bluuebunny/crossref_metadata_2025 huggingface dataset:
Free energies of carbon dioxide sequestration and methane recovery in clathrate hydrates
Authors: Dornan Peter , Alavi Saman , Woo T. K.
Published: September 2007

Classical molecular dynamics simulations are used to compare the stability of methane, carbon dioxide, nitrogen, and mixed CO2∕N2 structure I (sI) clathrates under deep ocean seafloor temperature and pressure conditions (275K and 30MPa) which were considered suitable for CO2 sequestration. Substitution of methane guests in both the small and large sI cages by CO2 and N2 fluids are considered separately to determine the separate contributions to the overall free energy of substitution. The structure I clathrate with methane in small cages and carbon dioxide in large cages is determined to be the most stable. Substitutions of methane in the small cages with CO2 and N2 have positive free energies. Substitution of methane with CO2 in the large cages has a large negative free energy and substitution of the methane in the large cages with N2 has a small positive free energy. The calculations show that under conditions where storage is being considered, carbon dioxide spontaneously replaces methane from sI clathrates, causing the release of methane. This process must be considered if there are methane clathrates present where CO2 sequestration is to be attempted. The calculations also indicate that N2 does not directly compete with CO2 during methane substitution or clathrate formation and therefore can be used as a carrier gas or may be present as an impurity. Simulations further reveal that the replacement of methane with CO2 in structure II (sII) cages also has a negative free energy. In cases where sII CO2 clathrates are formed, only single occupancy of the large cages will be observed.
Solitary pulses in linearly coupled Ginzburg-Landau equations
Authors: Malomed Boris A.
Published: September 2007

This article presents a brief review of dynamical models based on systems of linearly coupled complex Ginzburg-Landau (CGL) equations. In the simplest case, the system features linear gain, cubic nonlinearity (possibly combined with cubic loss), and group-velocity dispersion (GVD) in one equation, while the other equation is linear, featuring only intrinsic linear loss. The system models a dual-core fiber laser, with a parallel-coupled active core and an additional stabilizing passive (lossy) one. The model gives rise to exact analytical solutions for stationary solitary pulses (SPs). The article presents basic results concerning stability of the SPs; interactions between pulses are also considered, as are dark solitons (holes). In the case of the anomalous GVD, an unstable stationary SP may transform itself, via the Hopf bifurcation, into a stable localized breather. Various generalizations of the basic system are briefly reviewed too, including a model with quadratic (second-harmonic-generating) nonlinearity and a recently introduced model of a different but related type, based on linearly coupled CGL equations with cubic-quintic nonlinearity. The latter system features spontaneous symmetry breaking of stationary SPs, and also the formation of stable breathers.
Stark-assisted population control of coherent CS2 4f and 5p Rydberg wave packets studied by femtosecond time-resolved photoelectron spectroscopy
Authors: Knappenberger Kenneth L. , Lerch Eliza-Beth W. , Wen Patrick , Leone Stephen R.
Published: September 2007

A two-color (3+1′) pump-probe scheme is employed to investigate Rydberg wave packet dynamics in carbon disulfide (CS2*). The state superpositions are created within the 4f and 5p Rydberg manifolds by three photons of the 400nm pump pulse, and their temporal evolution is monitored with femtosecond time-resolved photoelectron spectroscopy using an 800nm ionizing probe pulse. The coherent behavior of the non-stationary superpositions are observed through wavepacket revivals upon ionization to either the upper (1∕2) or lower (3∕2) spin-orbit components of CS2+. The results show clearly that the composition of the wavepacket can be efficiently controlled by the power density of the excitation pulse over a range from 500GW∕cm2to10TW∕cm2. The results are consistent with the anticipated ac-Stark shift for 400nm light and demonstrate an effective method for population control in molecular systems. Moreover, it is shown that Rydberg wavepackets can be formed in CS2 with excitation power densities up to 10TW∕cm2 without significant fragmentation. The exponential 1∕e population decay (T1) of specific excited Rydberg states are recovered by analysis of the coherent part of the signal. The dissociation lifetimes of these states are typically 1.5ps. However, a region exhibiting a more rapid decay (≈800fs) is observed for states residing in the energy range of 74450–74550cm−1, suggestive of an enhanced surface crossing in this region.
Analysis of the Casson and Carreau-Yasuda non-Newtonian blood models in steady and oscillatory flows using the lattice Boltzmann method
Authors: Boyd Joshua , Buick James M. , Green Simon
Published: September 2007

The lattice Boltzmann method is modified to allow the simulation of non-Newtonian shear-dependent viscosity models. Casson and Carreau-Yasuda non-Newtonian blood viscosity models are implemented and are used to compare two-dimensional Newtonian and non-Newtonian flows in the context of simple steady flow and oscillatory flow in straight and curved pipe geometries. It is found that compared to analogous Newtonian flows, both the Casson and Carreau-Yasuda flows exhibit significant differences in the steady flow situation. In the straight pipe oscillatory flows, both models exhibit differences in velocity and shear, with the largest differences occurring at low Reynolds and Womersley numbers. Larger differences occur for the Casson model. In the curved pipe Carreau-Yasuda model, moderate differences are observed in the velocities in the central regions of the geometries, and the largest shear rate differences are observed near the geometry walls. These differences may be important for the study of atherosclerotic progression.
Coalescence of two equal-sized deformable drops in an axisymmetric flow
Authors: Yoon Yosang , Baldessari Fabio , Ceniceros Hector D. , Leal L. Gary
Published: October 2007

