bluuebunny/crossref_metadata_2025 huggingface dataset:
A simulation model for the density of states and for incomplete ionization in crystalline silicon. II. Investigation of Si:As and Si:B and usage in device simulation
Authors: Altermatt P. P. , Schenk A. , Schmithüsen B. , Heiser G.
Published: December 2006

Building on Part I of this paper [Altermatt et al., J. Appl. Phys. 100, 113714 (2006)], the parametrization of the density of states and of incomplete ionization (ii) is extended to arsenic- and boron-doped crystalline silicon. The amount of ii is significantly larger in Si:As than in Si:P. Boron and phosphorus cause a similar amount of ii although the boron energy level has a distinctly different behavior as a function of dopant density than the phosphorus level. This is so because the boron ground state is fourfold degenerate, while the phosphorus ground state is twofold degenerate. Finally, equations of ii are derived that are suitable for implementation in device simulators. Simulations demonstrate that ii increases the current gain of bipolar transistors by up to 25% and that it decreases the open-circuit voltage of thin-film solar cells by up to 10mV. The simulation model therefore improves the predictive capabilities of device modeling of p-n-junction devices.
Orientation and symmetry control of inverse sphere magnetic nanoarrays by guided self-assembly
Authors: Kiziroglou Michail E. , Li Xiaoli , Gonzalez David C. , de Groot Cornelis H. , Zhukov Alexander A. , de Groot Peter A. J. , Bartlett Philip N.
Published: December 2006

Inverse sphere shaped Ni arrays were fabricated by electrodeposition on Si through the guided self-assembly of polystyrene latex spheres in Si∕SiO2 patterns. It is shown that the size commensurability of the etched tracks is critical for the long range ordering of the spheres. Moreover, noncommensurate guiding results in the reproducible periodic triangular distortion of the close packed self-assembly. Magnetoresistance measurements on the Ni arrays were performed showing room temperature anisotropic magnetoresistance of 0.85%. These results are promising for self-assembled patterned storage media and magnetoresistance devices.
Modeling optical effects and thickness dependent current in polymer bulk-heterojunction solar cells
Authors: Sievers Douglas W. , Shrotriya Vishal , Yang Yang
Published: December 2006

Device characteristics of polymer based bulk-heterojunction photovoltaic cells incorporating poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] and methanofullerene ([6,6]-phenyl C61-butyric acid methyl ester) as the active materials are examined as a function of active layer thickness. The dependence of short circuit current on optical effects and its oscillatory variation on the polymer layer thickness is explained by solving the short circuit current using the drift-diffusion equations, where the light intensity calculated from the optical transfer matrix theory is used as the input for optical carrier generation. Furthermore, the effects of polymer layer thickness on other device operation parameters such as open-circuit voltage, fill factor, and series resistivity are measured. Considering the variation of above mentioned parameters, an optimized power conversion efficiency as high as 1.8% (under simulated air mass 1.5 global conditions) was achieved for a device with a polymer layer thickness of 55nm.
Generalized plane strain deformation of multilayer structures with initial strains
Authors: Nishidate Y. , Nikishkov G. P.
Published: December 2006

A closed-form solution for multilayer structures with initial strains under generalized plane strain conditions is presented. Such solutions can be useful for estimating the curvature radius and strains or stresses for self-positioning micro- and nanostructures with lattice mismatched layers. Comparison with finite element results shows that the developed solution predicts reasonable values of the curvature radius at the central part of the structure. Strains provided by the generalized plane strain solution are in agreement with those obtained by finite element analysis.
Effect of silicon nitride layers on the minority carrier diffusion length in c-Si wafers
Authors: Toušek Jiří , Toušková Jana , Poruba Aleš , Hlídek Pavel , Lörinčík Jan
Published: December 2006

Silicon nitride layers prepared from silane and ammonia based gases by microwave assisted plasma enhanced chemical vapor deposition (PECVD) and by low pressure chemical vapor deposition (LPCVD) techniques on p-type c-silicon substrates were studied via the methods of surface photovoltage (SPV), Fourier transform infrared (FTIR), and secondary-ion-mass spectroscopy (SIMS). The effective diffusion length in silicon was evaluated by the SPV method, and it was strongly influenced by the deposited SiNx layer. The FTIR spectra show the form of chemical bond of hydrogen in the layer. Two absorption bands belonging to Si–H and N–H groups and their modification after temperature treatment were found in the spectra of PECVD samples, while in the spectra of LPCVD samples only N–H bonds were recognized. Transport of H from PECVD silicon nitride into Si subsurface layer during the annealing process is shown by SIMS profiles of hydrogen. Positive influence of the penetrated H manifests in passivation of defects in the subsurface Si layer and, consequently, in better operation of the space charge region below the nitride and in longer effective diffusion length of minority carriers in the Si bulk. The average value of the diffusion length in the Si samples with the LPCVD nitride was shorter and dependent on the location of wafers in the reactor.
Laser ablation of a platinum target in water. I. Ablation mechanisms
Authors: Nichols William T. , Sasaki Takeshi , Koshizaki Naoto
Published: December 2006

