bluuebunny/crossref_metadata_2025 huggingface dataset:
Solid-state ultra-wideband pulse generator: Application in bioeffect research
Authors: Sunkam Ramana K. , Hill Jerimiah , Selmic Rastko R. , Haynie Donald T.
Published: April 2005

This article describes the design and characterization of an ultra-wideband (UWB) pulse generator suitable for biomedical research. The pulser generates nanosecond-long electromagnetic pulses of adjustable rise time, amplitude, and duration, permitting analysis of possible bioeffects over a range of pulse characteristics. Nanosecond rise-time pulses (2.4 ns) of 3.4 kV are achieved in a four-stage pulser circuit using ZTX415 transistors. This device has been used to assess exposure bioeffects to UWB pulses of the model system E. coli.
Measurement of CH density in a pulsed-dc hydrocarbon-gas- mixture discharge
Authors: Brown Michael S. , Forlines Robert A. , Ganguly Biswa N.
Published: April 2005

Optical and electrical measurements are performed in methane/diluent mixtures in a 250-ns pulsed-dc discharge using a fast-rise-time pulser. In particular, emission and laser-induced-fluorescence studies of CH produced by direct-electron-impact dissociation reveal the postdissociation kinetics of CH as well as a means of estimating the CH density. In a 20-Torr 10%∕20%∕70% mixture of argon/methane/nitrogen, the instantaneous CH density is determined to be 2.1×1013cm−3 at the end of the steady-state voltage and current condition. Kinetics analysis reveals that neutral chemistry with methane is the largest loss process for the CH fragments. Analysis of N2(C-B) rotational temperatures indicates that less than 30% of the input electrical energy is deposited as heat in the gas, with the balance going toward dissociation, excitation of internal molecular states, and ionization.
Numerical investigation of feedback control of thermocapillary instability
Authors: Shiomi Junichiro , Amberg Gustav
Published: April 2005

Control of oscillatory thermocapillary convection in an annular geometry with a horizontal free surface is investigated by means of a numerical simulation. The objective is to suppress oscillations using a feedback opposition control. The temperature is measured at certain positions on the interface, this signal is amplified and used to apply local heating on the free surface. Many features of the controlled system observed in previous experiments could be reproduced by the simulation. The numerical simulation allows us to clarify the picture of the spatial structures of the controlled oscillation, which was not accessible in the experiments. In addition to what was found in the previous experiments, the present simulations also permit us to investigate the importance of the positioning of sensors and heaters, and the influence of the properties of the heaters.
The magneto-optical properties of annealed Co96Pt4 nanowire arrays
Authors: Li Wuxia , Jones Grenville A. , Peng Yong , Shen Tiehan H. , Hill Geoff
Published: April 2005

The magneto-optical (MO) properties of highly ordered arrays of electrodeposited Co96Pt4 alloy nanowires embedded in anodic aluminum oxide (AAO) membranes have been studied as a function of annealing temperature (TA). The initial measurements yielded the four Stokes parameters from which the Faraday rotation angle (FRA) and ellipticity (tanχ) were extracted. The FRA was determined as 8.0×103deg∕cm for the as-deposited sample. After annealing at 300°C the FRA attained a maximum of 1.1×104deg∕cm (an increase of 38%), but thereafter declined monotonically as TA was increased to 650°C. The ellipticity shows a similar behavior with TA. However, the corresponding variation in saturation magnetization (Ms) with TA does not match that of the magneto-optical constants, which suggests that factors other than Ms also determine the values of FRA and tanχ. The data obtained indicate that a suitable annealing regime is an effective method to optimize the MO properties of AAO-based magnetic nanowire arrays.
Measurement of source profile of proton beams generated by ultraintense laser pulses using a Thomson mass spectrometer
Authors: Oishi Y. , Nayuki T. , Fujii T. , Takizawa Y. , Wang X. , Yamazaki T. , Nemoto K. , Sekiya T. , Horioka K. , Andreev A. A.
Published: April 2005

For measuring the source profile of proton beams, a method using a Thomson mass spectrometer, which has precise energy resolution and enables the discrimination of ion species, was proposed and applied in an experiment with irradiation of 55-fs, 6.6×1018-W∕cm2-laser pulses on a 5-μm-thick copper tape target. The typical measured source size was 150μm at E=400keV and 44μm at E=980keV in full width at half maximum.
Microstructure of epitaxial Ba0.7Sr0.3TiO3∕SrRuO3 bilayer films on SrTiO3 substrates
Authors: He J. Q. , Vasco E. , Jia C. L. , Dittmann R. , Wang R. H.
Published: April 2005

The thickness evolution of the microstructure of epitaxial Ba0.7Sr0.3TiO3 thin films grown on SrRuO3∕SrTiO3 was investigated by means of transmission electron microscopy. Within the Ba0.7Sr0.3TiO3 layer, a layered structure (three sublayers) is distinguished as for the configuration of lattice strain and defects. The first sublayer extends for 3nm from the lattice-coherent Ba0.7Sr0.3TiO3∕SrRuO3 interface. The second 13-nm-thick sublayer forms a semicoherent interface with the first sublayer due to the creation of a misfit dislocation network. The third sublayer extends beyond the second sublayer exhibiting a structure characterized by compact columnar features. Planar defects are formed at the boundaries between such features. The formation of a layered structure within the Ba0.7Sr0.3TiO3 films is discussed in the light of the growth modes of films on lattice-mismatched substrates.
Orientation distribution of sheared isotactic polypropylene plates through thickness in the presence of sodium benzoate
Authors: Zhu Peng-wei , Phillips Andrew , Tung Jason , Edward Graham
Published: April 2005

