bluuebunny/crossref_metadata_2025 huggingface dataset:
Moment variation in Er(Co1−xFex)2 Laves phase: Magnetic measurements and Mössbauer spectroscopy study
Authors: Liu X. B. , Altounian Z. , Ryan D. H.
Published: March 2009

Er ( Co 1 − x Fe x ) 2 cubic Laves phases have been studied by magnetic measurements and F57e Mössbauer spectroscopy. With increasing x from 0 to 0.8, the magnetization decreases from 6.8 to 4.8μB/f.u. (5 K), then it slightly increases to 5.2μB/f.u. as x increases from 0.8 to 1.0. Assuming a fixed Er moment of 9μB (Er metal value), the corresponding moment of Co(Fe) shows a maximum at x=0.8 due to the antiparallel coupling between the Er and Co(Fe) sublattices in the compounds. The F57e spectra can be analyzed in terms of two distinct six-line F57e patterns with intensities in the ratio of 1:3, indicating that the easy magnetization direction is along [111] in these compounds. The Fe content dependence of the hyperfine fields at the F57e nuclei in Er(Co1−xFex)2 shows the same tendency as that of the moment variation of Co(Fe), following a 3d-type Slater–Pauling dependence.
Parallel execution and scriptability in micromagnetic simulations
Authors: Fischbacher Thomas , Franchin Matteo , Bordignon Giuliano , Knittel Andreas , Fangohr Hans
Published: March 2009

We demonstrate the feasibility of an “encapsulated parallelism” approach toward micromagnetic simulations that combines offering a high degree of flexibility to the user with the efficient utilization of parallel computing resources. While parallelization is obviously desirable to address the high numerical effort required for realistic micromagnetic simulations through utilizing now widely available multiprocessor systems (including desktop multicore CPUs and computing clusters), conventional approaches toward parallelization impose strong restrictions on the structure of programs: numerical operations have to be executed across all processors in a synchronized fashion. This means that from the user’s perspective, either the structure of the entire simulation is rigidly defined from the beginning and cannot be adjusted easily, or making modifications to the computation sequence requires advanced knowledge in parallel programming. We explain how this dilemma is resolved in the NMAG simulation package in such a way that the user can utilize without any additional effort on his side both the computational power of multiple CPUs and the flexibility to tailor execution sequences for specific problems: simulation scripts written for single-processor machines can just as well be executed on parallel machines and behave in precisely the same way, up to increased speed. We provide a simple instructive magnetic resonance simulation example that demonstrates utilizing both custom execution sequences and parallelism at the same time. Furthermore, we show that this strategy of encapsulating parallelism even allows to benefit from speed gains through parallel execution in simulations controlled by interactive commands given at a command line interface.
Effect of compressibility on the Rayleigh–Taylor and Richtmyer–Meshkov instability induced nonlinear structure at two fluid interface
Authors: Gupta M. R. , Roy Sourav , Khan Manoranjan , Pant H. C. , Sarkar Susmita , Srivastava M. K.
Published: March 2009

The effect of compressibility and of density variation on Rayleigh–Taylor and Richtmyer–Meshkov instability of the temporal development of two fluid interfacial structures such as bubbles and spikes have been investigated. It is seen that the velocity of the tip of the bubble or spike increases (destabilization) if the local Atwood number increases due to density variation of either of the fluids. The opposite is the result, i.e., the bubble or spike tip velocity decreases (stabilization) if the density variation leads to lowering of the value of the local Atwood number. The magnitude of stabilization or destabilization is an increasing function of the product of the wave number k and interfacial pressure p0. The effect of compressibility is quite varied. If the heavier (upper) fluid alone is incompressible (γh→∞), but the lighter fluid is compressible the growth rate is higher (destabilization) than when both the fluids are incompressible. Moreover the heavier fluid remaining incompressible the growth rate decreases (stabilization) as γl (finite) increases and ultimately tends to the incompressible limit value as γl→∞. With γl→∞ but γh finite the growth increases (destabilization) as γh increases. When both γh and γl are finite (density ρh>density ρl) the growth is reduced when γh<γl compared to that when both fluids are incompressible and enhanced when γh>γl. The set of nonlinear equations describing the dynamics of bubbles and spikes in the presence of fluid density variations are not analytically integrable in closed form. The results derived by numerical solution methods are represented and interpreted in corresponding figures.
Local modes and two magnon scattering in ordered permalloy antidot arrays
Authors: Martens Stephan , Albrecht Ole , Nielsch Kornelius , Görlitz Detlef
Published: March 2009

Antidot arrays consisting of periodically arranged holes in continuous magnetic films exhibit periodic demagnetization field distributions, which have large impact on the magnetic structure and properties such as enhanced coercivity. The authors report on the investigation of the statics and the dynamics of ordered square antidot arrays with 180 nm period and 70 nm antidot diameter that have been prepared by vapor deposition of 20 nm Permalloy on porous anodic alumina substrates. Using superconducting quantum interference device magnetometry, magneto-optical Kerr effect measurements and angular variable ferromagnetic resonance (FMR) spectroscopy at 9.2 GHz, together with micromagnetic simulations several local magnetization modes, were identified. The enhanced FMR linewidth with increasing out-of-plane angle of the magnetic field is not due only to demagnetization effects, but is rather attributed to two magnon scattering processes. This feature is assumed to originate from the surface roughness [wrms(L=1.3 μm)∼5 nm] of the alumina substrate which is quantified by atomic force microscopy.
A comprehensive study of temperature-dependent reflectance and photoluminescence of Zn1−xMnxO thin films grown on c-Al2O3
Authors: Lin D. Y. , Lin H. J. , Wu J. S. , Chou W. C. , Yang C. S. , Wang J. S.
Published: March 2009