This is the first in a series of three papers aimed at better understanding the processes that lead to nanomaterial formation during laser ablation of solid targets in liquids. Here we study the variation of the target surface morphology versus laser fluence and wavelength in order to suggest an ablation mechanism. A key finding is that an explosive ablation mechanism is prominent for a wide range of laser fluences for all wavelengths tested. Interestingly, however, ultraviolet (355nm) and infrared (1064nm) wavelengths show characteristically different explosive behaviors. In the infrared case, numerous large craters with diameters around 20μm form at localized points within the laser irradiated area. In contrast, ultraviolet ablation results in a striking transition to nanoscale surface roughness across the entire irradiated area. This texture is attributed to spinodal decomposition at the molten target surface. We propose that the wavelength and fluence dependence of the ablation craters can be explained by the amount of energy absorbed in the target. The consequences of the ablation mechanism for nanomaterial synthesis are discussed.
Laser ablation of a platinum target in water. III. Laser-induced reactions
Authors: Nichols William T. , Sasaki Takeshi , Koshizaki Naoto
Published: December 2006

This is the third paper in our series studying the laser-target-liquid interactions occurring in laser ablation in liquids (LAL). Here, laser ablation of a platinum target in pure water at 355nm wavelength is studied as a function of laser energy. We describe three distinct reaction regimes between the ablated target species and water at different laser focusing conditions. At low laser fluence (<10J∕cm2), material removal is caused by laser heating of the platinum surface and the primary products are small clusters with a large percentage of platinum atoms in a nonzero oxidation state. At intermediate fluences (10–70J∕cm2), platinum nanoparticles are the primary products. Our previous studies demonstrated that in this fluence regime ablation occurs through both thermal vaporization and explosive ejection of molten droplets. In both cases reactivity is small due to the low reactivity of platinum with water. At high fluences (>70J∕cm2), we find large, faceted particles that are attributed to the drying of PtOx gels formed by reactive plasma etching of the target. Taken together these results demonstrate that significant tunability in the target-liquid interaction is possible during nanomaterial synthesis by LAL.
Field-effect mobility temperature modeling of 4H-SiC metal-oxide-semiconductor transistors
Authors: Pérez-Tomás A. , Brosselard P. , Godignon P. , Millán J. , Mestres N. , Jennings M. R. , Covington J. A. , Mawby P. A.
Published: December 2006

Here a physically based channel mobility model has been developed to investigate the temperature dependence of the field-effect mobility of 4H-SiC metal-oxide-semiconductor (MOS) transistors with thermally oxidized gate insulators. This model has been designed so that it accounts for the high density of traps at the MOS interface. This temperature dependence is a key issue for silicon carbide electronics, as its basic material properties make it the foremost semiconductor for high power/high temperature electronic devices in applications such as spacecraft, aircraft, automobile, and energy distribution. Our modeling suggests that the high density of charged acceptor interface traps, encountered in thermally grown gate oxides, modulates the channel mobility due to the Coulomb scattering of free carriers in the inversion layer. When the temperature increases, the field-effect mobility of these devices also increases, due to an increase in inversion charge and a reduction of the trapped charge. Experimental data of the field-effect mobility temperature dependence are in good agreement with this model.
Low temperature formation of SiO2∕Si structure by nitric acid vapor
Authors: Imamura Kentarou , Maida Osamu , Hattori Kensaku , Takahashi Masao , Kobayashi Hikaru
Published: December 2006

Si can be oxidized at temperatures between 350 and 500°C by use of nitric acid (HNO3) vapor, resulting in 5–10nm SiO2∕Si structure. The oxidation kinetics follows a parabolic law, indicating that diffusion of oxidizing species (i.e., oxygen atoms generated by decomposition of HNO3 molecules) through SiO2 is the rate-determining step. The leakage current density flowing through the SiO2 layer formed at 350°C follows the Poole-Frenkel mechanism, indicating the presence of trap states in the SiO2 band gap, and the trap energy is estimated to be 0.57eV below the SiO2 conduction band. On the other hand, the leakage current density for the SiO2 layer formed at 500°C follows the Fowler-Nordheim mechanism, showing the absence of trap states.
Exploring the effect of anharmonicity of molecular vibrations on thermodynamic properties
Authors: Njegic Bosiljka , Gordon Mark S.
Published: December 2006

Thermodynamic properties of selected small and medium size molecules were calculated using harmonic and anharmonic vibrational frequencies. Harmonic vibrational frequencies were obtained by normal mode analysis, whereas anharmonic ones were calculated using the vibrational self-consistent field (VSCF) method. The calculated and available experimental thermodynamic data for zero point energy, enthalpy, entropy, and heat capacity are compared. It is found that the anharmonicity and coupling of molecular vibrations can play a significant role in predicting accurate thermodynamic quantities. Limitations of the current VSCF method for low frequency modes have been partially removed by following normal mode displacements in internal, rather than Cartesian, coordinates.
Breakdown of the Coulomb friction law in TiC∕a-C:H nanocomposite coatings
Authors: Pei Y. T. , Huizenga P. , Galvan D. , De Hosson J. Th. M.
Published: December 2006

