coupling efficiency, the ratio of the capsule absorbed energy to the driver energy, is a key parameter in ignition target designs. The hohlraum originally proposed for the National Ignition Facility (NIF) [G. H. Miller, E. I. Moses, and C. R. Wuest, Nucl. Fusion 44, S228 (2004)] coupled ∼11% of the absorbed laser energy to the capsule as x rays. Described here is a second generation of the hohlraum target which has a higher coupling efficiency, ∼16%. Because the ignition capsule’s ability to withstand three-dimensional effects increases rapidly with absorbed energy, the additional energy can significantly increase the likelihood of ignition. The new target includes laser entrance hole (LEH) shields as a principal method for increasing coupling efficiency while controlling symmetry in indirect-drive inertial confinement fusion. The LEH shields are high Z disks placed inside the hohlraum on the symmetry axis to block the capsule’s view of the relatively cold LEHs. The LEH shields can reduce the amount of laser energy required to drive a target to a given temperature via two mechanisms: (1) keeping the temperature high near the capsule pole by putting a barrier between the capsule and the pole; (2) because the capsule pole does not have a view of the cold LEHs, good symmetry requires a shorter hohlraum with less wall area. Current integrated simulations of this class of target couple 140kJ of x rays to a capsule out of 865kJ of absorbed laser energy and produce ∼10MJ of yield. In the current designs, which continue to be optimized, the addition of the LEH shields saves ∼95kJ of energy (about 10%) over hohlraums without LEH shields.
Canonical transformation theory for multireference problems
Authors: Yanai Takeshi , Chan Garnet Kin-Lic
Published: May 2006

We propose a theory to describe dynamic correlations in bonding situations where there is also significant nondynamic character. We call this the canonical transformation (CT) theory. When combined with a suitable description of nondynamic correlation, such as given by a complete-active-space self-consistent Field (CASSCF) or density matrix renormalization group wave function, it provides a theory to describe bonding situations across the entire potential energy surface with quantitative accuracy for both dynamic and nondynamic correlation. The canonical transformation theory uses a unitary exponential ansatz, is size consistent, and has a computational cost of the same order as a single-reference coupled cluster theory with the same level of excitations. Calculations using the CASSCF based CT method with single and double operators for the potential energy curves for water and nitrogen molecules, the BeH2 insertion reaction, and hydrogen fluoride and boron hydride bond breaking, consistently yield quantitative accuracies typical of equilibrium region coupled cluster theory, but across all geometries, and better than obtained with multireference perturbation theory.
Classical and generalized Gibbs’ approaches and the work of critical cluster formation in nucleation theory
Authors: Schmelzer Jürn W. P. , Boltachev Grey Sh. , Baidakov Vladimir G.
Published: May 2006

In the theoretical interpretation of the kinetics of first-order phase transitions, thermodynamic concepts developed long ago by Gibbs are widely employed giving some basic qualitative insights into these processes. However, from a quantitative point of view, the results of such analysis, based on the classical Gibbs approach and involving in addition the capillarity approximation, are often not satisfactory. Some progress can be reached here by the van der Waals and more advanced density functional methods of description of thermodynamically heterogeneous systems having, however, its limitations in application to the interpretation of experimental data as well. Moreover, both mentioned theories—Gibbs’ and density functional approaches—lead to partly contradicting each other’s results. As shown in preceding papers, by generalizing Gibbs’ approach, existing deficiencies and internal contradictions of these two well-established theories can be removed and a new generally applicable tool for the interpretation of phase formation processes can be developed. In the present analysis, a comparative analysis of the basic assumptions and predictions of the classical and the generalized Gibbs approaches is given. It is shown, in particular, that—interpreted in terms of the generalized Gibbs approach—the critical cluster as determined via the classical Gibbs approach corresponds not to a saddle but to a ridge point of the appropriate thermodynamic potential hypersurface. By this reason, the classical Gibbs approach (involving the classical capillarity approximation) overestimates as a rule the work of critical cluster formation in nucleation theory and, in general, considerably.
On the structure of bilayer condensed phases confined between crystalline walls of triangular symmetry
Authors: Patrykiejew A. , Sokołowski S.
Published: May 2006

