bluuebunny/crossref_metadata_2025 huggingface dataset:
Control of nonadiabatic dissociation dynamics with the use of laser-induced wave packet interferences
Authors: Nagaya K. , Lin S. H. , Nakamura H.
Published: December 2006

Based on wave packet interferences induced by a stationary laser field, a simple way of controlling nonadiabatic dissociation dynamics is proposed. We treat a simple two-state model of diatomic molecules. In this model, there exist two dissociative potential energy curves which cross and are strongly coupled at an internuclear distance, and thus dissociations into one channel are predominant. We propose a control scheme to selectively dissociate a molecule into any favorite channel by choosing the laser frequency and intensity appropriately. The semiclassical estimation of desirable laser parameters can be performed easily by regarding the dissociation processes as nonadiabatic transitions between the Floquet states. The agreement between the semiclassical estimation and the quantum wave packet calculation is found to be satisfactory in the high frequency region (⩾1000cm−1) where the Floquet state picture is valid. In the low frequency region (<1000cm−1), on the other hand, there are discrepancies between them due to the invalidity of the Floquet picture and the dissociation probability is sensitive to the laser phase. This control scheme is applied to the predissociation dynamics of NaI, NaI→Na+I.
Infinitely many periodic orbits for the rhomboidal five-body problem
Authors: Corbera Montserrat , Llibre Jaume
Published: December 2006

We prove the existence of infinitely many symmetric periodic orbits for a regularized rhomboidal five-body problem with four small masses placed at the vertices of a rhombus centered in the fifth mass. The main tool for proving the existence of such periodic orbits is the analytic continuation method of Poincaré together with the symmetries of the problem.
The variant of post-Newtonian mechanics with generalized fractional derivatives
Authors: Kobelev V. V.
Published: December 2006

In this article, we investigate mathematically the variant of post-Newtonian mechanics using generalized fractional derivatives. The relativistic-covariant generalization of the classical equations for gravitational field is studied. The equations (i) match the weak Newtonian limit on the moderate scales and (ii) deliver a potential higher than Newtonian on certain large-distance characteristic scales. The perturbation of the gravitational field results in the tiny secular perihelion shift and exhibits some unusual effects on large scales. The general representation of the solution for the fractional wave equation is given in the form of retarded potentials. The solutions for the Riesz wave equation and classical wave equation are clearly distinctive in an important sense. The hypothetical gravitational Riesz wave demonstrates the space diffusion of the wave at the scales of metric constant. The diffusion leads to the blur of the peak and disruption of the sharp wave front. This contrasts with the solution of the D’Alembert classical wave equation, which obeys the Huygens principle and does not diffuse.
Analysis of the linewidth enhancement factor (α-factor) in compressively strained InGaAsP quantum wire lasers
Authors: Khayer M. Abul , Haque A.
Published: December 2006

The linewidth enhancement factor (α-factor) of compressively strained (CS) InGaAsP/InP quantum wire (QWire) lasers is theoretically studied using an eight-band k⋅p formalism. It is found that tensile strained (TS) barriers lower the value of the α-factor by suppressing elastic strain relaxation. When multiple QWire layers are stacked vertically, the α-factor decreases owing to the increase in the modal gain. However, the decrease is lower than expected, because enhanced strain relaxation in multiple-layer stack tends to increase the α-factor. Comparison of the α-factors of QWires and QWells at the lasing frequency shows that the peak value of the α-factor decreases monotonically with increasing confinement, but an improvement in the α-factor of QWires at threshold condition relative to the α-factor of QWell lasers is not achieved unless the QWire width is reduced below a critical value. These results provide useful guidelines for designing CS QWire lasers with reduced values of the α-factor.
Finite-size effects in the electrical transport properties of single bismuth nanowires
Authors: Cornelius T. W. , Toimil-Molares M. E. , Neumann R. , Karim S.
Published: December 2006

Single bismuth nanowires with diameters ranging between 150nm and 1μm were fabricated by electrochemical deposition in single-pore membranes. Using three different parameter sets of temperature and voltage, three groups of wires, each with a different mean grain size, were created. By measuring the electrical resistance of each nanowire individually, resistance-versus-diameter data enable us to assort the Bi nanowires in three different groups characterized by three distinct mean specific electrical resistivities. Each of these three groups is also characterized by a common mean grain size predetermined by the correspondent set of deposition parameters. Thus the results demonstrate that the resistivity is determined mainly by electron scattering at grain boundaries. Resistance-versus-temperature curves display a nonmonotonic behavior. The calculated charge carrier mobility saturates at low temperatures and is one to two orders of magnitude smaller than in bulk material depending on wire diameter and crystallinity. This saturation is attributed to finite-size effects.
Highly transparent cathodes comprised of rare earth and Au stacked layers for top-emission organic light emitting diodes
Authors: Ran G. Z. , Zhao W. Q. , Dai L. , Qin G. G.
Published: December 2006

We report the transparent cathodes comprised of a low work function rare earth element (RE) and a Au capping layer for top-emission organic light emitting diodes (TOLEDs). The Sm∕Au, Yb∕Au, Gd∕Au, and Dy∕Au cathodes possess a transmittance up to ∼60% when the RE/Au thickness is 5∕10nm. The light extraction efficiencies for the TOLEDs with a silicon anode and a RE/Au cathode are computed to be 9%–13%, markedly higher than that for a similar device but with an Al∕Au cathode. The surface plasmon polaritons at the Yb(orSm)∕Au cathode are found to be much weaker than those at the Al∕Au cathode.
Magnesium diboride superconductor thin film tunnel junctions for superconductive electronics
Authors: Kim Tae Hee , Moodera Jagadeesh S.
Published: December 2006