Advanced TiC∕a-C:H nanocomposite coatings have been produced via reactive deposition in a closed-field unbalanced magnetron sputtering system (Hauzer HTC-1000 or HTC 1200). In this paper, we report on the tribological behavior of TiC∕a-C:H nanocomposite coatings in which ultralow friction is tailored with superior wear resistance, two properties often difficult to achieve simultaneously. Tribotests have been performed at room temperature with a ball-on-disk configuration. In situ monitoring of the wear depth of the coated disk together with the wear height of the ball counterpart at nanometer scale reveals that the self-lubricating effects are induced by the formation of transfer films on the surface of the ball counterpart. A remarkable finding is a breakdown of the Coulomb friction law in the TiC∕a-C:H nanocomposite coatings. In addition, the coefficient of friction of TiC∕a-C:H nanocomposite coatings decreases with decreasing relative humidity. A superior wear resistance of the coated disk at a level of 10−17m3∕Nm (per lap) has been achieved under the condition of superlow friction and high toughness, both of which require fine TiC nanoparticles (e.g., 2nm) and a wide matrix separation that must be comparable to the dimensions of the nanoparticles.
Report of the Symposium on Interactions for Dispersed Systems in Newtonian and Viscoelastic Fluids, Guanajuato, Mexico, 2006
Authors: Denn Morton M. , Meiburg Eckart H. , Morris Jeffrey F. , Shaqfeh Eric S. G. , Squires Todd M.
Published: December 2006

This report summarizes the issues discussed during a Symposium of the International Union of Theoretical and Applied Mechanics, entitled “Interactions for Dispersed Systems in Newtonian and Viscoelastic Fluids,” which was held in March 2006 in Guanajuato, Mexico.
Dynamics of particle-particle collisions in a viscous liquid
Authors: Yang F.-L. , Hunt M. L.
Published: December 2006

When two solid spheres collide in a liquid, the dynamic collision process is slowed by viscous dissipation and the increased pressure in the interparticle gap as compared with dry collisions. This paper investigates liquid-immersed head-on and oblique collisions, which complements previously investigated particle-on-wall immersed collisions. By defining the normal from the line of centers at contact, the experimental findings support the decomposition of an oblique collision into its normal and tangential components of motion. The normal relative particle motion is characterized by an effective coefficient of restitution and a binary Stokes number with a correlation that follows the particle-wall results. The tangential motion is described by a collision model using a normal coefficient of restitution and a friction coefficient that are modified for the liquid effects.
Collapse and growth of cavity regions in granular media due to viscous flow
Authors: Sano Osamu , Nagata Yusaku
Published: December 2006

Experimental studies of viscous flow are made on the effect of macroscopic cavity regions in an otherwise homogeneous granular material. The presence of such cavity regions enhances local velocity, which can accelerate the collapse of their boundaries. At the same time, the mobilized regions grow toward the upstream direction. In a continuation of our previous paper [Kaneko and Sano, Phys. Fluids 17, 033102 (2005)], we focus our attention on the latter processes in this paper. For a certain configuration of two interacting cavities, mobilized regions spread faster and in larger scale, which is likely to play an important role in the network formation of a water channel and the onset of a landslide. A numerical simulation based on the two-fluid model is also performed, and is compared with our experimental results.
Selection of the ripple length on a granular bed sheared by a liquid flow
Authors: Charru François
Published: December 2006

The stability of a granular bed sheared by a liquid flow is investigated using an erosion-deposition model for the moving grains. Extending to arbitrary wave number a previous long wave analysis [F. Charru and E. J. Hinch, J. Fluid Mech. 550, 111 (2006)], this study shows that short waves are stabilized by a crest-erosion mechanism that is absent when the local particle flux is assumed to be in equilibrium with the local bed shear stress. The cutoff length associated with this mechanism scales on a deposition length of the particles. This characteristic length is similar, although of a different physical nature, to the inertial length involved in relaxation models of dune dynamics under the wind. Two stabilizing mechanisms cooperate for the most amplified wave number: the new crest-erosion mechanism and the gravity force parallel to the local slope of the inclined bed. The predictions of this model are compared to observations, showing better agreement than previous stability analyses, which strongly underpredict the observed lengths.
On velocity profiles and stresses in sheared and vibrated granular systems under variable gravity
Authors: Baran Oleh , Kondic Lou
Published: December 2006

We employ discrete element three-dimensional simulations that include realistic modeling of physical system boundaries to determine the influence of gravity on velocity profiles and stresses for frictional inelastic particles that are confined in an angular Couette cell, and sheared by a rotated upper wall. In addition to Earth gravity, we consider other gravitational fields, in particular those of the Moon and Mars. The computational techniques are based on hard-sphere simulations of polydisperse particles at relatively high volume fraction (50–55%). We find that the presence of gravity induces significant changes of the velocity profiles and stresses. One important nondimensional parameter in the problem is shown to be IΩ=γ̇d∕Pg∕ρs, where γ̇ is the imposed shear rate, Pg is the weight of the system per unit area due to gravity, and ρs is the solid density. We also consider systems that are vibrated in addition to being sheared, since vibrations are one of several important methods for agitating (e.g., fluidizing and/or unjamming) granular systems. We find that the introduction of nondimensional acceleration Γ=a(2πf)2∕g, where a,f,g are the amplitude and frequency of oscillations, and the acceleration of gravity, explains novel features that develop in these complex granular systems.
Entrapped air bubbles in piezo-driven inkjet printing: Their effect on the droplet velocity
Authors: de Jong Jos , Jeurissen Roger , Borel Huub , van den Berg Marc , Wijshoff Herman , Reinten Hans , Versluis Michel , Prosperetti Andrea , Lohse Detlef
Published: December 2006

