bluuebunny/crossref_metadata_2025 huggingface dataset:
Excitation of electron Bernstein waves by a gyrating relativistic electron beam in a plasma slab
Authors: Kumar Asheel , Tripathi V. K.
Published: January 2004

A nonlocal theory of excitation of electron Bernstein waves in a magnetized plasma column by a gyrating relativistic electron beam has been developed. The beam response is obtained using the Vlasov equation. For a one-dimensional parabolic density profile of the background plasma, the mode structure equation yields Hermite polynomial eigenfunctions. The growth of the Bernstein wave occurs via a fast cyclotron interaction. For a typical case when the electron cyclotron frequency is comparable to the electron plasma frequency and beam velocity vb≈0.8c, the growth rate is maximum for k⊥ρ0≈5. The nonlocal effects reduce the growth rate.
Comparison of linear microinstability calculations of varying input realism
Authors: Rewoldt G. , Kinsey J. E.
Published: January 2004

The effect of varying “input realism” or varying completeness of the input data for linear microinstability calculations, in particular on the critical value of the ion temperature gradient for the ion temperature gradient mode, is investigated using gyrokinetic and gyrofluid approaches. The calculations show that varying input realism can have a substantial quantitative effect on the results.
Magnetic diagnostic responses for compact stellarators
Authors: Hirshman Steven P. , Lazarus Edward A. , Hanson James D. , Knowlton Stephen F. , Lao Lang L.
Published: January 2004

The formulation of magnetic diagnostic response functions for a three-dimensional stellarator plasma is described. Reciprocity relations are used to compute unique response functions for each type of magnetic diagnostic. Green’s function response tables (databases) are generated from which both external coil and internal plasma current contributions to diagnostic signals can be rapidly computed. Applications to compact stellarators are described.
Phase diagram of mixtures of hard colloidal spheres and discs: A free-volume scaled-particle approach
Authors: Oversteegen S. M. , Lekkerkerker H. N. W.
Published: January 2004

Phase diagrams of mixtures of colloidal hard spheres with hard discs are calculated by means of the free-volume theory. The free-volume fraction available to the discs is determined from scaled-particle theory. The calculations show that depletion induced phase separation should occur at low disc concentrations in systems now experimentally available. The gas–liquid equilibrium of the spheres becomes stable at comparable size ratios as with bimodal mixtures of spheres or mixtures of rods and spheres. Introducing finite thickness of the platelets gives rise to a significant lowering of the fluid branch of the binodal.
Dynamics of chain closure: Approximate treatment of nonlocal interactions
Authors: Debnath Pallavi , Cherayil Binny J.
Published: January 2004

The Wilemski–Fixman model of diffusion controlled-reactions [J. Chem. Phys. 58, 4009 (1973)] is combined with a generalized random walk description of chain conformations to predict the dependence of the closure time τ on the chain length N of polymers with reactive end groups and nonlocal interactions. The nonlocal interactions are modeled by a modification to the connectivity term in the Edwards continuum representation of the polymer. The modification involves a parameter h lying between 0 and 1 that is a measure of the extent of correlation between adjacent monomers on the chain backbone. Different choices of h correspond to chain conformations of different average radial dimensions. In particular, the values 1/3, 1/2 and 3/5 provide approximations to the statistics of polymers in poor, theta and good solvents, respectively. The closure time τ of such chains is calculated analytically for different N. In all cases, τ is found to vary as a power law in N, Nb, with b a function of h. For the special case h=1/3, which models collapsed polymers and globular proteins, b is about 1.6–1.7.
Constrained fluid λ-integration: Constructing a reversible thermodynamic path between the solid and liquid state
Authors: Grochola Gregory
Published: January 2004

A novel λ-integration path is proposed for calculating the Gibbs free energy difference between any arbitrary solid and liquid state needed for the location of melting lines. This technique involves reversibly forcing a liquid state to a solid state across the phase transition along a nonphysical path, thermodynamically coupling the two states directly. The process eliminates the need for coupling to idealized reference states as is presently performed and hence simplifies the location of phase transitions for computer simulation systems. More specifically the path involves a three stage process, whereby, initially a liquid state is transformed to a weakly attractive fluid using linear λ-integration scaling of the intermolecular potential. In the second stage, the resulting fluid is then constrained to the required solid configurational phase space via the insertion of a periodic lattice of 3D Gaussian wells. The final stage involves reversing to full strength the main intermolecular potential while gradually turning off the constraining 3D Gaussian lattice finally resulting in a stable (or metastable) solid state. Each stage was found to be completely reversible and the resulting change in free energy was thermodynamically integrable. The methodology is demonstrated and validated by calculating solid–liquid coexistence points using the new technique and comparing to those in present literature for the truncated and shifted Lennard-Jones system. The results are found to be in good agreement. The new method is not limited to melting phase transitions and is readily applicable to any simulation methodology, simulation cell size and/or intermolecular potential including ab initio methods.
Vibrational relaxation and coupling of two OH-stretch oscillators with an intramolecular hydrogen bond
Authors: Lock A. J. , Gilijamse J. J. , Woutersen S. , Bakker H. J.
Published: January 2004