Based on superconducting MgB2 films with higher critical temperature of 39K and the advantage of the conventional superconductors, those that follow Bardeen-Cooper-Shrieffer theory, fabrication of quasiparticle, and Josephson tunnel junctions have been investigated. To explore the potential of MgB2 for superconductive electronics, the essentials such as clean high quality thin film surfaces and reproducible tunnel junction fabrication are addressed. Our results show clean tunneling characteristics for in situ prepared MgB2 junctions on Si wafer with the measured superconducting energy gap values in good agreement with theory and the feasibility of the technology. The recent results on all-epitaxial MgB2∕MgO∕MgB2 Josephson junctions will be also reported.
Autoresonant beat-wave generation
Authors: Lindberg R. R. , Charman A. E. , Wurtele J. S. , Friedland L. , Shadwick B. A.
Published: December 2006

Autoresonance offers an efficient and robust means for the ponderomotive excitation of nonlinear Langmuir waves by phase-locking of the plasma wave to the slowly chirped beat frequency of the driving lasers via adiabatic passage through resonance. This mechanism is analyzed for the case of a cold, relativistic, underdense electron plasma, and its suitability for particle acceleration is discussed. Compared to traditional approaches, this new autoresonant scheme achieves larger accelerating electric fields for given laser intensity; the plasma wave excitation is much more robust to variations in plasma density; it is largely insensitive to the precise choice of chirp rate, provided only that it is sufficiently slow; and the suitability of the resulting plasma wave for accelerator applications is, in some respects, superior. As in previous schemes, modulational instabilities of the ionic background ultimately limit the useful interaction time, but nevertheless peak electric fields approaching the wave-breaking limit seem readily attainable. The total frequency shift required is only of the order of a few percent of the laser carrier frequency, and might be implemented with relatively little additional modification to existing systems based on chirped pulse amplification techniques, or, with somewhat greater technological effort, using a CO2 or other gas laser system.
Analysis of threshold voltage of short channel polycrystalline silicon thin-film transistors fabricated on large grains
Authors: Kawachi Genshiro , Tsuboi Shinzo , Okada Takashi , Mitani Masahiro , Matsumura Masakiyo
Published: December 2006

Drain bias dependence of threshold voltages of short channel thin-film transistors (TFTs) fabricated on polycrystalline silicon (poly-Si) films with large grains has been investigated. The drain coefficient of the threshold voltage was found to be substantially larger in poly-Si TFTs than that in metal-oxide-semiconductor field-effect transistors on a silicon-on-insulator wafer. In addition, the drain coefficient of poly-Si TFTs shows asymmetric behavior with respect to source-drain swapping. The observed results can be explained in terms of body potential modulation caused by accumulation of excess holes generated by impact ionization and drain junction leakage. The asymmetry in drain coefficient with source-drain swapping can be attributed to the difference in the bipolar gain between the source-body and drain-body junctions.
Thermally synthesized ruby microstructures and luminescence centers
Authors: Zanatta A R. , Ribeiro C T. M. , Jahn U. , Aldabergenova S. B. , Strunk H P.
Published: December 2006

This work reports on the synthesis and the optical-structural investigation of light-emitting microstructures obtained from amorphous aluminum nitride (AlN) thin films doped with chromium. The samples were prepared by radio frequency sputtering an Al target, with approximately 0.8ppm of chromium, in an atmosphere of pure nitrogen. After deposition, the samples were thermally annealed up to 1050°C under a flow of dry oxygen. The experimental investigation included photoluminescence, cathodoluminescence, energy dispersive x-ray spectroscopy, and scanning and transmission electron microscopy techniques. According to the experimental data, films annealed at 1050°C for 15min exhibit relatively strong luminescence at ∼685nm. Further thermal annealing at this temperature induces the development of microscopic features onto the surface of the films that emit red light at approximately 693nm. In view of their distinctive luminescence characteristics and spatial dimensions, the observed features have been designated by ruby microstructures (RbMS’s). For the present AlN films, the occurrence of these RbMS’s is associated with Cr3+ ions in oxygen-rich environments (α-alumina) that arise in consequence of the thermal treatments and of the presence of microimperfections (pores, voids, etc.) in the samples.
Thermal diffusion of dilute polymer solutions: The role of solvent viscosity
Authors: Hartung M. , Rauch J. , Köhler W.
Published: December 2006

We have performed measurements of the thermal diffusion coefficient DT in the dilute limit on polystyrene in cyclo-octane, cyclohexane, benzene, toluene, tetrahydrofuran, ethyl acetate, and methyl ethyl ketone and of poly(dimethyl-siloxane) in toluene. These data have been combined with literature data to test various theoretical predictions. The viscosity is identified as the dominating and only relevant solvent parameter. On the polymer side, the size or mass of an effective correlated segment determines the strength of the Soret effect. Large and heavy effective segments, as found in stiffer chains, lead to higher DT.
Ultraviolet emission in Tm3+-doped fluoride fiber pumped with two infrared wavelengths
Authors: Mejía E. B.
Published: December 2006

