bluuebunny/crossref_metadata_2025 huggingface dataset:
An age extended progress variable for conditioning reaction rates
Authors: Grout R. W.
Published: October 2007

An aging progress variable (APV) is proposed as a convenient tool for conditioning quantities used to calculate reaction rates in premixed turbulent combustion. The APV is defined to obey an advection-diffusion-reaction equation where the source term is linearly related to the fuel consumption rate when the APV is less than a threshold representative of the trailing edge of the fuel consumption layer. Above this threshold, the APV has a constant source term. To test the proposal, three-dimensional fully compressible direct numerical simulation (DNS) is performed in an inflow-outflow configuration with inlet turbulence forcing using a modified version of the DNS code SENGA. A model chemical mechanism involving two steps and four species is used which has an order of magnitude difference between the time scales associated with the two steps. The inlet Taylor scale Reynolds number is 51 and the Damköhler numbers are 2.0 and 0.29 for the two steps. When conditionally averaged on the proposed APV the scalar fluctuations about the conditional average are negligible. Further, it is shown that the probability density function of the APV can be reasonably approximated based on the first two moments of the APV and the fuel mass fraction. The APV probability density function (PDF) is approximated on rectangular slabs of cells normal to the flow direction using only these moments as a test case. Convolution of the PDF so approximated with conditional mean reaction rates—calculated from conditionally averaged scalar fields where the averaging was carried out over the entire domain—leads to an approximation for the unconditional mean reaction rates on each of these slabs typically within 10% of the true value for both steps. That the correlation between the reaction rates and the APV is strong, and that the PDF can be approximated for a situation where approximating the PDF for a product based progress variable is nontrivial, makes the proposed APV a strong alternative to traditional progress variables for both flamelet models and premixed conditional moment closure (CMC) approaches.
Metastable and charged particle decay in neon afterglow studied by the breakdown time delay measurements
Authors: Marković V. Lj. , Gocić S. R. , Stamenković S. N. , Petrović Z. Lj.
Published: October 2007

Memory effect—the long time variation of the electrical breakdown time delay on the relaxation time td¯(τ) in neon—was explained by the Ne(P23) (1s5) metastable state remaining from the preceding glow [Dj. A. Bošan, M. K. Radović, and Dj. M. Krmpotić, J. Phys. D 19, 2343 (1986)]. However, the authors neglected the quenching processes that reduce the effective lifetime of metastable states several orders of magnitude below that of the memory effect observations. In this paper the time delay measurements were carried out in neon at the pressure of 6.6mbar in a gas tube with gold-plated copper cathode, and the approximate and exact numerical models are developed in order to study the metastable and charged particle decay in afterglow. It was found that the metastable hypothesis completely failed to explain the afterglow kinetics, which is governed by the decay of molecular neon ions and molecular nitrogen ions produced in Ne2+ collisions with nitrogen impurities; i.e., Ne2++N2→N2++2Ne. Charged particle decay is followed up to hundreds of milliseconds in afterglow, from ambipolar to the free diffusion limit. After that, the late afterglow kinetics in neon can be explained by the nitrogen atoms recombining on the cathode surface and providing secondary electrons that determine the breakdown time delay down to the cosmic rays and natural radioactivity level.
Molecular simulation of crystallization in n-alkane ultrathin films: Effects of film thickness and substrate attraction
Authors: Yamamoto Takashi , Nozaki Koji , Yamaguchi Aki , Urakami Naohito
Published: October 2007

Crystallization in n-alkane ultrathin films supported by solid substrates is investigated by molecular dynamics simulation. We consider a relatively short n-alkane, undecane C11H24, on a flat substrate of varied degree of attraction. By the use of the united atom model for n-alkane, we reveal several characteristics of the thin film crystallization. It is found that the crystalline films consist of thin crystalline lamellae where chains are either parallel or perpendicular to the substrate. The relative amount of both types of lamellae changes systematically with film thickness, substrate attraction, and crystallization temperature; thicker films on substrates of higher attraction comprise dominant parallel lamellae, while thinner films on substrates of weaker attraction prefer the perpendicular lamellae. A clue to the morphogenesis is suggested to be the marked preference of the chain ends to locate on the free surface and on the effectively repulsive substrate. It is also shown that the perpendicular crystals, both on the free surface and on the solid substrate, have melting points higher than that of the bulk.
Theoretical study of the structure and properties of Na-MOR and H-MOR zeolite models
Authors: Dominguez-Soria V. D. , Calaminici P. , Goursot A.
Published: October 2007

This work presents calculated structural parameters and energetic properties of Na- and H-mordenites (MORs) using cluster models with more than 400 atoms. These state of the art calculations were performed within the framework of density functional theory, using both the local density approximation and the generalized gradient approximation, employing all-electron basis sets. The most populated T3, T4, and T1 Al sites have been investigated using two different MOR models, each containing two isolated Al sites. A detailed analysis of the structures, three-dimensional contours of molecular electrostatic potential, binding energies of Na cations and protons, and BrØnsted O–H harmonic frequencies are presented. These properties are compared with available experimental results. The structural changes among Si∕Al substitution as well as Na∕H exchange are discussed.
Effect of cross-field transport on H− density profile in magnetized plasmas: Comparison between measurement and simulation
Authors: Kajita Shin , Kado Shinichiro , Ohno Noriyasu , Takamura Shuichi , Kurihara Kiminori , Kuwahara Yosuke
Published: October 2007

