bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of ion mixing on the energy dependence of the ion-assisted chemical etch rate in reactive plasmas
Authors: Stafford L. , Pearton S. J. , Margot J.
Published: October 2006

Recently, Stafford et al. [Appl. Phys. Lett. 87, 071502 (2005)] have shown that in contrast to the etch yield on a saturated surface, the ion-assisted chemical etch rate cannot universally be modeled by a simple square-root energy dependence. This results from the surface coverage by reactive neutral species being also a function of the ion energy. In this work, we further point out that depending on the plasma-material combination, the etch rate can exhibit two regimes that are characterized by different dependences on the ion energy. While these results are inconsistent with currently available models, we show that they can be interpreted by taking into account ion mixing effects on the desorption rate of volatile reaction products involved in the model of Stafford et al. Application of this rate model to the etching of Si, SiO2, HfO2, and ZrO2 in chlorine and fluorine plasma chemistries provides an excellent description of the simultaneous dependence of the etch rate on ion energy and on ion and reactive neutral fluxes.
Spectral simulation of GaAs and InAs quantum-dot terahertz detectors designed for higher-temperature operation
Authors: Vurgaftman I. , Meyer J. R. , Wu D. H. , Bussmann K. , Jonker B. T.
Published: October 2006

The spectral properties and other performance characteristics of single-photon quantum-dot terahertz detectors incorporating single-electron transistors are analyzed for the cases of GaAs and InAs electron channels. By defining the electrostatically confined double-quantum-dot structures in shallow two-dimensional electron gas structures, these devices have the potential to operate at temperatures above the dilution refrigerator range. We calculate the interlevel plasmon absorption by using the random-phase approximation in conjunction with realistic band structures and three-dimensional confinement geometries. While the level separations increase with decreasing dot diameter, carrier depletion eventually limits the minimum size that is viable. The maximum detection frequency is about 1THz for GaAs channels, but increases to 1.5–2THz for InAs channels having a lower effective mass.
Experimental investigations of elastic tail propulsion at low Reynolds number
Authors: Yu Tony S. , Lauga Eric , Hosoi A. E.
Published: October 2006

A simple way to generate propulsion at low Reynolds number is to periodically oscillate a passive flexible filament. Here we present a macroscopic experimental investigation of such a propulsive mechanism. A robotic swimmer is constructed and both tail shape and propulsive force are measured. Filament characteristics and actuation are varied, and the resulting data are quantitatively compared with existing linear and nonlinear theories.
An experimental study of the coalescence between a drop and an interface in Newtonian and polymeric liquids
Authors: Chen Xiaopeng , Mandre Shreyas , Feng James J.
Published: October 2006

When a water drop falls onto an oil-water interface, the drop usually rests for some time before merging with the water underneath the interface. We report experiments on this process using water- and oil-based Newtonian liquids and polymer solutions, with an emphasis on the non-Newtonian effects. We deduce that the drop surface is immobilized by contaminants pre-existing in the fluids, and find that the rest time scales with the matrix viscosity for Newtonian fluids. The results are compared with lubrication models for film drainage. If the surrounding matrix is a dilute polymer solution, the rest time is identical to that for a matrix of the solvent alone. Further investigation indicates that the polymer molecules have been cleared from the film by surface adsorption. Depending on the fluid properties and drop size, the drop-interface merging may be completed in one shot or through a cascade of partial coalescence. Partial coalescence occurs for an intermediate range of drop sizes; it is arrested by viscosity for smaller drops and by gravity for larger ones. When either the drop or the matrix phase is a polymer solution, viscoelasticity is shown to suppress partial coalescence for smaller drops. This is apparently due to the inhibition of capillary pinch-off which would otherwise produce a secondary drop before the merging is complete.
Lyapunov exponents of heavy particles in turbulence
Authors: Bec Jérémie , Biferale Luca , Boffetta Guido , Cencini Massimo , Musacchio Stefano , Toschi Federico
Published: October 2006

Lyapunov exponents of heavy particles and tracers advected by homogeneous and isotropic turbulent flows are investigated by means of direct numerical simulations. For large values of the Stokes number, the main effect of inertia is to reduce the chaoticity with respect to fluid tracers. Conversely, for small inertia, a counterintuitive increase of the first Lyapunov exponent is observed. The flow intermittency is found to induce a Reynolds number dependency for the statistics of the finite-time Lyapunov exponents of tracers. Such intermittency effects are found to persist at increasing inertia.
Temperature-controlled martensitic phase transformations in a model NiAl alloy
Authors: Lazarev N. P. , Abromeit C. , Schäublin R. , Gotthardt R.
Published: October 2006

Reversible martensitic phase transformations in a partially disordered Ni–Al alloy within the composition range from 60to65at.% of Ni are investigated using molecular dynamics simulation. During a complete temperature cycle a wide hysteresis in enthalpy, volume, and shape of the simulated crystals is observed. The temperature T0 of the phase transformation is found from the calculated free energy evolution. To investigate the atomic-scale development during the phase transformation a local order parameter is defined which is based on a combined method of Voronoy tessellation [J. Reine Angew. Math. 134, 198 (1908)] with common-neighbor analysis. This local order parameter allows us to get a detailed localized picture of nucleation and growth of the new phases. Both homogeneous formation of the new phase and heterogeneous nucleation are observed. The velocity of new phase growth front is estimated.
Formation of simple and compound drops in microfluidic devices
Authors: Zhou Chunfeng , Yue Pengtao , Feng James J.
Published: October 2006

