bluuebunny/crossref_metadata_2025 huggingface dataset:
Accelerating the molecular time steps for nanomechanical simulations: Hybrid Monte Carlo method
Authors: Tomar Vikas
Published: May 2007

A majority of computational mechanical analyses of nanocrystalline materials or nanowires have been carried out using classical molecular dynamics (MD). Due to the fundamental reason that the MD simulations must resolve atomic level vibrations, they cannot be carried out at a time scale of the order of microseconds in a reasonable computing time. Additionally, MD simulations have to be carried out at very high loading rates (∼108 s−1) rarely observed during experiments. In this investigation, a modified hybrid Monte Carlo (HMC) method that can be used to analyze time-dependent (strain-rate-dependent) atomistic mechanical deformation of nanostructures at higher time scales than currently possible using MD is established for a Cu nanowire and for a nanocrystalline Ni sample. In this method, there is no restriction on the size of MD time step except that it must ensure a reasonable acceptance rate between consecutive Monte Carlo (MC) steps. In order to establish the method, HMC analyses of a Cu nanowire deformation at two different strain rates, viz., 108 and 109 s−1, and of a nanocrystalline Ni sample deformation at a strain rate of 109 s−1 with three different time steps, viz., 2, 4, and 8 fs, are compared with the analyses based on MD simulations at the same strain rates and with a MD time step of 2 fs. MD simulations of the Cu nanowire as well as nanocrystalline Ni deformations reproduce the defect nucleation and propagation results as well as strength values reported in the literature. Defect formation and stress-strain responses of the Cu nanowire, as well as of the nanocrystalline Ni sample during HMC simulations with a time step of 8 fs, are similar to that observed in the case of MD simulations with the maximum permissible time step of 2 fs (for the interatomic potential used, 2 fs is the highest MD time step). Simulation time analyses show that by using HMC approximately 4 times saving in computational time can be achieved bringing the atomistic analyses closer to the continuum time scales.
Al doping in (1−101)GaN films grown on patterned (001)Si substrate
Authors: Hikosaka T. , Honda Y. , Yamaguchi M. , Sawaki N.
Published: May 2007

The effect of Al doping on crystalline and optical properties of semipolar (1−101)GaN was investigated. The samples were grown on a patterned (001)Si substrate by selective metal-organic vapor phase epitaxy. The x-ray analyses showed that the strain in the (1−101)GaN layer is reduced substantially by the Al doping. Moreover, the cathode-luminescence (CL) intensity of the band edge emission band was enhanced and the linewidth became narrow. The CL images showed the reduction of dislocation density. These results show that the small amount of Al atoms in GaN improves the crystalline and optical properties. The results are attributed to the dislocation pinning and solution hardening effect due to Al atoms.
Two dimensional hydrodynamic simulation of high pressures induced by high power nanosecond laser-matter interactions under water
Authors: Wu Benxin , Shin Yung C.
Published: May 2007

In laser shock peening (LSP) under a water-confinement regime, laser-matter interaction near the coating-water interface can induce very high pressures in the order of gigapascals, which can impart compressive residual stresses into metal workpieces to improve fatigue and corrosion properties. For axisymmetric laser spots with finite size, the pressure generation near the water-coating interface is a two dimensional process in nature. This is in particular the case for microscale LSP performed with very small laser spots, which is a very promising technique to improve the reliability performance of microdevices. However, models capable of predicting two dimensional (2D) spatial distributions of the induced pressures near the coating-water interface in LSP have rarely been reported in literature. In this paper, a predictive 2D axisymmetric model is developed by numerically solving the hydrodynamic equations, supplemented with appropriate equations of state of water and the coating material. The model can produce 2D spatial distributions of material responses near the water-coating interface in LSP, and is verified through comparisons with experimental measurements. The model calculation shows that the effect of radial release wave on pressure spatial distributions becomes more significant as the laser spot size decreases, indicating the importance of a 2D model, particularly for microscale LSP.
Morphological differences affecting the dielectric response of MoO3−WO3 and WO3 thick films
Authors: Malagù C. , Carotta M. C. , Cervi A. , Guidi V. , Martinelli G.
Published: May 2007

An impedance spectroscopic analysis has been performed on WO3 and MoO3−WO3 thick-film gas sensors, working under the flow through technique. The dielectric response of the metal oxides at typical operation temperature (250–300 °C) has been analyzed in the presence of a few parts per million of NO2, in dry air. The experimental data were processed to get the best equivalent circuit with known circuital elements such as resistance, capacitance, and constant phase element. The sensors have been modeled taking into account the contribution to the total impedance of the intergranular contacts and of the substrate. As a result, a bimodal distribution of grains was foreseen for the WO3 film, as confirmed by scanning electron microscope analysis. In the case of MoO3−WO3 it was also possible to determine the permittivity of the material and the depletion region width.
A study of H+H2 and several H-bonded molecules by phaseless auxiliary-field quantum Monte Carlo with plane wave and Gaussian basis sets
Authors: Al-Saidi W. A. , Krakauer Henry , Zhang Shiwei
Published: May 2007

