bluuebunny/crossref_metadata_2025 huggingface dataset:
Fluorescent probes and bioconjugation chemistries for single-molecule fluorescence analysis of biomolecules
Authors: Kapanidis Achillefs N. , Weiss Shimon
Published: December 2002

Fluorescence-based detection of single biomolecules in solution and at room temperature has opened new avenues for understanding biological mechanisms. Single-molecule fluorescence spectroscopy (SMFS) of biomolecules requires careful selection of fluorophores, sites of incorporation, and labeling chemistries. SMFS-compatible fluorophores should permit extended, uninterrupted observations of fluorescence with high signal-to-noise ratios; more stringent considerations apply for specific methodologies, such as fluorescence resonance energy transfer and fluorescence anisotropy. Strategies for site-specific in vitro labeling of small proteins exploit the reactivity of the amino acid cysteine (Cys), allowing incorporation of one or more fluorophores; labeling of closely spaced Cys residues using bis-functionalized fluorophores allows probing of the orientation of individual protein domains. For in vitro labeling of large proteins, the options include peptide ligation, intein-mediated labeling, puromycin-based labeling, unnatural amino acid mutagenesis, and reconstitution from individual subunits or subunit fragments. For in vivo analysis, one can use proteins that are labeled in vitro and then incorporated in cells; genetic encoding of specific protein sequences can also lead to in vivo labeling, either by in vivo targeting by fluorophores or by biosynthesis of protein fusions with natural fluorophores such as the green fluorescent protein. The existing methods, along with others under development, will bring SMFS to the mainstream and advance significantly our understanding of vital biological processes.
Analytical solutions for distributed multipolar vortex equilibria on a sphere
Authors: Crowdy Darren , Cloke Martin
Published: December 2002

Analytical solutions of the steady Euler equations corresponding to stationary multipolar vortices on a sphere are derived. The solutions represent localized regions of distributed vorticity consisting of uniform vortex patches with a finite set of superposed point vortices. The mathematical method combines stereographic projection with conformal mapping theory to generalize a class of exact solutions for planar multipolar vortices developed by Crowdy [Phys. Fluids 11, 2556 (1999)] to the physically more important scenario of multipolar vortices on a spherical surface. The solutions are believed to be the first examples of analytical solutions of the Euler equations on a sphere involving patches of distributed vorticity with nontrivial shape.
On the theoretical determination of the static dipole polarizability of intermediate size silicon clusters
Authors: Bazterra Vı́ctor E. , Caputo Marı́a C. , Ferraro Marta B. , Fuentealba Patricio
Published: December 2002

The B3PW91 method of the density functional theory has been applied to the study of the dipole polarizability of medium size silicon clusters employing pseudopotential on all of them. All electron calculations have been performed for those clusters with less than nine atoms. In addition, we have optimized the structures of the clusters with less than ten atoms. On using the modified genetic algorithm, fourteen conformers of silicon isomers with nine atoms have been determined. The corresponding geometry of these clusters was optimized and their relative stability determined. The calculated polarizabilities are compared with experimental data and previous theoretical results.
Computer simulations of pure and mixed systems of disklike particles interacting with the S-function Corner potential
Authors: Cinacchi Giorgio , Tani Alessandro
Published: December 2002

The S-function potential for single-site particles has been recently introduced as an extension of the Gay–Berne potential. With the S-function potential, we can model particles of other and, possibly, more realistic shapes than Gaussian overlap ellipsoids and we can simulate mixtures in a systematic manner. As an example of both applications, we have studied via Monte Carlo computer simulation assemblies of cylindrically symmetric disklike particles of two different types. They can be considered complementary perturbations of a disk, with, respectively, a depression (red-cell, R particles) and a protuberance (ufo, U particles) at the center. The former is meant to mimic the average effect of side chains of real diskotics, while the latter is a simple representation of diskotic metallomesogens. Four systems of these kind of particles have been studied as a function of temperature: two pure cases and two 1:2 binary mixtures, so that the combined effects of shape and concentration can be observed. We have found the largest variety of phases in the pure U system where also a nematic phase is present. No such phase is observed when 13 of the U particles are replaced by R particles, as well as in the mixture 23R-13U and in the pure R system. At the lowest temperatures investigated all systems show a lattice phase except the mixture 13R-23U, whose organization is better described as a disordered columnar phase.
Exploring the energy landscape of proteins: A characterization of the activation-relaxation technique
Authors: Wei Guanghong , Mousseau Normand , Derreumaux Philippe
Published: December 2002

Finding the global energy minimum region of a polypeptide chain, independently of the starting conformation and in a reasonable computational time, is of fundamental interest. As the energy landscape of proteins is very rugged, sampling is hindered by the vast number of minima existing on this multidimensional landscape. In this study, we use activation-relaxation technique (ART) to explore the energy landscape of a series of peptide models with 14, 26, and 28 amino acids. Peptides are modeled by a reduced off-lattice representation and a simplified OPEP-like (optimized potential for efficient peptide-structure prediction) energy model. ART defines moves directly in the energy landscape and can generate with equal efficiency events with root-mean-square deviation as small as 0.1 or as large as 4 Å. Our results show that (i) ART trajectories are reversible and provide real activated paths; (ii) ART simulations converge to the same low-energy minimum region, for a wide range of starting configurations; (iii) ART method can sample the phase space effectively, going through many hyper-basins, and can generate significant moves in a single event. Possible applications of ART method to biomolecules are discussed.
Theoretical and experimental studies of the opto-electronic properties of positively charged oligo(phenylene vinylene)s: Effects of chain length and alkoxy substitution
Authors: Grozema F. C. , Candeias L. P. , Swart M. , van Duijnen P. Th. , Wildeman J. , Hadziioanou G. , Siebbeles L. D. A. , Warman J. M.
Published: December 2002