Grand canonical ensemble Monte Carlo simulation method is used to study the structure of Lennard-Jones fluids confined between the parallel walls, formed by the (111) planes of the face centered cubic crystal. Thin slit pores with a width allowing for the formation of only two atomic layers are considered. It is shown that the structure of confined solidlike phases is very sensitive to the pore width, the misfit between the size of adsorbate atoms and the size of surface lattice, as well as the corrugation of the surface potential. In particular, when the misfit between the adsorbate atoms and the surface lattice is very small, even a weakly corrugated surface potential highly stabilizes solidlike phases of triangular symmetry. On the other hand, in the case of large misfit the packing effects become a dominating factor and lead to the appearance of solidlike phases of different structures and symmetries.
Size and charge effects on the binding of CO to late transition metal clusters
Authors: Fielicke André , von Helden Gert , Meijer Gerard , Pedersen David B. , Simard Benoit , Rayner David M.
Published: May 2006

We report on the size and charge dependence of the C–O stretching frequency, ν(CO), in complexes of CO with gas phase anionic, neutral, and cationic cobalt clusters (ConCO−∕0∕+), anionic, neutral, and cationic rhodium clusters (RhnCO−∕0∕+), and cationic nickel clusters (NinCO+) for n up to 37. We develop models, based on the established vibrational spectroscopy of organometallic carbonyl compounds, to understand how cluster size and charge relate to ν(CO) in these complexes. The dominating factor is the available electron density for backdonation from the metal to the CO π* orbital. Electrostatic effects play a significant but minor role. For the charged clusters, the size trends are related to the dilution of the charge density at the binding site on the cluster as n increases. At large n, ν(CO) approaches asymptotes that are not the same as found for ν(CO) on the single crystal metal surfaces, reflecting differences between binding sites on medium sized clusters and the more highly coordinated metal surface sites.
Fractional dynamics of coupled oscillators with long-range interaction
Authors: Tarasov Vasily E. , Zaslavsky George M.
Published: May 2006

We consider a one-dimensional chain of coupled linear and nonlinear oscillators with long-range powerwise interaction. The corresponding term in dynamical equations is proportional to 1∕∣n−m∣α+1. It is shown that the equation of motion in the infrared limit can be transformed into the medium equation with the Riesz fractional derivative of order α, when 0<α<2. We consider a few models of coupled oscillators and show how their synchronization can appear as a result of bifurcation, and how the corresponding solutions depend on α. The presence of a fractional derivative also leads to the occurrence of localized structures. Particular solutions for fractional time-dependent complex Ginzburg-Landau (or nonlinear Schrödinger) equation are derived. These solutions are interpreted as synchronized states and localized structures of the oscillatory medium.
The constrained space orbital variation analysis for periodic ab initio calculations
Authors: Cruz Hernández N. , Zicovich-Wilson Claudio Marcelo , Fdez. Sanz Javier
Published: May 2006

The constrained space orbital variation (CSOV) method for the analysis of the interaction energy has been implemented in the periodic ab initio CRYSTAL03 code. The method allows for the partition of the energy of two interacting chemical entities, represented in turn by periodic models, into contributions which account for electrostatic effects, mutual polarization and charge transfer. The implementation permits one to carry out the analysis both at the Hartree-Fock and density functional theory levels, where in the latter the most popular exchange-correlation functionals can be used. As an illustrating example, the analysis of the interaction between CO and the MgO (001) surface has been considered. As expected by the almost fully ionic character of the support, our periodic CSOV results, in general agree with those previously obtained using the embedded cluster approach, showing the reliability of the present implementation.
Brightness of micronozzle helium source
Authors: DePonte D. P. , Kevan S. D. , Patton F. S.
Published: May 2006