The orientation distribution of sheared isotactic polypropylene (iPP) containing different amount of sodium benzoate (SB) has been investigated through the gradient of shear flow field using microbeam of synchrotron wide-angle x-ray techniques. The degree of the overall orientation of α-phase crystal is found to increase with increasing concentration of SB. Compared with the sheared iPP in the absence of SB, the orientation of α-phase crystal is found to distribute over a broader range of shear flow field in the presence of SB. The overall orientation of α-phase crystal is explained in terms of a parent-daughter model or lamella-branched shish-kebab structure. As the concentration of SB increases, the contribution from the c-axis orientation of parent lamellae decreases in the flow direction. The contribution from the a*-axis orientation of daughter lamellae is developed to be dominant in the flow direction when the concentration of SB exceeds a critical value.
Preparation and characterization of ZnO particles embedded in SiO2 matrix by reactive magnetron sputtering
Authors: Ma J. G. , Liu Y. C. , Xu C. S. , Liu Y. X. , Shao C. L. , Xu H. Y. , Zhang J. Y. , Lu Y. M. , Shen D. Z. , Fan X. W.
Published: April 2005

ZnO particles embedded in SiO2 thin films were prepared by a radio-frequency magnetron sputtering technique. X-ray diffraction (XRD) and optical-absorption spectra showed that ZnO particles with hexagonal wurtzite structure had been embedded in the SiO2 matrix, and the size of ZnO particles increased with increasing annealing temperature from 773to973K. Raman-scattering and Fourier transform infrared (FTIR) spectrum measurements also confirmed the presence of ZnO particles. When the annealing temperature was lower than 973K, room-temperature photoluminescence (PL) spectra showed dominative deep-level emissions in the visible region and very weak ultraviolet emissions. As the annealing temperature increased to 973K, an emission band in the ultraviolet region besides the emissions from free and bound excitons recombination was observed in the low-temperature PL spectra. The origin of the ultraviolet emission bands was discussed with the help of temperature-dependent PL spectra. When the annealing temperature was higher than 973K, Zn2SiO4 particles were formed, as shown by XRD and FTIR results.
The large-Z Behavior of pseudorelativistic atoms
Authors: Sørensen Thomas Østergaard
Published: April 2005

In this paper we study the large-Z behavior of the ground state energy of atoms with electrons having relativistic kinetic energy p2c2+m2c4−mc2. We prove that to leading order in Z the energy is the same as in the nonrelativistic case, given by (nonrelativistic) Thomas–Fermi theory. For the problem to make sense, we keep the product Zα fixed (here α is Sommerfeld’s fine structure constant), and smaller than, or equal to, 2∕π, which means that as Z tends to infinity, α tends to zero.
Maximizing ion current by space-charge neutralization using negative ions and dust particles
Authors: Smirnov A. , Raitses Y. , Fisch N. J.
Published: April 2005

Ion current extracted from an ion source (ion thruster) can be increased above the Child–Langmuir limit if the ion space charge is neutralized. Similarly, the limiting kinetic energy density of the plasma flow in a Hall thruster might be exceeded if additional mechanisms of space-charge neutralization are introduced. Space-charge neutralization with high-mass negative ions or negatively charged dust particles seems, in principle, promising for the development of a high current or high energy density source of positive light ions. Several space-charge neutralization schemes that employ heavy negatively charged particles are considered. It is shown that the proposed neutralization schemes can lead, at best, only to a moderate but nonetheless possibly important increase of the ion current in the ion thruster and the thrust density in the Hall thruster.
Microbridge testing of plasma-enhanced chemical-vapor deposited silicon oxide films on silicon wafers
Authors: Cao Zhiqiang , Zhang Tong-Yi , Zhang Xin
Published: April 2005

Plasma-enhanced chemical-vapor deposited (PECVD) silane-based oxides (SiOx) have been widely used in both microelectronics and microelectromechanical systems (MEMS) to form electrical and/or mechanical components. In this paper, a nanoindentation-based microbridge testing method is developed to measure both the residual stresses and Young’s modulus of PECVD SiOx films on silicon wafers. Theoretically, we considered both the substrate deformation and residual stress in the thin film and derived a closed formula of deflection versus load. The formula fitted the experimental curves almost perfectly, from which the residual stresses and Young’s modulus of the film were determined. Experimentally, freestanding microbridges made of PECVD SiOx films were fabricated using the silicon undercut bulk micromachining technique. Some microbridges were subjected to rapid thermal annealing (RTA) at a temperature of 400 °C, 600 °C, or 800 °C to simulate the thermal process in the device fabrication. The results showed that the as-deposited PECVD SiOx films had a residual stress of −155±17MPa and a Young’s modulus of 74.8±3.3GPa. After the RTA, Young’s modulus remained relatively unchanged at around 75 GPa, however, significant residual stress hysteresis was found in all the films. A microstructure-based mechanism was then applied to explain the experimental results of the residual stress changes in the PECVD SiOx films after the thermal annealing.
Defect-induced photoluminescence from tetraethylorthosilicate thin films containing mechanically milled silicon nanocrystals
Authors: Lau H. W. , Tan O. K. , Liu Y. , Ng C. Y. , Chen T. P. , Pita K. , Lu D.
Published: April 2005