An infrared, two-wavelength pumping scheme for generating UV in Tm3+-doped fibers is investigated and proposed as an alternative because the pump wavelengths are accessible from laser diodes. Spectral characterizations of fiber samples with different concentrations revealed that moderate concentrations are best suitable to produce UV (348–362nm) emission when single—or double-line pumping with 1117 and 725nm. Detailed spectroscopic measurements realized to the fiber with the best performance, the 2000ppmwt, allowed to obtain the copumping wavelengths (in the ∼725nm region) that enhanced the UV emission. For example, when applying tens of milliwatts at 725nm, which represented a 28% increase of total pump power, the UV emission increased in an avalanchelike fashion up to three orders of magnitude. Then, a high-power 1117nm source that currently exists in the market and a moderate power 725nm source under development are possible to be used as pumps for this scheme.
Periodic nanostructuring of Er∕Yb-codoped IOG1 phosphate glass by using ultraviolet laser-assisted selective chemical etching
Authors: Pappas C. , Pissadakis S.
Published: December 2006

The patterning of submicron period (≈500nm) Bragg reflectors in the Er∕Yb-codoped IOG1 Schott, phosphate glass is demonstrated. A high yield patterning technique is presented, wherein high volume damage is induced into the glass matrix by exposure to intense ultraviolet 213nm, 150ps Nd:YAG laser radiation and, subsequently, a chemical development in potassium hydroxide (KOH)∕ethylenediamine tetra-acetic acid (EDTA) aqueous solution selectively etches the exposed areas. The electronic changes induced by the 213nm ultraviolet irradiation are examined by employing spectrophotometric measurements, while an estimation of the refractive index changes recorded is provided by applying Kramers-Kronig transformation to the absorption change data. In addition, real time diffraction efficiency measurements were obtained during the formation of the volume damage grating. After the exposure, the growth of the relief grating pattern in time was measured at fixed time intervals and the dependence of the grating depth on the etching time and exposure conditions is presented. The gratings fabricated are examined by atomic and scanning electron microscopies to reveal the relief topology of the structures. Gratings with average depth of 120nm and excellent surface quality were fabricated by exposing the IOG1 phosphate glass to 36 000 pulses of 208mJ∕cm2 energy density, followed by developing in the KOH∕EDTA agent for 6min.
Equilibrium statistical mechanics for single waves and wave spectra in Langmuir wave-particle interaction
Authors: Firpo M.-C. , Leyvraz F. , Attuel G.
Published: December 2006

Under the conditions of weak Langmuir turbulence, a self-consistent wave-particle Hamiltonian models the effective nonlinear interaction of a spectrum of M waves with N resonant out-of-equilibrium tail electrons. In order to address its intrinsically nonlinear time-asymptotic behavior, a Monte Carlo code was built to estimate its equilibrium statistical mechanics in both the canonical and microcanonical ensembles. First, the single wave model is considered in the cold beam-plasma instability and in the O’Neil setting for nonlinear Landau damping. O’Neil’s threshold, which separates nonzero time-asymptotic wave amplitude states from zero ones, is associated with a second-order phase transition. These two studies provide both a testbed for the Monte Carlo canonical and microcanonical codes, with the comparison with exact canonical results, and an opportunity to propose quantitative results to longstanding issues in basic nonlinear plasma physics. Then, the properly speaking weak turbulence framework is considered through the case of a large spectrum of waves. Focusing on the small coupling limit as a benchmark for the statistical mechanics of weak Langmuir turbulence, it is shown that Monte Carlo microcanonical results fully agree with an exact microcanonical derivation. The wave spectrum is predicted to collapse towards small wavelengths together with the escape of initially resonant particles towards low bulk plasma thermal speeds. This study reveals the fundamental discrepancy between the long-time dynamics of single waves, which can support finite amplitude steady states, and of wave spectra, which cannot.
Fluid modeling of the electron flow driven ion acoustic mode in a collisional plasma with magnetized electrons
Authors: Vranjes J. , Tanaka M. Y. , Poedts S.
Published: December 2006

A fluid analysis is presented of the ion sound mode in a weakly ionized collisional plasma. The ion-neutral collision frequency exceeds the ion gyrofrequency while the electrons remain magnetized. Under these conditions, an ion sound wave can propagate at arbitrary angles with respect to the direction of the magnetic field. In the presence of an electron flow along the magnetic lines the sound mode can grow. Due to the electron collisions the mode is unstable while ion collisions cause an angle dependent instability threshold which is such that the mode is most easily excited at very large angles. Hot ion effects are included in the study by means of an effective viscosity which effectively describes the ion Landau damping effect. In the presence of an additional light ion specie, the mode frequency and increment in a certain parameter range are increased.
Determination of dust particle charge using the deflection method in a plasma
Authors: Ashbourn J. M. A.
Published: December 2006

