bluuebunny/crossref_metadata_2025 huggingface dataset:
Seasonal variations of global lightning activity extracted from Schumann resonances using a genetic algorithm method
Authors: Yang Heng , Pasko Victor P. , Sátori Gabriella
Published: January 2009

A three‐dimensional Finite Difference Time Domain (FDTD) model of the Earth‐ionosphere cavity with a realistic conductivity profile is employed to study the global lightning activity using the observed intensity variations of Schumann resonances (SR). Comparison of the results derived from our FDTD model and the previous studies by other authors on related subjects shows that Schumann resonance is a good probe to indicate the seasonal variations of lightning activity in three main thunderstorm regions (Africa, southeast Asia, and South America). An inverse method based on genetic algorithms is developed to extract information on lightning intensity in these three regions from observed SR intensity data. Seasonal variations of the lightning activity in three thunderstorm centers are clearly observed in our results. Different SR frequency variations associated with seasonal variations of global lighting activity are also discussed.
Characterization of silicon-on-insulator films with pseudo-metal-oxide-semiconductor field-effect transistor: Correlation between contact pressure, crater morphology, and series resistance
Authors: Ionica I. , Savin I. , Van Den Daele W. , Nguyen T. , Mescot X. , Cristoloveanu S.
Published: January 2009

Pseudo-metal-oxide-semiconductor field-effect transistor (Ψ-MOSFET) is a simple and efficient technique for characterizing electrical properties of silicon-on-insulator wafers. The primary condition for reliable parameter extraction is to achieve ohmic contacts between Ψ-MOSFET probes and silicon film. This paper brings experimental arguments on the probe-pressure impact on contact nature, series resistance, and carrier mobility. The specificity of our study consists of the topographical analysis (by atomic force microscopy) of craters induced by the probes. We correlate the probe-pressure with morphology parameters (size of craters) and electrical parameters (series resistance, mobility). For higher pressures, contacts are improved, changing from Schottky-like to ohmic-like.
Effect of thermal vibrations on the resonant frequency of cantilever for scanning thermal microscopy nanomachining
Authors: Lee Haw-Long , Fang Te-Hua , Chang Win-Jin
Published: January 2009

The effect of thermal vibrations on modal frequencies of flexural vibration for a scanning thermal microscope (SThM) cantilever probe has been evaluated using the Timoshenko beam model, including the effects of rotary inertia and shear deformation, and an analytical expression for the frequencies of vibration modes has been obtained. According to this analysis, the thermal vibration effect makes the probe stiffer and leads to an increasing frequency in the nanomachining process. This phenomenon may result in a rough machined surface of the materials. The resonant frequency of vibration modes decreases as the length-to-thickness ratio of the probe increases. The mode shape decreases with increasing the operating temperature in SThM machining. In addition, the results based on Euler beam and Timoshenko beam models are compared. It can be found that the Timoshenko beam model is able to predict the frequencies of flexural vibration of the higher modes for the SThM cantilever probe nanomachining.
The effect of Mg doping on the thermoelectric performance of Zn4Sb3
Authors: Pedersen Birgitte L. , Birkedal Henrik , Nygren Mats , Frederiksen Poul Toft , Iversen Bo B.
Published: January 2009

Cation doping studies of high performance thermoelectric Zn4Sb3 have so far focused on elements with larger electronegativity than Zn. Mg has a lower electronegativity than Zn, and, in the present paper, the effect of Mg doping of Zn4Sb3 on phase transition temperatures and thermal and thermoelectric properties have been studied on five samples with a doping degree ranging from 0.1 to 2 at. % Mg. The results are compared with data obtained for a pure, undoped sample. Laboratory x-ray powder diffraction shows that all samples are single phase materials as further evidenced with higher confidence by Rietveld refinement of high resolution synchrotron powder diffraction of as synthesized 1 and 2 at. % Mg doped samples. Multitemperature synchrotron powder diffraction and differential scanning calorimetry reveal a significant effect of Mg-doping on the low temperature phase transitions in one sample (0.5 at. % Mg), whereas the other samples showed only small or no effect. Physical properties have been evaluated from 2–400 K for all samples. Some physical property parameters are affected by doping but no immediate improvement of ZT was achieved. The reason for this is discussed in relation to differences in sample compaction.
Generation and control of thoron emanated from lantern mantles
Authors: Sorimachi Atsuyuki , Sahoo Sarata Kumar , Tokonami Shinji
Published: January 2009

This paper describes the performance of a thoron (R220n) flowthrough source made of a commercially available lantern mantle. This R220n source is easy to construct and operate and has a negligible radon (R222n) gas generated when the air was passed through the source. We studied the R220n concentration generated from the lantern mantles in terms of the variability in concentration associated with both the total weight of the lantern mantles used in the source and the air flow rate used in the experimental chamber. We found that the concentration of R220n generated in air ranged from 0.9 to 150 kBq m−3 and exponentially depended on the absolute humidity. The R220n concentration increased linearly with increasing total weight of the lantern mantles, but variations in the rate of air flow passed through the source had no influence on the observed R220n concentration.
Virtual processes and super-radiance in spin-boson models
Authors: Alcalde M. Aparicio , Kullock R. , Svaiter N. F.
Published: January 2009

