bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of inertia on drop breakup under shear
Authors: Renardy Yuriko Y. , Cristini Vittorio
Published: July 2002

A spherical drop, placed in a second liquid of the same density and viscosity, is subjected to shear between parallel walls. The subsequent flow is investigated numerically with a volume-of-fluid continuous-surface-force algorithm. Inertially driven breakup is examined. The critical Reynolds numbers are examined for capillary numbers in the range where the drop does not break up in Stokes flow. It is found that the effect of inertia is to rotate the drop toward the vertical direction, with a mechanism analogous to aerodynamic lift, and the drop then experiences higher shear, which pulls the drop apart horizontally. The balance of inertial stress with capillary stress shows that the critical Reynolds number scales inversely proportional to the capillary number, and this is confirmed with full numerical simulations. Drops exhibit self-similar damped oscillations towards equilibrium analogous to a one-dimensional mass-spring system. The stationary drop configurations near critical conditions approach an inviscid limit, independent of the microphysical flow- and fluid-parameters.
Preisach modeling of ferroelectric pinched loops
Authors: Robert G. , Damjanovic D. , Setter N.
Published: July 2002

The Preisach approach is applied to model ferroelectric pinched (or constricted) loops. Starting from the existence of distributed switchable dipolar defects in the material and a corresponding distribution function, an analytical expression for the resulting ferroelectric loop is obtained. The corresponding hysteresis shape is indeed showing a clear pinching. Considerations about defect distribution expression and ways to characterize it from the loop are also made. It is finally suggested that such a description can be applied to any system exhibiting a pinched loop.
Ground state energy of massive scalar field inside a spherical region in the global monopole background
Authors: Bezerra de Mello E. R. , Bezerra V. B. , Khusnutdinov N. R.
Published: July 2002

Using the zeta function regularization method we calculate the ground state energy of scalar massive field inside a spherical region in the space–time of a pointlike global monopole. Two cases are investigated: (i) First, we calculate the Casimir energy inside a sphere of radius R and analyze the obtained result. We observe that this energy may be positive or negative depending on metric coefficient α and nonconformal coupling ξ. In the limit R→∞, this energy vanishes: (ii) In the second model, we surround the monopole by an additional sphere of radius r0<R and consider the scalar field confined in the region between these two spheres. In this case, the ground state energy presents an additional contribution due to boundary at r0 which is divergent for small radius. Additional comments about renormalization are considered.
Dynamic behavior in mathematical models of the tryptophan operon
Authors: Santillán Moisés , Mackey Michael C.
Published: July 2002

This paper surveys the general theory of operon regulation as first formulated by Goodwin and Griffith, and then goes on to consider in detail models of regulation of tryptophan production by Bliss, Sinha, and Santillán and Mackey, and the interrelationships between them. We further give a linear stability analysis of the Santillán and Mackey model for wild type E. coli as well as three different mutant strains that have been previously studied in the literature. This stability analysis indicates that the tryptophan production systems should be stable, which is in accord with our numerical results.
IR spectroscopy of aqueous alkali halide solutions: Pure salt-solvated water spectra and hydration numbers
Authors: Max Jean-Joseph , Chapados Camille
Published: July 2002

Extrapolation techniques were used to obtain pure salt-solvated water spectra from the attenuated total reflection infrared spectra (ATR-IR) of aqueous solutions of the nine alkali halide salts LiCl, NaCl, KCl, CsCl, NaBr, KBr, NaI, KI, and CsI and the alkaline–earth chloride salt MgCl2. These salts ionize completely in water. The ions by themselves do not absorb in the IR, but their interactions with water can be observed and analyzed. A pure salt-solvated water spectrum is easier to analyze than that of a combined solution of pure water and salt-solvated water. Although the salt-solvated water spectra examined have distinctive signatures, they can be classified in three categories: those similar to NaCl; those not similar to NaCl; and MgCl2, in a class by itself. Each of the pure salt-solvated water spectra differs from that of liquid water, though the number of bands is the same. From the Gaussian band fitting, we found that the positions of the bands were fairly constant, whereas their intensities differed. The salt hydration numbers were determined: for NaCl, KCl, NaBr, KBr, and CsI solutions it is 5; for KI and MgCL2 it is 4; for NaI it is 3.5; for CsCl it is 3; and for LiCl it is 2. From these results we found that each pair of ions (monoatomic ions) of the ten salt solutions studied are close bound and form a complex in a cluster organization with a fixed number of water molecules.
Simultaneous elemental analysis system using laser induced breakdown spectroscopy
Authors: Body Doug , Chadwick Bruce L.
Published: July 2002

A new instrumentation variation on laser induced breakdown spectroscopy (LIBS) is presented that allows simultaneous determination of all detectable elements using a multiple spectrograph and synchronous, multiple charge coupled device spectral acquisition system. The device has been designed to reduce the cost penalties often associated with the application of LIBS while allowing accurate and precise determination of the elemental composition of bulk materials. The system is particularly suited to the analysis of heterogeneous materials such as coal and mineral ores. In coal analysis detectable elements include the key inorganic components of coal—such as Al, Si, Mg, Ca, Fe, Na, and K—in addition to C and H. Detection limits vary depending on the particular element, but are typically of the order of 0.01% for as received, moist materials. Measurement repeatability and accuracy are typically within ±10% absolute, which is similar to results from standard analysis procedures for heterogeneous materials. Beta versions of the device have been produced and are currently being used routinely in two commercial coal-fired power stations.
Layered chiral metallic microstructures with inductive coupling
Authors: Svirko Yuri , Zheludev Nikolay , Osipov Michail
Published: July 2002