The radial density profile of negative hydrogen ions in magnetized plasma is investigated in the divertor simulator MAP (Material and plasma)-II [S. Kado et al., J. Plasma Fusion Res. 71, 810 (2005)] by measurement and numerical simulation. The laser photodetachment method is used to evaluate the H− density by considering the influence of the magnetic field. The density of H− has a hollow profile and exhibits a peak in the peripheral region, though the electron density and temperature exhibit a peak at the center of the plasma column. The density profile of H− does not agree with the calculation result obtained from the rate equation, in which the local production and extinction rates are balanced, under the present experimental condition. To understand the behavior of negative ions, their trajectories are calculated by numerically solving the equation of motion by considering the effect of collisions. The negative ion density profile calculated from the particle simulation agrees well with the measured negative ion density profile. It is shown that the cross-field H− transport due to the radial electric field with the assistance of the elastic collisions plays an important role in enhancing the negative ion density in the peripheral region.
Curvature particle pinch in tokamak and stellarator geometry
Authors: Mishchenko Alexey , Helander Per , Turkin Yuriy
Published: October 2007

A study of the curvature pinch effect in various fusion devices (both tokamaks and stellarators) is presented. Canonical density profiles are calculated employing the theory developed by M. B. Isichenko, A. V. Gruzinov, P. H. Diamond, and P. N. Yushmanov [Phys. Plasmas 3, 1916 (1995)]. In tokamaks, it is found that the curvature pinch is relatively strong (especially in a spherical tokamak) and usually leads to a peaked density profile. In stellarators, the curvature pinch is weaker and can have either sign.
Interaction of water, methanol, and ammonia with AlxOy−: A comparative theoretical study of Al5O4− versus Al3O3−
Authors: Das Ujjal , Raghavachari Krishnan
Published: October 2007

The chemical reactions of water, methanol, and ammonia with Al5O4− have been studied using electronic structure calculations. The chemistry of Al5O4− with these molecules is different from that of Al3O3−. While Al3O3− dissociatively adsorbs two water molecules (and methanol), Al5O4− reacts with only one. In addition, Al5O4− does not show any reaction with ammonia while recent experimental and theoretical studies suggest that Al3O3− chemisorbs ammonia. These apparent differences in their chemical reactivity have been explained based on the thermodynamic stability of the corresponding reaction products and kinetic barriers associated with their formation.
A comparison between plane wave and Gaussian-type orbital basis sets for hydrogen bonded systems: Formic acid as a test case
Authors: Tosoni Sergio , Tuma Christian , Sauer Joachim , Civalleri Bartolomeo , Ugliengo Piero
Published: October 2007

The formic acid molecule, its dimers, and its molecular crystal are adopted as test systems to compare results obtained with plane wave (PW) basis sets and norm-conserving pseudopotentials to all-electron Gaussian-type orbital (GTO) calculations. The CPMD and CRYSTAL06 codes, respectively, are applied with the PBE, PW91, and BLYP density functionals. Hydrogen bonding is the leading interaction in the dimers and the crystal. In the latter, dispersive and weak C–H⋯O interactions are also relevant. Irrespective of the adopted functional, for all considered structures PW and GTO results converge smoothly as a function of the quality of the adopted basis sets to the same values for structures, energies of interaction, and harmonic vibrational features. To achieve a high level of mutual agreement the use of GTO basis sets of at least of triple-zeta quality including one set of polarization functions and PW basis sets with a kinetic energy cutoff higher than 110Ry is recommended. Pros and cons of both approaches for studying molecular crystals are also discussed.
All-optical modulation of laser light in amorphous silicon-filled microstructured optical fibers
Authors: Won Dong-Jin , Ramirez Mariola O. , Kang Hoonsoo , Gopalan Venkatraman , Baril Neil F. , Calkins Jacob , Badding John V. , Sazio Pier J. A.
Published: October 2007

Amorphous silicon is deposited within optical fibers by a high pressure microfluidic deposition process and characterized via Raman spectroscopy. All-optical modulation of 1.55μm light guided through the silicon core is demonstrated using the free carrier absorption generated by a 532nm pump pulse. Modulation depths of up to 8.26dB and modulation frequencies of up to 1.4MHz are demonstrated.
Complete determination of the photoionization dynamics of a polyatomic molecule. I. Experimental photoelectron angular distributions from ÃAu1 acetylene
Authors: Hockett Paul , King Adrian K. , Powis Ivan , Reid Katharine L.
Published: October 2007

Angle-resolved photoelectron spectra from rotationally selected ÃAu1 state acetylene have been recorded using velocity-map imaging. Several Renner-Teller split vibrational bands have been observed and assigned, showing good agreement with previous zero kinetic energy photoelectron (ZEKE) work [S. T. Pratt, P. M. Dehmer, and J. L. Dehmer, J. Chem. Phys. 99, 6233 (1993); S.-J. Tang, Y.-C. Chou, J. J.-M. Lin, and Y.-C. Hsu, ibid. 125, 133201 (2006).] The extracted photoelectron angular distributions (PADs) corresponding to these bands show a strong dependence on the vibronic angular momentum projection quantum number K+. Subbands with odd K+ show PADs with maximum intensity along the polarization vector of the ionizing laser beam, while those with even K+ show PADs with maximum intensity perpendicular to this direction. Velocity-map images recorded at low photoelectron energies approach rotational resolution of the ion, and the evolution of the PADs with increasing rotational level prepared in the ÃAu1 state indicates the potential of a “complete” determination of the photoionization dynamics of the ÃAu1 state. This is further investigated in the following paper.
Complete determination of the photoionization dynamics of a polyatomic molecule. II. Determination of radial dipole matrix elements and phases from experimental photoelectron angular distributions from ÃAu1 acetylene
Authors: Hockett Paul , Reid Katharine L.
Published: October 2007

