bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical band gap and ultralow dielectric constant materials prepared by a simple dip coating process
Authors: Iskandar Ferry , Abdullah Mikrajuddin , Yoden Hiroyoshi , Okuyama Kikuo
Published: May 2003

A simple procedure for producing silica films containing ordered porous silica films, with pore sizes in the range of 40–1000 nm, is described. The precursors were prepared by mixing polystyrene latex (PSL) colloidal spheres and silica nanoparticles. The precursors were dipped vertically onto a silicon wafer or glass substrate and the PSL particles were then completely removed at a temperature of approximately 400 °C. This method permits the pore size to be selected by appropriate adjusting of the size of the PSL particles. The presence of an optical band gap, which is dependent on the dielectric constant periodicity of the material, was clearly observed. A simple medium effective model to calculate the effective refraction index of the film combined with a Bragg diffraction equation exhibit calculation results in agreement with the measured data. The film produced using this procedure has a dielectric constant as low as 1.192, thus confirming that the proposed method has also the potential for producing ultralow dielectric constant materials.
Application of Poisson maps on coadjoint orbits of Sp(6,R) group to many body dynamics
Authors: Cerkaski M.
Published: May 2003

The canonical transformation approach generated by the semisimple subgroup GCM(3)⊂Sp(6,R) is applied to reduction of the Lie–Poisson bracket on coadjoint orbits of the Sp(6,R) group and the Poisson coalgebra spO*(6) is determined. Investigating the construction of the N-particle phase, induced by this reduction, we identify the Poisson coalgebra spO*(6) as the algebra of quadratic O(K), K≡N−1 invariant forms on symplectic 6 K−12 dimensional phase space T*[O(K−3)\O(K)], K⩾3. The general classification scheme of Poisson orbits for spO*(6) is found and applied to the classification of coadjoint orbits of the Sp(6,R) group occurring in the decomposition of N-particle phase spaces. We show that the spO*(k), k=4,6 Poisson action on some class of surfaces determined by Casimir invariants is not transitive. The Poisson maps for all classes of orbits spO*(4) and spO*(6) are found. The quantum unitary irreducible representations of spO*(4) are obtained.
Optical gain in materials with indirect transitions
Authors: Trupke Thorsten , Green Martin A. , Würfel Peter
Published: May 2003

Based on a simple two-level system, we discuss optical gain in materials with indirect optical transitions. We show that contrary to direct materials the absolute value of the negative absorption coefficient, which describes the optical gain, can be orders of magnitude larger in indirect semiconductors than the positive absorption coefficient measured under equilibrium conditions. These results are in contrast to previously published work and suggest that the question as to whether laser operation based on band–band transitions in bulk crystalline silicon is possible or not, in principle, needs to be readdressed. In addition, it is shown that indirect optical transitions can provide negative absorption, i.e., optical gain without an electronic population inversion.
Effect of strain rate on the formation of nanocrystallites in an Al-based amorphous alloy during nanoindentation
Authors: Jiang W. H. , Pinkerton F. E. , Atzmon M.
Published: May 2003

The effect of deformation by nanoindentation on nanocrystallization in amorphous Al90Fe5Gd5 was investigated by transmission electron microscopy. Massive precipitation of nanocrystallites is observed within the indents. Under the quasistatic condition used, a temperature rise due to adiabatic heating is likely negligible, confirming that plastic deformation can induce crystallization without a heating effect. The nucleation of nanocrystallites is significantly affected by the strain rate.
Adsorption–desorption equilibrium and kinetics on surfaces undergoing self-similar reversible reconstruction
Authors: Cerofolini G. F.
Published: May 2003

A rate equation, which takes explicitly into account the surface reconstruction phenomena occurring (reversibly or irreversibly) while adsorption or desorption proceed, is proposed. The equation contains a few free parameters which specify the adsorption and desorption rate constants, and the reconstruction and irreversibility degrees of the process. A complete qualitative study of the solutions of the rate equation for two kinds of reversible and self-similar reconstruction is given.
Atomic layer deposition of Al2O3 on H-passivated Si. I. Initial surface reaction pathways with H/Si(100)-2×1
Authors: Halls Mathew D. , Raghavachari Krishnan
Published: May 2003

Aluminum oxide (Al2O3) grown by atomic layer deposition (ALD) is currently under investigation for use as a high-κ gate dielectric alternative to SiO2. Cluster calculations employing hybrid density functional theory have been carried out to examine the chemical reaction pathways between the ALD precursors, trimethylaluminum (TMA) and H2O, with the H/Si(100)-2×1 surface. Results obtained using Si9H14 and Si15H20, dimer and double dimer clusters to represent the surface active site are in good agreement, providing a consistent view of reaction energetics on the H/Si(100)-2×1 surface. The adsorption energies for TMA and H2O on the surface are calculated to be 0.02 and 0.15 eV, respectively. For the reaction between H2O and the H/Si(100)-2×1 surface, hydroxylation of the surface accompanied by loss of H2 was found to be the preferred pathway having an activation energy and overall reaction enthalpy of 1.60 eV and −0.75 eV, both of which are ⩾0.70 eV lower than the corresponding values for the possible H/D exchange reaction. TMA exposure of the H/Si(100)-2×1 surface favors the deposition of –Al(CH3)2 with loss of CH4, having a barrier height of 1.30 eV and reaction enthalpy of −0.31 eV, which are 0.10 and 0.40 eV lower than the surface methylation pathway (H/CH3 exchange) and 2.64 and 0.45 eV lower in energy than the H2 loss reaction, that results in the deposition of –CH2–Al(CH3)2 to the surface. Therefore, the dominant reactions identified in this work are those with direct implication in the Al2O3 ALD growth mechanism, leading to the formation of Si–O and Si–Al species on the H/Si(100)-2×1 surface.
Water heptamer with an excess electron: Ab initio study
Authors: Lee Han Myoung , Bum Suh Seung , Kim Kwang S.
Published: May 2003