Air bubbles entrapped in the ink channel are a major problem in piezo-driven inkjet printing. They grow by rectified diffusion and eventually counteract the pressure buildup at the nozzle, leading to a breakdown of the jetting process. Experimental results on the droplet velocity udrop as a function of the equilibrium radius R0 of the entrained bubble are presented. Surprisingly, udrop(R0) shows a pronounced maximum around R0=17μm before it sharply drops to zero around R0=19μm. A simple one-dimensional model is introduced to describe this counterintuitive behavior which turns out to be a resonance effect of the entrained bubble.
Microscale tipstreaming in a microfluidic flow focusing device
Authors: Anna Shelley L. , Mayer Hans C.
Published: December 2006

A microfluidic flow-focusing device is used to explore the use of surfactant-mediated tipstreaming to synthesize micrometer-scale and smaller droplets. By controlling the surfactant bulk concentration of a soluble nonionic surfactant in the neighborhood of the critical micelle concentration, along with the capillary number and the ratio of the internal and external flow rates, we observe several distinct modes of droplet breakup. For the most part, droplet breakup in microfluidic devices results in highly monodisperse droplets in the range of tens of micrometers in size. However, we observe a new mode of breakup called “thread formation” that resembles tipstreaming and yields tiny droplets in the range of a few micrometers in size or smaller. In this work, we characterize the growth of the thread and its maximum length as a function of flow variables and surfactant content, and we also characterize the period of droplet breakup as a function of these variables. Our results suggest possible methods for controlling the process. Using a simple flow visualization experiment as the basis, we report on preliminary efforts to model the thread formation process.
X-ray preheating of window materials in direct-drive shock-wave timing experiments
Authors: Theobald W. , Miller J. E. , Boehly T. R. , Vianello E. , Meyerhofer D. D. , Sangster T. C. , Eggert J. , Celliers P. M.
Published: December 2006

The optical properties of x-ray preheated planar-window materials relevant for shock-wave timing experiments were studied on the OMEGA Laser System. The x-ray radiation was generated by 100 ps, 1×1015W∕cm2 laser pulses incident on planar plastic targets, instantaneously affecting samples located ∼0.7mm away. An abrupt onset of strong absorption of an optical probe beam (λ=532nm) and a temporally varying refractive index were measured in polystyrene and diamond windows. The behavior of diamond windows exposed to x rays is consistent with a simple model based on the generation of free charge carriers. Polystyrene windows showed indications of optical transitions due to molecular states that are created by the ionizing radiation.
Properties of linear and nonlinear ion thermal waves in a pair ion plasma containing charged dust impurities
Authors: Moslem W. M. , Shukla P. K.
Published: December 2006

Properties of linear and nonlinear ion thermal waves (ITWs) in pair ion plasmas containing a fraction of stationary charged (positive or negative) dust grains are investigated. For this purpose, a linear dispersion relation, a Korteweg-de Vries equation and (an energy integral of a classical potential) for linear (nonlinear) ITWs are derived from the ion continuity and momentum equations together with the Poisson equation. It is found that both the ITW frequency and the profile of the ion thermal solitary waves are significantly affected by the presence of positively/negatively charged dust grains. The present results should be useful in understanding the salient features of finite amplitude localized ion thermal solitary pulses in a pair ion plasma containing charged dust impurities.
Coherent control of cross-peaks in chirality-induced two-dimensional optical signals of excitons
Authors: Voronine Dmitri V. , Abramavicius Darius , Mukamel Shaul
Published: December 2006

Polarization tuning of the interference of chirality-induced tensor components is used to simulate the suppression of diagonal peaks and amplification of cross peaks in femtosecond two-dimensional photon echo signals of excitons in a chiral porphyrin dimer. Superpositions of various tensor components which generate the optimized signals are constructed using a genetic learning algorithm. Exciton couplings and bath correlations may be extracted from these highly resolved signals.
InP-based lattice-matched InGaAsP and strain-compensated InGaAs∕InGaAs quantum well cells for thermophotovoltaic applications
Authors: Rohr Carsten , Abbott Paul , Ballard Ian , Connolly James P. , Barnham Keith W. J. , Mazzer Massimo , Button Chris , Nasi Lucia , Hill Geoff , Roberts John S. , Clarke Graham , Ginige Ravin
Published: December 2006

Quantum well cells (QWCs) for thermophotovoltaic (TPV) applications are demonstrated in the InGaAsP material system lattice matched to the InP substrate and strain-compensated InGaAs∕InGaAs QWCs also on InP substrates. We show that lattice-matched InGaAsP QWCs are very well suited for TPV applications such as with erbia selective emitters. QWCs with the same effective band gap as a bulk control cell show a better voltage performance in both wide and erbialike emission. We demonstrate a QWC with enhanced efficiency in a narrow-band spectrum compared to a bulk heterostructure control cell with the same absorption edge. A major advantage of QWCs is that the band gap can be engineered by changing the well thickness and varying the composition to the illuminating spectrum. This is relatively straightforward in the lattice-matched InGaAsP system. This approach can be extended to longer wavelengths by using strain-compensation techniques, achieving band gaps as low as 0.62eV that cannot be achieved with lattice-matched bulk material. We show that strain-compensated QWCs have voltage performances that are at least as good as, if not better than, expected from bulk control cells.
Hall assisted forced magnetic reconnection
Authors: Vekstein G. , Bian N. H.
Published: December 2006