We present a fit to photoelectron angular distributions (PADs) measured following the photoionization of rotationally selected ÃAu1 state acetylene. In the case of the 41Σu−2 vibronic state of the ion, we are able to use this fit to make a complete determination of the radial dipole matrix elements and phases connecting the prepared level to each photoelectron partial wave. We have also investigated other Renner-Teller subbands with a view to disentangling geometrical and dynamical contributions to the resulting PADs.
Separate control of direct and converse piezoelectric effects in flexoelectric piezoelectric composites
Authors: Fu John Y. , Cross L. Eric
Published: October 2007

The fabrication of piezoelectric composites driven by the flexoelectric charge separation has aroused considerable interest recently. In earlier oral presentations, we have posited that, in such composites, even though the direct and the converse flexoelectric effects are thermodynamically equivalent, it is possible to separately control the direct and converse piezoelectricities. Here, we would like to demonstrate how this separation can be accomplished through the control of the texture symmetry and that the result does not violate reciprocity as two different phases are involved in those two effects.
Initial interactions between water molecules and Ti-adsorbed carbon nanotubes
Authors: Lei Yang , Guo Zheng Xiao , Zhu Wenguang , Meng Sheng , Zhang Zhenyu
Published: October 2007

Spin-polarized first-principles calculations were employed to study the initial stages of water interactions with Ti-doped carbon nanotubes (CNTs), aiming to explore their potentials for water splitting to generate hydrogen or as practical hydrogen storage systems involving moisture. The rate-limiting barrier for splitting the first H2O is 0.48eV on a semiconducting (8,0) CNT, and 0.41eV on a metallic (5,5) CNT. The barrier for splitting the second H2O is only 0.12eV, accompanied by the formation and release of a H2 molecule.
The dynamics of intermittent strand separation in double-stranded DNA
Authors: Chatterjee Debarati , Chaudhury Srabanti , Cherayil Binny J.
Published: October 2007

The transient rupture and reformation of hydrogen bonds between base pairs on distinct chains of double-stranded DNA (“bubble” dynamics) is modeled in terms of the fluctuating distance between the bases. The fluctuations in the distance are assumed to be governed by a simple Langevin equation with a quadratic potential under conditions of high friction. A critical distance of separation L must be achieved before a bubble defect is considered to have been formed. The decay of the dynamic correlations between states of the DNA that have such defects and those that do not has been calculated from the above model and has been found to reproduce the trends in experimental measurements of the same quantity.
Transition from Turing stripe patterns to hexagonal patterns induced by polarized electric fields
Authors: Chen Wen-Qiang , Zhang Hong , Ying He-Ping , Li Bing-Wei , Chen Jiang-Xing
Published: October 2007

The effect of a circularly polarized electric field on the Turing stripe patterns is studied. The numerical results show that stripe patterns may change to hexagonal wave patterns by choosing the intensity and the frequency of the circularly polarized electric field suitably. Our findings indicate that a pattern tends to organize itself to the pattern with the same symmetry of the applied field with the fact that compared to the stripe patterns, hexagonal wave patterns possess hexagonal symmetry which is closer to the rotation symmetry of the circularly polarized electric field.
Vibrational characterization of different benzene phases on flat and vicinal Si(100) surfaces
Authors: Naydenov Borislav , Widdra Wolf
Published: October 2007

Based on high-resolution electron energy loss spectroscopy and temperature-programmable desorption, benzene chemisorption on vicinal and nominally flat Si(100) surfaces has been studied for various adsorption, annealing, and site blocking treatments. Three different chemisorbed benzene (C6H6 and C6D6) phases with distinct thermal desorption characteristics and different vibrational spectra have been separated and characterized on both substrates. All three phases are identified as 1,4-cyclohexadiene-like structures with butterfly geometry. Whereas the dominant phase is di-σ bonded to the two Si atoms of a single Si–Si dimer, the benzene orientation (double bond orientation) in the other phases is rotated. Di-σ bonding to Si atoms of adjacent Si–Si dimer for the latter cases is most likely. Coverage and temperature dependent conversions between the different phases have been addressed by vibrational spectroscopy.
Negative refraction and focusing in magnetically coupled L-C loaded transmission lines
Authors: Algredo-Badillo U. , Halevi P.
Published: October 2007

We study electromagnetic waves in circuits with one-dimensional and two-dimensional (2D) periodicities. The unit cell is composed of L-C elements, and neighboring cells are magnetically coupled. Simulations reveal passband response and are in excellent accord with dispersion relations for corresponding infinite circuits. We also consider refraction at the interface between two different 2D circuits for long wavelengths. The product of the phase and group velocities is proportional to the mutual inductance M providing the coupling. Hence, the sign of the effective refractive index is the same as the sign of M. Further, we simulate negative refraction (and focusing) of circular waves, excited by a point source, if M has opposite signs on the two sides of the border line.
The Casimir effect for parallel plates revisited
Authors: Kawakami N. A. , Nemes M. C. , Wreszinski Walter F.
Published: October 2007