The electron–water heptamer is one of the “magic” numbers in the mass spectra of electron–water clusters, but up to now the structure of the electron–water heptamer is not known. Thus we have investigated a number of low-energy structures, and report the lowest-energy structure using ab initio calculations. The relative energies, vertical electron-detachment energies, and OH vibrational frequencies of several lowest energy conformers are discussed. As in the case of e−(H2O)6, the electron affinity for e−(H2O)7 is predicted to be positive, which would explain the intense peak observed in the time-of-flight mass spectra.
Ti-chabazite as a model system of Ti(IV) in Ti-zeolites: A periodic approach
Authors: Damin Alessandro , Bordiga Silvia , Zecchina Adriano , Doll Klaus , Lamberti Carlo
Published: May 2003

For the first time, full ab initio periodic calculations on a Ti-chabazite with a very low content of Ti (Ti/Si=1/11) were done. The simulated crystal is less computationally demanding and it is similar to the real titanosilicates microporous materials such as TS-1 or Ti-β, where the Ti/Si is lower than 1/30, and hence it can be used as a model system. The geometric and energetic features of the interaction of Ti-CHA with NH3, H2O, H2CO, and CH3CN were studied, principally at the Hartree–Fock level. Bare Ti-CHA and its NH3, H2O, H2CO, and CH3CN complexes were fully optimized at Hartree–Fock level with an upgraded version of the CRYSTAL code [Chem. Phys. Lett. 348, 131 (2001)] and such optimized structures were employed to calculate BEc (binding energy corrected for the basis set superposition error) at the Hartree–Fock and B3-LYP levels.
Full-dimensionality quantum calculations of acetylene–vinylidene isomerization
Authors: Zou Shengli , Bowman Joel M. , Brown Alex
Published: May 2003

The isomerization of acetylene to vinylidene is examined theoretically in full dimensionality (six degrees of freedom), using a new ab initio potential energy surface [S. Zou and J. M. Bowman, Chem. Phys. Lett. 368, 421 (2003)]. Eigenfunctions and eigenvalues of the exact Hamiltonian, for zero total angular momentum, are obtained using a series of novel truncation/recoupling procedures that permits calculations up to very high energies. The Hamiltonian is given in diatom–diatom Jacobi coordinates, with the choice H2–C2 for the two diatoms in order to exploit the full permutational symmetry of the problem. By examining expectation values of the eigenfunctions, a number of states are definitely identified with vinylidenelike characteristics. Corresponding calculations are also done for C2D2. Full dimensional simulations of the photodetachment spectra of C2H2− and C2D2− are done (within the Franck–Condon approximation) and compared to the experimental ones. For this calculation the ground vibrational state wave function of the anion is obtained using a new force field, based on high quality ab initio calculations, which are also briefly reported.
The essential role of hydrogen atoms in the electron–phonon interactions in the monocation of cubic hydrocarbon cluster, (CH)8
Authors: Kato Takashi , Yamabe Tokio
Published: May 2003

Electron–phonon coupling in the monocations of cubic clusters such as cubane (CH)8 and octasilacubane (SiH)8 are studied. The total electron–phonon coupling constants for the monocations (lHOMO) are estimated and compared with those for the monoanions (lLUMO) in cubane and octasilacubane. The calculated lHOMO value for cubane (0.766 eV) is much larger than that for octasilacubane (0.327 eV). And the lHOMO values are larger than the lLUMO values both in cubane and octasilacubane. The frequencies (ωln,HOMO) of the vibrational modes which play an essential role in the electron–phonon interactions and the possible superconducting transition temperatures (Tcs) are estimated on the basis of the hypothesis that vibronic interactions between the highest occupied molecular orbitals (HOMO) and intramolecular vibrations would play an essential role in the occurrence of superconductivity in positively charged nanosized molecular systems. The phase pattern difference between the delocalized t2g HOMO in cubane and the t1u HOMO localized on silicon atoms in octasilacubane as well as the molecular weights difference between cubane and octasilacubane is the main reason why the lHOMO and ωln,HOMO values for cubane are much larger than those for octasilacubane, and the reason why the possible Tcs for the monocation of cubane are estimated to be much larger than those for the monocation of octasilacubane. In a similar way, the Tcs for the monocation are estimated to be larger than those for the monoanion in cubane. The important vibronic interaction effects originating from the orbital interactions between carbon and hydrogen atoms due to the delocalized electron density in the t2g HOMO in cubane are the main reason why the Tcs for the monocation are estimated to be larger than those for the monoanion in cubane.
On the properties of the seam and branching spaces of conical intersections in molecules with an odd number of electrons: A group homomorphism approach
Authors: Han Seungsuk , Yarkony David R.
Published: May 2003

