bluuebunny/crossref_metadata_2025 huggingface dataset:
Density functional theory of size-dependent surface tension of Lennard-Jones fluid droplets using a double well type Helmholtz free energy functional
Authors: Ghosh Satinath , Ghosh Swapan K.
Published: September 2011

A double well type Helmholtz free energy density functional and a model density profile for a two phase vapor-liquid system are used to obtain the size-dependent interfacial properties of the vapor-liquid interface at coexistence condition along the lines of van der Waals and Cahn and Hilliard density functional formalism of the interface. The surface tension, temperature-density curve, density profile, and thickness of the interface of Lennard-Jones fluid droplet-vapor equilibrium, as predicted in this work are reported. The planar interfacial properties, obtained from consideration of large radius of the liquid drop, are in good agreement with the results of other earlier theories and experiments. The same free energy model has been tested by solving the equations numerically, and the results compare well with those from the use of model density profile.
First principles calculations on structure, bonding, and vibrational frequencies of SiP2
Authors: Bachhuber F. , Rothballer J. , Pielnhofer F. , Weihrich R.
Published: September 2011

Pyrite type SiP2 is reinvestigated by first principles calculations on various levels of functionals including local density approximation, generalized gradient approximation, Becke-Lee-Yang-Parr hybrid functional, and the Hartree-Fock method. SiP2 is seen as a model compound with molecular [P–P] entities and [SiP6] octahedra. Structure and bonding are addressed by electronic structure calculations. Special attention is spent on P–P and Si–P bonds in terms of bond lengths and respective stretching modes from simulated Raman spectra. The electronic structure is analyzed in both direct and momentum space by the electron localization function and site projected density of states. The main goals of this work are to understand the nature of chemical bonding in SiP2 and to compare and contrast the different methods of calculation.
Quantitative analysis of the dripping and jetting regimes in co-flowing capillary jets
Authors: Cordero María Luisa , Gallaire François , Baroud Charles N.
Published: September 2011

We study a liquid jet that breaks up into drops in an external co-flowing liquid inside a confining microfluidic geometry. The jet breakup can occur right after the nozzle in a phenomenon named dripping or through the generation of a liquid jet that breaks up a long distance from the nozzle, which is called jetting. Traditionally, these two regimes have been considered to reflect the existence of two kinds of spatiotemporal instabilities of a fluid jet, the dripping regime corresponding to an absolutely unstable jet and the jetting regime to a convectively unstable jet. Here, we present quantitative measurements of the dripping and jetting regimes, both in an unforced and a forced state, and compare these measurements with recent theoretical studies of spatiotemporal instability of a confined liquid jet in a co-flowing liquid. In the unforced state, the frequency of oscillation and breakup of the liquid jet is measured and compared to the theoretical predictions. The dominant frequency of the jet oscillations as a function of the inner flow rate agrees qualitatively with the theoretical predictions in the jetting regime but not in the dripping regime. In the forced state, achieved with periodic laser heating, the dripping regime is found to be insensitive to the perturbation and the frequency of drop formation remains unaltered. The jetting regime, on the contrary, amplifies the externally imposed frequency, which translates into the formation of drops at the frequency imposed by the external forcing. In conclusion, the dripping and jetting regimes are found to exhibit the main features of absolutely and convectively unstable flows, respectively, but the frequency selection in the dripping regime is not ruled by the absolute frequency predicted by the stability analysis.
Electrical transport mechanisms in three dimensional ensembles of silicon quantum dots
Authors: Balberg I.
Published: September 2011

In this review, we try to derive a comprehensive understanding of the transport mechanisms in three dimensional ensembles of Si quantum dots (QDs) that are embedded in an insulating matrix. This understanding is based on our systematic electrical measurements as a function of the density of Si nanocrystallites as well as on a critical examination of the available literature. We conclude that in ensembles of low density QDs, the conduction is controlled by quantum confinement and Coulomb blockade effects while in the high density regime, the system behaves as a simple disordered semiconductor. In between these extremes, the transport is determined by the clustering of the QDs. In view of the clustering, two types of transitions in the electrical and optical properties of the system are identified. In order to understand them, we introduce the concept of “touching.” The application of this concept enables us to suggest that the first transition is a local carrier deconfinement transition, at which the concentration of the non “touching” QDs reaches its maximum, and that the other transition is associated with the onset of percolation in a continuous disordered network of “touching” QDs. It is hoped that our conclusions for the entire possible density range will provide guidance for the discussion and understanding of the transport in ensembles of semiconductor QDs in general and in ensembles of Si and Ge QDs in particular.
Modified Jiles-Atherton-Sablik model for asymmetry in magnetomechanical effect under tensile and compressive stress
Authors: Li Jianwei , Xu Minqiang
Published: September 2011

A robust magnetomechanical effect model is desirable for the measurement of magnetic properties in the evaluation of stress in ferromagnetic materials. The previous classical Jiles-Atherton-Sablik model has some limitations in describing the asymmetry in magnetic behavior under tensile and compressive stress. In this paper, the Jiles-Atherton-Sablik model has been modified by the incorporation of a stress demagnetization term, a variable pinning coefficient, a stress-dependent domain coupling coefficient, and stress-dependent saturation magnetostriction. It is found that the modified model provides a much better description of asymmetrical magnetic properties under tension and compression. Moreover, the accuracy of the calculations was improved considerably, particularly in the compressive stress situation.
Experimental observation of standing interfacial waves induced by surface waves in muddy water
Authors: Maxeiner Eric , Dalrymple Robert A.
Published: September 2011