The Casimir effect for a massless scalar field with Dirichlet and periodic boundary conditions (bc’s) on infinite parallel plates is revisited in the local quantum field theory (lqft) framework introduced by Kay [Phys. Rev. D 20, 3052 (1979)]. The model displays a number of more realistic features than the ones he treated. In addition to local observables, as the energy density, we propose to consider intensive variables, such as the energy per unit area ε, as fundamental observables. Adopting this view, lqft rejects Dirichlet (the same result may be proved for Neumann or mixed) bc, and accepts periodic bc: in the former case ε diverges, in the latter it is finite, as is shown by an expression for the local energy density obtained from lqft through the use of the Poisson summation formula. Another way to see this uses methods from the Euler summation formula: in the proof of regularization independence of the energy per unit area, a regularization-dependent surface term arises upon use of Dirichlet bc, but not periodic bc. For the conformally invariant scalar quantum field, this surface term is absent due to the condition of zero trace of the energy momentum tensor, as remarked by De Witt [Phys. Rep. 19, 295 (1975)]. The latter property does not hold in the application to the dark energy problem in cosmology, in which we argue that periodic bc might play a distinguished role.
A shock-induced phase transformation in a LiTaO3 crystal
Authors: Li Jun , Zhou Xianming , Zhu Wenjun , Li Jiabo , Jing Fuqian
Published: October 2007

The high-pressure phase transformation behavior of LiTaO3 crystal has been studied by both Hugoniot measurements and first-principle calculations. We observe a discontinuity in shock velocity (D) versus particle velocity (UP) relation, a two-wave structure below 37.9 GPa, and a three-wave structure above 37.9 GPa. These data confirm that a shock-induced phase transformation of LiTaO3 occurs. The onset pressure of the phase transformation (37.9 GPa) defined by our new shock compression data is higher than the early shock wave value (19 GPa) reported by Stantonand Graham [P. L. Stanton and R. A. Graham, J. Appl. Phys. 50, 6892 (1979)]. A first-principle calculation of the zero degree isotherm for rhombohedral phase (R3c space group) is in good agreement with our low-pressure experimental data. The calculated zero degree isotherm for orthorhombic phase (Pbnm space group) is in concord with our high-pressure shock compression data.
Annealing effects on the nanoscale indium and nitrogen distribution in Ga(NAs) and (GaIn)(NAs) quantum wells
Authors: Volz K. , Torunski T. , Rubel O. , Stolz W. , Kruse P. , Gerthsen D. , Schowalter M. , Rosenauer A.
Published: October 2007

III∕V semiconductors containing dilute amounts of nitrogen are metastable and need to be thermally treated after growth to optimize optoelectronic properties. The influence of thermal annealing on the nitrogen depth profile in metal organic vapor phase epitaxygrown Ga(NAs)∕GaAs as well as (GaIn)(NAs)∕GaAs heterostructures is examined on a nanometer scale by combining several high resolution transmission electron microscopy techniques, also with Rutherford backscattering spectrometry. Annealing conditions, which are optimized for quaternary alloys with respect to photoluminescence intensity, do not result in element redistribution for the In containing material. Contrary to the quaternary material, the result of annealing the ternary Ga(NAs) is a pronounced pileup of the nitrogen profile without any out diffusion of nitrogen. These findings have important influence on device structures, which often contain Ga(NAs) barriers for strain-compensation purposes together with (GaIn)(NAs) active regions. In the light of metastability considerations for the ternary and quaternary alloy, one can conclude that the In contained in the quaternary material stabilizes the material and suppresses phase separation. Consequently (GaIn)(NAs) is more stable than its ternary counterpart Ga(NAs).
Vibrational lifetimes of cyanide and carbon monoxide on noble and transition metal surfaces
Authors: Forsblom Mattias , Persson Mats
Published: October 2007

The electronic vibrational damping rates of the CN and CO internal stretch modes on the (111) surfaces of Ag, Cu, Au, and Pt were calculated using density functional theory calculations. Our calculated damping rates are in excellent agreement with experimental data obtained from pump-probe laser spectroscopy. The striking difference in trends and magnitudes between the internal stretch modes of CN and CO is in part rationalized in terms of the adsorbate-induced electronic structure within the framework of a simple Newns-Anderson model.
The free energy of the metastable supersaturated vapor via restricted ensemble simulations
Authors: Nie Chu , Geng Jun , Marlow W. H.
Published: October 2007

Pressure, excess chemical potential, and excess free energy, with respect to ideal gas data at different densities of the supersaturated Lennard-Jones particle vapor at the reduced temperature 0.7 are obtained by the restricted canonical ensemble Monte Carlo simulation method [D. S. Corti and P. Debenedetti, Chem. Eng. Sci. 49, 2717 (1994)]. The excess free energy values depend upon the constraints imposed on the system with local minima exhibited for densities below the spinodal density and monotonic variation for densities larger than the spinodal density. The results are compared with a molecular dynamics simulation study [A. Linharton et al., J. Chem. Phys. 122, 144506 (2005)] on the same system. The current study verifies the conclusion drawn by the simulation work that clustering of Lennard-Jones atoms exists even in the vicinity of spinodal. Our method gives an alternative to molecular dynamic simulations for the determination of equilibrium properties of a metastable fluid, especially close to the spinodal, and does not require a very large system to carry out the simulation.
X-ray photoelectron spectroscopy study of the oxidation of Se passivated Si(001)
Authors: Aguirre-Tostado F. S. , Layton D. , Herrera-Gomez A. , Wallace R. M. , Zhu J. , Larrieu G. , Maldonado E. , Kirk W. P. , Tao M.
Published: October 2007