A bilayered quasiplanar metallic microstructure, comprising an ensemble of fully metallic “molecules” with inductive coupling between two parts of the molecule, is predicted to show strong optical rotatory power.
Electrical conductivity relaxation in thin-film yttria-stabilized zirconia
Authors: Rivera A. , Santamarı́a J. , León C.
Published: July 2002

We report on complex admittance measurements on ZrO2:Y2O3 (YSZ) thin films in the parallel plate geometry. Highly textured YSZ thin films, grown by rf sputtering, allow measuring complex admittance free of the effect of charge blocking at grain boundaries. We have examined low-temperature (close to room temperature) regime dominated by association of oxygen vacancies. Complex admittance analyzed in terms of the modulus formalism supplies information on correlation effects in ion motion and allows obtaining an association energy for the oxygen vacancies of 0.45 eV, in agreement with previous theoretical calculations.
Reduction of dispersionless coupled Korteweg–de Vries equations to the Euler–Darboux equation
Authors: Matsuno Yoshimasa
Published: July 2002

A quasilinear hyperbolic system of two first-order equations is introduced. The system is linearized by means of the hodograph transformation combined with Riemann’s method of characteristics. In the process of linearization, the main step is to explicitly express the characteristic velocities in terms of the Riemann invariants. The procedure is shown to be performed by quadrature only for specific combinations of the parameters in the system. We then apply the method developed here to the dispersionless versions of the typical coupled Korteweg–de Vries (cKdV) equations including the Broer–Kaup, Ito, Hirota–Satsuma, and Bogoyavlenskii equations and show that these equations are transformed into the classical Euler–Darboux equation. A more general quasilinear system of equations is also considered with application to the dispersionless cKdV equations for the Jaulent–Miodek and Nutku–Ög̃uz equations.
Designer gene networks: Towards fundamental cellular control
Authors: Hasty Jeff , Isaacs Farren , Dolnik Milos , McMillen David , Collins J. J.
Published: July 2002

The engineered control of cellular function through the design of synthetic genetic networks is becoming plausible. Here we show how a naturally occurring network can be used as a parts list for artificial network design, and how model formulation leads to computational and analytical approaches relevant to nonlinear dynamics and statistical physics. We first review the relevant work on synthetic gene networks, highlighting the important experimental findings with regard to genetic switches and oscillators. We then present the derivation of a deterministic model describing the temporal evolution of the concentration of protein in a single-gene network. Bistability in the steady-state protein concentration arises naturally as a consequence of autoregulatory feedback, and we focus on the hysteretic properties of the protein concentration as a function of the degradation rate. We then formulate the effect of an external noise source which interacts with the protein degradation rate. We demonstrate the utility of such a formulation by constructing a protein switch, whereby external noise pulses are used to switch the protein concentration between two values. Following the lead of earlier work, we show how the addition of a second network component can be used to construct a relaxation oscillator, whereby the system is driven around the hysteresis loop. We highlight the frequency dependence on the tunable parameter values, and discuss design plausibility. We emphasize how the model equations can be used to develop design criteria for robust oscillations, and illustrate this point with parameter plots illuminating the oscillatory regions for given parameter values. We then turn to the utilization of an intrinsic cellular process as a means of controlling the oscillations. We consider a network design which exhibits self-sustained oscillations, and discuss the driving of the oscillator in the context of synchronization. Then, as a second design, we consider a synthetic network with parameter values near, but outside, the oscillatory boundary. In this case, we show how resonance can lead to the induction of oscillations and amplification of a cellular signal. Finally, we construct a toggle switch from positive regulatory elements, and compare the switching properties for this network with those of a network constructed using negative regulation. Our results demonstrate the utility of model analysis in the construction of synthetic gene regulatory networks.
Nuclear Fukui function and Berlin’s binding function: Prediction of the Jahn–Teller distortion
Authors: Balawender Robert , De Proft Frank , Geerlings Paul
Published: July 2002

The properties of the derivative of the total binding function (the virial of the forces) with respect to the number of electrons and its decomposition at local and atomic level have been analyzed. At local level the binding function is expressed by the Berlin function fv(r) and the electronic Fukui function f(r). The atomic analog is expressed in terms of the nuclear Fukui function (FF) and the nuclear position vectors. A relationship between the local maps of fv(r)f(r), the nuclear FF vectors, and the Jahn–Teller distortion direction is discussed. It is predicted that upon ionization the symmetry of the nearest local stationary point for BH3 is C2v, for AH4 molecules (CH4 and SiH4) D2d, and for C3H6 C2v. For the benzene anion a D2h symmetry is predicted.
Quasirandom distributed Gaussian bases for bound problems
Authors: Garashchuk Sophya , Light John C.
Published: July 2002