The coalescence of two equal-sized deformable drops in an axisymmetric flow is studied, using a boundary-integral method. An adaptive mesh refinement method is used to resolve the local small-scale dynamics in the gap and to retain a reasonable speed of computation. The thin film dynamics is successfully simulated, with sufficient stability and accuracy, up to a film thickness of O(10−4) times the undeformed drop radius, for a range of capillary numbers, Ca, from O(10−4–10−1) and viscosity ratios from O(0.1–10). The results are compared with experimental results from our earlier studies as well as the simple scaling theory for film drainage. The collisions for time-independent flow simulating head-on collisions in the experimental studies show two distinctively different regimes. At lower capillary numbers, the interfaces of the thin film between the colliding drops remain almost spherical up to the point of film rupture, and the dimensionless drainage time scales as tdG∼Ca. At higher capillary numbers, the film becomes dimpled at an early stage of the collision process, and the rate of the film drainage significantly slows down after the dimple is fully formed. In this case, the drainage time scales approximately as tdG∼Ca4∕3. The simulation, using a Hamaker constant with a fixed value calculated via Lifshitz theory, qualitatively agrees with the experimental results for the higher capillary numbers but not for the lower capillary numbers. The critical conditions for head-on collisions are also examined when the internal circulation within the drop, caused by the external flow, arrests the film drainage. Collisions in a time-dependent flow are also examined to simulate glancing collisions. Although the simulations predict many aspects of the experimental results, the results are quantitatively accurate, in comparison to the experimental data, only for the lowest viscosity ratio of 0.19. The interfaces of the thin film locally bulge outward when the drops are being pulled apart due to the suction pressure. This local deformation causes a local minimum in the film thickness. At the larger offsets, the coalescence angle continuously increases with Ca up to the separation angle (θ=55°–58°), for Ca45° cannot induce film rupture, even though coalescence is observed experimentally for the higher viscosity ratios.
Modeling pattern formation and cavity solitons in quantum dot optical microresonators in absorbing and amplifying regimes
Authors: Brambilla M. , Maggipinto T. , Perrini I. M. , Barbay S. , Kuszelewicz R.
Published: September 2007

We present a complete overview of our investigation past and present of the modelization and study of the spatiotemporal dynamics of a coherent field emitted by a semiconductor microcavity based on self-assembled quantum dots. The modelistic approach is discussed in relation to prospective growth and experimental research, and the model is then applied to resonators for which the medium is either passive (coherent photogeneration of carriers) or active (carrier pumping by current bias). The optical response of the system is investigated, especially in what concerns the linewidth enhancement factor, which turns out to be critical for the onset of self-organized patterns. The regimes in which one can expect bistable response, modulational instabilities, pattern formation, and cavity soliton formation are investigated. The pattern scenario is described, and experimentally achievable conditions are predicted for the occurrence of stable cavity solitons.
Depletion interaction mediated by polydisperse rods
Authors: Lang Peter R.
Published: September 2007

The interaction between a colloidal hard sphere of radius R and a wall or between two spheres in a dilute suspension of infinitely thin rods of length L is calculated numerically. The method allows the study of depletion potentials for any value of L∕R and, consequently, the influence of rod length polydispersity can be investigated. It was observed that both the depth and the range of the potential increase drastically if the relative standard deviation σ of the length distribution is larger than 0.25, while the potential is virtually indistinguishable from that caused by monodisperse rods, if σ≲0.1.
Core-annular flow through a horizontal pipe: Hydrodynamic counterbalancing of buoyancy force on core
Authors: Ooms G. , Vuik C. , Poesio P.
Published: September 2007

A theoretical investigation has been made of core-annular flow: the flow of a high-viscosity liquid core surrounded by a low-viscosity liquid annular layer through a horizontal pipe. Special attention is paid to the question of how the buoyancy force on the core, caused by a density difference between the core and the annular layer, is counterbalanced. From earlier studies it is known that at the interface between the annular layer and the core waves are present that move with respect to the pipe wall. In the present study the core is assumed to consist of a solid center surrounded by a high-viscosity liquid layer. Using hydrodynamic lubrication theory (taking into account the flow in the low-viscosity liquid annular layer and in the high-viscosity liquid core layer) the development of the interfacial waves is calculated. They generate pressure variations in the core layer and annular layer that can cause a net force on the core. Steady eccentric core-annular flow is found to be possible.
Configurational entropy of binary hard-disk glasses: Nonexistence of an ideal glass transition
Authors: Donev Aleksandar , Stillinger Frank H. , Torquato Salvatore
Published: September 2007

