bluuebunny/crossref_metadata_2025 huggingface dataset:
Tensile stress generation and dislocation reduction in Si-doped AlxGa1−xN films
Authors: Manning I. C. , Weng X. , Acord J. D. , Fanton M. A. , Snyder D. W. , Redwing J. M.
Published: July 2009

The effects of Si doping on the evolution of stress in AlxGa1−xN:Si thin films (x≈0.4–0.6) grown on 6H-SiC by metal organic chemical vapor deposition were investigated using in situ wafer curvature measurements. The results were correlated with changes in film microstructure as observed by transmission electron microscopy. The incorporation of Si into the films resulted in a compressive-to-tensile transition in the biaxial stress at the surface, and the magnitude of the tensile stress was found to increase in proportion to the Si concentration. The stress gradient was attributed to Si-induced dislocation inclination resulting from an effective climb mechanism. Si doping also resulted in a decrease in the threading dislocation density in the AlxGa1−xN layers, which was attributed to increased dislocation interaction and annihilation. The model describing tensile stress generated by dislocation effective climb was modified to account for the dislocation reduction and was found to yield an improved fit to the experimental stress-thickness data.
Density functional theory calculation on many-cores hybrid central processing unit-graphic processing unit architectures
Authors: Genovese Luigi , Ospici Matthieu , Deutsch Thierry , Méhaut Jean-François , Neelov Alexey , Goedecker Stefan
Published: July 2009

We present the implementation of a full electronic structure calculation code on a hybrid parallel architecture with graphic processing units (GPUs). This implementation is performed on a free software code based on Daubechies wavelets. Such code shows very good performances, systematic convergence properties, and an excellent efficiency on parallel computers. Our GPU-based acceleration fully preserves all these properties. In particular, the code is able to run on many cores which may or may not have a GPU associated, and thus on parallel and massive parallel hybrid machines. With double precision calculations, we may achieve considerable speedup, between a factor of 20 for some operations and a factor of 6 for the whole density functional theory code.
A review of solar water heating systems
Authors: Ogueke N. V. , Anyanwu E. E. , Ekechukwu O. V.
Published: July 2009

A review of solar water heating systems for domestic and industrial applications is presented. They are grouped into two broad categories as passive and active solar water heating systems. Each of them operates in either direct or indirect mode. Their performances, uses and applications, and factors considered for their selection are reported. The active systems generally have higher efficiencies, their values being 35%–80% higher than those of the passive systems. They are more complex and expensive. Accordingly, they are most suited for industrial applications where the load demand is quite high or in applications where the collector and service water storage tank need not be close to each other or for the applications in which the load requires more than one solar collector. On the other hand, the passive systems are less expensive and easier to construct and install. They are most suitable for domestic applications and in applications where load demand is low or medium. Generally more research and development work are needed to further improve the existing level of efficiency for it to serve effectively as a viable alternative to the conventional means of hot water generation. The actual field testing experiences, together with the prospects and economic problems that affect popularization of the systems, are also presented. Their possible solutions are suggested.
N
=
2
supersymmetric extension of a hydrodynamic system in Riemann invariants
Authors: Grundland A. M. , Hariton A. J.
Published: July 2009

In this paper, we formulate an N=2 supersymmetric extension of a hydrodynamic-type system involving Riemann invariants. The supersymmetric version is constructed by means of a superspace and superfield formalism, using bosonic superfields, and consists of a system of partial differential equations involving both bosonic and fermionic variables. We make use of group-theoretical methods in order to analyze the extended model algebraically. Specifically, we calculate a Lie superalgebra of symmetries of our supersymmetric model and make use of a general classification method to classify the one-dimensional subalgebras into conjugacy classes. As a result we obtain a set of 401 one-dimensional nonequivalent subalgebras. For selected subalgebras, we use the symmetry reduction method applied to Grassmann-valued equations in order to determine analytic exact solutions of our supersymmetric model. These solutions include traveling waves, bumps, kinks, double-periodic solutions, and solutions involving exponentials and radicals.
Electronic structure of aromatic amino acids studied by soft x-ray spectroscopy
Authors: Zhang Wenhua , Carravetta Vincenzo , Plekan Oksana , Feyer Vitaliy , Richter Robert , Coreno Marcello , Prince Kevin C.
Published: July 2009

The electronic structure of phenylalanine, tyrosine, tryptophan, and 3-methylindole in the gas phase was investigated by x-ray photoemission spectroscopy (XPS) and near edge x-ray absorption fine structure (NEXAFS) spectroscopy at the C, N, and O K-edges. The XPS spectra have been calculated for the four principal conformers of each amino acid, and the spectra weighted by the Boltzmann population ratios calculated from published free energies. Instead of the single peaks expected from the stoichiometry of the compounds, the N 1s core level spectra of phenylalanine and tryptophan show features indicating that more than one conformer is present. The calculations reproduce the experimental features. The C and O 1s spectra do not show evident effects due to conformational isomerism. The calculations predict that such effects are small for carbon, and for oxygen it appears that only broadening occurs. The carbon K-edge NEXAFS spectra of these aromatic amino acids are similar to the published data of the corresponding molecules in the solid state, but show more structure due to the higher resolution in the present study. The N K-edge spectra of tryptophan and 3-methylindole differ from phenylalanine and tyrosine, as the first two both contain a nitrogen atom located in a pyrrole ring. The nitrogen K-edge NEXAFS spectra of aromatic amino acids do not show any measurable effects due to conformational isomerism, in contrast to the photoemission results. Calculations support this result and show that variations of the vertical excitation energies of different conformers are small, and cannot be resolved in the present experiment. The O NEXAFS spectra of these three aromatic compounds are very similar to other, simpler amino acids, which have been studied previously.
Origin of large moments in MnxSi1−x at small x
Authors: Shaughnessy M. , Fong C. Y. , Snow Ryan , Liu Kai , Pask J. E. , Yang L. H.
Published: July 2009