In this paper a combined experimental and quantum chemical study of the geometry and opto-electronic properties of unsubstituted and dialkoxy-sustituted phenylene–vinylene oligomers (PV’s) is presented. The optical absorption spectra for PV cations with different chain lengths and substitution patterns were measured using pulse radiolysis with time-resolved spectrophotometric detection from 1380 to 500 nm (0.9 to 2.5 eV). The geometries of the PV’s studied were optimized using density functional theory (DFT) for both the neutral and singly charged molecule. The spectra for the PV radical cations were then calculated using singly excited configuration interaction with an intermediate neglect of differential overlap reference wave function method together with the DFT geometry. The agreement between experimental and theoretical absorption energies is excellent; most of the calculated radical cation absorption energies are within 0.15 eV of the experimental values. The pattern of dialkoxy-substitution is found to have a large effect on the optical absorption spectrum of the cation. Using the calculated charge distribution it is shown that the degree of delocalization of the charge correlates with the energy of the lowest absorption band. If alkoxy side chains are present on some of the rings the positive charge tends to localize at those sites.
Coupling between molecular rotations and OH⋯O motions in liquid water: Theory and experiment
Authors: Gallot G. , Bratos S. , Pommeret S. , Lascoux N. , Leicknam J-Cl. , Koziński M. , Amir W. , Gale G. M.
Published: December 2002

A new theory is proposed to describe spectral effects of the coupling between molecular rotations and OH⋯O motions in liquid water. The correlation function approach is employed together with a special type of development in which the coupling energy of these two motions is the expansion parameter. The isotropy of the liquid medium plays an essential role in this study. Based on this theory, a new infrared pump–probe experiment is described permitting a visualization of molecular rotations at subpicosecond time scales. Full curves relating the mean squared rotational angle and time, and not only the rotational relaxation time, are measured by this experiment. However, very short times where the incident pulses overlap must be avoided in this analysis. The lifetime of OH⋯O bonds in water is rotation–limited.
Asymptotic analysis of a surface-interfacial wave interaction
Authors: Jamali Mirmosadegh , Seymour Brian , Lawrence Gregory A.
Published: December 2002

The three-dimensional interaction of a surface wave with two oblique interfacial waves in a horizontally infinite two-layer fluid is analyzed asymptotically. The nondimensional density difference is taken as a perturbation parameter and simple expressions for the growth rates and kinematic properties of the waves are obtained. The results show that the interfacial wavelengths are an order smaller than the surface wavelength. Also, to the leading-order approximation, the interfacial waves have a frequency half that of the surface wave, and their directions differ by 180° in the horizontal plane. The interaction coefficients are found to be equal at the leading order. The asymptotic solution is compared with the exact solution, and an excellent agreement is obtained for the range of applicability of the asymptotic theory. The analysis is extended to interactions in a medium with sidewalls. A previous laboratory flume study is addressed, and the asymptotic theory is used to explain the experimental observations.
Carbon cluster coagulation and fragmentation kinetics in shocked hydrocarbons
Authors: Viecelli J. A. , Glosli J. N.
Published: December 2002

Molecular dynamics simulations of the dissociation of methane, ethylene, and benzene at 50 GPa show that thermally activated carbon–carbon bond breaking produces a stationary n−5/2 distribution of short-lived n-atom carbon chains. Chain fragmentation rates follow an Arrhenius law in temperature, and the fraction of carbon existing as small chains increases with increasing temperature, ultimately approaching 1 at some critical temperature. Mean-field coagulation theory with added fragmentation terms yields the stationary n−5/2 distribution and shows that the rate of carbon coagulation to bulk material slows down with increasing temperature, approaching zero at the critical temperature.
Nondissociative low-energy electron attachment to SF6, C6F6, C10F8, and c-C7F14: Negative ion lifetimes
Authors: Suess L. , Parthasarathy R. , Dunning F. B.
Published: December 2002

The lifetimes of long-lived parent anions formed by nondissociative electron attachment to SF6, C6F6, C10F8, and c-C7F14 are investigated. The ions are created via electron transfer in collisions with K(np) Rydberg atoms and their lifetimes determined by observing their decay using a Penning ion trap. The measured lifetimes vary widely from target to target and range from ∼10 μs for C6F6− to ∼10 ms for SF6−. The present results are compared with values obtained in earlier free-electron studies.
Spin–orbit interactions, new spectral data, and deperturbation of the coupled b 3∏u and A 1∑u+ states of K2
Authors: Manaa M. R. , Ross A. J. , Martin F. , Crozet P. , Lyyra A. M. , Li Li , Amiot C. , Bergeman T.
Published: December 2002