The authors present phaseless auxiliary-field (AF) quantum Monte Carlo (QMC) calculations of the ground states of some hydrogen-bonded systems. These systems were selected to test and benchmark different aspects of the new phaseless AF QMC method. They include the transition state of H+H2 near the equilibrium geometry and in the van der Walls limit, as well as the H2O, OH, and H2O2 molecules. Most of these systems present significant challenges for traditional independent-particle electronic structure approaches, and many also have exact results available. The phaseless AF QMC method is used either with a plane wave basis with pseudopotentials or with all-electron Gaussian basis sets. For some systems, calculations are done with both to compare and characterize the performance of AF QMC under different basis sets and different Hubbard-Stratonovich decompositions. Excellent results are obtained using as input single Slater determinant wave functions taken from independent-particle calculations. Comparisons of the Gaussian based AF QMC results with exact full configuration interaction show that the errors from controlling the phase problem with the phaseless approximation are small. At the large basis-size limit, the AF QMC results using both types of basis sets are in good agreement with each other and with experimental values.
Phase-ordering kinetics of the liquid-liquid transition in single-component molecular liquids
Authors: Kurita Rei , Tanaka Hajime
Published: May 2007

Recently it has been revealed that even a single-component liquid can have more than two liquid states. The transition between these liquid states is called the “liquid-liquid transition.” Most known liquid-liquid transitions occur at temperatures and pressures which are difficult to access experimentally, so the physical nature of the transition, particularly the kinetics, has remained elusive. However, the recent discovery of liquid-liquid transitions in molecular liquids opens up a possibility to study the kinetics in detail. Here, we report the first phase field simulation on the kinetics of a liquid-liquid transition and its direct comparison with experimental results of the molecular liquids. Both nucleation-growth-type and spinodal-decomposition-type liquid-liquid transformation observed experimentally are well reproduced by numerical simulation based on a two-order-parameter model of liquid that regards the liquid-liquid transition as the cooperative formation of locally favored structures. Thus, phase field calculations may allow us to predict the kinetics of liquid-liquid transitions and the resulting spatiotemporal change of various physical properties of the liquid, such as density and refractive index.
Experimental technique using FTIR to estimate IR optical properties at variable temperatures: Application to PMDA-ODA polyimide thin films from 100to380°C
Authors: Chandrasekharan Ramesh , Masel Richard I. , Shannon Mark A.
Published: May 2007

An experimental technique is presented to measure reflectance at high sample temperature with respect to room temperature in the infrared using Fourier transform infrared fitted with a reflectometer. Sample temperature artifacts are accounted for by sequential measurements taken with the lamp source on and with the lamp source off. The sequential measurements are shown mathematically to correct for the modulation of sample and detector thermal emissions. Further, the technique is applied to a polyimide (PMDA-ODA) film on a layer of gold deposited on a thermally oxidized Si wafer. It is shown that the optical properties (index of refraction and extinction coefficient) remain relatively constant with temperature (from room temperature to 380°C) in the 4000–6000cm−1 spectral region. The significant changes that occur with temperature are the change in thickness of the film and also the spectral properties in the 2000–4000cm−1 region. Also, by using a Lorentz oscillator model, it is shown that this method is able to discern that spectral features corresponding to the OH stretching bands at 3630 and 3470cm−1 show significant variation with increasing temperature.
Low frequency electrostatic waves in a cylindrically bounded dusty magnetoplasma with nonadiabatic dust charge fluctuation
Authors: Wei Nan-Xia , Xue Ju-Kui
Published: May 2007

A rigorous theoretical investigation is made for the low frequency electrostatic waves in a cylindrically bounded magnetized dusty plasmas with nonadiabatic dust charge variation, and the dispersion relation for the low frequency modes is derived. The combined effects of the cylindrical boundary, the external magnetized field, the nonadiabatic dust charge fluctuation, the nonthermally distributed ions and the ratio of electron to ion density on the low frequency waves are studied in great detail. It is shown that the mentioned effects have a strong influence on the dispersion properties of the low frequency modes.
Low-cost evaluation of the exchange Fock matrix from Cholesky and density fitting representations of the electron repulsion integrals
Authors: Aquilante Francesco , Pedersen Thomas Bondo , Lindh Roland
Published: May 2007

The authors propose a new algorithm, “local K” (LK), for fast evaluation of the exchange Fock matrix in case the Cholesky decomposition of the electron repulsion integrals is used. The novelty lies in the fact that rigorous upper bounds to the contribution from each occupied orbital to the exchange Fock matrix are employed. By formulating these inequalities in terms of localized orbitals, the scaling of computing the exchange Fock matrix is reduced from quartic to quadratic with only negligible prescreening overhead and strict error control. Compared to the unscreened Cholesky algorithm, the computational saving is substantial for systems of medium and large sizes. By virtue of its general formulation, the LK algorithm can be used also within the class of methods that employ auxiliary basis set expansions for representing the electron repulsion integrals.
Geometry and electronic structure of impurity-trapped excitons in Cs2GeF6:U4+ crystals. The 5f17s1 manifold
Authors: Ordejón Belén , Seijo Luis , Barandiarán Zoila
Published: May 2007

