bluuebunny/crossref_metadata_2025 huggingface dataset:
A Uniform Geometrical Theory of Diffraction for predicting fields of sources near or on thin planar positive/negative material discontinuities
Authors: Lertwiriyaprapa T. , Pathak P. H. , Volakis J. L.
Published: December 2007

Relatively simple and accurate closed form Uniform Geometrical Theory of Diffraction (UTD) solutions are obtained for describing the radiated and surface wave fields, respectively, which are excited by sources near or on thin, planar, canonical two‐dimensional (2‐D) double positive/double negative (DPS/DNG) material discontinuities. Unlike most previous works, which analyze the plane wave scattering by such DPS structures via the Wiener‐Hopf (W‐H) or Maliuzhinets methods, the present development can also treat problems of the radiation by and coupling between antennas near or on finite material coatings on large metallic platforms. The latter is made possible mainly through the introduction of important higher‐order UTD slope diffraction terms which are developed here in addition to first‐order UTD. The present solutions are simpler to use because, in part, they do not contain the complicated split functions of the W‐H solutions nor the complex Maliuzhinets functions. Unlike the latter methods based on approximate boundary conditions, the present solutions, which are developed via a heuristic spectral synthesis approach, recover the proper local plane wave Fresnel reflection and transmission coefficients and surface wave constants of the DPS/DNG material. They also include the presence of backward surface waves in DNG media. Besides being asymptotic solutions of the wave equation, the present UTD diffracted fields satisfy reciprocity, the radiation condition, boundary conditions on the conductor, and the Karp‐Karal lemma which dictates that the first‐order UTD space waves vanish on a material interface.
A Modified Mini-Stroke Model with Region-Directed Reperfusion in Rat Cortex
Authors: Luo Weihua , Wang Zhen , Li Pengcheng , Zeng Shaoqun , Luo Qingming
Published: December 2007

Mini-ischemia localized into a specific brain area has promoted understanding of the mechanisms underlying brain recovery in stroke. However, the conventional mini-stroke model adopted permanent arterial ligations but lacked controllable reperfusion, which is crucial for the outcome of delayed functional recovery. In this study, we devised a new rat mini-stroke model in which the vascular ligations can be easily reversed to induce targeted reperfusion. Specifically, a flexible ring was incorporated into the conventional small arterial ligations to tighten the ligating loops and facilitate cutting the ligatures for sufficient reperfusion afterwards. The distribution of cerebral blood flow was explored directly through a cranial window using laser speckle contrast imaging. A distinct ischemic core, which well fits the profile of the ligated ring, was bordered by a penumbral zone and then together surrounded by nonischemic tissue immediately after the arterial ligations involving the ring. After cutting the ligatures, post-recanalization hyperperfusion occurred in the previous ischemic core and to a greater extent at 24 h after reperfusion. In contrast, recirculation of common carotid artery in the conventional mini-stroke model hardly altered hypoperfusion status within the ischemic core. Evidence from two kinds of control groups indicated that the ring might produce a compression effect on the underlying cortex and then contribute to the more highly localized infarct that was identified by triphenyltetrazolium chloride staining. Our data suggest that this model provides opportunities for investigating the neurovascular dynamics in acute stroke and rehabilitation, especially with emerging optical imaging techniques.
Mechanistic Role of Calpains in Postischemic Neurodegeneration
Authors: Bevers Matthew B , Neumar Robert W
Published: December 2007

The calpain family of proteases is causally linked to postischemic neurodegeneration. However, the precise mechanisms by which calpains contribute to postischemic neuronal death have not been fully elucidated. This review outlines the key features of the calpain system, and the evidence for its causal role in postischemic neuronal pathology. Furthermore, the consequences of specific calpain substrate cleavage at various subcellular locations are explored. Calpain substrates within synapses, plasma membrane, endoplasmic reticulum, lysosomes, mitochondria, and the nucleus, as well as the overall effect of postischemic calpain activity on calcium regulation and cell death signaling are considered. Finally, potential pathways for calpain-mediated neurodegeneration are outlined in an effort to guide future studies aimed at understanding the downstream pathology of postischemic calpain activity and identifying optimal therapeutic strategies.
Spared Caudal Brainstem SERT Binding in Early Parkinson's Disease
Authors: Albin Roger L , Koeppe Robert A , Bohnen Nicolaas I , Wernette Kristine , Kilbourn Michael A , Frey Kirk A
Published: December 2007

Postmortem data indicate loss of serotoninergic neurons in Parkinson's disease (PD). We used the serotonin transporter (SERT) radioligand 3-amino-4-(2-dimethylaminomethyl-phenylsulfaryl)-benzonitril (DASB) and positron emission tomography to examine SERT distribution and changes in early PD subjects. We studied five PD subjects (H&Y 1 to 2.5) and eight normal controls. There is reduced SERT binding in PD. The magnitude of DASB binding reductions was greater in the forebrain than in the brainstem regions. There was no asymmetry of diminished SERT binding. DASB binding in the medulla was relatively spared, inconsistent with the description of early prominent pathologic study in these caudal brainstem nuclei.
Imaging studies of the photodissociation of H2S+ cations. I. Illustrations of the role of nuclear spin
Authors: Webb Alexander D. , Dixon Richard N. , Ashfold Michael N. R.
Published: December 2007