We present a systematic study of temperature-dependent reflectance (R) and photoluminescence (PL) measurements on ZnMnO films grown by plasma-assisted molecular beam epitaxy. For the first time, the three free-exciton transitions FXA (Γ7c-Γ7vu), FXB (Γ7c-Γ9v), and FXC (Γ7c-Γ7vl) and the longitudinal-optical phonon replicas of FXB and FXC of ZnMnO films have been clearly observed in the R spectra. The parameters describing the activation energy and the temperature dependence of the transition energy and broadening have been extracted by fitting the experimental R and PL spectra. The spectral data of ZnMnO films not only show the deterioration of crystalline quality with increasing Mn composition fraction but also indicate the Mn clustering caused by Mn atom segregation.
Microphase separation of rod-coil diblock copolymer in solution
Authors: Lin Jiaping , Lin Shaoliang , Zhang Liangshun , Nose Takuhei
Published: March 2009

Lattice theory of rigid rods is extended to describe the microphase separation behavior of a rod-coil diblock copolymer in solution. The free energy was formulated by inclusion of the energy terms arising from the core-corona interface between the rods and coils and the corona formed by the coils into the lattice model of rigid rods. The rod-coil diblock copolymer exhibits lyotropic mesophases with lamellar, cylindrical, and spherical structures when the copolymer concentration is above a critical value. The tendency of the rodlike blocks to form orientational order plays an important role in the formation of lyotropic phases. Influences of polymer-solvent interaction, surface free energy, and molecular architectures of the rod-coil diblock copolymer on the phase behaviors were studied, and phase diagrams were mapped accordingly. The theoretical results were compared with some existing experimental observations and a good agreement is shown.
Early post-impact time dynamics of viscous drops onto a solid dry surface
Authors: Mongruel A. , Daru V. , Feuillebois F. , Tabakova S.
Published: March 2009

The spreading dynamics of liquid drops normally impacting a solid dry surface at high Reynolds and Weber numbers is experimentally and numerically studied at early post-impact times starting from 10−5 s after impact. The focus is on the emergence and growing of the axisymmetric liquid lamella underneath the drop, that is, on the time evolution of its thickness, radius, and velocity, as a function of impact velocity U and liquid viscosity ν. The Navier–Stokes equations for two-phase flows are solved numerically by an artificial compressibility method. A “shock-capturing” method is used for the tracking of the gas-liquid interface, neglecting surface tension effects. Experimental and numerical results are interpreted using a simple scaling analysis that reveals the characteristic lengths and velocities of the spreading dynamics. In particular, a finite characteristic time of appearance for the lamella is found, which is of the order of ν/U2. Rescaling of the data works satisfactorily in the considered range of parameters. Thus, the lamella ejection is limited by viscosity.
Dispersion and spin wave “tunneling” in nanostructured magnetostatic spin waveguides
Authors: Kozhanov A. , Ouellette D. , Rodwell M. , Allen S. J. , Jacob A. P. , Lee D. W. , Wang S. X.
Published: March 2009

Magnetostatic spin wave dispersion and loss are measured in micron scale spin waveguides in ferromagnetic, metallic CoTaZr. Results are in good agreement with model calculations of spin wave dispersion. The measured attenuation lengths, of the order of 3 μm, are several of orders of magnitude shorter than that predicted from eddy currents in these thin wires. Spin waves effectively “tunnel” through air gaps, produced by focused ion beam etching, as large as 1.5 μm.
A note on the vibrational efficacy in molecule-surface reactions
Authors: Díaz C. , Olsen R. A.
Published: March 2009

The effectiveness of vibrational energy in promoting dissociation of molecules colliding with surfaces can be measured through the so-called vibrational efficacy. It is thought by many to be a pure “energetic” measure and therefore believed to be limited from below by zero (in the case that there is no increase in dissociation probability upon vibrational excitation) and from above by one (in the case that all of the vibrational excitation energy is used to promote reaction). However, the quantity vibrational efficacy is clearly linked to the detailed dynamics of the system, and straightforward considerations lead to the conclusion that it is not limited either from below or above. Here we discuss these considerations together with a quasiclassical dynamics study of a molecule-surface system, N2/Ru(0001), for which a vibrational efficacy bigger than one has been found both experimentally and theoretically. We show that an analysis of the vibrational efficacy only in terms of energy transfer from vibration to translation can be too simple to describe the behavior of systems for which the potential energy surfaces present (high) reaction barriers, potential corrugation and anisotropy, and curved reaction paths.
On the role of stochastic heating in experiments with complex plasmas
Authors: Marmolino C. , de Angelis U. , Ivlev A. V. , Morfill G. E.
Published: March 2009