The properties of the branching and seam spaces of conical intersections in a molecule with an odd number of electrons are explored for the general case, where the molecule has no spatial symmetry and the Hamiltonian explicitly includes the spin–orbit interaction. A realization of the homomorphism connecting the symplectic group of order 4, Sp(4), and the group of proper rotations in five dimensions SO(5) is used to find an orthogonal representation of the branching space that preserves the standard form of the electronic Hamiltonian near a conical intersection. An invariant property of the branching space is also identified. These findings extend previous results for the nonrelativistic Hamiltonian and the relativistic Hamiltonian with Cs symmetry. A model Hamiltonian representing a tetra-atomic molecule with three coupled doublet electronic states is used to demonstrate the efficacy of the approach and illustrate possible seam loci. The seam of conical intersection is shown to have two distinct branches, one bounded and one infinite in extent. The branching spaces of these seams are characterized.
Infrared photon echo signatures of hydrogen bond connectivity in the cyclic decapeptide antamanide
Authors: Moran Andrew M. , Park Seung-Min , Mukamel Shaul
Published: May 2003

Distinct hydrogen bonding patterns are predicted in the amide I and amide A vibrational bands of four dominant conformations of antamanide using anharmonic vibrational Hamiltonians constructed at the DFT and AM1 levels. We show how these conformations may be distinguished using coherent three pulse infrared measurements with several pulse polarization configurations in the amide I region. The amide A hydrogen bonded N–H stretching modes are highly localized and have conformation-dependent frequencies, but their anharmonicities are insensitive to local structure at the hydrogen bond distances in antamanide.
High-resolution threshold-ionization spectroscopy of NH3
Authors: Seiler R. , Hollenstein U. , Softley T. P. , Merkt F.
Published: May 2003

High-resolution photoionization, zero-kinetic-energy photoelectron and Rydberg-state-resolved threshold-ionization spectra of ammonia and its deuterated isotopomers have been recorded in the region of the lowest vibrational levels (v2+=0,1) of the X̃+ ground ionic state of NH3+ following single-photon excitation from the ground neutral state using a narrow bandwidth vacuum ultraviolet laser system (bandwidth 0.008 cm−1). The resolution enables the observation of photoionization transitions originating from distinct tunneling components of the ground neutral state and the measurement of the spin-rotational splittings of the ionic energy levels. A new value of the first adiabatic ionization potential of NH3 [I.P.=82 158.751(16) cm−1] has been derived which is more accurate than previous values by almost two orders of magnitude. The photoionization dynamics of NH3 to the lowest vibrational levels of the X̃+(2A2″) ground state of NH3+ is dominated by the emission of even ℓ photoelectron partial waves, and a strong propensity rule is observed that favors transitions with ΔK=K+−K″=0 and, to a lesser extent, ΔK=3.
On axisymmetric resistive magnetohydrodynamic equilibria with flow free of Pfirsch–Schlüter diffusion
Authors: Throumoulopoulos G. N. , Tasso H.
Published: May 2003

The equilibrium of an axisymmetric magnetically confined plasma with anisotropic resistivity and incompressible flows parallel to the magnetic field is investigated within the framework of the magnetohydrodynamic (MHD) theory by keeping the convective flow term in the momentum equation. It turns out that the stationary states are determined by a second-order elliptic partial differential equation for the poloidal magnetic flux function ψ along with a decoupled Bernoulli equation for the pressure identical in form with the respective ideal MHD equations; equilibrium consistent expressions for the resistivities η∥ and η⊥ parallel and perpendicular to the magnetic field are also derived from Ohm’s and Faraday’s laws. Unlike in the case of stationary states with isotropic resistivity and parallel flows [G. N. Throumoulopoulos and H. Tasso, J. Plasma Phys. 64, 601 (2000)] the equilibrium is compatible with nonvanishing poloidal current densities. Also, although exactly Spitzer resistivities either η∥(ψ) or η⊥(ψ) are not allowed, exact solutions with vanishing poloidal electric fields can be constructed with η∥ and η⊥ profiles compatible with roughly collisional resistivity profiles, i.e., profiles having a minimum close to the magnetic axis, taking very large values on the boundary and such that η⊥>η∥. For equilibria with vanishing flows satisfying the relation (dP/dψ)(dI2/dψ)>0, where P and I are the pressure and the poloidal current functions, the difference η⊥−η∥ for the reversed-field pinch scaling, Bp≈Bt, is nearly two times larger than that for the tokamak scaling, Bp≈0.1Bt (Bp and Bt are the poloidal and toroidal magnetic-field components). The particular resistive equilibrium solutions obtained in the present work, inherently free of—but not inconsistent with—Pfirsch–Schlüter diffusion, indicate that parallel flows might result in a reduction of the diffusion observed in magnetically confined plasmas.
Numerical simulation of the equilibrium and transport of a centrifugally confined plasma
Authors: Osborn B. R. , Ellis R. F. , Hassam A. B.
Published: May 2003

A centrifugally confined plasma utilizes centrifugal forces from supersonic plasma rotation to augment the conventional magnetic confinement of fusion plasmas. Appropriately used, plasma pressure can be contained along the magnetic field lines, allowing for “open” configurations, introducing flexibility in design. The axisymmetric equilibrium of one such system, a single coil configuration, is investigated numerically. Plasma “detachment” is shown at Mach numbers ranging from 3 to 4. It is also shown how flow shear heats the plasma by viscous dissipation. Profiles relaxed with respect to plasma transport are shown.
Laser pulse splitting and trapping in tenuous gases
Authors: Wu Jianzhou , Antonsen Thomas M.
Published: May 2003