The chemical and electronic passivation of semiconductors is an important issue for the fabrication of electronic devices. In this work we use angle resolved x-ray photoelectron spectroscopy to study the chemical passivation of silicon (001) surface with selenium at a surface coverage close to 1 monolayer. The interaction of Se with silicon breaks the Si–Si dimers leading to a change in the surface reconstruction from a (2×1) to (1×1) symmetry. The silicon surface covered with a selenium monolayer was exposed to dry oxygen at 300°C for 80min. We find that the presence of the Se monolayer does not appreciably reduce the formation of a SiO2 layer.
Polarization retention loss in PbTiO3 ferroelectric films due to leakage currents
Authors: Morelli A. , Venkatesan Sriram , Palasantzas G. , Kooi B. J. , De Hosson J. Th. M.
Published: October 2007

The relationship between retention loss in single crystal PbTiO3 ferroelectric thin films and leakage currents is demonstrated by piezoresponse and conductive atomic force microscopy measurements. It was found that the polarization reversal in the absence of an electric field followed a stretched exponential behavior 1−exp[−(t∕k)d] with exponent d>1, which is distinct from a dispersive random walk process with d<1. The latter has been observed in polycrystalline films for which retention loss was associated with grain boundaries. The leakage current indicates power law scaling at short length scales, which strongly depends on the applied electric field. Additional information of the microstructure, which contributes to an explanation of the presence of leakage currents, is presented with high resolution transmission electron microscopy analysis.
Optical patterning of silver nanoparticle Langmuir-Blodgett films
Authors: Kim Hyunjun , Shin Hyunkwon , Ha Jungmin , Lee Myeongkyu , Lim Ki-Soo
Published: October 2007

We report on the optical patterning of Ag nanoparticle Langmuir-Blodgett (LB) films deposited on glass substrates. Ag nanoparticles could be quickly desorbed from the substrate without surface damage, when exposed to a nanosecond Nd:YAG laser pulse over threshold energy density. Well-aligned one-dimensional and two-dimensional patterns at the micrometer scale were fabricated in LB films by a simple spatial modulation of the laser beam. This parallel process enables the period and feature size of the pattern to be independently controlled, and may provide an effective method for nanoparticle pattering.
The quantum harmonic oscillator on the sphere and the hyperbolic plane: κ-dependent formalism, polar coordinates, and hypergeometric functions
Authors: Cariñena José F. , Rañada Manuel F. , Santander Mariano
Published: October 2007

A nonlinear model representing the quantum harmonic oscillator on the sphere and the hyperbolic plane is solved in polar coordinates (r,ϕ) by making use of a curvature-dependent formalism. The curvature κ is considered as a parameter and then the radial Schrödinger equation becomes a κ-dependent Gauss hypergeometric equation. The energy spectrum and the wave functions are exactly obtained in both the sphere S2 (κ>0) and the hyperbolic plane H2 (κ<0). A comparative study between the spherical and the hyperbolic quantum results is presented.
Molecular-beam-epitaxy-grown CrSe∕Fe bilayer on GaAs(100) substrate
Authors: Wang C. , Zhang B. , You B. , Lok S. K. , Chan S. K. , Zhang X. X. , Wong G. K. L. , Sou I. K.
Published: October 2007

A novel CrSe∕Fe bilayer structure has been fabricated on a GaAs (100) substrate by the molecular beam epitaxy technique. Microstructural characterizations have revealed that the Fe layer is a single-crystalline bcc structure with the orientation relationship of (100)Fe‖(100)GaAs, while the top CrSe layer shows four preferred hexagonal domains with their c axis each along one of the four upward-pointing ⟨111⟩ directions of the underlying Fe lattice. The magnetic hysteresis loops of this bilayer structure measured by a superconducting quantum interference device magnetometer demonstrate a strong exchange bias effect with a negative exchange bias field as high as −48.4Oe at 5K. The magnetization reversal process shows an abrupt transition nature at temperature from 5to300K. An enhancement of the coercivity not accompanied by the exchange bias field was observed at temperature higher than and well above the blocking temperature. We have interpreted these observations based on the well-established exchange spring model for antiferromagnetic/ferromagnetic bilayer structures.
Evolution of nanodomains during the electric-field-induced relaxor to normal ferroelectric phase transition in a Sc-doped Pb(Mg1∕3Nb2∕3)O3 ceramic
Authors: Qu W. , Zhao X. , Tan X.
Published: October 2007

Sc doping in Pb(Mg1∕3Nb2∕3)O3 enhances the B-site 1:1 cation order significantly but promotes the ferroelectric polar order moderately. At low doping levels, the electrical polar domains remain at the nanometer scale and the relaxor ferroelectric behavior is preserved. A normal ferroelectric state can be triggered with electric fields from the relaxor state at lower temperatures. This electric-field-induced phase transition process was directly observed with an in situ transmission electron microscopy technique in a 4at.% Sc-doped Pb(Mg1∕3Nb2∕3)O3 polycrystalline ceramic under different conditions. It was found that the phase transition started at the grain boundary and took two steps to complete: The gradual coalescence of the polar nanodomains and the abrupt formation of the long-range ferroelectric domains. During the growth of the polar nanodomains, the morphology of the cation ordered chemical domains does not change. Furthermore, these chemical domains seem to have no strong resistance to the growth of polar domains in Sc-doped Pb(Mg1∕3Nb2∕3)O3.
Effect of thickness on the self-positioning of nanostructures
Authors: Nishidate Y. , Nikishkov G. P.
Published: October 2007