Stochastic heating of dust particles resulting from dust charge fluctuations is considered in some laboratory situations, where high kinetic temperatures of dust particles have been suggested or could be observed. A particular case, in the conditions of the scrape-off layer in tokamak plasmas, is also considered and it is shown that kinetic energies corresponding to velocities of ≃km/s can be reached in times of order ≃1 ms by micron-size particles interacting with a background of stochastically heated nanosize particles.
Intermediate vibronic coupling in sexithiophene single crystals
Authors: Stradomska Anna , Petelenz Piotr
Published: March 2009

A new approach is proposed to describe intermediate-to-strong linear vibronic coupling in an infinite molecular crystal. The Hamiltonian, transformed to the Lang–Firsov representation, is approximated by disregarding the terms involving more-than-two-particle excitations and block-diagonalized by the Fourier transformation. The spectroscopically relevant block corresponding to zero wave vector is further simplified by introducing a cutoff in the off-diagonal matrix elements and reduced to a manageable size by truncating the basis set, which enables one to diagonalize it numerically. The parametrization, based on independent experiments or theoretical estimates, is aimed to represent the sexithiophene crystal. The results, compared to those obtained for a finite cluster with equivalent material parameters, highlight the favorable convergence properties of the infinite-crystal approach.
Error-field induced electromagnetic torques in a large aspect-ratio, low-β, weakly shaped tokamak plasma
Authors: Fitzpatrick Richard
Published: March 2009

The toroidal electromagnetic braking torques exerted at the various internal rational surfaces of a large aspect-ratio, low-β, weakly shaped, tokamak plasma by a nonaxisymmetric error field are investigated using a semianalytic approach. It is found that there is an optimal error-field spectrum for exerting a torque at a given rational surface. This spectrum is dominated by the resonant harmonic, but also contains sideband harmonics induced by plasma toroidicity, pressure, ellipticity, and triangularity. These sidebands couple back to the resonant harmonic in such a manner as to reduce its amplitude. Provided that there is significant coupling to a (stable) ideal external kink mode which is close to its marginal stability boundary, the optimal error field predominately contains sideband harmonics whose poloidal mode numbers are more positive than the resonant mode number (which is assumed to be positive), and also tends to balloon on the outboard side of the plasma.
Direct measurement of Co-ion spin state transitions in Ca3Co4O9 using variable-temperature electron energy-loss spectroscopy
Authors: Yang Guang , Ramasse Quentin , Klie Robert F.
Published: March 2009

Previous studies of Ca3Co4O9 suggested that the abrupt changes in the magnetic susceptibility and electrical resistivity at ∼420 K can be attributed to Co spin state transitions. In this letter, we study the possible transitions above 420 K by variable-temperature Z-contrast imaging and electron energy-loss spectroscopy. We find that there is no observable change in the structure and Co valence state upon in situ heating to 500 K, compared to room temperature. However, an intensity decrease of the prepeak in the O K-edge near edge fine structure indicates a Co3+-ion spin state transition has occurred from a low to an intermediate spin state.
Relative importance of first and second derivatives of nuclear magnetic resonance chemical shifts and spin-spin coupling constants for vibrational averaging
Authors: Dračínský Martin , Kaminský Jakub , Bouř Petr
Published: March 2009

Relative importance of anharmonic corrections to molecular vibrational energies, nuclear magnetic resonance (NMR) chemical shifts, and J-coupling constants was assessed for a model set of methane derivatives, differently charged alanine forms, and sugar models. Molecular quartic force fields and NMR parameter derivatives were obtained quantum mechanically by a numerical differentiation. In most cases the harmonic vibrational function combined with the property second derivatives provided the largest correction of the equilibrium values, while anharmonic corrections (third and fourth energy derivatives) were found less important. The most computationally expensive off-diagonal quartic energy derivatives involving four different coordinates provided a negligible contribution. The vibrational corrections of NMR shifts were small and yielded a convincing improvement only for very accurate wave function calculations. For the indirect spin-spin coupling constants the averaging significantly improved already the equilibrium values obtained at the density functional theory level. Both first and complete second shielding derivatives were found important for the shift corrections, while for the J-coupling constants the vibrational parts were dominated by the diagonal second derivatives. The vibrational corrections were also applied to some isotopic effects, where the corrected values reasonably well reproduced the experiment, but only if a full second-order expansion of the NMR parameters was included. Contributions of individual vibrational modes for the averaging are discussed. Similar behavior was found for the methane derivatives, and for the larger and polar molecules. The vibrational averaging thus facilitates interpretation of previous experimental results and suggests that it can make future molecular structural studies more reliable. Because of the lengthy numerical differentiation required to compute the NMR parameter derivatives their analytical implementation in future quantum chemistry packages is desirable.
Generation of plasma intrinsic oscillation at the front surface of a target irradiated by a circularly polarized laser pulse
Authors: Zhang Xiaomei , Shen Baifei , Jin Zhangying , Wang Fengchao , Ji Liangliang
Published: March 2009

In laser-target interaction, the effects of laser intensity on plasma oscillation at the front surface of targets have been investigated by one-dimensional particle in cell simulations. The periodical oscillations of the ion density and electrostatic field at the front surface of the targets are reported for the first time, which is considered as an intrinsic property of the target excited by the laser. The oscillation period depends only on initial plasma density and is irrelevant with laser intensity. Flattop structures with curves in ion phase space are found with a more intense laser pulse due to the larger amplitude variation of the electrostatic field. A simple but valid model is proposed to interpret the curves.
Heat transport in the quasi-single-helicity islands of EXTRAP T2R
Authors: Frassinetti L. , Brunsell P. R. , Drake J.
Published: March 2009