This work is motivated by the recent experimental development of microfluidic flow-focusing devices that produce highly monodisperse simple or compound drops. Using finite elements with adaptive meshing in a diffuse-interface framework, we simulate the breakup of simple and compound jets in coflowing conditions, and explore the flow regimes that prevail in different parameter ranges. Moreover, we investigate the effects of viscoelasticity on interface rupture and drop pinch-off. The formation of simple drops exhibits a dripping regime at relatively low flow rates and a jetting regime at higher flow rates. In both regimes, drops form because of the combined effects of capillary instability and viscous drag. The drop size increases with the flow rate of the inner fluid and decreases with that of the outer fluid. Viscoelasticity in the drop phase increases the drop size in the dripping regime but decreases it in the jetting regime. The formation of compound drops is a delicate process that takes place in a narrow window of flow and rheological parameters. Encapsulation of the inner drop depends critically on coordination of capillary waves on the inner and outer interfaces.
Influence of surface Si–Ag bonds on photoluminescence of porous silicon
Authors: Lu Y. W. , Du X. W. , Sun J. , Han X. , Kulinich S. A.
Published: October 2006

Porous silicon with surface Si–Ag bonds was prepared by depositing Ag on conventional electrochemically prepared porous silicon using electrodeposition method, and the influence of surface-bonded Ag on photoluminescence properties of the material was studied. Photoluminescence intensity of porous silicon increases with the amount of surface Si–Ag bonds formed during electrochemical deposition. Further treatment leads to increased amounts of Ag–O and Ag–Ag bonds on the material surface. Such layers of oxygen-bonded and metallic Ag are shown to retard the light emission from porous silicon.
Oscillations of the large scale wind in turbulent thermal convection
Authors: Resagk Christian , du Puits Ronald , Thess André , Dolzhansky Felix V. , Grossmann Siegfried , Fontenele Araujo Francisco , Lohse Detlef
Published: October 2006

The large scale “wind of turbulence” of thermally driven flow is analyzed for very large Rayleigh numbers between 4∙1011 and 7∙1011 and Prandtl number of 0.71 (air at 40°C) and aspect ratios order of one. The wind direction near the upper plate is found to horizontally oscillate with a typical time scale very similar to the large eddy turnover time. The temporal autocorrelation of the wind direction reveals an extremely long memory of the system for the direction. We then apply and extend the dynamical model of Gledzer, Dolzhansky, and Obukhov to the flow, which is based on the Boussinesq equations in the bulk and which can be solved analytically in the inviscid and unforced limit, but which completely ignores the boundary layer and plume dynamics. Nevertheless, the model correctly reproduces both the oscillations of the horizontal wind direction and its very long memory. It is therefore concluded that the boundary layers and the plumes are not necessary to account for the oscillations of the wind direction. The oscillations rather occur as intrinsic precession of the bulk flow.
Optimal model parameters for multi-objective large-eddy simulations
Authors: Meyers Johan , Sagaut Pierre , Geurts Bernard J.
Published: October 2006

A methodology is proposed for the assessment of error dynamics in large-eddy simulations. It is demonstrated that the optimization of model parameters with respect to one flow property can be obtained at the expense of the accuracy with which other flow properties are predicted. Therefore, an approach is introduced which allows to assess the total errors based on various flow properties simultaneously. We show that parameter settings exist, for which all monitored errors are “near optimal,” and refer to such regions as “multi-objective optimal parameter regions.” We focus on multi-objective errors that are obtained from weighted spectra, emphasizing both large- as well small-scale errors. These multi-objective optimal parameter regions depend strongly on the simulation Reynolds number and the resolution. At too coarse resolutions, no multi-objective optimal regions might exist as not all error-components might simultaneously be sufficiently small. The identification of multi-objective optimal parameter regions can be adopted to effectively compare different subgrid models. A comparison between large-eddy simulations using the Lilly-Smagorinsky model, the dynamic Smagorinsky model and a new Re-consistent eddy-viscosity model is made, which illustrates this. Based on the new methodology for error assessment the latter model is found to be the most accurate and robust among the selected subgrid models, in combination with the finite volume discretization used in the present study.
On streamwise vortices and their role in the development of a curved wall jet
Authors: Han G. , de Zhou M. , Wygnanski I.
Published: October 2006

The streamwise growth of longitudinal vortices in a curved wall jet was investigated experimentally, mostly through Particle Image Velocimetry (PIV). This growth was partially achieved through amalgamation of neighboring vortices having the same sense of rotation and partially due to centrifugal instability, thus the average spacing of the vortices far downstream was independent of the protuberances (micro vortex generators) that initiated them at the slot lip. The spacing of these protuberances along the span established only the preferred distance from the nozzle where vortex amalgamations occurred. The streamwise vortices initiated spanwise undulations in the mean flow creating a radial component of vorticity that when examined at a given distance from the surface was found to be inviscidly unstable, and could enhance further this component of vorticity. A strong coupling was observed between plane periodic excitation of the nozzle-flow and the small stationary protuberances that were placed at the nozzle in the presence of curvature. This coupling enhanced the coherence and strength of the longitudinal vortices far downstream from the nozzle, while in the absence of curvature these structures simply decayed.
Microstructure and optical properties of Cu∕Al2O3 nanoarray composite structure
Authors: Tang H. J. , Wu F. Q. , Wang H. L. , Wei Y. H. , Li Q. S.
Published: October 2006

