bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetism and superconductivity in the Heusler alloy Pd2YbPb
Authors: Öner Y.
Published: February 2012

We report the temperature dependence (1.8–300 K) of the heat capacity (Cp) and electrical resistivity (ρ) for the Heusler alloy Pd2YbPb and its isomorphous nonmagnetic counterpart, Pd2YPb. Both compounds appear to be the same MnCu2Al structure with space group Fm3¯m. From the resistivity data, the superconducting transition for Pd2YbPb and Pd2YPb are observed at T = 2.85 K and T = 4.75 K, respectively. At low temperatures, the Cp/T versus T data exhibits a small jump at around 4 K for Pd2YPb, while, for Pd2YbPb, this superconducting transition is hidden by an upturn feature observed below T = 3 K, presumably due to nuclear spin polarization. From low temperature analysis of the Cp/T data, the electronic contribution to Cp and Debye temperature were determined to be γ = 4.5 mJ/mole-K and ΘD = 140 K for Pd2YbPb and γ = 1.6 mJ/mole-K and ΘD = 173 K for Pd2YPb. The magnetic contribution of the Yb+3 ions to the heat capacity and resistivity were calculated by subtracting the Pd2YPb data from that for Pd2YbPb. The scheme of the energy levels created by the crystal electric field (CEF) splitting for cubic symmetry was determined from the Schottky contribution to Cp. In addition, the effect of CEF on the resistivity has been investigated. Both analyses lead us to suggest that the ground state of Yb ions is a quartet Γ8. The first excited level between the ground state Γ8 and a Γ7 doublet, Δ1 = 125 K, and the second energy splitting between Γ8 and Γ6, Δ2 = 268 K, were deduced from the heat capacity data, while values of Δ1 = 155 K and Δ2 = 355 K were obtained from resistivity data. Furthermore, at higher temperatures, we observed that Cp for Pd2YbPb exceeds the Dulong-Petit limit, leading us to include a small anharmonic correction in the fitting analysis.
Efficient room-temperature spin detector based on GaNAs
Authors: Puttisong Y. , Buyanova I. A. , Geelhaar L. , Riechert H. , Tu C. W. , Chen W. M.
Published: February 2012

Efficient and highly spin-dependent recombination processes are shown to not only turn GaNAs into an efficient spin filter but also to make it an excellent spin detector functional at room temperature (RT). By taking advantage of the defect-engineered spin-filtering effect, the spin detection efficiency is no longer limited by the fast spin relaxation of conduction electrons. This leads to a significant enhancement in the optical polarization of the spin detector, making it possible to reliably detect even very weak electron spin polarization at RT, as demonstrated by a study of spin loss during optical spin injection across a GaAs/GaNAs interface.
Efficiently recyclable magnetic core-shell photocatalyst for photocatalytic oxidation of chlorophenol in water
Authors: Choi Kyong-Hoon , Oh Seung-Lim , Jung Jong-Hyung , Jung Jin-Seung
Published: February 2012

Multifunctional Fe3O4@TiO2 core-shell submicron particles were fabricated by a simple surface modification process that induces the magnetic submicron particles to be coated with a TiO2 shell. As characterized by field emission scanning electron microscopy, (FESEM), the as-synthesized Fe3O4@TiO2 particles exhibit a narrow size distribution with a typical size of 248 ± 19 nm and 8 nm in shell thickness. Magnetic measurement indicates that the as-synthesized Fe3O4@TiO2 core-shell particles are superparamagnetic at room temperature. Photocatalytic experiment is demonstrated by utilizing the oxidation reaction of 2,4,6-trichlorophenol (2,4,6-TCP) with the photofunctional magnetic nanoparticles.
Chiral magnetization configurations in magnetic nanostructures in the presence of Dzyaloshinskii-Moriya interactions
Authors: Grisewood Naoise , Eves John , Usher Tim , Braun Hans-Benjamin
Published: February 2012