We studied the vibrational dynamics of the OH-stretch oscillators of an alcohol with two vicinal OH groups using femtosecond midinfrared pump–probe spectroscopy. The absorption spectrum of pinacol (2,3-dimethyl-2,3-butanediol) in CDCl3 shows two OH-stretch peaks belonging to hydrogen bonded and free OH groups. The anharmonicities of the hydrogen-bonded and free OH-stretch vibrations are 180 and 160 cm−1, respectively. The lifetime T1 of the OH-stretch vibration is found to be 3.5±0.4 ps for the hydrogen bonded and 7.4±0.5 ps for the free OH group. We observed sidebands in the transient spectra after excitation of the bonded OH group, which we attribute to a progression in a low-frequency hydrogen-bond mode. The sideband is redshifted 60 cm−1 with respect to the 0→1 transition. Due to the coupling between the two OH groups and the presence of the sidebands, simultaneous excitation of both OH-stretch vibrations leads to oscillations on the pump–probe signal with frequencies of 40 and 60 cm−1.
A second quantization formulation of multimode dynamics
Authors: Christiansen Ove
Published: January 2004

A new formalism for calculating and analyzing many-mode quantum dynamics is presented. The formalism is similar in spirit to the second quantization formulation of electronic structure theory. The similarity means that similar techniques can be employed for calculating the many-mode nuclear wave function. As a consequence a new formulation of the vibrational self-consistent-field (VSCF) method can be developed. Another result is that the formalism opens up for the construction of new methods that go beyond the VSCF level. A vibrational coupled cluster (VCC) theory is constructed using the new formalism. The size-extensivity concept is introduced in the context of multimode wave functions and the size extensivity of approximate VCC methods is illustrated in comparison with the non-size-extensive vibrational configuration interaction method.
Vibrational coupled cluster theory
Authors: Christiansen Ove
Published: January 2004

The theory and first implementation of a vibrational coupled cluster (VCC) method for calculations of the vibrational structure of molecules is presented. Different methods for introducing approximate VCC methods are discussed including truncation according to a maximum number of simultaneous mode excitations as well as an interaction space order concept is introduced. The theory is tested on calculation of anharmonic frequencies for a three-mode model system and a formaldehyde quartic force field. The VCC method is compared to vibrational self-consistent-field, vibrational Møller–Plesset perturbation theory, and vibrational configuration interaction (VCI). A VCC calculation typically gives higher accuracy than a corresponding VCI calculation with the same number of parameters and the same formal operation count.
Electron localization–delocalization transitions in dissociation of the C4− anion: A large-D analysis
Authors: Shi Qicun , Kais Sabre , Herschbach Dudley R.
Published: January 2004

We present a study, employing high level ab initio methods, of electron localization–delocalization transitions along the dissociation path of the C4− anion to C2 and C2−. We find that at the equilibrium geometry, the symmetrical and nonsymmetrical configurations of the linear C4− anion are almost isoenergetic. However, along a collinear dissociation path, the dipole moment drops abruptly to zero when the separation between the two middle carbon nuclei reaches about R=2.15 Å. The dipole moment remains zero until about R=2.78 Å, and then continuously increases as dissociation proceeds. This behavior is analogous to critical phenomena: The abrupt drop to zero of the dipole moment resembles a first-order phase transition, the later steady rise resembles a continuous phase transition. We show that a simple sub-Hamiltonian model, corresponding to the large-dimension limit for an electron in the field of four collinear carbon atoms, exhibits both kinds of phase transitions along the dissociation path.
Ab initio study including spin–orbit effects on the B–X transition of AgI
Authors: Ramı́rez-Solı́s A.
Published: January 2004

The lowest Ω=0−,0+,1,2 fine-structure potential energy curves arising from the two lowest-lying singlet (X 1Σ+ and 2 1Σ+) and the first Π3 electronic states of AgI were obtained through an effective Hamiltonian; the purely electronic ΛSΣ energies were used as diagonal elements, which were calculated through extensive complete active space self-consistent field+averaged coupled pair functional calculations, with relativistic effective core potentials and optimized Gaussian basis sets for both atoms. The spin–orbit interactions were included using the Stuttgart effective spin–orbit potentials. For the excited Ω=0+ states, very strong mixtures were found of the 2 1Σ+ and Π3 parents that lead to the fine-structure (0+) single B state (dominated by the 2 1Σ+ parent at long distance), that explains the B←X transitions. The present results also explain the presence of a second long-distance minimum for the B0+ state, experimentally Rydberg–Klein–Rees fitted. These calculations produced, as a byproduct, a new lower-lying Ω=0+ yet unobserved fine-structure state predicted to exist around 22 000 cm−1. Our theoretical results are compared and discussed in the light of the experimental data for the B–X transitions in silver halides [J. Chem. Phys. 109, 9831 (1998)].
Imaging the quantum-state specific differential cross sections of HCl formed from reactions of chlorine atoms with methanol and dimethyl ether
Authors: Murray Craig , Orr-Ewing Andrew J. , Toomes Rachel L. , Kitsopoulos Theofanis N.
Published: January 2004