Recently, the magnetic moment/Mn, M, in MnxSi1−x was measured to be 5.0 μB/Mn, at x=0.1%. To understand this observed M, we investigate several MnxSi1−x models of alloys using first-principles density functional methods. The only model giving M=5.0 was a 513-atom cell having the Mn at a substitutional site, and Si at a second-neighbor interstitial site. The observed large moment is a consequence of the weakened d-p hybridization between the Mn and one of its nearest neighbor Si atoms, resulting from the introduction of the second-neighbor interstitial Si. Our result suggests a way to tune the magnetic moments of transition metal doped semiconductors.
Surface anchoring and dynamics of thiolated azobenzene molecules on Au(111)
Authors: Cho Jongweon , Levy Niv , Kirakosian Armen , Comstock Matthew J. , Lauterwasser Frank , Fréchet Jean M. J. , Crommie Michael F.
Published: July 2009

We have investigated the temperature-dependent behavior of thiolated azobenzene molecules on Au(111) using scanning tunneling microscopy. The addition of a thiol functional group to azobenzene molecules leads to increased surface anchoring of single azobenzene molecules to gold. Thiolated azobenzene shows diverse surface morphology and does not form well-ordered structures at low coverage. At elevated temperatures, anchored molecules are observed to spin in place via hindered rotation. By measuring the number of rotating molecules as a function of temperature and using a simple model, we are able to estimate the energy barrier and attempt frequency for thermally induced hindered rotation to be 102±3 meV and 110±2 GHz, respectively.
Photoluminescence properties of size-controlled silicon nanocrystals at low temperatures
Authors: Rinnert H. , Jambois O. , Vergnat M.
Published: July 2009

This study attempts to clarify the origin of the temperature dependence of the photoluminescence (PL) spectra of silicon nanocrystals (Si-ncs) embedded in SiO2 from 5 to 300 K. For this purpose, size-controlled Si-ncs with a narrow size distribution were fabricated, using the SiO/SiO2 multilayer structure. The PL intensity is strongly temperature dependent and presents a maximum at around 70 K, depending on the Si-nc size and on the excitation power. The origin of this maximum is first discussed thanks to PL dynamics study and power dependence study. The evolution of the PL energy with temperature is also discussed. In bulk semiconductors the temperature dependence of the gap is generally well represented by Varshni’s law. Taking into account the quantum confinement energy, the PL energy of Si-ncs follows very well this law in the range 50–300 K. Below 50 K, a strong discrepancy to this law is observed characterized by a strong increase in the PL energy at low temperature, which is dependent on the Si-nc size distribution. This temperature dependence of the PL energy is correlated with a decrease in the radiative rate at low temperature and is explained by a preferential saturation effect of the bigger Si-ncs.
Synthesis of DNA templated trifunctional electrically conducting, optical, and magnetic nanochain of Nicore–Aushell for biodevice
Authors: Mandal Madhuri , Mandal Kalyan
Published: July 2009

Synthesis of trifunctional, e.g., electrically conducting, optical, and magnetic nanochains of Nicore–Aushell, has been discussed here. Properties of the materials were investigated from the view of its application in bionanodevice. Our investigation indicates that such material attached to biomolecule “DNA chain” and having three main properties in one material will have great potentiality in medical instrumentation and biocomputer device.
The retarded van der Waals potential: Revisited
Authors: Miyao Tadahiro , Spohn Herbert
Published: July 2009

The retarded van der Waals potential, as first obtained by Casimir and Polder, is usually computed on the basis of nonrelativistic quantum electrodynamics . The Hamiltonian describes two infinitely heavy nuclei, charge e, separated by a distance R and two spinless electrons, charge −e, nonrelativistically coupled to the quantized radiation field. Casimir and Polder used the dipole approximation and small coupling to the Maxwell field. We employ here the full Hamiltonian and determine the asymptotic strength of the leading −R−7 potential, which is valid for all e. Our computation is based on a path integral representation and expands in 1/R, rather than in e.
Effects of Cu5Zn3 addition on the thermoelectric properties of Zn4Sb3
Authors: Cui J. L. , Fu H. , Mao L. D. , Chen D. Y. , Liu X. L.
Published: July 2009