Many low-dimensional systems, such as nanoscale islands, thin films, and multilayers, as well as bulk systems, such as multiferroics, are characterized by the lack of inversion symmetry, a fact that may give rise to a Dzyaloshinskii-Moriya (DM) interaction. For sufficient strength, the DM interaction will favor spiral spin configurations of definite chirality. In order to harness such systems for applications, it is important to understand the conditions under which these spiral spin configurations form and how they can be controlled via an external field. Here, we present exact solutions of the 1D magnetization profiles in such systems for arbitrary material parameters in closed form. Determining the energy per unit length exactly, we are able to present the critical strength of the DM interaction, at which spiral solutions are energetically favorable. These magnetization profiles, in general, take the form of a domain wall or soliton lattice, with all solitons having the same chirality, whose sign is dictated by DM interaction. Conversely, given an energetically favorable spiral solution, we determine quantitatively how the magnetization profile changes as a function of the applied field.
Strain-induced magnetoresistance and magnetic anisotropy properties of Co/Cu multilayers
Authors: Rizal C. , Gyawali P. , Kshattry I. , Pokharel R. K.
Published: February 2012

[Co (tCo) nm/Cu 1.5 nm]50 multilayers were grown onto 15-nm Cu/polyimide buffer layers. The relationship between stress, σ, and strain, ɛ, for the [Co 1.0 nm/Cu 1.5 nm]50 multilayers has been presented. The effects of induced strain on the magnetoresistance (MR) and magnetic anisotropy have been examined. The [Co 1.0 nm/Cu 1.5 nm]50 multilayer exhibited a maximum MR ratio of 3.4% at a Co layer thickness of 1.0 nm, β of 0.1, and a strain of 1.5%. The multilayers exhibited a remarkable magnetic anisotropy with the easy axis of magnetization always lying in a plane perpendicular to the direction of the induced strain.
Phase evolution and magnetocaloric effect of melt-spun Mn3Sn2−xMx (M = B, C; x = 0–0.5) ribbons
Authors: Zhao X. G. , Lee E. H. , Hsieh C. C. , Shih C. W. , Chang W. C. , Zhang Z. D.
Published: February 2012

The effects of B and C substitution for Sn on phase components, Curie temperature TC, and magnetocaloric effect of melt-spun Mn3Sn2-xMx (M = B, C; x = 0−0.5) ribbons have been investigated. X-ray diffraction (XRD) results show that the main phase in the as-spun Mn3Sn2−xMx (M = B, C; x = 0−0.5) ribbons is Mn3Sn2 of Ni3Sn2-type (Pnma). Minor Mn2B (when x ≥ 0.1) or Mn5C2 (when x > 0.1) secondary phase is formed, and their amounts increase with increasing B and C concentration, respectively. The Curie temperature TC of these ribbons varies in the temperature range of 240−250 K. The peak values of the maximal magnetic entropy change, −ΔSMmax, are about 13.6−18.3 mJ/cm3 K for B-substituted ribbons and 13.6–17.5 mJ/cm3 K for C-substituted ribbons, respectively, at a maximum applied field of 50 kOe. These values are about one fifth that of Gd (81.4 mJ/cm3 K). However, the relatively broader temperature range of the half maximum of ΔSM peak (∼100 K), low-cost and nontoxic elements still make Mn3Sn2-based ribbons the promising candidates for magnetic refrigeration applications close to room temperature.
Simulation of a spin field effect transistor based on magnetic impurity–doped ZnO
Authors: Thankalekshmi Ratheesh R. , Rastogi A. C.
Published: February 2012

This paper theoretically analyzes and simulates the conductance modulation of the electron channel of a spin-FET in terms of the variation in polarized spin density in response to the gate voltage. Spin-switched change in the channel charge has been modeled theoretically and is correlated with the channel conductance. Simulation studies establish the range of channel parameters, spin modulation relative to the sensor, the electron mobility, and doping concentration for the spin-switched changes in the electron channel conductance.
Development of a He- and He0 beam source for alpha particle measurement in a burning plasma
Authors: Tanaka N. , Sasao M. , Terai K. , Okamoto A. , Kitajima S. , Yamaoka H. , Wada M.
Published: February 2012

Proof of principle experiments of neutral helium beam production for alpha particle diagnostics was carried out on a test stand. Negative helium ions were produced in the Li charge exchange cell, in which stable and long time operation was possible. He− beam was accelerated to 157 keV. Finally, He0 beam was successfully produced after the flight in the drift-tube through the auto-electron-detachment process from He− to He0. A neutral beam detector using a pyroelectric device was also developed to measure He0 beam intensity. The metastable component in the neutral helium beam was found to be less than 2%.
Spatial distribution of the charged particles and potentials during beam extraction in a negative-ion source
Authors: Tsumori K. , Nakano H. , Kisaki M. , Ikeda K. , Nagaoka K. , Osakabe M. , Takeiri Y. , Kaneko O. , Shibuya M. , Asano E. , Kondo T. , Sato M. , Komada S. , Sekiguchi H. , Kameyama N. , Fukuyama T. , Wada S. , Hatayama A.
Published: February 2012