In this work, the unique synthesis of mechanically milled silicon nanocrystals (Si nc) embedded in tetraethylorthosilicate (TEOS) thin films is reported. A series of Si nc, with sizes ranging from 10to25nm, have been synthesized using mechanical milling. For both the milled Si nc and milled Si nc embedded in TEOS thin film, infrared absorption and photoluminescence results show that the photoluminescence (PL) is not a consequence of quantum confinement, amorphous Si component, or Si–OH or Si–H bonds. The defects, such as nonbridging oxide hole centers (NBOHCs), in amorphous SiO2 are probably the dominant mechanism for the PL of milled Si nc embedded in TEOS thin films. In addition, PL excitation results reveal oxidation-induced strain between the interfaces of milled Si nc∕SiO2 has also generated a new luminescence center. This luminescence center is similar to the NBOHCs attributed to interfacial strain.
Geometrical criteria required for the determination of the epitaxial stress from the transmission electron microscopy curvature method
Authors: Cabié M. , Ponchet A. , Rocher A. , Durand L. , Altibelli A.
Published: April 2005

The epitaxial stress of a Ga0.8In0.2As thin layer deposited on a GaAs substrate has been measured by the curvature method adapted to transmission electron microscopy. It is shown that even if the geometrical characteristics of the specimens thinned to be observed by transmission electron microscopy are very different from the ones of a thick sample, the conditions of validity of the model can still be verified. Finite element calculations have been performed to determine the geometry of the specimen answering to these conditions. Once these conditions are satisfied, the stress measured on a Ga0.8In0.2As layer is −1.30±0.13GPa.
Particle imaging velocimetry-based identification of coherent structures in normally impinging multiple jets
Authors: Geers L. F. G. , Tummers M. J. , Hanjalić K.
Published: April 2005

To gain a better insight into the effects of eddy structure and their thermal imprint on the impinging surface, we applied several methods to identify coherent structures in two arrangements of multiple jets. High-resolution particle imaging velocimetry measurements of the instantaneous fluid velocity have been performed to provide data for structure identification. Several methods based on critical point theory, i.e., vorticity magnitude, kinematic vorticity number, and second invariant of the velocity gradient tensor provided similar and generally useful information. However, in the flow considered they all appear inferior as compared with the proper orthogonal decomposition (POD). Despite lacking in conspicuous high energy modes, the flow showed to be well suited for POD, which provided clear identification of the near wall dominating vortical structure.
Broadband “in-series multistation” rf cavity with low voltage standing wave ratio
Authors: Misu Toshiyuki , Miyahara Nobuyuki , Sugiura Akinori , Hojo Satoru , Kanazawa Mitsutaka , Yamada Satoru
Published: April 2005

A configuration for an untuned broadband rf cavity with a low-voltage standing wave ratio (VSWR) is proposed. Although an untuned broadband cavity is currently implemented by loading magnetic alloy (MA) cores, the VSWR of such a cavity is expected to be no less than approximately 2.0 in the operational frequencies sweeping by a factor of about 10. A type of rf cavity, “in-series multistation” cavity, described here can cover a much broader frequency range sweeping by a factor of 20, while keeping the VSWR value below 1.2. The system consists of multiple stations, each of which is loaded with low-Q high-permeability MA cores. A “bench” test circuit was built and successfully tested.
Prediction of a linear polymer made of xenon and carbon
Authors: Brown Eric C. , Cohen Arik , Gerber R. Benny
Published: April 2005

Electronic structure calculations predict the existence of a novel type of a chemically bound noble gas compound. The predicted species is an extended linear and periodic polymer, made of the repeat unit –(XeCC)–, where CC is the acetylenic group. The polymer has a strong partly ionic nature, with positive partial charge on the xenon atoms and a negative one on the CC groups. High energy barriers are found for the removal of a Xe atom from the chain, indicating high stability. This is the first polymer with a noble-gas-containing building block.
Some useful combinatorial formulas for bosonic operators
Authors: Blasiak P. , Penson K. A. , Solomon A. I. , Horzela A. , Duchamp G. H. E.
Published: April 2005

We give a general expression for the normally ordered form of a function F[ŵ(a,a†)] where ŵ is a function of boson creation and annihilation operators satisfying [a,a†]=1. The expectation value of this expression in a coherent state becomes an exact generating function of Feynman-type graphs associated with the zero-dimensional quantum field theory defined by F(ŵ). This enables one to enumerate explicitly the graphs of given order in the realm of combinatorially defined sequences. We give several examples of the use of this technique, including the applications to Kerr-type and superfluidity-type Hamiltonians.
Three-dimensional force microscope: A nanometric optical tracking and magnetic manipulation system for the biomedical sciences
Authors: Fisher J. K. , Cummings J. R. , Desai K. V. , Vicci L. , Wilde B. , Keller K. , Weigle C. , Bishop G. , Taylor R. M. , Davis C. W. , Boucher R. C. , O’Brien E. Timothy , Superfine R.
Published: April 2005