A striking feature has been observed in a laboratory wave tank with a thin layer of clear water overlying a layer of mud. A piston-type wave maker is used to generate long monochromatic surface waves in a tank with a layer of kaolinite clay at the bottom. The wave action on the mud causes the clay particles to rise from the bottom into the water column, forming a lutocline. As the lutocline approaches the water surface, a set of standing interfacial waves form on the lutocline. The interfacial wave directions are oriented nearly orthogonal to the surface wave direction. The interfacial waves, which sometimes cover the entire length and width of the tank, are also temporally subharmonic as the phase of the interfacial wave alternates with each passing surface wave crest. These interfacial waves are the result of a resonant three-wave interaction involving the surface wave train and the two interfacial wave trains. The interfacial waves are only present when the lutocline is about 3 cm of the water surface and they can be sufficiently nonlinear as to exhibit superharmonics and a breaking-type of instability.
Single-shot velocity-map imaging of attosecond light-field control at kilohertz rate
Authors: Süßmann F. , Zherebtsov S. , Plenge J. , Johnson Nora G. , Kübel M. , Sayler A. M. , Mondes V. , Graf C. , Rühl E. , Paulus G. G. , Schmischke D. , Swrschek P. , Kling M. F.
Published: September 2011

High-speed, single-shot velocity-map imaging (VMI) is combined with carrier-envelope phase (CEP) tagging by a single-shot stereographic above-threshold ionization (ATI) phase-meter. The experimental setup provides a versatile tool for angle-resolved studies of the attosecond control of electrons in atoms, molecules, and nanostructures. Single-shot VMI at kHz repetition rate is realized with a highly sensitive megapixel complementary metal-oxide semiconductor camera omitting the need for additional image intensifiers. The developed camera software allows for efficient background suppression and the storage of up to 1024 events for each image in real time. The approach is demonstrated by measuring the CEP-dependence of the electron emission from ATI of Xe in strong (≈1013 W/cm2) near single-cycle (4 fs) laser fields. Efficient background signal suppression with the system is illustrated for the electron emission from SiO2 nanospheres.
Unveiling the (De)coupling of magnetostructural transition nature in magnetocaloric R5Si2Ge2 (R = Tb, Gd) materials
Authors: Pereira A. M. , Kampert E. , Moreira J. M. , Zeitler U. , Belo J. H. , Magen C. , Algarabel P. A. , Morellon L. , Ibarra M. R. , Gonçalves J. N. , Amaral J. S. , Amaral V. S. , Sousa J. B. , Araújo J. P.
Published: September 2011

We present a detailed study on the magnetization under high magnetic fields of Tb5Si2Ge2 and Gd5Si2Ge2 compounds. From the Arrott plot construction (A. Arrott, Phys. Rev. 108, 1394 (1957)), we were able to estimate the TC of each structure (M and O(I)) experimentally and found that the TC of the O(I) phase can be directly obtained by extrapolating the TC curve of this phase in the respective phase diagram. Using a physical model based on free energy considerations, one explains the (de)coupling of the magnetic and structural transitions in R5(SixGe1−x)4 (R = Tb, Gd) compounds.
High-precision semiconductor wavelength sensor based on a double-layer photo diode
Authors: Amthor Thomas , Hofmann Christoph S. , Knorz Jürgen , Weidemüller Matthias
Published: September 2011

We present a wavelength sensor setup for monochromatic visible light, based on the double-layer photo diode WS-7.56. Employing high-precision electronics and automatic compensation of different error sources, we achieve a measurement accuracy of ±0.025 nm with a resolution below 0.01 nm. The described apparatus is particularly suited for the determination of small laser frequency deviations in atomic physics experiments. Various design issues as well as error sources and diode characteristics are discussed.
A laboratory experiment to examine the effect of auroral beams on spacecraft charging in the ionosphere
Authors: Siddiqui M. U. , Gayetsky L. E. , Mella M. R. , Lynch K. A. , Lessard M. R.
Published: September 2011

A 2.54 cm diameter conducting electrically isolated Copper sphere is suspended in a low density (104 cm−3), low temperature (Te = 0.5 eV) Argon plasma, which mimics a spacecraft in an ionospheric plasma. An electron beam with current density of approximately 10−10 A/cm2 and beam spot of 10.2 cm diameter, which mimics an auroral electron beam, is fired at the sphere while varying the beam energy from 100 eV to 2 keV. The plasma potential in the sheath around the sphere is measured using an emissive probe as the electron beam energy is varied. To observe the effects of the electron beam, the experimental sheath potential profiles are compared to a model of the plasma potential around a spherically symmetric charge distribution in the absence of electron beams. Comparison between the experimental data and the model shows that the sphere is less negative than the model predicts by up to half a volt for beam energies that produce high secondary electron emission from the surface of the sphere. It is shown that this secondary emission can account for changes in potential of spacecraft in the ionosphere as they pass through auroral beams and thus helps to improve interpretations of ionospheric thermal ion distributions.
Evolution of magnetohydrodynamic waves and associated ultrarelativistic electron acceleration
Authors: Takeyama Yosuke , Nakayama Shun-ichi , Ohsawa Yukiharu
Published: September 2011

