bluuebunny/crossref_metadata_2025 huggingface dataset:
Contactless electroreflectance evidence for reduction in the surface potential barrier in AlGaN/GaN heterostructures passivated by SiN layer
Authors: Kudrawiec R. , Paszkiewicz B. , Motyka M. , Misiewicz J. , Derluyn J. , Lorenz A. , Cheng K. , Das J. , Germain M.
Published: November 2008

Contactless electroreflectance (CER) has been applied to study the AlGaN potential-barrier height in AlGaN/GaN heterostructures without and with a SiN passivation layer. In the case of an unpassivated structure, an AlGaN band-edge signal with a strong Franz–Keldysh oscillation (FKO) was observed. On the basis of the FKO period, the surface potential barrier has been determined to be ∼1.1 eV. For the SiN passivated structure, a broad CER signal without FKO appears at the AlGaN edge. This observation is associated with a decrease in the height of the surface potential barrier, i.e., a shift in the Fermi level position at the AlGaN surface toward the conduction band.
Gas motion induced by unsteady boundary heating in a small-scale slab
Authors: Manela Avshalom , Hadjiconstantinou Nicolas G.
Published: November 2008

We study the response of a gas confined in a small-scale gap to a small time-dependent change in the temperature of the gap boundaries. Using the collisionless Boltzmann equation, a general scheme for the calculation of the probability density function and the respective hydrodynamic fields in response to any heating history is developed. Asymptotic analytical results are obtained for the cases of “ramp” (linearly varying with a cutoff value) and oscillatory boundary heating. The ramp solution can be used to approximate the system response to arbitrarily slow and fast process time scales (compared to the mean free time) and thus complement previous analyses focusing on relatively slow or instantaneous heating. For oscillatory heating at frequencies significantly higher than the collision frequency, we find that, at late times, the hydrodynamic fields decay proportionally to exp[−(ωδw)2/3], where ω is the oscillation frequency and δw is the distance from the wall. As a result, for sufficiently high frequencies, the steady-state gas motion is confined to narrow “bounded” layers in the vicinity of each wall. The results are compared with low-variance particle simulations of the linearized Boltzmann equation using the recently developed low-variance deviational simulation Monte Carlo method. Good agreement is found between the analysis and simulations, suggesting the former to be an accurate and simple means for calculating the hydrodynamic response of systems of arbitrary size within one mean free path from the heated boundary.
Temperature dependent Raman spectroscopy of chemically derived graphene
Authors: Allen Matthew J. , Fowler Jesse D. , Tung Vincent C. , Yang Yang , Weiller Bruce H. , Kaner Richard B.
Published: November 2008

Reduced graphite oxide (GO) has shown promise as a scalable alternative to mechanically exfoliated specimens. Although many measurements show that reduced GO has properties approaching those of pristine graphene, it has been difficult to quantify the extent to which the graphitic network is restored upon reduction. Raman spectroscopy is widely used for the characterization of mechanically exfoliated graphene, but has not been fully explored for reduced GO. In this work, hydrazine suspensions of reduced GO are deposited on micro-hot-plates and examined over a range of temperatures by Raman spectroscopy. The work highlights the benefits of solution processing.
Intrinsic thermal expansivity and hydrational properties of amyloid peptide Aβ42 in liquid water
Authors: Brovchenko I. , Burri R. R. , Krukau A. , Oleinikova A. , Winter R.
Published: November 2008

Volumetric and conformational properties of the amyloid β(1–42) peptide (Aβ42) are studied in relation to the properties of hydration water in a wide temperature range by computer simulations. The apparent volume of Aβ42, which is the change in the total volume of the solution due to the presence of Aβ42, shows a quite different temperature dependence below and above T≈320 K. The apparent thermal expansion coefficient αapp(Aβ42) is about 1.5×10−3 K−1 at T≤320 K and about 0.6×10−3 K−1 at T>320 K. By evaluation of the thermal expansivity of hydration water, the intrinsic expansivity of the biomolecule in liquid water was determined for the first time. The intrinsic thermal expansion coefficient of Aβ42 is found to be negative: αint(Aβ42)≈−0.8×10−3 K−1. The negative thermal expansion coefficient of Aβ42 can be attributed to its rubberlike (entropic) elasticity and/or to a decreasing number of intrapeptide hydrogen bonds. Upon heating, Aβ42 transforms from an extended chain with a significant content of α-helices to a compact coil with noticeable content of β-structures. A hydrogen-bonded spanning network of hydration water envelops Aβ42 homogeneously at low temperatures but breaks into an ensemble of small water clusters upon heating via a percolation transition, whose midpoint is close to the temperature, where the apparent volume of Aβ42 changes its temperature behavior. The mutual relation between the volumetric properties of Aβ42, its conformational properties, and the properties of the hydration water is discussed.
The Cauchy problem of the Ward equation with mixed scattering data
Authors: Wu Derchyi
Published: November 2008

We solve the Cauchy problem of the Ward equation with both continuous and discrete scattering data.
Use of an enhanced bulk diffusion-based algorithm for phase separation of a ternary mixture
Authors: Gómez Jordi , Sagués Francesc , Reigada Ramon
Published: November 2008