The structures and thermoelectric properties of mCu5Zn3⋅nZn4Sb3 with multiphase coexistence are reported. Rietveld analysis reveals that at least 92.3% wt % β-Zn4Sb3 phase can be obtained with only small quantities of ZnSb and Cu5Zn8 phases precipitated after proper Cu5Zn3 addition. Measurements indicate that although the β-Zn4Sb3 phase plays a determining role in controlling the transport properties involving the Seebeck coefficient, electrical conductivity, and thermal conductivity, the impurity phases Cu5Zn8 and ZnSb with a crooked riverlike and intertwined tree stump morphologies, respectively, are still of great significance to tune the thermoelectric performance. The highest ZT value of 0.84 can be obtained for the alloy mCu5Zn3⋅nZn4Sb3 (m/n=1/200) at 631 K, approximately 1.8 times that of undoped β-Zn4Sb3, proving that a good combination between the transports of carriers and phonons can be achieved if a proper dopant is introduced in the Zn4Sb3 matrix.
Photoresistances of semi-insulating GaAs photoconductive switch illuminated by 1.064 μm laser pulse
Authors: Wu Minghe , Zheng Xiaoming , Ruan Chengli , Yang Hongchun , Sun Yunqing , Wang Shan , Zhang Kedi , Liu Hong
Published: July 2009

The Shockley–Read–Hall model (SRHM) and its simplified model (SSRHM) were used to describe the characteristics of a photoconductive semiconductor switch (PCSS) made from a semi-insulating (SI) gallium arsenide (GaAs) chip, biased at low voltage, and illuminated by a 1.064 μm laser pulse. These characteristics include the free carrier densities, dynamic photoresistance, and time evolution of output pulses of the PCSS. The deep donor EL2 centers in SI GaAs play a dominant role in both the SRHM and SSRHM as electrons at EL2 unionized centers are strongly excited by the subband-gap photons at the wavelength of 1.064 μm. Theoretical modeling on the evolution of the experimental measured output pulses led to a two-step micromechanism of electron excitation process within the GaAs chip. The minimum photoresistances predicted by the SSRHM are in good agreement with experimental measurements, which confirms the dominant role of EL2 in the generation of electric pulses from a SI GaAs photoconductivity switch on which the 1064 nm laser pulse is illuminated.
Magnetic-field-dependent morphology of self-organized Fe on stepped Si(111) surfaces
Authors: Cougo dos Santos M. , Geshev J. , Pereira L. G. , Schmidt J. E.
Published: July 2009

The present work reports on Fe thin films grown on vicinal Si(111) substrates via rf magnetron sputtering. The dependencies of the growth mode and magnetic properties of the obtained iron nanostructures on both crystallographic surface orientation and on the direction of the very weak stray magnetic field from the magnetron gun were studied. Scanning tunneling microscopy images showed strong dependence of the Fe grains’ orientation on the stray field direction in relation to the substrate’s steps demonstrating that, under appropriately directed magnetic field, Si surfaces can be used as templates for well-defined self-assembled iron nanostructures. Magneto-optical Kerr effect hysteresis loops showed an easy-axis coercivity almost one order of magnitude smaller for the film deposited with stray field applied along the steps, accompanied with a change in the magnetization reversal mode. Phenomenological models involving coherent rotation and/or domain-wall unpinning were used for the interpretation of these results.
Optical characterization of thin epitaxial GaAs films on Ge substrates
Authors: Wu J. D. , Huang Y. S. , Brammertz G. , Tiong K. K.
Published: July 2009

Two thin epitaxial GaAs films on Ge substrates were characterized by photoluminescence (PL), contactless electroreflectance (CER), and piezoreflectance (PzR) techniques. The GaAs films containing antiphase domains (APDs) and those APD free were grown at 650 °C by metal organic vapor phase epitaxy. The PL intensity of the APD-containing film is four orders of magnitude lower than that of the APD-free sample. The reduction in intensity is due to the electrically active Ga–Ga and As–As bonds at the boundaries between the different APDs. The electric fields deduced from Franz–Keldysh oscillations of the CER spectra were used to determine the net carrier concentration of the GaAs films. In addition, the conduction to light-hole band transitions and the conduction to heavy-hole band transitions that originated from strain induced valence band splitting in GaAs thin films were observed and identified through a comparison of the relative intensities of light- and heavy-hole related features in the PzR and CER spectra. The results demonstrate that PL, CER, and PzR are useful nondestructive tools to characterize thin epitaxial GaAs films on Ge substrates.
Model study of electroporation effects on the dielectrophoretic response of spheroidal cells
Authors: Hu Q. , Joshi R. P. , Beskok A.
Published: July 2009

Model-based numerical predictions of the dielectrophoretic behavior of spheroidal biological cells are carried out. Membrane electroporation has been specifically included to gauge its effect on the Clausius–Mossotti factor. It is shown that poration could be utilized as an additional flexible tool for cell separation. Furthermore, the simulation study focuses on differences in dielectrophoretic characteristics between normal and malignant cells, and that one cell type can be more prone to poration than another. These results point to the possibility of judiciously selecting electrical parameters for effective cell separation or tumor detection, especially in high conductivity media.
Magnetic properties and large reversible magnetocaloric effect in PrCo2B2 compound
Authors: Li Lingwei , Nishimura Katsuhiko
Published: July 2009

A single-phased PrCo2B2 compound was prepared by an arc-melting method. The magnetic properties and magnetocaloric effect (MCE) were studied by the magnetization and heat capacity measurement. The compound exhibits a single paramagnetic to ferromagnetic transition at the Curie temperature TC of 18 K. A large reversible MCE was observed which is related to a second order magnetic transition from paramagnetic to ferromagnetic state. The values of maximum magnetic entropy change (−ΔSMmax) reach 4.9 and 9.6 J Kg−1 K−1 for the field change of 2 and 7 T with no obvious hysteresis loss in the vicinity of the Curie temperature, respectively. The corresponding maximum adiabatic temperature changes (ΔTadmax) are evaluated to be 4.3 and 9.8 K. The magnetic transition and the origin of large MCE in PrCo2B2 were discussed.
Characterization of midwave infrared InAs/GaSb superlattice photodiode
Authors: Cervera C. , Rodriguez J. B. , Chaghi R. , Aït-Kaci H. , Christol P.
Published: July 2009