We consider spin-boson models composed by a single bosonic mode and an ensemble of N identical two-level atoms. The situation where the coupling between the bosonic mode and the atoms generates resonant and nonresonant processes is studied, where the whole system is in thermal equilibrium with a reservoir at temperature β−1. Phase transitions from ordinary fluorescence to super-radiant phase in three different models are investigated. First a model where the coupling between the bosonic mode and the jth atom is via the pseudo-spin-operator σ(j)z is studied. Second, we investigate the generalized Dicke model, introducing different coupling constants between the single mode bosonic-field and the environment, g1 and g2, for rotating and counter-rotating terms, respectively. Finally a modified version of the generalized Dicke model with intensity-dependent coupling in the rotating terms is considered. In the first model, for an arbitrary value for the coupling constant, the zero mode contributes in rendering the canonical entropy a negative quantity for low temperatures. For small coupling constants, the partition function is analytic for all temperatures. The last two models present phase transitions, even when only Hamiltonian terms which generate virtual processes are considered.
Quasiequilibrium states in thermotropic liquid crystals studied by multiple-quantum NMR
Authors: Buljubasich L. , Monti G. A. , Acosta R. H. , Bonin C. J. , González C. E. , Zamar R. C.
Published: January 2009

Previous work showed that by means of the Jeener–Broekaert (JB) experiment, two quasiequilibrium states can be selectively prepared in the proton spin system of thermotropic nematic liquid crystals (LCs) in a strong magnetic field. The similarity of the experimental results obtained in a variety of LC in a broad Larmor frequency range, with crystal hydrates, supports the assumption that also in LC the two spin reservoirs, into which the Zeeman order is transferred, originate in the dipolar energy and that they are associated with a separation in energy scales: A constant of motion related to the stronger dipolar interactions (S), and a second one (W) corresponding to the secular part of the weaker dipolar interactions with regard to the Zeeman and the strong dipolar part. We study the nature of these quasi-invariants in nematic 5CB (4′-pentyl-4-biphenyl-carbonitrile) and measure their relaxation times by encoding the multiple-quantum coherences of the states following the JB pulse pair on two orthogonal bases, Z and X. The experiments were also performed in powder adamantane at 301K which is used as a reference compound having only one dipolar quasi-invariant. We show that the evolution of the quantum states during the buildup of the quasiequilibrium state in 5CB prepared under the S condition is similar to the case of powder adamantane and that their quasiequilibrium density operators have the same tensor structure. In contrast, the second constant of motion, whose explicit operator form is not known, involves a richer composition of multiple-quantum coherences of even order on the X basis, in consistency with the truncation inherent in its definition. We exploited the exclusive presence of coherences of ±4,±6,±8, besides 0 and ±2 under the W condition to measure the spin-lattice relaxation time TW accurately, so avoiding experimental difficulties that usually impair dipolar order relaxation measurement such as Zeeman contamination at high fields and also superposition of the different quasi-invariants. This procedure opens the possibility of measuring the spin-lattice relaxation of a quasi-invariant independent of the Zeeman and S reservoirs, so incorporating a new relaxation parameter useful for studying the complex molecular dynamics in mesophases. In fact, we report the first measurement of TW in a LC at high magnetic fields. Comparison of the obtained value with the one corresponding to a lower field (16MHz) points out that the relaxation of the W-order strongly depends on the intensity of the external magnetic field, similarly to the case of the S reservoir, indicating that the relaxation of the W-quasi-invariant is also governed by the cooperative molecular motions.
Thermal cycling reliability of Cu/SnAg double-bump flip chip assemblies for 100 μm pitch applications
Authors: Son Ho-Young , Kim Ilho , Lee Soon-Bok , Jung Gi-Jo , Park Byung-Jin , Paik Kyung-Wook
Published: January 2009

A thick Cu column based double-bump flip chip structure is one of the promising alternatives for fine pitch flip chip applications. In this study, the thermal cycling (T/C) reliability of Cu/SnAg double-bump flip chip assemblies was investigated, and the failure mechanism was analyzed through the correlation of T/C test and the finite element analysis (FEA) results. After 1000 thermal cycles, T/C failures occurred at some Cu/SnAg bumps located at the edge and corner of chips. Scanning acoustic microscope analysis and scanning electron microscope observations indicated that the failure site was the Cu column/Si chip interface. It was identified by a FEA where the maximum stress concentration was located during T/C. During T/C, the Al pad between the Si chip and a Cu column bump was displaced due to thermomechanical stress. Based on the low cycle fatigue model, the accumulation of equivalent plastic strain resulted in thermal fatigue deformation of the Cu column bumps and ultimately reduced the thermal cycling lifetime. The maximum equivalent plastic strains of some bumps at the chip edge increased with an increased number of thermal cycles. However, equivalent plastic strains of the inner bumps did not increase regardless of the number of thermal cycles. In addition, the z-directional normal plastic strain ε22 was determined to be compressive and was a dominant component causing the plastic deformation of Cu/SnAg double bumps. As the number of thermal cycles increased, normal plastic strains in the perpendicular direction to the Si chip and shear strains were accumulated on the Cu column bumps at the chip edge at low temperature region. Thus it was found that the Al pad at the Si chip/Cu column interface underwent thermal fatigue deformation by compressive normal strain and the contact loss by displacement failure of the Al pad, the main T/C failure mode of the Cu/SnAg flip chip assembly, then occurred at the Si chip/Cu column interface shear strain deformation during T/C.
Investigation of the time evolution of Lane–Emden-type Kanai–Caldirola oscillator
Authors: Özeren Serhat F.
Published: January 2009