We report here the development of a three-dimensional (3D) magnetic force microscope for applying forces to and measuring responses of biological systems and materials. This instrument combines a conventional optical microscope with a free-floating or specifically bound magnetic bead used as a mechanical probe. Forces can be applied by the bead to microscopic structures of interest (specimens), while the reaction displacement of the bead is measured. This enables 3D mechanical manipulations and measurements to be performed on specimens in fluids. Force is generated by the magnetically permeable bead in reaction to fields produced by external electromagnets. The displacement is measured by interferometry using forward light scattered by the bead from a focused laser beam. The far-field interference pattern is imaged on a quadrant photodetector from which the 3D displacement can be computed over a limited range about the focal point. The bead and specimen are mounted on a 3D translation stage and feedback techniques are used to keep the bead within this limited range. We demonstrate the system with application to beads attached to cilia in human lung cell cultures.
High pressurein situx-ray absorption spectroscopy cell for studying simultaneously the liquid phase and the solid/liquid interface
Authors: Grunwaldt Jan-Dierk , Ramin Michael , Rohr Markus , Michailovski Alexej , Patzke Greta R. , Baiker Alfons
Published: April 2005

A high pressure in situ x-ray absorption spectroscopy cell with two different path lengths and path positions is presented for studying element-specifically both the liquid phase and the solid/liquid interface at pressures up to 250 bar and temperatures up to 220 °C. For this purpose, one x-ray path probes the bottom, while the other x-ray path penetrates through the middle of the in situ cell. The basic design of the cell resembles a 10 ml volume batch reactor, which is equipped with in- and outlet lines to dose compressed gases and liquids as well as a stirrer for good mixing. Due to the use of a polyetheretherketone inset it is also suitable for measurements under corrosive conditions. The characteristic features of the cell are illustrated using case studies from catalysis and solid state chemistry: (a) the ruthenium-catalyzed formylation of an amine in “supercritical” carbon dioxide in the presence of hydrogen; (b) the cycloaddition of carbon dioxide to propylene oxide in the presence of a solid Zn-based catalyst, and (c) the solvothermal synthesis of MoO3 nanorods from MoO3∙2H2O.
Domain rearrangement during nanoindentation in single-crystalline barium titanate measured by atomic force microscopy and piezoresponse force microscopy
Authors: Schneider G. A. , Scholz T. , Muñoz-Saldaña J. , Swain M. V.
Published: April 2005

Nanoindentation tests in an aa-in-plane domain area of an {001} oriented barium titanate single crystal were performed using a conical indenter with a tip radius of 800 nm. The topography and the polarization vectors of the area after indentation were imaged afterwards by both atomic force and piezoresponse force microscopy (PFM), respectively. Two perpendicular oriented cracks in the {110} planes were identified in the topographic image. An unexpected considerable uplift occurs inside the residual impression, which was correlated with a sharp pop-out-like behavior observed in the force-displacement curve just prior to unloading. Furthermore, PFM revealed an almost a twofold symmetric arrangement of the domains around the indent, which can be explained by residual circumferential tensile stresses around a residual impression and was unambiguously correlated to the crystal orientation.
Accurate determination of atomic structure of multiwalled carbon nanotubes by nondestructive nanobeam electron diffraction
Authors: Liu Zejian , Zhang Qi , Qin Lu-Chang
Published: April 2005

We report a method that allows direct, systematic, and accurate determination of the atomic structure of multiwalled carbon nanotubes by analyzing the scattering intensities on the nonequatorial layer lines in the electron diffraction pattern. Complete structure determination of a quadruple-walled carbon nanotube is described as an example, and it was found that the intertubular distance varied from 0.36nmto0.5nm with a mean value of 0.42nm.
Thermodynamic stability of Ga2O3(Gd2O3)∕GaAs interface
Authors: Huang Y. L. , Chang P. , Yang Z. K. , Lee Y. J. , Lee H. Y. , Liu H. J. , Kwo J. , Mannaerts J. P. , Hong M.
Published: April 2005

Ga 2 O 3 ( Gd 2 O 3 ) ∕ Ga As heterostructures have been annealed up to ∼780°C. Studies using x-ray reflectivity and high-resolution transmission electron microscopy have shown that the samples annealed under ultrahigh vacuum have maintained smooth and abrupt interfaces with the interfacial roughness being less than 0.2nm. The oxide remains amorphous, an important parameter for device consideration. Current–voltage and capacitance–voltage measurements have shown low leakage currents (10−8–10−9A∕cm2), a high dielectric constant of 15, and a low interfacial density of states (Dit) between gate dielectrics and GaAs. The attainment of a smooth interface between the gate dielectric and GaAs, even after high temperature annealing for activating implanted dopant, is a must to ensure the low (Dit) and to maintain a high carrier mobility in the channel of the metal–oxide–semiconductor field-effect transistor.
Nanoscale friction mapping
Authors: Tambe Nikhil S. , Bhushan Bharat
Published: April 2005

Contrary to classical friction laws postulated by Amontons and Coulomb centuries ago, nanoscale friction force is found to be strongly dependent on the normal load and sliding velocity. Many materials, coatings, and lubricants that have wide applications in micro/nanoelectromechanical systems show reversals in friction behavior corresponding to transitions between different friction mechanisms. We have developed a contour map to provide a fundamental insight into the normal load and sliding velocity dependence of friction force and thus help identify and classify dominant friction mechanisms.
Role of phonon scattering in carbon nanotube field-effect transistors
Authors: Guo Jing , Lundstrom Mark
Published: April 2005

