bluuebunny/crossref_metadata_2025 huggingface dataset:
Upgrades to the TEXTOR electron cyclotron emission imaging diagnostic
Authors: Domier C. W. , Xia Z. G. , Zhang P. , Luhmann N. C. , Park H. K. , Mazzucato E. , van de Pol M. J. , Classen I. G. J. , Donné A. J. H. , Jaspers R.
Published: November 2006

A 128 channel electron cyclotron emission imaging instrument has been routinely used to study magnetohydrodynamics physics such as m=1 and m=2 modes on the TEXTOR tokamak. As currently configured, each of the 16 mixer array elements measures plasma emission at 8 simultaneous frequencies to form a 16×8 image of electron temperature profiles and fluctuations over an area of 16cm (vertical) by 6cm (horizontal). A redesigned mixer array, coupled with new wideband electronics to be installed later this year, will increase the plasma coverage to 17cm(v)×9cm(h). The new arrangement offers increased temperature resolution together with new gain and video bandwidth controls in a highly modular configuration for ease of maintenance and facilitation of future upgrades both in frequency coverage as well as number of channels.
Electronic structure and optical properties of rare earth sesquioxides (R2O3, R=La, Pr, and Nd)
Authors: Singh Nirpendra , Saini Sapan Mohan , Nautiyal Tashi , Auluck Sushil
Published: November 2006

The electronic structure and optical properties of the rare earth sesquioxides (R2O3, R=La, Pr, and Nd) have been studied using the full potential linearized augmented plane wave method within local spin density approximation (LSDA) and Coulomb corrected local density approximation (LSDA+U) as implemented in the WIEN2K code. Our calculations show that the LSDA+U results give a better representation of the band structure compared to LSDA. Our calculated reflectivity and optical conductivity spectra are compared with the experimental data. The peaks in the optical conductivity can be explained as arising out of the transitions from R-p and O-p to R-d states. We have also calculated the dielectric function and the coefficient of refraction for these rare earth sesquioxides. We do not find any significant differences in the optical properties calculated using LSDA and LSDA+U.
On the systematics of particle velocity histories in the shock-to-detonation transition regime
Authors: James H. R. , Lambourn B. D.
Published: November 2006

Analysis of recent high quality, in-material gauge results from two cyclotetramethylene tetranitramine based explosives and one triamino trinitrobenzene based explosive has shown a number of significant correlations. These include the strong monotonic relationship between the local shock strength and the time to peak particle velocity along each particle path, and the simple scaling of velocity histories along the particle path that exists at a common local shock strength from shots with different initial conditions. Even shocks that have radically different evolutions, such as double shocks or those arising from thin pulses, show the same correlations once the catch-up of the second shock or rarefaction has occurred. From the correlations the strongest relationship is demonstrated to occur between the reaction and the local shock strength. Hence reaction, at least to first order, is a function of shock strength and time along the particle path, and is independent of local flow variables behind the shock such as pressure and temperature. Arguments are presented to suggest that shock entropy is the most likely measure of the shock strength which controls the reaction.
Development of electron reflection suppression materials for improved thermionic energy converter performance using thin film deposition techniques
Authors: Islam Mohammad , Inal Osman T. , Luke James R.
Published: November 2006

Nonideal electrode surfaces cause significant degree of electron reflection from collector during thermionic converter operation. The effect of the collector surface structure on the converter performance was assessed through the development of several electron reflection suppression materials using various thin film deposition techniques. The double-diode probe method was used to compare the J-V characteristics of converters with polished and modified collector surfaces for emitter temperature and cesium vapor pressure in the ranges of 900–2000K and 0.02–1.5torr, respectively. The coadsorption of cesium and oxygen with respective partial vapor pressures of ∼1.27torr and a few microtorrs reduced the emitter work function to a minimum value of 0.99eV. It was found that the collector surfaces with matte black appearance such as platinum black, voided nickel from radio-frequency plasma sputtering, and etched electroless Ni–P with craterlike pore morphology exhibited much better performance compared with polished collector surface. For these thin films, the increase in the maximum output voltage was up to 2.0eV. For optimum performance with minimum work function and maximum saturation emission current density, the emitter temperature was in the range of 1100–1500K, depending on the collector surface structure. The use of these materials in cylindrical converter design and/or in combination with hybrid mode triode configuration holds great potential in low and medium scale power generators for commercial use.
Calcium fluoride windows for high-energy chemical lasers
Authors: Klein Claude A.
Published: November 2006