We introduce quasirandom distributed Gaussian bases (QDGB) that are well suited for bound problems. The positions of the basis functions are chosen quasirandomly while their widths and density are functions of the potential. The basis function overlap and kinetic energy matrix elements are analytical. The potential energy matrix elements are accurately evaluated using few-point quadratures, since the Gaussian basis functions are localized. The resulting QDGB can be easily constructed and is shown to be accurate and efficient for eigenvalue calculation for several multidimensional model vibrational problems. As more demanding examples, we used a 2D QDGB-DVR basis to calculate the lowest 400 or so energy levels of the water molecule for zero total angular momentum to sub-wave-number precision. Finally, the lower levels of Ar3 and Ne3 were calculated using a symmetrized QDGB. The QDGB was shown to be accurate with a small basis.
Electrochemical carbon nanotube field-effect transistor
Authors: Krüger M. , Buitelaar M. R. , Nussbaumer T. , Schönenberger C. , Forró L.
Published: July 2002

We explore the electric-field effect of carbon nanotubes (NTs) in electrolytes. Due to the large gate capacitance, Fermi energy (EF) shifts of order ±1 V can be induced, enabling to tune NTs from p to n-type. Consequently, large resistance changes are measured. At zero gate voltage, the NTs are hole-doped in air with |EF|≈0.3–0.5 eV, corresponding to a doping level of ≈1013 cm−2. Hole-doping increases in the electrolyte.
Multistationarity, the basis of cell differentiation and memory. I. Structural conditions of multistationarity and other nontrivial behavior
Authors: Thomas R. , Kaufman M.
Published: July 2002

A biological introduction serves to remind us that differentiation is an epigenetic process, that multistationarity can account for epigenetic differences, including those involved in cell differentiation, and that positive feedback circuits are a necessary condition for multistationarity and, by inference, for differentiation. The core of the paper is comprised of a formal description of feedback circuits and unions of disjoint circuits. We introduce the concepts of full-circuit (a circuit or union of disjoint circuits which involves all the variables of the system), and of ambiguous circuit (a circuit whose sign depends on the location in phase space). We describe the partition of phase space (a) according to the signs of the ambiguous circuits, and (b) according to the signs of the eigenvalues or their real part. We introduce a normalization of the system versus one of the circuits; in two variables, this permits an entirely general description in terms of a common diagram in the “circuit space.” The paper ends with general statements concerning the requirements for multistationarity, stable periodicity, and deterministic chaos.
Quasi-classical treatment of the Stereodynamics of chemical reactions: k-r-k′ vector correlation for the Li+HF(v=1,j=1)→LiF+H reaction
Authors: Aoiz F. J. , Martı́nez M. T. , Sáez Rábanos V.
Published: July 2002

We present a classical treatment of the k-r-k′ vector correlation for atom–diatom reactions, involving the directions of the initial and final relative velocities and the internuclear axis of the diatom. The formalism is based on the expansion of the joint probability distributions in multipolar moments, and it is analogous to that developed for the vector correlations implying the initial or final rotational angular momentum. Within the framework of classical mechanics, the present treatment allows the determination of the differential cross section for any preparation of the internuclear axis and for any initial rotational angular momentum of the reactants. This methodology has been applied to the study of the steric effect in the Li+HF(v=1,j=1,m=0)→LiF+H reaction and the theoretical results have been compared via simulation with the recent experimental determination by Loesch and co-workers of laboratory angular distributions (LAB ADs) for several distributions of the internuclear axis of HF. Very good agreement has been found between experimental and simulated LAB AD, allowing an interpretation of the experimental results. Although the integral steric effect is very small, the differential and/or state resolved steric effects are more pronounced. Moreover, it has been found that by varying the preparation of the internuclear axis of the HF molecule, the population of final states of the LiF changes noticeably, which represents a clear case of control of the outcome of a chemical reaction.
Flat panel display prototype using gated carbon nanotube field emitters
Authors: Wang Q. H. , Yan M. , Chang R. P. H.
Published: July 2002

A flat panel display prototype has been fabricated using gated carbon nanotubes as electron emission source. The gate structure is made by a self-aligned method. The pixels are turned “on” and “off” by controlling the gate electrode with a 50 V source. The operating properties of the display show much promise for further development. It is believed that this technology can lead to easy-to-make and inexpensive flat panel displays with low driving voltage.
Development of selective lateral photoelectrochemical etching of InGaN/GaN for lift-off applications
Authors: Stonas A. R. , Margalith T. , DenBaars S. P. , Coldren L. A. , Hu E. L.
Published: July 2002

The authors have developed a wet band gap-selective photoelectrochemical etching process to produce deep undercuts (∼500 μm) into InGaN/GaN heterostructures. These undercuts were used in a lift-off process which successfully transferred device-scale (100 μm diameter, 5 μm thick) disks from their underlying sapphire substrates to another substrate. Experiments were conducted using a lamp-and-filter arrangement, employing n-type and p-type GaN pieces as filters. Polishing was conducted to smooth the resulting substrate-transferred GaN disks.
Clifford algebra as quantum language
Authors: Baugh James , Finkelstein David Ritz , Galiautdinov Andrei , Saller Heinrich
Published: July 2002