The role of phonon scattering in carbon nanotube field-effect transistors (CNTFETs) is explored by solving the Boltzmann transport equation using the Monte Carlo method. The results show that elastic scattering in a short-channel CNTFET has a small effect on the source-drain current due to the long elastic mean-free path (mfp) (∼1μm). If elastic scattering with a short mfp were to exist in a CNTFET, the on current would be severely degraded due to the one-dimensional channel geometry. At high drain bias, optical phonon scattering, which has a much shorter mfp (∼10nm), is expected to dominate, even in a short-channel CNTFET. We find, however, that inelastic optical scattering has a small effect in CNTFETs under modest gate bias.
Equivalent oxide thickness of a thin oxide interlayer in gate insulator stacks on silicon
Authors: Giustino Feliciano , Bongiorno Angelo , Pasquarello Alfredo
Published: April 2005

We investigate the equivalent oxide thickness of a thin oxide interlayer in gate insulator stacks on silicon. Through the use of a first-principles approach, we map the profile of the local permittivity across two interface models showing different suboxide structures. These models incorporate the available atomic-scale experimental data and account for the amorphous nature of the oxide. The equivalent oxide thickness of the interfacial oxide layer is found to be smaller than the corresponding physical thickness by 0.2–0.3nm. We discuss implications of these results for future device scaling.
Magnetic domain configurations in spark-eroded ferromagnetic shape memory Ni-Mn-Ga particles
Authors: Solomon Virgil C. , McCartney Martha R. , Smith David J. , Tang YunJun , Berkowitz Ami E. , O’Handley Robert C.
Published: April 2005

Spherical particles of the ferromagnetic shape memory material Ni51Mn29Ga20 obtained by spark erosion transform during cooling from the high-temperature Heusler L21 cubic phase into tetragonal martensite. Using the Fresnel (i.e., Lorentz) imaging mode, magnetic domains with an average width of 100nm are observed in the modulated martensitic phase in the absence of a magnetic field. The magnetization distribution within individual particles is determined using electron holography. The magnetic flux lines change direction when crossing boundaries between crystallographic twin variants. These boundaries, where the easy c-axis of the crystallographic variants rotate by 86.5°, coincide with quasi-90° magnetic domain walls, with thickness of approximately 17nm. The magnetization saturation determined by electron holography is about 0.57T.
Interlayer coupling in a trilayer junction having bismuth as spacer layer
Authors: Hsu Jen-Hwa , Sahu D. R.
Published: April 2005

A trilayer system (CoFe∕Bi∕Co) with bismuth as a spacer layer was synthesized using dc sputtering at room temperature. Magnetization studies show that the interlayer coupling between two ferromagnetic electrodes oscillates with the thickness of Bi spacer layer at room temperature, as seen in most metallic spacers, but it has a much longer period, which is estimated to be 180to200Å. Furthermore, the range of interlayer coupling between two ferromagnetic layers separated by Bi spacer can be extended up to 500Å. As the measuring temperature is reduced, the strength of interlayer coupling increases and varies with Bi thickness in a similar way, but the oscillation amplitude decreases.
Polarization reversal and capacitance-voltage characteristic of epitaxial Pb(Zr,Ti)O3 layers
Authors: Pintilie L. , Lisca M. , Alexe M.
Published: April 2005

Capacitance-voltage (C–V) measurements were performed on epitaxial layers of Pb(Zr0.2Ti0.8)O3 (PZT) with top and bottom SrRuO3 (SRO) electrodes. It is shown that the sharp capacitance peak∕discontinuity which is present in the C–V characteristics at different frequencies is directly associated with the polarization reversal. The ferroelectric film is assumed as a large band-gap semiconductor with Schottky contacts at the metal-ferroelectric interfaces. The capacitance discontinuity at the reversal (coercive) voltage is associated with a discontinuity in the built-in potential at the PZT∕SRO interfaces. The C–V characteristics for voltage ranges outside the coercive values can be used to extract the free carrier concentrations as in the case of Schottky metal-semiconductor contacts. The carrier concentration was found to be (2–4)×1018cm−3, independent of measuring frequency and temperature up to 1 MHz and 170 °C, respectively, suggesting completely ionized shallow impurities.
Activation characteristics of ion-implanted Si+ in AlGaN
Authors: Irokawa Y. , Fujishima O. , Kachi T. , Pearton S. J. , Ren F.
Published: April 2005

Multiple-energy Si+ implantation in the range 30–360 keV into Al0.13Ga0.87N for n-type doping was carried out at room temperature, followed by annealing at 1150–1375 °C for 5 min. Activation efficiencies close to 100% were obtained for ion doses of 1.0×1015cm−2 after annealing at 1375 °C, with a resulting sheet resistance of 74Ω∕square. By sharp contrast, the activation efficiency at 1150 °C was only 4% for this dose, with a sheet resistance of 1.63×104Ω∕square. The activation efficiency was also a function of dose, with a maximum activation percentage of only 55% for lower doses of 1.0×1014cm−2 annealed at 1375 °C. This is due to the comparatively larger effect of compensating acceptors at the lower dose and is also lower than the corresponding activation of Si in pure GaN under these conditions (78%). The measurement temperature dependence of sheet carrier density showed an activation energy of 23 meV, consistent with the ionization energy of Si in AlGaN.
Visual Impressions of Interactions between Objects When the Causal Object Does Not Move
Authors: White Peter A
Published: April 2005