In this study, we have investigated the time evolution of the first kind Lane–Emden-type Kanai–Caldirola oscillator. The total energy of the one dimensional classical harmonic oscillator with time-dependent mass is obtained. Then by using the SU(1,1) coherent states, the classical equation of motion of the oscillator has been analyzed and the effects of increasing mass have been clearly illustrated in the figures.
Edgeworth expansion of the largest eigenvalue distribution function of Gaussian orthogonal ensemble
Authors: Choup Leonard N.
Published: January 2009

In this paper we focus on the large n probability distribution function of the largest eigenvalue in the Gaussian orthogonal ensemble of n×n matrices (GOEn). We prove an Edgeworth-type theorem for the largest eigenvalue probability distribution function of GOEn. The correction terms to the limiting probability distribution are expressed in terms of the same Painlevé II functions appearing in the Tracy–Widom distribution. We conclude with a brief discussion of the GSEn case.
Spectroscopic study of hydrogen Balmer lines in a microwave-induced discharge
Authors: Jovićević S. , Sakan N. , Ivković M. , Konjević N.
Published: January 2009

We present the results of a spectroscopic study of hydrogen Balmer line shapes and intensities in a microwave-induced discharge generated in an Evenson resonant cavity at 2.45 GHz in pure hydrogen and hydrogen-argon and hydrogen-helium gas mixtures. The Balmer line shapes do not show signs of excessive Doppler broadening. The discharge parameters—Doppler temperature (980 K), electron excitation temperature (1460–1790 K), and electron density of about 8×1011 cm−3—point to a typical microwave-induced discharge.
Structural and electronic properties of identical-sized Zn nanoclusters grown on Si(111)-(7×7) surfaces
Authors: Zhou Changjie , Xue Qikun , Jia Jinfeng , Zhan Huahan , Kang Junyong
Published: January 2009

Identical-sized Zn nanoclusters have been grown on Si(111)-(7×7) surfaces at room temperature. In situ scanning tunneling microscopy (STM) studies and first-principles total energy calculations show that room-temperature grown Zn nanoclusters tend to form the seven-Zn-atom structure with one excess Zn atom occupying characteristically the center of the cluster. The evolution of the surface electronic structures measured by scanning tunneling spectroscopy reveals that the formation of Zn nanoclusters is responsible for the saturation of the metallic Si adatom dangling bond states at about −0.3 and +0.5 V and causes the semiconducting characteristics of the nanoclusters. Furthermore, the Zn nanocluster in a faulted half unit cell empties the filled surface dangling bond state of the closest edge Si adatoms in the nearest neighboring uncovered unfaulted half unit cells at about −0.3 V, leading to the suppressed height of the closest edge Si adatoms in the filled-state STM images.
The driving force of channel formation in triheteropolymers confined in nanocylindrical tubes
Authors: Chen Houyang , Ruckenstein Eli
Published: January 2009

The driving force for channel formation by triheteropolymers confined in hydrophobic nanocylinders was investigated by lattice Monte Carlo simulations. A variety of conditions, such as the interactions between different kinds of segments, the interactions between segments and the nanocylindrical walls, and the structure of the polymer, were considered. It was found that for a channel to be generated, the attractive interaction between one kind of segments and the nanocylindrical wall should be larger than a minimum value. The cross-sectional radius of a channel seen from above or below the ends of the nanotube as a function of the interactions between the segments and the wall as well as of the interactions between different kinds of segments was determined. The succession in the chain of different kinds of segments (the chain connectivity) also plays a significant role in channel formation. The present treatment may provide some insight regarding the formation of protein channels in the phospholipid bilayers of the cell membrane.
Finding elementary first integrals for rational second order ordinary differential equations
Authors: Duarte L. G. S. , da Mota L. A. C. P.
Published: January 2009

Here we present a semialgorithm to find elementary first integrals of a class of rational second order ordinary differential equations. The method is based on a Darboux-type procedure and it is an attempt to construct an analogous of the method built by Prelle and Singer [“Elementary first integral of differential equations,” Trans. Am. Math. Soc. 279, 215 (1983)] for rational first order ordinary differential equations.
Nitrogen diffusion and interaction with dislocations in single-crystal silicon
Authors: Alpass C. R. , Murphy J. D. , Falster R. J. , Wilshaw P. R.
Published: January 2009

The results of dislocation unlocking experiments are reported. The stress required to unpin a dislocation from nitrogen impurities in nitrogen-doped float-zone silicon (NFZ-Si) and from oxygen impurities in Czochralski silicon (Cz-Si) is measured, as a function of the unlocking duration. It is found that unlocking stress drops with increasing unlocking time in all materials tested. Analysis of these results indicates that dislocation locking by nitrogen in NFZ-Si is by an atomic species, with a similar locking strength per atom to that previously deduced for oxygen atoms in Cz-Si. Other experiments measure dislocation unlocking stress at 550 °C in NFZ-Si annealed at 500–1050 °C. The results allow an effective diffusivity of nitrogen in silicon at 500–750 °C to be inferred, with an activation energy of 3.24 eV and a diffusivity prefactor of approximately 200 000 cm2 s−1. This effective diffusivity is consistent with previous measurements made at higher temperatures using secondary ion mass spectrometry. When the results are analyzed in terms of a monomer-dimer dissociative mechanism, a nitrogen monomer diffusivity with an activation energy in the range of 1.1–1.4 eV is inferred. The data also show that the saturation dislocation unlocking stress measured at 550 °C in NFZ-Si is dependent on the anneal temperature, peaking at 600–700 °C and falling toward zero at 1000 °C.
Large eddy simulation of a swirling transverse jet into a crossflow with investigation of scalar transport
Authors: Denev Jordan A. , Fröhlich Jochen , Bockhorn Henning
Published: January 2009