The evolution of magnetosonic shock waves and Alfvén waves generated by a strong disturbance and electron acceleration occurring in these waves is studied with fully kinetic, relativistic, electromagnetic, particle simulations. If two plasmas collide, magnetic field lines are compressed near the initial boundary of the two, resulting in the formation of a strong-magnetic-field pulse, which reflects ions of the two plasmas in two opposite directions. These ion motions create forward and backward shock waves. Furthermore, large-amplitude Alfvén waves are produced, with their propagation speeds much lower than the shock speeds. In the Alfvén wave region, three types of ultrarelativistic electron acceleration are observed, which are analyzed in detail.
Use of vacuum arc plasma guns for a metal puff Z-pinch system
Authors: Rousskikh A. G. , Zhigalin A. S. , Oreshkin V. I. , Chaikovsky S. A. , Labetskaya N. A. , Baksht R. B.
Published: September 2011

The performance of a metal puff Z-pinch system has been studied experimentally. In this type of system, the initial cylindrical shell 4 cm in diameter was produced by ten plasma guns. Each gun initiates a vacuum arc operating between magnesium electrodes. The net current of the guns was 80 kA. The arc-produced plasma shell was compressed by using a 450-kA, 450-ns driver, and as a result, a plasma column 0.3 cm in diameter was formed. The electron temperature of the plasma reached 400 eV at an average ion concentration of 1.85 · 1018 cm−3. The power of the Mg K-line radiation emitted by the plasma for 15–30 ns was 300 MW/cm.
Analytical and computational study of the ideal full two-fluid plasma model and asymptotic approximations for Hall-magnetohydrodynamics
Authors: Srinivasan B. , Shumlak U.
Published: September 2011

The 5-moment two-fluid plasma model uses Euler equations to describe the ion and electron fluids and Maxwell’s equations to describe the electric and magnetic fields. Two-fluid physics becomes significant when the characteristic spatial scales are on the order of the ion skin depth and characteristic time scales are on the order of the ion cyclotron period. The full two-fluid plasma model has disparate characteristic speeds ranging from the ion and electron speeds of sound to the speed of light. Two asymptotic approximations are applied to the full two-fluid plasma to arrive at the Hall-MHD model, namely negligible electron inertia and infinite speed of light. The full two-fluid plasma model and the Hall-MHD model are studied for applications to an electromagnetic plasma shock, geospace environmental modeling (GEM challenge) magnetic reconnection, an axisymmetric Z-pinch, and an axisymmetric field reversed configuration (FRC).
Photon statistics in blinking fluorescence of single PPV-PPyV molecule
Authors: Fedyanin V. V. , Osad'ko I. S.
Published: September 2011

A theoretical six-level model for blinking fluorescence of single PPV-PPyV copolymer molecule excited by CW-laser light is proposed. The model has been chosen in accordance with the following facts found in the Paul Barbara group experiment: (i) alternation of two types of fluorescence with moderate and strong levels of emission, (ii) existence of “dark” states with no fluorescence, (iii) linear dependence of inverse on-interval duration on laser intensity, and (iv) existence of laser intensity independent off-intervals. Relations between the distribution function w′′(N, T) for photons emitted by a single molecule, the distribution function w′(N, T) for photons arriving at photomultiplier tube (PMT) and photo-electric pulse distribution w(N, T) created in a PMT are discussed. The theory is able to describe pulse distribution function w(N, T) measured experimentally at signal acquisition time T = 0.1 s. Values of all rate constants of the model have been found from comparison of the theory with the experiment. Distributions won, off(t) of on- and off-times and distribution w(N, T) of pulses have been calculated for infrequent and frequent inter-conformational jumps in single copolymer molecule.
Low energy (0–10 eV) electron driven reactions in the halogenated organic acids CCl3COOH, CClF2COOH, and CF3CHNH2COOH (trifluoroalanine)
Authors: Kopyra Janina , König-Lehmann Constanze , Illenberger Eugen
Published: September 2011

Negative ion formation following resonant electron attachment to the three title molecules is studied by means of a beam experiment with mass spectrometric detection of the anions. All three molecules exhibit a pronounced resonance in the energy range around 1 eV which decomposes by the loss of a neutral hydrogen atom thereby generating the closed shell anion (M–H)− (or RCOO−), a reaction which is also a common feature in the non-substituted organic acids. The two chlorine containing molecules CCl3COOH and CClF2COOH exhibit an additional strong and narrow resonance at very low energy (close to 0 eV) which decomposes by the cleavage of the C–Cl bond with the excess charge finally localised on either of the two fragments Cl− and (M–Cl)−. This reaction is by two to three orders of magnitude more effective than hydrogen loss. Apart from these direct bond cleavages (C–Cl, O–H) resonant attachment of subexcitation electrons trigger additional remarkably complex unimolecular decompositions leading, e.g., to the formation of the bihalide ions ClHCl− and ClHF− from CCl3COOH and CClF2COOH, respectively, or the loss of a neutral CF2 unit from trifluoroalanine thereby generating the fluoroglycine radical anion. These reactions require substantial rearrangement in the transitory negative ion, i.e., the cleavage of different bonds and formation of new bonds. F− from both chlorodifluoroacetic acid and trifluoroalanine is formed at comparatively low intensity (more than three orders of magnitude less than Cl− from the chlorine containing molecules) and predominantly within a broad resonant feature around 7–8 eV characterised as core excited resonance.
Mathematical analysis of the boundary-integral based electrostatics estimation approximation for molecular solvation: Exact results for spherical inclusions
Authors: Bardhan Jaydeep P. , Knepley Matthew G.
Published: September 2011