Center-of-mass frame scattering angle distributions obtained directly from crossed molecular beam velocity map images are reported for HCl formed in different rotational levels of its vibrational ground state by reaction of Cl atoms with CH3OH and CH3OCH3. Products are observed to scatter over all angles, with peaks in the distribution in the forward and backward directions (θ=0 and 180° with respect to the relative velocity vectors of the Cl atoms). Products of both reactions exhibit differential cross sections that vary with the rotational quantum number of the HCl, with a greater propensity for forward scatter for J=2, shifting to more pronounced backward scatter for J=5. This trend is, however, more evident for reaction of dimethyl ether than for methanol. The mean fractions of the available energy channeled into product kinetic energy vary with scattering angle, but the angle-averaged fractions are, respectively, 0.37 and 0.42 for the methanol and dimethyl ether reactions. On average, 46% or more of the available energy of the reactions becomes internal energy of the radical co-product. Results are interpreted with the aid of computed energies of transition states and molecular complexes along the reaction pathways, and comparisons are drawn with recent measurements of the scattering distributions and energy release for reactions of Cl atoms with small alkanes.
Quantum reactive scattering with a transmission-free absorbing potential
Authors: Gonzalez-Lezana Tomas , Rackham Edward J. , Manolopoulos David E.
Published: January 2004

A recently derived transmission-free absorbing potential is applied to the study of atom–diatom chemical reactions. This absorbing potential only depends on a single parameter—the width of the absorbing region—and its reflection properties are guaranteed to improve as this parameter is increased. Converged results can therefore be obtained very easily, as we illustrate with time-dependent wave packet calculations on the H+H2,F+H2, and H+O2 reactions.
Exciton exciton annihilation dynamics in chromophore complexes. II. Intensity dependent transient absorption of the LH2 antenna system
Authors: Brüggemann B. , May V.
Published: January 2004

Using the multiexciton density matrix theory of excitation energy transfer in chromophore complexes developed in a foregoing paper [J. Chem. Phys. 118, 746 (2003)], the computation of ultrafast transient absorption spectra is presented. Beside static disorder and standard mechanisms of excitation energy dissipation the theory incorporates exciton exciton annihilation (EEA) processes. To elucidate signatures of EEA in intensity dependent transient absorption data the approach is applied to the B850 ring of the LH2 found in rhodobacter sphaeroides. As main indications for two-exciton population and resulting EEA we found (i) a weakening of the dominant single-exciton bleaching structure in the transient absorption, and (ii) an intermediate suppression of long-wavelength and short-wavelength shoulders around the bleaching structure. The suppression is caused by stimulated emission from the two-exciton to the one-exciton state and the return of the shoulders follows from a depletion of two-exciton population according to EEA. The EEA-signature survives as a short-wavelength shoulder in the transient absorption if orientational and energetic disorder are taken into account. Therefore, the observation of the EEA-signatures should be possible when doing frequency resolved transient absorption experiments with a sufficiently strongly varying pump-pulse intensity.
On the ignition of high gain thermonuclear microexplosions with electric pulse power
Authors: Winterberg F.
Published: January 2004

It was recently shown that the ignition of thermonuclear microexplosions seems possible with two Marx generators of modest size, one with a high current lower voltage for compression and confinement, and one with a high voltage lower current for ignition, transmitting their energy to the thermonuclear target by two nested magnetically insulated transmission lines. Here it is shown in much greater detail how this concept has the potential for the ignition of high gain thermonuclear microexplosions with a yield sufficiently low for a thermonuclear reactor and rocket propulsion. The concept also offers the possibility for the concurrent burn of deuterium–tritium with natural uranium or thorium.
Spectral energy transfer and generation of turbulent structures in toroidal plasma
Authors: Xia H. , Shats M. G.
Published: January 2004

Nonlinear energy cascades in turbulent spectra are studied in the H-1 toroidal heliac [S. M. Hamberger et al., Fusion Technol. 17, 123 (1990)] using the spectral energy transfer estimation and the amplitude correlation technique. An inverse energy cascade of the spectral energy from the unstable range is shown to be responsible for the generation of the large-scale coherent structures dominating turbulence spectra. Among such structures are zonal flows which are also found to be generated via the inverse cascade. The generation of zonal flows is correlated with the increased strength in the nonlinear energy transfer. The onset of the strongly sheared radial electric field across the low–high (L–H) transition dramatically changes the energy transfer in the spectra and the spectral power of the fluctuations.
The polarity dependent effect of gyroviscosity on the flow shear stabilized Rayleigh–Taylor instability and an application to the plasma focus
Authors: Ruden E. L.
Published: January 2004