The flow field of a turbulent jet emerging from a straight round pipe into a laminar crossflow is investigated by means of large eddy simulations. The concentration of a passive scalar, introduced with the jet, is calculated in order to quantify the mixing of the jet and the crossflow. In the jet, swirl is introduced by means of body forces and a range of jet swirl numbers from S=0 up to S=0.6 is studied. The impact of the jet swirl on the flow field, on the coherent structures, and on the mixing efficiency is investigated and quantified by means of various analyses. It is found that for all swirl numbers larger than zero a clear asymmetry appears in all quantities studied. Additional to the two hanging vortices at both sides of the jet a third vortex is introduced by the swirling pipe flow which interacts with the former. This feature is described in detail as it is not mentioned in the literature. For the strongest swirl investigated a recirculation zone near the jet exit is observed. Despite the asymmetry and even with a recirculation zone at the outlet, the counter-rotating vortex pair still exists in all cases in the downstream flow, where it entrains a large amount of crossflow fluid into the jet. The near field, however, is altered by the jet swirl in several respects. The jet more and more approaches the bottom wall with increasing swirl. As a result, the entrainment is gradually attenuated due to the larger blocking of the secondary flow by the wall. Increased swirl increases both the turbulent kinetic energy in the pipe and the vorticity of the average flow field near the jet exit, and thus stimulates the mixing in these regions. However, this stimulating effect is overwhelmed by the closer position of the jet trajectory to the wall of the channel with increasing swirl, which in turn reduces entrainment of fresh crossflow fluid into the jet. As a final result of these two competing effects, the overall mixing efficiency of a jet into a crossflow is merely unchanged with the addition of swirl. Various mixing indices, both spatial and temporal, are used for this analysis. Their respective advantages and disadvantages are discussed and detailed illustrations provide a sound understanding of their behavior.
Influence of nitrogen on the growth and luminescence of silicon nanocrystals embedded in silica
Authors: Bolduc M. , Genard G. , Yedji M. , Barba D. , Martin F. , Terwagne G. , Ross G. G.
Published: January 2009

Silicon nanocrystals (Si-ncs) have been produced by implantation of Si+ in excess into SiO2 followed by both annealing and passivation using argon or nitrogen. Nitrogen increases the photoluminescence (PL) emission and shifts the spectra toward the blue. The measured Si-nc diameter is 4.3 and 3.8 nm after annealing performed under Ar and N2, respectively. A significant quantity of nitrogen atoms has been detected in all samples by resonant nuclear reaction analysis (RNRA). The nitrogen concentration is significantly higher when the annealing and passivation are performed in a nitrogen environment, in agreement with a larger Si–N vibration signal on the Raman spectra. The depth profiles of nitrogen are very similar to those of Si-nc, suggesting that the N2 molecules may diffuse in the SiO2 during the annealing and then are trapped in proximity to the Si-nc. In addition to Si+, the implantation of N2+ to concentrations of 3 and 6 at. % produced a decrease in the PL intensity (accentuated at the higher concentration) and an increase in the Raman signal associated to Si–N vibrations. These results suggest that a relatively low nitrogen atomic fraction enhances the PL emission, since a large nitrogen concentration impedes the formation of Si-nc thus significantly decreasing the PL intensity.
A theoretical study of linear beryllium chains: Full configuration interaction
Authors: Vetere Valentina , Monari Antonio , Scemama Anthony , Bendazzoli Gian Luigi , Evangelisti Stefano
Published: January 2009

We present a full configuration interaction study of BeN (N=2,3,4,5) linear chains. A comparative study of the basis-set effect on the reproduction of the energy profile has been reported. In particular, the 3s1p, 4s2p, 4s2p1d, 5s3p2d, and 5s3p2d1f bases were selected. For the smallest chains (i.e., Be2 and Be3), smaller basis sets give dissociative energy profiles, so large basis set is demanded for the reproduction of equilibrium minima in the structures. For Be4 and Be5 linear chains, the energy profiles show a minimum also by using the smallest basis sets, but the largest ones give a much stronger stabilization energy. For all the structures, two spin states have been studied: the singlet and the triplet. It is shown that the energy separation of the two states, in the equilibrium region, is small and decays exponentially with respect to the number of atoms in the chain. Finally an interpolative technique allowing for the estimation of the long-chain parameters from shorter ones is presented.
Plasmonic enhancement of emission from Si-nanocrystals
Authors: Gong Yiyang , Lu Jesse , Cheng Szu-Lin , Nishi Yoshio , Vučković Jelena
Published: January 2009

Plasmonic gratings of different periodicities are fabricated on top of silicon nanocrystals embedded in silicon dioxide. Total enhancements of up to 2 were observed, which matches the value from simulations. Plasmonic enhancements are observed for the first three orders of the plasmonic modes, with the peak enhancement wavelength varying with the periodicity. Biharmonic gratings are also fabricated to extract the enhanced emission from the first order plasmonic mode, resulting in enhancements with quality factors of up to 16.
Dynamic measurements of ultraviolet-enhanced silica contamination by photoluminescence-based diagnostic
Authors: Pereira Alexandre , Quesnel Etienne , Reymermier Maryse
Published: January 2009