We study the thermodynamics of a binary hard-disk mixture in which the ratio of disk diameters is κ=1.4. We use a recently developed molecular dynamics algorithm to calculate the free-volume entropy of glassy configurations and obtain the configurational entropy (degeneracy) of the supercompressed liquid as a function of density. We find that the configurational entropy of the glasses near the kinetic glass transition is very close to the mixing entropy, suggesting that the degeneracy is zero only for the phase-separated crystal. We explicitly construct an exponential number of jammed packings with densities spanning the spectrum from the accepted “amorphous” glassy state to the phase-separated crystal, thus showing that there is no ideal glass transition in binary hard-disk mixtures. This construction also demonstrates that the ideal glass, defined as having zero configurational entropy, is not amorphous, but instead is nothing more than a phase-separated crystal. This critique of the presumed existence of an ideal glass parallels our previous critique of the idea that there is a most-dense random (close) packing for hard spheres [Torquato et al., Phys. Rev. Lett. 84, 2064 (2000)]. We also perform free-energy calculations to determine the equilibrium phase behavior of the system. The calculations predict a first-order freezing transition at a density below the kinetic glass transition. However, this transition appears to be strongly kinetically suppressed and is not observed directly. New simulation techniques are needed in order to gain a more complete understanding of the thermodynamic and kinetic behavior of the binary disk mixture and, in particular, of the demixing process during crystallization.
Pattern formation in forced reaction diffusion systems with nearly degenerate bifurcations
Authors: Halloy José , Sonnino Giorgio , Coullet Pierre
Published: September 2007

The existence and stability of stable standing-wave patterns in an assembly of spatially distributed generic oscillators governed by a couple of complex Ginzburg-Landau equations, subjected to parametric forcing, are reported. The mechanism of a dispersion-induced pattern in dissipative oscillators parametrically forced near the degenerate Turing-Hopf bifurcation is also illustrated. We show that, when excitation occurs just after the Turing bifurcation and before the Hopf instability, the system exhibits a new type of stable standing-wave structures, namely the mixed-mode solutions. The Brussellator-model, parametrically forced below the threshold of oscillations, is analyzed as an example of calculation.
Nonlinear propagation of sinusoidal pulse laser beam in homogeneous plasma
Authors: Upadhyay Ajay K. , Raj Gaurav , Mishra Rohit K. , Malviya Amita , Jha Pallavi
Published: September 2007

The propagation of an intense laser beam, having a sinusoidal pulse profile, in underdense plasma, is analyzed. Applying variational technique and using a standard trial function for the laser pulse amplitude, simultaneous equations describing the evolution of pulse length and spot size are obtained. Relativistic nonlinearity, finite pulse length, and group velocity dispersion effects have been taken into account. Using numerical methods, a graphical analysis of simultaneous evolution of laser spot size and pulse length is presented.
Dynamics of traveling pulses in heterogeneous media
Authors: Nishiura Yasumasa , Teramoto Takashi , Yuan Xiaohui , Ueda Kei-Ichi
Published: September 2007

One of the fundamental issues of pulse dynamics in dissipative systems is clarifying how the heterogeneity in the media influences the propagating manner. Heterogeneity is the most important and ubiquitous type of external perturbation. We focus on a class of one-dimensional traveling pulses, the associated parameters of which are close to drift and/or saddle-node bifurcations. The advantage in studying the dynamics in such a class is twofold: First, it gives us a perfect microcosm for the variety of outputs in a general setting when pulses encounter heterogeneities. Second, it allows us to reduce the original partial differential equation dynamics to a tractable finite-dimensional system. Such pulses are sensitive when they run into heterogeneities and show rich responses such as annihilation, pinning, splitting, rebound, as well as penetration. The reduced ordinary differential equations (ODEs) explain all these dynamics and the underlying bifurcational structure controlling the transitions among different dynamic regimes. It turns out that there are hidden ordered patterns associated with the critical points of ODEs that play a pivotal role in understanding the responses of the pulse; in fact, the depinning of pulses can be explained in terms of global bifurcations among those critical points. We focus mainly on a bump and periodic types of heterogeneity, however our approach is also applicable to general cases. It should be noted that there appears to be spatio-temporal chaos for a periodic type of heterogeneity when its period becomes comparable with the size of the pulse.
Equation of state using scaled binding energy model
Authors: Bhattacharya Chandrani , Srivastava M. K.
Published: September 2007

Global equation of state models are currently used for hydrodynamic simulations of inertial confinement fusion systems. The quotidian equation of state (QEOS) is one of such models. In spite of a sound theoretical basis, QEOS uses two empirical corrections for obtaining agreement with measured shock wave data. While the first is used for all materials, the second is to account for structural phase transitions. Evaluation of these corrections requires a priori knowledge of experimental data. Two improvements to QEOS proposed in this article obviate the use of both corrections. First, a modified version of the universal scaled binding energy is used for the zero-temperature isotherm. Then, an expression is derived for the Grüneisen parameter γ(ρ), which also includes the noncentral features of interparticle potential. The Debye temperature and melting temperature, deduced from γ(ρ), show excellent agreement with experimental results or electronic structure calculations. Predictions of the improved EOS model also compare very well with measured Hugoniot for normal metals such as Al, Cu, Pd, Pt, etc., and Fe and Zr, which undergo shock induced phase transitions.
Linear Wegner estimate for alloy-type Schrödinger operators on metric graphs
Authors: Helm Mario , Veselić Ivan
Published: September 2007