We analyze the mathematically rigorous BIBEE (boundary-integral based electrostatics estimation) approximation of the mixed-dielectric continuum model of molecular electrostatics, using the analytically solvable case of a spherical solute containing an arbitrary charge distribution. Our analysis, which builds on Kirkwood's solution using spherical harmonics, clarifies important aspects of the approximation and its relationship to generalized Born models. First, our results suggest a new perspective for analyzing fast electrostatic models: the separation of variables between material properties (the dielectric constants) and geometry (the solute dielectric boundary and charge distribution). Second, we find that the eigenfunctions of the reaction-potential operator are exactly preserved in the BIBEE model for the sphere, which supports the use of this approximation for analyzing charge-charge interactions in molecular binding. Third, a comparison of BIBEE to the recent GBε theory suggests a modified BIBEE model capable of predicting electrostatic solvation free energies to within 4% of a full numerical Poisson calculation. This modified model leads to a projection-framework understanding of BIBEE and suggests opportunities for future improvements.
Fractal structure of ferromagnets: The singularity structure analysis
Authors: Kuetche Victor K. , Bouetou Thomas B. , Kofane Timoleon C.
Published: September 2011

Following the Weiss-Tabor-Carnevale approach [J. Weiss, M. Tabor, and G. Carnevale, J. Math. Phys. 24, 522 (1983)10.1063/1.525721; J. Weiss, M. Tabor, and G. Carnevale, J. Math. Phys. 25, 13 (1984).]10.1063/1.526009 designed for studying the integrability properties of nonlinear partial differential equations, we investigate the singularity structure of a (2+1)-dimensional wave-equation describing the propagation of polariton solitary waves in a ferromagnetic slab. As a result, we show that, out of any damping instability, the system above is integrable. Looking forward to unveiling its complete integrability, we derive its Bäcklund transformation and Hirota's bilinearization useful in generating a set of soliton solutions. In the wake of such results, using the arbitrary functions to enter into the Laurent series of solutions to the above system, we discuss some typical class of excitations generated from the previous solutions in account of a classification based upon the different expressions of a generic lower dimensional function. Accordingly, we unearth the nonlocal excitations of lowest amplitudes, the dromion and lump patterns of higher amplitudes, and finally the stochastic pattern formations of highest amplitudes, which arguably endow the aforementioned system with the fractal properties.
A thermochromic low-emittance coating: Calculations for nanocomposites of In2O3:Sn and VO2
Authors: Li S.-Y. , Niklasson G. A. , Granqvist C. G.
Published: September 2011

Calculations based on the Bruggeman effective medium theory were applied to thin films comprising a heavily doped wide band gap semiconductor (specifically In2O3:Sn (ITO)) and VO2. Films with ∼20 vol. % of VO2 can combine a 10% thermochromic modulation of the solar energy throughput with a luminous transmittance of 50%–60% and low thermal emittance. The maximum thermochromic modulation is ∼13% and occurs at ∼35 vol. % VO2. Coatings of ITO-VO2 are of interest for energy efficient fenestration.
Spectrographic temperature measurement of a high power breakdown arc in a high pressure gas switch
Authors: Yeckel Christopher , Curry Randy
Published: September 2011

A procedure for obtaining an approximate temperature value of conducting plasma generated during self-break closure of a RIMFIRE gas switch is described. The plasma is in the form of a breakdown arc which conducts approximately 12 kJ of energy in 1 μs. A spectrographic analysis of the trigger-section of the 6-MV RIMFIRE laser triggered gas switch used in Sandia National Laboratory's “Z-Machine” has been made. It is assumed that the breakdown plasma has sufficiently approached local thermodynamic equilibrium allowing a black-body temperature model to be applied. This model allows the plasma temperature and radiated power to be approximated. The gas dielectric used in these tests was pressurized SF6. The electrode gap is set at 4.59 cm for each test. The electrode material is stainless steel and insulator material is poly(methyl methacrylate). A spectrum range from 220 to 550 nanometers has been observed and calibrated using two spectral irradiance lamps and three spectrograph gratings. The approximate plasma temperature is reported.
Interplay between charge and vibrational delocalization in cationic helium clusters
Authors: Calvo F. , Naumkin F. Y. , Wales D. J.
Published: September 2011

The stable structures and low temperature thermodynamics of cationic helium clusters are investigated theoretically using a diatomics-in-molecules model for the potential energy surfaces and a computational framework in which both electronic and nuclear degrees of freedom are treated on a quantum mechanical footing. While the charge is generally carried by two atoms, vibrational delocalization significantly spreads out the charge over multiple isomers for clusters containing five or more helium atoms. Our calculations indicate that large clusters are essentially fluid with a well-defined solvation shell around the charged core.
Effect of inflow density on ion diffusion region of magnetic reconnection: Particle-in-cell simulations
Authors: Wu P. , Shay M. A. , Phan T. D. , Oieroset M. , Oka M.
Published: September 2011