Atomic-scale modeling of self-positioning GaAs–InAs nanostructures is performed. Curvature radius values obtained by the atomic-scale finite element method are compared with those obtained by a continuum mechanics solution under plane strain conditions. Atomic-scale modeling and continuum mechanics solution predict the same curvature radius for structures with large thickness. However, atomic-scale modeling shows significant decrease of the curvature radius for structures with thickness less than 40nm.
Impact of elasticity on the piezoresponse of adjacent ferroelectric domains investigated by scanning force microscopy
Authors: Jungk Tobias , Hoffmann Ákos , Soergel Elisabeth
Published: October 2007

As a consequence of elasticity, mechanical deformations of crystals occur on a length scale comparable to their thickness. This is exemplified by applying a homogeneous electric field to a multidomain ferroelectric crystal: As one domain is expanding, the adjacent ones are contracting, leading to clamping at the domain boundaries. The piezomechanically driven surface corrugation of micron-sized domain patterns in thick crystals using large-area top electrodes is thus drastically suppressed, barely accessible by means of piezoresponse force microscopy.
Paramagnetic-ferromagnetic phase transition and magnetic properties of ultrathin CoFe∕Cu(110) films
Authors: Küpper D. , Easton S. , Bland J. A. C.
Published: October 2007

We present the results of a systematic investigation of the coverage-dependent magnetic response and anisotropy of ultrathin Co75Fe25∕Cu(110) films using the magneto-optic Kerr effect. A paramagnetic-ferromagnetic phase transition is found to occur at a critical thickness dc=4.0±0.25 ML. In the vicinity of dc, the paramagnetic susceptibility χ follows a power law with a critical exponent of γ=2.385±0.069 in excellent agreement with the theoretical value for a perfect two-dimensional geometric percolation phase transition. At the very onset of ferromagnetic order, a mixture of cubic and uniaxial anisotropy was observed.
Observation of three weakly bound valence states of I2
Authors: Ridley Trevor , Lawley Kenneth P. , Donovan Robert J.
Published: October 2007

Structured emission in the gas phase to two weakly bound valence states that correlate with the third dissociation limit, I*(P1∕22)+I*(P1∕22), designated as (bb), from two third tier ion-pair states of I2 correlating with I−(S01)+I+(D21), the 1g(D21), and F′0u+(D21) states, has been observed for the first time. The 1u(bb) state is shown to be bound by 377±2cm−1 and molecular constants have been determined. Vibrational structure in the 0g+(bb) state could not be resolved but the spectrum is consistent with the state being bound by 435cm−1. The relative integrated intensities of the emissions from both ion-pair states to various valence states have also been measured, and some aspects are rationalized in terms of the electronic configurations of the upper and lower states. Bound levels of a previously uncharacterized 1g(ab) valence state have also been observed in emission from the γ1u(P23) ion-pair state. The lower state is shown to be bound by 270±2cm−1 and molecular constants have been determined.
Z-scan theory of two-photon absorption saturation and experimental evidence
Authors: Gu Bing , Fan Ya-Xian , Chen Jing , Wang Hui-Tian , He Jun , Ji Wei
Published: October 2007

In our article [B. Gu, Y. X. Fan, J. Wang, J. Chen, J. P. Ding, H. T. Wang, and B. Guo, Phys. Rev. A 73, 065803 (2006)] we have presented the theory of open-aperture Gaussian-beam Z-scan, based on the Adomian decomposition method, which is available only for the saturable absorption caused by single-photon absorption transition. In the present article, using the same technique (Adomian decomposition method, as a common technique), we develop an open-aperture Z-scan theory for evaluating the property of the saturable absorption originating from two-photon absorption (2PA) transition, when a spatial Gaussian beam is used as the excitation source. We find analytic polynomial expressions of the Z-scan traces for a continuous wave laser or a temporal Gaussian pulsed laser. As the experimental evidence, we investigate the saturable 2PA behaviors caused by the interband two-photon transition in the direct-gap II–VI semiconductors CdS, CdSe, ZnSe, and ZnTe, under the excitation condition of a femtosecond laser with a 140 fs pulse duration, a 1 kHz low repetition rate, and a 1.6 eV photon energy.
Interference filter properties of nonuniform Josephson junction arrays
Authors: Salez Morvan , Boussaha Faouzi , Loukitch Lionel , Caputo Jean-Guy
Published: October 2007

We experimentally and numerically study the zero-voltage supercurrent versus magnetic field of nonuniform arrays of Josephson junctions parallel-connected by a superconducting stripline. The measured curves are complex, geometry-dependent, and in excellent agreement with numerical simulations using a specially developed model. Such arrays can be optimized to have specific interference patterns, suited for applications in magnetometery, quasiparticle microwave sensors, Josephson oscillators, and superconducting electronics.
Nanocontact spin-transfer oscillators based on perpendicular anisotropy in the free layer
Authors: Consolo G. , Lopez-Diaz L. , Torres L. , Finocchio G. , Romeo A. , Azzerboni B.
Published: October 2007