We study spectra of alloy-type random Schrödinger operators on metric graphs. For finite edge subsets we prove a Wegner estimate which is linear in the volume (i.e., the total length of the edges) and the length of the energy interval. The single site potential needs to have fixed sign; the metric graph does not need to have a periodic structure. A further result is the existence of the integrated density of states for ergodic random Hamiltonians on metric graphs with a Zν structure. For certain models the two above results together imply the Lipschitz continuity of the integrated density of states.
Characterization of Si nanostructures using a noncontact mode scanning near-field optical Raman microscope, with 100nm spatial resolution and 5nm depth resolution, using ultraviolet resonant Raman scattering
Authors: Yoshikawa M. , Murakami M. , Ishida H.
Published: September 2007

The authors developed a noncontact mode scanning near-field Raman microscope (NC-SNORM) with a hollow pyramidal probe, using ultraviolet resonant Raman scattering, measured the stress distribution in very-large-scale integration standards made of silicon dioxide (SiO2) film and Si, and characterized Si nanodot structures. The authors found that the areas covered by SiO2 are under tensile stress and the areas not covered by SiO2 are under compressive stress. Compressive stress concentrates on the interface, about 100nm wide, between the covered and noncovered areas. They measured near-field Raman spectra of the Si nanodot structures and found that the Raman intensities change periodically approximately every 100nm.
Zero kinetic energy spectroscopy of hydroquinone-water (1:1) complex: A probe for conformer assignment
Authors: Chakraborty S. , Misra P. , Wategaonkar S.
Published: September 2007

Zero kinetic energy (ZEKE) photoelectron spectroscopy of the hydroquinone-water (HQW) complex was carried out to characterize its S1-S0 resonantly enhanced multiphoton ionization (REMPI) spectrum in terms of the cis and trans conformers. The ZEKE spectra of the hydroquinone isomers show differences in the Franck-Condon (FC) activity of a few ring modes, viz., modes 15, 9b, and 6b, due to the different symmetries of the two isomers. These modes were used as a “diagnostic tool” to carry out the categorical assignment of the REMPI spectrum of the HQW complex. It was found that the FC activity of these diagnostic modes in the cationic ground state (D0) of the water complex is similar as that of the monomer. The two lowest energy transitions in the REMPI spectrum of the water complex, 33 175 and 33209cm−1, were reassigned as the band origins of the cis and trans hydroquinone-water complexes, which is opposite of the previous assignment. The intermolecular stretching mode (σ) of the complex shows a long progression, up to v′=4, in the cationic ground state and is strongly coupled to other observed ring modes. The Franck-Condon factors for different members in the progression were calculated using the potential energy surfaces computed ab initio. These agree well with the observed intensity patterns in the progression. The ionization potential of the trans and cis complexes was determined to be 60071±4 and 60024±4cm−1, respectively.
Maximally complex simple attractors
Authors: Sprott J. C.
Published: September 2007

A relatively small number of mathematically simple maps and flows are routinely used as examples of low-dimensional chaos. These systems typically have a number of parameters that are chosen for historical or other reasons. This paper addresses the question of whether a different choice of these parameters can produce strange attractors that are significantly more chaotic (larger Lyapunov exponent) or more complex (higher dimension) than those typically used in such studies. It reports numerical results in which the parameters are adjusted to give either the largest Lyapunov exponent or the largest Kaplan-Yorke dimension. The characteristics of the resulting attractors are displayed and discussed.
Annealing properties of open volumes in strained SiN films studied by monoenergetic positron beams
Authors: Uedono A. , Ito K. , Narumi T. , Sometani M. , Yamabe K. , Miyagawa Y. , Murata T. , Honda K. , Hattori N. , Matsuura M. , Asai K. , Ohdaira T. , Suzuki R.
Published: September 2007

The effect of annealing on open volumes in strained SiN films deposited on Si by plasma enhanced chemical vapor deposition was studied using monoenergetic positron beams. For compressive SiN, the stress was reduced by postdeposition annealing; this effect was attributed to the relaxation of matrix structures accompanied by an expansion of small open spaces intrinsically existing in the matrix and the introduction of large open volumes. For tensile SiN, although annealing tends to decrease the concentration of large open volumes, the size of the small open spaces and the film stress were almost constant up to 1000°C annealing. This was attributed to the network structure related to the open spaces remaining stable even at 1000°C annealing, and this mainly determines the stress in the tensile film.
Spatial periodicity of spatially evolving turbulent flow caused by inflow boundary condition
Authors: Nikitin Nikolay
Published: September 2007

Space-time evolution of a flow in a semi-infinite circular pipe {x⩾0,y2+z2⩽R2} is investigated via direct numerical simulations. The unsteady turbulent inflow velocity u∣x=0 is extracted from a streamwise-periodic auxiliary simulation. It is shown that at t≫0 spatially evolving flow does not depend on initial state u∣t=0. It takes the form of spatially periodic flow of the auxiliary simulation with superimposed spatially growing perturbation. The amplitude of the perturbation increases exponentially with distance from the inlet. The growth rate is estimated for the considered Reynolds number. The obtained results suggest that the memory of the inflow boundary condition in spatial simulations is evident for a significant distance from the inlet. In the considered case an almost periodic flow is established (with less than 1% relative deviation from the periodicity) over a distance as large as 70 pipe radii.
Gyrotropic linear and nonlinear motions of a magnetic vortex in soft magnetic nanodots
Authors: Lee Ki-Suk , Kim Sang-Koog
Published: September 2007