We suggest Clifford algebra as a useful simplifying language for present quantum dynamics. Clifford algebras arise from representations of the permutation groups as they arise from representations of the rotation groups. Aggregates using such representations for their permutations obey Clifford statistics. The vectors supporting the Clifford algebras of permutations and rotations are plexors and spinors, respectively. Physical spinors may actually be plexors describing quantum ensembles, not simple individuals. We use Clifford statistics to define quantum fields on a quantum space–time, and to formulate a quantum dynamics-field-space–time unity that evades the compactification problem. The quantum bits of history regarded as a quantum computation seem to obey a Clifford statistics.
Bubble nucleation in binary mixtures: A semiempirical approach
Authors: Talanquer V. , Cunningham Crystal , Oxtoby D. W.
Published: July 2002

We present a semiempirical approach to the density functional theory of bubble nucleation in binary mixtures. Intermolecular forces between solvent and solute molecules are modeled using Lennard-Jones potentials whose interaction parameters are determined by equilibrium properties of the pure liquids and the liquid mixtures. We study the temperature variation of the critical supersaturation for carbon dioxide, oxygen, nitrogen, and hydrogen dissolved in water at 1 atm pressure. Our results are consistent with experimental trends that show an “inverse” dependence of the nucleation rates on temperature for the case of hydrogen. In the different mixtures, critical nuclei for gas formation exhibit fluid densities at their centers that are a substantial fraction of the density of the corresponding pure liquids.
Modeling of quasistatic magnetic hysteresis with feed-forward neural networks
Authors: Makaveev Dimitre , Dupré Luc , De Wulf Marc , Melkebeek Jan
Published: July 2002

A modeling technique for rate-independent (quasistatic) scalar magnetic hysteresis is presented, using neural networks. Based on the theory of dynamic systems and the wiping-out and congruency properties of the classical scalar Preisach hysteresis model, the choice of a feed-forward neural network model is motivated. The neural network input parameters at each time step are the corresponding magnetic field strength and memory state, thereby assuring accurate prediction of the change of magnetic induction. For rate-independent hysteresis, the current memory state can be determined by the last extreme magnetic field strength and induction values, kept in memory. The choice of a network training set is motivated and the performance of the network is illustrated for a test set not used during training. Very accurate prediction of both major and minor hysteresis loops is observed, proving that the neural network technique is suitable for hysteresis modeling.
Fabrication and characterization of ultraviolet-emitting diodes composed of transparent p-n heterojunction, p-SrCu2O2 and n-ZnO
Authors: Ohta Hiromichi , Orita Masahiro , Hirano Masahiro , Hosono Hideo
Published: July 2002

An ultraviolet light-emitting diode (UV-LED) was realized using a p-n heterojunction composed of the transparent oxide semiconductors p-SrCu2O2 and n-ZnO. A Ni/SrCu2O2/ZnO/ITO multilayered film was epitaxially grown on an extremely flat YSZ (111) surface by a pulsed-laser deposition technique. SrCu2O2 (112) was preferentially grown on ZnO (0001) at 350°C, while the preferential plane was changed into the (100) when the temperature was increased to 600 °C. The grown films were processed by conventional photolithography followed by reactive ion etching to fabricate heterojunction diodes. The resulting devices exhibited rectifying I-V characteristics inherent to p-n junctions. A relatively sharp electroluminescence band centered at 382 nm, attributed to transitions associated with exciton-exciton collision or electron-hole plasma in ZnO, was generated by applying a forward bias voltage greater than the turn-on voltage of 3 V. UV-LED performance characteristics such as threshold current and conversion efficiency improved with higher SrCu2O2 deposition temperatures. On the other hand, increased laser power density at 600 °C during deposition raised the incidence of insulating layer formation between the p and n layers, probably due to migration of K+ ions doped as an acceptor impurity. The resulting p-i-n diode emits broad luminescence centered at 500 nm for forward voltage greater than 14 V.
Monitoring unresolved scales in multiscale turbulence modeling
Authors: Collis S. Scott
Published: July 2002

This paper extends the two-level variational multiscale (VMS) method for large-eddy simulation, introduced by Hughes et al. [Comput. Visual. Sci. 3, 47 (2000)], to a three-level approach that clarifies the role of unresolved scales of motion on the resolved scales. It is shown that the multiscale framework does not obviate modeling on the large scales, but that VMS allows for different modeling assumptions to be used on each scale range. In particular, one can choose to neglect the effect of the unresolved scales on the large scales which is reasonable for sufficiently large-scale separation.
Universal solders for direct and powerful bonding on semiconductors, diamond, and optical materials
Authors: Mavoori Hareesh , Ramirez Ainissa G. , Jin Sungho
Published: July 2002