The ultraviolet-enhanced (UV-enhanced) contamination of optical components leads to an untimely aging of sealed laser systems, photolithography, and synchrotron installations. The laser-induced deposition of organic films on silica substrates and coatings significantly reduces their transmission and degrades their optical functions. In this paper, measurements of organic contaminant films growth under 213 nm laser irradiation performed on silica Corning 7980 grade ArF are reported. We present an in situ contaminant layer growth diagnostic based on silica photoluminescence measurements. The purpose was to determine the photodeposition kinetics as a function of controlled environmental conditions and fluence and to find out the experimental conditions in which the growth of contamination films was significantly reduced. We then demonstrated that with a low partial pressure of oxygen, the growth of carbonaceous films is drastically reduced during UV laser irradiation whereas with water and nitrogen it was not the case. We also proposed a physical modeling of the UV-enhanced silica contamination processes.
Fast detector of the ellipticity of infrared and terahertz radiation based on HgTe quantum well structures
Authors: Danilov S. N. , Wittmann B. , Olbrich P. , Eder W. , Prettl W. , Golub L. E. , Beregulin E. V. , Kvon Z. D. , Mikhailov N. N. , Dvoretsky S. A. , Shalygin V. A. , Vinh N. Q. , van der Meer A. F. G. , Murdin B. , Ganichev S. D.
Published: January 2009

We report a fast, room temperature detection scheme for the polarization ellipticity of laser radiation, with a bandwidth that stretches from the infrared to the terahertz range. The device consists of two elements, one in front of the other, that detect the polarization ellipticity and the azimuthal angle of the ellipse. The elements, respectively, utilize the circular photogalvanic effect in a narrow gap semiconductor and the linear photogalvanic effect in a bulk piezoelectric semiconductor. For the former we characterized both a HgTe quantum well and bulk Te, and for the latter, bulk GaAs. In contrast with optical methods we propose is an easy to handle all-electric approach, which is demonstrated by applying a large number of different lasers from low power, continuous wave systems to high power, pulsed sources.
Reduction in efficiency droop, forward voltage, ideality factor, and wavelength shift in polarization-matched GaInN/GaInN multi-quantum-well light-emitting diodes
Authors: Xu Jiuru , Schubert Martin F. , Noemaun Ahmed N. , Zhu Di , Kim Jong Kyu , Schubert E. Fred , Kim Min Ho , Chung Hun Jae , Yoon Sukho , Sone Cheolsoo , Park Yongjo
Published: January 2009

Blue light-emitting diodes (LEDs) with polarization-matched GaInN/GaInN multi-quantum-well (MQW) active regions are grown by metal-organic vapor-phase epitaxy. The GaInN/GaInN MQW structure reduces the magnitude of polarization sheet charges at heterointerfaces in the active region. The GaInN/GaInN MQW LEDs are shown to have enhanced light-output power, reduced efficiency droop, a lower forward voltage, a smaller diode ideality factor, and decreased wavelength shift, compared with conventional GaInN/GaN MQW LEDs.
Correlation of on-state conductance with referenced electrochemical potential in ion gel gated polymer transistors
Authors: Xia Yu , Cho JeongHo , Paulsen Bryan , Frisbie C. Daniel , Renn Michael J.
Published: January 2009

We report direct measurement of the electrochemical potential at organic semiconductor/gate dielectric interfaces in printed polymer transistors employing a gel electrolyte as the gate insulator. An oxidized silver wire reference electrode was embedded into the gel electrolyte, and its potential relative to the grounded source contact was measured simultaneously with the transistor transfer characteristics. The referenced turn-on voltages of transistors based on three common polymer semiconductors [(poly-3-hexylthiophene, poly(3,3‴-didodecylquaterthiophene), and poly(9,9′-dioctylfluorene-co-bithiophene)] were found to correlate with the reported highest occupied molecular orbital levels (ionization potentials) for these materials. Further, analysis of the transfer characteristics revealed a negative differential transconductance regime at high gate-induced carrier densities (∼1015 cm−2), which we attribute to a combination of band filling and a mobility lowering effect.
Two-photon patterning of a polymer containing Y-shaped azochromophores
Authors: Ambrosio A. , Orabona E. , Maddalena P. , Camposeo A. , Polo M. , Neves A. A. R. , Pisignano D. , Carella A. , Borbone F. , Roviello A.
Published: January 2009

We report on the patterning of the free surface of azo-based polymer films by means of mass migration driven by one- or two-photon absorption. A symmetric donor-acceptor-donor structured Y-shaped azochromophore is specifically synthesized to enhance two-photon absorption in the polymer. The exposure of the polymer film to a focused laser beam results in light-driven mass migration for both one- and two-photon absorptions. Features with subdiffraction resolution (250 nm) are realized and the patterning dynamics is investigated as a function of the light dose. Furthermore, functional photonic structures, such as diffraction gratings with periods ranging between 0.5 and 2.0 μm, have been realized.
Organic semiconductor composites: Influence of additives on the transient photocurrent
Authors: Day Jonathan , Platt Andrew D. , Ostroverkhova Oksana , Subramanian Sankar , Anthony John E.
Published: January 2009