We report on structural, electrical, and optical characterizations of midwave infrared InAs/GaSb superlattice (SL) p-i-n photodiodes. High-quality SL samples, with 1 μm thick active region (220 SL periods), exhibited a cut-off wavelength of 4.9 μm at 80 K. Using a capacitance-voltage measurement technique performed on mesa diode, the residual background concentration in the nonintentionally doped region was determined to be 3×1015 cm−3 at 80 K. Extracted from current-voltage characteristics, R0A products above 4×105 Ω cm2 at 80 K were measured, and the quantitative analysis of the J-V curves showed that the dark current density of SL photodiode is dominated by generation-recombination processes. Front-side illuminated photodiodes produced responsivity at 80 K equal to 360 mA/W at 4.5 μm.
Conductive oxide thin films: Model systems for understanding and controlling surface plasmon resonance
Authors: Losego Mark D. , Efremenko Alina Y. , Rhodes Crissy L. , Cerruti Marta G. , Franzen Stefan , Maria Jon-Paul
Published: July 2009

Degeneratively doped conductive oxides represent a unique host for exploring the inter-relationship between the properties of charge carriers and their collective plasmonic response. These materials often lack interband transitions that obfuscate interpretation of spectral response in elemental metals, and unlike metals, the electronic transport properties of conductive oxides are easily tunable. This work explores the process-structure-property relationships that regulate surface plasmon resonance (SPR) in sputter deposited indium tin oxide (ITO) thin films. Film deposition conditions are used to regulate film microstructure and tune the electronic mobility to between 7 and 40 cm2 V−1 s−1. Postdeposition annealing in low oxygen partial pressure atmospheres is used to engineer the ITO defect equilibrium and modulate carrier concentrations to between 1020 and 1021 cm−3. These electronic transport properties are modulated with near independence enabling straightforward interpretation of their influence on the SPR response observed in the infrared reflectivity spectrum. Higher electronic mobilities favor narrower surface plasmon absorption bands, while higher carrier concentrations favor higher absorption band frequencies. A simple free electron model, having only electronic carrier density and electronic mobility as variables, can be used to describe ITO’s dielectric response. Calculations that combine this dielectric function and the Fresnel equations provide simulated reflectivity spectra that match experimental data with remarkable accuracy. Because these spectra use no fitting parameters and are calculated with well-studied material properties, it opens the opportunity for future design of plasmonic response in advanced material systems including degeneratively doped semiconductors, silicides, and nitrides.
Energy and charge-transfer dynamics using projected modes
Authors: Pereverzev Andrey , Bittner Eric R. , Burghardt Irene
Published: July 2009

For electron-phonon Hamiltonians with the couplings linear in the phonon operators, we construct a class of unitary transformations that separate the normal modes into two groups. The modes in the first group interact with the electronic degrees of freedom directly. The modes in the second group interact directly only with the modes in the first group but not with the electronic system. These transformations can be carried out independently for different types of phonon modes, e.g., high- versus low-frequency phonon bands. This construction generalizes recently introduced transformations for systems exhibiting a conical intersection topology. The separation of the normal modes into several groups allows one to develop new approximation schemes. We apply one of such schemes to study electronic relaxation at a semiconducting polymer interface.
The near IR spectrum of the NO(XΠ2)–CH4 complex
Authors: Wen B. , Meyer H.
Published: July 2009

We report the first measurement of the near IR spectrum of the NO–CH4 complex in the region of the first vibrational NO overtone transition in an IR-resonance enhanced multiphoton ionization double resonance experiment. The origin band is located at 3723.26 cm−1, i.e., redshifted by 0.59 cm−1 from the corresponding NO monomer frequency. The observed spectrum consists of two bands assigned to the origin band and the excitation of hindered rotation of the NO monomer in the complex similar to z-axis rotation. The spacing and the relative intensity of the bands are consistent with a structure in which NO resides preferentially in a position perpendicular to the intermolecular axis. The deviation from the linear configuration with C3v symmetry can be regarded as a Jahn–Teller (JT) distortion. Each band is dominated by two broad peaks with a few resolved rotational structures. The large spacing between the two peaks is indicative of significant angular momentum quenching, possibly another manifestation of the JT effect. The delay dependence between the IR and UV laser pulses reveals a lifetime of about 10 ns for the vibrationally excited complex due to vibrational predissociation. On the other hand, the linewidth of the narrowest spectral features indicates a much shorter excited state lifetime of about 100 ps, most likely due to intramolecular vibrational redistribution.
Using a two-dimensional electron gas to study nonequilibrium tunneling dynamics and charge storage in self-assembled quantum dots
Authors: Marquardt B. , Geller M. , Lorke A. , Reuter D. , Wieck A. D.
Published: July 2009