The authors investigated the gyrotropic linear and nonlinear motions of a magnetic vortex in soft magnetic cylindrical nanodots under in-plane oscillating magnetic fields of different frequencies and amplitudes, by employing both micromagnetic simulations and the numerical solutions of Thiele’s equation of motion [Phys. Rev. Lett. 30, 230 (1973)]. Not only noncircular elliptical vortex-core orbital trajectories in the linear regime but also complex trajectories including stadiumlike shape in the nonlinear regime were observed from the micromagnetic simulations and were in excellent agreement with the numerical solutions of the analytical equations of motion. It was verified that the numerical solutions of Thiele’s equation are promisingly applicable in order to predict and describe well such complex vortex gyrotropic linear and nonlinear motions in both the initial transient and later steady states. These results enrich the fundamental understanding of the linear and nonlinear motions of vortices in confined magnetic elements in response to oscillating driving forces.
Taylor cones of ionic liquids from capillary tubes as sources of pure ions: The role of surface tension and electrical conductivity
Authors: Garoz D. , Bueno C. , Larriba C. , Castro S. , Romero-Sanz I. , Fernandez de la Mora J. , Yoshida Y. , Saito G.
Published: September 2007

The emissions of Taylor cones from a wide range of ionic liquids (ILs) have been tested in vacuo in an attempt to identify what physical properties favor the purely ionic regime (PIR). This regime is well known in the case of Taylor cones of liquid metals. For nonmetallic liquids, it has been previously observed in conventional (capillary tube) electrospray sources at room temperature only for the room temperature molten salt (ionic liquid) EMI–BF4 (EMI=1-ethyl-3-methylimidazolium). A large number of other ILs and their mixtures have been studied here, most of which (but not all) are unable to reach the PIR at room temperature. Based on these results and additional theoretical considerations, strong support is assembled for the notion that the PIR is favored by ILs not only of high electrical conductivity but also of high surface tension. This hypothesis is confirmed by tests with three recently synthesized ILs, EMI–GaCl4, EMI–C(CN)3, and EMI–N(CN)2, all of which combine exceptional surface tension and electrical conductivity, and all of which reach the PIR at room temperature far more readily than EMI–BF4.
Magnetoexcitons in near-surface symmetric and asymmetric double quantum wells: Optical properties
Authors: Flores-Desirena B. , Pérez-Rodríguez F.
Published: September 2007

We present a theoretical study of the optical properties of excitons in symmetric and asymmetric bulk and near-surface double quantum wells (DQWs) under the action of an external magnetic field oriented along the growth direction. The coupling of magnetoexcitons with light in the semiconductor heterostructure is described with Stahl’s real-space density-matrix approach. After solving the system of equations for the coherent wave amplitude and the electromagnetic fields, we calculate optical spectra (reflectivity and absorption) for specific GaAs/AlGaAs DQWs. Our results for the absorption of bulk symmetric DQWs are compared with those obtained by Vera and Barticevic [F. Vera and Z. Barticevic, J. Appl. Phys. 83, 7720 (1998)]. The effect of the surface-magnetoexciton interaction upon the reflectivity and absorption for near-surface DQWs is analyzed. It is established that such an interaction alters the symmetry or asymmetry of the DQWs, and therefore, the resonance structure of their optical spectra is noticeably modified as the thickness of the cap layer is reduced.
Measurement and modeling of Ar∕H2∕CH4 arc jet discharge chemical vapor deposition reactors. I. Intercomparison of derived spatial variations of H atom, C2, and CH radical densities
Authors: Rennick C. J. , Ma J. , Henney J. J. , Wills J. B. , Ashfold M. N. R. , Orr-Ewing A. J. , Mankelevich Yu. A.
Published: September 2007

Comparisons are drawn between spatially resolved absorption spectroscopy data obtained for a 6.4kW dc arc jet reactor, operating with Ar∕H2∕CH4 gas mixtures, used for deposition of thin, polycrystalline diamond films, and the results of a two-dimensional (r,z) computer model incorporating gas activation, expansion into the low pressure reactor, and the chemistry of the neutral and charged species. The experimental measurements, using either cavity ring-down spectroscopy or diode laser absorption spectroscopy, determined absolute number densities of H(n=2) atoms, and column densities of C2(aΠu3), C2(XΣg+1), and CH(XΠ2) radicals, with vibrational and rotational quantum state resolutions, and their variation with height through the horizontally propagating arc jet plume. Spectra were also analyzed to obtain temperatures and local electron densities [from Stark broadening of H(n=2) absorption lines]. The experimental data are directly compared with the output data of the model that returns spatially inhomogeneous temperature, flow velocities, and number densities of 25 neutral and 14 charged species. Under the base operating conditions of the reactor [11.4SLM (standard liters per minute) of Ar and 1.8SLM of H2 entering the primary torch, with addition of 80SCCM (SCCM denotes cubic centimeter per minute at STP) of CH4 downstream; 6.4kW input power; reactor pressure of 50Torr], the calculated and measured column and number densities agree to within factors of 2–3, the model reproduces the spatial dependence of column densities, and the mean temperatures of C2(a) and CH(X) radicals derived from spectra and model results are in good agreement. The model also captures the variation of these parameters with changes to operating conditions of the reactor such as flows of H2 and CH4, and input power. Further details of the model and the insights it provides are the subject of the accompanying paper [Mankelevich et al., J. Appl. Phys. 102, 063310 (2007) ].
Measurement and modeling of Ar∕H2∕CH4 arc jet discharge chemical vapor deposition reactors II: Modeling of the spatial dependence of expanded plasma parameters and species number densities
Authors: Mankelevich Yu. A. , Ashfold M. N. R. , Orr-Ewing A. J.
Published: September 2007