In this article an examination is made of the reported comparisons between the charge on a dust particle determined by the deflection method using a voltage applied perpendicular to the dust particle’s trajectory and the charge determined by the capacitor charging model using the floating potential measured by a Langmuir probe within the plasma. The reason for the observed discrepancy between the charge values from the two methods is discussed and a calculation is presented for the dust charge using the deflection method, which includes the ion drag force on the dust particle. This deflection calculation removes the discrepancy and shows good agreement with the reported capacitor model values.
Improved dielectric and ferroelectric properties of high Curie temperature (1−x)BiFeO3–xPbTiO3 ceramics by aliovalent ionic substitution
Authors: Zhu W.-M. , Ye Z.-G.
Published: December 2006

Ceramics of 0.67Bi(Fe1−xTix)O3+x∕2–0.33PbTiO3 (x=0, 0.005, 0.01, 0.02, 0.04) were prepared by solid state reactions, with a relative density of 94%–97%. The dielectric and ferroelectric properties of the ceramics have been significantly improved by means of aliovalent ionic substitution of Ti4+ for Fe3+, which reduces the electric conductivity. The temperature variation of the dielectric constant has been measured up to the Curie temperature as high as TC=650°C. A saturated dielectric hysteresis loop with a remnant polarization (Pr) of 9μC∕cm2 is displayed in the 0.5mol% Ti4+-substituted sample at room temperature. Pr reaches 17μC∕cm2 in the sample with 2mol% Ti4+ even though the hysteresis loop is not saturated, suggesting that a much higher saturated polarization can be expected in the Ti4+-substituted BiFeO3–PbTiO3 ceramics.
Dynamic shear modulus of glycerol: Corrections due to instrument compliance
Authors: Schröter K. , Hutcheson S. A. , Shi X. , Mandanici A. , McKenna G. B.
Published: December 2006

A recent article by Shi et al. [J. Chem. Phys.123, 174507 (2005)] reports results from mechanical measurements on three simple inorganic glass formers: glycerol, m-toluidine, and sucrose benzoate. The experiments carried out were stress relaxation, aging, and dynamic (all in shear) using a torsional rheometer, an advanced rheometric expansion system (TA Instruments). The original force rebalance transducer (2KFRT) supplied with the system was replaced with a custom-made load cell (Sensotec) that had a capacity of 20000gcm in torque and 5000g in normal force. The replacement of the load cell was done due to the belief that the main source of compliance in this instrument was from the 2KFRT. With this assumption, the authors published their results for the three materials of interest and compared their results with the techniques of Schröter and Donth [J. Chem. Phys.113, 9101 (2000)] for the measurements on glycerol and reported important differences. These differences were disputed by one of the present authors (Schröter), and the present report shows that the results from Schröter and Donth are correct. We show that the reasons have to do with the instrument compliance being greater than originally thought by Shi et al. Here we examine the effects of platen diameter/geometry on the glycerol dynamic moduli, describe a means to correct dynamic data, present a revised comparison of the corrected data with that of Schröter and Donth, and provide a discussion of future work and conclusions.
Shilnikov homoclinic orbit bifurcations in the Chua’s circuit
Authors: Medrano-T. R. O. , Baptista M. S. , Caldas I. L.
Published: December 2006

We analytically describe the complex scenario of homoclinic bifurcations in the Chua’s circuit. We obtain a general scaling law that gives the ratio between bifurcation parameters of different nearby homoclinic orbits. As an application of this theoretical approach, we estimate the number of higher order subsidiary homoclinic orbits that appear between two consecutive lower order subsidiary orbits. Our analytical finds might be valid for a large class of dynamical systems and are numerically confirmed in the parameter space of the Chua’s circuit.
Microfluidic cell fusion under continuous direct current voltage
Authors: Wang Jun , Lu Chang
Published: December 2006

The authors report a technique which produces cell-to-cell electrofusion using a common direct current power supply on a microfluidic platform. In the authors’ method, the cells were first conjugated based on biotin-streptavidin interaction. The electrofusion was then conducted by flowing the linked cells through a simple microfluidic channel with geometric variation under continuous direct current voltage. This microfluidics-based technique offers processing at the level of single cell pairs with efficiency comparable to that of conventional electrofusion technique based on electrical pulses.
Finite-size effects in simulations of nucleation
Authors: Wedekind Jan , Reguera David , Strey Reinhard
Published: December 2006

We investigate the importance of finite-size effects in simulations of nucleation processes. Most molecular dynamics simulations of first order phase transitions, such as vapor-liquid nucleation, are performed in the canonical NVT ensemble where, owing to the fixed total number of molecules N, the growth of the new phase causes the depletion of the metastable phase. This effect may lead to significant errors in the simulation and even to the impossibility of observing nucleation in a small finite system. We present a theory to estimate the system size beyond which these finite-size effects are expected to be negligible. This optimization saves valuable calculation time and can extend the range of supersaturations and rates attainable by simulations by several orders of magnitude. Our results are applicable to diverse situations, such as crystallization, capillary condensation, or the melting of nanoclusters.
Alternative search strategy for minimal energy nanocluster structures: The case of rhodium, palladium, and silver
Authors: Rogan José , García Griselda , Loyola Claudia , Orellana W. , Ramírez Ricardo , Kiwi Miguel
Published: December 2006