Stimuli were presented that consisted of a stationary row of black-bordered white rectangles. As observers watched, each rectangle in turn from left to right changed from white to black. The final rectangle did not change colour but moved off from left to right. The sequential colour change suggested motion from left to right, and observers reliably reported a visual impression that this illusory motion kicked or bumped the last rectangle, thereby making it move. The impression was stronger when the sequential colour change was faster, but was not significantly affected by the number of the rectangles in the row (ranging from 2 to 8). These results support the conclusion that neither contact nor motion of a causal object is necessary for a visual impression of causality to occur.
A New Variant of the Ouchi Illusion Reveals Fourier-Component-Based Processing
Authors: Ashida Hiroshi , Kitaoka Akiyoshi , Sakurai Kenzo
Published: April 2005

We report that anomalous motion illusion in a new variant of the Ouchi figure is well predicted by the strength of its Fourier fundamentals and harmonics. The original Ouchi figure consists of a rectangular checkerboard pattern surrounded by an orthogonal rectangular checkerboard pattern, in which illusory relative motion between the two regions is perceived. Although this illusion has been explained in terms of biases in integrating one-dimensional motion signals to determine the two-dimensional motion direction, the physiological mechanism has not been clarified. With our new stimuli, which consisted of thin lines instead of rectangles, we found that the perceived illusion is drastically reduced when the position of each line element is randomly shifted. This is not predicted by simple models of local motion integration along the visible edges. We demonstrate that the relative amplitude of the relevant Fourier fundamentals and harmonics leads to a quantitative prediction. Our analysis was successfully applied to other variants of the Ouchi figure (Khang and Essock 1997 Perception26 585–597), closely predicting the reported rating. The results indicate that the underlying physiological mechanism is sensitive to the Fourier components of the stimuli rather than the visible edges.
Handgrip Maximum Force and the Visual Horizontal – Vertical Illusion
Authors: Raudsepp Jaanus , Djupsjöbacka Mats
Published: April 2005

The visual horizontal–vertical illusion (HVI) refers to the tendency to overestimate vertical distances relative to horizontals in both 2-D and 3-D presentations. Although the HVI is evident across a wide range of different stimuli, no general theoretical account fully explains the illusion. Some recent authors have proposed the ‘effort’ account of HVI, contending that vertical overestimation is mediated by effort assessment of gravitational challenges offered by the stimulus. The theory has been supported by a set of studies showing that the height overestimation of large-scale 3-D objects is inversely related to perceivers' fitness and strength. We explored if the large-scale HVI/strength dependence extends to the evaluation of small-scale 2-D line stimuli, traditionally used in HVI studies. We measured the maximum handgrip strength, and assessed the HVI with a computerised line-adjustment task in thirty-two individuals. Compatible with earlier findings in the context of large-scale 3-D stimuli, a significant negative correlation was found between the strength of the dominant hand and amount of HVI. In addition, the variability of HVI was negatively correlated with maximum grip strength of both hands. The results are discussed with reference to the ‘effort’ account of HVI.
Motion Parallax Driven by Head Movements: Conditions for Visual Stability, Perceived Depth, and Perceived Concomitant Motion
Authors: Ono Hiroshi , Ujike Hiroyasu
Published: April 2005

Yoking the movement of the stimulus on the screen to the movement of the head, we examined visual stability and depth perception as a function of head-movement velocity and parallax. In experiment 1, for different head velocities, observers adjusted the parallax to find (a) the depth threshold and (b) the concomitant-motion threshold. Between these thresholds, depth was seen with no perceived motion. In experiment 2, for different head velocities, observers adjusted the parallax to produce the same perceived depth. A slower head movement required a greater parallax to produce the same perceived depth as faster head movements. In experiment 3, observers reported the perceived depth for different parallax magnitudes. Perceived depth covaried with smaller parallax without motion perception, but began to decrease with larger parallax and concomitant motion was seen. Only motion was seen with the larger parallax.
Comparing the Spatial-Frequency Response of First-Order and Second-Order Lateral Visual Interactions: Grating Induction and Contrast – Contrast
Authors: McCourt Mark E
Published: April 2005

The magnitudes of two suprathreshold lateral spatial-interaction effects—grating induction and contrast–contrast—were compared with regard to their dependence upon inducing-grating spatial frequency. Both effects cause the contrast of target stimuli embedded in surrounding patterns to be matched nonveridically. The magnitudes of each effect were measured in a common unit that indexed the degree of nonveridical contrast matching across a large range of target-grating contrasts (±0.80). Grating induction was a low-pass effect with respect to spatial frequency, whereas contrast–contrast was bandpass, peaking at approximately 4.0 cycles deg−1. The magnitude of grating induction exceeded that of contrast – contrast, both overall and at their optimal frequencies (0.03125 and 4.0 cycles deg−1, respectively); the two effects are equipotent at an inducing-grating spatial frequency of 1.0 cycle deg−1. A significant negative correlation between the magnitudes of the two effects suggests a link whereby activation of second-order normalization mechanisms may inhibit first-order mechanisms.
Spatiotemporal maturation patterns of murine brain quantified by diffusion tensor MRI and deformation-based morphometry
Authors: Verma Ragini , Mori Susumu , Shen Dinggang , Yarowsky Paul , Zhang Jiangyang , Davatzikos Christos
Published: April 2005