We have measured the brightness of several helium free jet sources. Five converging nozzles with diameters between 0.6 and 5μm, and three tube nozzles with diameters between 2 and 10μm were studied at stagnation temperatures of 77 and 300K and at stagnation pressures depending on nozzle size from 350to17000kPa. Smaller nozzles produced higher brightness beams with values approaching 1028(ssrm2)−1. At low-temperature quantum effects on the helium collision cross section significantly decreased the source brightness. We explore the possibility of producing even higher brightness sources with smaller diameter.
Spatiotemporal control and synchronization of flute modes and drift waves in a magnetized plasma column
Authors: Brochard F. , Bonhomme G. , Gravier E. , Oldenbürger S. , Philipp M.
Published: May 2006

An open-loop spatiotemporal synchronization method is applied to flute modes in a cylindrical magnetized plasma. It is demonstrated that synchronization can be achieved only if the exciter signal rotates in the same direction as the propagating mode. Moreover, the efficiency of the synchronization is shown to depend on the radial properties of the instability under consideration. It is also demonstrated that the control disposition can alternatively be used to produce strongly developed turbulence of drift waves or flute instabilities.
Formation of a self-magnetic cusp in a highly bunched relativistic annular electron beam
Authors: Hess Mark , Chen Chiping
Published: May 2006

The formation of a self-magnetic cusp that develops in a highly bunched, relativistic, annular electron beam as the beam current approaches a critical current limit is demonstrated. The self-magnetic cusp is calculated within the framework of a fluid equilibrium model for a beam of periodic, azimuthally symmetric, relativistic bunched annular disks propagating in a perfectly conducting cylindrical pipe with a uniform magnetic focusing field. Magnetic cusp formation may play an important role in determining the operational current limit of high-current bunched relativistic annular beam experiments.
Remote chemical sensing and recognition by acoustic mapping of photothermal fields
Authors: Su Ming , Thundat Thomas
Published: May 2006

A remote chemical sensing and recognition approach is developed based on the acoustic mapping of photothermal fields, which has been verified in transmission and reflection modes using a system with ultrasonic transducers, a light source, and light filters. Light absorption increases sample temperature, which changes the phase angle of reflected or transmitted ultrasonic waves. The chemical natures of samples are determined from the wavelength dependent phase change spectrum. The method does not require resonance chambers to amplify photogenerated signals. The light source and ultrasonic transducers can be arranged to enable compact device construction and remote field operation in stand-off manner.
In situ synthesis of In2O3 nanowires with different diameters from indium film
Authors: Liang Y. X. , Li S. Q. , Nie L. , Wang Y. G. , Wang T. H.
Published: May 2006

Several kinds of In2O3 nanowires (NWs) with different shapes and diameters were in situ synthesized from indium films with different thicknesses at a low temperature of 400°C. The NWs had screw structures, and they grew along each axial screw direction by vapor-solid growth mechanism. The thin NWs were synthesized with low indium vapor supersaturation. Two-dimensional nucleation and radial growth trends occurred when the supersaturation increased. The field emission results showed that the turn-on and threshold fields of the thick conical In2O3 NWs were lower than those of the thin ones.
High-pressure x-ray diffraction study of Ta4AlC3
Authors: Manoun Bouchaib , Saxena S. K. , El-Raghy T. , Barsoum M. W.
Published: May 2006

Using a synchrotron radiation source and a diamond anvil cell, we measured the pressure dependence of the lattice parameters of a recently discovered phase, Ta4AlC3. This phase adopts a hexagonal structure with the space group P63∕mmc; at room conditions, the a and c parameters are 3.087(5) and 23.70(4)Å, respectively. Up to a pressure of 47GPa, no phase transformations were observed. Like Ta2AlC, but unlike many related phases such as Ti4AlN3, Ti3SiC2, Ti3GeC2, and Zr2InC, the compressibility of Ta4AlC3 along the c and a axes are almost identical. The bulk modulus of Ta4AlC3, 261±2GPa, is ≈4% greater than that of Ta2AlC. Both, however, are ≈37% lower than the 345±9GPa of TaC.
Insights into phase formation in fluorochlorozirconate glass-ceramic storage phosphors
Authors: Chen Gang , Johnson Jacqueline , Woodford John , Schweizer Stefan
Published: May 2006