Detailed methodology and results are presented for a two-dimensional (r,z) computer model applicable to dc arc jet reactors operating on argon/hydrogen/hydrocarbon gas mixtures and used for chemical vapor deposition of micro- and nanocrystalline diamond and diamondlike carbon films. The model incorporates gas activation, expansion into the low pressure reactor chamber, and the chemistry of the neutral and charged species. It predicts the spatial variation of temperature, flow velocities and number densities of 25 neutral and 14 charged species, and the dependence of these parameters on the operating conditions of the reactor such as flows of H2 and CH4 and input power. Selected outcomes of the model are compared with experimental data in the accompanying paper [C. J. Rennick et al., J. Appl. Phys. 102, 063309 (2007)]. Two-dimensional spatial maps of the number densities of key radical and molecular species in the reactor, derived from the model, provide a summary of the complicated chemical processing that occurs. In the vortex region beyond the plume, the key transformations are CH4→CH3↔C2H2↔large hydrocarbons; in the plume or the transition zone to the cooler regions, the chemical processing involves C2Hx↔(CHyandCHz), C3Hx↔(CHyandC2Hz), (C2HyandC2Hz)↔C4Hx↔(CHyandC3Hz). Depending on the local gas temperature Tg and the H∕H2 ratio, the equilibria of H-shifting reactions favor C, CH, and C2 species (in the hot, H-rich axial region of the plume) or CH2, C2H, and C2H2 species (at the outer boundary of the transition zone). Deductions are drawn about the most abundant C-containing radical species incident on the growing diamond surface (C atoms and CH radicals) within this reactor, and the importance of chemistry involving charged species is discussed. Modifications to the boundary conditions and model reactor geometry allow its application to a lower power arc jet reactor operated and extensively studied by Jeffries and co-workers at SRI International, and comparisons are drawn with the reported laser induced fluorescence data from these studies.
Ion-beam-induced phase transformations in δ-Sc4Zr3O12
Authors: Ishimaru Manabu , Hirotsu Yoshihiko , Tang Ming , Valdez James A. , Sickafus Kurt E.
Published: September 2007

Structural changes in ion-beam-irradiated rhombohedral Sc4Zr3O12 (δ-Sc4Zr3O12) have been examined using transmission electron microscopy (TEM). Polycrystalline δ-Sc4Zr3O12 samples were irradiated at cryogenic temperature with 300keV Kr2+ ions to a fluence of 3×1016 Kr∕cm2 (equivalent to a peak dose of ∼70 displacements per target atom). High-resolution TEM and nanobeam electron diffraction experiments revealed a phase transformation to another ordered crystalline phase in the near-surface region of the irradiated sample. We propose an atomistic model for this crystalline phase, based on the bixbyite structure, and discuss its formation process. The phase transformation (occurring during irradiation) that produces this bixbyite structure is unusual in the fact that a more highly ordered structure is the product of the transformation.
Surface influence on stability and structure of hexagon-shaped III-V semiconductor nanorods
Authors: Leitsmann R. , Bechstedt F.
Published: September 2007

We report ab initio investigations of hexagon-shaped, [111]∕[0001]-oriented III-V semiconductor nanowires with varying crystal structure, surface passivation, surface orientation, and diameter. Their stability is dominated by the free surface energies of the corresponding facets, which differ only weakly from those of free surfaces. We observe a phase transition between local zinc-blende and wurtzite geometry versus preparation conditions of the surfaces, which is accompanied by a change in the facet orientation. The influence of the actual III-V compound remains small. The atomic relaxation of nanowires gives rise to smaller bond lengths in comparison to the bulk zinc-blende structures, resulting in somewhat reduced bilayer thicknesses parallel to the growth direction.
Artificial introduction of defects into vertically aligned multiwalled carbon nanotube ensembles: Application to electrochemical sensors
Authors: Nichols Jeffrey A. , Saito Hiroaki , Deck Christian , Bandaru Prabhakar R.
Published: September 2007

Carbon nanotubes (CNTs) inevitably contain defects that exert a significant influence on their physical, electrical, and electrochemical properties. In this study, we subject vertically aligned multiwalled CNT ensembles to argon and hydrogen ion irradiation, to artificially introduce defects into the structure. Subsequently, Raman spectroscopy in conjunction with electrochemical analyses was used to characterize the amount and nature of disorder within the CNTs. While an increased disorder with ion irradiation was generally observed, argon and hydrogen exhibit different effects on the Raman intensity spectra. Argon irradiation seems to cause charged defects, e.g., in the form of dangling bonds, and increases the in-plane correlation length (La), while hydrogen irradiation passivates residual defects and decreases the La. It was noted that hydrogen treated CNTs could serve as electrochemical sensors with quicker response time.
Ultralow k using a plasma enhanced chemical vapor deposition porogen approach: Matrix structure and porogen loading influences
Authors: Favennec Laurent , Jousseaume Vincent , Gerbaud Guillaume , Zenasni Aziz , Passemard Gérard
Published: September 2007