An alternative strategy to find the minimal energy structure of nanoclusters is presented and implemented. We use it to determine the structure of metallic clusters. It consists in an unbiased search, with a global minimum algorithm: conformational space annealing. First, we find the minima of a many-body phenomenological potential to create a data bank of putative minima. This procedure assures us the generation of a set of cluster configurations of large diversity. Next, the clusters in this data bank are relaxed by ab initio techniques to obtain their energies and geometrical structures. The scheme is successfully applied to magic number 13 atom clusters of rhodium, palladium, and silver. We obtained minimal energy cluster structures not previously reported, which are different from the phenomenological minima. Moreover, they are not always highly symmetric, thus casting some doubt on the customary biased search scheme, which consists in relaxing with density functional theory global minima chosen among high symmetry structures obtained by means of phenomenological potentials.
Wannier-Stark ladder conditions in 4H-SiC p-n junctions grown on off axis substrates
Authors: Sankin V. I. , Shkrebiy P. P. , Yakimova R.
Published: December 2006

4 H - Si C  p+-n−-n+ junctions have been prepared on 8° off oriented plane aiming to investigate conditions of Wannier-Stark localization (WSL), and respectively the occurrence of a negative temperature coefficient of avalanche breakdown voltage (TCABV). A method is proposed that can enable obtaining important results without reaching the regime of destructive avalanche breakdown. By analyzing the photocurrent of light with different wavelengths and at strong electric fields, a negative differential photoconductance has been discovered testifying that the WSL is not suppressed at the field inclination in 8° off axis, and a negative TCABV is expected in commercial p-n junctions.
Electrical characteristics of flexible organic static induction transistors under bending conditions
Authors: Watanabe Yasuyuki , Iechi Hiroyuki , Kudo Kazuhiro
Published: December 2006

Organic static induction transistors (OSITs) with vertical channel structures that operate at low voltages have potential as components of portable or ubiquitous devices. In this study, the static characteristics of pentacene OSITs on flexible substrates were examined under bending conditions. Bending tests of the flexible substrate were carried out under various compressive and tensile strains and at various bending radii. The results demonstrate that OSITs exhibit stable electrical characteristics at compressive and tensile strains up to a bending radius of 5mm, suggesting a possible application as transistors in the field of flexible electronics.
Glass transition temperature of water confined in lipid membranes as determined by anelastic spectroscopy
Authors: Castellano C. , Generosi J. , Congiu A. , Cantelli R.
Published: December 2006

The research of gene delivery vehicles used in gene therapy is focused on nonviral vectors like lipid membranes. Such vectors, nonimmunogenic and biodegradable, are formed by complexation of DNA with a mixture of cationic lipids and a neutral colipid which improve the transfection efficiency. A main topic related to lipid membrane dynamics is their capability to spontaneously confine water. At present the value of the glass transition temperature (Tg) is largely debated and determined only by some indirect methods. Here the authors show that anelastic spectroscopy allows the confined water Tg value to be directly identified in several lipid mixtures.
Broad wavelength modulating and design of organic white diode based on lighting by using exciplex emission from mixed acceptors
Authors: Wang D. , Li W. L. , Su Z. S. , Li T. L. , Chu B. , Bi D. F. , Chen L. L. , Su W. M. , He H.
Published: December 2006

Modulating electroluminescent (EL) spectra from interfacial exciplex emissions were observed by varying the ratios of two acceptors of exciplex-type devices in which the emissive wavelengths were tuned from 530to656nm. In the devices 4,4′,4″-tris[3-methyl-pheny(phenyl)amino]triphenylamine and (bathocuproine: scdolinium-(dibenzoyl-methane)3bathophenyl-phenathroline) mixtures were used as donor and acceptor materials, respectively. In terms of the exciplex broad band emission a white organic light emitting diode was demonstrated by skillfully designing the structure when blue subband was subjoined in the white spectrum. The white device behaves the Commission Internationale de l’Eclairage coordinates of (0.32, 0.35) with higher color stability at various biases, a color rendering index of 90.4, and a maximum luminance of 425cd∕m2, respectively, although the EL efficiency needs to be further improved. The emission mechanism of the broad exciplex band formed by two mixed acceptors was also discussed.
Anomalous conical menisci under an ac field-departure from the dc Taylor cone
Authors: Maheshwari Siddharth , Chang Hsueh-Chia
Published: December 2006

Observation of steady conical menisci with longitudinal growth is reported for electrospraying under high frequency alternating current (ac) electric fields. The authors report conical shapes that are analogous to the well-known direct-current (dc) cone-jet mode in the occurrence of conical menisci, but have some very distinctively ac features. The ac cones show a unique longitudinal growth that result in very high aspect ratio cones. The cone half angle for ethanol is approximately 9° for the ac case as compared to 47° for the dc case. These dissimilarities point to different mechanisms for dc and ac sprayings.
Giant optical birefringence in ensembles of semiconductor nanowires
Authors: Muskens O. L. , Borgström M. T. , Bakkers E. P. A. M. , Gómez Rivas J.
Published: December 2006

Giant birefringence is demonstrated in ensembles of randomly grown, vertically aligned semiconductor nanowires. Bottom-up fabrication of epitaxial semiconductor nanowires grown using the vapor-liquid-solid mechanism yields a highly anisotropic photonic metamaterial. The birefringence in the material is shown to depend on the nanowire length. Short nanowires with a volume filling fraction around 50% exhibit the strongest birefringence, in agreement with effective medium theories. A large difference is found between the in-plane and out-of-plane refractive indices of Δn=0.8, exceeding by a factor of 75 the natural birefringence of quartz and by more than a factor of 2 that of inverted artificial materials.
Computation of the nonhomogeneous equilibrium states of a rigid-rod solution
Authors: Green Micah J. , Armstrong Robert C. , Brown Robert A.
Published: December 2006