The heat transport inside the magnetic island generated in a quasi-single-helicity regime of a reversed-field pinch device is studied by using a numerical code that simulates the electron temperature and the soft x-ray emissivity. The heat diffusivity χe inside the island is determined by matching the simulated signals with the experimental ones. Inside the island, χe turns out to be from one to two orders of magnitude lower than the diffusivity in the surrounding plasma, where the magnetic field is stochastic. Furthermore, the heat transport properties inside the island are studied in correlation with the plasma current and with the amplitude of the magnetic fluctuations.
Diffusion-influenced reactions involving a reactant with two active sites
Authors: Kang Aeri , Kim Ji-Hyun , Lee Sangyoub , Park Hwangseo
Published: March 2009

We consider the kinetics of diffusion-influenced reactions which involve a reactant species that can be modeled as a sphere with two reactive patches located on its surface at an arbitrary angular distance. An approximate analytic expression for the rate coefficient is derived based on the Wilemski–Fixman–Weiss decoupling approximation and a multivariable Padé approximation. The accuracy of the rate expression is evaluated against computer simulations as well as an exact analytic expression available for a special case. The present theory provides accurate estimates for the magnitude of diffusive interference effects between the two reactive patches. We also present an efficient Brownian dynamics method for calculating the time-dependent rate coefficient, which is applicable when the reactants involve multiple active sites.
Onset of oscillatory convection in two liquid layers with phase change
Authors: McFadden G. B. , Coriell S. R.
Published: March 2009

We perform linear stability calculations for horizontal fluid bilayers that can undergo a phase transformation in the presence of a vertical temperature gradient. We reconsider the oscillatory instability calculated by Huang and Joseph [J. Fluid Mech. 242, 235 (1992)] for the water-steam system cooled from below at temperatures near 100 °C, where there is a large difference in the densities of the two fluids. We find that buoyancy and surface tension gradients are unimportant for this instability. Numerical solutions demonstrate that the properties of the vapor and liquid systems at these temperatures are sufficiently different that an approximate treatment is possible in which the equations for the vapor phase can be eliminated from the overall governing equations. Further analytical approximations suggested by the numerical solution are also presented, and the results are in good agreement with the numerical solution for the full set of governing equations. A simple model of the oscillatory instability is developed which gives insight into its origins.
Improving photovoltaic properties by incorporating both single walled carbon nanotubes and functionalized multiwalled carbon nanotubes
Authors: Khatri Ishwor , Adhikari Sudip , Aryal Hare Ram , Soga Tetsuo , Jimbo Takashi , Umeno Masayoshi
Published: March 2009

Single-walled carbon nanotubes (SWCNTs) and functionalized multiwalled carbon nanotubes (f-MWCNTs) are introduced together for photovoltaic application in a poly(3-octylthiophene)/n-Si heterojunction solar cell. The performance of the device was improved by manyfold by the incorporation of both SWCNTs and f-MWCNTs. The open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), and power conversion efficiency (η) were 0.44 V, 6.16 mA/cm2, 36%, and 0.98%, respectively. Here, we expect that SWCNTs help in exciton dissociation and provide percolation paths for electron transfer, whereas f-MWCNTs provide efficient hole transportation. CNT incorporation yields better carrier mobility, easy exciton splitting, and suppression of charge recombination, thereby improving photovoltaic action.
Stretched-exponential increase in the open-circuit voltage induced by thermal annealing of amorphous silicon-carbide heterojunction solar cells
Authors: Pysch Damian , Ziegler Johannes , Becker Jan-Philipp , Suwito Dominik , Janz Stefan , Glunz Stefan W. , Hermle Martin
Published: March 2009

In this letter, the annealing behavior of the open-circuit voltage improvement of n-doped amorphous silicon-carbide heterojunction solar cells is investigated in detail. We present our results of a significant open-circuit voltage improvement of more than 100 mV up to a maximum value of 675.5 mV triggered by thermal annealing on a hotplate. The observed open-circuit voltage behavior can be described very well by a stretched-exponential function, which in general describes relaxation rates in complex systems. Therefore, we suggest a diffusion of weakly bonded hydrogen, activated by the annealing, which saturates dangling bonds in the amorphous layer itself and at the heterojunction interface, to be responsible for the strong Voc improvement.
A theoretical study on the frequency-dependent electric conductivity of electrolyte solutions. II. Effect of hydrodynamic interaction
Authors: Yamaguchi T. , Matsuoka T. , Koda S.
Published: March 2009

The theory on the frequency-dependent electric conductivity of electrolyte solutions proposed previously by Yamaguchi et al. [J. Chem. Phys. 127, 234501 (2007)] is extended to include the hydrodynamic interaction between ions. The theory is applied to the aqueous solution of NaCl and the concentration dependence of the conductivity agrees well with that determined by experiments. The effects of the hydrodynamic and relaxation effects are highly nonadditive in the concentrated solution, because the hydrodynamic interaction between ions affects the time-dependent response of the ionic atmosphere. The decrease in the electric conductivity is divided into the contributions of ion pair distribution at various distances. The long-range ionic atmosphere plays a major role at the concentration as low as 0.01 mol/kg, whereas the contribution of the contact ion pair region is important at 1 mol/kg. The magnitude of the contribution of the contact ion pair region is scarcely dependent on the presence of the hydrodynamic interaction. The transport number of cation is calculated to be a decreasing function of concentration as is observed in experiments.
Physics of thermoelectric cooling: Alternative approach
Authors: Lashkevych Igor , Cortes Carlos , Gurevich Yuri G.
Published: March 2009