We demonstrate a strong influence of charged self-assembled quantum dots (QD) on the conductance of a nearby two-dimensional electron gas (2DEG). A conductance measurement of the 2DEG allows us to probe the charge tunneling dynamics between the 2DEG and the QDs in nonequilibrium as well as close to equilibrium. Measurements of hysteresis curves with different sweep times and time-resolved conductance measurements enable us to unambiguously identify the transients as tunneling events between the 2DEG and QD states.
Umbrella integration in two or more reaction coordinates
Authors: Kästner Johannes
Published: July 2009

Umbrella integration is a method to analyze umbrella sampling simulations by calculating and integrating the mean force. Here, the method is extended to multidimensional reaction coordinates. Approximation of the probability distribution obtained from sampling by a multivariate normal distribution allows to calculate the mean force from the average and the covariance matrix of the reaction coordinate. Integration schemes of the free-energy gradient field are discussed. Integration on a real-space grid is compared to expansion of the gradient in a series of analytic functions (such as a Fourier analysis), which can be integrated, and the expansion of the gradient only at the window means in a series of analytic functions. The Fourier analysis was found particularly useful for periodic reaction coordinates, such as torsion angles. An expression is provided to calculate the Hessian of the free energy with respect to the reaction coordinates from sampling data. The utility of the method is demonstrated at the example of the free-energy surface of the alanine dipeptide in vacuum calculated with respect to the backbone torsion angles Φ and Ψ. Relevance of the Jacobian term for non-Cartesian reaction coordinates is discussed.
Structure of liquid n-hexane
Authors: Venturi G. , Formisano F. , Cuello G. J. , Johnson M. R. , Pellegrini E. , Bafile U. , Guarini E.
Published: July 2009

The structure of deuterated liquid n-hexane has been investigated at room temperature by neutron diffraction and molecular dynamics simulations. By carrying out a careful analysis of the measurements, experimental data were obtained in very good agreement with the simulated data. This allowed a thorough analysis of the simulation results aiming at the evaluation of the partial, intra-, and intermolecular components of the n-hexane structure. We finally compare the intramolecular differential cross sections calculated from the most probable n-hexane molecular configurations with the measured and simulated data.
A hexapole magnetic guide for neutral atomic beams
Authors: Beardmore J. P. , Palmer A. J. , Kuiper K. C. , Sang R. T.
Published: July 2009

In this paper we present a multiple element magnetic device to guide atoms using a spatially inhomogeneous magnetic field formed by a series of permanent hexapole magnets. The operation of the device is demonstrated using an enhanced beam of neon atoms in the P32 metastable state. These atoms are guided around a bend of 30° from their original path. A flux of 4.35×109±2×107 atoms s−1 was measured after the device yielding a transmission efficiency of approximately 9% of the input flux. Simulations of the center of mass motion of the atoms through the magnetic guide have been performed giving reasonable agreement with the experimental results.
Eddy genesis and manipulation in plane laminar shear flow
Authors: Scholle M. , Haas A. , Aksel N. , Wilson M. C. T. , Thompson H. M. , Gaskell P. H.
Published: July 2009

Eddy formation and presence in a plane laminar shear flow configuration consisting of two infinitely long plates orientated parallel to each other is investigated theoretically. The upper plate, which is planar, drives the flow; the lower one has a sinusoidal profile and is fixed. The governing equations are solved via a full finite element formulation for the general case and semianalytically at the Stokes flow limit. The effects of varying geometry (involving changes in the mean plate separation or the amplitude and wavelength of the lower plate) and inertia are explored separately. For Stokes flow and varying geometry, excellent agreement between the two methods of solution is found. Of particular interest with regard to the flow structure is the importance of the clearance that exists between the upper plate and the tops of the corrugations forming the lower one. When the clearance is large, an eddy is only present at sufficiently large amplitudes or small wavelengths. However, as the plate clearance is reduced, a critical value is found, which triggers the formation of an eddy in an otherwise fully attached flow for any finite amplitude and arbitrarily large wavelength. This is a precursor to the primary eddy to be expected in the lid-driven cavity flow, which is formed in the limit of zero clearance between the plates. The influence of the flow driving mechanism is assessed by comparison with corresponding solutions for the case of gravity-driven fluid films flowing over an undulating substrate. When inertia is present, the flow generally becomes asymmetrical. However, it is found that for large mean plate separations the flow local to the lower plate becomes effectively decoupled from the inertia dominated overlying flow if the wavelength of the lower plate is sufficiently small. In such cases the local flow retains its symmetry. A local Reynolds number based on the wavelength is shown to be useful in characterizing these large-gap flows. As the mean plate separation is reduced, the form of the asymmetry caused by inertia changes and becomes strongly dependent on the plate separation. For lower plate wavelengths which do not exhibit a kinematically induced secondary eddy, an inertially induced secondary eddy can be created if the mean plate separation is sufficiently small and the global Reynolds number is sufficiently large.
Effects of low-temperature-buffer, rf-power, and annealing on structural and optical properties of ZnO/Al2O3(0001) thin films grown by rf-magnetron sputtering
Authors: Liu H. F. , Chua S. J.
Published: July 2009