The nonhomogeneous equilibrium phase behavior of a solution of rigid rods is analyzed for a periodic one-dimensional system. Stable and unstable equilibrium solutions for the distribution function are computed as extrema of the free energy of the system expressed by the nonhomogeneous generalization of Onsager’s [Ann. N.Y. Acad. Sci. 51, 627 (1949)] theory, which models interaction between rods on the scale of a single rod length. Biaxial equilibrium solutions are computed in a periodic system by discretizing the Euler-Lagrange nonlinear integral equation by the finite-element method and using Newton’s method to solve the resulting set of nonlinear equations. Stable states for isotropic-nematic coexistence are computed in a periodic system rather than the semi-infinite system used in previous calculations. The density and order parameter profiles evolve monotically from the isotropic phase to the nematic phase. Unstable, nonhomogeneous, equilibrium states are also computed for concentrations of rods that exceed the value for spinodal decomposition. These nonhomogeneous states are characterized by combinations of bend, twist, and splay distortions in physical space and correspond to unstable attractors in the dynamic process of isotropic-nematic spinodal decomposition. For large systems, the nonhomogeneous states develop wide, bulklike nematic regions separated by thin regions with sharp gradients in orientation. The free energy formulation was also used to compute the accurate neutral stability curve; this curve shows the limits of applicability of the low-wave-number approximations frequently used in the study of spinodal decomposition.
Tunnel magnetoresistance in GaCrN∕AlN∕GaCrN ferromagnetic semiconductor tunnel junctions
Authors: Kim Moo-Seong , Zhou Yi-Kai , Funakoshi Masayuki , Emura Shuichi , Hasegawa Shigehiko , Asahi Hajime
Published: December 2006

Ga Cr N ∕ Al N ∕ Ga Cr N trilayer structures with different Cr concentrations were grown by molecular-beam epitaxy. Well-defined hysteresis loop was observed in the magnetization versus magnetic field curves even at room temperature. By the current versus voltage characteristics it is confirmed that the AlN layer behaves as a tunnel barrier with an estimated barrier height of 2.1eV in the GaCrN∕AlN∕GaCrN structures. For these trilayer tunnel junction diodes, clear hysteresis characteristics were observed in the resistance versus magnetic field curves at 77K when the current flow and the applied magnetic field were perpendicular to and parallel to the junction plane, respectively.
Pseudomorphic molecular beam epitaxy growth of γ-Al2O3(001) on Si(001) and evidence for spontaneous lattice reorientation during epitaxy
Authors: Merckling C. , El-Kazzi M. , Delhaye G. , Gendry M. , Saint-Girons G. , Hollinger G. , Largeau L. , Patriarche G.
Published: December 2006

Single crystal γ-Al2O3 thin films have been epitaxially grown by molecular beam epitaxy at 850°C on Si(001) substrates. Reflection high energy electron diffraction and transmission electron microscopy experiments evidence the good crystalline quality of the Al2O3 layer. The present study shows that the two first monolayers of γ-Al2O3 are (001) oriented and coherently strained on Si. For larger thickness, a transition from (001)- to (111)-oriented Al2O3 occurs, together with the apparition of domains in the layer. In-plane epitaxial relationship between Al2O3 and Si(001) are deduced from these observations.
Sub-Kelvin transport spectroscopy of fullerene peapod quantum dots
Authors: Utko Pawel , Nygård Jesper , Monthioux Marc , Noé Laure
Published: December 2006

The authors have studied electrical transport properties of individual C60 fullerene peapods, i.e., single-wall carbon nanotubes encapsulating C60 molecules. Their measurements indicated power lawlike temperature dependencies of linear conductance similar to those for empty nanotubes. At temperatures below 30K, peapod devices behaved as highly regular individual quantum dots showing regular Coulomb blockade oscillations. Signatures of Kondo physics appeared at the lowest measurement temperature of 315mK.
Local environment of Er3+ in Er-doped Si nanoclusters embedded in SiO2 films
Authors: Bian Liu-Fang , Zhang C. G. , Chen W. D. , Hsu C. C. , Shi Tongfei
Published: December 2006

The local environment of Er3+ in heavily Er-doped (Er, 2.5at.%) Si nanoclusters embedded in SiO2 films annealed at various temperatures was investigated by using the fluorescence-extended x-ray absorption fine structure spectroscopy. The results show that annealing caused a large effect on the local environment of Er3+ surrounded by O atoms and the 1.54μm photoluminescence intensity. The correlation between the local environment around Er3+ and the corresponding 1.54μm photoluminescence was discussed.
Metal-insulator-semiconductor structure on low-temperature grown GaAs
Authors: Chen A. , Young M. , Li W. , Ma T. P. , Woodall J. M.
Published: December 2006