To improve integrated circuit performance, microelectronic chip interconnections need dielectric materials with ultralow k (ULK) values. Porous a-SiOCH, an ULK material, can be created using a two step strategy called a porogen approach. The first step consists of a hybrid film deposition, which is an a-SiOCH matrix containing an organic sacrificial phase. Afterwards, the organic phase is removed during a post-treatment to generate the porosity. In this work, hybrid deposition was performed by plasma enhanced chemical vapor deposition and the post-treatment was a thermal annealing. Firstly, hybrid films with different a-SiOCH matrix structures were created using two matrix precursors [decamethylcyclopentasiloxane (DMCPS) and diethoxymethylsilane (DEMS)] and an O2 addition in a plasma gas feed. For the same porogen loading, the shrinkage behavior during the porogen removal is correlated to the matrix structure. Fourier transform infrared spectroscopy and Si29 solid nuclear magnetic resonance are both used to determine the structure. The O3SiMe1 is favorable to prevent high film shrinkage and the O2SiMe2 leads to high film shrinkage and absence of porosity generation. To prevent the existence of this type of structure, DEMS+O2 appears more appropriate as a matrix precursor than DEMS only or DMCPS. Secondly, using the appropriate matrix structure, the influence of the porogen loading on the porosity creation is studied. Hybrids with different porogen loadings are achieved by changing the porogen precursor ratio in the plasma gas feed. The porogen conversion into porosity is studied for different porogen loadings and the results indicate the existence of a porogen loading threshold. Above it, there is no more porosity generation because of the too high film shrinkage during the porogen removal. This behavior is explained by a too low matrix ratio in the hybrid film. For the high porogen loadings, the matrix skeleton (mainly constituted by SiOSi bridging bonds) is not in sufficient quantity to prevent such film shrinkage. The porous a-SiOCH created with the most favorable matrix structure and porogen loading has the ultralow k value of 2.3.
Three-dimensional synchrotron x-ray particle image velocimetry
Authors: Fouras A. , Dusting J. , Lewis R. , Hourigan K.
Published: September 2007

There is great potential to vastly improve biological flow measurement by using a combination of synchrotron imaging and the latest experimental flow measurement techniques. In the current paper, the three-dimensional velocity field within a cylindrical tube is measured using a combination of phase-contrast x-ray imaging and particle image velocimetry (PIV). We greatly refine the techniques previously used to undertake velocity measurements with a synchrotron light source, substantially enhancing accuracy. Furthermore a PIV correlation peak analysis is developed to allow three-dimensional measurement of the velocity field.
Spectral properties of phase-locked flux flow oscillator
Authors: Pankratov Andrey L. , Vaks Vladimir L. , Koshelets Valery P.
Published: September 2007

In the present paper, the results of theoretical modeling of flux flow oscillator (FFO) included into phase-locked loop (PLL) system and its comparison with experiment are outlined. To describe theoretically the dynamics of phase-locked FFO, we have considered two models: first order PLL system and the PLL system with integral-proportional filter. While the first order PLL system may be treated analytically even in the frame of nonlinear PLL model and gives satisfactory agreement with experimental results, it does not describe all peculiarities of spectral density, which may be described well by a more sophisticated model with the integral-proportional filter. The quantitative prediction of spectral ratio improvement due to increase of PLL regulation bandwidth is given.
Kerr microscopy studies of microwave assisted switching
Authors: Martín Pimentel P. , Leven B. , Hillebrands B. , Grimm H.
Published: September 2007

We study microwave assisted magnetization reversal of elliptical Ni81Fe19 thin film elements employing Kerr microscopy. A microwave field is applied along the short axis of the element and orthogonally to a quasistatic magnetic field, which is parallel to the long axis of the element. The reversal process is characterized by the formation of a complex multidomain structure. We demonstrate that the reversal process in total is compressed by applying a sufficiently high microwave field, which assists to overcome the effective energy barrier of the domain nucleation, and thus reduces the coercive field.
Effect of molecular interactions on carbon nanotube friction
Authors: Heo SeongJun , Sinnott Susan B.
Published: September 2007

The mechanical responses of carbon nanotubes (CNTs) under compressive and sliding forces are examined using classical molecular dynamics simulations. Several different types of nanotube systems are considered, including pristine single-walled tubes that are empty, filled with fullerenes to form nanopeapods, and filled with other nanotubes to form multiwalled tubes. In addition, the responses of systems where benzene molecules are introduced between and around the walls of single-walled nanotubes and peapods are considered. The results indicate that the filled CNT bundles can sustain higher compressive forces than the hollow CNT bundles. However, the filled CNT bundle has a similar friction coefficient to the hollow CNT bundles during sliding at low compressive pressures. The simulations further demonstrate the way in which the introduction of additive lubricants influences the friction coefficient of CNT bundles.
Stability of nanocrystalline silicon bottom-gate thin film transistors with silicon nitride gate dielectric
Authors: Esmaeili-Rad Mohammad R. , Li Flora , Sazonov Andrei , Nathan Arokia
Published: September 2007