The role of the Hall effect in forced magnetic reconnection is investigated analytically for the so-called Taylor problem. In the latter, a tearing stable slab plasma equilibrium, which is chosen here to be a simple magnetic field reversal, is subjected to a small-amplitude boundary deformation that drives magnetic reconnection (hence the adjective “forced” ) at the neutral surface within the plasma. It is shown that such reconnection becomes substantially accelerated by the Hall effect when the nondimensional parameter di=(c∕ωpi)∕a exceeds S−1∕5. Here, c∕ωpi is the ion inertial skin depth, a is the width of the plasma slab, and S≫1 is the Lundquist number of a highly conducting plasma. Two different types of external perturbation are considered. In the case of continuous quasistatic driving, with a frequency ω such that ωτA≪1, τA being the Alfvén transit time, various reconnection regimes are identified. The corresponding heating rates, which are determined by the parameters di, S, and ωτA, are derived. In the case of a “one-off” reconnection event, we demonstrate when and how the transition from the Hall regime to the magnetohydrodynamic regime occurs in the course of the reconnection process. It is found that the peak instantaneous reconnection rate scales as dψ1(0)∕dt∼di1∕2S−1∕2(B0δ0∕τA), where ψ1(0) is the reconnected magnetic flux, B0 is the magnetic field strength, and δ0 is the amplitude of the boundary deformation.
Ferroelectric plasma thruster for microspacecraft propulsion
Authors: Kemp Mark A. , Kovaleski Scott D.
Published: December 2006

This paper presents a technology in microthruster design: the ferroelectric plasma thruster (FEPT). The FEPT utilizes an applied rf electric field to create plasma on the surface of a ferroelectric dielectric. Acceleration of ions from this plasma provides thrust. Advantages of the FEPT include emission of both electrons and ions leading to self-neutralization, creation of plasma, and acceleration of ions with a single power supply, and application of thrust in a short amount of time. We present the concept of the thruster, operational physics, as well as experimental results demonstrating plasma creation and ion acceleration. These results along with plasma spectroscopy allow us to calculate thruster parameters.
Spectroscopic properties of Pr3+ ions in NaGd(WO4)2 crystal
Authors: Wang Hongyan , Jia Guohua , Yang Fugui , Wei Yanping , Tu Chaoyang , You Zhenyu , Wang Yan , Zhu Zhaojie , Li Jianfu
Published: December 2006

A Pr3+-doped NaGd(WO4)2 single crystal has been grown by the Czochralski technique along the (0 0 1) orientation. The refractive indices were measured precisely as a function of wavelength. A Raman spectrum, polarized absorption spectra, fluorescence spectra, and fluorescence decay curve have been recorded at room temperature. The standard and modified Judd-Ofelt theories have been performed to analyze the absorption spectra to determine the spectroscopic parameters, including the Judd-Ofelt intensity parameters Ωt (t=2,4,6), radiative transition probabilities, radiative lifetimes, and branching ratios. The stimulated emission cross sections and fluorescence lifetime of the promising laser level were obtained.
Ab initiotreatment of time-resolved x-ray scattering: Application to the photoisomerization of stilbene
Authors: Debnarova Andrea , Techert Simone , Schmatz Stefan
Published: December 2006

In this work we present a general theoretical outline for calculating time-dependent x-ray scattering signal changes from first principles. We derive a formalism for the description of atom-atom correlation functions as Fourier transforms of quantum-chemically calculated electron densities and show their proportionality to the molecular form factor. The formalism derived in this work is applied to the photoisomerization of stilbene. We can demonstrate that wide-angle x-ray scattering offers a possibility to study the changes in electron densities in nonperiodic complex systems, which renders it a suitable technique for the investigation of (bio)organic systems.
Radial propagation of structures in drift wave turbulence
Authors: Windisch T. , Grulke O. , Klinger T.
Published: December 2006

The formation and propagation of spatiotemporal fluctuation structures in weakly developed drift-wave turbulence in a linearly magnetized helicon device is investigated. Turbulent density fluctuations in the far edge plasma display an intermittent character with large-amplitude positive density bursts. Their peak amplitudes correspond to the time-averaged density in the maximum radial plasma pressure gradient. The conditional average technique is applied to reconstruct the dynamics of turbulent coherent structures in the azimuthal plane. The formation of turbulent structures is closely linked to a quasicoherent m=1 drift wave mode, which is generally observed in the radial density gradient region in the weakly developed turbulent state. It is demonstrated that every positive high amplitude density burst in the plasma edge is due to the radial propagation of a turbulent structure. The typical scale size of the turbulent structures is 4ρs and their lifetime exceeds the eddy turnover time by orders of magnitude, thereby characterizing them as coherent structures. Although the turbulent structures propagate mainly azimuthally in the direction of the E×B drift they are observed to have a radial velocity, which is typically 10% of the ion sound speed.
Thermodynamics of water octamer in a uniform electric field
Authors: Hernández-Rojas Javier , González Briesta S. , James Tim , Wales David J.
Published: December 2006

We study the water octamer in a uniform electric field using the all-exchanges parallel tempering Monte Carlo method in the canonical ensemble. The heat capacity, quenched energy configurations, and the order parameter Q4 are employed to understand the phase changes observed as a function of temperature and the strength of the applied electric field. At a low field strength of 0.1VÅ−1 a solidlike to liquidlike “melting” transition is detected. The corresponding heat capacity peak appears around 206K, where Q4 shows a significant change of slope. For E⩾0.5VÅ−1 such features are absent. However, at E=0.5VÅ−1 we find a solidlike to solidlike transition between cubic and extended structures around T∼25K.
Shape and stability of two-dimensional lipid domains with dipole-dipole interactions
Authors: Iwamoto Mitsumasa , Liu Fei , Ou-Yang Zhong-can
Published: December 2006