Phase separation kinetics of two-dimensional ternary mixtures have been studied by means of a Monte Carlo approach. Standard Kawasaki kinetics are impractical to study the late stages of the segregation process at deep quenches. An extension of the accelerated algorithm for binary mixtures proposed by Marko and Barkema [Phys. Rev. E 52, 2522 (1995)] is presented to overcome this problem in a three-component system discretized with two coupled lattices. We study the domain growth and the scaling behavior over a wide range of quench depths. Computer performances of the Kawasaki and the accelerated schemes are compared.
Isomorphic classical molecular dynamics model for an excess electron in a supercritical fluid
Authors: Miller Thomas F.
Published: November 2008

Ring polymer molecular dynamics (RPMD) is used to directly simulate the dynamics of an excess electron in a supercritical fluid over a broad range of densities. The accuracy of the RPMD model is tested against numerically exact path integral statistics through the use of analytical continuation techniques. At low fluid densities, the RPMD model substantially underestimates the contribution of delocalized states to the dynamics of the excess electron. However, with increasing solvent density, the RPMD model improves, nearly satisfying analytical continuation constraints at densities approaching those of typical liquids. In the high-density regime, quantum dispersion substantially decreases the self-diffusion of the solvated electron. In this regime where the dynamics of the electron is strongly coupled to the dynamics of the atoms in the fluid, trajectories that can reveal diffusive motion of the electron are long in comparison to βℏ.
Robust and leaky transport barriers in unstable baroclinic flows
Authors: Esler J. G.
Published: November 2008

Unstable baroclinic jets undergoing life cycles of wave growth and nonlinear equilibration are investigated numerically in a quasigeostrophic two-layer model. The focus is on understanding the properties of the upper layer transport barrier that emerges in the ensuing turbulent flow. The transport barriers in the simulated flows are representative of observed atmospheric and oceanic transport barriers, for example, at the extratropical tropopause, or at the Atlantic Gulf stream or the Antarctic circumpolar current. The simulations reveal that, depending on the value of the (inverse) criticality parameter β associated with the initial jet, the developing transport barriers either remain almost entirely impermeable to transport or “leak” by allowing vortices to be shed across them. A dynamical theory to predict the final flow, based on minimization of potential energy subject to relevant kinematic and dynamical constraints, is extended to make predictions about the transport barrier behavior. The theory is able to predict the value of β for which the barrier begins to leak and provides accurate estimates for the ensuing potential vorticity exchange across the barrier, which is primarily due to the vortex shedding.
Comparative study of laser produced Li plasma plumes from thin film and solid target
Authors: Kumar Ajai , Singh R. K. , Prahlad V. , Joshi H. C.
Published: November 2008

The evolution features of lithium ions and neutrals generated by two different schemes viz. the laser-blow-off (LBO) of multicomponent LiF–C thin film and conventional laser ablation (referred here as LPP) from solid lithium have been studied using optical emission spectroscopic technique. The optical signal emitted by Li I (at 670.8 nm) and Li II (548.4 nm) was monitored as a function of laser fluence, ambient gas pressure, and distance “z” from the target. Apart from their similarities, some interesting differences were noticed in temporal profiles of the plumes generated by LPP and LBO both in vacuum as well as in the presence of the ambient gas. A comparative analysis of experimental results indicates that the ablation mechanism and subsequent laser-plume interaction were responsible for the observed differences in LPP and LBO plumes. Expansion features of the plume are discussed in the light of two different models.
Pressure shifts and broadening of the Cs D1 and D2 lines by He, N2, and Xe at densities used for optical pumping and spin exchange polarization
Authors: Couture A. H. , Clegg T. B. , Driehuys B.
Published: November 2008

The production of hyperpolarized gases by spin-exchange optical pumping (SEOP) requires exact knowledge of the alkali metal’s D1 absorption profile and the degree to which it is broadened and shifted by varying buffer gas composition and pressure. We have measured these parameters for cesium (Cs) in the presence of Xe, N2, and H4e perturber gases at densities up to 10 amagats. The effects of these gases are important as Cs is attracting increasing interest for SEOP applications. Our measurements were made using simple white-light illumination of the Cs vapor while characterizing the D1 (6S1/2 to 6P1/2) and D2 (6S1/2 to 6P3/2) resonances using a high-resolution optical spectrometer. For the Cs D1 resonance at T=120 °C, we report shifts from the 894.59 nm vacuum wavelength caused by H3e, H4e, N2, and Xe of −0.017±0.003, −0.013±0.002, 0.026±0.002, and 0.029±0.002 nm/amagat. We also report the shifts for the D2 resonance as well as pressure broadening coefficients for both resonances.
Effect of small size on wave propagation in double-walled carbon nanotubes under temperature field
Authors: Tounsi A. , Heireche H. , Berrabah H. M. , Benzair A. , Boumia L.
Published: November 2008

The effect of small size on wave propagation in double-walled carbon nanotubes (DWCNTs) under temperature field is investigated using the Euler–Bernoulli beam model. Dynamic governing equations of the carbon nanotube are formulated on the basis of nonlocal thermal elastic theory. The effects of temperature change and van der Waals forces between the inner and outer nanotubes are taken into account. Results show the significance of the small-scale effect on wave propagation in DWCNTs and that some properties of transverse vibrations of DWCNTs are dependent on the change in temperature. The results demonstrate the great potential of the proposed nonlocal beam theory in studying wave propagation in CNTs including thermal effects and also indicate the limitations of local continuum mechanics in analysis of small-scale effects. The work should be useful in the design and application of nanoelectronics and nanoelectromechanical devices.
Selective transmittance of linearly polarized light in thin films rationally designed by FDTD and FDFD theories and fabricated by glancing angle deposition
Authors: Leontyev Viktor , Wakefield Nicholas G. , Tabunshchyk Kyrylo , Sit Jeremy C. , Brett Michael J. , Kovalenko Andriy
Published: November 2008