Micromagnetic simulations are used to predict the behavior exhibited by spin-transfer oscillators when materials with perpendicular anisotropy are introduced in the “free” layer of nanocontact devices. Under a perpendicular-to-plane bias field, the frequency exhibits nonlinear dependence on the anisotropy field, mostly originated by the exchange-dominated propagating nature of spin-wave modes. The increase of frequency without using large bias fields makes it suitable for potential technological applications. A study of the feasibility of bias-field-free devices has been also performed deriving multiharmonic signals at gigahertz frequencies. Here, the magnetization describes a gyrotropic motion where both vortex-core polarization and rotation sense switch periodically.
Stability of the flow of a viscoelastic fluid past a deformable surface in the low Reynolds number limit
Authors: Chokshi Paresh , Kumaran V.
Published: October 2007

The stability of the plane Couette flow of a viscoelastic fluid adjacent to a flexible surface is analyzed with the help of linear and weakly nonlinear stability theory in the limit of zero Reynolds number. The fluid is described by an Oldroyd-B model, which is parametrized by the viscosity η, the relaxation time λ, and the parameter β, which is the ratio of solvent-to-solution viscosity; β=0 for a Maxwell fluid and β=1 for a Newtonian fluid. The wall is modeled as an incompressible neo-Hookean solid of finite thickness and is grafted to a rigid plate at the bottom. The neo-Hookean constitutive model parametrized by the shear modulus G, augmented to include the viscous dissipation, is used for the solid medium. Previous studies for the Newtonian flow past a compliant wall predict an instability as the dimensionless shear rate Γ=(ηV∕GR) is increased beyond the critical value Γc. The present analysis investigates the effect of fluid elasticity, in terms of the Weissenberg number W=λG∕η, on the critical value of the imposed shear rate Γc for various parameters. The fluid elasticity is found to increase Γc, indicating the stabilizing influence of the polymer addition on the viscous instability. For dilute polymeric solutions with β⩾0.5, the flow is stable when the Weissenberg number is increased beyond a maximum value Wmax, and Wmax increases proportional to the ratio of solid-to-fluid thickness H. For concentrated polymer solutions and melts with β<0.5, the flow becomes unstable when the strain rate increases beyond a critical value for any large Weissenberg number. The weakly nonlinear analysis reveals that the bifurcation of the linear instability is subcritical when there is no dissipation in the solid. The nature of bifurcation, however, changes to supercritical when the viscous effects in the solid are taken into account and the relative solid viscosity ηr is large such that ηr∕H>1. The equilibrium amplitude and the threshold strain energy for the solid have been calculated, and the effect of parameters H, β, ηr, and interfacial tension on these quantities is analyzed.
Surface plasmon assisted photoluminescence in GaAs–AlGaAs quantum well microstructures
Authors: Lepage Dominic , Dubowski Jan J.
Published: October 2007

Surface plasmon resonance has been investigated in a quantum well (QW) GaAs microstructure the photoluminescence (PL) of which is coupled via a submicrometer period grating with surface plasmons (SPs) propagating at SiO2–Au-dielectric interfaces. Introduction of the SiO2 layer allowed to increase both the propagation length and the penetration depth of SPs and, consequently, achieve their enhanced interaction with the QW PL signal. For a QW GaAs–Al0.5Ga0.5As microstructure emitting at 822nm, a modulated PL emission has been observed in agreement with the calculated resonance conditions expected for such a microstructure and the 375nm period grating.
Comment on “The effects of buoyancy convection on the measured solute diffusion coefficients in dilute metallic liquids” [J. Appl. Phys. 96, 6213 (2004)]
Authors: Shirkhanzadeh M.
Published: October 2007

Two main problems have been identified with this recently published paper: (1) The authors claim that the terrestrial and microgravity experiments were performed under isothermal conditions to render any thermotransport insignificant. Close examination of the problems encountered during processing of the capillary diffusion couples on the MIR space station reveals that this claim cannot be justified. (2) On the basis of experimental results, the authors conclude that the diffusion coefficients of gold and silver in lead measured on the ground are much higher than those obtained under microgravity. This conclusion is questionable because the methodology used to determine the processing time and temperatures on the MIR space station does not seem to be scientific.
Rotational diffusion of ionic and neutral solutes in mixed micelles: Effect of surfactant to block copolymer mole ratio on solute rotation
Authors: Mali K. S. , Dutt G. B. , Mukherjee T.
Published: October 2007

Rotational diffusion of an ionic solute rhodamine 110 and a neutral solute 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) has been investigated in aqueous mixtures of cetyltrimethylammonium chloride (CTAC) and poly(ethyleneoxide)20-poly(propyleneoxide)70-poly(ethyleneoxide)20 (P123). The purpose of this work is to understand how an increase in the mole ratio of surfactant to block copolymer from low to high influences the dynamics of ionic and neutral solute molecules. The variation in the mole ratio of CTAC to P123 from low to high has resulted in a drastic increase in the average reorientation time of rhodamine 110. In contrast, an exactly opposite trend has been noticed in the case of DMDPP. In the low mole ratio regime, rhodamine 110 and DMDPP are located at the interface and palisade layer, respectively, of P123 micelle–CTAC complexes. On the other hand, in the high mole ratio regime, both the probes are located in the Stern layer of CTAC-P123 complexes. The enhancement in the average reorientation time of rhodamine 110 with an increase in the mole ratio of surfactant to block copolymer has been rationalized on the basis of formation of rhodamine 110–Cl ion pair, which in turn associates with the cationic head groups of CTAC-P123 complexes. The observed decrease in the average reorientation time of DMDPP with an increase in the mole ratio of CTAC to P123 is a consequence of lower microviscosity of the Stern layer of CTAC-P123 complexes compared to the palisade layer of P123 micelle–CTAC complexes.
Optical gain at 1535nm in LaF3:Er,Yb nanoparticle-doped organic-inorganic hybrid material waveguide
Authors: Zhang Dan , Chen Cong , Chen Changming , Ma Chunsheng , Zhang Daming , Bo Shuhui , Zhen Zhen
Published: October 2007