The linear dispersion relation is derived for modes of an isothermal finite Larmor radius incompressible plasma with an equilibrium density and horizontal fluid velocity varying with depth in a uniform gravitational field. The velocity and magnetic field are assumed parallel and transverse to the wave number, respectively. Stability criteria are derived and unstable growth rate diagrams plotted for the combined Rayleigh–Taylor/Kelvin–Helmholtz modes for two and three region piecewise uniform cases representing an accelerated plasma layer with sheared flow. The effect of gyroviscosity on wave numbers larger than a critical value is shown to differ if the direction of the magnetic field is reversed, all else being equal, being either stabilizing or destabilizing depending on direction. This implies an electrode polarity dependence for a magnetically accelerated plasma with sheared flow consistent with the observation that plasma foci generally have superior performance if the center conductor is the anode. Characteristic properties of the shocked plasma layer of a plasma focus during the accretion phase are inferred for use with the model. Given a plasma focus with a central anode, a maximum B0t product is derived for high wave number stability for a given current waveform, where B0 is the driving magnetic field magnitude and t is the current risetime. When combined with a recognized empirical scaling law for neutron yield optimized D2 plasma foci, a maximum current for high wave number stability is implied independent of t. For a linearly rising current, for example, this is 2 MA. Strategies for mitigating the constraints are discussed, such as applying an exponentially increasing current waveform. This and other parametric relationships of the model may lead to designs with higher performance than would otherwise be possible for plasma foci and other devices such as flow shear stabilized Z-pinches.
Synthesis and magnetic properties of polymer nanocomposites with embedded iron nanoparticles
Authors: Wilson J. L. , Poddar P. , Frey N. A. , Srikanth H. , Mohomed K. , Harmon J. P. , Kotha S. , Wachsmuth J.
Published: January 2004

Magnetic nanoparticles embedded in polymer matrices have excellent potential for electromagnetic device applications like electromagnetic interference suppression. We have synthesized polymer nanocomposites of poly(methylmethacrylate) doped with varying concentrations of iron nanoparticles (∼20 nm in size). The iron nanoparticles were produced using a microwave plasma technique and have a natural oxide surface layer for passivation. These nanocomposites were processed using melt blending technique. The polymer processing conditions were optimized to achieve good uniform dispersion of the nanoparticles in the polymer matrix. The concentration and dispersion of nanoparticles were varied in a controlled way. Surface characterization with scanning electron microscopy indicates that, to a large extent, the iron nanoparticles are embedded in the bulk; the surface mainly showed features associated with the polymer surface. Static magnetic properties such as susceptibility and M–H loops were studied using a physical property measurement system. The variation of the ferromagnetic response was consistent with the varying volume concentration of the nanoparticles, the polymer itself contributing a diamagnetic response. At room temperature, hysteresis loops exhibited a somewhat large coercivity (260 Oe) associated with a surface oxide layer on the particles. Overall, the excellent dispersion coupled with reasonable control over magnetic properties achieved in our experiments is promising for electromagnetic applications of these materials.
Photoconducting polymer–liquid crystal structure studied by electroreflectance
Authors: Miniewicz A. , Michelotti F. , Belardini A.
Published: January 2004

Polarization sensitive electroreflectance was used to study a typical hybrid photoconducting polymer–liquid crystal (LC) structure. Similar structures serve as optically addressed liquid crystalline spatial light modulators in real-time holography. It was shown that this technique might be used for the evaluation of internal electric-field changes in the vicinity of LC layer due to light-induced photogeneration and transport of charge carriers in the photoconducting polymer layer. A simplified theory presented in this work clarifies the experimental technique and the obtained results for the planar nematic LC panel. Employing the formalism of nonlinear optics for a particular type of LC (seven-component nematic mixture), we measured that the low-frequency (Ω/2π=1680 Hz) main component of a cubic optical susceptibility tensor χijkl(3)(−ω;0,Ω,ω) along the director direction is χa(3)=(7.4±1.5)×10−16 m2/V2.
Simple model for magnetization ratios in doped nanocrystals
Authors: Schrier Joshua , Whaley K. Birgitta
Published: January 2004

Recent experiments on Mn-doped ZnS nanocrystals have shown unusual magnetization properties. We describe a nearest-neighbor Heisenberg exchange model for calculating the magnetization ratios of these antiferromagnetically doped crystals, in which the dopant atoms are distributed inhomogeneously within the nanocrystal. This simple inhomogeneous doping model is capable of reproducing the experimental results, and suggests that interior dopant atoms are localized within the crystal.
Transport and stability properties of the charge ordered state for Sm1−xCaxMnO3 (0.4⩽x⩽0.8)
Authors: Qian T. , Li G. , Han Q. , Guo X. Y. , Liu Y. , Qu J. F. , Li X. G.
Published: January 2004

The structure and transport properties of Sm1−xCaxMnO3 with 0.4⩽x⩽0.8 were investigated. With decreasing temperature from 350 K, cooperative Jahn–Teller (JT) distortion develops, leading to the formation of a charge ordered (CO) state and anomalies in the lattice parameters and resistivities. At low temperature the magnetoresistance effect originates from the polarization of the localized t2g spins under the magnetic field and decreases with the increase in Ca2+ content x, whereas the stability of the CO state at the CO transition temperature TCO strongly depends on both Ca2+ content and cooperative JT distortion. These interesting phenomena sufficiently confirm the close relationship among charge, lattice, spin, and orbital degrees of freedom.
Dielectric property and structure of anodic alumina template and their effects on the electrophoretic deposition characteristics of ZnO nanowire arrays
Authors: Wang Y. C. , Leu I. C. , Hon M. H.
Published: January 2004