The surfaces of electronic and optical materials such as nitrides, carbides, oxides, sulfides, fluorides, selenides, diamond, silicon, and GaAs are known to be very difficult to bond with low melting point solders (<300 °C). We have achieved a direct and powerful bonding on these surfaces by using low temperature solders doped with rare-earth elements. The rare earth is stored in micron-scale, finely-dispersed intermetallic islands (Sn3Lu or Au4Lu), and when released, causes chemical reactions at the interface producing strong bonds. These solders directly bond to semiconductor surfaces and provide ohmic contacts. They can be useful for providing direct electrical contacts and interconnects in a variety of electronic assemblies, dimensionally stable and reliable bonding in optical fiber, laser, or thermal management assemblies.
Computation of the methane–water potential energy hypersurface via ab initio methods
Authors: Cao Zhitao , Tester Jefferson W. , Trout Bernhardt L.
Published: July 2002

Accurate intermolecular potentials between hydrocarbons and water are essential for the prediction of properties of systems containing these components. Unfortunately, current experimental techniques are unable to measure directly the interaction potential between methane and water molecules. Therefore we have used quantum mechanical calculations, both ab initio and density functional theory (DFT) calculations, in order to determine the H2O–CH4 potential energy surface (PES) accurately for use in modeling gas hydrates. Ab initio methods were found to be more accurate than DFT methods, which do not account for the substantial dispersion interactions that exist between methane and water. Electron correlation was found to be treated accurately by MP2. However, a large basis set, cc-pVQZ was found to be necessary to compute the binding energies to within 0.1 kcal/mol of the basis set limit. In order to sample accurately the PES, the H2O–CH4 binding energy was computed at 18 000 points. For these computations to be feasible, a new method was developed in which all 18 000 points were computed using MP2/6-31++G(2d,2p) and then corrected to near the accuracy of MP2/cc-pVQZ. The PES calculated from the six-dimensional numerical potential agrees very well with far infrared vibration-rotation-tunneling spectroscopic data and experimental second virial coefficient data at the potential minimum and larger separations. The exp-6 potential proves to be the best representation of the H2O–CH4 interaction.
Polyelectrolyte–macroion complexation. I. Effect of linear charge density, chain length, and macroion charge
Authors: Jonsson Marie , Linse Per
Published: July 2002

The complexation between a linear flexible polyelectrolyte and one or several oppositely charged macroions was examined by employing a simple model system with focus on the electrostatic interactions. The composition and the structure of the complex as well as conformational data of the polyelectrolyte were obtained by using Monte Carlo simulations. These properties were investigated at different linear charge densities of the polyelectrolyte, different chain lengths of the polyelectrolyte, and different macroion charges, all at different numbers of macroions at constant volume. The binding isotherms obtained are Langmuir type, and in excess of macroions the polyelectrolyte–macroion complex displays a charge reversal. Upon complexation, the polyelectrolyte extension first reduces and thereafter increases as the number of complexed macroions increases, the minimal extension appearing for a neutral complex. Macroions prefer to complex to central polyelectrolyte segments, but for a neutral or an overcharged complex the entire polyelectrolyte chain is involved. In the complex, the macroions are able to come into direct contact with each other, despite their mutual electrostatic repulsion. The distribution of the small ions is also presented.
New implementation of the trajectory surface hopping method with use of the Zhu–Nakamura theory
Authors: Zhu Chaoyuan , Nobusada Katsuyuki , Nakamura Hiroki
Published: July 2002

A new implementation of the trajectory surface hopping (TSH) method is proposed to treat multidimensional nonadiabatic dynamics by incorporating the analytical Zhu–Nakamura semiclassical theory of nonadiabatic transition. The problem of classically forbidden hops in the TSH method can now be solved and dealt with just as easily as the classically allowed hops by introducing nonvertical hopping techniques. This is made possible, because the theory can treat both classically allowed and forbidden hops accurately in a unified way. The Zhu–Nakamura theory also enables us to predetermine important regions of potential energy surface before carrying out any dynamics calculations, and thus to save a lot of computational efforts. The charge transfer processes in the collinear H3+ system are studied numerically to test the new TSH method. Comparing the new and old versions of TSH with exact quantum calculations, the new method shows much better agreement with the exact calculations. It also works well when all trasitions are classically forbidden and the old method fails completely.
Giant isotropic magnetostriction of itinerant-electron metamagnetic La(Fe0.88Si0.12)13Hy compounds
Authors: Fujieda S. , Fujita A. , Fukamichi K. , Yamazaki Y. , Iijima Y.
Published: July 2002

La(Fe x Si 1−x ) 13 compounds exhibit an itinerant-electron metamagnetic (IEM) transition above Curie temperature TC. The IEM transition in the compound with x=0.88 is accompanied by a giant volume change. From a practical viewpoint, TC was controlled by hydrogen absorption in order to obtain such a giant volume magnetostriction at room temperature. For the La(Fe0.88Si0.12)13H1.0 compound, the IEM transition occurs above TC=278 K, and a significant isotropic linear magnetostriction of about 0.3% at 7 T is induced in the vicinity of room temperature. This large magnetostriction is attributed to the giant volume magnetostriction of about 1% by the IEM transition.
Molecular dynamics simulation of the linear low-density polyethylene crystallization
Authors: Zhang Xiu-bin , Li Ze-sheng , Lu Zhong-yuan , Sun Chia-chung
Published: July 2002