LaF 3 : Er , Yb nanoparticle-doped organic-inorganic hybrid material waveguides were demonstrated using reactive ion etching technology. The absorption and photoluminescence spectra were observed on a 140μm thick film of the nanoparticle-doped hybrid material. Under excitation at 976nm, the fluorescence was obtained at 1535nm, and its full width at half maximum was about 83nm. A relative optical gain of about 5dB was measured at 1535nm in a 22-mm-long waveguide.
Femtosecond real-time single-shot digitizer
Authors: Chou Jason , Boyraz Ozdal , Solli Daniel , Jalali Bahram
Published: October 2007

We demonstrate a single-shot digitizer with a 10Tsample∕s sampling rate. This feat is accomplished with a photonic time stretch preprocessor that slows down the electrical waveform by an unprecedented factor of 250 before it is captured by a commercial electronic digitizer. To achieve such a large stretch factor, distributed Raman optical amplification is realized inside the dispersive element that performs the time stretch.
The residual anisotropy at small scales in high shear turbulence
Authors: Casciola C. M. , Gualtieri P. , Jacob B. , Piva R.
Published: October 2007

It has always been believed that turbulence in fluids can achieve a universal state at small scales with fluctuations that, becoming statistically isotropic, are characterized by universal scaling laws. In fact, in different branches of physics it is common to find conditions such that statistical isotropy is never recovered and the anisotropy induced by large scale shear contaminates the entire range of scales up to velocity gradients. We address this issue here, of particular significance, for wall bounded flows. The systematic decomposition in spherical harmonics of the correlation functions of velocity fluctuations enables us to extract the different anisotropic contributions. They vanish at small scale at a relatively fast rate under weak shear. Under strong shear instead they keep a significant amplitude up to viscous scales, thus leaving a persistent signature on the gradients which can be detected even in the statistics of low order, e.g., in the energy dissipation tensor.
Terahertz spectroscopic study of vertically aligned InN nanorods
Authors: Ahn H. , Ku Y.-P. , Wang Y.-C. , Chuang C.-H. , Gwo S. , Pan Ci-Ling
Published: October 2007

Terahertz time-domain spectroscopy has been used to investigate terahertz conductivity and dielectric response of indium nitride (InN) nanorod array and epitaxial film. The complex terahertz conductivity of InN film is well fitted by the Drude model, while the negative imaginary conductivity of the InN nanorods can be described by using the Drude-Smith model. The electron mobility of the InN film is 1217±58cm2∕Vs, while that of the InN nanorods is 80±5cm2∕Vs. The reduced mobility of carriers for the latter can be attributed to the restricted carrier transport within the nanorods.
Thermo- and photoinduced voltages in Ag heterodimensional junctions
Authors: Xu Jia , Sun Jia-Lin , Zhu Jia-Lin
Published: October 2007

Macroscopic-long Ag nanowires have been synthesized through an improved solid-state ionics method and characterized at different spatial scales. Using a bundle of as-fabricated Ag nanowires to connect with two bulk Ag electrodes, two Ag heterodimensional junctions are formed into an electrical circuit. Thermo- and photoinduced electromotive forces are observed and the corresponding voltages are measured in the circuit on the macroscale. For the photoinduced voltage, the fast dynamic response of the circuit, irradiated locally by 532nm and 10.6μm lasers, is exhibited. The studied low-dimensional effects need to be taken into account in future applications of Ag nanowires.
Simulated experiment for elimination of air contaminated with odorous chemical agents by microwave plasma burner
Authors: Hong Yong Cheol , Shin Dong Hun , Uhm Han Sup
Published: October 2007

An experimental study on elimination of odorous chemical agent was carried out by making use of a microwave plasma burner, which consists of a microwave plasma torch and a reaction chamber with a fuel injector. Injection of hydrocarbon fuels into a high-temperature microwave torch plasma generates a plasma flame. The plasma flame can eliminate the odorous chemical agent diluted in air or purify the interior air of a large volume in isolated spaces. The specially designed reaction chamber eliminated H2S and NH3 diluted in airflow rate of 5000lpm (liters per minute), showing β values of 46.52 and 39.69J∕l, respectively.
Determination of the valence band offset of wurtzite InN∕ZnO heterojunction by x-ray photoelectron spectroscopy
Authors: Zhang Riqing , Zhang Panfeng , Kang Tingting , Fan Haibo , Liu Xianglin , Yang Shaoyan , Wei Hongyuan , Zhu Qinsheng , Wang Zhanguo
Published: October 2007

The valence band offset (VBO) of the wurtzite InN∕ZnO heterojunction is directly determined by x-ray photoelectron spectroscopy to be 0.82±0.23eV. The conduction band offset is deduced from the known VBO value to be 1.85∓0.23eV, which indicates a type-I band alignment for InN∕ZnO heterojunction.