The nonlinear propagation over long distances of moderate intensity laser pulses in tenuous gases is studied. The dynamics of these pulses will be affected by nonlinear focusing and dispersion due to the background gas, and by plasma induced refraction and dispersion. Laser propagation is studied numerically using the simulation code WAKE [P. Mora and T. M. Antonsen, Jr., Phys. Plasmas 4, 217 (1997)]. Different phenomena are found for different regimes of peak input power. For powers near the critical power, temporal pulse narrowing and splitting due to phase modulation and group velocity dispersion is seen. For slightly higher powers, plasma generation and the formation of a trailing pulse, which is guided off axis by plasma refraction and nonlinear gas focusing, is observed. For even higher powers, the laser pulse is partially trapped by the plasma and then exhibits a form of self interference.
On the convergence to statistical equilibrium for harmonic crystals
Authors: Dudnikova T. V. , Komech A. I. , Spohn H.
Published: May 2003

We consider the dynamics of a harmonic crystal in d dimensions with n components, d,n arbitrary, d,n⩾1, and study the distribution μt of the solution at time t∈R. The initial measure μ0 has a translation-invariant correlation matrix, zero mean, and finite mean energy density. It also satisfies a Rosenblatt—resp. Ibragimov–Linnik type mixing condition. The main result is the convergence of μt to a Gaussian measure as t→∞. The proof is based on the long time asymptotics of the Green’s function and on Bernstein’s “room-corridors” method.
Tensor operators and constructing indecomposable representations of semidirect product groups
Authors: Conidis Chris , Repka Joe
Published: May 2003

Consider a semidirect product group G=H⋉V, where H is reductive and V is a vector group. Two irreps π1 and π2 of H can be “assembled” into a representation of G if it is possible to construct an indecomposable representation Π of G whose restriction to H is π1⊕π2. It is shown that this is equivalent to the existence of a tensor operator from π2 to π1 carrying a representation of H which is equivalent to a nontrivial quotient of the representation which defines the semidirect product. This provides a systematic method for deciding whether two irreps can be assembled, and, if so, in how many inequivalent ways. The method is applied in many of the standard examples that arise in physical questions.
A simulation study of lyotropic isotropic–nematic phase transitions in polydisperse chain systems
Authors: Escobedo Fernando A.
Published: May 2003

The isotropic–nematic (I–N) phase behavior of athermal chain-length polydisperse systems was investigated by molecular simulation. The approach adopted involved the formulation of a novel semigrand Gibbs-type of ensemble for polydisperse chain systems, the use of extrapolation methods (like histogram reweighting) to target polydispersities of interest, and the implementation of both intermutation moves among species and expanded-ensemble moves (for the transfer of the shortest chains) to attain chemical potential equilibration. This approach was first used to illustrate how the results of a system with bimodal polydispersity distribution can be used to get coexistence data for a bidisperse system of short and long chains, thereby circumventing the need to insert/delete the longest molecules. Simulations were then used to examine the effect of osmotic pressure (concentration) and the shape of the chain-length polydispersity function on the I–N phase transitions. In agreement with the predictions of Flory theory, the phase behavior of a unimodal (Poisson) parent distribution showed significant partitioning of the components between the coexisting phases, wherein longer chains concentrate in the nematic phase. Within the biphasic region, the order parameter in the nematic phase increased with pressure despite the growing proportion of shorter chains. Very short chains, like dimers and trimers, were found to be disordered in the nematic phase. Partitioning effects are more pronounced when the parent system had a flatter or a bimodal chain-length distribution. Flory theory gives a good description of the partition effects, but significantly overestimates the coexisting concentrations and the nematic ordering.
Comment on “Direct determination of kinetic fragility indices of glassforming liquids by differential scanning calorimetry: Kinetic versus thermodynamic fragilities” [J. Chem. Phys. 117, 10184 (2002)]
Authors: Roland C. M. , Santangelo P. G. , Robertson C. G. , Ngai K. L.
Published: May 2003

Recently Wang et al. [J. Chem. Phys. 117, 10184 (2002)] reported fragilities for various molecular liquids, as determined by calorimetry. The technique is based on measuring the change in fictive temperature with cooling rate. We point out that the same method was previously applied to polymers [Robertson, Santangelo, and Roland, J. Non-Cryst. Solids 275, 153 (2000)], yielding good agreement with fragility values obtained by mechanical or dielectric spectroscopy.
Ice nucleation in aqueous solutions of poly[ethylene glycol] with different molar mass
Authors: Zobrist Bernhard , Weers Uwe , Koop Thomas
Published: May 2003