By means of molecular dynamics simulation, the crystallization process of the copolymer (ethylene-co-propene) with different branch distributions is studied at the molecular level. It is shown that subglobules formed at the branch sites along the copolymer chains and subsequently coalesced into a single globule and then developed to a lamellar structure in the end. This process can be considered as crystal nucleation and growth at the early stage of the copolymer crystallization. In the nucleation the branch acts as a nucleating seed and in the crystal growth the branch is rejected from the crystal region as a defect. The driving force for the relaxation process is the attractive van der Waals interaction between the chain segments. Furthermore, it is found that the branch distribution is an important factor in determining the crystallinity of the copolymer, when the comonomer composition and the molecular weight of the copolymers are fixed, as a blocky type of copolymer will show nearly unhampered crystallization and a true random copolymer will show a hindered one.
n
-symplectic algebra of observables in covariant Lagrangian field theory
Authors: Norris L. K.
Published: July 2002

n -symplectic geometry on the adapted frame bundle λ:LπE→E of an n=(m+k)-dimensional fiber bundle π:E→M is used to set up an algebra of observables for covariant Lagrangian field theories. Using the principle bundle ρ:LπE→J1π we lift a Lagrangian L:J1π→R to a Lagrangian L≔ρ*(L):LπE→R, and then use L to define a “modified n-symplectic potential” θ̂L on LπE, the Cartan–Hamilton–Poincaré (CHP) Rn-valued 1-form. If the lifted Lagrangian is nonzero, then (LπE,dθ̂L) is an n-symplectic manifold. To characterize the observables we define a lifted Legendre transformation φL from LπE into LE. The image QL≔φL(LπE) is a submanifold of LE, and (QL,d(θ̂|QL)) is shown to be an n-symplectic manifold. We prove the theorem that θ̂L=φL*(θ|QL), and pull back the reduced canonical n-symplectic geometry on QL to LπE to define the algebras of observables on the n-symplectic manifold (LπE,dθ̂L). To find the reduced n-symplectic algebra on QL we set up the equations of n-symplectic reduction, and apply the general theory to the model of a k-tuple of massless scalar fields on Minkowski space–time. The formalism set forth in this paper lays the ground work for a geometric quantization theory of fields.
Nondestructive high-resolution and absolute mass determination of single charged particles in a three-dimensional quadrupole trap
Authors: Schlemmer Stephan , Illemann Jens , Wellert Stefan , Gerlich Dieter
Published: July 2002

A method for the nondestructive high-resolution and absolute mass determination of charged particles is described. It is based on the detection of light scattered from a single particle in an electrodynamical three-dimensional quadrupole trap. From the amplitude modulation of this signal, the eigenfrequencies of the secular motion are determined with high precision. For 500 nm in diameter SiO2 particles, having a mass of 130 fg (≈1011 u), a resolution of 10−4 is achieved in a 10 s measurement. On a longer time scale, the 10−6 range can be accessed. Key features of the method such as reproducibility, long term stability, accuracy, and linearity are characterized in detail. The extension of the mass range of quadrupole traps from atomic masses to micrometer sized particles as well as potential applications are discussed.
Optical gain and saturation in nitride-based laser structures
Authors: Vehse M. , Michler P. , Lange O. , Röwe M. , Gutowski J. , Bader S. , Lugauer H.-J. , Brüderl G. , Weimar A. , Lell A. , Härle V.
Published: July 2002

We have performed systematic studies of the optical gain and its saturation in (In, Ga)N/GaN/(Al, Ga)N laser structures that depend on the excitation density and number of quantum wells. The unsaturated gain factor which was obtained by the variable stripe-length method increases with excitation power, i.e., increasing modal gain. The gain factor also increases with a decreasing number of quantum wells, as is shown by the investigation of a series of laser structures with 3, 4, 5, and 10 quantum wells for fixed modal gain. Values up to 40 dB at 300 K were measured. Thermal activation energies obtained by temperature dependent photoluminescence measurements yield information on the influence of nonradiative recombination processes on optical gain saturation.
Modeling of thermal magnetization fluctuations in thin-film magnetic devices
Authors: Smith Neil
Published: July 2002

It has been recently shown that broad-band, thermally induced magnetization fluctuations in submicron magnetoresistive (MR) read sensors (used in all present and foreseeable magnetic hard-disk-drives), will serve as a fundamental limit to their achievable signal-to-noise ratio, independent of how large the intrinsic sensitivity parametrics, e.g., ΔR/R, may be. This type of magnetization noise may also be of consequence and/or interest for a broader class of future nanoscale magnetic devices. The foundation for understanding and quantitatively modeling this phenomenon on a practical device-physics level is the fluctuation-dissipation theorem (FDT). This paper develops a theoretical methodology for application of the FDT to the problem of micromagnetic modeling of thermal magnetization fluctuations in small, magnetically soft thin-film magnetic devices, using a generalized Gilbert formulation of magnetization dynamics with phenomenological damping.
Infrared spectroscopy study of low-dielectric-constant fluorine-incorporated and carbon-incorporated silicon oxide films
Authors: Kim Yoon-Hae , Hwang Moo Sung , Kim Hyeong Joon , Kim Jin Yong , Lee Young
Published: July 2002