We have studied the effect of oxide impurities on the formation of BaCl2 nanocrystals in Eu-doped fluorochlorozirconate glass-ceramic storage phosphors using x-ray absorption near-edge structure and x-ray diffraction analyses. It is found that annealing the glass with a relatively high level of oxide impurities causes crystallization of BaCl2 and a phase transition from hexagonal to orthorhombic BaCl2. The latter phase is responsible for the storage phosphor effect. In contrast, the glass with a low impurity concentration forms hexagonal BaCl2, but no such phase transition. Our study provides structural clues for understanding the x-ray storage mechanism in fluorozirconate-based glass ceramics.
Magnetocaloric properties of Ni2Mn1−xCuxGa
Authors: Stadler Shane , Khan Mahmud , Mitchell Joseph , Ali Naushad , Gomes Angelo M. , Dubenko Igor , Takeuchi Armando Y. , Guimarães Alberto P.
Published: May 2006

We present the magnetocaloric properties of the substituted Heusler alloy Ni2Mn1−xCuxGa, which shows a maximum magnetic entropy change of ΔSM≈−64J∕KgK=−532mJ∕cm3K at 308K for a magnetic field change ΔH=5T. The dependence of ΔSM on ΔH is approximately linear and does not reach saturation at 5T. It is demonstrated that the temperature at which ΔSM occurs can be tuned through subtle variations in composition.
Investigation of work function adjustments by electric dipole formation at the gate/oxide interface in preimplanted NiSi fully silicided metal gates
Authors: Liu J. , Kwong D. L.
Published: May 2006

The work function adjustments by implanted dopants (P, As, and Sb) in NiSi fully silicided metal gates were investigated. It was found that the work function adjustments only occurred when dopants were segregated at the silicide/dielectric interface after the fully silicided process. The dopant specie, size, and peak concentration at the interface were found to have impacts on work function tuning. The theoretical estimates of work function tuning were calculated using electric dipole domination assumption and confirmed by experimental measurements.
Ultrasensitive and highly selective gas sensors using three-dimensional tungsten oxide nanowire networks
Authors: Ponzoni Andrea , Comini Elisabetta , Sberveglieri Giorgio , Zhou Jun , Deng Shao Zhi , Xu Ning Sheng , Ding Yong , Wang Zhong Lin
Published: May 2006

Three-dimensional (3D) tungsten oxide nanowire networks have been demonstrated as a high-surface area material for building ultrasensitive and highly selective gas sensors. Utilizing the 3D hierarchical structure of the networks, high sensitivity has been obtained towards NO2, revealing the capability of the material to detect concentration as low as 50ppb (parts per billion). The distinctive selectivity at different working temperatures is observed for various gases. The results highlight that the nanobelts (nanowires) technology can be adopted for the development of gas sensors with performances suitable for practical applications.
1.54
μ
m
Si:Er light emitting diode with memory function
Authors: Andreev B. A. , Krasilnik Z. F. , Kryzhkov D. I. , Kuznetsov V. P. , Gregorkiewicz T. , Jantsch W.
Published: May 2006

We report a memory effect in electroluminescence of Er-doped silicon light emitting diodes: applying a voltage pulse in reverse-bias direction below breakdown we observe 1.54μm emission only, if a forward pulse was issued before. This effect occurs for temperatures T⩽120K in sublimation molecular-beam epitaxy grown structures. This finding opens perspectives for the development of a fully complementary metal-oxide-semiconductor compatible electro-optical converter with a memory function, operating in the 1.5μm telecommunication band. Such an element could find numerous applications in telecommunication and silicon photonics and optoelectronics circuitry.
Ultraviolet lasing in high-order bands of three-dimensional ZnO photonic crystals
Authors: Scharrer Michael , Yamilov Alexey , Wu Xiaohua , Cao Hui , Chang Robert P. H.
Published: May 2006