Ion imaging methods have been used to study the dynamics of H2(D2) molecular elimination from H2S+(D2S+) cations following photoexcitation to the ÃA12 state in the wavelength range 300<λ<360nm. Ground (X̃B12) state parent ions were formed by multiphoton ionization of a jet cooled H2S(D2S) sample, resonance enhanced at the two photon energy by the v=0 level of the A21(⋯2b114pb21) Rydberg state. This Rydberg excited state predissociates sufficiently slowly that the 2+1 resonance enhanced multiphoton ionization (REMPI) spectrum shows resolved rovibronic structure, thereby allowing full quantum state selectivity at this intermediate stage of the cation preparation process. Analysis of the S+ ion images following one photon excitation of the resulting H2S+(D2S+) cations shows that these fragments are formed in their ground S4 state, and that the H2(D2) cofragments are formed predominantly (if not exclusively) in rotational states with either odd or even J rotational quantum number—depending on the chosen REMPI preparation wavelength. This striking specificity for forming ortho- or para-H2(D2) products can be traced back to the state selectivity introduced in the REMPI preparation step. In the case of H2S, therefore, the nuclear spin symmetry of the two equivalent H nuclei in ortho-H2S (and H2S+) carries through into ortho-H2 products, and para-H2S molecules map into para-H2 fragments, surviving photoionization of the Rydberg state, photolysis of the resulting parent cation, and two subsequent radiationless transitions during the evolution from the photoexcited (ÃA12 state) cation through to S(S4)+H2 products. We identify two distinct fragmentation pathways. One, which we term route I, involves nonadiabatic (Renner-Teller) coupling to the X̃ state at near linear configurations and subsequent (spin-orbit induced) coupling to the repulsive A24 potential energy surface (PES) at smaller bond angles. This process operates throughout the photolysis wavelength range investigated and yields rotationally “cool” and vibrationally “cold” H2 products. The second (route II) shows a long wavelength threshold λ∼335nm, and gradually becomes dominant as the photolysis wavelength is reduced. Route II dissociation involves vibronically facilitated nonadiabatic transfer from the Ã to the B̃B22 state, followed by spin-orbit induced transfer to the A24 PES; the route II fragmentation dynamics results in H2 products carrying higher levels of rotational and vibrational excitation.
Liquid methanol Monte Carlo simulations with a refined potential which includes polarizability, nonadditivity, and intramolecular relaxation
Authors: Valdéz-González Maximiliano , Saint-Martin Humberto , Hernández-Cobos Jorge , Ayala Regla , Sanchez-Marcos Enrique , Ortega-Blake Ivan
Published: December 2007

Monte Carlo simulations of liquid methanol were performed using a refined ab initio derived potential which includes polarizability, nonadditivity, and intramolecular relaxation. The results present good agreement between the energetic and structural properties predicted by the model and those predicted by ab initio calculations of methanol clusters and experimental values of gas and condensed phases. The molecular level picture of methanol shows the existence of both rings and linear polymers in the methanol liquid phase.
The vibrationless Ã←X̃ transition of the jet-cooled deuterated methyl peroxy radical CD3O2 by cavity ringdown spectroscopy
Authors: Wu Shenghai , Dupré Patrick , Rupper Patrick , Miller Terry A.
Published: December 2007

The nearly rotationally resolved spectrum of the ÃA′2←X̃A″2 000 transition of perdeutero methyl peroxy near 1.35μm has been studied via pulsed cavity ringdown spectroscopy. Albeit, this is a weak transition, it is possible to observe the spectrum under jet-cooled conditions (≈15K) by combining a source of narrow-bandwidth radiation (≈250MHz) with a supersonic slit-jet expansion incorporating an electric discharge. The near infrared radiation was obtained by using stimulated Raman scattering and a pulsed, nearly Fourier-transform-limited Ti:sapphire amplifier seeded by a scanable cw Ti:sapphire ring laser. The experimental spectrum has been fitted using a model Hamiltonian that includes the rigid body rotation of an asymmetric top and the spin-rotation interaction. An excellent quality fit was obtained resulting in the determination of 15molecular parameters characterizing the Ã and X̃ states. Other results reported for CD3O2 include an estimate of the radical concentration and the vibronic transition dipole from the observed absorption intensities. Details about the spectral linewidths are also discussed.
Femtosecond synchronism of x-rays and visible/infrared light in an x-ray free-electron laser
Authors: Adams B. W.
Published: December 2007

A way is proposed to obtain ultrashort pulses of intense infrared/visible light in few-femtosecond synchronism with x-rays from an x-ray free-electron laser (XFEL). It makes use of the recently proposed emittance-slicing technique [Emma et al., Phys. Rev. Lett. 92, 074801 (2004)] to both restrict the duration of self-amplified spontaneous emission (SASE) to a few femtoseconds and to lead to a coherence enhancement of near-infrared transition undulator radiation (CTUR). The x-rays and the near-infrared light originate within the XFEL undulator from the same slice of electrons within a bunch and are therefore perfectly synchronized with each other. An example of realizing the scheme at the Linac Coherent Light Source is presented. A few side issues are explored briefly, such as the magnitude of the velocity term versus the acceleration term in the Lienard-Wiechert fields and the possible use of the CTUR as a diagnostic tool for the SASE process itself.
How cationic gold clusters respond to a single sulfur atom
Authors: Woldeghebriel Hagos , Kshirsagar Anjali
Published: December 2007