Excitons trapped at impurity centers in highly ionic crystals were first described by McClure and Pédrini [Phys. Rev. B 32, 8465 (1985)] as excited states consisting of a bound electron-hole pair with the hole localized on the impurity and the electron on nearby lattice sites, and a very short impurity-ligand bond length. In this work the authors present a detailed microscopic characterization of impurity-trapped excitons in U4+-doped Cs2GeF6. Their electronic structure has been studied by means of relativistic ab initio model potential embedded cluster calculations on (UF6)2− and (UF6Cs8)6+ clusters embedded in Cs2GeF6, in combination with correlation methods based on multireference wave functions. The local geometry of the impurity-trapped excitons, their potential energy curves, and their multielectronic wave functions have been obtained as direct, nonempirical results of the methods. The calculated excited states appear to be significantly delocalized outside the UF6 volume and their U–F bond length turns out to be very short, closer to that of a pentavalent uranium defect than to that of a tetravalent uranium defect. The wave functions of these excited states show a dominant U 5f17s1 configuration character. This result has never been anticipated by simpler models and reveals the unprecedented ability of diffuse orbitals of f-element impurities to act as electron traps in ionic crystals.
High-energy x-ray microbeam with total-reflection mirror optics
Authors: Suzuki Yoshio , Takeuchi Akihisa , Terada Yasuko
Published: May 2007

Total-reflection mirror optics for high-energy x-ray microfocusing have been developed, and tested in the energy range of 30–100keV at beamline 20XU of Synchrotron Radiation Facility SPring-8. The optical system consists of a Kirkpatrick-Baez-type [J. Opt. Soc. Am. 38, 766 (1548)] focusing optics with aspherical total-reflection mirrors for the purpose of reducing the spherical aberrations. A focused beam size of 0.35×0.4μm2 has been achieved at an x-ray energy of 80keV, and the measured spot size was less than 1μm in the x-ray energy region below 90keV.
Slice imaging of the quantum state-to-state cross section for photodissociation of state-selected rovibrational bending states of OCS (v2=,1,2∣JlM)+hν→CO(J)+S(D21)
Authors: Lipciuc M. Laura , Janssen Maurice H. M.
Published: May 2007

Using hexapole quantum state-selection of OCS (v2=0,1,2∣JlM) and high-resolution slice imaging of quantum state-selected CO(J), the state-to-state cross section OCS (v2=0,1,2∣JlM)+hν→CO(J)+S(D21) was measured for bending states up to v2=2. The population density of the state-selected OCS (v2=0,1,2∣JlM) in the molecular beam was obtained by resonantly enhanced multiphoton ionization of OCS and comparison with room temperature bulk gas. A strong increase of the cross section with increasing bending state is observed for CO(J) in the high J region, J=60–67. Integrating over all J states the authors find σ(v2=0):σ(v2=1):σ(v2=2)=1.0:7.0:15.0. A quantitative comparison is made with the dependence of the transition dipole moment function on the bending angle.
Vapor-liquid coexistence in fluids of charged hard dumbbells
Authors: Ganzenmüller Georg , Camp Philip J.
Published: May 2007

Vapor-liquid coexistence in fluids of charged hard dumbbells, each made up of two oppositely charged hard spheres with diameters σ and separation d, has been studied using grand-canonical Monte Carlo simulations. In the limit d∕σ→0, and with the temperature scaled accordingly, the system corresponds to dipolar hard spheres. For separations in the range 0.3<d∕σ⩽1 the coexisting vapor phase contains compact clusters. For separations in the range 0.1⩽d∕σ<0.3 the coexistence is between a chainlike vapor and a networklike liquid. Finite-size effects preclude the simulation of the coexistence in systems with d∕σ<0.1, but extrapolations of the results to d∕σ→0 yield estimates of the apparent critical parameters for dipolar hard spheres.
Time-resolved dynamics of coherent acoustic phonons in CdMnTe diluted-magnetic single crystals
Authors: Wang D. , Cross A. , Guarino G. , Wu S. , Sobolewski Roman , Mycielski A.
Published: May 2007

The authors report on observation of pronounced coherent acoustic phonon (CAP) oscillations in diluted-magnetic Cd1−xMnxTe (x=0.09) single crystals by means of femtosecond, optical single-color, and two-color pump-probe spectroscopy. The observed properties of CAP oscillations are consistent with the propagating-strain-pulse model, with electronic stress as the main CAP generation mechanism. The experimental dispersion relation between the CAP oscillation frequency and the probe-beam wave vector allowed them to extract the Cd0.91Mn0.09Te refractive index dependence on wavelength, while the dependence of oscillation amplitudes on the photon energy near the Cd0.91Mn0.09Te band gap edge indicated that the material energy gap is 1.63eV. Two-color pump-probe studies showed that the speed of sound in our material is 3590m∕s and that the absorption depth of probe light, rather than the decay time of CAPs, is the dominant source of the experimentally observed oscillation dephasing time. The intrinsic lifetime of CAPs in CdMnTe crystals has been found to be at least in the nanosecond range.
Anisotropic bulletlike emission of terminal ethynyl fragment ions: Ionization of ethynylbenzene-d under intense femtosecond laser fields
Authors: Yatsuhashi Tomoyuki , Murakami Masanao , Nakashima Nobuaki
Published: May 2007

The authors investigated Coulomb explosions of ethynylbenzenes under intense femtosecond laser fields. Deuteration on the edge of the triple bond gave information about specific fragment emissions and the contribution of hydrogen migration. Some fragments not resulting from migration were emitted in the direction of laser polarization. These were ethynyl fragment ions (D+, CD+, C2D+, and C3D+). Although two bonds have to be cleaved to produce C3D+, the rigid character of the triple bond was maintained in the Coulomb explosion process. In contrast, fragment ions, which are formed after single or double hydrogen migration, showed isotropic emissions with distinct kinetic energies. The character of the substituents has been found to hold even under strong laser light fields where violent fragmentation took place. The ethynyl parts were emitted like bullets from the molecular frame of ethynylbenzene despite the explosion into pieces of the main body of benzene ring.
Electron collisions with furan
Authors: Bettega M. H. F. , Lima M. A. P.
Published: May 2007