The development of high-energy lasers requires optical windows capable of handling megajoule beam energies without compromising the system’s performance. Calcium fluoride (CaF2) has been identified as a prime candidate for windows operating at chemical laser wavelengths due to very low bulk absorption and exceptionally small thermal lensing coefficients; it is, however, vulnerable to structural failure owing to poor mechanical strength characteristics and a large thermal stress factor. It is, therefore, essential to properly assess the ultimate potential of this material, which we attempt to do here in the following manner: (a) We assemble reliable numbers for all pertinent properties of (111)-oriented CaF2 single crystals and polycrystalline isotropic aggregates (PIAs), such as fusion-cast CaF2, which requires addressing issues relating to the elastic properties, the stress-optic coefficients, and the flexural strength. (b) We provide correct analytical expressions for evaluating the impact of pressure- and beam-induced effects on wave-front phase distortions and mechanical failure modes, taking advantage of a previous investigation [J. Appl. Phys. 98, 043103 (2005)]. (c) We perform detailed calculations on “model” windows made of either (111)CaF2 or (PIA)CaF2 that transmit optimally truncated Gaussian beams at wavelengths of 1.15 and 3.39μm, for run times such that lateral heat conduction and surface cooling can be ignored. Our main conlusions are as follows: (a) With CaF2 windows thermal lensing, as measured in terms of the Strehl ratio and on assuming coating absorptances of no more than 3×10−5, is of no consequence in the sense that catastrophic failure may occur at fluence levels way below the threshold for optical distortion. (b) Evidence of a poor Weibull shape factor (m≃3.5) degrades the design safety margins, which requires operating at peak intensities of no more than 100kW∕cm2 to achieve optimum on-target fluences. (c) Regarding the issue of (111)CaF2 vs (PIA)CaF2, we note that fusion-cast material outperforms single crystals based on the figure of merit for distortion, as well as fracture and yield strengths, but contrary to (111)-oriented material, it exhibits birefringence that may rule out its use if depolarization is of concern.
Inertialess temporal and spatio-temporal stability analysis of the two-layer film flow with density stratification
Authors: Hu J. , Millet S. , Botton V. , Ben Hadid H. , Henry D.
Published: November 2006

This paper presents a temporal, spatial, and spatio-temporal linear stability analysis of the two-layer film flow down a plate tilted at an angle θ. It is based on a zero Reynolds number approximation to the Orr-Sommerfeld equations and a zero surface tension approximation to both surface boundary conditions. The combined effects of density and viscosity stratifications are systematically investigated. The subtle influence of density stratification is first put into light by a temporal analysis for θ=0.2; when increasing/decreasing the density ratio (upper fluid/lower fluid), the two-layer film flow becomes much more unstable/stable with respect to the finite wavelength instability. Moreover, below a critical density ratio this finite wavelength instability even disappears, whatever the viscous ratio. Concerning the long wave instability, it becomes dominant when decreasing the density ratio below 1 and is even triggered in a region which was stable for equal density layers. The spatio-temporal analysis shows that the instability is convective for incline angles that are not too small as θ=0.2. The study of the local growth rates of the spatio-temporal instability as a function of the ray velocity V shows that there is a transition between long wave and short wave instabilities which has been determined by using the Briggs-Bers collision criterion. Accordingly, there exists a jump for the local oscillatory frequency, spatial amplification rate, and spatial wave number due to this transition. Due to the existence of the absolute Rayleigh-Taylor instability for θ=0, the transition from convective to absolute instability can be detected for values of θ smaller than 0.2, and absolute/convective instability boundary curves have been obtained for varying characteristic parameters.
Terahertz absorption spectrum of para and ortho water vapors at different humidities at room temperature
Authors: Xin X. , Altan H. , Saint A. , Matten D. , Alfano R. R.
Published: November 2006

Terahertz time-domain spectroscopy has been used to measure the absorption of water vapor in 0.2–2.4THz range from low to high humidity at room temperature. The observed absorption lines are due to the water molecular rotations in the ground vibrational state. We find that the absorption strength of para transitions increases as humidity increases, while the absorption strength of ortho transitions increases and then decreases in intensity with increasing humidity. We explain this difference based on the nuclear spin statistics based ratio of ortho to para water monomer populations at room temperature. The preferential adsorption on the solid surfaces of para water leads to an ortho dominated vapor cloud whose monomer rotational absorption intensity decreases due to the effects of dimerization, molecular collisions, clustering, and interactions with liquid droplets at high concentrations.
Nanoscale banded 90° domain patterns in (111) oriented (Pb,La)TiO3 thin films
Authors: Liu Hong , Pu Zhaohui , Xiao Dingquan , Zhu Jianguo , Wang Zhihong , Huang Huidong , Li Yanrong
Published: November 2006

Lanthanum doped lead titanate ferroelectric thin films were grown on Pt∕Ti∕SiO2∕Si(100) substrates using rf magnetron sputtering. X-ray diffraction patterns show that the main phase of the films is tetragonal perovskite with preferred orientation of (111) perpendicular to the substrate. We investigated the ferroelectric domain patterns and the corresponding polarization arrangements of the films using piezoresponse force microscopy. Head-to-tail polarization configurations were often found to exist, indicating that the low electrical energy configurations are quite common in these thin films. Nanoscale banded 90° a-a domain patterns as small as 30–60nm in width were observed. The banded domains alternated with adjacent lamellae can be attributed to mechanical strains at the surface and at the interface between the substrate and the film.
Realization of ultrawide omnidirectional photonic band gap in multiple one-dimensional photonic crystals
Authors: Lee Hyun-Yong , Nam Gi-Yeon
Published: November 2006