Highly heterogeneous spatiotemporal patterns of maturation of the murine brain during the first 80 postnatal days were examined by high-dimensional deformation-based morphometry applied to high-resolution diffusion tensor MRIs. The maturation profile revealed a sharp contrast between tissue anisotropy changes in the cortex and in major white-matter fibers. Radially oriented tissue anisotropy was measured during the first postnatal week in cortical regions, reflecting the underlying columnar organization of the cortex. Subsequently, tissue anisotropy reduced rapidly, potentially reflecting the growth of randomly oriented dendritic trees that reduce tissue coorientation. Distinct anisotropy patterns were also observed along layer I of the cortex and were attributed to thin fibers oriented parallel to the outer surface. Last, spatially complex patterns of maturation were measured in all major axonal pathways and in the hippocampus, caudate putamen, and cerebellum. This analysis provides a framework for quantifying normative maturation patterns against which phenotypes of mice of different genetic and environmental backgrounds can be contrasted.
Interfacial folding and membrane insertion of designed peptides studied by molecular dynamics simulations
Authors: Im Wonpil , Brooks Charles L.
Published: April 2005

The mechanism of interfacial folding and membrane insertion of designed peptides is explored by using an implicit membrane generalized Born model and replica-exchange molecular dynamics. Folding/insertion simulations initiated from fully extended peptide conformations in the aqueous phase, at least 28 Å away from the membrane interface, demonstrate a general mechanism for structure formation and insertion (when it occurs). The predominately hydrophobic peptides from the synthetic WALP and TMX series first become localized at the membrane-solvent interface where they form significant helical secondary structure via a helix–turn–helix motif that inserts the central hydrophobic residues into the membrane interior, and then fluctuations occur that provide a persistent helical structure throughout the peptide and it inserts with its N-terminal end moving across the membrane. More specifically, we observed that: (
i
) the WALP peptides (WALP16, WALP19, and WALP23) spontaneously insert in the membrane as just noted; (
ii
) TMX-1 also inserts spontaneously after a similar mechanism and forms a transmembrane helix with a population of ≈50% at 300 K; and (
iii
) TMX-3 does not insert, but exists in a fluctuating membrane interface-bound form. These findings are in excellent agreement with available experimental data and demonstrate the potential for new implicit solvent/membrane models together with advanced simulation protocols to guide experimental programs in exploring the nature and mechanism of membrane-associated folding and insertion of biologically important peptides.
Leptosphaeria
rhodopsin: Bacteriorhodopsin-like proton pump from a eukaryote
Authors: Waschuk Stephen A. , Bezerra Arandi G. , Shi Lichi , Brown Leonid S.
Published: April 2005

Bacteriorhodopsin-like proteins provide archaea and eubacteria with a unique bioenergetic pathway comprising light-driven transmembrane proton translocation by a single retinal-binding protein. Recently, homologous proteins were found to perform photosensory functions in lower eukaryotes, but no active ion transport by eukaryotic rhodopsins was detected. By demonstrating light-driven proton pumping in a fungal rhodopsin from
Leptosphaeria maculans
, we present a case of a retinal-based proton transporter from a eukaryote. This result implies that in addition to oxidative phosphorylation and chlorophyll photosynthesis, some lower eukaryotes may have retained the archaeal route of building an electrochemical transmembrane gradient of protons.
A genetically defined mouse ovarian carcinoma model for the molecular characterization of pathway-targeted therapy and tumor resistance
Authors: Xing Deyin , Orsulic Sandra
Published: April 2005

Cell lines and tumors with defined genetic alterations provide ideal systems in which to test the molecular mechanisms of tumor sensitivity to pathway-targeted therapy. We have generated mouse ovarian epithelial tumor cell lines that contain various combinations of genetic alterations in the
p53
,
c-myc
, K-
ras
and
Akt
genes. Using both
in vitro
and
in vivo
approaches, we investigated the effect of rapamycin on cell proliferation, tumor growth, and the accumulation of peritoneal ascites. We demonstrated that rapamycin effectively inhibits the growth of tumors that rely on Akt signaling for proliferation, whereas tumors in which Akt signaling is not the driving force in proliferation are resistant to rapamycin. The introduction of activated
Akt
to the rapamycin-resistant cells does not render the cells susceptible to rapamycin if they can use alternative pathways for survival and proliferation. Accordingly, the rapamycin-sensitive tumors develop resistance to rapamycin when presented with alternative survival pathways, such as the mitogen-activated extracellular kinase signaling pathway. The combination of rapamycin and the mitogen-activated extracellular kinase inhibitor PD98059 is required to diminish proliferation in these cell lines. Our results indicate that mammalian target of rapamycin inhibitors may be effective in a subset of tumors that depend on Akt activity for survival but not effective in all tumors that exhibit Akt activation. Tumors with alternative survival pathways may require the inactivation of multiple individual pathways for successful treatment.
The Sca-1 cell surface marker enriches for a prostate-regenerating cell subpopulation that can initiate prostate tumorigenesis
Authors: Xin Li , Lawson Devon A. , Witte Owen N.
Published: April 2005