The authors report integral, differential and momentum transfer cross sections for elastic scattering of low-energy electrons by C4H4O (furan) molecules. Their calculations employed the Schwinger multichannel method with pseudopotentials and were performed in the static-exchange and in the static-exchange plus polarization approximations. The authors found two shape resonances located around 2.1 and 4.2eV that belong to the B1 and A2 symmetries of the C2v group, respectively. The authors’ results are consistent with recent measurements of vertical electron attachment energies.
Single-layer organic light-emitting diodes using naphthyl diamine
Authors: Tse S. C. , Tsung K. K. , So S. K.
Published: May 2007

N , N ′ -diphenyl-N,N′-bis(1-naphthyl)(1,1′-biphenyl)-4,4′diamine (NPB), a common hole transporter, was employed to fabricate single-layer organic light-emitting diodes (OLEDs). With a quasi-Ohmic anode, NPB device exhibited a bulk-limited hole current in the low-voltage region. Electron injection and light emission were clearly observed for applied voltages exceeding 4V. In order to confine the recombination zone, intentional doping was applied to the single-layer device. After doping with perylene, the luminance and current efficiency of NPB device increased dramatically. It is expected that more efficient single-layer OLEDs can be achieved by using the doping strategy.
Transmission through randomly arranged microcells of subwavelength holes on an aluminum film
Authors: Chang Yi-Tsung , Chuang Tzu-Hung , Yang Chieh-Hung , Tsai Ming-Wei , Lee Si-Chen
Published: May 2007

This investigation presents an observation of enhanced optical transmission through an Al film that is perforated with microcells that are arranged in random structures. The dispersion relations of the Al∕p-Si surface plasmon polariton in these structures with individual microcells with 3×3, 6×6, 9×9, 12×12, and 16×16 hole arrays of hexagonal were deduced. The transmission peak wavelength is determined from the spatial period of the microcell arrays. The random structure provides multicolor light transmission, which can be exploited in infrared wavelength-selective devices.
Closed-loop wavelength stabilization of an optical parametric oscillator as a front end of a high-power iodine laser chain
Authors: Kral L.
Published: May 2007

We present a complex stabilization and control system for a commercially available optical parametric oscillator. The system is able to stabilize the oscillator’s output wavelength at a narrow spectral line of atomic iodine with subpicometer precision, allowing utilization of this solid-state parametric oscillator as a front end of a high-power photodissociation laser chain formed by iodine gas amplifiers. In such setup, a precise wavelength matching between the front end and the amplifier chain is necessary due to extremely narrow spectral lines of the gaseous iodine (∼20pm). The system is based on a personal computer, a heated iodine cell, and a few other low-cost components. It automatically identifies the proper peak within the iodine absorption spectrum, and then keeps the oscillator tuned to this peak with high precision and reliability. The use of the solid-state oscillator as the front end allows us to use the whole iodine laser system as a pump laser for the optical parametric chirped pulse amplification, as it enables precise time synchronization with a signal Ti:sapphire laser.
Passive photonic elements based on dielectric-loaded surface plasmon polariton waveguides
Authors: Krasavin A. V. , Zayats A. V.
Published: May 2007

The authors present full three-dimensional numerical modeling of passive photonic elements based on dielectric-loaded surface plasmon polariton waveguides (DLSPPWs). They demonstrate that at telecom wavelengths a highly confined SPP mode can be guided in a single mode DLSPPW of subwavelength cross section and estimate the achievable density of photonic integration. The size of bending and splitting photonic elements based on DLSPPW can be as small as a few micrometers with pure bend loss less than 10% (0.4dB) and the transmission efficiency exceeding 70% (total loss of about 1.3dB). Such DLSPPW elements are important for implementation of photonic integrated circuits, guiding optical and electric signals in the same circuitry, and lab-on-a-chip applications.
Magnetism in dodecanethiol-capped gold nanoparticles: Role of size and capping agent
Authors: Dutta P. , Pal S. , Seehra M. S. , Anand M. , Roberts C. B.
Published: May 2007

In gold nanoparticles (Au NPs) capped with dodecanethiol (DT), the authors report the observation of superparamagnetic blocking temperature TB≃50K in D≃5nm NPs but only diamagnetism in 12nm NPs. For T<TB=50K, the strong temperature dependence of coercivity Hc, saturation magnetization Ms, and exchange bias He (in the field-cooled sample) confirm the blocked state resembling ferromagnetism with Hc≃250Oe, He≃−40Oe, and Ms≃10−2emu∕g at 5K. The observed electron magnetic resonance line shows expected shift, broadening, and reduced intensity below TB. A magnetic moment μ≃0.006μB per Au atom attached to DT is determined using a model which yields Ms varying as 1∕D, with its source being holes in the 5d band of Au produced by charge transfer from Au to S atoms in DT.
Foamability and structure analysis of foams in Hele-Shaw cell
Authors: Caps H. , Vandewalle N. , Broze G. , Zocchi G.
Published: May 2007