Bonding characteristics of low-dielectric-constant (low-k) fluorine-incorporated silicon oxide (SiOF) and carbon-incorporated silicon oxide (SiOC) films prepared by plasma enhanced chemical vapor deposition were investigated by Fourier transform infrared spectroscopy (FTIR). The frequency of Si–O stretching vibration mode in SiOF film shifted to higher wave number (blueshift) with the increase of fluorine incorporation, while that in SiOC film shifted to lower wave number (redshift) as the carbon content increased. In N2-annealed SiOC film, the Si–O stretching frequency slightly shifted to lower wave number. To elucidate these phenomena, we have developed the “bonding structure model” based on the electronegativity of an atom. The frequency shifts observed in the FTIR spectra of SiOF and SiOC films were well explained by this model.
Biexciton luminescence in high-quality ZnO epitaxial thin films
Authors: Yamamoto Aishi , Miyajima Kensuke , Goto Takenari , Ju Ko Hang , Yao Takafumi
Published: July 2002

A photoluminescence (PL) band, the M band, was observed in photoluminescence spectra for various excitation densities in high-quality ZnO epitaxial thin films. The M band intensity increased superlinearly with an increase in the excitation intensity, suggesting that the observed PL band is due to a biexciton state. In order to prove this, the photoluminescence excitation (PLE) spectrum of the M band and time dependence of the PL intensity were measured. A shoulder that originates from two-photon absorption of the biexciton state appeared in the PLE spectrum. The biexciton binding energy was estimated to be 15 meV. The temporal behavior of the PL intensities of the M and free-exciton bands can be explained by modified rate equations assuming that the M band is caused by radiative annihilation of a biexciton leaving a free exciton and that the rate of creation of biexcitons is proportional to the 1.5th power of the exciton density. This power dependence is consistent with experimental results showing that the M band intensity is proportional to the 1.5th power of the excitation density.
High strength, low dielectric constant fluorinated silica xerogel films
Authors: Gorman B. P. , Orozco-Teran Rosa A. , Roepsch Jodi A. , Dong Hanjiang , Reidy Richard F. , Mueller D. W.
Published: July 2002

The mechanical, electrical, and microstructural properties of low-k fluorinated silica xerogels produced using a one step spin-on process are reported. Derived from a fluorinated silane monomer, these films are easily processed and exhibit very low dielectric constants (2.1 as processed and 2.3 after heat treating at 450 °C in air). Structural determination by Fourier transform infrared spectrophotometry indicates a fluorinated silica structure with shortened Si–O bonds; however, some of the fluorine is lost during annealing. Nanoindentation studies show high elastic moduli (12 GPa) and hardness (1 GPa). Microstructural analyses by transmission and scanning electron microscopy indicate an unusual morphology with highly linked features and pore sizes in the 20–30 nm range. We believe the low dielectric constants and robust mechanical properties are due to the unusual microstructure of these films.
An intermediate state representation approach to K-shell ionization in molecules. I. Theory
Authors: Schirmer J. , Thiel A.
Published: July 2002

The general intermediate state representation (ISR) for single-electron ionization is adapted to the case of K-shell (or core-level) ionization in molecules. The development is based on the so-called core–valence separation (CVS) approximation leading to a considerable simplification of the ISR secular equations. Using the CVS approximation the core-level ISR can be formulated entirely in terms of the intermediate states of the valence electron excitation problem, which allows one to construct consistent nth-order approximation schemes for the (single-hole) ionization energies by a specific extension of the (n−2)-nd order ISR approximation for electronic excitation. In particular, the CVS-ISR concept is used to derive a consistent fourth-order approximation for core-level ionization based on the existing second-order algebraic-diagrammatic construction [ADC(2)] approximation to electron excitation. The computational scheme combines the diagonalization of a Hermitian secular matrix with finite perturbation expansions for the secular matrix elements. The explicit configuration space is spanned by one-hole (1h), two-hole-one-particle (2h-1p), and (3h-2p) ionic states with exactly one hole in the core-level shell of interest, while the configurations considered implicitly via perturbation theory extend to the class of 5h-4p states. A characteristic of the method is that the dominant valence electron relaxation effect is accounted for at the post-Hartree–Fock (HF) level. This calls for the relatively high order of perturbation-theoretical consistency, but avoids, on the other hand, the necessity of a localized (symmetry breaking) one-particle representation in the case of molecules with equivalent 1s orbitals. The method is size consistent and thus suitable for applications to large systems.
Mobility enhancement limit of excimer-laser-crystallized polycrystalline silicon thin film transistors
Authors: Hara Akito , Takeuchi Fumiyo , Sasaki Nobuo
Published: July 2002