Sca-1 (stem cell antigen-1) enriches for murine prostate cells capable of regenerating tubular structures containing basal and luminal cell lineages in a dissociated cell prostate regeneration system. Sca-1
+
fractions are enriched for cells at the G
0
stage of the cell cycle, and Sca-1
+
cells cluster in the proximal region of prostatic tubules where replication-quiescent cells have been localized. Castration-induced enrichment for androgen-independent cells results in a concomitant enrichment for Sca-1
+
cells. Genetic perturbations of PTEN/AKT signaling in prostate-regenerating cells leads to the initiation of tumorigenesis, and cancer progression is associated with a dramatic increase in Sca-1
+
cells. Sca-1-enriched prostate-regenerating cells possess multiple stem/progenitor cell properties and can serve as targets for cancer initiation.
Representations of faces and body parts in macaque temporal cortex: A functional MRI study
Authors: Pinsk Mark A. , DeSimone Kevin , Moore Tirin , Gross Charles G. , Kastner Sabine
Published: April 2005

Human neuroimaging studies suggest that areas in temporal cortex respond preferentially to certain biologically relevant stimulus categories such as faces and bodies. Single-cell studies in monkeys have reported cells in inferior temporal cortex that respond selectively to faces, hands, and bodies but provide little evidence of large clusters of category-specific cells that would form “areas.” We probed the category selectivity of macaque temporal cortex for representations of monkey faces and monkey body parts relative to man-made objects using functional MRI in animals trained to fixate. Two face-selective areas were activated bilaterally in the posterior and anterior superior temporal sulcus exhibiting different degrees of category selectivity. The posterior face area was more extensively activated in the right hemisphere than in the left hemisphere. Immediately adjacent to the face areas, regions were activated bilaterally responding preferentially to body parts. Our findings suggest a category-selective organization for faces and body parts in macaque temporal cortex.
The Interaction of Human and Physical Capital Accumulation: Evidence from Sub‐Saharan Africa*
Authors: Grier Robin
Published: April 2005

In this paper, a simultaneous model of the evolution of human and physical capital in Sub‐Saharan Africa is estimated. It can be shown that the two types of capital are jointly endogenous, in that increases in human capital significantly raise the per‐worker physical capital stock, and increases in the physical capital significantly raise primary education levels. Unlike the implications of other recent papers, there is no evidence that tropical climates and ethnic diversity have a negative effect on the accumulation of capital in the region.
Analysis of vegetation distribution in Interior Alaska and sensitivity to climate change using a logistic regression approach
Authors: Calef Monika P. , David McGuire A. , Epstein Howard E. , Scott Rupp T. , Shugart Herman H.
Published: April 2005

Aim  To understand drivers of vegetation type distribution and sensitivity to climate change.

Location  Interior Alaska.

Methods  A logistic regression model was developed that predicts the potential equilibrium distribution of four major vegetation types: tundra, deciduous forest, black spruce forest and white spruce forest based on elevation, aspect, slope, drainage type, fire interval, average growing season temperature and total growing season precipitation. The model was run in three consecutive steps. The hierarchical logistic regression model was used to evaluate how scenarios of changes in temperature, precipitation and fire interval may influence the distribution of the four major vegetation types found in this region.

Results  At the first step, tundra was distinguished from forest, which was mostly driven by elevation, precipitation and south to north aspect. At the second step, forest was separated into deciduous and spruce forest, a distinction that was primarily driven by fire interval and elevation. At the third step, the identification of black vs. white spruce was driven mainly by fire interval and elevation. The model was verified for Interior Alaska, the region used to develop the model, where it predicted vegetation distribution among the steps with an accuracy of 60–83%. When the model was independently validated for north‐west Canada, it predicted vegetation distribution among the steps with an accuracy of 53–85%. Black spruce remains the dominant vegetation type under all scenarios, potentially expanding most under warming coupled with increasing fire interval. White spruce is clearly limited by moisture once average growing season temperatures exceeded a critical limit (+2 °C). Deciduous forests expand their range the most when any two of the following scenarios are combined: decreasing fire interval, warming and increasing precipitation. Tundra can be replaced by forest under warming but expands under precipitation increase.

Main conclusion  The model analyses agree with current knowledge of the responses of vegetation types to climate change and provide further insight into drivers of vegetation change.
Colonization of dynamic Mediterranean landscapes: where do birds come from after fire?
Authors: Brotons Lluís , Pons Pere , Herrando Sergi
Published: April 2005

Aim Two main mechanisms may explain post‐disturbance species colonization patterns of early successional habitats such as those originated by wildfires. First, post‐disturbance colonization is not limited by the dispersal ability of the species to reach the newly created open areas and, secondly, colonization is limited by dispersal. Under the first hypothesis, we expect, at a regional scale, to find similar post‐disturbance communities to develop on recently burned sites. However, colonization limited by dispersal will lead to strong between‐site variations in species composition.

Location To test these hypotheses, we studied the post‐fire colonization patterns of nine open‐habitat bird species in eight distantly located wildfires in the north‐eastern Iberian Peninsula.

Methods We censused post‐fire bird composition by means of field transects and identified potential colonization sources from species–habitat suitability maps derived from atlas data.

Results Our results showed strong significant differences in post‐fire species composition between burnt areas. Burnt areas located in areas with low probability of species presence before the fire event showed lower species occurrence and richness after the fire.

Main conclusions These results do not support the idea that early successional stages and open habitats have a homogeneous community structure at regional scales and suggest that dispersal is a key constraint determining bird colonization of post‐fire habitats. Further attention should be paid to landscape heterogeneity as a key factor in determining population dynamics of open‐habitat species in the light of current and future land‐use changes in Mediterranean regions.