UV lasing in three-dimensional ZnO photonic crystals is demonstrated at room temperature. The photonic crystals are inverse opals with high refractive index contrast that simultaneously confine light and provide optical gain. Highly directional lasing with tunable wavelength is obtained by optical pumping. Comparison of the experimental results to the calculated band structure shows that lasing occurs in high-order bands with abnormally low group velocity. This demonstrates that the high-order band structure of three-dimensional photonic crystals can be used to effectively confine light and enhance emission. Our findings may also impact other applications of photonic crystal devices.
Room temperature interactions of water vapor with HfO2 films on Si
Authors: Driemeier C. , Gusev E. P. , Baumvol I. J. R.
Published: May 2006

Hf O 2 ∕ Si O 2 ∕ Si ( 001 ) thin film structures were exposed at room temperature to water vapor isotopically enriched in H2 and O18 followed by quantification and profiling of these nuclides by nuclear reaction analysis. We showed (i) the formation of strongly bonded hydroxyls at the HfO2 surface; (ii) room temperature migration of oxygen and water-derived oxygenous species through the HfO2 films, indicating that HfO2 is a weak diffusion barrier for these oxidizing species; (iii) hydrogenous, water-derived species attachment to the SiO2 interlayer, resulting in detrimental hydrogenous defects therein. Consequences of these results to HfO2-based metal-oxide-semiconductor devices are discussed.
Nanocrystalline dye-sensitized solar cell/copper indium gallium selenide thin-film tandem showing greater than 15% conversion efficiency
Authors: Liska P. , Thampi K. R. , Grätzel M. , Brémaud D. , Rudmann D. , Upadhyaya H. M. , Tiwari A. N.
Published: May 2006

Multijunction stacked (tandem) solar cells can increase the overall photovoltaic conversion efficiency by optimal utilization of the solar spectrum in individual cells. We demonstrate that a photovoltaic tandem cell comprising a nanocrystalline dye-sensitized solar cell as a top cell for high-energy photons and a copper indium gallium selenide thin-film bottom cell for lower-energy photons produces AM 1.5 solar to electric conversion efficiencies greater than 15%.
Ni-based bulk glassy alloys with superhigh strength of 3800MPa in Ni–Fe–B–Si–Nb system
Authors: Shen Baolong , Chang Chuntao , Inoue Akihisa
Published: May 2006

The effects of replacement of Ni by Fe on the glass-forming ability (GFA) and mechanical properties of Ni–B–Si–Nb glassy alloys were examined. The substitution of Fe for Ni caused increases of GFA and mechanical properties. Ni-based bulk glassy alloys (BGAs) with diameters up to 3mm were formed in a [(Ni1−xFex)0.75B0.2Si0.05]96Nb4 alloy system. The BGAs exhibit a superhigh fracture strength of 3680–3840MPa and Young’s modulus of 172–186GPa, which are the highest values for Ni-based BGAs. The improvement of GFA and fracture strength is interpreted to result from the enhanced atomic bonding nature among the constituent elements by adding Fe.
Step ordering induced by nonplanar patterning of GaAs surfaces
Authors: Dalla Volta A. , Vvedensky D. D. , Gogneau N. , Pelucchi E. , Rudra A. , Dwir B. , Kapon E. , Ratsch C.
Published: May 2006

We report the observation and theory of the morphological evolution of vicinal (001) ridges on V-grooved GaAs surfaces during metal organic vapor-phase epitaxy. The pattern of the nonplanar substrate induces unusual ordering of monatomic steps, different from the free step flow observed on a nonpatterned vicinal surface. The step edges develop profiles that kinetic Monte Carlo simulations reveal are determined by the width of the ridges between neighboring V grooves and the kinetics of interfacet mass migration between the ridge and the bounding sidewalls of the V groove.
Fabrication of metallic air bridges using multiple-dose electron beam lithography
Authors: Girgis E. , Liu J. , Benkhedar M. L.
Published: May 2006