Theoretical and experimental investigations have shown that the omnidirectional photonic band gap (omni-PBG) of one-dimensional photonic crystals (1D PCs) can be enlarged by including multiple-periodic structures. These structures, consisting of Si–SiO2 alternating layers with multiple periods, Λ1, Λ2,…,ΛM, have great potential for ultrawide omnireflectors operating in the infrared frequency range. To confirm the effect of multiple periods in the 1D PCs, we prepared two types of 16-pair, Si∕SiO2 1D PCs with a single period of Λ1=426.9nm and Λ2=306.9nm, and one type of 16-pair 1D PC with double periods—the combination of eight-pair Λ1 and eight-pair Λ2. Theoretically, the normalized frequency range for omni-PBG (Δω) in double PC is enhanced by approximately twice that in the single PC. That is, Δω increases from 0.086 to 0.166, which corresponds to the wavelength range (Δλ) from 520to980nm for Λ=407.7nm. Measured reflectance (R) spectra are in a good agreement with the calculated results. For example, the R spectrum of Si∕SiO2 double 1D PC (8-pair Λ1+8-pair Λ2: Λ1=426.9nm, Λ2=306.9nm, and filling factor η=0.406) exhibits an ultrawide PBG over the wavelength range of 1050–2500nm for TE polarization at the incident angle of 5°.
Equilibrium hydrodynamics concept for developing dunes
Authors: Coleman S. E. , Nikora V. I. , McLean S. R. , Clunie T. M. , Schlicke T. , Melville B. W.
Published: November 2006

Experiments utilizing two-dimensional fixed dune profiles and varying flow depth (dune regime flows) highlight the equilibrium (self-similar) nature of the near-bed boundary layer over developing dunes with flow separation in the dune lee. The negligible variation in roughness layer (comprising the interfacial and form-induced layers) flow structure for developing dunes was confirmed in terms of spatial fields of time-averaged velocities and stresses; and vertical distributions of: (a) double-averaged (in time and space) longitudinal velocity, (b) double-averaged normal stresses, and (c) the components of the momentum balance for the flow. The finding of an equilibrium nature for the near-bed flow over developing dunes is significant in its centrality to understanding the feedback loop between flow, bed morphology, and sediment transport that controls erodible-bed development. Further research is required into the form of the distribution of double-averaged velocity in the form-induced layer above roughness tops, and also to complete generalization for varying dune steepness of the universal expression for double-averaged longitudinal velocity (varying linearly with elevation) determined herein for the interfacial layer (below roughness tops). Work is presently focusing on the additional effects on flow structure due to sediment transport and three-dimensional flow and bed morphology, although it is expected that the equilibrium boundary layer flow structure patterns identified herein will still be evident for these more complex systems.
Parameter determination from resistance-voltage curve for long-wavelength HgCdTe photodiode
Authors: Quan Z. J. , Li Z. F. , Hu W. D. , Ye Z. H. , Hu X. N. , Lu W.
Published: November 2006

A data-processing approach has been developed to obtain device parameters from resistance-voltage (R-V) curves measured on long-wavelength HgCdTe n-on-p photodiodes. The physical model used for R-V curve fitting includes the dark current mechanisms induced by diffusion, generation recombination, trap-assisted tunneling, and band-to-band tunneling. Moreover, the series resistance effect is also taken into account. Six parameters, which include the dopant density Nd in the n region, the ratio of mobility to lifetime of electrons μn∕τn in the p region, the effective lifetime τ0 in the depletion region, the relative energy position of trap level Et∕Eg and its density Nt in the depletion region, and the series resistance Rs, can be extracted from measured R-V curves. The fitting procedure has been presented in detail and the error ranges of the extracted parameters have been discussed. By fitting to the R-V characteristics of three long-wavelength devices with different Cd compositions, the applicability of our data-processing approach has been verified for obtaining those basic parameters.
Smectic order parameters from diffusion data
Authors: Cifelli Mario , Cinacchi Giorgio , De Gaetani Luca
Published: November 2006

Microcanonical molecular dynamics simulations have been performed in the smectic A phase of an elementary liquid-crystal model. Smectic order parameters and diffusion coefficients along directions parallel and perpendicular to the director have been calculated during the same trajectory for a number of state points. This has permitted the satisfactory testing of a procedure, adopted in the analysis of experimental self-diffusion coefficients, leading to an estimate of the temperature dependence of the smectic order parameters. This methodology has been then confidently applied to two smectogenic thermotropic liquid crystals belonging to the 4,4′-di-n-alkyl-azoxybenzene series. The derived smectic order parameters are larger for the homologue compound with the longest alkyl chains. This is consistent with the well-established increased tendency, for members of a homologue series, to form a smectic phase as their alkyl chains become longer.
Retraction quasi-phase-matching properties of congruent periodically poled LiNbO3 in type-I second harmonic generation
Authors: Uesugi Naoshi , Harada Shyoko , Sasaki Yuzo , Ito Hiromasa
Published: November 2006

The quasi-phase-matching temperature characteristics of congruent periodically poled LiNbO3 (CPPLN) with various grating periods in type-I second harmonic generation were experimentally and theoretically clarified. The fundamental wave in the wavelength range from 1.42to1.75μm was successfully quasi-phase-matched by changing the temperature of a CPPLN with a grating period of 24.5μm from 25to72°C. The retraction quasi-phase-matching property at 72°C was experimentally observed in the 1.6μm wavelength region. Two fundamental waves with different wavelengths were phase matched below the retraction temperature. The fundamental wavelength dependence on the grating period of CPPLN at the retraction temperature was also revealed.
Topological mixing study of non-Newtonian duct flows
Authors: Speetjens Michel , Metcalfe Guy , Rudman Murray
Published: November 2006