Cu ∕ Al 2 O 3 nanoarray composite structure was obtained by alternating-current electrodeposition Cu into the pores of anodic alumina. The ordered wire-grid structure of Cu nanowires formed along a preferential direction, typically 20nm in diameter, possesses a face-centered-cubic crystalline structure. Optical properties of Cu∕Al2O3 were investigated in detail. An apparent resonance absorption peak near 560nm was observed, and with the increasing amount of electrodeposition Cu, the blueshift of the peak and the redshift of the absorption band edge were observed. Experimental results also indicate that Cu nanowires have good transmittance characteristic in near-IR wave band and do not exhibit obvious absorption action in mid-IR wave band; in the range of 800–2000nm, Cu∕Al2O3 present a good polarization property, and with the increase of electrodeposition time, the polarization increases.
Imaging buried defects in a three dimensional magnetically permeable medium using pseudoinverse technique
Authors: Baskaran R. , Pattabiraman M. , Janawadkar M. P.
Published: October 2006

Pseudoinverse technique has been used to identify the location and distribution of buried defects inside a magnetically permeable medium by measuring the leakage fields outside the medium. Defects have been modeled as localized antidipoles whose moments and locations have to be determined. Use of pseudoinverse technique has enabled us to separate the moment variables and space variables and factor out the moment variables in the least square minimum norm solution. The leakage fields from two different flaws with an additive noise were simulated and utilized for inversion using this approach. It has been possible to identify the locations of defects and quantify the strength of the moments in the three dimensional medium even in the presence of reasonably large noise. This imaging technique has also been utilized to identify a single defect located 20mm below the top surface in the presence of noise.
Modifying the electronic properties of GaAs∕AlAs superlattices with low-density nitrogen doping
Authors: Spasov S. , Allison G. , Patanè A. , Eaves L. , Hopkinson M. , Airey R.
Published: October 2006

We investigate how low concentrations of N impurities (x<1%) modify the electrical and optical properties of a GaAs∕AlAs superlattice positioned in the intrinsic i region of a p-i-n diode. Increasing x from 0% to 0.9% enhances the integrated intensity of the low temperature electroluminescence emission from the superlattice by more than a factor of 104. We attribute this behavior to reduced transmission of electrons through the superlattice due to the N-induced carrier localization.
A high rate flow-focusing foam generator
Authors: Lorenceau Elise , Sang Yann Yip Cheung , Höhler Reinhard , Cohen-Addad Sylvie
Published: October 2006

We use a rigid axisymmetric microfluidic flow-focusing device to produce monodisperse bubbles, dispersed in a surfactant solution. The gas volume fraction of the dispersion collected out of this device can be as large as 90%, demonstrating that foam with solid-like viscoelastic properties can be produced in this way. The polydispersity of the bubbles is so low that we observe crystallization of our foam. We measure the diameter of the bubbles and compare these data to recent theoretical predictions. The good control over bubble size and foam gas volume fraction shows that our device is a flexible and promising tool to produce calibrated foam at a high flow rate.
The sensitivity of volumetric flow rate to applied voltage in cone-jet mode electrospray and the influence of solution properties and emitter geometry
Authors: Smith Katharine L. , Alexander Matthew S. , Stark John P. W.
Published: October 2006

A high accuracy online flow rate measurement system has been used to demonstrate the effect of applied voltage, Vapp, on the volumetric flow rate, Q, through an electrospray system. Several solutions of the organic solvents ethylene and triethylene glycols doped with sodium iodide to give varying conductivities in the range of 0.0025–0.23S∕m have been sprayed. It was established for the first time that solution conductivity has no appreciable effect on the sensitivity of flow rate to applied voltage in the cone-jet mode of electrospray. However, it appears that even when the hydraulic resistance is taken into account, the sensitivity of flow rate as controlled by the applied voltage is additionally related to the emitter exit geometry. These findings are of particular importance to both spacecraft propulsion and electrospray mass spectrometry technologies and suggest careful emitter geometry design considerations will lead to greater control over electrospray properties.
Control of electron transport related defects in in situ fabricated single wall carbon nanotube devices
Authors: Zhou Zhixian , Jin R. , Eres Gyula , Subedi Alaska , Mandrus D.
Published: October 2006

Metallic single wall carbon nanotube devices were characterized using low temperature transport measurements to study how the growth conditions affect defect formation in carbon nanotubes. Suspended carbon nanotube devices were grown in situ by a molecular beam growth method on a pair of catalyst islands located on opposing Au electrodes fabricated by electron beam lithography. The authors present experimental evidence that defect formation in carbon nanotubes, in addition to the well known growth temperature dependence, is also affected by the nature and the composition of the carbon growth gases.
Pair correlation functions and structural building schemes in amorphous alloys
Authors: Luo W. K. , Sheng H. W. , Ma E.
Published: October 2006