We study the general energy and shape of the two-dimensional solid monolayer domains with the dipole-dipole interactions. Compared with the domain energy without tilted dipole moments [M. Iwamoto and Z. C. Ou-Yang, Phys. Rev. Lett. 93, 206101 (2004)], the general dipolar energy is not only shape and size but also boundary orientation dependent. The general shape equation derived by this energy using variational approach predicts a circular solution and an equilibrium shape grown from this circle. In particular, the latter is composed of two branches: a translation-induced growth of all odd harmonic modes and a pressure-induced cooperative deformation by all even harmonic modes. The good qualitative agreement between our prediction and the experimental observations shows the validity of the present theory.
Self-trapping of the N–H vibrational mode in α-helical polypeptides
Authors: Tsivlin Dmitry V. , May Volkhard
Published: December 2006

Recent calculations on the formation of self-trapped amide group vibrational states in α-helical polypeptides [J. Chem. Phys. 124, 134907 (2006)] are extended to the amide N–H normal mode vibrations. First, the adiabatic N–H vibrational single- and two-exciton states are examined by treating the longitudinal chain coordinates as parameters. Then, in using the multiconfiguration time-dependent Hartree method coupled exciton-chain vibrational quantum dynamics are accounted for. Based on the respective exciton-chain vibrational wave function propagation the infrared transient absorption related to a sequential pump-probe experiment is calculated. The modulation of local amide vibrational energies by the longitudinal chain coordinates is found to have a pronounced effect on the broadening of absorption lines. Moreover, the ultrafast exciton transfer in the system is studied in order to characterize the dynamics of the self-trapped single-exciton states on a time scale below 10ps.
Reaction of chlorine atom with trichlorosilane from 296to473K
Authors: DeSain John D. , Valachovic Linda , Jusinski Leonard E. , Taatjes Craig A.
Published: December 2006

The reaction of trichlorosilane (HSiCl3) with atomic chlorine (Cl) has been investigated by using infrared kinetic spectroscopy of the HCl product. The overall second order rate constant for the reaction has been determined as a function of temperature by using pseudo-first-order kinetic methods. Formation of HCl (ν=0) was monitored on the (ν=1←0) R(2) line at 2944.914cm−1 and that of HCl (ν=1) on the (ν=2←1) R(2) line at 2839.148cm−1. The overall second order rate constant was determined to be (2.8±0.1)×10−11cm3molecule−1s−1 at 296K. The rate constant shows no pressure dependence and decreases slightly with increased temperature [k=(2.3±0.2)×10−11e(66±3)∕Tcm3molecule−1s−1]. Substantial vibrational excitation is measured in the HCl product, with the fraction of HCl (ν=1)∕HCl (total)=0.41±0.08. These observations are consistent with the reaction being a barrierless hydrogen abstraction reaction. The experimental results are supported by ab initio quantum chemical calculations that show the transition state for abstraction to lie below the energy of the reactants, in disagreement with previously published calculations.
Optical cavity with a double-layered cholesteric liquid crystal mirror and its prospective application to solid state laser
Authors: Jeong Soon Moon , Sonoyama Koji , Takanishi Yoichi , Ishikawa Ken , Takezoe Hideo , Nishimura Suzushi , Suzaki Goro , Song Myoung Hoon
Published: December 2006

The authors have fabricated an optical cavity with silver (Ag) and double-layered cholesteric liquid crystal (CLC) mirrors facing each other. This CLC mirror consists of left-handed CLC and right-handed CLC films for high light reflection irrespective of polarization states. A single-mode lasing was observed in dye-doped CLC sandwiched between Ag and double-layered CLC mirrors. The authors also fabricated a flexible solid state device with a spin-coated dye molecular film sandwiched between Ag and double-layered CLC mirrors. Amplified spontaneous emission was observed from the solid state device, suggesting a possible structure for a flexible and tunable solid state laser.
Modeling the release of nanoparticles from mobile microcapsules
Authors: Verberg Rolf , Alexeev Alexander , Balazs Anna C.
Published: December 2006

The authors present a novel computational approach to simulate both the release of nanoparticles from a microcapsule, which is moving through a microchannel, and the adsorption of the released particles onto the channel walls. By integrating the lattice spring model for the micromechanics of elastic solids and the lattice Boltzmann model for fluid dynamics, they simulate the relevant fluid-structure interactions in the system. In particular, they capture the dynamic interactions among the capsule’s elastic shell, the encapsulated fluid, and the external, host solution. The nanoparticles are treated as “tracer particles” and their motion is modeled via a Brownian dynamics simulation. An imposed pressure gradient drives the capsule to move along an adhesive substrate and the particles are released from the surface of this mobile capsule. The authors determine how the elasticity of the capsule, the strength of the capsule–surface adhesion and the diffusion coefficient of the nanoparticles affect the relative amount of particles that are adsorbed onto the substrate. In addition to showing that the compliant nature of the capsule can significantly affect the nanoparticle deposition, they isolate a range of parameters for maximizing the adsorbed amount. The findings yield guidelines for optimizing the efficiency of microcapsule carriers in the targeted delivery of nanoparticles.
Piezoelectric fields in GaInN∕GaN quantum wells on different crystal facets
Authors: Feneberg M. , Lipski F. , Sauer R. , Thonke K. , Wunderer T. , Neubert B. , Brückner P. , Scholz F.
Published: December 2006