Tracer advection of non-Newtonian fluids in reoriented duct flows is investigated in terms of coherent structures in the web of tracer paths that determine transport properties geometrically. Reoriented duct flows are an idealization of in-line mixers, encompassing many micro and industrial continuous mixers. The topology of the tracer dynamics of reoriented duct flows is Hamiltonian. As the stretching per reorientation increases from zero, we show that the qualitative route from the integrable state to global chaos and good mixing does not depend on fluid rheology. This is due to a universal symmetry of reoriented duct flows, which we derive, controlling the topology of the tracer web. Symmetry determines where in parameter space global chaos first occurs, while increasing non-Newtonian effects delays the quantitative value of onset. Theory is demonstrated computationally for a representative duct flow, the rotated arc mixing flow.
Nanotubes in liquids: Effective thermal conductivity
Authors: Strauss Michael T. , Pober Richard L.
Published: November 2006

The thermal conductivities of nanotube and liquid dispersions are described with a simple physical model without any adjustable parameters. The geometric model uses periodic lattices with equal surface-to-surface distances between the nanotubes and their nearest neighbors in all directions. A combination of series model in the direction of the thermal gradient and a parallel model perpendicular to it is used along with averaging over all orientations to derive an effective bulk thermal conductivity. This model closely matches the experimental data to date.
Influence of the Al distribution on the structure, elastic properties, and phase stability of supersaturated Ti1−xAlxN
Authors: Mayrhofer P. H. , Music D. , Schneider J. M.
Published: November 2006

Ti 1 − x Al x N films and/or their alloys are employed in many industrial applications due to their excellent mechanical and thermal properties. Synthesized by plasma-assisted vapor deposition, Ti1−xAlxN is reported to crystallize in the cubic NaCl (c) structure for AlN mole fractions below 0.4–0.91, whereas at larger Al contents the hexagonal ZnS-wurtzite (w) structure is observed. Here we use ab initio calculations to analyze the effect of composition and Al distribution on the metal sublattice on phase stability, structure, and elastic properties of c-Ti1−xAlxN and w-Ti1−xAlxN. We show that the phase stability of supersaturated c-Ti1−xAlxN not only depends on the chemical composition but also on the Al distribution of the metal sublattice. An increase of the metastable solubility limit of AlN in c-Ti1−xAlxN from 0.64 to 0.74 is obtained by decreasing the number of Ti–Al bonds. This can be understood by considering the Al distribution induced changes of the electronic structure, bond energy, and configurational entropy. This may in part explain the large variation of the metastable solubility limit reported in the literature.
The surface structure of ionic liquids: Comparing simulations with x-ray measurements
Authors: Sloutskin E. , Lynden-Bell R. M. , Balasubramanian S. , Deutsch M.
Published: November 2006

The surface-normal electron density profile of an ionic liquid, [bmim][PF6], derived from x-ray reflectivity measurements, is compared with two independent molecular-dynamics simulations. It is shown that a meaningful comparison requires a detailed accounting for both thermal and nonthermal surface roughening effects. The former is due to thermally excited capillary waves, and the latter is due to the molecular zero-point motion and form. These quantities influence very significantly, but differently, the simulated and measured density profiles. Stripping off these effects from both types of profiles yields the intrinsic structure factor of the surface. The simulated intrinsic structure factors are found to deviate considerably from the measured one. The introduction of additional ad hoc surface roughness to the simulated profiles greatly reduces the deviation, however, no physical origin for this effect can be identified. The method employed in this study should prove useful for simulation-experiment comparisons of other liquid surfaces, provided they obey capillary-wave theory, as do almost all liquid surfaces studied to date by x-ray reflectivity.
Evolution equation for bidirectional surface waves in a convecting fluid
Authors: Depassier M. C.
Published: November 2006

Surface waves in a heated viscous fluid exhibit a long-wave oscillatory instability. The non-linear evolution of unidirectional waves is known to be described by a modified Korteweg-deVries-Kuramoto-Sivashinsky equation. In the present work, we eliminate the restriction of unidirectional waves and find that the evolution of the wave is governed by a modified Boussinesq system. A perturbed Boussinesq equation of the form ytt−yxx−ϵ2[yxxtt+(y2)xx]+ϵ3[yxxt+yxxxxt+(y2)xxt]=0, which includes instability and dissipation, can be derived from this system.
Nonsteady dust charge variation induced ion acoustic monotonic shock structure in dusty plasma: Roles of ionization, ion loss, and collision
Authors: Ghosh Samiran , Khan Manoranjan , Gupta M. R.
Published: November 2006

The effects of nonsteady dust charge variation, ionization, ion loss, and collision on finite amplitude nonlinear ion acoustic wave are investigated in a complex (dusty) plasma. The dynamics of the nonlinear wave is governed by a Burger equation with linear damping or growth term. The anomalous dissipation originating from the nonsteady dust charge variation is responsible for the Burger term, whereas the other dissipative mechanisms (ionization, ion loss, and collision) are responsible for the linear damping (collision and ion loss are the dominant) or growth term (ionization is the dominant). Analytical approximate time evolution solution reveals that the wave possesses monotonic shock structure with exponentially growing or decaying wave amplitude. Its implications in gas discharge laboratory plasma are discussed.
Single molecule electron transport junctions: Charging and geometric effects on conductance
Authors: Andrews David Q. , Cohen Revital , Van Duyne Richard P. , Ratner Mark A.
Published: November 2006