We report on the effect of various guest molecules added to a functionalized anthradithiophene (ADT) host on photoexcited charge carrier dynamics in solution-deposited thin films, from ∼100 ps to >100 μs after photoexcitation with 100 fs laser pulses. An addition of 2 and 5 wt % of C60 to a fluorinated ADT derivative, ADT-TES-F, resulted in transient photocurrent amplitude enhancement by a factor of ∼3 and 10, respectively. In contrast, an addition of 10 wt % of another ADT derivative, ADT-TIPS-CN, or a functionalized pentacene derivative (TIPS pentacene) to ADT-TES-F led to a decrease in the amplitude by a factor of ∼3–4 and dramatically different photocurrent dynamics.
Monoclinic optical constants, birefringence, and dichroism of slanted titanium nanocolumns determined by generalized ellipsometry
Authors: Schmidt Daniel , Booso Benjamin , Hofmann Tino , Schubert Eva , Sarangan Andrew , Schubert Mathias
Published: January 2009

Generalized spectroscopic ellipsometry determines the principal monoclinic optical constants of thin films consisting of slanted titanium nanocolumns deposited by glancing angle deposition under 85° incidence and tilted from the surface normal by 47°. Form birefringence measured for wavelengths from 500 to 1000 nm renders the Ti nanocolumns monoclinic absorbing crystals with c-axis along the nanocolumns, b-axis parallel to the film interface, and 67.5° monoclinic angle between the a- and c-axes. The columnar thin film reveals anomalous optical dispersion, extreme birefringence, strong dichroism, and differs completely from bulk titanium. Characteristic bulk interband transitions are absent in the spectral range investigated.
Three point method to characterize low-pressure electronegative discharges using electrostatic probe
Authors: Kang S. Y. , Chung T. H. , Chung K.-S.
Published: January 2009

Electrostatic probe measurements for low-pressure inductively coupled SF6 plasmas are performed. From the current-voltage (I-V) curves of probe, the saturation currents of the positive ions and electrons and the electron temperature are measured. The electronegativity and the negative ion density are deduced by using the ratios of these parameters at three adjacent pressure points. The positive ion density is calculated by the orbital-motion-limited theory, and the electron temperatures are given either by the slope of the I-V curves or by the electron energy distribution function with the second derivative of I-V curves. The variations in the charged species density with pressure and power are investigated.
A lightweight balloon-carried cloud charge sensor
Authors: Nicoll K. A. , Harrison R. G.
Published: January 2009

Despite the importance of microphysical cloud processes on the climate system, some topics are under-explored. For example, few measurements of droplet charges in nonthunderstorm clouds exist. Balloon carried charge sensors can be used to provide new measurements. A charge sensor is described for use with meteorological balloons, which has been tested over a range of atmospheric temperatures from −60 to 20 °C, in cloudy and clear air. The rapid time response of the sensor (to >10 V s−1) permits charge densities from 100 fC m−3 to 1 nC m−3 to be determined, which is sufficient for it to act as a cloud edge charge detector at weakly charged horizontal cloud boundaries.
Laminated, microfluidic-integrated carbon nanotube based biosensors
Authors: Tey Ju Nie , Wijaya I Putu Mahendra , Wang Zongbin , Goh Wei Hau , Palaniappan Alagappan , Mhaisalkar Subodh G. , Rodriguez Isabel , Dunham Simon , Rogers John A.
Published: January 2009

In this communication, a laminated, flexible, microfluidic-integrated, all CNT based liquid-gated transistor and biosensor are reported that comprises single walled CNTs for both the semiconducting channel as well as the contact electrodes. The proposed architecture eliminates the need for lithography, electrode definition processes, and also circumvents substrate surface compatibility issues. Real-time detection of 1 pM poly-L-lysine in a liquid-gated transistor comprising only two materials, single walled CNTs and polydimethoxysilane substrate with microfluidic channel, is demonstrated.
Specific contact resistance at metal/carbon nanotube interfaces
Authors: Jackson Roderick , Graham Samuel
Published: January 2009

In this report, the specific contact resistance between a thin film single wall carbon nanotube electrode and a deposited silver contact was measured. The specific contact resistance was found to be 20 mΩ cm2, which is an order of magnitude higher than typically observed in standard Si photovoltaic technology. We demonstrate that when utilized as the transparent anode in organic photovoltaics, the specific contact resistance has the potential to induce non-negligible resistive power losses. Thus, specific contact resistance will adversely affect the performance of these systems and should therefore be addressed.
Theory of an electrically injected bulk polariton laser
Authors: Solnyshkov Dmitry , Petrolati Eleonora , Di Carlo Aldo , Malpuech Guillaume
Published: January 2009

We present the first simulation of an electrically injected polariton laser device based on Bose–Einstein condensation of exciton polaritons in a bulk GaN microcavity. Our simulator self-consistently describes carrier flow, exciton formation and dissociation, and relaxation of exciton polaritons toward the ground state. We find a density-current threshold of about 50 A/cm2 at room temperature, which could be further improved by using polariton lasers based on quantum wells.
The formation of light emitting cerium silicates in cerium-doped silicon oxides
Authors: Li Jing , Zalloum Othman , Roschuk Tyler , Heng Chenglin , Wojcik Jacek , Mascher Peter
Published: January 2009