Columnar thin films exhibiting linear polarization selectivity have been theoretically investigated and fabricated using the glancing angle deposition (GLAD) technique. The film structure employed an s-shaped columnar morphology that may be either smoothly bent or composed of discrete segments. Finite-difference time-domain and frequency-domain methods have been used to model these thin film structures numerically. Simulation results have yielded optimal geometries in which the films exhibit either a single frequency band with polarization-selective transmittance or two separated frequency bands each transmitting one of two orthogonal linearly polarized modes. Based on these designs, a series of TiO2 films were grown by GLAD with continuous and discrete s-shaped columnar morphology. Experimental measurements by spectrophotometry verified the presence of selectivity for the orthogonal linearly polarized modes. Films with more then 24 periods were found to have polarization selectivity approaching unity. The agreement between the simulation and experimental results demonstrates the potential for future theoretical development of highly selective polarization filters based on GLAD thin films.
Newton’s cradle undone: Experiments and collision models for the normal collision of three solid spheres
Authors: Donahue C. M. , Hrenya C. M. , Zelinskaya A. P. , Nakagawa K. J.
Published: November 2008

Using an apparatus inspired by Newton’s cradle, the simultaneous, normal collision between three solid spheres is examined. Namely, an initially touching, motionless pair of “target” particles (doublet) is impacted on one end by a third “striker” particle. Measurements of postcollisional velocities and collision durations are obtained via high-speed photography and an electrical circuit, respectively. Contrary to intuition, the expected Newton's cradle outcome of a motionless, touching particle pair at the bottom of the pendulum arc is not observed in either case. Instead, the striker particle reverses its direction and separates from the middle particle after collision. This reversal is not observed, however, if the target particles are separated by a small distance (not in contact) initially, although a separation still occurs between the striker and middle particle after the collision, with both particles traveling in the same direction. For the case of initially touching target particles, contact duration measurements indicate that the striker separates from the three particles before the two target particles separate. However, when the targets are slightly separated, a three-particle collision is never observed, and the collision is, in fact, a series of two-body collisions. A subsequent implementation of a variety of hard-sphere and soft-sphere collision models indicates that a three-body (soft-sphere) treatment is essential for predicting the velocity reversal, consistent with the experimental findings. Finally, a direct comparison between model predictions and measurements of postcollisional velocities and contact durations provides a gauge of the relative merits of existing collision models for three-body interactions.
Thermal peculiarity of AlAs-capped InAs quantum dots in a GaAs matrix
Authors: Dorogan V. G. , Mazur Yu. I. , Lee J. H. , Wang Zh. M. , Ware M. E. , Salamo G. J.
Published: November 2008

GaAs and AlAs thin capping layers as well as postgrowth rapid thermal annealing (RTA) were applied to InAs quantum dots (QDs) grown by molecular beam epitaxy to study the tunability of optical properties of QDs by photoluminescence (PL) methods. The PL of AlAs-capped QDs shows double-peak structure, as opposed to GaAs-capped QDs, which is due to the formation of two families of QDs in the AlAs-capped sample confirmed by the power dependent PL measurements. The PL peak of the GaAs-capped samples subjected to RTA showed blueshift and narrowing with an increase in RTA temperature. This is the result of thermally enhanced In–Ga intermixing. More complex changes in the PL spectrum of AlAs-capped QDs during the RTA procedure were found and explained by the different In compositions in two branches of QDs. The features observed in the temperature dependences of PL peak energy of GaAs- and AlAs-capped samples were interpreted in terms of thermal escape of carriers from smaller QDs with further redistribution between larger QDs and different InAs content in two families of QDs.
The dissociation of excitons at indium tin oxide-copper phthalocyanine interface in organic solar cells
Authors: Sun X. Y. , Song Q. L. , Wang M. L. , Ding X. M. , Hou X. Y. , Zhou Z. G. , Li F. Y.
Published: November 2008

Exciton dissociation process at indium tin oxide (ITO)/copper phthalocyanine (CuPc) interface of ITO/CuPc(370 nm)/Al is studied by transient photovoltage method. A negative-to-positive change in the polarity of photovoltage upon pulsed laser irradiation is observed in CuPc thin film. The polarity change is regarded as a summation of the effect of exciton dissociation at ITO/CuPc interface (fast process) and that of free carrier separation by built-in field (slow process). Further experiments confirm the existence of exciton dissociation at ITO/CuPc interface, and the direction of which is electron injected into ITO, with holes left in CuPc film. This is opposite to that of the interfacial dissociation at donor/acceptor (D/A) interface in single heterojunction cells (ITO/D/A/buffer/Al). 3-nm-thick LiF insulating layer is inserted between ITO and CuPc to inhibit the exciton dissociation at ITO/CuPc interface. Thereby, the open-circuit voltage and power conversion efficiency of the single layer cell have been increased by several times.
Al-doped ZnO nanocrystals
Authors: Kadam Pratibha , Agashe Chitra , Mahamuni Shailaja
Published: November 2008