Using molecular dynamics simulations of a model binary amorphous alloy, the short-to-medium range pair correlations observed in the pair distribution functions are attributed to well-defined connection schemes of local structural motifs. The peak splitting features, particularly those in the solute-solute correlation function, are interpreted as due to the ordering of quasi-equivalent clusters in several specific sharing arrangements.
Experimental characterization of drift-interchange instabilities in a simple toroidal plasma
Authors: Poli F. M. , Brunner S. , Diallo A. , Fasoli A. , Furno I. , Labit B. , Müller S. H. , Plyushchev G. , Podestà M.
Published: October 2006

Low frequency electrostatic instabilities are investigated on TORPEX [Fasoli, Labit, McGrath, Müller, Podestà, and Poli, Bull. Am. Phys. Soc. 48, 119 (2003)], a toroidal device for basic plasma physics experiments with a toroidal magnetic field 100mT and a small vertical magnetic field (⩽4mT). A two-dimensional (2D) profile of the frequency and amplitude of density and potential fluctuations is reconstructed using electrostatic probes with high space and time resolution. The measured phase velocity, corrected for the Doppler shift induced by the E×B drift, is consistent with the electron diamagnetic drift velocity. The local dispersion relation, measured along and across the magnetic field, is in agreement with the predictions of a linear kinetic slab model for drift waves. Unstable modes are generated in regions of unfavorable curvature, where the pressure gradient is colinear with the magnetic field gradient. It is demonstrated that the curvature of the magnetic field lines is essential for driving the observed instabilities, which are therefore identified as drift-interchange modes.
Wideband and hysteresis-free regulation of piezoelectric actuator based on induced current for high-speed scanning probe microscopy
Authors: Kageshima Masami , Togo Shinsuke , Li Yan Jun , Naitoh Yoshitaka , Sugawara Yasuhiro
Published: October 2006

A novel closed-loop regulation of a piezoelectric actuator is presented to implement wideband and hysteresis-free motion required for high-speed operation of scanning probe microscopy. Velocity of the actuator’s displacement detected via the induced current and its integration giving the displacement were used to actively compensate the resonances and hysteresis. The validity of the idea was demonstrated using a prototype circuit composed of operational amplifiers and multilayered piezoelectric actuators. The fundamental resonance of the actuator at 260kHz was completely eliminated from the actuator’s displacement with an appropriate velocity feedback. With an additional displacement feedback the gain error was suppressed within ±5dB over a frequency range from dc to 1MHz, while a −45° bandwidth was also maintained as wide as 250kHz. In addition, intrinsic hysteresis of the actuator measured with an 8kHz sinusoidal signal was suppressed to below 1%. Responses in displacement of the actuator to burst oscillation and square wave inputs exhibited clear improvement from the uncompensated responses.
Versatile magneto-optic Kerr effect polarimeter for studies of domain-wall dynamics in magnetic nanostructures
Authors: Nistor Corneliu , Beach Geoffrey S. D. , Erskine James L.
Published: October 2006

This article describes a versatile instrument capable of probing magnetic domain-wall dynamics in microstructured thin films. The instrument combines a state-of-the-art scanning magneto-optic Kerr effect polarimeter that incorporates high-bandwidth signal detection, an integrated broadband magnet system, and a microwave probe station. Together, these subsystems enable a broad range of studies of field and current-driven domain-wall dynamics in submicrometer magnetic structures and devices. Domain-wall motion can be probed with ≈2μm spatial resolution and less than 2ns temporal resolution. That motion can be driven by magnetic fields of up to ≈100Oe amplitude with sinusoidal (>20MHz) or user-defined wave forms (20ns rise time) or by electric currents from dc to ≈10GHz. A detailed description of the instrument is provided as well as several experiments highlighting its capabilities, including hysteresis loop shape and magnetic energy loss measurements spanning ten decades of drive frequency; spatially and temporally resolved measurements of domain-wall propagation in submicrometer magnetic wires; and mobility measurements of field- and current-driven domain-wall motion.
Multipole moments and polarizabilities of nonpolar diatomic molecules through quantum solvation in solid parahydrogen: The quadrupole moment of N2
Authors: Mishra Adya P. , Balasubramanian T. K.
Published: October 2006

It is shown in this paper that from the study of the induced infrared absorption spectra of homonuclear diatomic molecules solvated as impurities in a molecular quantum solid, it is possible to extract information about the rovibrational matrix elements of the multipole moments and polarizability of the embedded molecule. Theoretical expressions are derived for the integrated absorption coefficients of various multipole-field-induced double transitions involving guest-host pairs in a solid para-H2 matrix. The intensities of some of the quadrupole moment induced transitions involving the N2-para-H2 pair have been measured. From a comparison of the experimental and theoretical intensities, rovibrational matrix elements of the quadrupole moment of N2 are determined in its ground vibrational state.
Luminescence properties of ZnO layers grown on Si-on-insulator substrates
Authors: Kumar Bhupendra , Gong Hao , Vicknesh S. , Chua S. J. , Tripathy S.
Published: October 2006