Homogeneous ice nucleation was investigated in aqueous solutions of poly[ethylene glycol] (PEG) with a molar mass between 300 and 6000 g mol−1. Experiments were performed with a differential scanning calorimeter using emulsified aqueous PEG solutions with concentrations of 0–44 wt %. Equilibrium phase transition temperatures are determined and discussed, in particular the simultaneous occurrence of metastable and stable eutectic temperatures. The observed homogeneous freezing temperatures of ice reveal a continuous increase in the supercooling of PEG solutions with increasing molar mass of the PEG. The freezing behavior was investigated within the framework of water-activity-based ice nucleation theory. The latter predicts that homogeneous ice nucleation in aqueous solutions is independent of the nature of the solute, but depends only on the water activity of the solution. Water activity data of various PEG solutions in the stable and supercooled range were compared. It was found that the water activity of PEG solutions decreases with decreasing temperature. This trend is stronger for the PEGs of larger molar mass, and can explain why solutions of larger PEGs supercool more readily. Our study shows that the predictions of water-activity-based ice nucleation theory are consistent with the observed freezing behavior and with the thermodynamic properties of PEG solutions. It is also in agreement with the suggestion of a thermodynamic control of ice nucleation in aqueous systems due to the existence of a liquid phase spinodal.
A quantum mechanical/molecular mechanical approach to the investigation of particle–molecule interactions
Authors: Sloth Marianne , Bilde Merete , Mikkelsen Kurt V.
Published: May 2003

A quantum mechanical/molecular mechanical aerosol model is developed to describe the interaction between gas phase molecules and atmospheric particles. The model enables the calculation of interaction energies and time-dependent properties. We use the model to investigate how a succinic acid molecule interacts with an aqueous particle. We show how the interaction energies and linear response properties (excitation energies, transition moments, and polarizabilities) depend on the distance between aerosol particle and molecule and on their relative orientation. The results are compared with those obtained previously using a dielectric continuum model [Sloth et al., J. Phys. Chem. (submitted)].
Probing hydrogen bond potential surfaces for out-of-plane geometries: Near-infrared combination band torsional (ν6) spectroscopy in (HCl)2
Authors: Fárnı́k Michal , Davis Scott , Nesbitt David J.
Published: May 2003

High-resolution near-infrared (IR) spectra of jet cooled (HCl)2 are obtained in the 3000–3070 cm−1 combination band region corresponding to simultaneous excitation of intermolecular out-of-plane torsion (ν6) with intramolecular HCl stretching (ν1,ν2) degrees of freedom. Two (H35Cl)2 bands (plus the isotopically mixed H35Cl–H37Cl bands) due to ν6 torsional excitation are observed. High-resolution spectral analysis of ground-state combination differences and nuclear spin intensity alternation effects permits unambiguous identification of J labeling and tunneling level symmetry, yielding precise rotational constants and vibrational band origins. Comparison with previous far-IR studies suggests assignment of the 3036.52(5) and 3014.86 cm−1 bands to “bound HCl stretch+torsion” (ν2+ν6) combination excitation out of the lower (Γvt=A−←A+) and upper (Γvt=B−←B+) tunneling levels, respectively. A third much weaker band (3035.37 cm−1) is also observed in close proximity with the ν2+ν6 (Γvt=A−←A+) origin, which most likely acquires oscillator strength by state-mixing between the “dark” perturbing state and the ν2+ν6 upper level. These data provide the first opportunity for comparison with exact quantum calculations by Qui and Bačić and co-workers on high level ab initio/semiempirical (HCl)2 potential surfaces, which should facilitate further refinement of intermolecular potentials for out-of-plane twisting of the hydrogen bond.
Improved light-output and electrical performance of InGaN-based light-emitting diode by microroughening of the p-GaN surface
Authors: Huh Chul , Lee Kug-Seung , Kang Eun-Jeong , Park Seong-Ju
Published: May 2003

We report on an InGaN-based light-emitting diode (LED) with a top p-GaN surface microroughened using the metal clusters as a wet etching mask. The light-output power for a LED chip with microroughening was increased compared to that for a LED chip without one. This indicates that the scattering of photons emitted in the active layer was much enhanced at the microroughened top p-GaN surface of a LED due to the angular randomization of photons inside the LED structure, resulting in an increase in the probability of escaping from the LED structure. By employing the top surface microroughened in a LED structure, the power conversion efficiency was increased by 62%.
Tensile stress generation during island coalescence for variable island-substrate contact angle
Authors: Seel Steven C. , Thompson Carl V.
Published: May 2003

During the deposition of films that grow by the Volmer–Weber mechanism, crystallites nucleate as isolated islands, grow larger to impinge with other islands, and eventually coalesce into a continuous film. Tensile stress generation due to grain boundary formation during island coalescence, commonly referred to as island zipping, has been cited as a probable stress generation mechanism in a wide variety of materials, including low- and high-melting point metals, polycrystalline diamond, amorphous semiconductors, and amorphous metallic alloys. For all zipping-based tensile stress models in the literature, with the notable exception of the study of faceted diamond films [Sheldon et al., J. Appl. Phys. 90, 5097 (2001)], the surface of the islands are assumed to approach the substrate at normal incidence, so that the island-substrate contact angle is 90°. We have investigated the importance of the island-substrate contact angle on the tensile stress generated during island coalescence using finite-element calculations. The equilibrium tensile stress as a function of island-substrate contact angle and island radius was calculated numerically. For islands of a given size, the equilibrium stress was found to be strongly dependent on the contact angle, with the stress decreasing with decreasing contact angle. The scaling of the equilibrium stress with island radius was also found to depend on the contact angle. These results suggest that, for a given material, the coalescence stress associated with grain boundary formation should vary significantly for different substrates, depending on detailed differences in wetting behavior.
Isentropic compression of irradiated stainless steel on the Z accelerator
Authors: Reisman D. B. , Wolfer W. G. , Elsholz A. , Furnish M. D.
Published: May 2003