Al 3 + -doped ZnO nanocrystals were differently obtained by wet chemical and an electrochemical route. An increase in forbidden gap due to change in crystal size and also due to Al3+ doping in ZnO is critically analyzed. The Moss–Burstein type shift in Al3+-doped ZnO nanocrystals provides an evidence of successful Al3+ doping in ZnO nanocrystals. The possibility of varying the carrier concentration in ZnO nanocrystals is the indirect implication of the present investigations.
Influence of growth conditions and polarity on interface-related electron density in InN
Authors: King P. D. C. , Veal T. D. , Gallinat C. S. , Koblmüller G. , Bailey L. R. , Speck J. S. , McConville C. F.
Published: November 2008

Electron accumulation at the oxidized surface of In- and N-polarity indium nitride is shown to exhibit no dependence on the growth conditions (varied from In- to N-rich), revealing the surface Fermi level to be pinned 1.4±0.1 eV above the valence band maximum for all cases. This is in contrast to the interpretation of recent multiple-field Hall effect measurements, which suggested almost an order of magnitude increase in the sheet density of the accumulation layer upon moving from In-rich to N-rich growth conditions, and sample thickness dependent single-field Hall effect measurements which suggested different surface sheet densities for In- and N-polarity samples. However, an increase in the electron density approaching the InN/GaN (buffer layer) interface was not considered in the analysis of these Hall effect measurements, and this is invoked here to reconcile the constant surface Fermi level with the variations in “excess” sheet density observed in the previous Hall effect studies.
Lattice contraction during amorphization by mechanical alloying
Authors: Suryanarayana C. , Sharma Satyajeet
Published: November 2008

Amorphization has been achieved in blended elemental Fe-based multicomponent alloy powders by mechanical alloying. The effect of Nb addition to the Fe42Ni28Zr10−xNbxB20 alloy in the composition range of 1–6 at. % Nb has been investigated and it was shown that the glass-forming ability (GFA) of the alloys, defined as the milling time required to produce an amorphous phase, improved with Nb addition. The improvement was not regular; the highest GFA was achieved at an Nb level of 2 at. %. Associated with the amorphization process, lattice contraction was noted. The processes of occurrence of the amorphous phase in this alloy system, maximum GFA in the alloy with 2 at. % Nb, and lattice contraction were explained on the basis of the atomic strain model developed first for binary alloys and extended later to ternary and multicomponent alloys, and the change in coordination number with the size ratio of the constituent atoms.
Oscillating streaming potential measurement system for macroscopic surfaces
Authors: Reischl Martin , Köstler Stefan , Kellner Gerhard , Stana-Kleinschek Karin , Ribitsch Volker
Published: November 2008

A method and instrumentation is described capable of streaming potential measurements of various macroscopic surfaces. It differs from other approaches due to the creation of an oscillatory flow of electrolyte solutions through or alongside the sample. This technique offers a wide range of applied flow frequency and amplitude resulting in a fast and highly accurate measurement. This enables the streaming potential detection at rather high ionic strength and in a short time regime, which allows the monitoring of adsorption processes. Streaming potential and applied pressure are measured simultaneously, together with the specific conductivity of the bulk solution, pH value, and temperature. Combining these data, the zeta potential (ζ) for many different material types (fibers, films, foils, granules, and particles) can be calculated. The apparatus comprises reliable and robust measurements, simple handling, a high degree of automation, and advanced software control. With this setup, automated pH and concentration dependent ζ-potential measurements are possible for a variety of analytes and adsorbing species (e.g., ionic strength, surfactants, polyelectrolytes, and proteins); time-resolved measurements are facilitated down to the seconds time scale. The device allows the necessary sample preparation and equilibration outside the instrument using exchangeable sample holders. This offers the opportunity of high sample throughput.
High precision differential measurement of surface photovoltage transients on ultrathin CdS layers
Authors: Dittrich Th. , Bönisch S. , Zabel P. , Dube S.
Published: November 2008

Time-resolved surface photovoltage (SPV) is an important method for studying charge separation, for example, in nanostructured semiconductors. High precision differential measurement of SPV transients was realized with two identical measurement capacitors and high-impedance buffers. In addition, logarithmic readout and averaging procedures were implemented for single transients over eight magnitudes in time. As a model system ultrathin CdS layers were investigated. The thickness dependencies of the SPV amplitudes and that of the dominating relaxation mechanisms are demonstrated and discussed.
Design principles for particle plasmon enhanced solar cells
Authors: Catchpole K. R. , Polman A.
Published: November 2008

We develop fundamental design principles for increasing the efficiency of solar cells using light trapping by scattering from metal nanoparticles. We show that cylindrical and hemispherical particles lead to much higher path length enhancements than spherical particles, due to enhanced near-field coupling, and that the path length enhancement for an electric point dipole is even higher than the Lambertian value. Silver particles give much higher path length enhancements than gold particles. The scattering cross section of the particles is very sensitive to the thickness of a spacer layer at the substrate, which provides additional tunability in the design of particle arrays.
Thermal phonon resonance observed with millisecond optical beating Brillouin spectroscopy
Authors: Minami Yasuo , Sakai Keiji
Published: November 2008