We perform a systematic study of the effect of inflow density on reconnection diffusion regions using a 2.5-D particle-in-cell code. The diffusion region structures are analyzed at times when all simulations have reconnected the same amount of magnetic flux. We find that reducing the inflow density from 1 to 1/100th of the current sheet density dramatically increases the diffusion region physical size and the reconnection rate. The width of the diffusion region scales with the upstream ion inertial length systematically. Consistent with the presence of counter-streaming inflowing ion beams near the x-line, the ion meandering width in the diffusion region also scales with the ion inertial length. The aspect ratio of the ion diffusion region remains a constant, independent of the inflow density. The quadrupole Hall magnetic field is reduced. The upstream magnetic field deviates from its asymptotic value by ∼50% at the lowest simulated inflow density. The downstream ion outflow velocity scales linearly with the upstream Alfvén speed with a multiplication factor ∼0.4 < 1. When applied to magnetic reconnection in the Earth’s magnetotail, this factor of 0.4 is a possible explanation as to why bulk flow velocities in the magnetotail are typically on the order of 500 km/s, while the Alfvén speeds of inflowing plasmas can exceed 2000 km/s.
Effects of adsorbate coverage and capillary on nano-asperity friction in atmosphere containing organic vapor
Authors: Asay David B. , Hsiao Erik , Kim Seong H.
Published: September 2011

The influence of alcohol adsorption on the nano-asperity friction of silicon oxide surfaces under equilibrium conditions was studied with atomic force microscopy (AFM). In the intermediate regime of the relative partial pressure (P/Psat) of alcohol, the friction versus applied load (F-L) curve deviates from the expected DMT behavior, while the F-L curve in dry and near saturation vapor conditions follows the DMT contact mechanics. The full analysis of the observed P/Psat dependence of the F-L data with theoretical models reveals clearly that the shear stress of the contact is governed by the coverage of the adsorbed alcohol on the surface while the friction near the critical snap-off is governed by the capillary meniscus formed at the nano-asperity contact.
Abrupt suppression of the exchange bias across a non-magnetic insulator spacer
Authors: Nicolodi S. , Harres A. , Pereira L. G. , Schmidt J. E. , de Sousa M. A. , Pelegrini F. , Viegas A. D. C. , Deranlot C. , Petroff F. , Geshev J.
Published: September 2011

This paper reports results obtained on exchange-biased IrMn/Al2O3/Co films deposited by magnetron sputtering, where the thickness of the non-magnetic insulator layer, tAl2O3, was varied. Ferromagnetic resonance and static magnetization measurements were used to study the exchange interaction between the antiferromagnet (IrMn) and ferromagnet (Co) layers. X-ray diffractometry and x-ray reflectometry as well as high-resolution transmission electron microscopy were employed for structural characterization of the films. It was found that the IrMn/Co exchange coupling decreases very abruptly with tAl2O3 being the exponential decay length at least four times smaller than the values previously reported for exchange-bias systems. Such a rapid suppression of the coupling is explained in terms of the prompt loss of the direct contact between the antiferromagnet and the ferromagnet with tAl2O3.
Note: Binding energy scale calibration of electron spectrometers for photoelectron spectroscopy using a single sample
Authors: Helander M. G. , Greiner M. T. , Wang Z. B. , Lu Z. H.
Published: September 2011

The feasibility of calibrating the binding energy scale for photoelectron spectroscopy using a single sample without sputter cleaning was investigated. By measuring the Fermi level of a Au film with both monochromatic Al Kα (hν = 1486.7 eV) and He Iα (hν = 21.22 eV), the binding energy scale was simply and accurately calibrated. This method is found to yield binding energy values for the Cu 2p3/2, Ag 3d5/2, and Au 4f7/2 peaks that agree with the standard tabulated values defined in International Organization for Standardization 15472 to within ±0.02 eV.
Improved calculation of vacancy properties in Ge using the Heyd-Scuseria-Ernzerhof range-separated hybrid functional
Authors: Śpiewak P. , Vanhellemont J. , Kurzydłowski K. J.
Published: September 2011

The revised Heyd-Scuseria-Ernzerhof screened hybrid functional (HSE06) is used for calculating the formation and migration energies of the vacancy in Ge, and the results are compared with those previously obtained using the local density approximation with the on-site Coulomb interaction U (LDA+U) approach and with other published results. It is demonstrated that using HSE06 gives a much more accurate electronic description of the vacancy and yields an excellent estimate of the activation energy of self-diffusion in Ge consistent with experimental data. The migration energies of the vacancy in different charge states calculated with the HSE06 approach agree well with the results of low-temperature infrared-absorption measurements. In contrast to previous results, the HSE06 calculations suggest that vacancies in Ge are multiple-level acceptors with levels located in the upper half of the bandgap. This can explain the observed high density of acceptor-like interface traps near the conduction band, pinning the Fermi level and inhibiting the formation of an inversion layer in n-channel devices in Ge.
A molecular dynamics investigation of the structural and dynamic properties of the ionic liquid 1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide
Authors: Liu Hongjun , Maginn Edward
Published: September 2011