Results describing the interaction of a single sulfur atom with cationic gold clusters (Aun+, n=1–8) using density functional theory are described. Stability of these clusters is studied through their binding energies, second order differences in the total energies, fragmentation behavior, and atom attachment energies. The lowest energy structures for these clusters appear to be three dimensional right from n=3. In most cases the sulfur atom in the structure of AunS+ is observed to displace the gold atom siting at the peripheral site of the Aun+ cluster. The dissociation channels of AunS+ clusters follow the same trend as Aun+ cluster, based on the even/odd number of gold atoms in the cluster, with the exception of Au3S+. This cluster dissociates into Au and Au2S+, signifying the relative stability of Au2S+ cluster regardless of having an odd number of valence electrons. Clusters with an even number of gold atoms dissociate into Au and Aun−1(S)+ and clusters with an odd number of gold atoms dissociate into Au2 and Aun−2(S)+ clusters. An empirical relation is found between the conduction molecular orbital and the number of atoms in the AunS+ cluster.
The use of atomic intrinsic polarizabilities in the evaluation of the dispersion energy
Authors: Olasz András , Vanommeslaeghe Kenno , Krishtal Alisa , Veszprémi Tamás , Van Alsenoy Christian , Geerlings Paul
Published: December 2007

The recent approach presented by Becke and Johnson [J. Chem. Phys. 122, 154104 (2005); 123, 024101 (2005); 123, 154101 (2005); 124, 174104 (2006); 124, 014104 (2006)] for the evaluation of dispersion interactions based on the properties of the exchange-hole dipole moment is combined with a Hirshfeld-type partitioning for the molecular polarizabilities into atomic contributions, recently presented by some of the present authors [A. Krishtal et al., J. Chem. Phys. 125, 034312 (2006)]. The results on a series of nine dimers, involving neon, methane, ethene, acetylene, benzene, and CO2, taken at their equilibrium geometry, indicate that when the C6, C8, and C10 terms are taken into account, the resulting dispersion energies can be obtained deviating 3% or 8% from high level literature data [E. R. Johnson and A. D. Becke, J. Chem. Phys. 124, 174104 (2006)], without the use of a damping function, the only outlier being the parallel face-to-face benzene dimer.
Blue-emitting InGaN–GaN double-heterostructure light-emitting diodes reaching maximum quantum efficiency above 200A∕cm2
Authors: Gardner N. F. , Müller G. O. , Shen Y. C. , Chen G. , Watanabe S. , Götz W. , Krames M. R.
Published: December 2007

Auger recombination is determined to be the limiting factor for quantum efficiency for InGaN–GaN (0001) light-emitting diodes (LEDs) at high current density. High-power double-heterostructure (DH) LEDs are grown by metal-organic chemical vapor deposition. By increasing the active layer thickness, DH LEDs can reach a maximum in quantum efficiency at current densities above 200A∕cm2. Encapsulated thin-film flip-chip DH LEDs with peak wavelength of 432nm have an external quantum efficiency of 40% and output power of 2.3W at 2A.
Weakly relativistic dielectric tensor for arbitrary wavenumbers
Authors: Volpe Francesco
Published: December 2007

The validity of Shkarofsky’s dielectric tensor is extended by taking the strictly weakly relativistic limit and removing, when possible, assumptions on the wavenumbers along and across the ambient magnetic field, k∥ and k⊥. An approximation of the time integral is retained, but is shown to be valid under more benign assumptions than those of quasiperpendicular incidence and small Larmor radius. The increased generality with respect to k∥ permits to handle cases of comparable Doppler and relativistic widths of electron cyclotron resonances. The tensor also suits Bernstein waves, as it captures both their natural large k⊥ and the finite k∥ that is typical of some mode conversions, or acquired as a consequence of the large k⊥ when propagating in curved magnetic fields. Finally, relativistic corrections to the optimal angle for the ordinary-extraordinary Bernstein mode conversion are presented.
Zonal flows and ion temperature gradient instabilities in multiple-helicity magnetic fields
Authors: Ferrando-Margalet S. , Sugama H. , Watanabe T.-H.
Published: December 2007

The effects of multiple-helicity magnetic fields on the ion temperature gradient (ITG) instability and on the zonal flow (ZF) evolution are studied with the linear gyrokinetic Vlasov code GKV [T.-H. Watanabe and H. Sugama, Nucl. Fusion 46, 24 (2006)]. The model helical fields corresponding to the standard and inward-shifted axis configurations of the Large Helical Device [O. Motojima, N. Ohyabu, A. Komori et al., Nucl. Fusion 43, 1674 (2003)] are used to investigate how ITG mode properties and ZF evolution response to a given source are influenced by the field geometry. It is shown that, in the inward-shifted configuration, the ITG mode growth rate increases slightly while the ZF is sustained for a longer time. In addition, velocity-space structures of the ion perturbed distribution function are numerically obtained which illustrate the validity of the analytical prediction that the plasma inward shift retards the radial drift of the helically trapped particles leading to the enhancement of the ZF response. This supports the conjecture that anomalous transport can be reduced by the ZF generated in the configurations optimized to decrease the neoclassical transport.
An apparatus for the study of high temperature water radiolysis in a nuclear reactor: Calibration of dose in a mixed neutron/gamma radiation field
Authors: Edwards Eric J. , Wilson Paul P. H. , Anderson Mark H. , Mezyk Stephen P. , Pimblott Simon M. , Bartels David M.
Published: December 2007