We observed resonating thermal phonons in a cavity. The combination of our millisecond Brillouin scattering technique and this phonon resonance observation enables the determination of phonon velocity with an accuracy of 10−4 within an observation time of 30 ms. In the experiment, phonon propagation in a confined space filled with liquid toluene was observed. The observed phonon resonance spectra agree with theoretical predictions.
Ab initio study of electron-phonon coupling in boron-doped SiC
Authors: Margine E. R. , Blase X.
Published: November 2008

Density functional theory calculations have been used to study the electronic structure, lattice dynamics, and electron-phonon coupling in boron-doped silicon carbide in the cubic phase. Our results provide evidence that the recently discovered superconducting transition in boron-doped silicon carbide can be explained within a standard phonon-mediated mechanism. For the same doping rate, the coupling constant λ in B-doped SiC is very close to that of doped diamond and twice as large as that of B-doped silicon. However, doped silicon carbide differs from its diamond counterpart as most of the electron-phonon coupling originates from low energy vibrational modes.
Thermal transpiration in zeolites: A mechanism for motionless gas pumps
Authors: Gupta Naveen K. , Gianchandani Yogesh B.
Published: November 2008

We explore the use of a naturally occurring zeolite, clinoptilolite, for a chip-scale, thermal transpiration-based gas pump. The nanopores in clinoptilolite enable the required free-molecular flow, even at atmospheric pressure. The pump utilizes a foil heater located between zeolite disks in a plastic package. A 2.3mm thick zeolite disk generates a typical gas flow rate of 6.6×10−3 cc/min-cm2 with an input power of <300mW∕cm2. The performance is constrained by imperfections in clinoptilolite, which provide estimated leakage apertures of 10.2–13.5μm∕cm2 of flow cross section. The transient response of the pump is studied to quantify nonidealities.
Structural and transport properties of epitaxial niobium-doped BaTiO3 films
Authors: Shao Y. , Hughes R. A. , Dabkowski A. , Radtke G. , Gong W. H. , Preston J. S. , Botton G. A.
Published: November 2008

Ba Ti 1 − x Nb x O 3 films, spanning the entire range of x, have been deposited on MgAl2O4 substrates. X-ray diffraction measurements indicate single phase films for all values of x, contrary to a previously reported niobium solubility limit. Films show extreme sensitivity to high temperature oxygen exposure, which destroys conductivity and severely disrupts crystallinity. Under optimum growth conditions increasing x gives rise to a Ti4+ to Ti3+ transformation in the oxidation state accompanied by increased conductivity with a semiconductor-metal transition near x=0.2. Temperature dependent magnetic measurements show an anomalous rise in the spin moment, a feature supportive of the small singlet bipolaron model.
Flow transition in a small scale microwave plasma jet at atmospheric pressure
Authors: Arnoult G. , Cardoso R. P. , Belmonte T. , Henrion G.
Published: November 2008

A small-scale microwave postdischarge at atmospheric pressure is studied. Its shape evolves from a straight beam of light to a diverging cone. By using the method of the synthetic spectrum, the mean temperature is shown to vary nonlinearly between 1000 and 2000K. Evaluation of the Reynolds number shows a linear dependence on flow rate and suggests a transition from laminar to turbulent flow. Simultaneously, the gas velocity increases and comes to exceed sound velocity. As the flow rate increases, three flow regimes are distinguished: laminar and subsonic, turbulent and transonic, and turbulent and supersonic.
Monte Carlo analysis of stress-directed phase segregation in binary thin film alloys under nonisothermal annealing
Authors: Nieves Alex M. , Vitek V. , Sinno T.
Published: November 2008

The use of patterned stress fields to direct phase separation in thin film alloys is investigated computationally with Monte Carlo simulations in which atomic interactions are represented by a Lennard-Jones potential. We show that careful design of annealing schedules based on consideration of the system phase diagram can lead to vastly enhanced patterning kinetics. In particular, by avoiding the low temperature formation of highly stable nuclei within the entire system, the kinetics of patterning are accelerated by rapid monomer diffusion, rather than classical Ostwald ripening in which small precipitates must dissolve to feed larger ones.
Role of field enhanced mechanisms and impact ionization on the threshold voltage of short channel polycrystalline silicon thin film transistors
Authors: Gaucci P. , Valletta A. , Mariucci L. , Pecora A. , Cuscunà M. , Maiolo L. , Fortunato G.
Published: November 2008

Drain bias induced threshold voltage variation in short channel polycrystalline silicon (polysilicon) thin-film transistors (TFTs) are investigated, combining experimental measurements and two-dimensional numerical simulations. We show that drain induced barrier lowering and floating body effects, induced by impact ionization, are the main causes of such variations. Field enhanced mechanisms, causing leakage current, tend to mitigate the effect of impact ionization, since they provide an enhanced recombination rate in the high field region. This result is in contrast to what is suggested for silicon-on-insulator devices and also applied to polysilicon TFTs, where leakage current is assumed to contribute to floating body effects.
Observation of injection locking in an optomechanical rf oscillator
Authors: Hossein-Zadeh Mani , Vahala Kerry J.
Published: November 2008