Molecular dynamics simulations have been performed to investigate the structure and dynamics of the ionic liquid, 1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C4mim][Tf2N]) in the temperature range of 283–460 K. Extensive analysis was carried out to characterize a number of structural and dynamic features. Transport properties were computed using a variety of equilibrium methods that employed the Green-Kubo and Einstein formulations. Nonequilibrium techniques were also used. In general, different methods mostly yielded consistent results, although some differences were observed. Computed self-diffusivities and ionic conductivities tended to be slightly lower than experimental values, while computed viscosities were significantly higher than experiment. Computed thermal conductivities agreed reasonably well with experimental data. Despite these discrepancies, the simulations capture the experimental temperature-dependent trends for all these transport properties. Single ion dynamics were studied by examining diffusional anisotropy, the self-part of the van Hove function, non-Gaussian parameters, and incoherent intermediate scattering functions. It is found that cations diffuse faster than anions and are more dynamically heterogeneous. A clear anisotropy is revealed in cation displacement, with the motion normal to the imidazolium ring plane being the most hindered and the motion along the alkyl chain in the plane of the ring being the most facile. Cations structurally relax faster than anions but they rotationally relax slower than anions. There is a pronounced temperature dependence to the rotational anisotropy of the cations, but only a weak temperature dependence for the anions. The ionic conductivity deviates from the Nernst–Einstein relation due to the correlated motion of cations and anions. The results suggest that the dynamical behavior of this and related ionic liquids is extremely complex and consists of many different modes with widely varying timescales, making the prediction of dynamical trends extremely difficult.
Excited-state switching by per-fluorination of para-oligophenylenes
Authors: Milián-Medina Begoña , Varghese Shinto , Ragni Roberta , Boerner Herbert , Ortí Enrique , Farinola Gianluca M. , Gierschner Johannes
Published: September 2011

Fluorination has become a versatile route to tune the electronic and optical properties of organic conjugated materials. Herein we report a new phenomenon, excited-state switching by per-fluorination of para-oligophenylenes, placing a low intensity 11B2 state below the 11B1 state, giving rise to large Stokes shifts. The switching is attributed to the specific impact of fluorine on the delocalized and localized frontier orbitals as elucidated by quantum-chemical calculations. The sterical demands of the fluorine atom additionally diminish efficient conjugation along the chain, leading to hypsochromic shifts with respect to the unsubstituted counterparts and to a weak chain length dependence of the absorption and unstructured emission spectra and enhanced internal conversion.
Accelerating atomic-level protein simulations by flat-histogram techniques
Authors: Jónsson Sigurður Æ. , Mohanty Sandipan , Irbäck Anders
Published: September 2011

Flat-histogram techniques provide a powerful approach to the simulation of first-order-like phase transitions and are potentially very useful for protein studies. Here, we test this approach by implicit solvent all-atom Monte Carlo (MC) simulations of peptide aggregation, for a 7-residue fragment (GIIFNEQ) of the Cu/Zn superoxide dismutase 1 protein (SOD1). In simulations with 8 chains, we observe two distinct aggregated/non-aggregated phases. At the midpoint temperature, these phases coexist, separated by a free-energy barrier of height 2.7 $k_{\text{B}}T$kBT. We show that this system can be successfully studied by carefully implemented flat-histogram techniques. The frequency of barrier crossing, which is low in conventional canonical simulations, can be increased by turning to a two-step procedure based on the Wang-Landau and multicanonical algorithms.
Energy levels, Auger branching ratios, and radiative rates of the core-excited states of B-like carbon
Authors: Sun Yan , Chen Feng , Gou Bing Cong
Published: September 2011

Energy levels, Auger branching ratios, and radiative rates of the core-excited states of B-like carbon are calculated by the saddle-point variation and saddle-point complex-rotation methods. Relativistic and mass polarization corrections are included using first-order perturbation theory. Calculated Auger channel energies and branching ratios are used to identify high-resolution Auger spectrum in the 300-keV C+ → CH4 collision experiment. It is found that Auger decay of these five-electron core-excited states gives significant contributions to Auger spectrum in the range of 238–280 eV.
The effect of doping on the energetics and quantum conductance in graphene nanoribbons with a metallocene adsorbate
Authors: Kang Hong Seok , Pramanik Anup
Published: September 2011

Based on a PBE-D2 calculation that empirically includes van der Waals interactions to the standard GGA approximation of Perdew, Berke, and Ernzerhof, we have investigated the adsorption of ferrocene or ruthenocene on pristine and X-doped graphene (GrS) or graphene nanoribbons (GNRs), where X (=B or N) is a p-type or n-type heteroatom. First, we find that van der Waals interactions play a dominant role in the adsorption. Second, we find that metallocene adsorption on doped GNRs introduces different effects in the low-bias conductance, not far from the linear response regime, of GNRs depending upon the doping type. Adsorption on undoped or p-type GNRs brings about a slight reduction in conductance due to an introduction of quasi-bound states just below the Fermi level. No appreciable reduction is expected in n-type GNRs because those states are introduced far below the Fermi level.
Controlled positions and kinetic analysis of spontaneous tin whisker growth
Authors: Su Chien-Hao , Chen Hao , Lee Hsin-Yi , Wu Albert T.
Published: September 2011

This study achieved controlling the positions of spontaneous growth of tin whiskers. We surmounted the unpredictable growing nature of such whiskers and performed accurately quantitative analyses of the growth kinetics and yielded precise measurement of the growth rate. Furthermore, using synchrotron radiation x-ray, this study determined the stress variations in conjunction with whisker growth that fitted appropriately to the model. Accordingly, the results could address the debate held for decades and prove that forming a surface oxide layer is one of the required and necessary conditions for controlling the positions of spontaneous growth of tin whiskers.
Scanning capacitance microscopy of ErAs nanoparticles embedded in GaAs pn junctions
Authors: Park K. W. , Nair H. P. , Crook A. M. , Bank S. R. , Yu E. T.
Published: September 2011