We report on the characteristics of the electronegative plasma in a large-scale hydrogen negative ion (H−) source. The measurement has been made with a time-resolved Langmuir probe installed in the beam extraction region. The H− density is monitored with a cavity ring-down system to identify the electrons in the negative charges. The electron-saturation current decreases rapidly after starting to seed Cs, and ion-ion plasma is observed in the extraction region. The H− density steps down during the beam extraction and the electron density jumps up correspondingly. The time integral of the decreasing H− charge density agrees well with the electron charge collected with the probe. The agreement of the charges is interpreted to indicate that the H− density decreasing at the beam extraction is compensated by the electrons diffusing from the driver region. In the plasmas with very low electron density, the pre-sheath of the extraction field penetrates deeply inside the plasmas. That is because the shielding length in those plasmas is longer than that in the usual electron-ion plasmas, and furthermore the electrons are suppressed to diffuse to the extraction region due to the strong magnetic field.
Effect of beam limiting aperture and collector potential on multi-element focused ion beams
Authors: Paul Samit , Chowdhury Abhishek , Bhattacharjee Sudeep
Published: February 2012

A compact microwave driven plasma based multi-element focused ion beam system has been developed. In the present work, the effect of reduced beam limiter (BL) aperture on the focused ion beam parameters, such as current and spot size, and a method of controlling beam energy independently by introducing a biased collector at focal point (FP) are investigated. It is found that the location of FP does not change due to the reduction of BL aperture. The location of FP and beam size are found to be weakly dependent on the collector potential in the range from –8 kV to –18 kV.
A compact neutron generator using a field ionization source
Authors: Persaud Arun , Waldmann Ole , Kapadia Rehan , Takei Kuniharu , Javey Ali , Schenkel Thomas
Published: February 2012

Field ionization as a means to create ions for compact and rugged neutron sources is pursued. Arrays of carbon nano-fibers promise the high field-enhancement factors required for efficient field ionization. We report on the fabrication of arrays of field emitters with a density up to 106 tips/cm2 and measure their performance characteristics using electron field emission. The critical issue of uniformity is discussed, as are efforts towards coating the nano-fibers to enhance their lifetime and surface properties.
A collimator for hot plasma x-ray diagnostics in ion sources
Authors: Bernhardi K. , Wiesemann K.
Published: February 2012

We present a collimator configuration for measuring energy resolved x-ray plasma volume bremsstrahlung emitted, e.g., by an ECRIS. Special attention we paid to shielding the detector against interfering Compton scattered radiation and wall bremsstrahlung stemming from the collimator entrance aperture. We estimate the efficiency for shielding of Compton scattered radiation at least attainable by this arrangement.
Enhancement of omnidirectional photonic band gaps in one-dimensional dielectric plasma photonic crystals with a matching layer
Authors: Zhang Hai-Feng , Liu Shao-Bin , Kong Xiang-Kun , Zou Liang , Li Chun-Zao , Qing Wu-shu
Published: February 2012

In this paper, we demonstrate by theoretical analysis a novel way to enhance the omnidirectional photonic band gap (OBG) in a type of photonic structure made of dielectric and plasma one-dimensional (1D) photonic crystals (1D PCs) by introducing a matching layer. Simulations by the transfer matrix method (TMM) show that such an OBG is insensitive to the incident angle and the polarization of electromagnetic (EM) wave; the frequency range and central frequency of OBG are significantly enlarged by introducing a matching layer in the heterostructure compared to 1D conventional binary dielectric photonic crystals (DPCs). The photonic band gap (PBG) of both polarizations also can be obviously enlarged as the incident angle is relatively small. The OBG originates from a Bragg gap in contrast to zero-ñ gap or single negative (negative permittivity or negative permeability) gap. From the numerical results, it has been shown that introducing a matching layer in such a heterostructure has a superior feature in the enhancement relative bandwidth of OBG compared with the conventional ternary plasma photonic crystals (PPCs); the frequency range of OBG can be notably enlarged by increasing the thickness and density of plasma layer.
Inertial-range anisotropy in Rayleigh-Taylor turbulence
Authors: Soulard Olivier , Griffond Jérôme
Published: February 2012