We report calculations of the spin–orbit energy as a function of internuclear distance R within the b 3∏u state of K2, and between the b 3∏0u and A 1∑u+ states, together with new spectroscopic data on the b state and previously unpublished data on the A state. Both the new data and previous data are fitted to Hamiltonian parameters using the discrete variable representation (DVR) method. The DVR matrix includes nonrelativistic Born–Oppenheimer potentials and spin–orbit interactions, which are scaled to match the known asymptotic limits and to best fit the experimental data. We report fitted Dunham coefficients that yield the A and b state potentials by means of the Rydberg–Klein–Rees method. These parameters thus take into account second-order spin–orbit perturbation shifts (from the vibrational levels of these two states) which are normally not considered in band-by-band fits to spectroscopic data.
Thermal decomposition of iso-propanol: First-principles prediction of total and product-branching rate constants
Authors: Bui B. H. , Zhu R. S. , Lin M. C.
Published: December 2002

The unimolecular decomposition of iso-C3H7OH has been studied with a modified GAUSSIAN-2 method. Among the six low-lying product channels identified, the H2O-elimination process (2) via a four-member-ring transition state is dominant below 760 Torr over the temperature range 500–2500 K. At higher pressures and over 1200 K, the cleavage of a C-C bond by reaction (1) producing CH3+CH3C(H)OH is predicted to be dominant. The predicted low- and high-pressure limit rate constants for these two major product channels can be given by k10=6.3×1042 T−16.21 exp(−47 400/T), k20=7.2×1044 T−14.70 exp(−35 700/T) cm3 molecule−1 s−1, k1∞=8.0×1029 T−3.75 exp(−45 800/T), and k2∞=2.0×106 T2.12 exp(−30 700/T) s−1, respectively. Predicted k1 values compare reasonably with available experimental data; however, k2 values are lower than the experimentally determined apparent rate constant for C3H6 formation, which may derive in large part from secondary radical reactions. Other minor decomposition products were predicted to have the barriers H2+CH3C(O)CH3, E30=82.8 kcal/mol; H2O+1CH3CCH3, E40=77.9 kcal/mol; CH4+CH3C(H)O, E50=84.3 kcal/mol; and CH4+1CH3COH, E60=81.9 kcal/mol. The triplet-singlet energy gap for CH3CCH3 was predicted to be 5.2 kcal/mol, favoring the singlet state.
Density dependence of the hydrodynamic response to SF6 rotation in superfluid helium
Authors: Huang Patrick , Whaley K. Birgitta
Published: December 2002

We examine the density dependence of the hydrodynamic response of superfluid helium to classical rotational motion of a molecular impurity within the assumption of an adiabatically following helium density. The sensitivity of the hydrodynamic response to small changes in the helium density is assessed by performing three-dimensional hydrodynamic calculations for different fits to a microscopic helium density around the octahedral SF6 molecule generated by finite-temperature path integral Monte Carlo simulations. The sensitivity to systematic errors in the helium solvation density is assessed by comparing the hydrodynamic results obtained with finite-temperature path integral Monte Carlo densities to the corresponding results obtained using zero-temperature diffusion Monte Carlo densities that possess trial function bias. Our analysis shows that the finite-temperature densities provide a robust upper bound on the hydrodynamic response that amounts to at most 10% of the experimentally measured moment of inertia increment for SF6 at low temperature. We also address theoretical consistency requirements on the numerically derived velocity flows and present modifications of the hydrodynamic equations that are required by incorporation of higher-order quantum phase correlations.
Microchannel flow of a macromolecular suspension
Authors: Fan Xijun , Phan-Thien Nhan , Yong Ng Teng , Wu Xuhong , Xu Diao
Published: December 2002

In the delivery of DNA molecules by microfluidic devices, the channel width is very often in the same order as the size of the DNA molecules and the applicability of continuum mechanics at this level may be questioned. In this paper we use finitely extendable nonlinear elastic (FENE) chains to model the DNA molecules and employ the dissipative particle dynamics (DPD) method to simulate their behavior in the flow. Simple DPD fluids are found to behave just like a Newtonian fluid in Poiseuille flow. However, the velocity profiles of FENE chain suspensions can be fitted with power-law curves, especially for dilute suspensions. Some results on the conformation and migration of FENE chains are also reported.
Linear stability analysis and numerical calculations of the lid-driven flow in a toroidally shaped cavity
Authors: Spasov Yulian , Herrero Joan , Grau Francesc X. , Giralt Francesc
Published: December 2002