ZnO thin films were epitaxially grown on Al2O3 (0001) substrates in a radio-frequency (rf) magnetron sputtering chamber. The surface morphology of ZnO was remarkably affected by the incorporation of a low-temperature grown ZnO buffer as well as the changes in rf-power. X-ray diffractions, combined with the surface micropits, revealed strain relaxations in the ZnO epilayers grown with higher rf-powers, which in turn caused a redshift to the intrinsic exciton absorption peak. Strain relaxations were also observed in the ZnO epilayers upon thermal annealing, which led to a redshift in the E2high Raman mode. A factor of ∼0.7 cm−1 GPa−1, i.e., a biaxial stress of 1 GPa can shift the E2high mode by 0.7 cm−1, was obtained. The point defects related absorptions and the exciton localizations were suppressed by annealing, which, in conjunction with the strain-relaxation induced redshift in the intrinsic-exciton absorptions, steepened the absorption edge and increased the optical bandgap energy of the ZnO epilayer.
“Compressing liquid”: An efficient global minima search strategy for clusters
Authors: Zhou R. L. , Zhao L. Y. , Pan B. C.
Published: July 2009

In this paper we present a new global search strategy named as “compressing liquid” for atomic clusters. In this strategy, a random fragment of liquid structure is adopted as a starting geometry, followed by iterative operations of “compressing” and Monte Carlo adjustment of the atom positions plus structural optimization. It exhibits fair efficiency when it is applied to seeking the global minima of Lennard-Jones clusters. We also employed it to search the low-lying candidates of medium silicon clusters Sin(n=40–60), where the global search is absent. We found the best candidates for most sizes. More importantly, we obtained non-fullerene-based structures for some sized clusters, which were not found from the endohedral-fullerene strategy. These results indicate that the “compressing-liquid” method is highly efficient for global minima search of clusters.
The effects of a complexing agent on the transverse stability of cubic autocatalytic reaction fronts
Authors: Merkin J. H.
Published: July 2009

The effects of adding a complexing agent on the propagation and transverse stability of reaction fronts in a system based on cubic autocatalysis is considered. Adding the complexing agent is seen to reduce the propagation speed, alter the reaction dynamics and the concentration of the final reaction product of the propagating reaction fronts. A linear stability analysis (LSA) is considered to determine how the complexing agent affects the stability of planar reaction fronts through the numerical calculation of dispersion curves, plots of the growth rate σ against wavenumber k. These dispersion curves show that adding the complexing agent can make the system unstable when it would otherwise be stable and, when the system is diffusionally unstable without the complexing agent, weaken this instability. An analysis valid for small values of k is undertaken, which confirms the results from the LSA and indicates how the critical value Dc of the diffusion coefficient ratio D for the onset of an instability is changed by the addition of the complexing agent.
Dynamics of gas bubble growth in a supersaturated solution with Sievert’s solubility law
Authors: Gor G. Yu. , Kuchma A. E.
Published: July 2009

This paper presents a theoretical description of diffusion growth of a gas bubble after its nucleation in supersaturated liquid solution. We study systems where gas molecules completely dissociate in the solvent into two parts, thus making Sievert’s solubility law valid. We show that the difference between Henry’s and Sievert’s laws for chemical equilibrium conditions causes the difference in bubble growth dynamics. Assuming that diffusion flux is steady we obtain a differential equation on bubble radius. Bubble dynamics equation is solved analytically for the case of homogeneous nucleation of a bubble, which takes place at a significant pressure drop. We also obtain conditions of diffusion flux steadiness. The fulfillment of these conditions is studied for the case of nucleation of water vapor bubbles in magmatic melts.
Template-directed self-assembled magnetic nanostructures for probe recording
Authors: Kappenberger P. , Luo F. , Heyderman L. J. , Solak H. H. , Padeste C. , Brombacher C. , Makarov D. , Ashworth T. V. , Philippe L. , Hug H. J. , Albrecht M.
Published: July 2009

We employ a template-directed self-assembly process to arrange nanospheres as small as 20 nm in a polymer resist template with regular hole arrays with periods down to 42 nm produced using extreme ultraviolet interference lithography. To demonstrate magnetic probe recording employing magnetic tips, an array of magnetic caps with perpendicular magnetic anisotropy was then created by depositing Co/Pt multilayer films on 60 nm nanospheres, arranged on a square lattice with 100 nm period. The magnetic nanocaps can be switched individually in a controlled fashion, with recognition of a successful switching event realized by measurement of a force-distance curve.
A high-performance long wavelength superlattice complementary barrier infrared detector
Authors: Ting David Z.-Y. , Hill Cory J. , Soibel Alexander , Keo Sam A. , Mumolo Jason M. , Nguyen Jean , Gunapala Sarath D.
Published: July 2009

We describe a long wavelength infrared detector where an InAs/GaSb superlattice absorber is surrounded by a pair of electron-blocking and hole-blocking unipolar barriers. A 9.9 μm cutoff device without antireflection coating based on this complementary barrier infrared detector design exhibits a responsivity of 1.5 A/W and a dark current density of 0.99×10−5 A/cm2 at 77 K under 0.2 V bias. The detector reaches 300 K background limited infrared photodetection (BLIP) operation at 87 K, with a black-body BLIP D∗ value of 1.1×1011 cm Hz1/2/W for f/2 optics under 0.2 V bias.
Nonpolar growth and characterization of InN overlayers on vertically oriented GaN nanorods
Authors: Sun Xian , Jiang Desheng , Liu Wenbao , Zhu Jihong , Wang Hui , Liu Zongshun , Zhu Jianjun , Wang Yutian , Zhao Degang , Zhang Shuming , You Liping , Ma Renmin , Yang Hui
Published: July 2009