The paper is devoted to the analysis of thermoelectric cooling phenomena in semiconductors containing potential barriers (p-n-junction). The formulation of an adequate self-consistent theoretical model describing the effect is presented. The role of the recombination rate as a new source of heat in linear approximation of the electric current was discussed, leading to a reformulation of the heat balance equations. The importance of redistribution of nonequilibrium charge carriers, which has been ignored in most publications on this subject, is also shown. The conventional theory of thermoelectric cooling, not taking into account the influence of the nonequilibrium charge carriers, is shown to be inadequate. Besides, when the recombination rate decreases, cooling changes to heating.
Thermal expansion characteristics of Al2O3 nanofluids: More to understand than understood
Authors: Nayak A. K. , Singh R. K. , Kulkarni P. P.
Published: March 2009

In this paper, the thermal expansion characteristics of Al2O3 nanofluids have been studied and compared with the base fluid, i.e., water. The nanoparticles used were in the range of 40–80 nm, which were dispersed in water by sonication. Interestingly, it was found that the volumetric rise of the metallic oxide nanofluids is much larger than water for the corresponding temperature rise, which is opposite to what has been interpreted so far. This is one of the wonderful properties of nanofluids that would find tremendous application in many heat extraction systems using natural convection as mode of heat removal.
Fast optical spectrometer for the charge exchange diagnostic on RFX-mod
Authors: Gazza E. , Valisa M.
Published: March 2009

A new fast visible spectrometer built for the charge exchange diagnostic system of the RFX-mod Reversed Field pinch experiment is described. The optical mounting is of the Littrow type. The spectral resolution is defined by a 3000 grooves/mm, 180×143 mm2 wide reflection grating, and a focal length of 400 mm. The collimating optics is based on a commercial high quality f/2.8 telephoto lens, so that the input fibers can be vertically stacked without space in between. The detector is a two-dimensional charge coupled device back-illuminated sensor to ensure high quantum efficiency. Thus the spectrometer combines high speed, high spectral resolution, and excellent imaging quality.
A system to investigate the remediation of organic vapors using microwave-induced plasma with fluidized carbon granules
Authors: Dawson Elizabeth A. , Parkes Gareth M. B. , Bond Gary , Mao Runjie
Published: March 2009

This article describes a system to investigate the parameters for the remediation of organic vapors using microwave-induced plasma on fluidized carbon granules. The system is based on a single mode microwave apparatus with a variable power (2.45 GHz) generator. Carbon granules are fluidized in a silica tube situated in the sample section of a waveguide incorporating two additional ports to allow plasma intensity monitoring using a light sensor and imaging with a digital camera. A fluoroptic probe is used for in situ measurement of the carbon granule temperature, while the effluent gas temperature is measured with a thermocouple situated in the silica tube outside the cavity. Data acquisition and control software allow experiments using a variety of microwave power regimes while simultaneously recording the light intensity of any plasma generated within the carbon bed, together with its temperature. Evaluation using two different granular activated carbons and ethyl acetate, introduced as a vapor into the fluidizing air stream at a concentration of 1 ppm, yielded results which indicated that significant destruction of ethyl acetate, as monitored using a mass spectrometer, was achieved only with the carbon granules showing high plasma activity under pulsed microwave conditions. The system is therefore suitable for comparison of the relative microwave activities of various activated carbon granules and their performance in microwave remediation and regeneration.
Position sensitive, continuous wavelength tunable laser based on photopolymerizable cholesteric liquid crystals with an in-plane helix alignment
Authors: Yoshida Hiroyuki , Inoue Yo , Isomura Takuya , Matsuhisa Yuko , Fujii Akihiko , Ozaki Masanori
Published: March 2009

Position sensitive lasing with continuous wavelength tunability and emission in the cell-plane direction is demonstrated from a photopolymerized cholesteric liquid crystal film. The device has a gradually dilating helix lying in the cell-plane direction and is fabricated by applying a vertical electric field in a conventionally rubbed wedge cell while cooling the sample from the isotropic phase. Tuning range of ∼100nm is achieved by translating the device with respect to the pump beam. Photopolymerizable materials are especially useful in this configuration since a freestanding film, not requiring any external voltage to maintain the molecular ordering, can be prepared.
The roles of grain boundary and interfacial energies in the grain growth of nanocomposite films
Authors: Hu X. J. , Shen Y. G.
Published: March 2009

We report the roles of grain boundary and interfacial energies in the grain growth of nanocomposite films comprising of nanocrystalline (nc) grains and amorphous (a) matrix by using Monte Carlo simulations. With the energy difference (ΔJ) between nc/nc boundary energy and nc/a interfacial energy increasing to ∼7kBT, the microstructure in the system with the fixed amorphous amounts has smaller grain size, longer grain boundaries (GBs), and approximately lognormal grain size distribution. The change in energy exerted on each GB during the growth evolution is responsible for the formed microstructures.
Periodically poled silicon
Authors: Hon Nick K. , Tsia Kevin K. , Solli Daniel R. , Jalali Bahram
Published: March 2009