The authors have generated two-dimensional foams by imposing an intermittent drainage in a Hele-Shaw cell partially filled with a detergent/water mixture. The foam generation associated with this process is reproducible and depends on the surfactant molecules composing the solution. A kinetic model can be proposed for the foam evolution. The structure of the foam is also investigated: the average bubble side number and correlation functions are measured. Distinguishable behaviors are observed for different surfactant molecules. This way of producing a foam is thus adequate for applied foam structure characterizations and fundamental studies.
Thermal and elastic properties of Cu–Zr–Be bulk metallic glass forming alloys
Authors: Duan Gang , Lind Mary Laura , De Blauwe Katrien , Wiest Aaron , Johnson William L.
Published: May 2007

The compositional dependence of thermal and elastic properties of Cu–Zr–Be ternary bulk metallic glass forming alloys was systematically studied. There exists a linear relationship between the glass transition temperature Tg and the total Zr concentration. G decreases linearly with increasing Zr concentration as well. The results also show that Tg, shear modulus G, and Poisson’s ratio ν are very sensitive to changes in compositions. Low Tg, low G, and relatively high ν can be achieved with high Zr and Ti concentration.
Mechanism for persistent hexagonal island formation in AlN buffer layer during growth on Si (111) by plasma-assisted molecular beam epitaxy
Authors: Hsu Kuang-Yuan , Chung Hung-Chin , Liu Chuan-Pu , Tu Li-Wei
Published: May 2007

The characteristics of structure and morphology of AlN grown by a growth interruption method on Si (111) with plasma-assisted molecular beam epitaxy are investigated. It is found that the growth interruption method would improve the surface flatness of the AlN layer without the formation of Al droplets. However, AlN hexagonal islands were present and persistent throughout the entire growth owing to effective strain relaxation and Eherlich-Schowebel barrier effect of preexistent surface islands grown on higher terraces of the Si substrate. The density of threading dislocations underneath the hexagonal islands is much less than elsewhere in the film, which is presumably due to dislocation annihilation during the island growth process.
Experimental demonstration of the polarization-dependent photon-mediated carrier redistribution in tunneling injection InP quantum-dot lasers with external-grating feedback
Authors: Kim J. , Kondratko P. K. , Chuang S. L. , Walter G. , Holonyak N. , Heller R. D. , Zhang X. B. , Dupuis R. D.
Published: May 2007

Measured modal gain spectra of the tunneling injection InP quantum-dot (QD) laser with and without an external feedback are presented to experimentally demonstrate the polarization-dependent photon-mediated carrier redistribution in the tunneling injection QD laser. The peak gain wavelength in the transverse-electric gain narrowing spectra near threshold follows the external feedback wavelength. This indicates that the carrier redistribution in the QD-coupled-quantum well structure is determined by the spectral distribution of the stimulated emission, which can be controlled by external grating.
Magnetocaloric effect of Ho-, Dy-, and Er-based bulk metallic glasses in helium and hydrogen liquefaction temperature range
Authors: Luo Q. , Zhao D. Q. , Pan M. X. , Wang W. H.
Published: May 2007

The authors study the magnetocaloric effect and refrigerant capacity of Ho30Y26Al24Co20, Dy50Gd7Al23Co20, and Er50Al24Co20Y6 bulk metallic glasses. Their magnetic entropy changes associated with spin glass to paramagnetic transition are larger than those of Gd, Gd5Si2Ge1.9Fe0.1, and many other intermetallic compounds reported in the same temperature range. The good refrigerant efficiency combined with their high electrical resistivity, high thermal stability, outstanding mechanical properties, and tunable nature makes these glassy materials be attractive candidates for magnetic refrigerants in helium and hydrogen liquefaction temperature range of 2–50K.
Improved exchange-correlation potential for polarizability and dissociation in density functional theory
Authors: Maitra Neepa T. , van Faassen Meta
Published: May 2007

The authors propose a novel approach to the problem of polarizabilities and dissociation in electric fields from the static limit of the Vignale-Kohn (VK) functional. The response to the purely scalar part of the VK response potential is considered. This potential has ground-state properties that notably improve over the full VK response density and over usual (semi-)local functionals. The correct qualitative behavior of our potentials means that it is expected to work well for polarizabilities in cases such as the H2 chain, and it will also correctly dissociate open-shell fragments in a field.
Development of exchange-correlation functionals with minimal many-electron self-interaction error
Authors: Cohen Aron J. , Mori-Sánchez Paula , Yang Weitao
Published: May 2007

New exchange-correlation functionals that address the important issue of many-electron self-interaction are developed. This is carried out by considering the performance of the functional on systems with fractional numbers of electrons at the same time as more standard thermochemical tests. The inclusion of Coulomb-attenuated exchange in the functional is facilitated by use of the adiabatic connection coupled with a short-range and long-range splittings. The new functionals have a good performance on thermochemistry and a much improved description of the total energy versus number of electrons and henceforth a much smaller many-electron self-interaction error.
Aggregation-based growth and magnetic properties of inhomogeneous Cu-doped ZnO nanocrystals
Authors: Wang Xuefeng , Xu J. B. , Cheung W. Y. , An Jin , Ke Ning
Published: May 2007