The effects of various carrier scattering mechanisms on excimer-laser-crystallized polycrystalline silicon (poly-Si) thin film transistors (TFTs) fabricated using 450 °C processes on a glass substrate were studied. Good performance of a separated by ion implanted oxygen (SIMOX) metal–oxide–semiconductor field-effect transistor (MOSFET) with field-effect mobility of 670 cm2/V s and a subthreshold swing value of 0.087 V/dec was obtained using these 450 °C processes. The results showed the formation of a good silicon/silicon dioxide (SiO2) interface that is comparable to that of thermal oxide, as well as the high capability of 450 °C processes. The performance of the above SIMOX-MOSFET is superior to that of excimer-laser-crystallized poly-Si TFTs fabricated using the same 450 °C processes. This shows that poorer performance of poly-Si TFTs is caused by the poor crystalline quality of the poly-Si film. The field-effect mobility is affected little by the in-grain microdefects and surface morphology of the excimer-laser-crystallized poly-Si film, but it is highly sensitive to the grain size. A field-effect mobility of 320 cm2/V s was obtained for an average grain size of 700 nm. The increase in field-effect mobility began to saturate with grain sizes of approximately 1000 nm. It is not necessary to enlarge the grain size beyond the saturation point of the field-effect mobility to improve performance, because the field-effect mobility of an average grain size of 700 nm is limited by phonon scattering, but not by the grain boundary.
Methane transformation to carbon and hydrogen on Pd(100): Pathways and energetics from density functional theory calculations
Authors: Zhang C. J. , Hu P.
Published: July 2002

Density functional theory with gradient corrections has been employed to study the reaction pathways and the reaction energetics for the transformations of CH4 to C and H on a Pd(100) surface. On examination of transition state structures identified in each elementary reaction, a clear relationship between the valencies of the CHx fragments and the locations of the transition states emerges. The higher the valency of the CHx fragment, the higher the coordination number of the CHx with the surface atoms. The calculated reaction energetics are in good agreement with the experiments. In addition, calculation results are also used to illustrate an interesting issue concerning the CH3 stability on Pd surfaces.
Instrumentation for fast functional optical tomography
Authors: Schmitz Christoph H. , Löcker Mario , Lasker Joseph M. , Hielscher Andreas H. , Barbour Randall L.
Published: July 2002

In this article, we describe the design rationale and performance features of an integrated multichannel continuous wave (cw) near-infrared (NIR) optical tomographic imager capable of collecting fast tomographic measurements over a large dynamic range. Fast data collection (∼70 Hz/channel/wavelength) is achieved using time multiplexed source illumination (up to 25 illumination sites) combined with frequency encoded wavelength discrimination (up to four-wavelength capability) and parallel detection (32 detectors). The described system features a computerized user interface that allows for automated system operation and is compatible with various previously described measuring heads. The results presented show that the system exhibits a linear response over the full dynamic measuring range (180 dB), and has excellent noise (∼10 pW noise equivalent power) and stability performance (<1% over 30 min). Recovered images of laboratory vessels show that dynamic behavior can be accurately defined and spatially localized.
Investigation of cation-exchanged montmorillonites by combined x-ray diffraction and positron annihilation lifetime spectroscopy
Authors: Consolati G. , Natali-Sora I. , Pelosato R. , Quasso F.
Published: July 2002

A characterization of some mineral and organophilic montmorillonites has been carried out using x-ray diffraction and positron annihilation lifetime spectroscopy. The combination of the two techniques supplies information on the microscopic structure of clays, in particular on the fraction of interlayer gap which is occupied by water molecules in the mineral clays. Nanocavities among cations of quaternary ammonium salts inserted in the organophilic clays were also investigated and compared with those existing in the free salt.
Stress hysteresis during thermal cycling of plasma-enhanced chemical vapor deposited silicon oxide films
Authors: Thurn Jeremy , Cook Robert F.
Published: July 2002

The mechanical response of plasma-enhanced chemical vapor deposited SiO2 to thermal cycling is examined by substrate curvature measurement and depth-sensing indentation. Film properties of deposition stress and stress hysteresis that accompanied thermal cycling are elucidated, as well as modulus, hardness, and coefficient of thermal expansion. Thermal cycling is shown to result in major plastic deformation of the film and a switch from a compressive to a tensile state of stress; both athermal and thermal components of the net stress alter in different ways during cycling. A mechanism of hydrogen incorporation and release from as-deposited silanol groups is proposed that accounts for the change in film properties and state of stress.
Theoretical study of single molecule fluorescence in a metallic nanocavity
Authors: Enderlein Jörg
Published: July 2002

A theoretical study is presented on the fluorescence excitation and emission of a molecule embedded within a nanometric metal-coated dielectric bead. It occurs that, for favorable values of bead radius and metal layer thickness, photostability of the molecule is improved significantly, due to a dramatic decrease of the fluorescence lifetime caused by the strong electromagnetic interaction between the emitting molecule and the metal layer. Moreover, the metal shell enhances the exciting electric field intensity leading to an increase in fluorescence brightness by more than one order of magnitude.
Bonding nature of rare-earth-containing lead-free solders
Authors: Ramirez Ainissa G. , Mavoori Hareesh , Jin Sungho
Published: July 2002

The ability of rare-earth-containing lead-free solders to wet and bond to silica was investigated. Small additions of Lu (0.5–2 wt. %) added to eutectic Sn–Ag or Au–Sn solder render it directly solderable to a silicon oxide surface. The bonding is attributed to the migration of the rare-earth element to the solder–silica interface for chemical reaction and the creation of an interfacial layer that contains a rare-earth oxide. It was found that additions of rare-earth materials did not significantly modify the solidification microstructure or the melting point. Such oxide-bondable solders can be useful for assembly of various optical communication devices.