The metal-insulator-semiconductor (MIS) capacitors on low-temperature grown (LTG) GaAs were made using the jet-vapor-deposition (JVD) silicon nitride as the gate dielectrics. The unpinned JVD-SiN/LTG-GaAs interface was shown by the capacitance-voltage characterization. The observed Fermi level “pinning” in the unannealed samples was caused by the bulk point defects in LTG GaAs. Annealing reduced the bulk defect density in LTG GaAs and revealed the intrinsically unpinned Fermi level at the interface. The result is consistent with the expected unpinning of LTG GaAs surface, and JVD SiN appears to play a critical role to enable this unpinned interface. JVD SiN showed low leakage current (∼10nA∕cm2 at 2MV∕cm) and high breakdown electric field (∼9.8MV∕cm), promising for MIS device applications.
Electroluminescence from ZnO nanoparticles/organic nanocomposites
Authors: Lee Chun-Yu , Haung Yau-Te , Su Wei-Fang , Lin Ching-Fuh
Published: December 2006

The authors report ultraviolet electroluminescence from ZnO nanoparticle-based devices prepared by the phase-segregation technique. The conditions for phase segregation are investigated using confocal microscopy. With proper parameters for phase segregation, the ZnO nanoparticles and N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine: poly(methyl methacrylate) can be separated into two layers upon spin-coating process. The method allows electrons and holes to recombine in the ZnO nanoparticles. The I-V curve shows stable and excellent rectification. For the device with 90nm ZnO nanoparticles, it exhibits a very narrow spectrum with a peak at 392nm and no defect-related emission. The emission peak well corresponds to the ZnO band-gap energy.
Geniculocortical relay of blue-off signals in the primate visual system
Authors: Szmajda Brett A. , Buzás Péter , FitzGibbon Thomas , Martin Paul R.
Published: December 2006

A fundamental dichotomy in the subcortical visual system exists between on- and off-type neurons, which respectively signal increases and decreases of light intensity in the visual environment. In primates, signals for red-green color vision are carried by both on- and off-type neurons in the parvocellular division of the subcortical pathway. It is thought that on-type signals for blue-yellow color vision are carried by cells in a distinct, diffusely projecting (koniocellular) pathway, but the pathway taken by blue-off signals is not known. Here, we measured blue-off responses in the subcortical visual pathway of marmoset monkeys. We found that the cells exhibiting blue-off responses are largely segregated to the koniocellular pathway. The blue-off cells show relatively large receptive fields, sluggish responses to maintained contrast, little sign of an inhibitory receptive-field surround mechanism, and negligible functional input from an intrinsic (melanopsin-based) phototransductive mechanism. These properties are consistent with input from koniocellular or “W-like” ganglion cells in the retina and suggest that blue-off cells, as previously shown for blue-on cells, could contribute to cortical mechanisms for visual perception via the koniocellular pathway.
Predicting the global spread of H5N1 avian influenza
Authors: Kilpatrick A. Marm , Chmura Aleksei A. , Gibbons David W. , Fleischer Robert C. , Marra Peter P. , Daszak Peter
Published: December 2006

The spread of highly pathogenic H5N1 avian influenza into Asia, Europe, and Africa has resulted in enormous impacts on the poultry industry and presents an important threat to human health. The pathways by which the virus has and will spread between countries have been debated extensively, but have yet to be analyzed comprehensively and quantitatively. We integrated data on phylogenetic relationships of virus isolates, migratory bird movements, and trade in poultry and wild birds to determine the pathway for 52 individual introduction events into countries and predict future spread. We show that 9 of 21 of H5N1 introductions to countries in Asia were most likely through poultry, and 3 of 21 were most likely through migrating birds. In contrast, spread to most (20/23) countries in Europe was most likely through migratory birds. Spread in Africa was likely partly by poultry (2/8 introductions) and partly by migrating birds (3/8). Our analyses predict that H5N1 is more likely to be introduced into the Western Hemisphere through infected poultry and into the mainland United States by subsequent movement of migrating birds from neighboring countries, rather than from eastern Siberia. These results highlight the potential synergism between trade and wild animal movement in the emergence and pandemic spread of pathogens and demonstrate the value of predictive models for disease control.
Environmental change and infectious disease: How new roads affect the transmission of diarrheal pathogens in rural Ecuador
Authors: Eisenberg Joseph N. S. , Cevallos William , Ponce Karina , Levy Karen , Bates Sarah J. , Scott James C. , Hubbard Alan , Vieira Nadia , Endara Pablo , Espinel Mauricio , Trueba Gabriel , Riley Lee W. , Trostle James
Published: December 2006

Environmental change plays a large role in the emergence of infectious disease. The construction of a new road in a previously roadless area of northern coastal Ecuador provides a valuable natural experiment to examine how changes in the social and natural environment, mediated by road construction, affect the epidemiology of diarrheal diseases. Twenty-one villages were randomly selected to capture the full distribution of village population size and distance from a main road (remoteness), and these were compared with the major population center of the region, Borbón, that lies on the road. Estimates of enteric pathogen infection rates were obtained from case-control studies at the village level. Higher rates of infection were found in nonremote vs. remote villages [pathogenicEscherichia coli: odds ratio (OR) = 8.4, confidence interval (CI) 1.6, 43.5; rotavirus: OR = 4.0, CI 1.3, 12.1; andGiardia: OR = 1.9, CI 1.3, 2.7]. Higher rates of all-cause diarrhea were found in Borbón compared with the 21 villages (RR = 2.0, CI 1.5, 2.8), as well as when comparing nonremote and remote villages (OR = 2.7, CI 1.5, 4.8). Social network data collected in parallel offered a causal link between remoteness and disease. The significant and consistent trends across viral, bacterial, and protozoan pathogens suggest the importance of considering a broad range of pathogens with differing epidemiological patterns when assessing the environmental impact of new roads. This study provides insight into the initial health impacts that roads have on communities and into the social and environmental processes that create these impacts.
Forest fire increases mercury accumulation by fishes via food web restructuring and increased mercury inputs
Authors: Kelly Erin N. , Schindler David W. , St. Louis Vincent L. , Donald David B. , Vladicka Katherine E.
Published: December 2006