The template-mediated preparation for ZnO nanowire arrays was conducted by electrophoretic deposition (EPD) within the channel of anodic alumina membrane (AAM) template in this study. The influence of annealing temperature on the structure and dielectric properties of AAM template is investigated, and the related deposition mechanisms and morphologies of ZnO nanowire arrays at different AAM annealing temperatures and applied voltages are also discussed. The results show that heat treatment strongly affects the microstructure and dielectric properties of AAM templates, which in turn determines the deposition characteristics of ZnO nanoparticles within template during EPD process. The phase of AAM is changed from amorphous to crystalline structure after annealing. In addition, the grain size and dielectric constant of AAM increase as annealing temperature increases. The breakdown characteristic of AAM is closely related to the oxygen vacancy and grain size of AAM itself, and the applied voltages. The morphologies of ZnO nanowire arrays and current-time curve during EPD are determined by the breakdown behaviors of AAM templates and the applied voltages.
Polarization spectroscopy of InGaAs/GaAs quantum wires grown on (331)B GaAs templates with nanoscale fluctuations
Authors: Wang X. Y. , Wang Z. M. , Yazdanpanah V. R. , Salamo G. J. , Xiao Min
Published: January 2004

Using (331)B GaAs templates with nanoscale fluctuations, we have fabricated InGaAs/GaAs quantum wires (QWRs) with a density of ∼2.0×106 cm−1 and the degree of polarization as high as ∼28%. In the samples with weak lateral confinement, we observed thermal delocalization of carriers from the one-dimensional QWR states to the two-dimensional quantum-well states with increasing temperature, which is almost absent in QWR samples with strong lateral confinement.
Localization effects in a two-electron quantum dot molecule: The case of AC–DC driving fields
Authors: Paspalakis Emmanuel , Terzis Andreas F.
Published: January 2004

The combined interaction of an AC and a DC field with two electrons in a double quantum dot structure is studied. Under appropriate conditions, the system can be described by an effective two-level model. Conditions for localization of the electrons in the same quantum dot are readily derived. The validity of the conditions is assessed by numerical calculations.
p
H
dependence of synchrotron x-ray induced electroless nickel deposition
Authors: Borse P. H. , Yi J. M. , Je J. H. , Tsai W. L. , Hwu Y.
Published: January 2004

We investigated the room temperature electroless nickel deposition on glass, induced by synchrotron x ray. By irradiating electrolytes with intense x ray the onset time for Ni reduction disappears at high electrolyte pH (>6.0) in sharp contrast with conventional electroless deposition. The time for Ni reduction in irradiated solution also decreases with increase in electrolyte pH. Consequently higher reduction rates in alkaline solutions (pH>8.0) raise the Ni nucleation density during deposition, as illustrated by homogeneous and complete coverage of the substrate by nanoparticles within a short period of 1 min. The enhancement in reduction rate is attributed to high redox efficiency of hydrated electrons produced by x ray as well as their redox potential enhancements under higher electrolyte pH conditions.
Bandgap energies and refractive indices of Pb1−xSrxSe
Authors: Majumdar A. , Xu H. Z. , Zhao F. , Keay J. C. , Jayasinghe L. , Khosravani S. , Lu X. , Kelkar V. , Shi Z.
Published: January 2004

Optical transmission measurements were carried out on Pb1−xSrxSe samples, grown by molecular beam epitaxy, with different Sr compositions (x) ranging from 0 to 1. Refractive indices were calculated for all the samples at room temperature and at 77 K by fitting the transmittance data. Bandgap energies of all compositions were calculated by fitting the absorption coefficients to theoretical models of direct and indirect transitions. A distinct bandgap inversion from the direct to the indirect was observed at a Sr composition of approximately x=0.20. The direct and indirect bandgaps of SrSe calculated from the experimental results were found to be 3.81 and 1.82 eV, respectively.
Structures and properties of dielectrics in Pb(Mg1/3Nb2/3)O3–La(Mg1/2Ti1/2)O3 system
Authors: Hu X. , Chen X. M.
Published: January 2004

(1−x) Pb(Mg 1/3 Nb 2/3 ) O 3 –x La(Mg 1/2 Ti 1/2 ) O 3 dielectrics were prepared by a solid-state reaction process and the structures were evaluated together with the dielectric characterization. The pyrochlore-free dielectrics were obtained with increasing La(Mg1/2Ti1/2)O3 content due to the increased electronegativity difference. The 1/2(111) superlattice reflection showed 1:1 ordering on the B site in all compositions. Antiphase tilting of oxygen octahedra and B-site cation displacing appeared at x=0.9. Low loss (tan δ∼10−4) dielectrics with high dielectric constant (36–900) were obtained in the present system, and the good microwave dielectric properties were achieved in the compositions with x⩾0.7. The best combination of microwave dielectric properties was obtained for x=0.9: ε=34, Qf=21 280 GHz, and τf=−60 ppm/°C.
Reversible stress changes at all stages of Volmer–Weber film growth
Authors: Friesen C. , Seel S. C. , Thompson C. V.
Published: January 2004