A p-benzenedithiolate (BDT) molecule covalently bonded between two gold electrodes has become one of the model systems utilized for investigating molecular transport junctions. The plethora of papers published on the BDT system has led to varying conclusions with respect to both the mechanism and the magnitude of transport. Conductance variations have been attributed to difficulty in calculating charge transfer to the molecule, inability to locate the Fermi energy accurately, geometric dispersion, and stochastic switching. Here we compare results obtained using two transport codes, TRANSIESTA-C and HÜCKEL-IV, to show that upon Au–S bond lengthening, the calculated low bias conductance initially increases by up to a factor of 30. This increase in highest occupied molecular orbital (HOMO) mediated conductance is attributed to charging of the terminal sulfur atom and a corresponding decrease in the energy gap between the Fermi level and the HOMO. Addition of a single Au atom to each terminal of the extended BDT molecule is shown to add four molecular states near the Fermi energy, which may explain the varying results reported in the literature.
Direct numerical study of a liquid droplet impulsively accelerated by gaseous flow
Authors: Quan Shaoping , Schmidt David P.
Published: November 2006

A liquid spherical droplet impulsively accelerated by a gaseous flow is simulated in order to investigate the drag force and the deformation. The dynamics of the droplet immersed in a gaseous flow are investigated by solving the incompressible Navier-Stokes equations using a finite volume staggered mesh method coupled with a moving mesh interface tracking scheme. The benefit of the current scheme is that the interface conditions are implemented directly on an explicitly located interface with zero thickness. The droplet shape changes as it is accelerated, and the deformation factor of the droplet is as small as 0.2, so mesh adaptation methods are employed to achieve good mesh quality and to capture the interface curvature. The total drag coefficients are found to be larger than typical steady-state drag coefficients of solid spheres at the same Reynolds numbers. This agrees with the observation of Temkin et al. [J. Fluid Mech. 96, 133 (1980)] that the unsteady drag of decelerating relative flows was always larger than the steady drag. The large recirculation region behind the deformed droplet may explain this greater drag force. The effects of the viscosity ratio, density ratio, and initial Weber number on the droplet dynamics are also studied. It is found that the initial Weber number and the viscosity ratio have significant effects on the droplet dynamics, while the density ratio does not.
Generalization of the Gaussian electrostatic model: Extension to arbitrary angular momentum, distributed multipoles, and speedup with reciprocal space methods
Authors: Cisneros G. Andrés , Piquemal Jean-Philip , Darden Thomas A.
Published: November 2006

The simulation of biological systems by means of current empirical force fields presents shortcomings due to their lack of accuracy, especially in the description of the nonbonded terms. We have previously introduced a force field based on density fitting termed the Gaussian electrostatic model-0 (GEM-0) J.-P. Piquemal et al. [J. Chem. Phys. 124, 104101 (2006)] that improves the description of the nonbonded interactions. GEM-0 relies on density fitting methodology to reproduce each contribution of the constrained space orbital variation (CSOV) energy decomposition scheme, by expanding the electronic density of the molecule in s-type Gaussian functions centered at specific sites. In the present contribution we extend the Coulomb and exchange components of the force field to auxiliary basis sets of arbitrary angular momentum. Since the basis functions with higher angular momentum have directionality, a reference molecular frame (local frame) formalism is employed for the rotation of the fitted expansion coefficients. In all cases the intermolecular interaction energies are calculated by means of Hermite Gaussian functions using the McMurchie-Davidson [J. Comput. Phys. 26, 218 (1978)] recursion to calculate all the required integrals. Furthermore, the use of Hermite Gaussian functions allows a point multipole decomposition determination at each expansion site. Additionally, the issue of computational speed is investigated by reciprocal space based formalisms which include the particle mesh Ewald (PME) and fast Fourier-Poisson (FFP) methods. Frozen-core (Coulomb and exchange-repulsion) intermolecular interaction results for ten stationary points on the water dimer potential-energy surface, as well as a one-dimensional surface scan for the canonical water dimer, formamide, stacked benzene, and benzene water dimers, are presented. All results show reasonable agreement with the corresponding CSOV calculated reference contributions, around 0.1 and 0.15kcal∕mol error for Coulomb and exchange, respectively. Timing results for single Coulomb energy-force calculations for (H2O)n, n=64, 128, 256, 512, and 1024, in periodic boundary conditions with PME and FFP at two different rms force tolerances are also presented. For the small and intermediate auxiliaries, PME shows faster times than FFP at both accuracies and the advantage of PME widens at higher accuracy, while for the largest auxiliary, the opposite occurs.
Soft core thermodynamics from self-consistent hard core fluids
Authors: Schöll-Paschinger Elisabeth , Reiner Albert
Published: November 2006