The authors report on the photoluminescence properties of polycrystalline ZnO thin films grown on compliant silicon-on-insulator (SOI) substrates by radio frequency magnetron sputtering. The ZnO thin films on SOI were characterized by micro-Raman and photoluminescence (PL) spectroscopy. The observation of E2high optical phonon mode near 438cm−1 in the Raman spectra of the ZnO samples represents the wurtzite crystal structure. Apart from the near-band-edge free exciton (FX) transition around 3.35eV at 77K, the PL spectra of such ZnO films also showed a strong defect-induced violet emission peak in the range of 3.05–3.09eV. Realization of such ZnO layers on SOI would be useful for heterointegration with SOI-based microelectronics and microelectromechanical systems.
High-resolution piezoresponse force microscopy investigation of imprint in ferroelectric thin films
Authors: Anbusathaiah V. , Nagarajan V. , Aggarwal S.
Published: October 2006

High-resolution piezoresponse force microscopy is used to visualize imprint in polycrystalline PbZr0.25Ti0.75O3 thin films. Three-dimensional domain images show the formation of a thin bright band (∼8nm in width) running along the grain boundary after local application of a negative bias. Such bands extend completely over the region under local bias thereby forming networks. Cross-section profile analysis reveals that these are not pinned regions, rather they are formed during the switching process. This demonstrates an active role of grain boundaries in pinning a preferential polarization state. Piezoresponse hysteresis loops confirm that these regions are imprinted.
Exchange spring driven spin flop transition in ErFe2∕YFe2 multilayers
Authors: Martin K. N. , Wang K. , Bowden G. J. , Zhukov A. A. , de Groot P. A. J. , Zimmermann J. P. , Fangohr H. , Ward R. C. C.
Published: October 2006

Magnetization loops for (110) ErFe2∕YFe2 multilayer films grown by molecular beam epitaxy are presented and discussed. The direction of easy magnetization for the Er layers is out of plane, near a ⟨111⟩-type crystal axis. For fields applied along the (110) crystal growth axis, out-of-plane magnetic exchange springs are set up in the magnetically soft YFe2 layers. For multilayer films that display negative coercivity at low temperatures, there is a crossover temperature above which the coercivity becomes positive, with additional transitions at high fields. These features are interpreted using micromagnetic modeling. At sufficiently high fields, applied perpendicular to the multilayer film plane, the energy is minimized by an exchange spring driven multilayer spin flop. In this state, the average magnetization of the ErFe2 layers switches into a nominally hard in-plane ⟨111⟩ axis, perpendicular to the applied field.
Ballistic admittance: Periodic variation with frequency
Authors: Dmitriev A. P. , Shur M. S.
Published: October 2006

The authors show that both real and imaginary parts of the admittance of short (ballistic or quasiballistic) semiconductor structures are oscillatory functions of the frequency with the period determined by the inverse electron transit time with the Fermi velocity. This oscillatory dependence is caused by the phase difference between the electrons injected from the opposite contacts.
Current-voltage characteristics of layer-by-layer self-assembled colloidal thin films
Authors: Jafri S. H. M. , Dutta J. , Sweatman D. , Sharma A. B.
Published: October 2006

Self-organized construction of advanced materials and devices has been carried out starting with tailor-made colloidal nanoparticles as building blocks. Multilayer thin films of gold nanoparticles stabilized by glutamates and zinc sulfide nanoparticles capped with chitosan were self-organized by a modified polyelectrolyte deposition process. Resistive current-voltage characteristic was observed in devices (less than 50 layers). The conduction onset in thicker devices (>50 layers) was found to be at applied voltages of ∼1.6, ∼1.94, and ∼2.79V for 75, 100, and 150 layer structures, respectively. Devices exhibit similar behavior in forward and reverse biases and the electrical characteristics were repeatable.
Quantification of multicomponent droplet evaporation through optical resonances
Authors: Tu H. , Ray A. K.
Published: October 2006

The evaporation dynamics of a binary droplet is established by aligning resonances observed in light scattering with those calculated from an evaporation model. For an ideal solution droplet the size and composition as functions of time are determined with high precision from the observed resonances, and the underlying evaporation model is unquestionably validated by accurately recovering the properties of the individual components of the droplet. The results suggest that experimental resonances, each containing a unique relation between the size parameter and the refractive index, can be used to accurately quantify multicomponent droplet evaporation through complex models.
Buckling and competition of energy and entropy lead conformation of single-walled carbon nanocones
Authors: Zhang Shengli , Yao Zhenwei , Zhao Shumin , Zhang Erhu
Published: October 2006

Using a continuum model, expressions for the elastic energy, defect energy, structure entropy, and mixing entropy of carbon nanocones are proposed analytically. The optimal conformation of carbon nanocones is studied by imposing minimization of free energy and analyzing the effects that the buckling of a nanocone’s walls have during formation. The model explains the experimentally observed preference of 19.2° for the cone angle of carbon nanocone. Furthermore, it predicts the optimal conformation of carbon nanocones to result in a cone angle of 19.2°, radius of 0.35nm, and critical length of 24nm, all of which agree very well with experimental observations.
Exciton resonance tuning for the generation of subpicosecond pulses from a mode-locked semiconductor disk laser
Authors: Saas Florian , Steinmeyer Günter , Griebner Uwe , Zorn Martin , Weyers Markus
Published: October 2006