The cooling water of nuclear reactors undergoes radiolytic decomposition induced by gamma, fast electron, and neutron radiation in the core. To model the process, recombination reaction rates and radiolytic yields for the water radical fragments need to be measured at high temperature and pressure. Yields for the action of neutron radiation are particularly hard to determine independently because of the beta/gamma field also present in any reactor. In this paper we report the design of an apparatus intended to measure neutron radiolysis yields as a function of temperature and pressure. A new methodology for separation of neutron and beta/gamma radiolysis yields in a mixed radiation field is proposed and demonstrated.
Experimental requirements for measuring pneumatochemical impedances
Authors: Millet P. , Decaux C. , Ngameni R. , Guymont M.
Published: December 2007

Hydrogen storage remains a bottleneck process on the way to the hydrogen economy. For practical applications, metal hydride systems offer interesting features, in particular, the possibility of reversibly storing large amounts of hydrogen at low or moderate pressure. However, they still suffer from unfavorable specific energy, with mass-fraction values ranging from 0 up to 5wt% whereas transport applications require 6wt% and more. Besides this, higher sorption/desorption kinetics and better chemical stability over long-term cycling are also needed. This is why many studies are carried out in the research community on hydride-forming systems, to develop new materials meeting these requirements. Development and optimization of metal hydride reactors require coupled thermodynamic and kinetic characterization of metal-hydrogen systems. In particular, it is necessary to analyze the kinetics in terms of reaction mechanism, in order to identify the different steps of commonly observed multistep reaction paths, and to measure their individual rate parameters. By analyzing hydriding kinetics in the frequency (Fourier) domain, pneumatochemical impedance spectroscopy (PIS) now offers the possibility of measuring experimental impedances and identifying reaction steps. However, measurement of such impedances is indirect and nontrivial. The purpose of this paper is to detail the experimental requirements needed for correctly measuring gas-phase impedance diagrams. In particular, practical conditions of data sampling and data treatment are described. Experimental results obtained with the model LaNi5–H2(g) system are presented to illustrate the potentialities of PIS analysis.
Quasilinear dynamics of a cloud of hot electrons propagating through a plasma in the presence of an externally applied uniform electric field
Authors: Foroutan G. , Robinson P. A. , Zahed H. , Li B. , Cairns I. H.
Published: December 2007

The propagation of a cloud of hot electrons through a plasma and the generation of Langmuir waves are investigated in the presence of an externally applied uniform electric field. Using numerical simulations of the quasilinear equations the evolution of the electron distribution function and the spectral density of Langmuir waves are monitored in coordinate and velocity space. It is found that the Langmuir waves are enhanced in the presence of the electric field and the distribution functions of the beam and Langmuir waves diffuse toward large velocities. The overall self-similar characteristic of the system is preserved in the presence of the electric field. The average beam velocity is no longer constant and increases with time along its trajectory, but the acceleration is much less than that of free streaming particles. The beam number density plateaus in coordinate space and large scale, small amplitude fluctuations develop on the top of this plateau. The level of the fluctuations depends on the strength of the electric field. We also investigated the influence of the external electric field on the evolution of gas-dynamical parameters such as the height of the plateau in the beam distribution function in velocity space, its upper velocity boundary, and the local velocity spread of the beam. Due to the finite quasilinear relaxation time and spatial inhomogeneity of the electron beam, different parts of the beam are in different states of relaxation. In the region of partial relaxation the plateau is specified by both upper and lower velocity boundaries. The upper boundary of plateau increases linearly with the strength of the electric field but the lower boundary is independent of it. Contrary to the free streaming of a beam in an electric field or quasilinear relaxation in the absence of the electric field, the local velocity spread of the beam increases during its propagation. Some of the electrons at the back of the beam are also transferred by the electric field to its front, so that the height of plateau increases at large distances.
Effect of toxic gases on humidity sensing property of nanocrystalline ZnO film
Authors: Dixit Shobhna , Srivastava Anchal , Srivastava Atul , Shukla R. K.
Published: December 2007

Humidity sensing property of nanocrystalline ZnO film has been investigated over a range of 5–90RH%. Contrary to more popular electrical approach, a novel optoelectronic sensing method has been used. Performance of the humidity sensor is affected by the presence of toxic gases in atmosphere. To gauge this effect, the humidity sensing properties of the film are studied by exposing it to the environment of H2S, SO2, and CH3OH one by one. The zinc oxide film is deposited using print and fire technology. X-ray diffraction of the film shows polycrystalline structure with average grain size of 33.5nm. The effect of toxic gases on the humidity sensing behavior of the film is explained while studying surface morphology of the film. The least detectable change (LDC) in humidity is evaluated for each case. For unexposed film it is found to be 0.14RH%. The LDC in the case of SO2-exposed film is recovered back almost completely by heat treatment.
Experimental studies of low density and temperature ion and electron sheaths
Authors: Frederick-Frost K. M. , Lynch K. A.
Published: December 2007