The lid-driven incompressible flow in a toroidally shaped cavity of square cross-section (d×d) and radius of curvature rc is studied. For discrete values of the curvature ratio (δ=d/rc), the Reynolds number at which the flow becomes three-dimensional (Rec) is determined by means of linear stability analysis. Present results show that both Rec and the wavelength of the critical mode (λc) depend strongly on curvature. For small curvatures (δ=0.125) steady modes of short wavelength (λc≈0.4) render the flow three-dimensional. For δ=0.25 the dominant modes are unsteady and of longer wavelength (λc≈0.8). For larger curvatures (δ⩾0.5), the first active modes are stationary and of even longer wavelength (λc>2.0). Numerical solutions of the Navier–Stokes equations for slightly to moderately supercritical conditions show a good agreement with the structure of the critical mode predicted by linear analyses. The flow is unstable for all curvatures studied due to instability of centrifugal type. The slightly supercritical flow presents pairs of Görtler-like vortices and the perturbation flow draws its energy from regions that are located along the outermost streamlines of the primary vortex. For small curvatures, these regions are located near the upstream wall where the Görtler vortices are stronger and the perturbation flow is similar to that in a straight cavity. For high curvatures, the region of maximum production of kinetic energy occurs instead near the downstream wall, but the pairs of counter-rotating Görtler-like vortices still appear near the upstream wall. The slightly supercritical (global) flow is characterized by a complex structure that is highly three-dimensional throughout most of the cavity.
On smoothed turbulent shear flows: Bounds, numerics and stress-reducing additives
Authors: Kerswell R. R. , Obrist D. , Schmid P. J.
Published: December 2002

We derive a rigorous upper bound on the energy dissipation rate in a smoothed version of plane Poiseuille flow in which a minimum length scale parallel to the walls is imposed. Due to the smaller number of degrees of freedom, much higher Reynolds numbers can be reached in numerical simulations of this system. These indicate that the turbulent energy dissipation rate achieves the upper bound scaling except for an extra logarithmic factor of 1/(log Re)2. Assuming that this discrepancy in scaling between the best known upper bound and true dissipation carries over to the full plane Poiseuille problem, this result presents further evidence that the viscous dissipation rate there is O(1/(log Re)2) (in inertial units) and hence vanishes as the Reynolds number Re→∞. The mean flow profiles which emerge from this smoothed system are strikingly similar to those which arise by adding stress-reducing additives to a wall-bounded shear flow. Both show familiar log regions with essentially the same gradient as the undisturbed (polymer free or no minimum length scale) system but which are now displaced vertically in the traditional U+−z+ diagram.
Predicting continuum breakdown in hypersonic viscous flows
Authors: Wang Wen-Lan , Boyd Iain D.
Published: December 2002

This paper presents a study of the breakdown of the Navier–Stokes equations in hypersonic viscous flows over a sharp cone tip and a hollow cylinder/flare geometry. Investigations are performed through detailed comparisons of the numerical results obtained with continuum and particle techniques. The objective of the study is to predict conditions under which the continuum approach may be expected to fail. A modified breakdown parameter is proposed that can predict the failure of the continuum approach accurately for the simple cone flow and fairly well for the more complex cylinder/flare flow. The study of continuum breakdown is the first step toward development of a hybrid numerical code.
Singularity formation in three-dimensional vortex sheets
Authors: Hou Thomas Y. , Hu Gang , Zhang Pingwen
Published: December 2002

We study singularity formation of three-dimensional (3-D) vortex sheets without surface tension using a new approach. First, we derive a leading order approximation to the boundary integral equation governing the 3-D vortex sheet. This leading order equation captures the most singular contributions of the integral equation. By introducing an appropriate change of variables, we show that the leading order vortex sheet equation degenerates to a two-dimensional vortex sheet equation in the direction of the tangential velocity jump. This change of variables is guided by a careful analysis based on properties of certain singular integral operators, and is crucial in identifying the leading order singular behavior. Our result confirms that the tangential velocity jump is the physical driving force of the vortex sheet singularities. We also show that the singularity type of the three-dimensional problem is similar to that of the two-dimensional problem. Moreover, we introduce a model equation for 3-D vortex sheets. This model equation captures the leading order singularity structure of the full 3-D vortex sheet equation, and it can be computed efficiently using fast Fourier transform. This enables us to perform well-resolved calculations to study the generic type of 3-D vortex sheet singularities. We will provide detailed numerical results to support the analytic prediction, and to reveal the generic form of the vortex sheet singularity.
Second-harmonic generation in amorphous silicon nitride microcavities
Authors: Lettieri S. , Finizio S. Di , Maddalena P. , Ballarini V. , Giorgis F.
Published: December 2002

We report an experimental observation of optical second-harmonic generation in an amorphous silicon–nitride based planar microcavity. The spectral dependence of the second-harmonic signal is investigated as a function of both wavelength and polarization state of the fundamental beam. The second-harmonic signal is enhanced by two orders of magnitude at the Fabry–Perot resonance. We ascribe the origin of the optical nonlinearity to surface second-harmonic generation occurring at the interfaces of the multilayered structure, where the resonant enhancement is due to the high intracavity fundamental light intensity and to the increase of the photonic density of states. Analysis of polarization dependence shows that interfacial second-order nonlinearity results from a distribution of dipoles directed perpendicularly to the plane of the film and distributed accordingly to an ∞mm point group symmetry.
Laminar boundary layer response to rotation of a finite diameter surface patch
Authors: Klewicki J. C. , Hill R. B.
Published: December 2002