The authors report on the ferromagnetic ordering in 1.86at.% Cu-doped ZnO nanocrystals synthesized by a solvothermal route. The distribution of Cu in ZnO lattice was inhomogeneous. In the Cu-poor cores, a spin-split acceptor impurity band model was proposed to explain the ferromagnetism. While in the Cu-rich surface layers, the shallow donorlike defects generated by imperfect oriented attachment could not only compensate a few holes but also readily occupy the Cu 3d states to destroy CuO4 tetrahedra, hence resulting in the short-range antiferromagnetic clusters. As a result, an interesting exchange biasing phenomenon took place at the ferromagnetic/antiferromagnetic interface.
Low threshold quantum-cascade lasers of room temperature continuous-wave operation grown by metal-organic chemical-vapor deposition
Authors: Wang X. J. , Fan J. Y. , Tanbun-Ek T. , Choa F.-S.
Published: May 2007

Very low threshold current densities are reported from buried heterojunction quantum-cascade laser (QCL) grown by metal-organic chemical-vapor deposition at an emission wavelength of 5.07μm. Continuous-wave operation threshold current densities as low as 0.75 and 0.83kA∕cm2, efficiencies of 1079 and 879mW∕A, and total output powers of 116 and 74mW have been achieved for temperatures of 288 and 298K. These cw threshold current densities are lower than all previously reported values for Fabry-Pérot QCLs. Internal loss as low as 1.84cm−1 has been extracted; also the lowest value thus far reported in the literature.
Higher nonlinear indices, Raman gain coefficients, and bandwidths in the TeO2–ZnO–Nb2O5–MoO3 quaternary glass system
Authors: Jose Rajan , Ohishi Yasutake
Published: May 2007

A quaternary glass system, TeO2–ZnO–Nb2O5–MoO3, with higher linear and nonlinear indices, third order optical susceptibility (χ3), and Raman gain coefficient and gain bandwidth is reported. The χ3 of these glasses are ∼80 times higher than the silica glass and 1.6 times higher than pure TeO2 glass; Raman gain coefficient is 40 times higher than the silica glass. Amplification bandwidth of one of the glasses is 1.7 times larger than the silica glass. The present glass has a glass transition temperature 80°C lower than the previously reported tellurite showing large Raman amplification bandwidth. These features make the TeO2–ZnO–Nb2O5–MoO3 glass a potential material for photonics devices such as fiber Raman amplifiers and/or nonlinear optical signal processing devices.
High-performance bottom-contact devices based on an air-stable n-type organic semiconductor N,N-bis (4-trifluoromethoxybenzyl)-1,4,5,8-naphthalene-tetracarboxylic di-imide
Authors: Kao Chia-Chun , Lin Pang , Lee Cheng-Chung , Wang Yi-Kai , Ho Jia-Chong , Shen Yu-Yuan
Published: May 2007

N , N -bis(4-trifluoromethoxybenzyl)-1,4,5,8-naphthalene-tetracarboxylic di-imide was applied to organic semiconductors for bottom-contact thin-film transistors. The carrier mobility was 1.6×10−2cm2V−1s−1, the threshold voltage (VT) was +5.5V, and the on/off current ratio was 8.6×105. Devices without any further surface treatments were tested in an ambient environment. The threshold voltage shift (ΔVT) was verified by gate bias stress measurements. A prototype compound, N,N-bis(4-trifluoromethylbenzyl)naphthalene-1,4,5,8-tetracarboxylic di-imide, shows direct correlation to the bottom-contact device with the varied molecular structure.
Crystallization at the glass transition in supercooled thin films of methanol
Authors: Dounce Susan M. , Mundy Julia , Dai Hai-Lung
Published: May 2007

The stability of an amorphous material depends on how fast and by what mechanism crystallization occurs. Based on crystallization rate measurements through optical reflectivity changes in supercooled methanol thin films, it is observed for the first time that there is a definitive and detectable change of the crystallization mechanism at the glass transition temperature Tg. For methanol glasses below Tg=103.4K, crystallization occurs as an interface controlled, one-dimension process at frozen-in embryo sites, while in the deep supercooled liquid phase above Tg crystallization is diffusion controlled in two dimensions with a constant nucleation rate and an activation energy of 107.8(±4.7)kJ∕mol.
Ferroelectric phase transitions near ionic liquid/vacuum interfaces
Authors: Tsekov Roumen
Published: May 2007

A simple theoretical model is developed describing ionic liquids as regular solutions. The separation of these ionic mixtures is studied on the base of the Cahn-Hilliard theory coupled with electrostatics. It is shown that the ionic liquids decompose to thin layers of oppositely charged liquids at low temperatures. At larger temperatures the separation occurs only near the ionic liquid/vacuum surface, thus explaining the oscillatory-decaying structure of the electric double layer observed via computer simulations. In contrast to noncharged liquids the ionic ones exhibit two critical temperatures, where the temperature coefficients of all characteristic lengths possess singularities. These second order ferroelectric phase transitions are possible explanations of the experimentally measured via light scattering peculiar temperature dependence of the interfacial dipole moment density on several ionic liquid/vacuum interfaces.
Vibrational spectroscopy of hydrated electron clusters(H2O)15–50− via infrared multiple photon dissociation
Authors: Asmis Knut R. , Santambrogio Gabriele , Zhou Jia , Garand Etienne , Headrick Jeffrey , Goebbert Daniel , Johnson Mark A. , Neumark Daniel M.
Published: May 2007