The authors investigate the spectral tuning of the exciton resonance of a surface quantum well saturable absorber mirror for the generation of subpicosecond pulses, employing an optically pumped semiconductor disk laser in the 1μm spectral range. The all-semiconductor laser generates transform-limited pulses as short as 590fs at a 3GHz repetition rate and an output power of 30mW. The pulse duration of the mode-locked laser was supported by the fast relaxation time of the saturable absorber of 1.0ps.
High-performance GaAs metal-insulator-semiconductor field-effect transistors enabled by self-assembled nanodielectrics
Authors: Lin H. C. , Ye P. D. , Xuan Y. , Lu G. , Facchetti A. , Marks T. J.
Published: October 2006

High-performance GaAs metal-insulator-semiconductor field-effect-transistors (MISFETs) fabricated with very thin self-assembled organic nanodielectrics (SANDs), deposited from solution at room temperature, are demonstrated. A submicron gate-length depletion-mode n-channel GaAs MISFET with SAND thicknesses ranging from 5.5to16.5nm exhibit a gate leakage current density <10−5A∕cm2 at a gate bias smaller than 3V, a maximum drain current of 370mA∕mm at a forward gate bias of 2V, and a maximum intrinsic transconductance of 170mS∕mm. The importance of appropriate GaAs surface chemistry treatments on SAND/GaAs interface properties is also presented. Application of SANDs to III-V compound semiconductors affords more opportunities to manipulate the complex III-V surface chemistry with broad materials options.
Optical and magnetic properties of Eu-doped GaN
Authors: Hite J. , Thaler G. T. , Khanna R. , Abernathy C. R. , Pearton S. J. , Park J. H. , Steckl A. J. , Zavada J. M.
Published: October 2006

GaN films were doped with Eu to a concentration of ∼0.12at.% during growth at 800°C by molecular beam epitaxy, with the Eu cell temperature held constant at 470°C. All samples were postannealed at 675°C. The films exhibited strong photoluminescence (PL) in the red (622nm) whose absolute intensity was a function of the Ga flux during growth, which ranged from 3.0×10−7to5.4×10−7Torr. The maximum PL intensity was obtained at a Ga flux of 3.6×10−7Torr. The samples showed room temperature ferromagnetism with saturation magnetization of ∼0.1–0.45emu∕cm3, consistent with past reports where the Eu was found to be predominantly occupying substitutional Ga sites. There was an inverse correlation between the PL intensity and the saturation magnetization in the films. X-ray diffraction showed the presence of EuGa phases under all the growth conditions but these cannot account for the observed magnetic properties.
“Needle beam:” Beyond-diffraction-limit concentration of field and transmitted power in dielectric waveguide
Authors: Bondarenko Victor , Zhao Yang
Published: October 2006

The authors report the “needle beam,” which is an eigenmode of J0-Y0-K0 profile propagating in a purely dielectric three-medium coaxial cylindrical waveguide, whose field and transmitted power are concentrated within the central medium of diameter beyond the diffraction limit. The huge concentration can be achieved for the TE0m and TM0m eigenmodes within a cylindrical hole of nanometer-scale diameter along the waveguide axis. A difference between refractive indices in the hole and neighbor media can considerably enhance the field and the power transmission along the hole.
Effects of deformation on band-edge luminescence of hexagonal boron nitride single crystals
Authors: Watanabe Kenji , Taniguchi Takashi , Kuroda Takashi , Kanda Hisao
Published: October 2006

The authors observed a drastic change in exciton-related luminescence in deformed hexagonal boron nitride single crystals. High quality single crystals that showed a free exciton luminescence band at 215nm were used as a starting material. They were pinched between aluminum plates and pressed with the tips of the first two fingers. The pressed crystals dominantly showed band-edge luminescence at the 227 instead of the 215nm band. The authors attribute this 227nm band to bound exciton luminescence caused by the stacking disorder produced by the mechanical deformation.
Space-charge-limited current in nanowires depleted by oxygen adsorption
Authors: Gu Y. , Lauhon L. J.
Published: October 2006

The effects of oxygen adsorption on the conductivity and carrier transport mechanisms in CdS nanowires were established through variable temperature electrical transport and photoconductivity measurements. n-type CdS nanowires were found to be significantly less conductive in the ambient air than in the vacuum due to electron depletion induced by adsorbed oxygen. The current-voltage characteristics of depleted nanowires exhibited a power law behavior consistent with space-charge-limited conduction in the presence of traps. Analysis of the voltage and temperature dependencies of the space-charge-limited current showed that the nanowire surface traps are exponentially distributed in energy with a characteristic depth of ∼0.28±0.04eV.
Efficient dispersion relations for terahertz spectroscopy
Authors: Gornov E. , Peiponen K.-E. , Svirko Y. , Ino Y. , Kuwata-Gonokami M.
Published: October 2006

The authors introduce dispersion relations that provide an efficient tool in analysis of the experimental data in reflection terahertz spectroscopy when both the real and imaginary parts of the response function can be measured. These dispersion relations allow one to check the consistency of the measured spectra of the real and imaginary parts of the complex reflection coefficient, and moreover to remove the sample misplacement error from the measured spectra.
Resonant and voltage-tunable terahertz detection in InGaAs∕InP nanometer transistors
Authors: El Fatimy A. , Teppe F. , Dyakonova N. , Knap W. , Seliuta D. , Valušis G. , Shchepetov A. , Roelens Y. , Bollaert S. , Cappy A. , Rumyantsev S.
Published: October 2006