Full characterization of nighttime ionospheric plasma requires access to both the ion and electron thermal core populations. Efforts to measure particle distributions with rocket and satellite detectors designed to study low energy particles are hindered because the magnitude of spacecraft charging is on the order of or greater than the energy of the bulk of ionospheric particles. This paper presents initial laboratory investigations exploring the formation of plasma sheaths with ionospheric electron energies, densities, and Debye lengths. The goal is to identify the difficulties and solutions to obtaining both thermal electron and ion velocity distributions on one payload. Sheaths around a long cylinder biased with respect to the vacuum vessel wall are studied to verify measurement procedures. An experimental setup in which two conducting spheres, with area ratios in excess of 100, are biased with respect to one another and not referenced to the wall simulates a payload-detector system. Data are compared with simple planar, cylindrical, and spherical sheath models. Ion-rich sheaths conformed with expectations. The criterion for the formation of electron-rich sheaths was found not only to depend on the ratio of ion collector area to electron collector area but also the ratio of the collector area to sheath area. Nonmonotonic electron sheaths obtained by embedding a positively biased electrode within the sheath of a more negative conductor are also explored. It was found that the superposition of two different potential geometries led to the formation of the nonmonotonic potential structures. These initial plasma ion and electron sheath investigations both clarify the behavior of a thermal electron detector previously flown in the ionosphere and explore a low energy parameter regime that is understudied in the laboratory.
Diagnosing direct-drive, shock-heated, and compressed plastic planar foils with noncollective spectrally resolved x-ray scattering
Authors: Sawada H. , Regan S. P. , Meyerhofer D. D. , Igumenshchev I. V. , Goncharov V. N. , Boehly T. R. , Epstein R. , Sangster T. C. , Smalyuk V. A. , Yaakobi B. , Gregori G. , Glenzer S. H. , Landen O. L.
Published: December 2007

The electron temperature (Te) and average ionization (Z) of nearly Fermi-degenerate, direct-drive, shock-heated, and compressed plastic planar foils were investigated using noncollective spectrally resolved x-ray scattering on the OMEGA Laser System [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. Plastic (CH) and Br-doped CH foils were driven with six beams, having an overlapped intensity of ∼1×1014W∕cm2 and generating ∼15 Mbar pressure in the foil. The plasma conditions of the foil predicted with a one-dimensional (1-D) hydrodynamics code are Te∼10eV, Z∼1, mass density ρ∼4g∕cm3, and electron density ne∼3×1023cm−3. The uniformly compressed portion of the target was probed with 9.0-keV x rays from a ZnHeα backlighter created with 18 additional tightly focused beams. The x rays scattered at either 90° or 120° were dispersed with a Bragg crystal spectrometer and recorded with an x-ray framing camera. An examination of the scattered x-ray spectra reveals that an upper limit of Z∼2 and Te=20eV are inferred from the spectral line shapes of the elastic Rayleigh and inelastic Compton components. Low average ionizations (i.e., Z<2) cannot be accurately diagnosed in this experiment due to the difficulties in distinguishing delocalized valence and free electrons. Trace amounts of Br in the CH foil (i.e., 2% atomic concentration) are shown to increase the sensitivity of the noncollective, spectrally resolved x-ray scattering to changes in the average ionization. The experimentally inferred electron temperatures are comparable to the 1-D predictions.
Control of valence band states in pyramidal quantum dot-in-dot semiconductor heterostructures
Authors: Troncale V. , Karlsson K. F. , Pelucchi E. , Rudra A. , Kapon E.
Published: December 2007

The character of the hole states in a pyramidal GaAs∕AlGaAs quantum dot-in-dot (DiD) heterostructure is shown to be controllable by tailoring the confinement potential shape. The change in ground valence band state from heavy hole like to light hole like is demonstrated by side-view polarization resolved photoluminescence measurements. The experimental findings are supported by three-dimensional numerical model calculations. The results are applicable for polarization control in quantum dot photonic devices.
Invited article: Vector and Bragg Magneto-optical Kerr effect for the analysis of nanostructured magnetic arrays
Authors: Westphalen A. , Lee M.-S. , Remhof A. , Zabel H.
Published: December 2007

Experimental and theoretical aspects of obtaining the magnetic information carried by laser beams diffracted from an array of micro- or nanosized magnetic objects are reviewed. We report on the fundamentals of vector magneto-optic Kerr effect (MOKE), Bragg-MOKE, and second-order effects in the Kerr signal in longitudinal Kerr geometry as well as on an experimental setup used for vector and Bragg-MOKE experiments. The vector and Bragg-MOKE technique in combination with micromagnetic simulation is a reliable tool for measuring the complete magnetization vector and for characterizing the reversal mechanism of lateral magnetic nanostructures. We discuss the Bragg-MOKE effect for three standard domain configurations during the magnetization reversal process and present the expected behavior of the magnetic hysteresis loops.
Does an enhanced yellow luminescence imply a reduction of electron mobility in n-type GaN?
Authors: Zhao D. G. , Jiang D. S. , Zhu J. J. , Liu Z. S. , Zhang S. M. , Liang J. W. , Yang H.
Published: December 2007