We have performed quasi-isentropic compression experiments on radiation-damaged stainless steel. The samples were dynamically loaded by Sandia National Laboratory’s Z accelerator with a ramp compression wave. Sample/window interface velocities were recorded with VISAR. The velocity histories suggest a sudden volume reduction of the material above 40 kbar caused by the collapse of nanosized voids. This is predicted by a theoretical model of void collapse based on the emission of vacancy-type dislocations loops. We compare the results of these experiments to hydrodynamic calculations performed using a constitutive model which is derived from the atomistic void collapse mechanism.
Intermolecular reactivity study to scale adsorption property of para- and meta-substituted nitrobenzene over 2:1 dioctahedral smectite
Authors: Chatterjee Abhijit , Ebina Takeo , Iwasaki Takashi , Mizukami Fujio
Published: May 2003

Substituted nitrobenzene, a toxic series of molecules with its wide application in pesticides needs to be removed by a suitable sorbent, whose characteristic is to be determined using the reactivity index theory within the helm of the density-functional theory and as well inside the broad subject range of the hard–soft acid-base (HSAB) principle. We studied a series of meta- and para-substituted nitrobenzene molecules (NBMs) starting from –NH2, –OH, –OCH3, –CHO, –OCOCH3, –COCH3, –CF3, –CN, and –COOCH3 to monitor the effect of electrophilicity and nucleophilicity in their adsorption properties. From the values of the local softness and the charge on the hydrogen atom of the bridging/structural (occurring on the surface) hydroxyl attached to the octahedral/tetrahedral metal site present in smectite, it is concluded that the local acidities of the inorganic material systems are dependent on several characteristics which are of importance within the framework of HSAB principle. We used “group softness” for the interacting molecules, whose values were generated by adding the local softness values of constituent atoms of the respective group and found it can be a better descriptor for intermolecular interactions occurring in these NBMs. A semiquantitave scale is proposed from the group reactivity index present in NBM and the clay cluster.
Calculation of bound and resonance states of HO2 for nonzero total angular momentum
Authors: Zhang Hong , Smith Sean C.
Published: May 2003

Bound and resonance states of HO2 have been calculated quantum mechanically by the Lanczos homogeneous filter diagonalization method [Zhang and Smith, Phys. Chem. Chem. Phys. 3, 2282 (2001); J. Chem. Phys. 115, 5751 (2001)] for nonzero total angular momentum J=1,2,3. For lower bound states, agreement between the results in this paper and previous work is quite satisfactory; while for high lying bound states and resonances these are the first reported results. A helicity quantum number Ω assignment (within the helicity conserving approximation) is performed and the results indicate that for lower bound states it is possible to assign the Ω quantum numbers unambiguously, but for resonances it is impossible to assign the Ω helicity quantum numbers due to strong mixing. In fact, for the high-lying bound states, the mixing has already appeared. These results indicate that the helicity conserving approximation is not good for the resonance state calculations and exact quantum calculations are needed to accurately describe the reaction dynamics for HO2 system. Analysis of the resonance widths shows that most of the resonances are overlapping and the interferences between them lead to large fluctuations from one resonance to another. In accord with the conclusions from earlier J=0 calculations, this indicates that the dissociation of HO2 is essentially irregular.
Electron acceleration by an intense short-pulse laser in underdense plasma
Authors: Yu M. Y. , Yu Wei , Chen Z. Y. , Zhang J. , Yin Y. , Cao L. H. , Lu P. X. , Xu Z. Z.
Published: May 2003

Electron acceleration from the interaction of an intense short-pulse laser with low density plasma is considered. The relation between direct electron acceleration within the laser pulse and that in the wake is investigated analytically. The magnitude and location of the ponderomotive-force-caused charge separation field with respect to that of the pulse determine the relative effectiveness of the two acceleration mechanisms. It is shown that there is an optimum condition for acceleration in the wake. Electron acceleration within the pulse dominates as the pulse becomes sufficiently short, and the latter directly drives and even traps the electrons. The latter can reach ultrahigh energies and can be extracted by impinging the pulse on a solid target.
Design of foam-buffered high gain target with Fokker–Planck implosion simulation for thermal insulation and imprint mitigation
Authors: Takeda T. , Mima K. , Norimatsu T. , Nagatomo H. , Nishiguchi A.
Published: May 2003

It is proposed that a thick foam layer on a plastic capsule of fusion pellet is effective not only for reducing the initial imprint, but also for solving the melting problem of cryogenic deuterium–tritium layer, in a reactor chamber. Investigated are the dependences of gain, thermal insulation for preventing the melting, and imprint mitigation of a foam-buffered target on the foam layer thickness. The imprint mitigation, the Rayleigh–Taylor growth factor and the fusion gain of a foam-buffered target are evaluated by the hydrodynamic implosion code HIMICO [A. Nishiguchi et al., Phys. Fluids B 4, 417 (1992)], which includes a Fokker–Planck transport code. As the result, it is found that high gain can be achieved by the foam-buffered target together with thermal insulation and imprint mitigation.
Infrared absorption bands associated with native defects in ZnGeP2
Authors: Giles N. C. , Bai Lihua , Chirila M. M. , Garces N. Y. , Stevens K. T. , Schunemann P. G. , Setzler S. D. , Pollak T. M.
Published: May 2003