Nanostructured hexagonal InN overlayers were heteroepitaxially deposited on vertically oriented c-axis GaN nanorods by metal-organic chemical vapor deposition. InN overlayers grown in radial directions are featured by a nonpolar heteroepitaxial growth mode on GaN nanorods, showing a great difference from the conventional InN growth on (0001) c-plane GaN template. The surface of InN overlayers is mainly composed of several specific facets with lower crystallographic indices. The orientation relationship between InN and GaN lattices is found to be [0001]InN∥[0001]GaN and [11̱00]InN∥[11̱00]GaN. A strong photoluminescence of InN nanostructures is observed.
Early events associated with the excited state proton transfer in 2-(2′-pyridyl)benzimidazole
Authors: Burai Tarak Nath , Mukherjee Tushar Kanti , Lahiri Priyanka , Panda Debashis , Datta Anindya
Published: July 2009

2-(2′-pyridyl)benzimidazole (2PBI) undergoes excited state proton transfer (ESPT) in acidic solutions, leading to a tautomer emission at 460 nm. This photoprocess has been studied using ultrafast fluorescence spectroscopic techniques in acidic neat aqueous solutions, in viscous mixtures of glycerol with water, as well as in sucrose solutions. The tautomer is found to be stabilized in the more viscous medium, leading to a greater relative quantum yield as well as lifetime. The long rise time in tautomer emission is not affected by viscosity though. Rather, it appears to have the same value as the long component of the decay of the cationic excited state (C∗). In addition to the subnanosecond lifetime reported earlier, C∗ is found to exhibit a decay time of 2 ps. This is assigned to its protonation to form the nonfluorescent dication in its excited state (D∗) considering the ground and excited state pKa values reported earlier. An additional rising component of 100 ps is observed in the region of C∗ emission. This is likely to arise from a structural change or charge redistribution in C∗ immediately after its creation and before the phototautomerization.
Layer-by-layer self-assembled conductor network composites in ionic polymer metal composite actuators with high strain response
Authors: Liu Sheng , Montazami Reza , Liu Yang , Jain Vaibhav , Lin Minren , Heflin James R. , Zhang Q. M.
Published: July 2009

We investigate the electromechanical response of conductor network composite (CNC) fabricated by the layer-by-layer (LbL) self-assembly method. The process makes it possible for CNCs to be fabricated at submicron thickness with high precision and quality. This CNCs exhibits high strain ∼6.8% under 4 V, whereas the RuO2/Nafion CNCs exhibit strain ∼3.3%. The high strain and submicron thickness of the LbL layers in an ionic polymer metal composite (IPMC) yield large and fast actuation. The response time of a 26 μm thick IPMC with 0.4 μm thick LbL CNCs to step voltage of 4 V is 0.18 s.
Exchange and correlation in molecular wire conductance: Nonlocality is the key
Authors: Evans Jeremy S. , Vydrov Oleg A. , Van Voorhis Troy
Published: July 2009

We study real-time electron dynamics in a molecular junction with a variety of approximations to the electronic structure, toward the ultimate aim of determining what ingredients are crucial for the accurate prediction of charge transport. We begin with real-time, all electron simulations using some common density functionals that differ in how they treat long-range Hartree–Fock exchange. We find that the inclusion or exclusion of nonlocal exchange is the dominant factor determining the transport behavior, with all semilocal contributions having a smaller effect. In order to study nonlocal correlation, we first map our junction onto a simple Pariser–Parr–Pople (PPP) model Hamiltonian. The PPP dynamics are shown to faithfully reproduce the all electron results, and we demonstrate that nonlocal correlation can be readily included in the model space using the generator coordinate method (GCM). Our PPP-GCM simulations suggest that nonlocal correlation has a significant impact on the I-V character that is not captured even qualitatively by any of the common semilocal approximations to exchange and correlation. The implications of our results for transport calculations are discussed.
Nonlocal collisionless power absorption using effective viscosity model in inductively coupled plasma discharges
Authors: Aman-ur-Rehman  , Lee J. K.
Published: July 2009

Effective viscosity model for inductively coupled plasma (ICP) discharges has been used to calculate the power absorbed inside ICP discharges. It is found that it can be used to calculate collisionless heating, which is a warm plasma effect. The validity of effective viscosity model has been checked by comparing it with kinetic model for warm ICP discharges. For very small plasma lengths, the results of both models are the same. At intermediate lengths where bounce resonance heating is important, results of the two models are not the same. Bounce resonance length given by effective viscosity model does not match very well with that given by the kinetic model. It shows that bounce resonance heating cannot be taken care of accurately using the effective viscosity model. For large plasma length, when driving frequencies are low, power absorbed calculated using the kinetic model is more than that calculated by the effective viscosity model. For high driving frequencies, power absorbed calculated using the kinetic model is less than that calculated by the effective viscosity model. The best match between the results of two models (for large plasma length) is obtained if the combination of plasma density, electron temperature, driving frequency, and speed of light is such that the relation K=ωpvth/ωc≃1 holds. It is concluded that computationally less extensive effective viscosity model can be used to estimate power absorption in ICP discharges by calibrating it with the help of computationally intensive kinetic model. Once calibration is done a lot of computational effort can be avoided by using effective viscosity model instead of kinetic model.
Spin polarization control by electric stirring: Proposal for a spintronic device
Authors: Pershin Yu. V. , Sinitsyn N. A. , Kogan A. , Saxena A. , Smith D. L.
Published: July 2009