In this work, the spectral equilibrium theory of Ishihara et al. [Phys. Rev. Lett. 88, 154501 (2002)10.1103/PhysRevLett.88.154501] is applied to Rayleigh-Taylor turbulence. With the help of Canuto and Dubovikov's model [V. Canuto and M. Dubovikov, Phys. Fluids 8, 571 (1996)10.1063/1.868842] closed expressions for the anisotropic spectra of velocity and density, valid in the inertial range, are derived. Based on this result, the main properties of Rayleigh-Taylor turbulence at small scales are discussed. These theoretical results are compared against a direct numerical simulation of a Rayleigh-Taylor mixing zone.
Photonically enhanced flow boiling in a channel coated with carbon nanotubes
Authors: Kousalya Arun S. , Hunter Chad N. , Putnam Shawn A. , Miller Timothy , Fisher Timothy S.
Published: February 2012

High heat dissipation rates are enabled by multi-phase cooling schemes owing to latent heat uptake. We demonstrate enhanced flow boiling from a carbon nanotube (CNT)-coated copper surface exposed to low-intensity ultraviolet (UV)-visible excitation. Compared to non-illuminated results, the average boiling incipience temperature decreased by 4.6 °C and heat transfer coefficients improved by 41.5% with light exposure. These improved results are attributed to augmented hydrophilicity upon exposure to UV light and possible nanoscale opto-thermal effects, and suggest opportunities for active temperature control of temperature-sensitive devices.
Local quasi hidden variable modelling and violations of Bell-type inequalities by a multipartite quantum state
Authors: Loubenets Elena R.
Published: February 2012

We introduce for a general correlation scenario a new simulation model, a local quasi hidden variable (LqHV) model, where locality and the measure-theoretic construction inherent to a local hidden variable (LHV) model are preserved but positivity of a simulation measure is dropped. We specify a necessary and sufficient condition for LqHV modelling and, based on this, prove that every quantum correlation scenario admits a LqHV simulation. Via the LqHV approach, we construct analogs of Bell-type inequalities for an N-partite quantum state and find a new analytical upper bound on the maximal violation by an N-partite quantum state of S1 × ⋯ × SN-setting Bell-type inequalities – either on correlation functions or on joint probabilities and for outcomes of an arbitrary spectral type, discrete or continuous. This general analytical upper bound is expressed in terms of the new state dilation characteristics introduced in the present paper and not only traces quantum states admitting an S1 × ⋯ × SN-setting LHV description but also leads to the new exact numerical upper estimates on the maximal Bell violations for concrete N-partite quantum states used in quantum information processing and for an arbitrary N-partite quantum state. We, in particular, prove that violation by an N-partite quantum state of an arbitrary Bell-type inequality (either on correlation functions or on joint probabilities) for S settings per site cannot exceed (2S − 1)N − 1 even in case of an infinite dimensional quantum state and infinitely many outcomes.
Thin films flowing down inverted substrates: Three-dimensional flow
Authors: Lin T.-S. , Kondic L. , Filippov A.
Published: February 2012

We study contact line induced instabilities for a thin film of fluid under destabilizing gravitational force in three-dimensional setting. In the previous work [T.-S. Lin and L. Kondic, Phys. Fluids 22, 052105 (2010)], we considered two-dimensional flow, finding formation of surface waves whose properties within the implemented long-wave model depend on a single parameter, \documentclass[12pt]{minimal}\begin{document}$D=(3Ca)^{1/3}\cot \alpha$\end{document}D=(3Ca)1/3cotα, where Ca is the capillary number and α is the inclination angle. In the present work we consider fully 3D setting and discuss the influence of the additional dimension on stability properties of the flow. In particular, we concentrate on the coupling between the surface instabilities and the transverse (fingering) instabilities of the film front. We furthermore consider these instabilities in the setting where fluid viscosity varies in the transverse direction. It is found that the flow pattern strongly depends on the inclination angle and the viscosity gradient.
2.5D magnetohydrodynamic simulation of the Kelvin-Helmholtz instability around Venus—Comparison of the influence of gravity and density increase
Authors: Zellinger M. , Möstl U. V. , Erkaev N. V. , Biernat H. K.
Published: February 2012