An optical absorption investigation from 10 to 296 K has been performed on bulk crystals of ZnGeP2 grown by the horizontal-gradient-freeze method. We identify three broad absorption bands in the spectral range from 1 to 4 μm that are due to native defects. At low temperature, a band peaking near 1.2 μm and another band peaking near 2.2 μm have intensities that correlate. The strength of these two absorption bands can be increased or decreased by illuminating the crystal with selected laser wavelengths. The 2.2 μm band is strongly polarized, with its greatest intensity occurring when E is parallel to the c axis of the crystal. A third absorption band, peaking near 2.3 μm and extending from 1.5 μm to beyond 4 μm, was observed at low temperature, during and immediately after illumination. Comparison of photoinduced changes in absorption with photoinduced changes in electron paramagnetic resonance spectra allowed specific defects to be associated with each of the three absorption bands. Both the 1.2 and the 2.2 μm bands involve the singly ionized zinc vacancy. The third band at 2.3 μm involves neutral phosphorus vacancies. Specific models for the optical transitions are discussed.
Double modulated thermoreflectance microscopy of semiconductor devices
Authors: Dietzel D. , Bein B. K. , Pelzl J.
Published: May 2003

Photothermal microscopy based on combined optical and electrical excitation has been applied to insulating lines and conducting channels on SIMOX mesas prepared by focused ion beam implantation. The double excitation technique permits imaging of electrical properties of the implanted structures yielding complementary information achievable by a single excitation defect tracing. In addition, the contrast of the images of implanted structures can be increased considerably. The best contrast for the observation of insulating lines adjacent to a conducting channel has been achieved by recording the modulated reflectance signal at the fourth harmonic of the modulation frequency used for electrical and optical excitation. The contrast enhancement is found to be mainly due to thermal origins caused by the photoinduced currents, which can act as an additional heat source.
An algebraic method for solving the SU(3) Gauss law
Authors: Salmela Antti
Published: May 2003

A generalization of existing SU(2) results is obtained. In particular, the source-free Gauss law for SU(3)-valued gauge fields is solved using a non-Abelian analog of the Poincaré lemma. When sources are present, the color-electric field is divided into two parts in a way similar to the Hodge decomposition. Singularities due to coinciding eigenvalues of the color-magnetic field are also analyzed.
An invariant action for noncommutative gravity in four dimensions
Authors: Chamseddine A. H.
Published: May 2003

Two main problems face the construction of noncommutative actions for gravity with star products: the complex metric and finding an invariant measure. The only gauge groups that could be used with star products are the unitary groups. I propose an invariant gravitational action in D=4 dimensions based on the constrained gauge group U(2,2) broken to U(1,1)×U(1,1). No metric is used, thus giving a naturally invariant measure. This action is generalized to the noncommutative case by replacing ordinary products with star products. The four-dimensional noncommutative action is studied and the deformed action to first order in deformation parameter is computed.
Dramatic response effects in the photoionization of the second-row hydrides: A time-dependent density functional investigation
Authors: Fronzoni G. , Stener M. , Decleva P.
Published: May 2003

Cross section and asymmetry parameter profiles for the ionizations of the valence and core shells of the second-row hydrides have been calculated by the time-dependent density functional theory (TD-DFT) and Kohn–Sham (KS) schemes. The importance of the screening effects has been pointed out by the comparison of the KS and TD-DFT results and has been analyzed in detail for the HCl molecule. The results have been discussed with respect to the nature of the ionized orbitals along the series in order to assess the observed trends. This behavior of the Cooper minima in the outer valence molecular orbitals has been considered in detail both in the cross section and asymmetry parameter profiles. The TD-DFT results, compared with the experimental data, show quantitative agreement with the experiment for the outer-valence and core ionizations. A deterioration of the quality of the TD-DFT results is still present in the inner-valence region.
Anion electric field is related to hydration energy
Authors: Chipman Daniel M.
Published: May 2003

Electronic structure calculations that utilize dielectric continuum theory to mimic solvent often considerably underestimate the magnitudes of anion solvation energies in water, unless unphysically small cavities are utilized. It is found that a simple correction term can account for most of this error, while retaining cavities of physically reasonable size for evaluating the bulk dielectric contributions. This correction is based on linear correlation with the minimum value of the outgoing normal electric field produced by the anion on the surface of the dielectric cavity separating it from solvent. Values of the correlation parameters vary somewhat among different chemical classes of anions and are probably related to strong hydrogen bonding interactions between the anions and first-shell water molecules that are otherwise difficult to describe with dielectric continuum theory alone.
Equation of state theory based on excluded volume
Authors: Rusanov Anatoly I.
Published: May 2003

A thermodynamic approach to deriving an equation of state has been elaborated on the basis of excluded volume. A master differential equation relating pressure to excluded volume has been derived whose particular solutions in the zero and first approximation are the famous Planck and van der Waals equations, respectively. The second approximation yields a new more accurate equation of state covering a wider state range from rarefied gas to the critical state. At the same time, the equation maintains the original simplicity that characterizes the Planck and van der Waals equations.
Coarse Brownian dynamics for nematic liquid crystals: Bifurcation, projective integration, and control via stochastic simulation
Authors: Siettos C. I. , Graham M. D. , Kevrekidis I. G.
Published: May 2003