Cerium-doped silicon oxides with cerium concentrations of up to 0.9 at. % were deposited by electron cyclotron resonance plasma enhanced chemical vapor deposition. Bright cerium related photoluminescence, easily seen even under room lighting conditions, was observed from the films and found to be sensitive to film composition and annealing temperature. The film containing 0.9 at. % Ce subjected to anneal in N2 at 1200 °C for 3 h showed the most intense cerium-related emission, easily visible under bright room lighting conditions. This is attributed to the formation of cerium silicate [Ce2Si2O7 or Ce4.667 (SiO4)3O], the presence of which was confirmed by high resolution transmission electron microscopy.
Enhancement of spin-asymmetry by L21-ordering in Co2MnSi/Cr/Co2MnSi current-perpendicular-to-plane magnetoresistance devices
Authors: Sakuraba Y. , Iwase T. , Saito K. , Mitani S. , Takanashi K.
Published: January 2009

Co 2 MnSi / Cr / Co 2 MnSi (001)-fully epitaxial current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices were fabricated via an UHV magnetron sputtering system. The relationship between the degree of chemical ordering in Co2MnSi (CMS) and the CPP-GMR characteristics was investigated systematically against the annealing temperature of the devices. X-ray diffraction profiles and reflection high-energy electron diffraction images indicated that annealing improved L21-ordering. The MR ratio also increased upon annealing and the maximum MR ratio of 5.2% and ΔRA of 6.5 mΩ μm2 were achieved by annealing at 400 °C. These results indicate that promoting the degree of L21-ordering in CMS enhances the bulk and/or interface spin-asymmetry coefficients.
Location of KCNE1 relative to KCNQ1 in the I
KS
potassium channel by disulfide cross-linking of substituted cysteines
Authors: Chung David Y. , Chan Priscilla J. , Bankston John R. , Yang Lin , Liu Guoxia , Marx Steven O. , Karlin Arthur , Kass Robert S.
Published: January 2009

The cardiac-delayed rectifier K
+
current (I
KS
) is carried by a complex of KCNQ1 (Q1) subunits, containing the voltage-sensor domains and the pore, and auxiliary KCNE1 (E1) subunits, required for the characteristic I
KS
voltage dependence and kinetics. To locate the transmembrane helix of E1 (E1-TM) relative to the Q1 TM helices (S1–S6), we mutated, one at a time, the first four residues flanking the extracellular ends of S1–S6 and E1-TM to Cys, coexpressed all combinations of Q1 and E1 Cys-substituted mutants in CHO cells, and determined the extents of spontaneous disulfide-bond formation. Cys-flanking E1-TM readily formed disulfides with Cys-flanking S1 and S6, much less so with the S3-S4 linker, and not at all with S2 or S5. These results imply that the extracellular flank of the E1-TM is located between S1 and S6 on different subunits of Q1. The salient functional effects of selected cross-links were as follows. A disulfide from E1 K41C to S1 I145C strongly slowed deactivation, and one from E1 L42C to S6 V324C eliminated deactivation. Given that E1-TM is between S1 and S6 and that K41C and L42C are likely to point approximately oppositely, these two cross-links are likely to favor similar axial rotations of E1-TM. In the opposite orientation, a disulfide from E1 K41C to S6 V324C slightly slowed activation, and one from E1 L42C to S1 I145C slightly speeded deactivation. Thus, the first E1 orientation strongly favors the open state, while the approximately opposite orientation favors the closed state.
A distinct H2A.X isoform is enriched inXenopus laeviseggs and early embryos and is phosphorylated in the absence of a checkpoint
Authors: Shechter David , Chitta Raghu K. , Xiao Andrew , Shabanowitz Jeffrey , Hunt Donald F. , Allis C. David
Published: January 2009

Histone H2A.X is an H2A variant present in multicellular organisms that is specifically phosphorylated on the serine in the C-terminal consensus sequence, canonically “SQEY,” in response to DNA damage. We have recently shown the significance of phosphorylation of the penultimate tyrosine for maintenance and processing of the DNA damage response in mammalian cells. Here, we report the identification of distinct H2A.X variants in the eggs and early embryos of the frogXenopus laevisthat contain a C-terminal SQEF, among other changes; we have denoted these proteins as “H2A.X-F.” H2A.X-F is present only in late-staged oocytes, eggs, and premidblastula transition embryos and is not present in somatic cells. Similar unannotated isoforms were identified in other rapidly developing aquatic species, such asXenopus tropicalis, goldfish, and zebrafish, and inArabidopsisand chickpea. Furthermore, we demonstrate by mass spectrometry and phospho-specific antibodies that H2A.X-F is phosphorylated in the absence of exogenous DNA damage, in both actively dividing, unperturbed embryos and cell-free egg extract in the absence and presence of DNA damage and S-phase checkpoint conditions. We propose that this isoform may be involved in modulating the cellular response to the rapid early cell cycles in externally developing species.
Robust production and passaging of infectious HPV in squamous epithelium of primary human keratinocytes
Authors: Wang Hsu-Kun , Duffy Aaron A. , Broker Thomas R. , Chow Louise T.
Published: January 2009