We present a numerical study of the 2.5D Kelvin-Helmholtz instability and its vortices, where an initial plasma configuration appropriate for the situation around unmagnetized planets is assumed. We solve the set of ideal magnetohydrodynamic equations numerically with the total variation diminishing Lax-Friedrichs algorithm. Our density profile is such that the mass density increases toward the planet. A high density leads to smaller growth rates of the instability and, thus, has a stabilizing effect for the boundary layer. Moreover, we include source terms in the equations, enabling us to study the influence of gravity. Our results show that gravity affects the evolution of the Kelvin-Helmholtz instability. However, the effect is not very significant. We thus conclude that the density increase toward the planet stabilizes the boundary layer around Venus more than gravity does.
The three-dimensional transition stages over the NACA-0009 airfoil at Reynolds numbers of several ten thousand
Authors: Elimelech Y. , Arieli R. , Iosilevskii G.
Published: February 2012

This experimental study addresses late transition stages over a conventional airfoil at Reynolds numbers of several ten thousands. The study is based on extensive volumetric flow measurements using constant temperature anemometry. This technique provided both the time-averaged description of the flow field in the vicinity of the wing and the high fidelity spectral analysis inside the separated boundary layer. Large cellular flow structures were observed above the wing. They resemble the stall cells at Reynolds numbers of a few hundred thousands and separation cells at Reynolds numbers of a few hundred. It is shown that most of the energy of the velocity fluctuations within the boundary layer over the forward half of the wing is concentrated in the low-frequency range of the spectrum, where the chord-based Strouhal number is small as compared with unity.
Draining of a thin film on the wall of a conical container set into rapid rotation about its vertical axis
Authors: Ungarish Marius , Sherwood John D.
Published: February 2012

A theory for how rotation modifies the draining of a thin fluid film on the surface of a conical container is developed. Rapid rotation, imposed instantaneously, creates an Ekman layer that pumps fluid to the outer edge of the container. This fluid is flung out of the vessel, rather than being transferred to the interior of the container. In consequence, fluid in the vessel but outside the Ekman layer does not spin up and continues to drain towards the container base. The net result is that draining is enhanced by the outward Ekman flux and finishes in finite time (when only the thin Ekman layer remains). The governing equations can be solved by the method of characteristics, to within numerical quadrature. The amount of fluid pumped outwards (rather than draining towards the base of the container) depends upon a single parameter that characterizes the ratio of the Ekman flux to gravitational draining.
Electromagnetically induced transparency spectroscopy
Authors: Eilam Asaf , Shapiro Evgeny A. , Shapiro Moshe
Published: February 2012

We propose a method based on the electromagnetically induced transparency (EIT) phenomenon for the detection of molecules which exist as a small minority in the presence of a majority of absorbers. The EIT effect we employ effectively eliminates the absorption of the majority species in the spectral region where it overlaps with the absorption of the minority species. The method can also be used to enhance local-modes transitions which overlap spectrally with a background of other local-modes transitions of the same molecule. The general theory is applied to the case of sparse and congested background spectra within the same molecule and to the recording of the spectra of isotopomers (of chlorine and methanol) that are in minority relative to other isotopomers which constitute the majority of molecules present.
Long-range interactions between adjacent and distant bases in a DNA and their impact on the ribonucleic acid polymerase-DNA dynamics
Authors: Saha M. , Kofane T. C.
Published: February 2012

When an inhomogeneous RNA-polymerase (RNAP) binds to an inhomogeneous DNA at the physiological temperature, we propose a spin-like model of DNA nonlinear dynamics with long-range interactions (LRI) between adjacent and distant base pairs to study RNAP-DNA dynamics. Using Holstein-Primakoff’s representation and Glauber’s coherent state representation, we show that the model equation is a completely integrable nonlinear Schrödinger equation whose dispersive coefficient depends on LRI’s parameter. Inhomogeneities have introduced perturbation terms in the equation of motion of RNAP-DNA dynamics. Considering the homogeneous part of that equation, a detailed study of the solution shows that the number of base pairs which form the bubble, the height, and the width of that bubble depend on the long-range parameter. The results of the perturbation analysis show that the inhomogeneities due to the DNA and RNAP structures do not alter the velocity and amplitude of the soliton, but introduce some fluctuations in the localized region of the soliton. The events that happen in the present study may represent binding of an RNAP to a promoter site in the DNA during the transcription process.
Theoretical analysis and experimental validation of a novel multilevel inverter topology for renewable energy interfacing applications
Authors: Gupta Krishna Kumar , Jain Shailendra
Published: February 2012