We demonstrate how time integration of stochastic differential equations (i.e., Brownian dynamics simulations) can be combined with continuum numerical analysis techniques to analyze the dynamics of liquid crystalline polymers (LCPs). Sidestepping the necessity of obtaining explicit closures, the approach analyzes the (unavailable in closed form) “coarse” macroscopic equations, estimating the necessary quantities through appropriately initialized, short “bursts” of Brownian dynamics simulation. Through this approach, both stable and unstable branches of the equilibrium bifurcation diagram are obtained for the Doi model of LCPs and their “coarse stability” is estimated. Additional macroscopic computational tasks enabled through this approach, such as coarse projective integration and coarse stabilizing controller design, are also demonstrated.
Dependence of the viscosity on the chain end dynamics in polymer melts
Authors: Paeßens Matthias
Published: May 2003

We compare the Rubinstein–Duke model for reptation to a model where the boundary dynamics are modified by calculating the viscosity of polymer melts. The question is investigated whether the viscosity is determined by details of the dynamics of the polymer ends or by the stretching of the polymer. Toward this end, the dependence of the viscosity on the particle density of the lattice gas models which can be identified by the stretching is determined. We show that the influence of the stretching of the polymer on the absolute value of the viscosity in the scaling limit of very long chains is much bigger than the influence of the boundary dynamics, whereas the corrections of the scaling of the viscosity depends significantly on the details of the boundary dynamics.
Coarse-grained molecular dynamics simulations of polymer melts in transient and steady shear flow
Authors: Padding J. T. , Briels W. J.
Published: May 2003

By use of nonequilibrium simulations a coarse-grained model of polyethylene, developed in our previous work [J. Chem. Phys. 115, 2846 (2001); 117, 925 (2002)], is subjected to a planar Couette flow. Both transient and steady-state nonlinear flow properties are investigated for shear rates varying from 30 to 3000 μs−1 and chain lengths varying from C80H162 to C800H1602. We report rheological data (shear viscosity, normal stress differences) and structural data (chain dimensions and the order tensor), and compare them with experimental results, where available. The locations of maxima and magnitudes of overshoots in the shear stress and normal stress difference are in agreement with experimental results. We also observe an undershoot in the transient extinction angle and a decrease of the steady-state extinction angle with shear rate, both of which are in very good agreement with recent experiments. Two rheological “rules,” the stress–optical rule and the Cox–Merz rule, are tested. It is shown that the extinction angle, as calculated from stress components, remains equal to the optical extinction angle even for high shear rates, where the stress–optical rule is no longer strictly valid.
Stabilization of ion channels due to membrane-mediated elastic interaction
Authors: Partenskii Michael B. , Miloshevsky Gennady V. , Jordan Peter C.
Published: May 2003

Recent work shows that linked gramicidin channels may have much longer lifetimes than single channels. We establish that the stabilization of the individual channels can be caused by membrane-mediated elastic interactions between such inclusions. In linear elastic theory, interaction can be rigorously described in terms of coupled harmonic oscillators. We determine the “effective spring constants” for various assemblies using the smectic bilayer model. We consider a range of aggregates; in clusters, channel lifetimes may increase by several orders of magnitude, an effect that is especially pronounced for a channel with many near neighbors.
Nucleation of stable cylinders from a metastable lamellar phase in a diblock copolymer melt
Authors: Wickham Robert A. , Shi An-Chang , Wang Zhen-Gang
Published: May 2003

The nucleation of a droplet of stable cylinder phase from a metastable lamellar phase is examined within the single-mode approximation to the mean-field Landau–Brazovskii model for diblock copolymer melts. By employing a variational ansatz for the droplet interfacial profile, an analytic expression for the interfacial free energy of an interface of arbitrary orientation between cylinders and lamellae is found. The interfacial free energy is anisotropic and is lower when the cylinder axis is perpendicular to the interface than when the cylinders lie along the interface. Consequently, the droplet shape computed via the Wulff construction is lens like, being flattened along the axis of the cylinders. The size of the critical droplet and the nucleation barrier are determined within classical nucleation theory. Near the lamellar–cylinder phase boundary, where classical nucleation theory is applicable, critical droplets of size 30–400 cylinders across with aspect ratios of 4–10 and nucleation barriers of (30–40)kBT are typically found. The general trend is to larger critical droplets, higher aspect ratios, and smaller nucleation barriers as the mean-field critical point is approached.
Energetic electron beam generation and anisotropy of hard x-ray emission from 0.9 to 1.0 MA high-Z X pinches
Authors: Kantsyrev V. L. , Fedin D. A. , Shlyaptseva A. S. , Hansen S. , Chamberlain D. , Ouart N.
Published: May 2003

An analysis of the energy, spatial character, and temporal evolution of electron beams and hard x rays from 0.9 to 1.0 MA high-Z X pinches is presented. Experimental results from Ti, Fe, Mo, and W X pinches show that X pinches are an effective source of hard x rays with energies greater than 30 keV. Electron beams with energies up to 2 MeV higher than the applied anode-cathode voltage are generated along the pinch axis before the maximum current is reached. The beams have diameters of about 3 mm and generate bursts of hard x rays with sizes between 1 and 2 mm and total time durations of up to 150 ns. The measured 100–500 keV x-ray distribution is spectrally anisotropic. Hard x-ray synchrotron radiation in the side-on direction is proposed as a possible explanation of this anisotropy.