We propose a spintronic device to generate spin polarization in a mesoscopic region by purely electric means. We show that the spin Hall effect in combination with the stirring effect are sufficient to induce measurable spin polarization in a closed geometry. Our device structure does not require the application of magnetic fields, external radiation or ferromagnetic leads, and can be implemented in standard semiconducting materials.
Study of the ground multiplet of Kramers rare earth ions in solid matrices by multifrequency electron paramagnetic resonance spectroscopy: Nd3+ in PbWO4 single-crystals
Authors: Popescu F. F. , Bercu V. , Barascu J. N. , Martinelli M. , Massa C. A. , Pardi L. A. , Stefan M. , Nistor S. V. , Nikl M. , Bohacek P.
Published: July 2009

A multifrequency electron paramagnetic resonance (EPR) investigation of Nd3+ impurities in PbWO4 single-crystals at the conventional microwave frequency (MF) 9.43 GHz, and at the 95, 190, and 285 GHz high frequencies was carried out. The resulting spectra are well described at all frequencies by an axial spin-Hamiltonian corresponding to an effective electron spin of one-half and to a tetragonal symmetry. For the magnetic field along the tetragonal axis, the g∥-factor and the hyperfine constant A∥ of the lowest doublet of the ground multiplet decreases with frequency increase. For the magnetic field perpendicular to the tetragonal axis, the g⊥-factor exhibits a small azimuthal angular dependence that increases with increasing the frequency due to the S4 site symmetry. The azimuthal angular dependence allows to clearly distinguish between different local axial symmetries. These properties are interpreted as high field/frequency (HF) effects associated with the mixing by the large Zeeman interaction of some of the upper-lying doublets of the ground multiplet into the lowest-lying doublet states. We show that from the combined analysis of the multifrequency MF- and HF-EPR spectra and of the optical data, an accurate description of the ground multiplet of the Kramers rare earth ions in solid matrices can be derived.
Tuning the termination of the SrTiO3(110) surface by Ar+ sputtering
Authors: Wang Zhiming , Wu Kehui , Guo Qinlin , Guo Jiandong
Published: July 2009

We report a scanning tunneling microscopy study on the SrTiO3(110) surface treated with Ar+ sputtering followed by annealing. Two types of termination coexist on the surface, which are spatially identified as the 4×1 reconstructed SrTiO layer and the O layer covered by Ti-rich oxide clusters, respectively. The relative areal ratio of the two types is tuned by sputtering dose reproducibly, and monophased surface with either SrTiO or O termination is obtained. The surface is stable at temperatures up to 1100 °C and under oxygen partial pressures from 6×10−5 mbar to ultra high vacuum, providing us a flexible epitaxial growth template.
Microstructure and infrared spectral properties of porous polycrystalline and nanocrystalline cubic silicon carbide
Authors: Fan J. Y. , Li H. X. , Cui W. N.
Published: July 2009

We investigated the structural and infrared spectral properties of porous polycrystalline 3C-SiC and 3C-SiC nanoparticles produced via electrochemical method. The porous sample consisted of parallel nanowires with periodic beadlike structures. It exhibited infrared spectral features quite different from that of single crystal. The 3C-SiC crystallites with an average size of 4 nm showed simple surface chemistry with the surfaces well passivated by dissociation of surrounding water molecules. Our result explains the distinctive optical properties in porous polycrystalline and nanocrystalline 3C-SiC and reveals the crucial conditions for quantum confinement photoluminescence to arise.
Large magnetoresistances and non-Ohmic conductivity in EuWO1+xN2−x
Authors: Kusmartseva A. , Yang M. , Oró-Solé J. , Bea A. M. , Fuertes A. , Attfield J. P.
Published: July 2009

The magnetic field and voltage dependent electronic transport properties of EuWO1+xN2−x ceramics are reported. Large negative magnetoresistances are observed at low temperatures, up to 70% in the least doped (x=0.09) material. Non-Ohmic conduction emerges below the 12 K Curie transition. This is attributed to a microstructure of ferromagnetic conducting and antiferromagnetic insulating regions resulting from small spatial fluctuations in the chemical doping.
Evaluation of master equations for the droplet size distribution in condensing flow
Authors: Sidin Ryan S. R. , Hagmeijer Rob , Sachs Ulrich
Published: July 2009

The kinetic equation (KE) and its first- and second-order approximations, the general dynamic equation (GDE), and the Fokker–Planck equation (FPE), respectively, have been evaluated based on (a) their equilibrium distributions, (b) a nucleation pulse experiment, and (c) an expanding nozzle flow. Large differences are observed between the equilibrium distributions of the FPE and KE, whereas the GDE does not have an equilibrium distribution at all. For the nucleation pulse experiment, good agreement is found between the KE, FPE, and GDE due to quasisteady nucleation. For the condensing nozzle flow, the difference between the GDE and the KE distributions is large, although the relevant flow variables show fair agreement. A sensitivity study of the KE solution with respect to uncertainties in (a) the surface tension model, (b) the sticking probability, and (c) the equilibrium distribution revealed that both the sticking probability and the equilibrium distribution have a significant influence on the predicted condensation onset. Furthermore, it is found that the proposed Wölk and Strey-corrected Courtney equilibrium distribution yields the best agreement with the reported measurements.