The techniques of fabricating metallic air bridges using different resists in a one-step electron beam lithography are presented. The exposure process employed a single-layer polymethyl methacrylate (PMMA) or photoresists with either different doses in the span and feet areas or with varying acceleration voltage of the electron beam. The process using photoresists with different doses has produced air bridges more stable than what the PMMA method using various acceleration voltages would achieve. Using this method, air bridges up to 12μm long have been fabricated. The length and height of these metallic air bridges vary with the photoresist thickness.
Determination of acceptor concentration in GaN from photoluminescence
Authors: Reshchikov M. A.
Published: May 2006

The concentration of the acceptor responsible for the yellow luminescence (YL) band at about 2.2eV in GaN is determined from photoluminescence. The YL band intensity increases linearly with excitation power density and partially saturates above some critical value. The dependence is quantitatively described within a phenomenological model accounting for recombination statistics in GaN layer and saturation of acceptors with photogenerated holes. The incomplete saturation of the YL intensity at high excitation intensities is explained by gradual saturation of acceptors at different distances from the sample surface. The identity of deep and shallow acceptors in GaN is discussed.
Surface energy and the equilibrium shape of hexagonal structured Ge2Sb2Te5 grain
Authors: Park Yu Jin , Lee Jeong Yong , Kim Yong Tae
Published: May 2006

Surface energy and the equilibrium shape of hexagonal structured grain in Ge2Sb2Te5 were investigated by a transmission electron microscopy (TEM) study. As a result, we have found that the equilibrium shape of hexagonal structured Ge2Sb2Te5 is a lengthened shape with (0001) plane at the major axis, which was experimentally revealed by TEM images. The hexagonal structured Ge2Sb2Te5 grain has low surface energy in {0001}, {11¯03}, and {1¯106} planes which have no additional broken bonds from the atoms on the steps. Therefore, it is strongly facetted by the plane having low surface energy, thus, becoming an equilibrium shape.
Effect of sprinkler irrigation system on air temperatures and use of chemicals to protect cherry and peach trees from early spring frost
Authors: Tsipouridis C. , Thomidis T. , Xatzicharisis I.
Published: May 2006

The use of some chemicals including vitamins C and E, copper hydroxide, gibberellic acid, pinolene, urea, captan, ethephon, chlormequat chloride and paclobutrazol for early spring frost protection of cherry and peach trees was studied. Applications before frost of urea, copper hydroxide and a mix of glycerine or captan + vitamins C and E reduced the damage to flowers (at rates up to 40–60%), while application of gibberellic acid in combination with captan and pinoline 6 h after frost, reduced the damage by 40–50%. Ethephon delayed the bloom time of peach and cherry trees for about a week. Placing 2 sprinkler heads (200 L/h) on the tree at 40 or 280 cm height raised the temperatures 2.0 and 1.0–1.5°C, respectively.
Influence of forage legume species, seeding rate and seed size on competitiveness with annual ryegrass (Lolium rigidum) seedlings
Authors: Dear B. S. , Hodge A. , Lemerle D. , Pratley J. E. , Orchard B. A. , Kaiser A. G.
Published: May 2006

Annual legumes sown as short-term forage crops are an important non-chemical option for the control of herbicide-resistant annual ryegrass (Lolium rigidum L.). The relative ability of 5 annual forage legume species (Trifolium subterraneum L., T. michelianum Savi., T. alexandrinum L., Medicago murex Wild and Vicia benghalensis L.) to suppress annual ryegrass seedlings was examined in a glasshouse study. The experiment investigated the importance of legume species, seed size and seeding rate in influencing the relative competitiveness of the legumes. Balansa clover (T. michelianum) and berseem clover (T. alexandrinum), the smallest seeded of the legume species, were the most effective of the 5 legume species, when compared at equivalent seeding rates, at reducing ryegrass biomass. Legume leaf area and biomass were the 2 factors measured that were most closely associated with depressing ryegrass biomass and were themselves most influenced by legume species and seeding rate. Balansa clover and vetch (V. benghalensis) were the most effective at restricting the amount of light penetrating to 50% of the height of the canopy over a range of seeding rates. The study showed that the competitiveness of forage break crops can be optimised by maximising legume biomass through selecting small-seeded legume species with high relative growth rates and dense canopies and by increasing the seeding rate where seed costs are low.