Stresses caused by Volmer–Weber growth of polycrystalline Cu films have been measured in situ during: Island nucleation and growth, island coalescence, and post-coalescence film thickening. Growth interruptions followed by resumption of growth resulted in the observation of reversible stress changes in all regimes. Reversible stress changes in the pre-coalescence and post-coalescence regimes are similar in that: The stress evolves in the tensile direction during growth interruptions, the initial rate of stress evolution is significantly faster when growth is resumed than when growth is first interrupted, and the magnitude of the reversible stress change increases with increasing pre-interruption deposition rate. It is argued that reversible stress changes are associated with changes in adatom and other surface defect concentrations, corresponding with changes in the growth flux. It is shown that the change in stress-thickness product with changing film thickness (the instantaneous stress) can be related to the adatom–surface interaction energy. High sensitivity stress measurements were made at a rate of 1000 measurements per second, and the instantaneous stress at the initiation of growth was measured at all stages of growth. The initial instantaneous stress and the adatom–surface interaction energy increased in the pre-coalescence regime and reached a fixed, maximum value once coalescence had occurred. The measured interaction energy in the post-coalescence regime is 0.67±0.1 eV, which corresponds well with values calculated using molecular dynamics.
Monte Carlo investigations of dense copolymer systems. III. Properties of triblock copolymers in good and theta solvent
Authors: Zifferer Gerhard , Neubauer Brigitte , Olaj Oskar Friedrich
Published: January 2004

The present article gives an analysis of XYX triblock copolymers in a good solvent and in a theta solvent, the segments of type X and type Y being repulsive for each other. The results are compared to homopolymers as well as to copolymers in a selective solvent that is a good one for the outer blocks and a theta solvent for the inner one and vice versa, the strength of repulsion between blocks being the same as in the present types of copolymers. A lattice model is used for the investigations and the concentration ranges from a volume fraction φ=0 up to φ=0.8. In the limit φ→0 the triblocks in good solvent are slightly more expanded than homopolymers and in theta solvent mean square dimensions of triblocks are considerably increased compared to homopolymers due to the repulsion between blocks. With increasing concentration the dimensions decrease but then they increase again and for large concentrations they become similar for all types of copolymers studied, as the effect of the solvent levels off making the repulsive interaction between blocks the dominant interaction. This leads to an orientation effect and as a consequence to microphase separation which is demonstrated by the concentration dependence of various quantities as well as by visualization of snapshots.
Lattice Monte Carlo simulations of three-dimensional charged polymer chains
Authors: Klos J. , Pakula T.
Published: January 2004

The configurational properties of strongly charged polyelectrolytes accompanied by neutralizing counterions in dilute solutions are simulated using the cooperative motion algorithm on the face-centered-cubic lattice. The full Coulomb potential and the excluded volume condition between different ions/beads are taken into account and the reduced temperature T* is considered the main, variable parameter. The calculations that have been carried out for solutions of both single and several chains indicate a few regions of their behavior: (1) for T*→∞, it corresponds to that of neutral, self-avoiding polymers under good solvent conditions; (2) for T*≈1, due to the electrostatic interactions being effectively stronger, the chains are more outstretched compared to their size at other temperatures; (3) for T* well below one, the counterion condensation becomes more and more dominant, which gradually leads to strongly collapsed chains; and (4) at the lowest temperatures the chains and counterions assume low-energy configurations in the form of neutral, compact aggregates.
Lattice Monte Carlo simulations of three-dimensional charged polymer chains. II. Added salt
Authors: Klos J. , Pakula T.
Published: January 2004

The configurational properties of strongly charged polyelectrolytes accompanied by counterions and salt ions in dilute solutions are simulated using the cooperative motion algorithm on the face-centered-cubic lattice. The calculations indicate that both positive and negative ions condense on the chains at sufficiently low temperatures and their amount depends on the concentration of added salt. As the temperature decreases from high to low, the chains undergo conformational changes from neutral-like, self-avoiding polymers by more outstretched forms to compact globules. The observed extension of the chains at intermediate temperatures is also affected by the amount of salt. Furthermore, configurations with the lowest energies recorded at the lowest temperatures are aggregates of single or more entangled chains and ions of both types.
Numerical study of tokamak equilibria with arbitrary flow
Authors: Guazzotto L. , Betti R. , Manickam J. , Kaye S.
Published: January 2004

The effects of toroidal and poloidal flows on the equilibrium of tokamak plasmas are numerically investigated using the code FLOW. The code is used to determine the changes in the profiles induced by large toroidal flows on NSTX-like equilibria [with NSTX being the National Spherical Torus Experiment, M. Ono, S.M. Kaye, Y.-K.M. Peng et al., Nucl. Fusion 40, 557 (2000)] where flows exceeding the sound speed lead to a considerable outward shift of the plasma. The code is also used to study the effects of poloidal flow when the flow velocity profile varies from subsonic to supersonic with respect to the poloidal sound speed. It is found that pressure and density profiles develop a pedestal structure characterized by radial discontinuities at the transonic surface where the poloidal velocity abruptly jumps from subsonic to supersonic values. These results confirm the conclusions of the analytic theory of R. Betti and J. P. Freidberg [Phys. Plasmas 7, 2439 (2000)], derived for a low-β, large aspect ratio tokamak with a circular cross section.
Magnetic islands and plasma rotation in the Tokamak Chauffage Alfvén Brésilien tokamak
Authors: Severo J. H. F. , Nascimento I. C. , Tsypin V. S. , Kuznetsov Y. K. , Saettone E. A. , Vannucci A. , Galvão R. M. O. , Tendler M. , Mikhailovskii A. B.
Published: January 2004