This paper presents a new topological structure for symmetrical cascaded multilevel inverters. The proposed circuit consists of cross-connected separate symmetrical DC sources which can generate number of levels equal to that of a cascaded H-bridge topology with reduced number of power switches. It is especially suitable for utility interfacing of renewable energy sources. It also offers the possibility of a transformer less and filter less operation. The working principle of the proposed topology is explained with the help of a single-phase five level inverter. Simulation results are obtained in matlab/simulink environment and are verified experimentally on a laboratory prototype. An exhaustive comparison of the proposed topology is made with the classical cascaded H-bridge topology, especially with the considerations of conduction losses and cost-effectiveness. The proposed topology results in the reduction of number of power switches, losses, installation area, and converter cost and thus can be implemented as an effective and efficient power electronic interface for renewable sources of energy with the utility grid.
The interaction of a vortex ring with a sloped sediment layer: Critical criteria for incipient grain motion
Authors: Munro R. J.
Published: February 2012

Experiments were performed to analyse the interaction between a vortex ring and a sloped sediment layer. Attention focussed on interactions under “critical” conditions, in which sediment motion was only just induced by the ring's flow field. Both hydraulically smooth and hydraulically rough bedforms were analysed, using near-spherical monodisperse sediments with relative densities of 1.2 and 2.5 and mean diameters (dp) ranging between 80 and 1087 μm. Measurements of the vortex-ring flow field were obtained, during the interaction, using two-dimensional particle imaging velocimetry. The threshold conditions for incipient sediment motion were analysed in terms of the critical Shields parameter (Nc), defined in terms of the peak tangential velocity measured adjacent to the bed surface. Bed-slope effects were investigated by tilting the sediment layer at various angles between the horizontal and the repose limit for the sediment. In all cases, the propagation axis of the vortex ring was aligned normal to the bed surface. The measured values of Nc were compared with a force-balance model based on the conditions for incipient grain motion on a sloping bed. For hydraulically smooth bedforms, where the bed roughness is small compared to the boundary-layer depth, the model was derived to account for how viscous stresses affect the drag and lift forces acting on the near surface sediment. For hydraulically rough bedforms, where this viscous-damping effect is not present, the model assumes the drag and lift forces scale with the square of the near-bed (inviscid) velocity scale. In both cases, the model predicts that bedforms become more mobile as the bed slope is increased. However, the damping effect of the viscous sublayer acts as a stabilizing influence for hydraulically smooth bedforms, to reduce the rate at which the bed mobility increases with bed slope. The measured values of Nc were in agreement with the trends predicted by this model, and exhibit a transition in behaviour between the smooth-bed and rough-bed cases when dp/δs ≈ 20 (where δs is the viscous-sublayer length scale).
Influence of the oscillatory quantum screening on the occurrence scattering time in electron-ion quantum plasmas
Authors: Hong Woo-Pyo , Jung Young-Dae
Published: February 2012

The oscillatory quantum screening effects on the occurrence scattering time advance for the electron-ion collision are investigated in dense quantum plasmas. The eikonal method is applied to obtain the scattering amplitude and occurrence scattering time advance for the electron-ion collision in quantum plasmas. The result shows that the oscillatory screening effect suppresses the occurrence scattering time advance for the electron-ion collision in quantum plasmas, especially, for large scattering angles. It is also found that the oscillatory screening effect on the occurrence scattering time advance increases with an increase of the collision energy.
A model electronic Hamiltonian to study low-lying electronic states of [Fe(bpy)3]2+ in aqueous solution
Authors: Iuchi Satoru
Published: February 2012

A simple model electronic Hamiltonian to describe the potential energy surfaces of several low-lying d−d states of the [Fe(bpy)3]2+ complex is developed for use in molecular dynamics (MD) simulation studies. On the basis of a method proposed previously for first-row transition metal ions in aqueous solution, the model Hamiltonian is constructed using density functional theory calculations for the lowest singlet and quintet states. MD simulations are then carried out for the two spin states in aqueous solution in order to examine the performance of the model Hamiltonian. The simulation results indicate that the present model electronic Hamiltonian reasonably describes the potential energy surfaces of the two spin states of the aqueous [Fe(bpy)3]2+ system, while retaining sufficient simplicity for application in simulation studies on excited state dynamics.
Particle size dependent hysteresis loss in La0.7Ce0.3Fe11.6Si1.4C0.2 first-order systems
Authors: Hu F. X. , Chen L. , Wang J. , Bao L. F. , Sun J. R. , Shen B. G.
Published: February 2012