Injection locking of a radiation-pressure optomechanical oscillator is demonstrated through external modulation of the optical pump power near the optomechanical oscillation frequency. It is shown that the frequency and phase of a microtoroidal optomechanical oscillator can be locked to those of an electronic oscillator (or any other signal) that can modulate the optical input power and whose frequency is within the lock range.
Omnidirectional photonic bandgaps in porous silicon based mirrors with a Gaussian profile refractive index
Authors: Estevez J. O. , Arriaga J. , Blas A. Méndez , Agarwal V.
Published: November 2008

We have designed and fabricated one dimensional photonic bandgap (PBG) structures from dielectric multilayers of porous silicon, with a periodic repetition of a unit cell consisting of 21 layers (95%) with the refractive index varying according to the envelope of a Gaussian function and another layer (5%) with a fixed refractive index. The structures can be designed to demonstrate the wavelength scalability within the visible as well as near infrared region. Three different structures have been stacked together to enhance the width of the PBG. The omnidirectional nature of the PBG was verified experimentally and theoretically up to 68° and 89° angles of incidence, respectively.
Strong magnetoelectric coupling in multiferroic BiFeO3–Pb(Zr0.52Ti0.48)O3 composite films derived from electrophoretic deposition
Authors: Wu Yujie , Wan Jian-guo , Huang Chuanfu , Weng Yuyan , Zhao Shifeng , Liu Jun-ming , Wang Guanghou
Published: November 2008

The BiFeO3–Pb(Zr0.52Ti0.48)O3 composite film in which BiFeO3 nanoparticles are distributed in the Pb(Zr0.52Ti0.48)O3 matrix was prepared by electrophoretic deposition method. The microstructural characterization revealed the structural distortion of the BiFeO3 phase, induced by the unique BiFeO3–Pb(Zr0.52Ti0.48)O3 core/shell structure. The composite film exhibited evidence of ferroelectricity with enhanced ferromagnetism as well as a magnetoelectric effect. The two origins for the magnetoelectric effect are presented. This present work provides a promising roadmap to improve the magnetoelectric effect of BiFeO3-based multiferroics for actual applications.
Single shot measurement of a silicon single electron transistor
Authors: Hasko D. G. , Ferrus T. , Morrissey Q. R. , Burge S. R. , Freeman E. J. , French M. J. , Lam A. , Creswell L. , Collier R. J. , Williams D. A. , Briggs G. A. D.
Published: November 2008

We have fabricated a custom cryogenic complementary metal-oxide-semiconductor integrated circuit that has a higher measurement bandwidth compared to conventional room temperature electronics. This allowed implementing single shot operations and observe the real-time evolution of the current of a phosphorus-doped silicon single electron transistor that was irradiated with a microwave pulse. Relaxation times up to 90μs are observed, suggesting the presence of well isolated electron excitations within the device. It is expected that these are associated with long decoherence time and the device may be suitable for quantum information processing.
Transformable broad-band transparency and amplification in negative-index films
Authors: Popov Alexander K. , Myslivets Sergey A.
Published: November 2008

The possibility to produce laser-induced optical transparency of a metamaterial slab through the entire negative-index frequency domain is shown above a certain control laser field intensity threshold.
Low threshold lasing of GaN-based vertical cavity surface emitting lasers with an asymmetric coupled quantum well active region
Authors: Zhang Jiang-Yong , Cai Li-E , Zhang Bao-Ping , Li Shui-Qing , Lin Feng , Shang Jing-Zhi , Wang Du-Xiang , Lin Ke-Chuang , Yu Jin-Zhong , Wang Qi-Ming
Published: November 2008

We have fabricated and characterized GaN-based vertical cavity surface emitting lasers (VCSELs) with a unique active region structure, in which three sets of InGaN asymmetric coupled quantum wells are placed in a half-wavelength (0.5λ) length. Lasing action was achieved under optical pumping at room temperature with a threshold pumping energy density of about 6.5mJ∕cm2. The laser emitted a blue light at 449.5nm with a narrow linewidth below 0.1nm and had a high spontaneous emission factor of about 3.0×10−2. The results indicate that this active region structure is useful in reducing the process difficulties and improving the threshold characteristics of GaN-based VCSELs.
Domain wall displacement in Py square ring for single nanometric magnetic bead detection
Authors: Vavassori P. , Metlushko V. , Ilic B. , Gobbi M. , Donolato M. , Cantoni M. , Bertacco R.
Published: November 2008

An approach based on domain wall displacement in confined ferromagnetic nanostructures for attracting and sensing a single nanometric magnetic particle is presented. We modeled and experimentally demonstrated the viability of the approach using an anisotropic magnetoresistance device made by a micron-sized square ring of Permalloy. This detection concept can be suitable to biomolecular recognition and, in particular, to single molecule detection.
A Dnv point group structure possessing complete band gap based on gradual heterostructure and self-simulating sphere
Authors: Zhai Tianrui , Wang Zhaona , Zhao Rongkuo , Zhou Jing , Liu Dahe , Zhang Xiangdong
Published: November 2008

A structure with Dnv point group was fabricated by compounding a gradual heterostructure and a self-simulating sphere. The gradual heterostructure has two-dimensional omnidirectional band gap and the self-simulating sphere makes it possess a complete band gap. The experimental results were explained by theoretical analysis. This kind of structure can be implemented using low refractive index materials.
Additional toxins for feral pig (Sus scrofa) control: identifying and testing Achilles' heels
Authors: Cowled Brendan D. , Elsworth Peter , Lapidge Steven J.
Published: November 2008