Collisional plasma rotation in the Tokamak Chauffage Alfvén Brésilien (TCABR) tokamak [J. H. F. Severo, I. C. Nascimento, V. S. Tsypin, and R. M. O. Galvão, Nucl. Fusion 43, 1047 (2003)] has been experimentally studied. It was found that the measured plasma poloidal rotation velocity agrees within error limits with neoclassical theoretical predictions, and toroidal velocity with experimental results obtained in analogous tokamaks, almost everywhere along the minor radius r, except for measurements at r/a≃0.56 and r/a≃0.89 (the minor radius of TCABR tokamak a=18 cm). For the first point, the measured plasma rotation velocities are higher than the velocity of the background plasma, respectively ∼30% and ∼10% for the poloidal and toroidal rotation velocities. Using a set of 22 Mirnov coils displaced poloidally, magnetic field perturbations were measured in shots adjusted to reproduce the ones of the previous plasma rotation measurements, and the results confirm that in the region r/a≃0.89 the plasma rotates together with the magnetic island (3,1).
Streamer formation and collapse in electron temperature gradient driven turbulence
Authors: Gürcan Ö. D. , Diamond P. H.
Published: January 2004

A simple model is useful to understand the formation and persistence of radially elongated structures (streamers) in electron temperature gradient (ETG) driven modes. The ETG model is very similar to the thermal Rossby wave model, a system of broad interest. The detailed correspondence of these two models is discussed. Streamer formation in this simple model is analyzed using the modulational stability method. In the inviscid limit of the model, an amplitude equation similar to the nonlinear Schrödinger equation (NLS) is derived. This equation has a second derivative cubic nonlinearity and is identified as a special case of a more general higher order NLS. Analytical solutions are found in the form of travelling waves and a localized thorn. Using the Lagrangian structure of the amplitude equation, it is shown that one-dimensional collapse in the poloidal direction is possible in this system for certain parameter values, and for sufficiently localized inital flow. This identifies a parameter regime basin in which there is an attractor with the structure of a thin extended streamer. In the viscous limit, another amplitude equation, which is a certain special case of the generalized complex Ginzburg–Landau equation, is obtained. Fixed points of the corresponding dynamical system are identified and their stability is investigated.
Preparation of carbon nanoparticles with strong optical limiting properties by laser ablation in water
Authors: Chen G. X. , Hong M. H. , Chong T. C. , Elim H. I. , Ma G. H. , Ji W.
Published: January 2004

Carbon nanoparticle colloids were prepared by Nd:YAG laser ablation of a carbon target immersed in water. The nanoparticles were collected on substrates and analyzed with micro-Raman spectroscopy and electron microscopy. Optical limiting properties of the carbon nanoparticle colloids towards 532 nm wavelength were characterized using a nanosecond Nd:YAG laser. A strong optical limiting response was detected for the carbon nanoparticle colloids with C60 as reference at input laser fluence above 0.6 J/cm2. The effects of carbon nanoparticle concentrations upon the optical limiting behavior were also investigated. A convenient method for preparing carbon nanoparticle based optical limiters is proposed in this investigation.
Plastic strain relaxation of nitride heterostructures
Authors: Bellet-Amalric E. , Adelmann C. , Sarigiannidou E. , Rouvière J. L. , Feuillet G. , Monroy E. , Daudin B.
Published: January 2004

Thick layers of GaN on AlN, AlN on GaN, and InN on GaN were grown by plasma-assisted molecular beam epitaxy. Their plastic strain relaxation was studied by reflection high-energy electron diffraction (RHEED) and high resolution x-ray diffraction (HRXRD). The results are consistent with a mechanism of progressive introduction of misfit dislocations based on the coalescence of dynamically formed platelets. Due to the lack of proper gliding planes in the wurtzite structure, such dislocations are not mobile, leading to inhomogeneity of the strain state along the growth axis. The agreement between in situ RHEED and ex situ HRXRD measurements provides evidence that the strain state is frozen in during growth.
Growth optimization and electrical characteristics of VO2 films on amorphous SiO2/Si substrates
Authors: Youn Doo-Hyeb , Lee Ju-Wook , Chae Byung-Gue , Kim Hyun-Tak , Maeng Sung-Lyul , Kang Kwang-Yong
Published: January 2004

In order to fabricate an electronic device, we have optimized growth conditions of VO2 films on amorphous SiO2/Si structure substrates by pulsed laser deposition. The phase of VO2 films observed consisted of multiphases such as VO2, V2O5, and V2O3. The correlation between phase changes and growth conditions of VO2 films was analyzed. Also, the electrical characteristics of VO2-based three terminal devices, attributed to structural and phase changes, are discussed. VO2 films under optimal growth conditions have a change in resistivity of the order of 102 near a critical temperature, Tc=340 K. Furthermore, an abrupt jump in current was observed when an electric field was applied to the three terminal device in the VO2-based SiO2/Si structure. The source–drain voltage, in which the abrupt current jump occurred, changed with application of a gate electric field.
Photoluminescence and photoreflectance study of InGaAs/AlAsSb quantum wells grown by molecular-beam epitaxy
Authors: Mozume T. , Kasai J.
Published: January 2004