Here, we report particle size dependent hysteresis loss in La0.7Ce0.3Fe11.6Si1.4C0.2. Hysteresis loss was getting smaller with reducing the particle size. The reduced ratio can be as high as ∼61% as the sample is ground from bulk into small particles (20-50 μm). Such reduction can be ascribed to the notably increased surface area of sample and the partially removed internal strain and grain boundaries, other than nucleation factors and electronic band structure. Meanwhile, entropy change |ΔS| slightly decreases, but the effective refrigeration capacity shows an increase due to the notable reduction of hysteresis loss. Our investigations also reveal particle size limitation. When the size is below 10 μm (average ∼ 4 μm), the sample may lose its stability and the |ΔS| notably reduces.
The effect of crystallite size on thermoelectric properties of bulk nanostructured magnesium silicide (Mg2Si) compounds
Authors: Satyala Nikhil , Vashaee Daryoosh
Published: February 2012

In nanostructured bulk materials, the additional interfaces in the material enhance phonon scattering and reduce the thermal conductivity. However, interfaces also scatter electrons and deteriorate charge carrier transport. In order to benefit from the interfacial effects, the crystallite size in the material must be small compared with phonon mean free path (PMFP) and large compared with the charge carrier mean free path (CMFP). In this paper, we solve the Boltzmann transport equation for charge carriers and phonons. We show that bulk nanostructuring of Mg2Si is not an efficient method to enhance the figure-of-merit as the PMFP and CMFP are in the same range.
Measurements of liquid film thickness for a droplet at a two-fluid interface
Authors: Oldenziel G. , Delfos R. , Westerweel J.
Published: February 2012

Coalescence of a droplet at a two-fluid interface is studied at Bond numbers larger than one and at three different values of the viscosity ratio. Both the thickness of the liquid film between the rising droplet and the two-fluid interface, and the location of film rupture are measured using laser induced fluorescence. Particle image velocimetry was applied to the flow in the film. It is found that the film thins asymetrically, and that the time interval between collision and film rupture is shorter than predicted by commonly used models. The film ruptures at an off-center location. It can be concluded that asymmetric film drainage speeds up coalescence.
Emerging dynamics in surfactant-based liquid mixtures: Octanoic acid/bis(2-ethylhexyl) amine systems
Authors: Calandra Pietro , Mandanici Andrea , Turco Liveri Vincenzo , Pochylski Mikolaj , Aliotta Francesco
Published: February 2012

This work focuses on the dynamic phenomena emerging in self-assembled transient intermolecular networks formed when two different surfactants are mixed. In particular, the relaxation processes in liquid mixtures composed by bis(2-ethylhexyl)amine (BEEA) and octanoic acid (OA) in the whole composition range has been investigated by dielectric spectroscopy and Brillouin spectroscopy. A thorough analysis of all the experimental data consistently suggests that, mainly driven by acid-base interactions arising when the two surfactants are mixed, supra-molecular aggregates formation causes the slowing down of molecular dynamics. This, in turn, reflects to longer-range relaxations. These changes have been found to be composition-dependent, involving strong departures of the mixture physico-chemical properties from an ideal behaviour, and reflecting the structural and dynamical features of local structures. In particular, the peculiar dynamic processes occurring in these local inter-molecular structures, have been found to be the factors responsible for the observed and quite surprising increase of direct-current conductivity which occurs when two different (and pretty non-conductive) surfactants are mixed. The discovery can be used not only to design novel materials for application purposes but also to shed more light on the basic principles regulating charge migration in structured liquid systems.
The final stages of capillary break-up of polymer solutions
Authors: Sattler R. , Gier S. , Eggers J. , Wagner C.
Published: February 2012