A literature review was conducted in order to identify unique weaknesses in the physiology or metabolism of pigs that could be targeted with specific chemicals (i.e. an ‘Achilles’ heel’ search). A promising weakness identified was the species’ susceptibility to methaemoglobin-forming compounds, most likely related to their uniquely low levels of methaemoglobin reductase. Further examination revealed that sodium nitrite is a cost-effective, readily available methaemoglobin-forming compound that is highly toxic to domestic pigs, which has caused numerous accidental poisonings. Pen trials on pigs showed that sodium nitrite delivered by gavage (>90 mg kg−1) and freely consumed in bait (>400 mg kg−1) caused rapid and lethal rises in methaemoglobin. Sodium nitrite appeared to be more humane than currently used toxins, with deaths following bait consumption being considerably quicker and with fewer symptoms (within 80 min of clinical signs beginning; clinical signs including infrequent vomiting, lethargy, ataxia and dyspnoea). The review also identified a second deficiency in the metabolism of pigs, namely high sensitivity to selective inhibition of cytochrome P450 liver enzymes. This leads to potentially lethal interactions between various drugs, such as two antibiotics, monensin and tiamulin. A pen trial confirmed that the antibiotic combination in a single gavage dose was reliably and rapidly lethal to pigs. However, its utility as a pig toxin is low, because it was unpalatable to pigs when delivered in bait and appeared to cause pain and suffering (leading to the early termination of pen trials). The findings presented here demonstrate the potential of sodium nitrite as an additional feral pig toxin.
Distribution and abundance of roadkill on Tasmanian highways: human management options
Authors: Hobday Alistair J. , Minstrell Melinda L.
Published: November 2008

An obvious sign of potential human impact on animal populations is roadkill. In Tasmania, this impact is perceived as relatively greater than in other Australian states, and is often noted by visitors and locals alike, such that calls for management action are common in the popular press. The goal of this three-year study was to assess the frequency and distribution of species killed on Tasmanian roads. Seasonal surveys were completed along five major routes, for a total of 154 trips. Over 15 000 km of road were surveyed and 5691 individuals in 54 taxa were recorded for an average roadkill density of 0.372 km−1. Over 50% of encountered roadkill could be identified to species, with common brushtail possums (Trichosurus vulpecula) and Tasmanian pademelon (Thylogale billardierii) the most common species identified, both in overall numbers and frequency of trips encountered. The 10 most common taxa accounted for 99% of the items observed. The seasonal occurrence, relationship with vehicle speed, and clustering in local hotspots for particular taxa all suggest that mitigation measures, such as vehicle speed reduction in specific areas, may be effective in reducing the number of animals killed. Mitigation measures, however, will not apply equally to all species and, in particular, success will depend on changing human behaviours.
Change-in-ratio density estimator for feral pigs is less biased than closed mark - recapture estimates
Authors: Hanson Laura B. , Grand James B. , Mitchell Michael S. , Jolley D. Buck , Sparklin Bill D. , Ditchkoff Stephen S.
Published: November 2008

Closed-population capture–mark–recapture (CMR) methods can produce biased density estimates for species with low or heterogeneous detection probabilities. In an attempt to address such biases, we developed a density-estimation method based on the change in ratio (CIR) of survival between two populations where survival, calculated using an open-population CMR model, is known to differ. We used our method to estimate density for a feral pig (Sus scrofa) population on Fort Benning, Georgia, USA. To assess its validity, we compared it to an estimate of the minimum density of pigs known to be alive and two estimates based on closed-population CMR models. Comparison of the density estimates revealed that the CIR estimator produced a density estimate with low precision that was reasonable with respect to minimum known density. By contrast, density point estimates using the closed-population CMR models were less than the minimum known density, consistent with biases created by low and heterogeneous capture probabilities for species like feral pigs that may occur in low density or are difficult to capture. Our CIR density estimator may be useful for tracking broad-scale, long-term changes in species, such as large cats, for which closed CMR models are unlikely to work.
Neutral evolution in paroxysmal nocturnal hemoglobinuria
Authors: Dingli David , Luzzatto Lucio , Pacheco Jorge M.
Published: November 2008

Paroxysmal nocturnal hemoglobinuria is an acquired hematopoietic stem cell (HSC) disorder characterized by the partial or complete deficiency of glycosyl-phosphatidylinositol (GPI)-linked membrane proteins, which leads to intravascular hemolysis. A loss of function mutation in the
PIG-A
gene, required for GPI biosynthesis, explains how the deficiency of many membrane proteins can result from a single genetic event. However, to date the mechanism of expansion of the GPI
−
clone has not been fully understood. Two hypotheses have been proposed: A selective advantage of GPI
−
cells because of a second mutation or a conditional growth advantage of GPI
−
cells in the presence of an immune attack on normal (GPI
+
) HSCs. Here, we explore a third possibility, whereby the PNH clone does not have a selective advantage. Simulations in a large virtual population accurately reproduce the known incidence of the disease; and the fit is optimized when the number of stem cells is decreased, reflecting a component of bone marrow failure in PNH. The model also accounts for the occurrence of spontaneous cure in PNH, consequent on clonal extinction. Thus, a clonal advantage may not be always necessary to explain clonal expansion in PNH.
Tracking the inflammatory response in stroke in vivo by sensing the enzyme myeloperoxidase
Authors: Breckwoldt Michael O. , Chen John W. , Stangenberg Lars , Aikawa Elena , Rodriguez Elisenda , Qiu Shumei , Moskowitz Michael A. , Weissleder Ralph
Published: November 2008