Infrared multiple photon dissociation spectra for size-selected water cluster anions (H2O)n−, n=15–50, are presented covering the frequency range of 560–1820cm−1. The cluster ions are trapped and cooled by collisions with ambient He gas at 20K, with the goal of defining the cluster temperature better than in previous investigations of these species. Signal is seen in two frequency regions centered around 700 and 1500–1650cm−1, corresponding to water librational and bending motions, respectively. The bending feature associated with a double-acceptor water molecule binding to the excess electron is clearly seen up to n=35, but above n=25; this feature begins to blueshift and broadens, suggesting a more delocalized electron binding motif for the larger clusters in which the excess electron interacts with multiple water molecules.
Temperature dependence of the pyroelectric coefficient and the spontaneous polarization of AlN
Authors: Yan W. S. , Zhang R. , Xiu X. Q. , Xie Z. L. , Han P. , Jiang R. L. , Gu S. L. , Shi Y. , Zheng Y. D.
Published: May 2007

Using the Debye model and existing experimental data of the pyroelectric coefficient of AlN, the temperature dependence of the pyroelectric coefficient as well as the spontaneous polarization of AlN is calculated over a wide temperature range from 0to1000K. The pyroelectric coefficient is proportional to T3 at low temperature and increases acutely from 0 to around 400K, and then increases gently from 400to1000K. It makes AlN uniquely suitable for application in high temperature pyroelectric sensors. The spontaneous polarization of AlN changes a little from 0to1000K, which indicates that the features of III-nitrides based devices will hardly be degraded by the change of the spontaneous polarization.
Fast reversible thermoelectrical switching in manganite thin films
Authors: Balevičius Saulius , Žurauskienė Nerija , Stankevič Voitech , Cimmperman Piotras , Keršulis Skirmantas , Česnys Antanas , Tolvaišienė Sonata , Altgilbers Larry Lee
Published: May 2007

The authors report on fast reversible switching of epitaxial and polycrystalline La0.83Sr0.17MnO3 films from the ferromagnetic to the paramagnetic state induced by high power nanosecond electrical pulses. It was demonstrated that for the epitaxial films adiabatic and uniform Joule heating is the reason of the switching. For polycrystalline films, electronic processes reduce the film resistance within a subnanosecond. For longer times, adiabatic nonhomogeneous heating plays the main role in the switching dynamics. Possible device applications of this phenomenon are also discussed.
Atomic layer deposition of insulating nitride interfacial layers for germanium metal oxide semiconductor field effect transistors with high-κ oxide/tungsten nitride gate stacks
Authors: Kim Kyoung H. , Gordon Roy G. , Ritenour Andrew , Antoniadis Dimitri A.
Published: May 2007

Atomic layer deposition (ALD) was used to deposit passivating interfacial nitride layers between Ge and high-κ oxides. High-κ oxides on Ge surfaces passivated by ultrathin (1–2nm) ALD Hf3N4 or AlN layers exhibited well-behaved C-V characteristics with an equivalent oxide thickness as low as 0.8nm, no significant flatband voltage shifts, and midgap density of interface states values of 2×1012cm−1eV−1. Functional n-channel and p-channel Ge field effect transistors with nitride interlayer/high-κ oxide/metal gate stacks are demonstrated.
Time dependent density of free carriers generated by two photon absorption in silicon waveguides
Authors: Liu Y. , Tsang H. K.
Published: May 2007

Carrier diffusion and recombination are considered in calculating the time dependent density of free carriers generated by two photon absorption in silicon waveguides. The calculations are compared to experimental results from optical pump-probe measurements of the induced loss from free carrier absorption. A generalized definition of nonlinear effective length is proposed to describe the effects of nonlinear loss. The authors experimentally demonstrate that helium-ion implantation can reduce the effective carrier lifetime and report on the observation of enhanced self-phase modulation in the helium implanted silicon waveguide.
In situ investigation of the discharge of alkaline Zn–MnO2 batteries with synchrotron x-ray and neutron tomographies
Authors: Manke I. , Banhart J. , Haibel A. , Rack A. , Zabler S. , Kardjilov N. , Hilger A. , Melzer A. , Riesemeier H.
Published: May 2007

Zn – Mn O 2 alkaline batteries were investigated in situ at different stages of electric discharge by synchrotron tomography with monochromatic x rays and by neutron tomography. The spatial distribution and the changes in the morphology of different components of a battery caused by the reduction of MnO2, the dissolution of Zn, and the nucleation and growth of ZnO are investigated with high spatial resolution around several micrometers with x rays. Neutron tomography is used to monitor the changes in the spatial distribution of hydrogen in the MnO2 matrix and provides complementary information about the process.
Observation of cross-sectional electric field for GaN-based field effect transistor with field-modulating plate
Authors: Wakejima Akio , Ota Kazuki , Nakayama Tatsuo , Ando Yuji , Okamoto Yasuhiro , Miyamoto Hironobu , Kamiya Shinichi , Suzuki Akira
Published: May 2007

Cross-sectional potential distribution of a GaN-based field effect transistor with a field-modulating plate has been measured by Kelvin prove force microscopy to investigate the effect of the field-modulating plate on an electric field under high-voltage biasing conditions. The observed potential distribution clearly revealed that the introduction of the field-modulating plate is effective in relaxing the peak electric field intensity at the drain side of the gate edge. Furthermore, the authors experimentally observed that the peak of electric field splits into two positions: drain-side edges of the gate and field-modulating plate electrodes.
Recovery of remanent polarization of poly(vinylidene fluoride-co-trifluoroethylene) thin film after high temperature annealing using topographically nanostructured aluminium bottom electrode
Authors: Park Youn Jung , Kang Seok Ju , Park Cheolmin , Woo Euntaek , Shin Kyusoon , Kim Kap Jin
Published: May 2007