In an effort to generalize the self-consistent Ornstein-Zernike approximation (SCOZA)—an accurate liquid state theory that has been restricted so far to hard core systems—to arbitrary soft core systems we study a combination of SCOZA with a recently developed perturbation theory. The latter was constructed by Ben-Amotz and Stell [J. Phys. Chem. B 108, 6877 (2004)] as a reformulation of the Weeks-Chandler-Andersen [J. Chem. Phys. 54, 5237 (1971)] perturbation theory directly in terms of an arbitrary hard sphere reference system. We investigate the accuracy of the combined approach for the Lennard-Jones fluid in comparison with simulation data and pure perturbation theory predictions and determine the dependence of the thermodynamic properties and the phase behavior on the choice of the effective hard core diameter of the reference system.
Effect of SnO2 addition on the dielectric properties of Ba2Ti9O20 ceramics in the high-frequency regime
Authors: Liu Hsiang-Lin , Chen Hsin-I , Tzeng Yuan-Tai , Chia Chia-Ta , Hsu Kuo-Chien , Chi Chen-Chung , Chang Chi-Ben , Leou Keh-Chyang , Lin I-Nan
Published: November 2006

The effect of SnO2 addition on the high-frequency dielectric properties of Ba2Ti9O20 ceramics has been investigated using terahertz time-domain spectroscopy, far-infrared (FIR) reflectivity, and Raman-scattering measurements. The dielectric constants determined from the terahertz and FIR spectra are smaller than that taken in the microwave frequency region. In contrast, the increase of the dielectric loss with increasing frequency is due to some resonance modes above 1THz. Importantly, our data clearly show that the SnO2 (2.4mole%) addition degraded the dielectric properties of Ba2Ti9O20 ceramics, which is ascribed to the deterioration on the coherency of lattice vibrational characteristics for these materials.
Efficiency analysis of reaction rate calculation methods using analytical models I: The two-dimensional sharp barrier
Authors: van Erp Titus S.
Published: November 2006

We analyze the efficiency of different methods for the calculation of reaction rates in the case of a simple two-dimensional analytical benchmark system. Two classes of methods are considered: the first is based on the free energy calculation along a reaction coordinate and the calculation of the transmission coefficient, the second on the sampling of dynamical pathways. We give scaling rules for how this efficiency depends on barrier height and width, and we hand out simple optimization rules for the method-specific parameters. We show that the path sampling methods, using the transition interface sampling technique, become exceedingly more efficient than the others when the reaction coordinate is not the optimal one.
Influence of the initial vibrationally nonequilibrium state of a medium on the structure of von Karman vortex street
Authors: Osipov A. I. , Uvarov A. V. , Vinnichenko N. A.
Published: November 2006

Numerical simulations of the flow of a vibrationally nonequilibrium gas around a circular cylinder have been undertaken. Distributions of velocity, temperature, and pressure in the wake behind the cylinder have been obtained, and the velocity fluctuation spectrum, which is measured in the experiment, has been examined. Three different models of medium and, accordingly, three different numerical schemes have been used. Comparison of the results enables one to identify the most important processes associated with the influence of relaxation, which result in some changes of the vortex street structure in comparison with the equilibrium case. Besides, such an approach enables one to verify the validity of the simplified methods used in numerical simulation of similar problems. Different conditions of heterogeneous relaxation on the surface of a cylinder are considered, and some comparison with experimental data is made.
High resolution photofragment translational spectroscopy studies of the near ultraviolet photolysis of imidazole
Authors: Devine Adam L. , Cronin Bríd , Nix Michael G. D. , Ashfold Michael N. R.
Published: November 2006

The fragmentation dynamics of imidazole molecules following excitation at 193.3nm and at many wavelengths in the range of 210⩽λphot⩽240nm have been investigated by H Rydberg atom photofragment translational spectroscopy. Long wavelength excitation within this range results in population of the 1A″1(π1σ*) excited state, but the 2A′1←XA′1(π*←π) transition becomes the dominant absorption once λphot⩽220nm. The measured energy disposals show parallels with those found in recent studies of the UV photolysis of pyrrole [Cronin et al., Phys Chem. Chem. Phys. 6, 5031 (2004)]. The total kinetic energy release (TKER) spectra display a “fast” feature, centred at TKER ∼9200cm−1. The analysis of the structure evident in the fast feature reveals the selective population of specific in-plane stretching vibrational levels of the imidazolyl cofragment; these fragments are deduced to carry only modest amounts of rotational excitation. Comparison with calculated normal mode vibrational frequencies allows the assignment of the populated levels and a precise determination of the N–H bond strength in imidazole: D0=33240±40cm−1. The observed energy disposal can be rationalized using Franck-Condon arguments, assuming that the potential energy surface (PES) for the 1A″1(π1σ*) state has a topology similar to that of the corresponding π1σ* state of pyrrole. As in pyrrole, photoexcitation populates skeletal motions in the S1 state (in-plane motions in the present case) that are only weakly coupled to the N–H dissociation coordinate and thus map through into the corresponding product vibrations. A second, “slow” feature is increasingly evident in TKER spectra recorded at shorter λphot. This component, which exhibits no recoil anisotropy, is attributed to H atoms formed by the “statistical” decay of highly vibrationally excited ground state molecules. The form of the TKER spectra observed at short λphot is rationalized by assuming two possible decay routes for imidazole molecules excited to the 2A′1(π1π*) state. One involves fast 2A′1(π1π*)⇝1A″1(π1σ*) radiationless transfer and subsequent fragmentation on the 1A′1(π1σ*) PES, yielding fast H atoms (and imidazolyl cofragments)—reminiscent of behavior seen at longer excitation wavelengths where the 1A″1(π1σ*) PES is accessed directly. The second is assumed to involve radiationless transfer to the ground state, most probably by successive 2A′1⇝1A″1⇝XA′1 couplings, mediated by conical intersections between the relevant PESs and the subsequent unimolecular decay of the resulting highly vibrationally excited ground state molecules yielding slow H atoms.
Photonic superdiffusive motion in resonance radiation trapping
Authors: Berberan-Santos M. N. , Nunes-Pereira E. J. , Martinho J. M. G.
Published: November 2006