Scanning capacitance microscopy is used to characterize the electronic properties of ErAs nanoparticles embedded in GaAs pn junctions grown by molecular beam epitaxy. Voltage-dependent capacitance images reveal localized variations in subsurface electronic structure near buried ErAs nanoparticles at lateral length scales of 20-30 nm. Numerical modeling indicates that these variations arise from inhomogeneities in charge modulation due to Fermi level pinning behavior associated with the embedded ErAs nanoparticles. Statistical analysis of image data yields an average particle radius of 6-8 nm—well below the direct resolution limit in scanning capacitance microscopy but discernible via analysis of patterns in nanoscale capacitance images.
Nanoscale phase separation in quasi-uniaxial and biaxial strained multiferroic thin films
Authors: Qi Yajun , Huang Chuanwei , Chen Zuhuang , Luo Zhenlin , Wang Yiqian , Guo Jun , White Tim , Wang Junling , Gao Chen , Sritharan Thirumany , Chen Lang
Published: September 2011

Nanoscale phase separation was investigated in epitaxial strained BiFeO3 thin films on LaAlO3 single crystal substrate. In biaxial strained thin films, nanoscale mixtures of the tetragonal-like and rhombohedral-like phases occur with a film thickness above 35 nm. For 10-30 nm ultrathin ones, tetragonal-like single phase is confirmed using synchrotron x-ray and the atomic force microscopy studies. However, nanoscale phase separations are still observed in quasi-uniaxial transmission electron microscopy foil specimens for those ultrathin films, indicating the phase separation emerges in a much smaller thickness in uniaxial constraint films than that in biaxial ones.
Improved light output of GaN-based vertical light emitting diodes using SiO2 nanotube arrays and transparent metal oxide current conduction layer
Authors: Wang Shui-Jinn , Wang Pei-Ren , Kuo Der-Ming , Kuo Hsiun-Rong , Kuo Jian-Shian
Published: September 2011

The use of a refractive index matching (RIM) structure with indium zinc oxide (IZO) transparent conduction layer and SiO2 nanotube (SiO2-NT) arrays to improve light extraction of vertical structure KOH-etched GaN-based light emitting diodes (VLEDs) is demonstrated. Compared to regular VLED with KOH-roughened surface, it shows considerable gains in light emitted critical angle and light output power by 21.3° and 103% at 350 mA, respectively. These improvements could be attributed to the effectiveness of the IZO/SiO2-NT RIM scheme in ameliorating current crowding and significantly minimizing the total internal reflection effect.
Spin-flop driven magneto-dielectric effect in Co4Nb2O9
Authors: Kolodiazhnyi T. , Sakurai H. , Vittayakorn N.
Published: September 2011

Co4Nb2O9 becomes antiferromagnetic (AFM) below 27.4 K with a spin-flop transition at a critical field, Hc, of 12 kOe. Room-temperature dielectric properties are dominated by finite electronic conductivity. Below 125 K, the charge carriers are frozen-out and the dielectric constant is controlled by the lattice phonons. A large (12%) spin flop-driven enhancement in dielectric constant is found in the very narrow temperature interval (Δ T = 1.6 K) in the vicinity of the AFM phase transition. Magneto-dielectric anomaly shows low-frequency dispersion; therefore, the H-induced changes in the phonon eigenfrequencies are unlikely. Other possible reasons for unusual magneto-dielectric effect in Co4Nb2O9 are discussed.
Communication: Strong excitonic and vibronic effects determine the optical properties of Li2O2
Authors: Garcia-Lastra J. M. , Bass J. D. , Thygesen K. S.
Published: September 2011

The band structure and optical absorption spectrum of lithium peroxide (Li2O2) is calculated from first-principles using the G0W0 approximation and the Bethe-Salpeter equation, respectively. A strongly localized (Frenkel type) exciton corresponding to the π*→σ* transition on the O2−2 peroxide ion gives rise to a narrow absorption peak around 1.2 eV below the calculated bandgap of 4.8 eV. In the excited state, the internal O2−2 bond is significantly weakened due to the population of the σ* orbital. As a consequence, the bond is elongated by almost 0.5 Å leading to an extreme Stokes shift of 2.6 eV. The strong vibronic coupling entails significant broadening of the excitonic absorption peak in good agreement with diffuse reflectance data on Li2O2 which shows a rather featureless spectrum with an absorption onset around 3.0 eV. These results should be important for understanding the origin of the high potential losses and low current densities, which are presently limiting the performance of Li-air batteries.
Electrical properties of atomic layer deposited aluminum oxide on gallium nitride
Authors: Esposto Michele , Krishnamoorthy Sriram , Nath Digbijoy N. , Bajaj Sanyam , Hung Ting-Hsiang , Rajan Siddharth
Published: September 2011

We report on our investigation of the electrical properties of metal/Al2O3/GaN metal-insulator-semiconductor capacitors. We determined the conduction band offset and interface charge density of the alumina/GaN interface by analyzing the capacitance-voltage characteristics of atomic layer deposited Al2O3 films on GaN substrates. The conduction band offset at the Al2O3/GaN interface was calculated to be 2.13 eV, in agreement with theoretical predications. A non-zero field of 0.93 MV/cm in the oxide under flat-band conditions in the GaN was inferred, which we attribute to a fixed net positive charge density of magnitude 4.60 × 1012 cm−2 at the Al2O3/GaN interface. We provide hypotheses to explain the origin of this charge by analyzing the energy band line-up.
Chemical insight into origin of forming-free resistive random-access memory devices
Authors: Wu X. , Fang Z. , Li K. , Bosman M. , Raghavan N. , Li X. , Yu H. Y. , Singh N. , Lo G. Q. , Zhang X. X. , Pey K. L.
Published: September 2011