We propose a new class of photonic devices based on periodic stress fields in silicon that enable second-order nonlinearity as well as quasi-phase matching. Periodically poled silicon (PePSi) adds the periodic poling capability to silicon photonics and allows the excellent crystal quality and advanced manufacturing capabilities of silicon to be harnessed for devices based on second-order nonlinear effects. As an example of the utility of the PePSi technology, we present simulations showing that midwave infrared radiation can be efficiently generated through difference frequency generation from near-infrared with a conversion efficiency of 50%.
Midinfrared GaInSb/AlGaInSb quantum well laser diodes operating above 200 K
Authors: Nash G. R. , Przeslak S. J. B. , Smith S. J. , de Valicourt G. , Andreev A. D. , Carrington P. J. , Yin M. , Krier A. , Coomber S. D. , Buckle L. , Emeny M. T. , Ashley T.
Published: March 2009

Electroluminescence from GaInSb/AlGaInSb type I quantum well diode lasers, grown on GaAs, has been investigated as a function of strain in the quantum wells. Lasing was observed, in pulsed operation, up to temperatures of 161, 208, 219, and 202 K for structures containing 0.55%, 0.62%, 0.78%, and 1.1% strain, respectively, with lasing occurring at ∼3.3 μm at 200 K for the 1.1% structure.
Positron generation through laser Compton scattering gamma ray
Authors: Li D. , Imasaki K. , Miyamoto S. , Horikawa K. , Amano S. , Mochizuki T.
Published: March 2009

The positron source is developed through a high brightness gamma-ray beam, which is produced from a laser light scattering off a high-energy electron beam circulating in a storage ring. A thin Pb target is irradiated by the gamma ray to generate positrons via pair creation. By using imaging plate, we measured the generation rate and worked out the energy spectrum of generated positrons and electrons. The experimental results are close to that from Monte Carlo simulation. About 3618 positrons/s at several MeV can be generated by the present experimental setup, and this value can be improved by optimizing experimental conditions.
Ab initio study of energy band structures of GaAs nanoclusters
Authors: Jiang J. , Gao B. , Han T.-T. , Fu Y.
Published: March 2009

Electronic states and optical transitions of hydrogen terminated GaAs nanoclusters up to 16.9 nm in diameter were studied using a large-scale quantum chemistry approach called central insertion scheme by which the quantum confinement effect is shown to quantitatively agree with experimental results. The ab initio study further reveals that the effective mass of the conduction-band electron (valence-band hole) in the GaAs nanocluster is larger (smaller) than the bulk material value.
Common signature of many-body thermal excitation in structural relaxation and crystallization of chalcogenide glasses
Authors: Ielmini Daniele , Boniardi Mattia
Published: March 2009

The structural stability of amorphous chalcogenides used in electrical and optical phase-change devices is critically affected by structural relaxation (SR) and crystallization. We studied the temperature activation of SR and crystallization in amorphous Ge2Sb2Te5. We demonstrate that SR and crystallization coherently obey the same Meyer–Neldel (MN) rule, evidencing the key role of many-body thermal excitation in these transformations. The different activation energies for SR and crystallization are discussed based on the strength and number of bonds to be rearranged during the transitions. The MN rule provides a straightforward explanation of the unphysical pre-exponential times (10−24–10−22 s) observed in chalcogenide glasses.
Optical modeling of the plasmon band of monolayer-protected nanometal clusters in pure and in polymer matrix thin films as a function of heat treatment
Authors: Sivaramakrishnan Sankaran , Anto Bibin T. , Ho Peter K.-H.
Published: March 2009

The plasmon band shape of thin films of gold nanoparticles and their composites can be quantitatively modeled in a surprisingly simple way by taking into account (i) quantum-size effect of the Au core on its permittivity, (ii) nanostructure effect of the core shell and matrix on the effective medium, and (iii) optical properties of thin films in a transfer-matrix formalism. From the excellent agreement achieved with the optical spectra of these films, neat and when dispersed in poly(3,4-ethylenedioxythiophene) matrices, details of the nanocrystal relaxation, desorption of the ligand shell, and ultimate surface melting and core-core coalescence to give percolating conductive paths during heat treatment were extracted.
Control of threshold voltage and hysteresis in organic field-effect transistors
Authors: Kawaguchi Hideyuki , Taniguchi Masateru , Kawai Tomoji
Published: March 2009

We have fabricated organic field-effect transistors that have hybrid gate insulating films, consisting of an inorganic insulating film and a cross-linked insulating polymer. The hybrid insulating films allow the capacitance to be controlled easily, which makes it possible to reduce the influence of trapped charge at the insulator-semiconductor interface. As a result, the threshold voltage and the threshold voltage shift can be controlled. The increased capacitance of the insulating film results in low driving voltages and small hysteresis in the transfer characteristics.
Individual addressing of ions using magnetic field gradients in a surface-electrode ion trap
Authors: Wang Shannon X. , Labaziewicz Jaroslaw , Ge Yufei , Shewmon Ruth , Chuang Isaac L.
Published: March 2009