In this work we consider the relation between the jump length probability density function and the line shape function in resonance radiation trapping in atomic vapors. The two-sided jump length probability density function suitable for a unidimensional formulation of radiative transfer is also derived. As a side result, a procedure to obtain the Maxwell distribution of velocities from the Maxwell-Boltzmann distribution of speeds was obtained. General relations that give the asymptotic jump length behavior and the Lévy flight parameter μ for any line shape are obtained. The results are applied to generalized Doppler, generalized Lorentz, and Voigt line shape functions. It is concluded that the lighter the tail of the line shape function, the less heavy the tail of the jump length probability density function, although this tail is always heavy, with μ⩽1.
Reaction coordinate analysis of the S1-S internal conversion of azulene
Authors: Amatatsu Yoshiaki , Komura Yoko
Published: November 2006

The reaction coordinate of the S1-S0 internal conversion of azulene has been analyzed using ab initio complete active space self-consistent field method. The stable geometry in S0 (S0 geometry) takes a bond-equalized structure where all the peripheral skeletal bond distances are similar to an aromatic CC bond distance. The transannular bond is similar to a normal C–C single bond. The first event upon electronic excitation into S1 at S0 geometry is characterized by the following two simultaneous changes in the skeletal bonds; the transannular bond in S1 increases its double bond character and the aromaticity of the peripheral bonds disappears. In consequence, the most stable azulene in S1 (S1 geometry) has a biradical character. To reach the conical intersection between S1 and S0 (S1∕S0-CIX) where radiationless relaxation takes place, the seven-membered ring greatly deviates from a planar structure. After a transition into S0 at S1∕S0-CIX, the bond-equalized structure is recovered immediately and then the nonplanarity decreases so that azulene again takes the stable planar S0 geometry. In order to deepen the understanding of the S1-S0 internal conversion, the dipole moments along the reaction coordinate have been analyzed.
Accelerated molecular dynamics simulation of the thermal desorption of n-alkanes from the basal plane of graphite
Authors: Becker Kelly E. , Fichthorn Kristen A.
Published: November 2006

We utilize accelerated molecular dynamics to simulate alkane desorption from the basal plane of graphite. Eight different molecules, ranging from n-pentane to n-hexadecane, are studied in the low coverage limit. Acceleration of the molecular dynamics simulations is achieved using two different methods: temperature acceleration and a compensating potential scheme. We find that the activation energy for desorption increases with increasing chain length. The desorption prefactor increases with chain length for molecules ranging from pentane to decane. This increase subsides and the value of the preexponential factor fluctuates about an apparently constant value for decane, dodecane, tetradecane, and hexadecane. These trends are consistent with data obtained in experimental temperature-programed desorption (TPD) studies. We explain the dependence of the preexponential factor on alkane chain length by examining conformational changes within the alkane molecules. For the shorter molecules, torsional motion is not activated over experimental temperature ranges. These molecules can be treated as rigid rods and their partial loss in translational and rotational entropies upon adsorption increases as chain length increases, leading to an increasing preexponential factor. At their typical TPD peak temperatures, torsions are activated in the longer adsorbed chain molecules to a significant extent which increases with increasing chain length, increasing the entropy of the adsorbed molecule. This increase counteracts the decrease in entropy due to a loss of translation and rotation, leading to a virtually constant prefactor.
Transport properties of CdS nanowire embedded poly(3-hexyl thiophene) nanocomposite
Authors: Bhattacharya S. , Malik S. , Nandi A. K. , Ghosh A.
Published: November 2006

Electrical transport properties of CdS nanowire embedded regioregular poly(3-hexyl thiophene) (P3HT) nanocomposite are investigated and the results are compared with those of the pure P3HT polymer. Both dc and ac conductivities of P3HT and CdS-P3HT nanocomposites show semiconducting nature and conductivity of CdS-P3HT is much less than that of the pure P3HT conducting polymer. The difference in the electrical behavior is attributed to the formation of nanowires in the CdS-P3HT nanocomposite. The dc conductivity data at high and low temperatures are qualitatively consistent with the optical-photon-assisted hopping and variable-range hopping models, respectively. However, the multiphonon-assisted hopping models of small polarons in nonadiabatic regime, which consider strong interactions with electrons with the optical as well as acoustical phonons, are the best to interpret quantitatively the experimental data in the entire temperature range for both P3HT and CdS-P3HT nanocomposites. We observe that the quantum tunneling model of small polarons is consistent with almost every respect of the loss data of P3HT conducting polymer, while for CdS-P3HT nanocomposite the ac conductivity varies with temperature in a different manner, which is explained in terms of simple quantum tunneling model of electrons in contrast to that of pure P3HT conducting polymer.
Entropy, thermostats, and chaotic hypothesis
Authors: Gallavotti Giovanni
Published: November 2006