The authors report on detection of terahertz radiation by high electron mobility nanometer InGaAs∕AlInAs transistors. The photovoltaic type of response was observed at the 1.8–3.1THz frequency range, which is far above the cutoff frequency of the transistors. The experiments were performed in the temperature range from 10to80K. The resonant response was observed and was found to be tunable by the gate voltage. The resonances were interpreted as plasma wave excitations in the gated two-dimensional electron gas. The minimum noise equivalent power was estimated, showing possible application of these transistors in sensing of terahertz radiation.
Ferromagnetism in Mg-doped AlN from ab initio study
Authors: Wu R. Q. , Peng G. W. , Liu L. , Feng Y. P. , Huang Z. G. , Wu Q. Y.
Published: October 2006

Ab initio calculations based on spin density functional theory were carried out to investigate Mg-doped AlN as a possible dilute magnetic semiconductor. It was found that both Al vacancy and substitutional Mg impurity in AlN lead to spin-polarized ground states. However, sufficient Al vacancy concentration may be difficult to achieve under thermal equilibrium because of the high formation energy of Al vacancy. On the other hand, formation energy of Mg defect is fairly low and the authors’ calculations predict a ferromagnetic coupling among MgN4 tetrahedra. Based on the analysis on Cu-doped ZnO [L. H. Ye et al., Phys. Rev. B 73, 033203 (2006)], room temperature ferromagnetism can be expected in AlN doped with 7% of Mg which can be incorporated at a growth temperature of 2000K under N-rich condition.
Optical signatures of asymmetric fractal diffusion of electron-hole plasma in semiconductor quantum wells
Authors: Borges J. B. , da Silva S. W. , Morais P. C. , Monte A. F. G.
Published: October 2006

The lateral electron-hole transport in intrinsic In0.53Ga0.47As∕InP quantum well, grown on tilted InP substrate, was investigated by scanning the photoluminescence intensity profile on the sample’s surface. Simulations of the asymmetric carrier density profile, using a non-Fickian diffusion equation described by the Lévy statistics [S. I. Denisov et al., Phys. Rev. E 73, 036120 (2006)], show that anomalous diffusion observed along the y direction (perpendicular to the terrace edges) is associated with the presence of Auger recombination, more likely induced by the fractal interface morphology.
First-principles calculations on the stability of Al∕TiB2 interface
Authors: Han Yanfeng , Dai Yongbing , Shu Da , Wang Jun , Sun Baode
Published: October 2006

The atomic structure, adhesion, and interfacial energy of Al∕TiB2 interfaces were studied using first-principles calculations. The preferable interfacial atomic structure is that wherein the Al atoms continue the natural stacking sequence of the bulk TiB2. The interface adhesion for Ti-terminated interface is stronger than that for B-terminated interface. The structural stabilities of two interface terminations depend greatly on the Ti interfacial chemical potential. Calculation of the interfacial energies provides theoretical evidence for the hypernucleation and duplex nucleation grain refinement mechanisms.
Nonradiative traps in InGaN∕GaN multiple quantum wells revealed by two wavelength excitation
Authors: Chen I. J. , Chen T. T. , Chen Y. F. , Lin T. Y.
Published: October 2006

The authors report an investigation of two wavelength excited photoluminescence on InGaN∕GaN multiple quantum wells. It is found that with an addition below-gap excitation the photoluminescence intensity can be quenched by up to 50%. In addition, the decay time of localized carriers changes from 173.9to7.9ns. The experimental results can be well explained in terms of the electronic transitions involving deep defect states in the GaN barrier. Based on the variation of the photon energy of the below-gap excitation, the origin of the deep trap can be identified. The authors point out here that two wavelength excitation spectroscopy is a powerful tool to reveal nonradiative defects in optoelectronic devices.
Selective fluorescent imaging of superoxide
in vivo
using ethidium-based probes
Authors: Robinson Kristine M. , Janes Michael S. , Pehar Mariana , Monette Jeffrey S. , Ross Meredith F. , Hagen Tory M. , Murphy Michael P. , Beckman Joseph S.
Published: October 2006

The putative oxidation of hydroethidine (HE) has become a widely used fluorescent assay for the detection of superoxide in cultured cells. By covalently joining HE to a hexyl triphenylphosphonium cation (Mito-HE), the HE moiety can be targeted to mitochondria. However, the specificity of HE and Mito-HE for superoxide
in vivo
is limited by autooxidation as well as by nonsuperoxide-dependent cellular processes that can oxidize HE probes to ethidium (Etd). Recently, superoxide was shown to react with HE to generate 2-hydroxyethidium [Zhao, H., Kalivendi, S., Zhang, H., Joseph, J., Nithipatikom, K., Vasquez-Vivar, J. & Kalyanaraman, B. (2003)
Free Radic. Biol. Med.
34, 1359–1368]. However, 2-hydroxyethidium is difficult to distinguish from Etd by conventional fluorescence techniques exciting at 510 nm. While investigating the oxidation of Mito-HE by superoxide, we found that the superoxide product of both HE and Mito-HE could be selectively excited at 396 nm with minimal interference from other nonspecific oxidation products. The oxidation of Mito-HE monitored at 396 nm by antimycin-stimulated mitochondria was 30% slower than at 510 nm, indicating that superoxide production may be overestimated at 510 nm by even a traditional superoxide-stimulating mitochondrial inhibitor. The rate-limiting step for oxidation by superoxide was 4 × 10
6
M
−1
·s
−1
, which is proposed to involve the formation of a radical from Mito-HE. The rapid reaction with a second superoxide anion through radical–radical coupling may explain how Mito-HE and HE can compete for superoxide
in vivo
with intracellular superoxide dismutases. Monitoring oxidation at both 396 and 510 nm of excitation wavelengths can facilitate the more selective detection of superoxide
in vivo
.
Neuropathy target esterase catalyzes osmoprotective renal synthesis of glycerophosphocholine in response to high NaCl
Authors: Gallazzini Morgan , Ferraris Joan D. , Kunin Margarita , Morris Ryan G. , Burg Maurice B.
Published: October 2006