The responses of the flat plate laminar boundary layer to perturbations generated by rotating a finite patch of the bounding surface are explored experimentally. The size of the surface patch was of the same order as the boundary layer thickness. The displacement thickness Reynolds number range of the boundary layers explored was 72–527. The rotation rates of the surface patch ranged from 2.14 to 62.8 s−1. Qualitative flow visualizations and quantitative molecular tagging velocimetry measurements revealed that rotation of a finite surface patch generates an asymmetric loop-like vortex. Significant features of this vortex include that, (i) the sign of the vorticity in the vortex head is opposite that of the boundary layer vorticity regardless of the sign of the input rotation, (ii) one leg of the vortex exhibits motion akin to solid body rotation while the other leg is best characterized as a spanwise shear layer, (iii) the vortex leg exhibiting near solid body rotation lifts more rapidly from the surface than the leg more like a shear layer, and (iv) the vortex leg exhibiting near solid body rotation always occurs on the side of the surface patch experiencing downstream motion. These asymmetries switch sides depending on the sign of the input rotation. The present results are interpreted and discussed relative to analytical solutions for infinite geometries. By way of analogy, plausible connections are drawn between the present results and the influences of wall normal vortices in turbulent boundary layer flows.
Detailed features of one-dimensional detonations
Authors: Daimon Yu , Matsuo Akiko
Published: December 2002

The oscillation mechanism and reignition process of one-dimensional unsteady detonations are numerically studied using a one-step chemical reaction model governed by Arrhenius kinetics. A series of simulations, without perturbations from the outflow boundary to the detonation front, are carried out while the degree of overdrive, f, is varied between 1.10 and 1.74 (f=D2/DCJ2; where D is detonation velocity). Shock pressure histories and x–t diagrams are utilized in order to attain precise understanding of the one-dimensional unsteady detonations. At higher degrees of overdrive, f=1.40–1.74, shock pressure histories agree with those of previous studies. The oscillation mechanism is the same as that of the large-disturbance regime of unsteady shock-induced combustion around a projectile. At lower degrees of overdrive, f<1.30, grid resolution affects the eventual results, because half reaction time in the shock pressure exhibits considerable variation. Four typical kinds of oscillation pattern are discussed and are explained by their x–t diagrams and shock pressure histories. Each oscillation mechanism is essentially the same as that of the large-disturbance regime. The reignition process in the failed regime was numerically investigated at f=1.01–1.25. The reignition points tend to converge on a specified point in study of grid refinement, although the oscillation of the shock pressure histories becomes chaotic, suggesting the existence of a unique solution for reignition. All the simulation results for f=1.01–1.20 show the failed regime after initial disturbance at the early stage. The failed regime is compared with the solution of the Riemann problem, and analysis consisting of a Rayleigh line for weak leading shock and a partially burnt Hugoniot curve is adopted. Analysis suggests the concept of partial chemical heat release, indicating the possibility of discontinuous change in conditions, and, indeed, simulation indicates occurrence of explosion. The explosion time derived from the analysis agrees well with the results of simulation.
Effect of a localized time-periodic wall motion on a turbulent boundary layer flow
Authors: Kim Chulkyu , Jeon Woo-Pyung , Park Jinil , Choi Haecheon
Published: December 2002

The effect of a localized time-periodic wall motion on a turbulent boundary layer flow is experimentally investigated in this study. A part of the wall is replaced with a circular membrane (560 wall units in diameter) which moves up and down periodically in the wall-normal direction (56 wall units in amplitude) at three different frequencies, corresponding to one, two, and three times the bursting frequency, by a wall-deformation actuator. The phase-averaged mean velocity decreases and increases with some phase delay, respectively, when the wall moves up and down. At all the actuation frequencies investigated, the region of the mean-velocity decrease is more widely distributed than that of the mean-velocity increase, indicating that the upward wall motion affects the downstream flow more greatly than the downward wall motion. The effect of the wall motion persists in the downstream, but the recovery of the flow to an equilibrium boundary layer flow becomes slower at a lower actuation frequency. At relatively high actuation frequencies, the upward wall motion generates a pair of streamwise vortices, providing a possibility of generating large-scale streamwise vortices from the present type of actuator.
Mode switching of an optical motor
Authors: Harada Takahiro , Yoshikawa Kenichi
Published: December 2002

A dielectric object irradiated with a strongly focused laser beam is affected by the trapping force as well as the scattering force. When the object is chirally shaped, it can be rotated in a certain direction. Here, we show that the rotational direction is controlled as desired only by changing the position of the focal spot on the same chiral object. We show the experimental evidence of the mode switching by using a helical chromosome under steady illumination of a Nd3+:YAG laser beam.
Trapping of cubic ZnO nanocrystallites at ambient conditions
Authors: Decremps F. , Pellicer-Porres J. , Datchi F. , Itié J. P. , Polian A. , Baudelet F. , Jiang J. Z.
Published: December 2002