It is studied whether there is any regular relationship between the yellow luminescence band and electron mobility of n-type GaN. For a series of GaN samples grown with the same Si doping, it is found that the electron mobility decreases with an increase of relative intensity of yellow luminescence, accompanied by an increase of edge dislocation density. Further research indicates that it is acceptors introduced by edge dislocations which lead to the concomitant changes of yellow luminescence and electron mobility. Similar changes are induced by Si doping in the n-type GaN samples with relatively low edge dislocation density. However, the relationship between the yellow luminescence and electron mobility of n-type GaN is not a simple one. A light Si doping may simultaneously increase yellow luminescence and electron mobility when Si doping plays a dominant role in reducing the carrier scattering. This means that even the intensity of yellow luminescence is often used as an indicator of material quality for GaN, it does not have any monotonous correlation with the electron mobility of GaN.
Effects of strain and interface on work function of a Nb–W metal gate system
Authors: Gong H. R. , Nishi Yoshio , Cho Kyeongjae
Published: December 2007

First principles calculation reveals that the compressed (tensile) strain increases (decreases) the work function of Nb and W (110) surfaces, and that such a work function change is due to the combined effects of the surface dipole and bulk electronic structure toward the same direction. Calculation also shows that the interface dipole is formed in the Nb–W interface due to an unequal loss of the electrons from Nb and W interface atoms, and that the formation of the Nb–W interface is energetically favorable with negative interface energy, implying a tendency of interface interdiffusion of Nb and W atoms.
Multistate fluorescence switching of s-triazine-bridged p-phenylene vinylene polymers
Authors: Yoo Jungmok , Kwon Taechang , Sarwade Bhimrao D. , Kim Yuna , Kim Eunkyoung
Published: December 2007

A polymer switch was explored utilizing the redox reaction of iodide ions with highly fluorescent s-triazine (ST)-bridged p-phenylene vinylene (PPV) polymers. The fluorescent polymers were synthesized through the Wittig polycondensation to yield ST-bridged PPV polymers. Utilizing different quenching properties of the polymer with iodine derivatives, the fluorescence of the polymers was modulated electrochemically in a two-electrode device consisting of the fluorescent polymer-doped gel layer containing an iodide and iodine couple and a solid polymer electrolyte layer. The cell showed reversible multistate fluorescence switching in the working potential range of +2V.
A study of the metal to nonmetal transition in Bi-doped β-PbO2 by high resolution x-ray photoemission
Authors: Rothenberg S. , Payne D. J. , Bourlange A. , Egdell R. G.
Published: December 2007

The influence of Bi doping on the electronic structure of β-PbO2 has been studied by high resolution x-ray photoemission spectroscopy. Doped films were prepared on Pt substrates by electrochemical deposition from solutions of Pb(NO3)2 and Bi(NO3)3 in HNO3. Bi doping was found to lead to a lowering of the density of states at the Fermi energy in valence region x-ray photoemission and to suppression of final state screening by mobile conduction electrons in Pb 4f core level photoemission. A metal to nonmetal transition was found to occur for bulk doping levels around 5at.% Bi. There is evidence of pronounced surface segregation of Bi. In contrast to host Pb ions, core holes on surface Bi ions do not couple to the mobile conduction electrons in the metallic state. It is concluded that Bi acts as a p-type acceptor in β-PbO2 and traps charge carriers introduced by oxygen deficiency in PbO2−x.
Electret mechanism, hysteresis, and ambient performance of sol-gel silica gate dielectrics in pentacene field-effect transistors
Authors: Cahyadi T. , Tan H. S. , Mhaisalkar S. G. , Lee P. S. , Boey F. , Chen Z.-K. , Ng C. M. , Rao V. R. , Qi G. J.
Published: December 2007

The electret induced hysteresis was studied in sol-gel silica films that result in higher drain currents and improved device performance in pentacene field-effect transistors. Vacuum and ambient condition studies of the hysteresis behavior and capacitance-voltage characteristics on single layer and varying thicknesses of bilayer dielectrics confirmed that blocking layers of thermal oxide could effectively eliminate the electret induced hysteresis, and that thin (25nm) sol-gel silica dielectrics enabled elimination of nanopores thus realizing stable device characteristics under ambient conditions.
Mechanism of aluminum-induced layer exchange upon low-temperature annealing of amorphous Si/polycrystalline Al bilayers
Authors: Wang J. Y. , Wang Z. M. , Mittemeijer E. J.
Published: December 2007

The aluminum-induced layer exchange (ALILE) process occurring upon annealing amorphous Si/polycrystalline Al bilayers (a-Si/c-Al) has been observed at a temperature as low as 165 °C. The diffusion length of Si along Al grain boundaries is proposed as a tool for determining the annealing conditions, i.e., temperature and time, for the occurrence of the c-Al→c-Si layer exchange. Analysis of the local and global energy changes upon layer exchange reveals that a tiny driving force controls the kinetics of layer exchange and leads to a general interpretation of the mechanism of the ALILE process.
Direct observation of field-induced variant transformation in Fe3Pt using pulsed magnetic field x-ray diffraction
Authors: Ouyang Z. W. , Matsuda Y. H. , Nojiri H. , Inami T. , Ohwada K. , Tsubota M. , Sakon T. , Fukuda T. , Kakeshita T.
Published: December 2007