Glycerophosphocholine (GPC) is an osmoprotective compatible and counteracting organic osmolyte that accumulates in renal inner medullary cells in response to high NaCl and urea. We previously found that high NaCl increases GPC in renal [Madin–Darby canine kidney (MDCK)] cells. The GPC is derived from phosphatidylcholine, catalyzed by a phospholipase that was not identified at that time. Neuropathy target esterase (NTE) was recently shown to be a phospholipase B that catalyzes production of GPC from phosphatidylcholine. The purpose of the present study was to test whether NTE contributes to the high NaCl-induced increase of GPC synthesis in renal cells. We find that in mouse inner medullary collecting duct cells, high NaCl increases
NTE
mRNA within 8 h and NTE protein within 16 h. Diisopropyl fluorophosphate, which inhibits NTE esterase activity, reduces GPC accumulation, as does an siRNA that specifically reduces NTE protein abundance. The 20-h half-life of
NTE
mRNA is unaffected by high NaCl. TonEBP/OREBP is a transcription factor that is activated by high NaCl. Knockdown of TonEBP/OREBP by a specific siRNA inhibits the high NaCl-induced increase of
NTE
mRNA. Further, the lower renal inner medullary interstitial NaCl concentration that occurs chronically in ClCK1
−/−
mice and acutely in normal mice given furosemide is associated with lower
NTE
mRNA and protein. We conclude that high NaCl increases transcription of NTE, likely mediated by TonEBP/OREBP, and that the resultant increase of NTE expression contributes to increased production and accumulation of GPC in mammalian renal cells in tissue culture and
in vivo
.
Orientations in adolescent use of information and communication technology: A digital divide by sociodemographic background, educational career, and health
Authors: Koivusilta Leena K. , Lintonen Tomi P. , Rimpelä Arja H.
Published: October 2006

Aims: The role of information and communication technology (ICT) in adolescents' lives was studied, with emphasis on whether there exists a digital divide based on sociodemographic background, educational career, and health. The assumption was that some groups of adolescents use ICT more so that their information utilization skills improve (computer use), while others use it primarily for entertainment (digital gaming, contacting friends by mobile phone). Methods: Data were collected by mailed survey from a nationally representative sample of 12- to 18-year-olds (n=7,292; response 70%) in 2001 and analysed using ANOVA. Results: Computer use was most frequent among adolescents whose fathers had higher education or socioeconomic status, who came from nuclear families, and who continued studies after compulsory education. Digital gaming was associated with poor school achievement and attending vocational rather than upper secondary school. Mobile phone use was frequent among adolescents whose fathers had lower education or socioeconomic status, who came from non-nuclear families, and whose educational prospects were poor. Intensive use of each ICT form, especially of mobile phones, was associated with health problems. High social position, nuclear family, and a successful educational career signified good health in general, independently of the diverse usage of ICT. Conclusions: There exists a digital divide among adolescents: orientation to computer use is more common in educated well-off families while digital gaming and mobile phone use accumulate at the opposite end of the spectrum. Poorest health was reported by mobile phone users. High social background and success at school signify better health, independently of the ways of using ICT.
Kidney failure in mice lacking the tetraspanin CD151
Authors: Sachs Norman , Kreft Maaike , van den Bergh Weerman Marius A. , Beynon Andy J. , Peters Theo A. , Weening Jan J. , Sonnenberg Arnoud
Published: October 2006

The tetraspanin CD151 is a cell-surface molecule known for its strong lateral interaction with the laminin-binding integrin α3β1. Patients with a nonsense mutation in CD151 display end-stage kidney failure associated with regional skin blistering and sensorineural deafness, and mice lacking the integrin α3 subunit die neonatally because of severe abnormalities in the lung and kidney epithelia. We report the generation of Cd151-null mice that recapitulate the renal pathology of human patients, i.e., with age they develop massive proteinuria caused by focal glomerulosclerosis, disorganization of the glomerular basement membrane, and tubular cystic dilation. However, neither skin integrity nor hearing ability are impaired in the Cd151-null mice. Furthermore, we generated podocyte-specific conditional knockout mice for the integrin α3 subunit that show renal defects similar to those in the Cd151 knockout mice. Our results support the hypothesis that CD151 plays a key role in strengthening α3β1-mediated adhesion in podocytes.