A dense array of ions in microfabricated traps represents one possible way to scale up ion trap quantum computing. The ability to address individual ions is an important component of such a scheme. We demonstrate individual addressing of trapped ions in a microfabricated surface-electrode trap using a magnetic field gradient generated on-chip. A frequency splitting of 310(2) kHz for two ions separated by 5 μm is achieved. Selective single qubit operations are performed on one of two trapped ions with an average of 2.2(±1.0%) crosstalk. Coherence time is reduced by the magnetic field gradient, but the spin-echo technique effectively restores the coherence time.
Dielectric relaxation behavior of undoped, Ce-, and Cr-doped Sr0.5Ba0.5Nb2O6 at high temperatures
Authors: Fang Tsang-Tse , Chung Han-Yang
Published: March 2009

The dielectric relaxation behavior of undoped, Ce-, and Cr-doped Sr0.5Ba0.5Nb2O6 in the nondispersion region below Burns temperature [G. Burns and F. H. Dacol, Phys. Rev. B 28, 2527 (1983)] has been examined by the equation ε(T)=εl[T/(T−Tc)] (ε is the dielectric constant, εl is the high-frequency permittivity, T is the temperature, and Tc is a parameter of the high temperature expansion of the dielectric constant) underlying Vugmeister’s theory [B. E. Vugmeister, Phys. Rev. B 73, 174117 (2006)]. The validity of this equation has been justified, and the physical meaning of εl has also been confirmed. The variation in Tc with the dopant content has been discussed.
Optical properties of Bi3.15Nd0.85Ti3O12 nanostructures
Authors: Cai Wei , Lu Xiaomei , Li De , Bo Huifeng , Peng Ruwen , Wu Xiaobo , Liu Yunfei , Zhu Jinsong
Published: March 2009

Ferroelectric materials are widely researched because of their application in electronic devices. A ferroelectric nanotube is important both theoretically and experimentally because of its nonplanar structure. Bi3.15Nd0.85Ti3O12 nanotubes are fabricated utilizing templates of porous anodic aluminum oxide and thin films are synthesized as well for comparison. Atomic force microscopy, scanning electron microscopy, and x-ray diffraction reveal the microstructure of the samples. The optical properties are investigated carefully to reveal the size effect with the decrease in sample dimensionality. The observed shift of the absorption edge for different nanostructures is discussed.
Two-dimensional electro-optic sampling of terahertz radiation using high-speed complementary metal-oxide semiconductor camera combined with arrayed polarizer
Authors: Kitahara Hideaki , Tani Masahiko , Hangyo Masanori
Published: March 2009

We developed a terahertz imaging system based on two-dimensional electro-optic (EO) sampling using a high speed complementary metal-oxide semiconductor (CMOS) camera and an arrayed polarizer matched to the camera. By operating the CMOS camera in a normalized differential mode on a single-shot basis, two-dimensional EO sampling is demonstrated.
Noise and terahertz rectification linked by geometry in planar asymmetric nanodiodes
Authors: Iñiguez-de-la-Torre I. , Mateos J. , Pardo D. , Song A. M. , González T.
Published: March 2009

In this work, by means of Monte Carlo simulations, we evidence the presence of a terahertz resonant peak in the ac to dc rectification of planar asymmetric nanodiodes which, remarkably, is linked to a noise mechanism, collective charge fluctuations in the space-charge region around the active channel of the device. The current noise spectral density of the diodes is compared with the frequency-dependent ac to dc rectification with the aim of identifying the signature of the phenomenon in both quantities. The frequency and magnitude of the resonance can be tuned by modifying the diode geometry. Results are interpreted in terms of an equivalent circuit model.
Polarization rotation of shape resonance in Archimedean spiral slots
Authors: Tzuang Lawrence Dah-Ching , Jiang Yu-Wei , Ye Yi-Han , Chang Yi-Tsung , Wu Yi-Ting , Lee Si-Chen
Published: March 2009

The experimental results of linear polarized light transmitting through an array of Archimedean spiral slots perforated on a thin metal film in the far-infrared region are demonstrated. The wavelength selective transmission spectra are examined, and the polarization selective shape resonance will lead to the polarization rotation. The sign of rotation not only depend on the handedness of the spiral but also related to specific initial polarization position. This phenomenon is different from the conventional optical activity in three-dimensional chiral molecules but rather similar to the polarization selective features in rectangular or elliptical slots.
Assessment of therapeutic efficacy and fate of engineered human mesenchymal stem cells for cancer therapy
Authors: Sasportas Laura S. , Kasmieh Randa , Wakimoto Hiroaki , Hingtgen Shawn , van de Water Jeroen A. J. M. , Mohapatra Gayatry , Figueiredo Jose Luiz , Martuza Robert L. , Weissleder Ralph , Shah Khalid
Published: March 2009