Facile recovery of ferroelectric polarization after high temperature annealing was observed in a poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) thin film on an etched Al bottom electrode which has a topographically nanostructured surface with hexagonal registry of the recessed hemispherical bowls of approximately 100nm diameter. Fairly large remanent polarization of 10μC∕cm2 was obtained after annealing up to 185°C with the etched Al electrode, while the polarization rapidly dropped near the melting temperature of P(VDF-TrFE) (∼150°C) with a flat Al electrode. The topographic electrode is found to facilitate the reorganization of P(VDF-TrFE) crystal under electric field.
Three-level structure design and optically controlled current in coupled quantum dots
Authors: Chu Weidong , Duan Suqing , Zhu Jia-Lin
Published: May 2007

Two-mode photon-assisted transport through coupled nonidentical quantum dots is studied by designing the level structure of the system. It is found that a Λ-type three-level structure can be formed by modulating the shape and size of dots or by applying electric field. Based on the three-level system, current antiresonance phenomenon occurs due to the effect of coherent population trapping. Using the asymmetry of quantum dots, oscillating current can be produced by rotating the polarization direction of linearly polarized microwave.
Application of a 'case-building approach' to investigate the age distributions and growth dynamics of Australian sardine (Sardinops sagax) off South Australia
Authors: Rogers P. J. , Ward T. M.
Published: May 2007

Like many pelagic fish, sardine Sardinops sagax is difficult to age by counting structures in otoliths. Of 12968 whole otoliths analysed in the present study, <25% could be counted with accuracies of greater than ±1 zone. Estimates of average percentage error (APE) were high (11.37%). We used a ‘case building approach’ to corroborate a method for estimating age. Regressions of otolith weight–age from otoliths with high readabilities were used to estimate the age of fish with lower or unreadable otoliths. Growth rates determined from daily increments ranged from 0.36 to 0.75 mm day–1 for larvae and from 0.22 to 0.47 mm day–1 for juveniles. Estimates of von Bertalanffy growth coefficients were k = 0.32–1.07 year–1 and L∞ = 166.96–195.68 mm. Growth parameters estimated during the present study support previous assertions that levels of pelagic production in South Australian (SA) waters lie between those recorded in the predominantly oligotrophic waters off Western Australia (WA) and the productive waters off southern California and South Africa. Disparities in the age distributions of inshore and offshore samples suggest that age data from commercial catch samples may not be representative of the population. This finding has implications for the use of age-structured models to assess of stocks of S. sagax and other small pelagic fishes.
Impact of tadpoles and mosquito larvae on ephemeral pond structure and processes
Authors: Mokany Allie
Published: May 2007

Competition between organisms can influence both the abundance of species and the function of ecosystems. Here, I report the results of a field-based aquatic microcosm experiment, where the timing of arrival and abundance of two herbivores, tadpoles (Limnodynastes tasmaniensis) and mosquito larvae (Ochlerotatus notoscriptus), were manipulated to determine their impact on invertebrate community structure and ecosystem processes. Although successful establishment decreased with experimental time, there was no evidence that interactions between tadpoles and mosquitoes decreased the other species' subsequent survival. However, there were negative effects of tadpole addition on other invertebrates, with decreases in the abundance of zooplankton (Moina australiensis) and dipterans (Ephydridae and Chironomus oppositus). The addition of both tadpoles and mosquito larvae also increased ecosystem productivity. The negative effect of tadpoles on invertebrate abundance may result from competition for food or space, while complementary tadpole and mosquito effects on ecosystem processes are likely to result from changes in the dominant pond state. Tadpoles and mosquito larvae might influence the development of the dominant pond state through preferential consumption of edible phytoplankton and bacteria, reducing the competitive pressure on relatively inedible metaphyton, which forms dominating mats.
Affinity of extracellular phosphatases for ELF97 phosphate in aquatic environments
Authors: Nedoma Jiří , Van Wambeke France , Štrojsová Alena , Štrojsová Martina , Duhamel Solange
Published: May 2007

Recently, the phosphatase substrate ELF97 phosphate (ELFP) has been employed to study the presence of extracellular phosphatases in different plankton populations in natural aquatic environments. Kinetic properties of ELFP hydrolysis by natural extracellular phosphatases are, however, mostly unknown. We indirectly studied the affinity of extracellular phosphatases for ELFP in different aquatic environments through its ability to inhibit the hydrolysis of 4-methylumbelliferyl phosphate (4MUP). Values of inhibition constants, Ki, which correspond to the concentrations necessary for half saturation of phosphatases by ELFP, were lowest (0.18–4.5 µmol L–1) in the oligotrophic Mediterranean Sea. We found higher values (i.e. lower affinity) in oligo- to mesotrophic acidified lakes (5.2–14 µmol L–1), in a eutrophic reservoir (13–35 µmol L–1) and in a pure culture of the marine bacterium Alteromonas infernus (29 µmol L–1). ELFP had a pronounced effect on the parameter KM (Michaelis constant) of 4MUP saturation kinetics, while its effect on the parameter Vmax was low. This behaviour is compatible with the assumption of competitive interaction between 4MUP and ELFP. Our experiments indicated that the assay ELFP concentration in the detection kit used was 250–500 µmol L–1 (after the recommended dilution to a ratio of 1:20), which would ensure >99% saturation of extracellular phosphatases in marine environments and >90% saturation in the studied fresh waters.