X-ray diffraction experiments in pulsed magnetic fields were performed to study the giant magnetic-field-induced strain in the martensitic phase of ordered Fe3Pt alloy. The field dependence of the arrangement of the variants (crystallographic domains) was observed directly in pulsed magnetic fields for the first time. The variants with the c axis perpendicular to the magnetic field are transformed into variants with the c axis nearly parallel to the magnetic field. Moreover, we found that the reorientation behaviors are different for different variants with the same crystallographic direction. The results demonstrate the crucial importance of microscopic investigation, such as x-ray diffraction analysis, of the martensitic transition. We also found that the magnetic-field-induced strain associated with the rearrangement of the variants occurs even in a very fast sweeping pulsed field of up to 1.9×103 T/s at 1 T.
The pressure-dependent performance of a substrate-free focal plane array in an uncooled infrared imaging system
Authors: Xiong Zhiming , Zhang Qingchuan , Gao Jie , Wu Xiaoping , Chen Dapeng , Jiao Binbin
Published: December 2007

Uncooled focal plane arrays (FPAs) are being developed for a wide range of infrared imaging applications. A substrate-free FPA for optical readout infrared imaging is fabricated with a pixel pitch of 120 μm. The pressure dependences of thermal conductance of a FPA with/without substrate are studied by modeling analysis. Infrared imaging experiments are performed to validate the modeling analysis. At atmospheric pressure the total thermal conductance of the substrate-free FPA is only 1/1000 time of the traditional FPA which has a 2 μm air gap between the cantilever beam and the substrate. Room temperature object’s thermal images are obtained even if the FPA is placed in the atmosphere. The air conductance of a substrate-free FPA at atmospheric pressure could be as small as that of a traditional FPA with a substrate in high vacuum (about 1 Pa). The experimental result also shows that the system noise keeps almost unchanged with the pressure. These characters will decrease the vacuum packaging request of the FPA and should be valuable for practical applications.
Giant tunnel magnetoresistance in codeposited fullerene-cobalt films in the low bias-voltage regime
Authors: Sakai Seiji , Sugai Isamu , Mitani Seiji , Takanashi Koki , Matsumoto Yoshihiro , Naramoto Hiroshi , Avramov Pavel V. , Okayasu Satoru , Maeda Yoshihito
Published: December 2007

Magnetotransport properties in the low bias-voltage regime were investigated for codeposited C60–Co films. A giant tunnel magnetoresistance (MR) ratio (ΔR∕Rmax) of 80%, which is the highest in ferromagnetic metal/organic molecule systems, was found at low temperatures. The observed bias-voltage dependence of the MR ratio is expressed by an unusual exponential form, suggesting that the MR ratio of nearly 100% can be realized in the low bias-voltage limit.
Optical transitions in polarized CdSe, CdSe∕ZnSe, and CdSe∕CdS∕ZnS quantum dots dispersed in various polar solvents
Authors: Thuy Ung Thi Dieu , Liem Nguyen Quang , Thanh Do Xuan , Protière Myriam , Reiss Peter
Published: December 2007

The optical transitions in ensembles of colloidal CdSe-based quantum dots (QDs) have been systematically studied as a function of the net QDs’ polarity/polarization and of the solvent’s polarity. While the general trend observed for all QD systems dispersed in different solvents is similar, the spectral shifts are more pronounced in core QDs than in core/shell structures. Our results can be rationalized by taking account of the electric field experienced by the QDs that results from their effective polarization in solvents of different polarities (quantum confined Stark effect) as well as from the effect of the external dielectric environment (solvatochromatic effect).
Reduced boron diffusion under interstitial injection in fluorine implanted silicon
Authors: Kham M. N. , Matko I. , Chenevier B. , Ashburn P.
Published: December 2007

Point defect injection studies are performed to investigate how fluorine implantation influences the diffusion of boron marker layers in both the vacancy-rich and interstitial-rich regions of the fluorine damage profile. A 185keV, 2.3×1015cm−2 F+ implant is made into silicon samples containing multiple boron marker layers and rapid thermal annealing is performed at 1000°C for times of 15–120s. The boron and fluorine profiles are characterized by secondary ion mass spectroscopy and the defect structures by transmission electron microscopy (TEM). Fluorine implanted samples surprisingly show less boron diffusion under interstitial injection than those under inert anneal. This effect is particularly noticeable for boron marker layers located in the interstitial-rich region of the fluorine damage profile and for short anneal times (15s). TEM images show a band of dislocation loops around the range of the fluorine implant and the density of dislocation loops is lower under interstitial injection than under inert anneal. It is proposed that interstitial injection accelerates the evolution of interstitial defects into dislocation loops, thereby giving transient enhanced boron diffusion over a shorter period of time. The effect of the fluorine implant on boron diffusion is found to be the opposite for boron marker layers in the interstitial-rich and vacancy-rich regions of the fluorine damage profile. For marker layers in the interstitial-rich region of the fluorine damage profile, the boron diffusion coefficient decreases with anneal time, as is typically seen for transient enhanced diffusion. The boron diffusion under interstitial injection is enhanced by the fluorine implant at short anneal times but suppressed at longer anneal times. It is proposed that this behavior is due to trapping of interstitials at the dislocation loops introduced by the fluorine implant. For boron marker layers in the vacancy-rich region of the fluorine damage profile, suppression of boron diffusion is seen for short anneals and then increased diffusion after a critical time, which is longer for inert anneal than interstitial injection. This behavior is explained by the annealing of vacancy-fluorine clusters, which anneal quicker under interstitial injection because the injected interstitials annihilate vacancies in the clusters.
Differential single living cell refractometry using grating resonant cavity with optical trap
Authors: Chin L. K. , Liu A. Q. , Lim C. S. , Zhang X. M. , Ng J. H. , Hao J. Z. , Takahashi S.
Published: December 2007