Direction and strength of piezoelectric built-in fields of GaInN quantum wells have been experimentally determined. The quantum wells have been grown either on the conventional {0001} crystal plane of GaN or on {11¯01} facets of selectively grown GaN stripes. The emission peak position of the electric-field-dependent photoluminescence can be modeled and yields value and sign of the piezoelectric field dependent on the strain of the quantum wells. On the semipolar {11¯01} facets, the quantum wells show a much weaker field (−0.1MV∕cm) compared to quantum wells grown on polar {0001} planes (−1.9MV∕cm), consistent with theoretic predictions.
Gene cooption without duplication during the evolution of a male-pregnancy gene in pipefish
Authors: Harlin-Cognato April , Hoffman Eric A. , Jones Adam G.
Published: December 2006

Comparative studies of developmental processes suggest that novel traits usually evolve through the cooption of preexisting genes and proteins, mainly via gene duplication and functional specialization of paralogs. However, an alternative hypothesis is that novel protein function can evolve without gene duplication, through changes in the spatiotemporal patterns of gene expression (e.g., via cis-regulatory elements), or functional modifications (e.g., addition of functional domains) of the proteins they encode, or both. Here we present an astacin metalloprotease, dubbed patristacin, which has been coopted without duplication, via alteration in the expression of a preexisting gene from the kidney and liver of bony fishes, for a novel role in the brood pouch of pregnant male pipefish. We examined the molecular evolution of patristacin and found conservation of astacin-specific motifs but also several positively selected amino acids that may represent functional modifications for male pregnancy. Overall, our results pinpoint a clear case in which gene cooption occurred without gene duplication during the genesis of an evolutionarily significant novel structure, the male brood pouch. These findings contribute to a growing understanding of morphological innovation, a critically important but poorly understood process in evolutionary biology.
Azoospermia in mice with targeted disruption of theBrek/Lmtk2(brain-enriched kinase/lemur tyrosine kinase 2) gene
Authors: Kawa Seiji , Ito Chizuru , Toyama Yoshiro , Maekawa Mamiko , Tezuka Tohru , Nakamura Takahisa , Nakazawa Takanobu , Yokoyama Kazumasa , Yoshida Nobuaki , Toshimori Kiyotaka , Yamamoto Tadashi
Published: December 2006

Brek/Lmtk2 (brain-enriched kinase/lemur tyrosine kinase 2) is a member of the Aatyk family of kinases that comprises Aatyk1, Brek/Lmtk2/Aatyk2, and Aatyk3. Although several potential roles have been proposed for Brek and other Aatyk family members, the physiological functions of these kinases remain unclear. Here, we report thatBrek−/−male mice are infertile, with azoospermia. Detailed histological analysis revealed thatBrek−/−germ cells differentiated normally until the round-spermatid stage, but failed to undergo the normal change in morphology to become elongated spermatids. Testicular somatic cells appeared normal in these mice. Expression of Brek in testis was restricted to the germ cells, suggesting that the maturations of germ cells inBrek−/−mice are affected in a cell-autonomous manner. On the basis of these findings, we concluded that Brek is essential for a late stage of spermatogenesis. Further clarification of the mechanism by which Brek regulates spermatogenesis may help identify new targets for reproductive contraceptives and treatments against infertility.
Insulin protects islets from apoptosis via Pdx1 and specific changes in the human islet proteome
Authors: Johnson James D. , Bernal-Mizrachi Ernesto , Alejandro Emilyn U. , Han Zhiqiang , Kalynyak Tatyana B. , Li Hong , Beith Jennifer L. , Gross Julia , Warnock Garth L. , Townsend R. Reid , Permutt M. Alan , Polonsky Kenneth S.
Published: December 2006

Insulin is both a hormone regulating energy metabolism and a growth factor. We and others have shown that physiological doses of insulin initiate complex signals in primary human and mouse β-cells, but the functional significance of insulin's effects on this cell type remains unclear. In the present study, the role of insulin in β-cell apoptosis was examined. Exogenous insulin completely prevented apoptosis induced by serum withdrawal when given at picomolar or low nanomolar concentrations but not at higher concentrations, indicating that physiological concentrations of insulin are antiapoptotic and that insulin signaling is self-limiting in islets. Insulin treatment was associated with the nuclear localization of Pdx1 and the prosurvival effects of insulin were largely absent in islets 50% deficient in Pdx1, providing direct evidence that Pdx1 is a signaling target of insulin. Physiological levels of insulin did not increase Akt phosphorylation, and the protective effects of insulin were only partially altered in islets lacking 80% of normal Akt activity, suggesting the presence of additional insulin-regulated antiapoptotic pathways. Proteomic analysis of insulin-treated human islets revealed significant changes in multiple proteins. Bridge-1, a Pdx1-binding partner and regulator of β-cell survival, was increased significantly at low insulin doses. Together, these data suggest that insulin can act as a master regulator of islet survival by regulating Pdx1.
TNFα kinoid vaccination-induced neutralizing antibodies to TNFα protect mice from autologous TNFα-driven chronic and acute inflammation
Authors: Le Buanec Hélène , Delavallée Laure , Bessis Natacha , Paturance Sébastien , Bizzini Bernard , Gallo Robert , Zagury Daniel , Boissier Marie-Christophe
Published: December 2006