We report here on a photoluminescence (PL) and photoreflectance (PR) study in which we examined InGaAs/AlAsSb multiple quantum wells (MQWs) with both arsenic- and antimony-terminated interfaces that were grown by molecular-beam epitaxy. The PL spectra of the As-terminated MQWs are dominated by near-band-edge PL peaks over the temperature range between 8 and 300 K. The PL line shape and the temperature evolution of their PL peak energy show clear evidence of a feature that is characteristic of a band-tail localized exciton emission below 40 K. Conversely, the PL spectra of the Sb-terminated MQWs show broad spectra and also show excitation power dependence. Both samples show clear features that are characteristic of QW-related interband transitions in the PR spectra. The observed QW transition energies correspond well with the calculated interband transition energies. The temperature evolutions of the PR peak energies for both samples correspond well with the temperature dependence of the InGaAs bandgap. Although the PL peak energies correspond well with the PR peak energies at high temperature, Stokes shifts of 20 to 30 meV were observed at lower temperature, indicating strong localization of excitons due to potential fluctuations. The broad PL peak and the low activation energy that we observed for the PL integrated intensity quenching process suggest that enhanced incorporation of nonradiative centers at the InGaAs/AlAsSb interfaces was induced by the Sb interface termination.
Electrical resistivity in Zr48Nb8Cu12Fe8Be24 glassy and crystallized alloys
Authors: Bai H. Y. , Tong C. Z. , Zheng P.
Published: January 2004

The electrical resistivity of Zr48Nb8Cu12Fe8Be24 bulk metallic glassy and crystallized alloys in the temperature range of 4.2–293 K is investigated. It is found that the resistivity in glassy and crystallized states shows opposite temperature coefficients. For the metallic glass, the resistivity shows a negative logarithmic dependence at temperatures below 16 K, whereas it has more normal behavior for the crystallized alloy. At higher temperatures, the resistivity in both glassy and crystallized alloys shows dependence upon both T and T2, but the signs of the T and T2 terms are opposite. The results are interpreted in terms of scattering from two-level tunneling states in glasses and the generalized Ziman diffraction model.
Properties of As+-implanted and annealed GaAs and InGaAs quantum wells: Structural and band-structure modifications
Authors: Tomm J. W. , Strelchuk V. , Gerhardt A. , Zeimer U. , Zorn M. , Kissel H. , Weyers M. , Jiménez J.
Published: January 2004

Both crystal structure and energy band-structure changes caused by As+ implantation and by subsequent annealing in GaAs and in an In0.253Ga0.747As quantum well are studied. We demonstrate that the main implantation impact to the crystal structure is the creation of a large number of point defects and strong compressive strain of up to −0.1%. Raman and x-ray data demonstrate almost complete structural recovery for rapid thermal annealing temperatures⩾600 °C. While the lattice expansion becomes relaxed by annealing, the implantation-induced ionized point defects are still present up to the highest annealing temperatures applied. Under these circumstances, a 22 meV blueshift of the heavy-hole–electron (1hh–1e) transition within the quantum well and a substantial reduction of the nonequilibrium carrier lifetime remain as consequence of implantation.
Transmission of light through quantum heterostructures modulated by coherent acoustic phonons
Authors: Chern Gia-Wei , Lin Kung-Hsuan , Sun Chi-Kuang
Published: January 2004

We investigate the optical transmission oscillation of multiple quantum wells (MQWs) under the modulation of coherent acoustic phonons. We treat the coherent acoustic phonons as a semiclassical wave obeying continuum elastic equations. Starting from the microscopic electron–phonon interaction Hamiltonian, we obtain expressions for optical absorption modulation of the MQWs due to coherent acoustic phonons. The acoustic phonons introduce a renormalization to single-particle energy and furthermore modify the resonant condition of photon absorption. The optical transmission modulation can be conveniently expressed with the use of sensitivity functions. We derive the analytical expressions for the sensitivity functions by which we calculate the optical transient transmission changes. The calculated results are in good agreement with the experimental observations.
Optically pumped ultraviolet AlGaInN quantum well laser at 340 nm wavelength
Authors: He Y. , Song Y.-K. , Nurmikko A. V. , Su J. , Gherasimova M. , Cui G. , Han J.
Published: January 2004

We have demonstrated and characterized an optically pumped room temperature pulsed laser at 340 nm, based on a separate confinement AlGaInN multiple quantum well heterostructure design. The lasing was achieved at threshold peak power of ∼800 kW/cm2. Gain spectroscopy showed modest peak modal gain on the order of 5 cm−1, suggesting that the combination of weak optical confinement and nonradiative recombination would lead to high threshold currents for comparably constructed diode lasers.
A self-consistent theory of the plasma wakefield of an electron beam
Authors: Lin Hai , Xu Zhizhan , Li Ruxin , Chen Li-Ming
Published: January 2004

The generation of the plasma wakefield of an electron beam is investigated in a self-consistent method. The effect of the distortion of the driving beam shape on the strength of the driven plasma wave is taken into account. In addition to the loss of the kinetic energy of the beam, the variation in the shape, as well as that in its electric potential energy, also influences the strength of the driven plasma wave. The dependence of the amplitude of the wakefield on some controllable parameters of the beam is investigated via numerical experiment.