Using Cre-loxP-mediated recombination, we established a highly efficient and reproducible system that generates autonomous HPV-18 genomes in primary human keratinocytes (PHKs), the organotypic raft cultures of which recapitulated a robust productive program. While E7 promoted S-phase re-entry in numerous suprabasal differentiated cells, HPV DNA unexpectedly amplified following a prolonged G2 arrest in mid- and upper spinous cells. As viral DNA levels intensified, E7 activity diminished and then extinguished. These cells then exited the cell cycle to undergo virion morphogenesis. High titers of progeny virus generated an indistinguishable productive infection in naïve PHK raft cultures as before, never before achieved until now. An immortalization-defective HPV-18 E6 mutant genome was also characterized for the first time. Numerous cells accumulated p53 protein, without inducing apoptosis, but the productive program was severely curtailed. Complementation of mutant genomes by E6-expressing retrovirus restored proper degradation of p53 as well as viral DNA amplification and L1 production. This system will be invaluable for HPV genetic dissection and serves as a faithful ex vivo model for investigating infections and interventions.
Making synaptic plasticity and memory last: mechanisms of translational regulation
Authors: Richter Joel D. , Klann Eric
Published: January 2009

Synaptic transmission in neurons is a measure of communication at synapses, the points of contact between axons and dendrites. The magnitude of synaptic transmission is a reflection of the strength of these synaptic connections, which in turn can be altered by the frequency with which the synapses are stimulated, the arrival of stimuli from other neurons in the appropriate temporal window, and by neurotrophic factors and neuromodulators. The ability of synapses to undergo lasting biochemical and morphological changes in response to these types of stimuli and neuromodulators is known as synaptic plasticity, which likely forms the cellular basis for learning and memory, although the relationship between any one form synaptic plasticity and a particular type of memory is unclear. RNA metabolism, particularly translational control at or near the synapse, is one process that controls long-lasting synaptic plasticity and, by extension, several types of memory formation and consolidation. Here, we review recent studies that reflect the importance and challenges of investigating the role of mRNA translation in synaptic plasticity and memory formation.
Transcription is required for establishment of germline methylation marks at imprinted genes
Authors: Chotalia Mita , Smallwood Sebastien A. , Ruf Nico , Dawson Claire , Lucifero Diana , Frontera Marga , James Katherine , Dean Wendy , Kelsey Gavin
Published: January 2009

Genomic imprinting requires the differential marking by DNA methylation of genes in male and female gametes. In the female germline, acquisition of methylation imprint marks depends upon the de novo methyltransferase Dnmt3a and its cofactor Dnmt3L, but the reasons why specific sequences are targets for Dnmt3a and Dnmt3L are still poorly understood. Here, we investigate the role of transcription in establishing maternal germline methylation marks. We show that at the Gnas locus, truncating transcripts from the furthest upstream Nesp promoter disrupts oocyte-derived methylation of the differentially methylated regions (DMRs). Transcription through DMRs in oocytes is not restricted to this locus but occurs across the prospective DMRs at many other maternally marked imprinted domains, suggesting a common requirement for transcription events. The transcripts implicated here in gametic methylation are protein-coding, in contrast to the noncoding antisense transcripts involved in the monoallelic silencing of imprinted genes in somatic tissues, although they often initiate from alternative promoters in oocytes. We propose that transcription is a third essential component of the de novo methylation system, which includes optimal CpG spacing and histone modifications, and may be required to create or maintain open chromatin domains to allow the methylation complex access to its preferred targets.
Mini-beam collimator enables microcrystallography experiments on standard beamlines
Authors: Fischetti Robert F. , Xu Shenglan , Yoder Derek W. , Becker Michael , Nagarajan Venugopalan , Sanishvili Ruslan , Hilgart Mark C. , Stepanov Sergey , Makarov Oleg , Smith Janet L.
Published: January 2009

The high-brilliance X-ray beams from undulator sources at third-generation synchrotron facilities are excellent tools for solving crystal structures of important and challenging biological macromolecules and complexes. However, many of the most important structural targets yield crystals that are too small or too inhomogeneous for a `standard' beam from an undulator source, ∼25–50 µm (FWHM) in the vertical and 50–100 µm in the horizontal direction. Although many synchrotron facilities have microfocus beamlines for other applications, this capability for macromolecular crystallography was pioneered at ID-13 of the ESRF. The National Institute of General Medical Sciences and National Cancer Institute Collaborative Access Team (GM/CA-CAT) dual canted undulator beamlines at the APS deliver high-intensity focused beams with a minimum focal size of 20 µm × 65 µm at the sample position. To meet growing user demand for beams to study samples of 10 µm or less, a `mini-beam' apparatus was developed that conditions the focused beam to either 5 µm or 10 µm (FWHM) diameter with high intensity. The mini-beam has a symmetric Gaussian shape in both the horizontal and vertical directions, and reduces the vertical divergence of the focused beam by 25%. Significant reduction in background was achieved by implementation of both forward- and back-scatter guards. A unique triple-collimator apparatus, which has been in routine use on both undulator beamlines since February 2008, allows users to rapidly interchange the focused beam and conditioned mini-beams of two sizes with a single mouse click. The device and the beam are stable over many hours of routine operation. The rapid-exchange capability has greatly facilitated sample screening and resulted in several structures that could not have been obtained with the larger focused beam.
I Am All about the Future World: Cambodian Children’s Views on Their Status as Orphans
Authors: Emond Ruth
Published: January 2009

The dominant representation of children living in majority world orphanages highlights their vulnerability and fragility. However, little is known about their lived experiences of orphanage care and their perspectives on being regarded as ‘orphans’. This article draws on data from a pilot project undertaken in one orphanage in Cambodia to highlight such perspectives. It presents the three discourses the children themselves used to make sense of their status; the orphan as becoming, as powerless and finally as ‘lucky’. These findings draw attention to the complex nature of the orphan identity and the juxtaposed discourses of orphan care that the children employ.