Dense powder of nanocrystalline ZnO has been recovered at ambient conditions in the metastable cubic structure after a heat treatment at high pressure (15 GPa and 550 K). Combined x-ray diffraction (XRD) and x-ray absorption spectroscopy (XAS) experiments have been performed to probe both long-range order and local crystallographic structure of the recovered sample. Within uncertainty of these techniques (about 5%), all the crystallites are found to adopt the NaCl structure. From the analysis of XRD and XAS spectra, the cell volume per chemical formula unit is found to be 19.57(1) and 19.60(3) Å3, respectively, in very good agreement with the zero-pressure extrapolation of previously published high-pressure data.
Microstructural dependence of electron and hole transport in low-temperature-grown polycrystalline-silicon thin-film solar cells
Authors: Matsui Takuya , Muhida Riza , Kawamura Tomohiro , Toyama Toshihiko , Okamoto Hiroaki , Yamazaki Tsutomu , Honda Shinya , Takakura Hideyuki , Hamakawa Yoshihiro
Published: December 2002

Carrier transport properties of undoped polycrystalline silicon (poly-Si) thin films prepared by SiH4–H2 plasma at low temperature have been investigated. The ac-conductivity measurement technique has been applied to poly-Si i layers with an n-i-n junction structure in order to characterize the electron conductivity along the growth direction. Furthermore, the hole conductivity has been measured with p-i-p junction structures. The temperature dependence of ac conductivity reveals that poly-Si films with relatively low crystalline volume fraction (Xc∼50%) exhibit intrinsic character, while the poly-Si films with high Xc (>50%) exhibit n-type character with activation energies less than 0.15 eV. Based on these results, the relationship among microstructure, carrier transport, and photovoltaic performance of poly-Si solar cells is discussed.
Magnesium segregation and the formation of pyramidal defects in p-GaN
Authors: Figge Stephan , Kröger Roland , Böttcher Tim , Ryder Peter L. , Hommel Detlef
Published: December 2002

Magnesium doping of GaN was found to generate extended defects with a pyramidal shape. Transmission electron micrographs of layers with different doping levels typically showed a defect-free region at the start of doping and a modulation of the defect density in the subsequent film. We developed a rate equation model based on the segregation of Mg to explain the formation process of these defects. The model explains the dependence of the defect-free thickness on the doping level and yields a criterion to avoid the defect formation. Hall measurements show a significant reduction of the free hole concentration for samples grown at doping levels beyond defect formation.
Heat capacity of mesoscopically disordered superconductors: implications for MgB2
Authors: Gabovich A. M. , Voitenko A. I. , Li Mai Suan , Szymczak H.
Published: December 2002

The electronic specific heat C as a function of temperature T is calculated for a mesoscopically disordered s-wave superconductor treated as a spatial ensemble of domains with continuously varying superconducting properties. Each domain is characterized by a certain critical temperature Tc0 in the range [0,Tc] and is supposed to have a size L>ξ, where ξ is the coherence length. Specific calculations are performed for exponential and Gaussian distributions of Tc0. For low T, the spatially averaged 〈C(T)〉 is proportional to T2, whereas the anomaly at Tc is substantially smeared even for small dispersions. For narrow gap distributions there exists an intermediate T range, where the curve 〈C(T)〉 can be well approximated by an exponential Bardeen–Cooper–Schrieffer-like dependence with an effective gap smaller than the weak-coupling value. The results obtained successfully reproduce the salient features of the C(T) data for MgB2, where a wide superconducting gap distribution has been observed previously in the tunneling, point-contact, photoemission and Raman spectra. The conclusion is reached that the multiple-gap behavior of superconducting MgB2 is due to the spatial distribution of dissimilar domains. Intrinsic nonstoichiometry of the compound or possible electronic phase separation may be the origin of the mesoscopic inhomogeneities. The same model describes the low-T heat capacity of cuprates, although the sources of inhomogeneity are different from those in MgB2.
Fluctuation conductivity in Y–Ba–Cu–O films with artificially produced defects
Authors: Solovjov A. L.
Published: December 2002

The fluctuation-induced conductivity (paraconductivity) measured in YBa2Cu3O7−δ (YBCO) films grown on 10° miscut SrTiO3 (001) substrates is analyzed using various theoretical models describing weak fluctuations in high-Tc superconductors and considering both Aslamazov-Larkin and Maki-Thompson fluctuation contributions in the clean limit approach. The analysis reveals a highly anisotropic pair-breaking caused by structural defects produced. This result is in favor of an idea that pseudogap in high-Tc oxydes is mainly governed by the fluctuating pairing.
Exact solution of the problem of an electron in a magnetic field consisting of a uniform field and arbitrarily arranged magnetic strings parallel to the uniform field
Authors: Dubrovskiı̆ I. M.
Published: December 2002

It is shown that the requirements of finiteness, single-valuedness, and definiteness of the wave function and probability current density necessarily lead to the fact that as a magnetic string is approached the wave functions of the electron must decrease in modulus more rapidly than the square root of the distance from the string (a magnetic string is an infinitely thin solenoid carrying a finite magnetic flux). The energy spectrum of an electron is obtained. In general the spectrum is identical to the spectrum in the absence of strings. The general form of the eigenfunctions of the ground state and an operator whose powers give the eigenfunctions of the excited states are found. When there is only one string with magnetic flux which is not a multiple of twice the flux quantum another equidistant sequence of eigenvalues appears in the energy spectrum. This sequence is shifted with respect to the main sequence by a fraction equal to the positive fractional part of the quotient obtained by dividing the magnetic flux by twice the flux quantum. This sequence starts from a level whose number equals the number of the remaining magnetic strings. The wave functions for these special states are also obtained.
Low-temperature deformation and fracture of bulk nanostructural titanium obtained by intense plastic deformation using equal channel angular pressing
Authors: Bengus V. Z. , Tabachnikova E. D. , Natsik V. D. , Mishkuf Ĭ. , Chakh K. , Stolyarov V. V. , Valiev R. Z.
Published: December 2002