The chaotic hypothesis is proposed as a basic for a general theory of nonequilibrium stationary states.
The cumulant two-particle reduced density matrix as a measure of electron correlation and entanglement
Authors: Juhász Tamás , Mazziotti David A.
Published: November 2006

Several measures of electron correlation are compared based on two criteria: (i) the presence of a unique mapping between the reduced variables in the measure and the many-electron wave function and (ii) the linear scaling of the measure and its variables with system size. We propose the squared Frobenius norm of the cumulant part of the two-particle reduced density matrix (2-RDM) as a measure of electron correlation that satisfies these criteria. An advantage of this cumulant-based norm is its ability to measure the correlation from spin entanglement, which is not contained in the correlation energy. Alternative measures based on the 2-RDM, such as the von Neumann entropy, do not scale linearly with system size. Properties of the measures are demonstrated with Be, F2, HF, N2, and a hydrogen chain.
Enhancement of electrical properties in polycrystalline BiFeO3 thin films
Authors: Yun Kwi Young , Ricinschi Dan , Kanashima Takeshi , Okuyama Masanori
Published: November 2006

Ferroelectric BiFeO3 thin films were grown on Pt∕TiO2∕SiO2∕Si substrates by pulsed-laser deposition. From the x-ray diffraction analysis, the BiFeO3 thin films consist of perovskite single phase, and the crystal structure shows the tetragonal structure with a space group P4mm. The BiFeO3 thin films show enhanced electrical properties with low leakage current density value of ∼10−4A∕cm2 at a maximum applied voltage of 31V. This enhanced electrical resistivity allowed the authors to obtain giant ferroelectric polarization values such as saturation polarizations of 110 and 166μC∕cm2 at room temperature and 80K, respectively.
Silicon-based injection lasers using electronic intersubband transitions in the L valleys
Authors: Driscoll Kristina , Paiella Roberto
Published: November 2006

The authors investigate the use of electronic intersubband transitions in Ge∕SiGe quantum wells on SiGe (001) virtual substrates for the development of silicon-based long-wavelength quantum cascade lasers. These heterostructures can provide relatively strong quantum confinement in the Ge L valleys particularly if the SiGe layers are sufficiently thin so that L-to-Δ intervalley scattering paths are suppressed. Numerical simulations indicate that low-threshold operation can be obtained from these devices, thanks to the nonpolar nature of SiGe. Furthermore, the tensor properties of the L-valley effective mass are favorable for the development of vertical emitting intersubband lasers.
Volumes of activation for the reaction of 1-chloro-2,4-dinitrobenzene and 2-chloro-1,3,5-trinitrobenzene with piperidine in aqueous ethanol
Authors: Alexander RD , Fall WA , Holper PN
Published: November 2006

Rate constants for the
reaction of 1-chloro-2,4-dinitrobenzene and 2- chloro-1,3,5-trinitrobenzene
with piperidine in 80% v/v aqueous ethanol have been measured as a function of
pressure up to 1.7 kbar. The volumes of activation are -35.0 and -20.7 cm3
mol-1 respectively, consistent with greater delocalization of charge
and less solvent electrostriction in the transition state for the second
reaction. The applicability of quadratic functionality for 1n k(P) is briefly
discussed.
'Selenation' as a diagnostic aid to reinvestigate the infrared spectra of benzimidazole-, benzoxazole- and benzothiazole-2-thiones
Authors: Devillanova FA , Verani G
Published: November 2006

The substitution of sulfur
with selenium in benzimidazole- (bnt), benzoxazole- (bot) and benzothiazole-2-thione (btt)
has been used to locate the vibrations related to the >C=S group. The
thioketone bnt behaves like imidazolidine-2-thione,
since it gives typical >C=S absorptions below 700 cm-1, while btt, like thiazolidine-2-thione, also gives rise to >C=S
absorptions around 1000 cm-1. In bot the selenation also affects the NH modes, thus producing
differences in several ranges of the infrared spectrum. The S- and
Se-methiodides and the complexes obtained from these ligands with CdCl2
have been prepared and their infrared spectra compared with the free ligands;
but it proves difficult to choose the site of coordination.
The color of rhodopsins at theab initiomulticonfigurational perturbation theory resolution
Authors: Coto Pedro B. , Strambi Angela , Ferré Nicolas , Olivucci Massimo
Published: November 2006

We demonstrate that “brute force” quantum-mechanics/molecular-mechanics computations based onab initio(i.e., first principles) multiconfigurational perturbation theory can reproduce the absorption maxima of a set of modified bovine rhodopsins with an accuracy allowing for the analysis of the factors determining their colors. In particular, we show that the theory accounts for the changes in excitation energy even when the proteins display the same charge distribution. Three color-tuning mechanisms, leading to changes of close magnitude, are demonstrated to operate in these conditions. The first is based on the change of the conformation of the conjugated backbone of the retinal chromophore. The second operates through the control of the distance between the positive charge residing on the chromophore and the carboxylate counterion. Finally, the third mechanism operates through the changes in orientation of the chromophore relative to the protein. These results offer perspectives for the unbiased computational design of mutants or chemically modified proteins with wanted optical properties.