The proinflammatory cytokine TNFα is a potent mediator of septic shock and a therapeutic target for chronic inflammatory pathologies including rheumatoid arthritis and Crohn's disease. As an alternative to anti-human TNFα (hTNFα) mAbs and other hTNFα blocker approved drugs, we developed an active anti-hTNFα immunotherapy, based on a vaccine comprised of a keyhole limpet hemocyanin-hTNFα heterocomplex immunogen (hTNFα kinoid) adjuvanted in incomplete Freund's adjuvant. In mice transgenic for hTNFα (TTg mice), hTNFα kinoid vaccination elicited high titers of Abs that neutralized hTNFα bioactivities but did not result in a cellular response to hTNFα. The vaccine was safe and effective in two experimental models. Kinoid-immunized but not control TTg mice resisted hTNFα-driven shock in one model and were prevented from spontaneous arthritis, inflammatory synovitis, and articular destruction in a second model. These data demonstrate an anti-cytokine induction of autoimmune protection against both acute and chronic hTNFα exposure. They show that active vaccination against a human cytokine can be achieved, and that the immune response can be effective and safe.
Developmental stage-specific biphasic roles of Wnt/β-catenin signaling in cardiomyogenesis and hematopoiesis
Authors: Naito Atsuhiko T. , Shiojima Ichiro , Akazawa Hiroshi , Hidaka Kyoko , Morisaki Takayuki , Kikuchi Akira , Komuro Issei
Published: December 2006

Although Wingless (Wg)/Wnt signaling has been implicated in heart development of multiple organisms, conflicting results have been reported regarding the role of Wnt/β-catenin pathway in cardiac myogenesis: Wg/armadillo signaling promotes heart development in
Drosophila
, whereas activation of Wnt/β-catenin signaling inhibits heart formation in avians and amphibians. Using an
in vitro
system of mouse ES cell differentiation into cardiomyocytes, we show here that Wnt/β-catenin signaling exhibits developmental stage-specific, biphasic, and antagonistic effects on cardiomyogenesis and hematopoiesis/vasculogenesis. Activation of the Wnt/β-catenin pathway in the early phase during embryoid body (EB) formation enhances ES cell differentiation into cardiomyocytes while suppressing the differentiation into hematopoietic and vascular cell lineages. In contrast, activation of Wnt/β-catenin signaling in the late phase after EB formation inhibits cardiomyocyte differentiation and enhances the expression of hematopoietic/vascular marker genes through suppression of bone morphogenetic protein signaling. Thus, Wnt/β-catenin signaling exhibits biphasic and antagonistic effects on cardiomyogenesis and hematopoiesis/vasculogenesis, depending on the stage of development.
p21 delays tumor onset by preservation of chromosomal stability
Authors: Barboza Juan A. , Liu Geng , Ju Zhenlin , El-Naggar Adel K. , Lozano Guillermina
Published: December 2006

The p53 protein suppresses tumorigenesis by initiating cellular functions such as cell cycle arrest and apoptosis in response to DNA damage. A p53 mutant, p53R172P, which is deficient for apoptosis but retains a partial cell cycle arrest function, delays tumor onset in mice. Remarkably, lymphomas arising in
Trp53
515C/515C
mice (encoding p53R172P) retain stable genomes. Given the dominant role of p21 in p53 cell cycle control, we crossed
Trp53
515C/515C
mice onto a
p21
-null background to determine whether p21 was required for maintaining chromosomal stability and delaying tumor onset. Loss of
p21
completely abolished the cell cycle arrest function of p53R172P and accelerated tumor onset in
Trp53
515C/515C
mice. Cytogenetic examination of
Trp53
515C/515C
p21
−/−
sarcomas and lymphomas revealed aneuploidy and chromosomal aberrations that were absent in
Trp53
515C/515C
malignancies. Thus, p21 coupled p53-dependent checkpoint control and preservation of chromosomal stability, and cooperated with apoptosis in suppressing tumor onset in mice.
Analysis of cartilage matrix fixed charge density and three-dimensional morphology via contrast-enhanced microcomputed tomography
Authors: Palmer Ashley W. , Guldberg Robert E. , Levenston Marc E.
Published: December 2006

Small animal models of osteoarthritis are often used for evaluating the efficacy of pharmacologic treatments and cartilage repair strategies, but noninvasive techniques capable of monitoring matrix-level changes are limited by the joint size and the low radiopacity of soft tissues. Here we present a technique for the noninvasive imaging of cartilage at micrometer-level resolution based on detecting the equilibrium partitioning of an ionic contrast agent via microcomputed tomography. The approach exploits electrochemical interactions between the molecular charges present in the cartilage matrix and an ionic contrast agent, resulting in a nonuniform equilibrium partitioning of the ionic contrast agent reflecting the proteoglycan distribution. In an
in vitro
model of cartilage degeneration we observed changes in x-ray attenuation magnitude and distribution consistent with biochemical and histological analyses of sulfated glycosaminoglycans, and x-ray attenuation was found to be a strong predictor of sulfated glycosaminoglycan density. Equilibration with the contrast agent also permits direct
in situ
visualization and quantification of cartilage surface morphology. Equilibrium partitioning of an ionic contrast agent via microcomputed tomography thus provides a powerful approach to quantitatively assess 3D cartilage composition and morphology for studies of cartilage degradation and repair.