We report on the stability of nanocrystalline silicon (nc-Si) bottom-gate (BG) thin film transistors (TFTs) with various compositions of hydrogenated amorphous silicon nitride (a-SiNx:H) gate dielectric. TFTs with nitrogen-rich nitride exhibit higher output transconductance, threshold voltage stability, and effective field effect mobility (μFE) than the devices with silicon-rich gate dielectric. For example, μFE drops from 0.75to0.2cm2∕Vs when the gate dielectric composition [N]∕[Si] changes from 1.3 to 1. The corresponding threshold voltages (VT) are 4 and −2V. Following 5h electrical stress tests, the shift in threshold voltage (ΔVT) is larger for dielectrics with lower [N]∕[Si] content, regardless of the operating regime. Indeed, ΔVT in the saturation regime is considerably less and correlates with the charge concentration in the channel, i.e., ΔVT in saturation is about 2∕3 of that in the linear regime. Relaxation tests on the stressed TFTs show that the charge trapping is the instability mechanism in nc-Si BG TFTs.
Viscoelastic effects on the free retraction of rubber
Authors: Bogoslovov R. B. , Roland C. M.
Published: September 2007

Wave propagation and retraction velocities were measured for two elastomers, a 1,4-polybutadiene and a polyurea, freely retracting from large tensile strains (⩽2). From these data the stress-strain response was calculated. The achievable strain rate depends on the initial strain and the viscoelasticity of the material, with values exceeding 1800s−1 attained herein. Thus, the method can be used to characterize the mechanical behavior at high strain rates, as well as high strains. A drawback is that the strain rate is not constant during the retraction. The kinetic energy of retraction reflects the unrelaxed stress, providing a straightforward determination of strain energy and its dissipation. The two elastomers represent extremes of viscoelastic behavior, as reflected in the retraction response at both low and high strain rates.
Porous silicon omnidirectional mirrors and distributed Bragg reflectors for planar waveguide applications
Authors: Xifré-Pérez E. , Marsal L. F. , Ferré-Borrull J. , Pallarès J.
Published: September 2007

The use of omnidirectional mirrors (an special case of distributed Bragg reflectors) as cladding for planar waveguides is proposed and analyzed. The proposed structure is an all-porous silicon multilayer consisting of a core layer inserted between two omnidirectional mirrors. The transfer matrix method is applied for the modal analysis. The influence of the parameters of the waveguide structure on the guided modes is analyzed. These parameters are the layer thickness and number of periods of the omnidirectional mirror, and the refractive index and thickness of the core layer. Finally, the confinement of the omnidirectional mirror cladding is analyzed with respect to two other different distributed Bragg reflector claddings.
Charge compensation mechanism for thermo-electric oxidization of lithium niobate crystals
Authors: Falk Matthias , Woike Theo , Buse Karsten
Published: September 2007

The charge compensation mechanism for thermo-electric oxidization of iron-doped lithium niobate crystals is investigated. Iron ions are oxidized, i.e., Fe2+ ions are transformed to Fe3+. It is shown by birefringence and absorption measurements that lithium and hydrogen ions leave the crystal during the thermo-electric oxidization process for charge compensation.
Simulation and measurement of the optical excitation of the S1 zero group velocity Lamb wave resonance in plates
Authors: Balogun O. , Murray T. W. , Prada C.
Published: September 2007

Recent reports on the thermoelastic generation of Lamb waves in isotropic elastic plates show that a laser source efficiently excites a resonance that occurs at the minimum frequency of the first order symmetric (S1) Lamb mode. The group velocity of the Lamb wave goes to zero at this frequency while the phase velocity remains finite, and the resonance is referred to as the S1 zero group velocity (S1 ZGV) resonance. The S1 ZGV resonance can be employed for the nondestructive evaluation of the elastic properties of plates or plate thickness. A model for the generation of elastic waves in plates using an intensity-modulated continuous wave laser source is developed and used to study the behavior of the S1 ZGV resonance. The effects of the laser source parameters on the generation of the S1 ZGV resonance are explored, and the spatial distribution of the displacement produced at the resonance frequency is determined. The predicted displacement spectrum of Lamb waves generated in micron scale plates is found to compare well with experimental measurements. In addition, experimental measurements demonstrate that the S1 ZGV resonance can be used to map subsurface features in thin (4μm) membranes at high ultrasonic frequencies (700MHz).
Magnetic relaxation and dissipative heating in ferrofluids
Authors: Vaishnava P. P. , Tackett R. , Dixit A. , Sudakar C. , Naik R. , Lawes G.
Published: September 2007

We have investigated the ac magnetic susceptibility and magnetic heating of aqueous suspensions of γ-Fe2O3 nanoparticles embedded in alginate hydrogel matrix and isolated γ-Fe2O3 and Fe3O4 nanoparticles coated with tetramethyl ammonium hydroxide. All three ferrofluids were characterized by measuring the dc magnetization, ac susceptibility, and magnetic heating. We found that significant Néel relaxation is present in all samples, but only the isolated nanoparticle ferrofluids show any significant feature associated with Brownian relaxation near the freezing temperature of the carrier liquid. The heating rate of the ferrofluids varies systematically with the magnitude of the Brownian relaxation peak, despite similar values of the absolute magnetization. These results highlight the importance of the Brownian relaxation for heating applications incorporating magnetic nanoparticles.