Recent findings indicate that fishes from lakes in partially burned catchments contain greater mercury (Hg) concentrations than fishes from reference catchments. Increased methyl Hg (MeHg) concentrations in fishes can result in serious health problems for consumers. Here we show that a forest fire caused a 5-fold increase in whole-body Hg accumulation by rainbow trout (Oncorhynchus mykiss) and smaller Hg increases in muscle of several fish species in a mountain lake. The enhanced Hg accumulation was caused primarily by increased nutrient concentrations in the lake, which enhanced productivity and restructured the food web through increased piscivory and consumption ofMysis. This restructuring resulted in increases to the trophic positions and Hg concentrations of fishes. Forest fire also caused a large short-term release of total Hg (THg) and MeHg to streams and the lake. This release initiated a small pulse of MeHg in invertebrates that contributed to enhanced Hg accumulation by fishes. Climate change and prescribed burning to compensate for past fire suppression are predicted to increase future forest fire occurrence in North America, and increased Hg accumulation by fishes may be an unexpected consequence.
Commercial breaks: an overview of corporate opportunities for commercializing education in US and English schools
Authors: Wilkinson Gary
Published: December 2006

This article outlines how the commercialization of structures and management in education risks accelerating the intensification of corporate marketing in schools. It is argued that marketing promotes materialistic values which are harmful to children and US and UK examples are offered
to demonstrate how companies seek to use schools as brand-building sites. The article argues that these activities pose a threat to children and to the integrity and purposes of education.
Students as rational decision-makers: the question of beliefs and attitudes
Authors: Sullivan Alice
Published: December 2006

Rational choice theorists have analysed rates of participation in post-compulsory education, and, in particular, class differentials in these rates. Various claims have been made about the motivations of student decision-makers, but these claims have not been grounded empirically. This
paper will assess the question of whether students' attitudes to education and beliefs about their own academic abilities vary according to social background and gender. Evidence is presented that students' attitudes to education do not vary greatly according to gender or social background,
but that both the social background and gender of students affect their perception of their own abilities.
Cadherins in development: cell adhesion, sorting, and tissue morphogenesis
Authors: Halbleib Jennifer M. , Nelson W. James
Published: December 2006

Tissue morphogenesis during development is dependent on activities of the cadherin family of cell–cell adhesion proteins that includes classical cadherins, protocadherins, and atypical cadherins (Fat, Dachsous, and Flamingo). The extracellular domain of cadherins contains characteristic repeats that regulate homophilic and heterophilic interactions during adhesion and cell sorting. Although cadherins may have originated to facilitate mechanical cell–cell adhesion, they have evolved to function in many other aspects of morphogenesis. These additional roles rely on cadherin interactions with a wide range of binding partners that modify their expression and adhesion activity by local regulation of the actin cytoskeleton and diverse signaling pathways. Here we examine how different members of the cadherin family act in different developmental contexts, and discuss the mechanisms involved.
A bifunctional O-GlcNAc transferase governs flagellar motility through anti-repression
Authors: Shen Aimee , Kamp Heather D. , Gründling Angelika , Higgins Darren E.
Published: December 2006

Flagellar motility is an essential mechanism by which bacteria adapt to and survive in diverse environments. Although flagella confer an advantage to many bacterial pathogens for colonization during infection, bacterial flagellins also stimulate host innate immune responses. Consequently, many bacterial pathogens down-regulate flagella production following initial infection. Listeria monocytogenes is a facultative intracellular pathogen that represses transcription of flagellar motility genes at physiological temperatures (37°C and above). Temperature-dependent expression of flagellar motility genes is mediated by the opposing activities of MogR, a DNA-binding transcriptional repressor, and DegU, a response regulator that functions as an indirect antagonist of MogR. In this study, we identify an additional component of the molecular circuitry governing temperature-dependent flagellar gene expression. At low temperatures (30°C and below), MogR repression activity is specifically inhibited by an anti-repressor, GmaR. We demonstrate that GmaR forms a stable complex with MogR, preventing MogR from binding its DNA target sites. GmaR anti-repression activity is temperature dependent due to DegU-dependent transcriptional activation of gmaR at low temperatures. Thus, GmaR production represents the first committed step for flagella production in L. monocytogenes. Interestingly, GmaR also functions as a glycosyltransferase exhibiting O-linked N-acetylglucosamine transferase (OGT) activity for flagellin (FlaA). GmaR is the first OGT to be identified and characterized in prokaryotes that specifically β-O-GlcNAcylates a prokaryotic protein. Unlike the well-characterized, highly conserved OGT regulatory protein in eukaryotes, the catalytic activity of GmaR is functionally separable from its anti-repression function. These results establish GmaR as the first known example of a bifunctional protein that transcriptionally regulates expression of its enzymatic substrate.