The poor prognosis of patients with aggressive and invasive cancers combined with toxic effects and short half-life of currently available treatments necessitate development of more effective tumor selective therapies. Mesenchymal stem cells (MSCs) are emerging as novel cell-based delivery agents; however, a thorough investigation addressing their therapeutic potential and fate in different cancer models is lacking. In this study, we explored the engineering potential, fate, and therapeutic efficacy of human MSCs in a highly malignant and invasive model of glioblastoma. We show that engineered MSC retain their “stem-like” properties, survive longer in mice with gliomas than in the normal brain, and migrate extensively toward gliomas. We also show that MSCs are resistant to the cytokine tumor necrosis factor apoptosis ligand (TRAIL) and, when engineered to express secreted recombinant TRAIL, induce caspase-mediated apoptosis in established glioma cell lines as well as CD133-positive primary glioma cells in vitro. Using highly malignant and invasive human glioma models and employing real-time imaging with correlative neuropathology, we demonstrate that MSC-delivered recombinant TRAIL has profound anti-tumor effects in vivo. This study demonstrates the efficacy of diagnostic and therapeutic MSC in preclinical glioma models and forms the basis for developing stem cell-based therapies for different cancers.
MLL5, a trithorax homolog, indirectly regulates H3K4 methylation, represses cyclin A2 expression, and promotes myogenic differentiation
Authors: Sebastian Soji , Sreenivas Prethish , Sambasivan Ramkumar , Cheedipudi Sirisha , Kandalla Prashanth , Pavlath Grace K. , Dhawan Jyotsna
Published: March 2009

Most cells in adult tissues are nondividing. In skeletal muscle, differentiated myofibers have exited the cell cycle permanently, whereas satellite stem cells withdraw transiently, returning to active proliferation to repair damaged myofibers. We have examined the epigenetic mechanisms operating in conditional quiescence by analyzing the function of a predicted chromatin regulator mixed lineage leukemia 5 (MLL5) in a culture model of reversible arrest. MLL5 is induced in quiescent myoblasts and regulates both the cell cycle and differentiation via a hierarchy of chromatin and transcriptional regulators. Knocking down MLL5 delays entry of quiescent myoblasts into S phase, but hastens S-phase completion. Cyclin A2 (CycA) mRNA is no longer restricted to S phase, but is induced throughout G
0
/G
1
, with activation of the cell cycle regulated element (CCRE) in the CycA promoter. Overexpressed MLL5 physically associates with the CCRE and impairs its activity. MLL5 also regulates CycA indirectly: Cux, an activator of CycA promoter and S phase is induced in RNAi cells, and Brm/Brg1, CCRE-binding repressors that promote differentiation are repressed. In knockdown cells, H3K4 methylation at the CCRE is reduced, reflecting quantitative global changes in methylation. MLL5 appears to lack intrinsic histone methyl transferase activity, but regulates expression of histone-modifying enzymes LSD1 and SET7/9, suggesting an indirect mechanism. Finally, expression of muscle regulators Pax7, Myf5, and myogenin is impaired in MLL5 knockdown cells, which are profoundly differentiation defective. Collectively, our results suggest that MLL5 plays an integral role in novel chromatin regulatory mechanisms that suppress inappropriate expression of S-phase-promoting genes and maintain expression of determination genes in quiescent cells.
Plant root growth and the marginal value theorem
Authors: McNickle Gordon G. , Cahill James F.
Published: March 2009

All organisms must find and consume resources to live, and the strategies an organism uses when foraging can have significant impacts on their fitness. Models assuming optimality in foraging behavior, and which quantitatively account for the costs, benefits, and biological constraints of foraging, are common in the animal literature. Plant ecologists on the other hand have rarely adopted an explicit framework of optimality with respect to plant root foraging. Here, we show with a simple experiment that the marginal value theorem (MVT), one of the most classic models of animal foraging behavior, can provide novel insights into the root foraging behavior of plants. We also discuss existing data in the literature, which has not usually been linked to MVT to provide further support for the benefits of an optimal foraging framework for plants. As predicted by MVT, plants invest more time and effort into highly enriched patches than they do to low-enriched patches. On the basis of this congruency, and the recent calls for new directions in the plant foraging literature, we suggest plant ecologists should work toward a more explicit treatment of the idea of optimality in studies of plant root foraging. Such an approach is advantageous because it forces a quantitative treatment of the assumptions being made and the constraints on the system. While we believe significant insight can be gained from the use of preexisting models of animal foraging, ultimately plant ecologists will have to develop taxa-specific models that account for the unique biology of plants.
Up-regulating BDNF with an ampakine rescues synaptic plasticity and memory in Huntington's disease knockin mice
Authors: Simmons Danielle A. , Rex Christopher S. , Palmer Linda , Pandyarajan Vijay , Fedulov Vadim , Gall Christine M. , Lynch Gary
Published: March 2009

Cognitive problems occur in asymptomatic gene carriers of Huntington's disease (HD), and mouse models of the disease exhibit impaired learning and substantial deficits in the cytoskeletal changes that stabilize long-term potentiation (LTP). The latter effects may be related to the decreased production of brain-derived neurotrophic factor (BDNF) associated with the HD mutation. This study asked whether up-regulating endogenous BDNF levels with an ampakine, a positive modulator of AMPA-type glutamate receptors, rescues plasticity and reduces learning problems in HD (CAG140) mice. Twice-daily injections of a short half-life ampakine normalized BDNF levels, activity-driven actin polymerization in dendritic spines, and LTP stabilization in 8-week-old mutants. Comparable results were obtained in 16-week-old HD mice with more severe LTP deficits. Ampakine treatments had no measurable effect on the decreased locomotor activity observed in the mutants but offset their impairments in long-term memory. Given that ampakines are well tolerated in clinical trials and were effective in this study after brief exposures, these results suggest a novel strategy for chronic treatment of the cognitive difficulties that occur in the early stages of HD.