This letter reports the measurement of single living cell’s refractive index (RI) using the optical grating resonant cavity with buffer modulation on a biochip. The cell’s RI is a significant cell biophysical property which has potential in cancer and disease diagnoses. The single cell is trapped optically within the cavity, and the transmission spectra shift due to the change of surrounding buffer, without/with cell, are used to determine the cell’s effective RI and effective thickness. The measurement shows that Madin-Darby canine kidney cell has an effective RI of 1.383±0.001. Moreover, the optical trapping technique eliminates uncertainty and avoids cell damage.
Negative and positive electroluminescence from a compensated p-type germanium in terahertz frequencies
Authors: Chung P. K. , Lin J. M. , Yen S. T. , Wang T. H.
Published: December 2007

Negative luminescence (NL) and positive luminescence in terahertz frequencies were observed from a compensated p-type Ge under various levels of pulse excitations at cryogenic temperatures. The NL spectra accompanying an S-type negative differential resistance phenomenon were attributed to the reduced emission transitions from the excited states to the ground states of the acceptors during the excitation. A possible excitation mechanism is also presented according to the measured data.
Effect of mechanical strain on mobility of polycrystalline silicon thin-film transistors fabricated on stainless steel foil
Authors: Kuo Po-Chin , Jamshidi-Roudbari Abbas , Hatalis Miltiadis
Published: December 2007

The effect of uniaxial tensile strain parallel to the channel on mobility of polycrystalline silicon thin-film transistors (TFTs) on stainless steel foil has been investigated. The electron mobility increases by 20% while the hole mobility decreases by 6% as the strain increases to 0.5%, and both followed by saturation as the strain increases further. The off current decreases for both types of TFTs under strain. All TFTs remained functional at the applied strain of 1.13%.
Effects of Ta seed layer and annealing on magnetoresistance in CoFe∕Pd-based pseudo-spin-valves with perpendicular anisotropy
Authors: Law Randall , Sbiaa Rachid , Liew Thomas , Chong Tow Chong
Published: December 2007

We have studied the switching characteristics and magnetoresistance of pseudo-spin-valves with perpendicular anisotropy based on CoFe∕Pd multilayers. In unpatterned thin films without exchange biasing, a maximum current-in-plane giant magnetoresistance of 7% was achieved, the highest reported to date in perpendicular pseudo-spin-valves. A Ta seed layer and the fcc (111) orientation of Pd was shown to be important in order to achieve good perpendicular anisotropy and sharp switching behavior. The improvement in perpendicular anisotropy and decay in magnetoresistance upon postdeposition annealing have been attributed to the formation of CoPd alloys at the CoFe∕Pd interfaces.
Molecular dynamics simulation of interfacial thermal conductance between silicon and amorphous polyethylene
Authors: Hu Ming , Shenogin Sergei , Keblinski Pawel
Published: December 2007

Using nonequilibrium molecular dynamics simulations, we study the interfacial thermal conductance between single crystal silicon and amorphous polyethylene (PE). We estimate that the silicon-PE interfacial thermal conductance is about 20MW∕m2K at room temperature, which is equivalent to the resistance of 16nm thick layer of bulk amorphous polyethylene. We also study the role of solid stiffness and the bonding strength across the interface on the interfacial thermal resistance. With strong interfacial bonding, our results are in agreement with the diffuse mismatch model and phonon radiation limit predictions, suggesting that in this case, heat carrying acoustic phonons in solids have transmission coefficients to polymer equal almost to unity.
Niche shifts and local competition between two coral reef fishes at their geographic boundary
Authors: Limbourn Andrew J. , Jones Geoffrey P. , Munday Philip L. , Srinivasan Maya
Published: December 2007

Competition theory predicts that, when resources are limiting, interacting species should differ more in resource use where they co-occur, compared with where they do not (resource partitioning). The damselfishes Dascyllus aruanus and D. melanurus provide a useful test of this prediction because they exhibit largely allopatric geographic ranges that overlap near Port Moresby, Papua New Guinea. To test whether this species pair exhibited responses in resource use that were consistent with competition theory, the depth distributions, coral substrata and diet of each species at allopatric and sympatric locations were compared. Where sympatric, the frequency of co-occurrence was examined and foraging ranges and diet on corals where the species do and do not co-occur were compared. Contrary to expectations, the species were more similar in depth, habitat use and diet in the sympatric region. Within this location, they sometimes co-existed in the same coral shelters, but much less often than would be expected with random assortment. Where they did co-exist, their diets converged. D. melanurus exhibited restricted foraging ranges and consumed smaller prey in the presence of D. aruanus, but not vice versa. It was concluded that niche shifts along different resource axes at the geographic boundary are explained both by large-scale constraints in resource availability and local-scale competitive interactions.