Inflammation can extend ischemic brain injury and adversely affect outcome in experimental animal models. A key difficulty in translating animal studies to humans is the lack of a definitive method to confirm and track inflammation in the brain in vivo. Myeloperoxidase (MPO), a key inflammatory enzyme secreted by activated neutrophils and macrophages/microglia, can generate highly reactive oxygen species to cause additional damage in cerebral ischemia. We report here that a functional, enzyme-activatable MRI agent can accurately track the oxidative activity of MPO noninvasively in stroke in living animals. We found that MPO is widely distributed in ischemic tissues, correlates positively with infarct size, and is detected even 3 weeks postinfarction. The peak level of MPO activity, determined by activation of the MPO-sensing agent in vivo and confirmed by MPO activity and quantitative RT-PCR assays, occurred on day 3 after ischemia. Both neutrophils and macrophages/microglia contribute to secrete MPO in the ischemic brain, although neutrophils peak earlier (days 1–3) whereas macrophages/microglia are most abundant later (days 3–7). In contrast to the conventional MRI agent diethylenetriamine-pentatacetate gadolinium, which reports blood–brain barrier disruption, MPO imaging is able to additionally track MPO activity and confirm inflammation on the molecular level in vivo, information that was previously only possible to obtain on ex vivo brain sections and impossible to assess in living human patients. Our findings could allow efficient noninvasive serial screening of therapies targeting inflammation and the use of MPO imaging as an imaging biomarker to risk-stratify patients.
Divalent metal transporter 1 (DMT1) contributes to neurodegeneration in animal models of Parkinson's disease
Authors: Salazar Julio , Mena Natalia , Hunot Stephane , Prigent Annick , Alvarez-Fischer Daniel , Arredondo Miguel , Duyckaerts Charles , Sazdovitch Veronique , Zhao Lin , Garrick Laura M. , Nuñez Marco T. , Garrick Michael D. , Raisman-Vozari Rita , Hirsch Etienne C.
Published: November 2008

Dopaminergic cell death in the substantia nigra (SN) is central to Parkinson's disease (PD), but the neurodegenerative mechanisms have not been completely elucidated. Iron accumulation in dopaminergic and glial cells in the SN of PD patients may contribute to the generation of oxidative stress, protein aggregation, and neuronal death. The mechanisms involved in iron accumulation also remain unclear. Here, we describe an increase in the expression of an isoform of the divalent metal transporter 1 (DMT1/Nramp2/Slc11a2) in the SN of PD patients. Using the PD animal model of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) intoxication in mice, we showed that DMT1 expression increases in the ventral mesencephalon of intoxicated animals, concomitant with iron accumulation, oxidative stress, and dopaminergic cell loss. In addition, we report that a mutation in DMT1 that impairs iron transport protects rodents against parkinsonism-inducing neurotoxins MPTP and 6-hydroxydopamine. This study supports a critical role for DMT1 in iron-mediated neurodegeneration in PD.
The ferritin Fe2 site at the diiron catalytic center controls the reaction with O
2
in the rapid mineralization pathway
Authors: Tosha Takehiko , Hasan Mohammad R. , Theil Elizabeth C.
Published: November 2008

Oxidoreduction in ferritin protein nanocages occurs at sites that bind two Fe(II) substrate ions and O
2
, releasing Fe(III)
2
–O products, the biomineral precursors. Diferric peroxo intermediates form in ferritins and in the related diiron cofactor oxygenases. Cofactor iron is retained at diiron sites throughout catalysis, contrasting with ferritin. Four of the 6 active site residues are the same in ferritins and diiron oxygenases; ferritin-specific Gln
137
and variable Asp/Ser/Ala
140
substitute for Glu and His, respectively, in diiron cofactor active sites. To understand the selective functions of diiron substrate and diiron cofactor active site residues, we compared oxidoreductase activity in ferritin with diiron cofactor residues, Gln
137
→ Glu and Asp
140
→ His, to ferritin with natural diiron substrate site variations, Asp
140
, Ser
140
, or Ala
140
. In Gln
137
→ Glu ferritin, diferric peroxo intermediates were undetectable; an altered Fe(III)–O product formed, Δ
A
350
= 50% of wild type. In Asp
140
→ His ferritin, diferric peroxo intermediates were also undetectable, and Fe(II) oxidation rates decreased 40-fold. Ferritin with Asp
140
, Ser
140
, or Ala
140
formed diferric peroxo intermediates with variable kinetic stabilities and rates:
t
1/2
varied 1- to 10-fold;
k
cat
varied approximately 2- to 3-fold. Thus, relatively small differences in diiron protein catalytic sites determine whether, and for how long, diferric peroxo intermediates form, and whether the Fe–active site bonds persist throughout the reaction cycle (diiron cofactors) or break to release Fe(III)
2
–O products (diiron substrates). The results and the coding similarities for cofactor and substrate site residues—e.g., Glu/Gln and His/Asp pairs share 2 of 3 nucleotides—illustrate the potential simplicity of evolving active sites for diiron cofactors or diiron substrates.