The low-temperature plasticity and fracture of polycrystals of coarse-grained (CG) and nanostructural (NS) technical-grade titanium of two structural modifications with grain size 0.3 and 0.1 μm, which were prepared by equal channel angular pressing (ECAP) with additional thermomechanical treatment are studied. The measurements are performed at temperatures 300, 77, and 4.2 K with uniaxial compression at deformation rate 4×10−4 s−1. The “stress-plastic deformation” hardening curves are obtained, the macroscopic yield stress, and the ultimate plasticity are measured for samples with compression axis orientations parallel and transverse to the ECAP axis. It is found that the yield stress for NS titanium is 1.5–2 times higher than for CG titanium and the yield stress on cooling from 300 to 4.2 K. Plasticity anisotropy is also observed in NS titanium—the yield stress is 1.2–1.5 times greater when the compression axis is oriented perpendicular to the ESAP axis than for parallel orientation. The ultimate plasticity with such changes in the structure of samples and under the experimental conditions systematically decreases, but the deformation to fracture remains above 4%. Nanostructural titanium does not show cold-brittleness right down to liquid-helium temperatures, but at 4.2 K plastic flow becomes jumplike, just as in CG titanium. It is established that for low-temperature uniaxial compression NS titanium fractures as a result of unstable plastic shear accompanied by local adiabatic heating of the material. This phenomenon is not characteristic of CG titanium. A study of the morphology of the shear-fracture surfaces using a scanning electron microsope shows a characteristic “vein” pattern, attesting to local heating at temperatures ⩾800 °C. It is established that plastic deformation in NS titanium is thermally activated at low temperatures. It is shown that microstructural internal stresses due to thermal anisotropy and possible microtwinning affect the yield stress.
The effect of microbubbles on developed turbulence
Authors: Mazzitelli Irene M. , Lohse Detlef , Toschi Federico
Published: December 2002

The motion and the action of microbubbles in homogeneous and isotropic turbulence are investigated through (three-dimensional) direct numerical simulations of the Navier–Stokes equations and applying the Lagrangian approach to track the bubble trajectories. The forces acting on the bubbles are added mass, drag, lift, and gravity. The bubbles are found to accumulate in vortices, preferably on the side with downward velocity. This effect, mainly caused by the lift force, leads to a reduced average bubble rise velocity. Once the reaction of the bubbles on the carrier flow is embodied using a point-force approximation, an attenuation of the turbulence on large scales and an extra forcing on small scales is found.
The effects of surfactants on the formation and evolution of capillary waves
Authors: Ceniceros Hector D.
Published: December 2002

The effects of surface-active agents on the formation and evolution of small capillary ripples developing in the forward front of short water waves is investigated numerically. The capillary waves, believed to have a significant relevance in the process of wave breaking and the onset of turbulence, accompany the initial development of spilling breakers. A novel hybrid numerical methodology is introduced to couple the full two-fluid Navier–Stokes equations with the free boundary motion and with the surfactant dynamics. The hybrid method uses dynamically adaptive front-tracking to accurately represent interfacial quantities and forces and to aid in treating the numerical difficulties associated with surface tension. At the same time the method employs the level set approach to efficiently update the material properties of the flow. It is found that the capillaries are dramatically affected by the presence of surfactants. The capillary region is invariably marked by accumulation of surfactants that reduces locally the interfacial tension. The size of the wave roller (bulge) diminishes and both the amplitude and wavelength of the capillary ripples also decrease as interfacial tension gradients increase. When surface convection dominates over diffusion, the accumulation of surfactants in the capillary region intensifies and the roller gets smaller and flatter. Large concentration gradients can be produced and these lead to a spread of vorticity along the spilling breakers as a result of the tangential Marangoni stress. In addition to the full two-phase viscous flow simulations, boundary integral computations of the corresponding potential inviscid flow are also performed to compare and contrast the two models in the case of uniform interfacial tension. Differences between the potential and the viscous flows are observed as soon as the wave steepens and develops high-curvature regions.
Diffraction behavior of photorefractive molecular materials containing phenothiazine derivatives
Authors: Choi Dong Hoon , Oh Kwang Yong , Jun Woong Gi , Kim Jae Hong , Choi Suk-Ho
Published: December 2002

We report the diffraction behavior of the photorefractive (PR) molecular materials that contain the phenothiazine derivatives. Diphenylhydrazine and malononitrile were reacted with N-alkyl substituted phenothiazinyl aldehyde to provide charge transporting and nonlinear optical (NLO) molecules, respectively. In the molecular materials prepared with phenothiazine derivatives, we observed very unusual complementary gratings formed by the space charge fields of two types of photocarriers, which can be explained based on a bipolar two-trap PR model. Adding the specific molecules to the host PR materials increased the trapped hole density during grating formation and erasing processes.
Field-emission induced growth of nanowires
Authors: Thong J. T. L. , Oon C. H. , Yeadon M. , Zhang W. D.
Published: December 2002