We demonstrate the realization of a forming-step free resistive random access memory (RRAM) device using a HfOx/TiOx/HfOx/TiOx multilayer structure, as a replacement for the conventional HfOx-based single layer structure. High-resolution transmission electron microscopy (HRTEM), along with electron energy loss spectroscopy (EELS) analysis has been carried out to identify the distribution and the role played by Ti in the RRAM stack. Our results show that Ti out-diffusion into the HfOx layer is the chemical cause of forming-free behavior. Moreover, the capability of Ti to change its ionic state in HfOx eases the reduction-oxidation (redox) reaction, thus lead to the RRAM devices performance improvements.
Analytical expressions for spectral composition of band photoluminescence from silicon wafers and bricks
Authors: Green Martin A.
Published: September 2011

Photoluminescence imaging is now a widely used characterization tool for silicon bricks, wafers sliced from these bricks and for completed silicon devices, particularly solar cells. Analytical solutions for the spectral distribution of photoluminescence are reported showing the dependence on wavelengths of both exciting and emitted light and on test specimen doping levels, surface recombination velocities, carrier diffusion lengths, and front and rear reflection.
pn-homojunction formation in single fullerene films
Authors: Kubo Masayuki , Kaji Toshihiko , Hiramoto Masahiro
Published: September 2011

A pn-homojunction was fabricated in a single fullerene (C60) film containing MoO3- and Ca-doped regions. The clear observation of masking effects under light irradiation to both sides of the electrode confirmed the existence of a pn-homojunction in the bulk cell. The position of the pn-homojunction was intentionally controlled by changing the thickness of the MoO3/Ca doped regions.
Variation in mussel and barnacle recruitment parallels a shift in intertidal community structure in the Cook Strait region of New Zealand
Authors: Demello Rahul , Phillips Nicole E.
Published: September 2011

Recruitment influences populations and communities of marine organisms to varying degrees and across a range of spatial scales. We hypothesised that recruitment plays a role in maintaining different intertidal invertebrate assemblages between two nearby locations in New Zealand (Wellington Harbour and the south coast), long reported to have dramatically different communities (with greater cover of sessile invertebrates in the Harbour). Sites in Wellington Harbour were hypothesised to have higher monthly recruitment rates of mussels and barnacles and greater barnacle colonisation after 1 year. Surveys were conducted to quantify community differences. In Wellington Harbour, the mid-intertidal zone was dominated by the mussel Mytilus galloprovincialis and the barnacle Chamaesipho columna and the high intertidal zone by C. columna. In contrast, on the south coast mussels were almost completely absent from both tidal heights and barnacles (predominantly Chamaesipho brunnea) were sparse. In the high zone, monthly recruitment and long term colonisation (over 1 year) of barnacles was much greater in the Harbour; in the mid-intertidal zone, mussel recruitment was up to two orders of magnitude greater in the Harbour than the south coast. Species-specific recruitment patterns differed between the locations, however and were consistent with those of adult abundance.
Learned recognition and avoidance of invasive mosquitofish by the shrimp, Paratya australiensis
Authors: Bool Joshua D. , Witcomb Kristen , Kydd Erin , Brown Culum
Published: September 2011

Little is known about the learning ability of crustaceans, especially with respect to their anti-predator responses to invasive species. In many vertebrates, anti-predator behaviour is influenced by experience during ontogeny. Here, predator-naïve glass shrimp (Paratya australiensisis) were exposed to a predatory, invasive fish species, Gambusia holbrooki, to determine whether shrimp could learn to: (1) avoid the scent of Gambusia via classical conditioning; and (2) restrict their activity patterns to the night to reduce predatory encounters. Conditioned shrimp were placed in containers in aquaria containing Gambusia for 3 days during which time they could be harassed but not consumed by Gambusia. When tested in a Y-maze, conditioned shrimp showed a long delay before making a choice between Gambusia scented water and aged tap water but chose an arm at random. Control shrimp showed a brief delay in emergence and also chose at random. In a second experiment, we housed shrimp with a single Gambusia and observed their activity patterns. In the presence of Gambusia, shrimp switched from diurnal to nocturnal foraging. These results show that naïve shrimp learn to recognise novel predatory species via chemical cues and adjust their activity patterns to coincide with periods when Gambusia are inactive.
Political Party Development in Iran
Authors: Cottam Richard W.
Published: September 2011

The period of reasonably free and vigorous political party activity in Iran was remarkably brief when viewed in the perspective of the very long history of that ancient land. Beginning slowly after the abdication of Reza Shah in 1941, party activity became steadily more intense until August 1953 when it was suddenly suppressed. But as brief as this period was, these twelve years witnessed patterns of political party development which deserve comparison with the experience of other developing states.

The term “developing” has been in vogue throughout the post-World War II era. But for all that the term remains a vague one. Why Iran with its recorded history of over twenty five hundred years should be classified as “developing” while the United States with a history of less than two hundred years should be classified as “developed” is not immediately obvious.