Nanowires are grown from a cold-field-emission tip in the presence of a precursor, typically an organometallic or organic compound. Electron emission from the newly grown nanowire tip continues the growth and can give rise to nanowires that are tens of microns long. Single nanowires are obtained by limiting the field-emission current to values of typically 100 nA or less. Tungsten nanowires with diameters of less than 4 nm have been grown from W(CO)6. Other nanowires grown include cobalt, iron, and carbon. Composite wires can be fabricated by continued growth with different precursors. Nanowires have been grown on etched wire tips, carbon nanotubes and scanning probe tips. Voltages applied to electrodes on an integrated circuit die can be used to attract a nanowire towards and contact a biased electrode. By such means, it is possible to connect the end of a pointed structure, such as a carbon nanotube, to an electrode.
Electronic structure transformation from a quantum-dot to a quantum-wire system: Photoluminescence decay and polarization of colloidal CdSe quantum rods
Authors: Wang Xiao-Yong , Zhang Jia-Yu , Nazzal A. , Darragh M. , Xiao Min
Published: December 2002

Radiative lifetime, polarization, and the global Stokes shift of colloidal CdSe quantum rods with aspect ratios from 1.9 to 3.8 are measured at room temperature. The radiative lifetime and the global Stokes shift show nonmonotonous dependence on the aspect ratio of the nanorods, and strong linear polarization in emission sharply appears as the aspect ratio crosses a turning point. The features of radiative lifetime and polarization versus aspect ratio in these nanorods indicate a transformation of the electronic structure from a zero-dimensional quantum-dot system to a one-dimensional quantum-wire system.
Nonvertical ascension or fall of a free sphere in a Newtonian fluid
Authors: Jenny Mathieu , Bouchet Gilles , Dušek Jan
Published: December 2002

It is shown that the system represented by a free sphere ascending or falling in a Newtonian fluid under the action of gravity buoyancy undergoes a regular, symmetry breaking bifurcation making the trajectory deviate from the vertical direction. The instability threshold expressed in terms of the asymptotic Reynolds number lies below that of a fixed sphere wake. The instability is shown to saturate and reach a fixed point corresponding to a straight oblique ascension (fall).
Terrace grading of SiGe for high-quality virtual substrates
Authors: Capewell A. D. , Grasby T. J. , Whall T. E. , Parker E. H. C.
Published: December 2002

Silicon germanium (SiGe) virtual substrates of final germanium composition x=0.50 have been fabricated using solid-source molecular beam epitaxy with a thickness of 2 μm. A layer structure that helps limit the size of dislocation pileups associated with the modified Frank–Read dislocation multiplication mechanism has been studied. It is shown that this structure can produce lower threading dislocation densities than conventional linearly graded virtual substrates. Cross-sectional transmission electron microscopy shows the superior quality of the dislocation network in the graded regions with a lower rms roughness shown by atomic force microscopy. X-ray diffractometry shows these layers to be highly relaxed. This method of Ge grading suggests that high-quality virtual substrates can be grown considerably thinner than with conventional grading methods.
Nonlinear optical absorption of antimony and lead oxyhalide glasses
Authors: de Araujo Renato E. , de Araújo Cid B. , Poirier Gäel , Poulain Marcel , Messaddeq Younes
Published: December 2002

Nonlinear optical properties and optical limiting behavior of antimony and lead oxyhalide glasses were studied using laser pulses of 80 ps at 532 nm. The standard Z-scan technique was used to determine large nonlinear absorption coefficients which range from 11 to 20 cm/GW, using intensities of 100 MW/cm2. Photodarkening was observed when intensities larger than ≈200 MW/cm2 were incident on the samples.
Transferable tight-binding parametrization for the group-III nitrides
Authors: Jancu J.-M. , Bassani F. , Sala F. Della , Scholz R.
Published: December 2002

We present accurate tight-binding (TB) parametrizations of the quasi-particle band structures of AlN, GaN, and InN, both for the zincblende and wurtzite phases. For this purpose, an empirical sp3d5s* nearest-neighbor TB model including the spin–orbit interaction is used, ensuring the transferability of the Slater–Koster parameters to changes in the structural environment. The present approach results in excellent agreement with experimental band positions and ab initio calculations for the lowest conduction band and the valence-band splittings. As a first application, we investigate the electronic band structure of an ordered Al0.5Ga0.5N alloy in the wurtzite phase.
Interface properties in metal-oxide-semiconductor structures on n-type 4H-SiC(033̄8)
Authors: Yano Hiroshi , Hirao Taichi , Kimoto Tsunenobu , Matsunami Hiroyuki , Shiomi Hiromu
Published: December 2002

The interface properties of SiO2/4H-SiC(033̄8) were characterized using n-type metaloxide-semiconductor structures fabricated by wet oxidation. The interface states near the conduction band edge are discussed based on the capacitance and conductance measurements at a low temperature and room temperature. 4H-SiC(033̄8) was found to have different energy distribution of the interface state density from the (0001) face. The shallow interface state density on (033̄8) is lower than on (0001) by a factor of 4 to 8.