The capillary break-up of a polymer solution evolves via a series of stages. After the initial instability a long-lived cylindrical filament is formed, which thins exponentially in time, while the flow is purely extensional. During the final stages of the thinning process, at which the polymers are stretched sufficiently for the filament to become unstable to a Rayleigh–Plateau-like instability, a complex flow pattern develops, which we describe here. Achieving a high spatial resolution well below the optical Rayleigh limit, we describe both the formation of individual droplets as well as that of periodic patterns. Following the periodic instability, a blistering pattern appears, with different generations of smaller droplets. At sufficiently high polymer concentrations, the filament does not break at all, but a solid polymeric fiber with a thickness well below a micron remains. The experiments were performed for various polymer and solvent systems, all of which showed the same qualitative behavior for most of the observed features.
TetraMag: A compact magnetizing device based on eight rotating permanent magnets
Authors: Gilbert M. , Mertins H.-Ch. , Tesch M. , Berges O. , Feilbach Herbert , Schneider C. M.
Published: February 2012

In this paper we describe a novel magnetizing device based on eight rotatable permanent magnets arranged in a quadrupolar configuration, which is termed the TetraMag. TetraMag creates stable and homogeneous magnetic fields at the sample position with a resolution of 0.02 mT tunable between –570 mT and +570 mT. The field direction is continuously rotatable between 0° and 360° within the sample plane, while the field strength is maintained. A simplified mathematical description of TetraMag is developed leading to magnetic field calculations which are in good agreement with the experimental results. This versatile device avoids electrical energy dissipation, cooling mechanisms, and hysteresis effects known from classical electromagnets. It is ultrahigh vacuum compatible and it offers a completely free optical path over 180° for magneto-optical experiments. It is suitable for scattering experiments with synchrotron radiation and neutrons and may be employed in a large class of magnetization experiments.
Exact solutions of coupled Liénard-type nonlinear systems using factorization technique
Authors: Hazra Tamaghna , Chandrasekar V. K. , Pradeep R. Gladwin , Lakshmanan M.
Published: February 2012

General solutions of nonlinear ordinary differential equations (ODEs) are in general difficult to find; although, powerful integrability techniques exist in the literature for this purpose. It has been shown that in some scalar cases particular solutions may be found with little effort if it is possible to factorize the equation in terms of first-order differential operators. In our present study, we use this factorization technique to address the problem of finding solutions of a system of general two-coupled Liénard-type nonlinear differential equations. We describe a generic algorithm to identify specific classes of Liénard-type systems for which solutions may be found. We demonstrate this method by identifying a class of two-coupled equations for which the particular solution can be found by solving a Bernoulli equation. This class of equations include coupled generalization of the modified Emden equation. We further deduce the general solution of a class of coupled ODEs using the factorization procedure discussed in this paper.
How Péclet number affects microstructure and transient cluster aggregation in sedimenting colloidal suspensions
Authors: Moncho-Jordá A. , Louis A. A. , Padding J. T.
Published: February 2012

We study how varying the Péclet number (Pe) affects the steady state sedimentation of colloidal particles that interact through short-ranged attractions. By employing a hybrid molecular dynamics simulation method we demonstrate that the average sedimentation velocity changes from a non-monotonic dependence on packing fraction ϕ at low Pe numbers, to a monotonic decrease with ϕ at higher Pe numbers. At low Pe number the pair correlation functions are close to their equilibrium values, but as the Pe number increases, important deviations from equilibrium forms are observed. Although the attractive forces we employ are not strong enough to form permanent clusters, they do induce transient clusters whose behaviour is also affected by Pe number. In particular, clusters are more likely to fragment and less likely to aggregate at larger Pe numbers, and the probability of finding larger clusters decreases with increasing Pe number. Interestingly, the lifetime of the clusters is more or less independent of Pe number in the range we study. Instead, the change in cluster distribution occurs because larger clusters are less likely to form with increasing Pe number. These results illustrate some of the subtleties that occur in the crossover from equilibrium like to purely non-equilibrium behaviour as the balance between convective and thermal forces changes.
Penning traps with unitary architecture for storage of highly charged ions
Authors: Tan Joseph N. , Brewer Samuel M. , Guise Nicholas D.
Published: February 2012

Penning traps are made extremely compact by embedding rare-earth permanent magnets in the electrode structure. Axially-oriented NdFeB magnets are used in unitary architectures that couple the electric and magnetic components into an integrated structure. We have constructed a two-magnet Penning trap with radial access to enable the use of laser or atomic beams, as well as the collection of light. An experimental apparatus equipped with ion optics is installed at the NIST electron beam ion trap (EBIT) facility, constrained to fit within 1 meter at the end of a horizontal beamline for transporting highly charged ions. Highly charged ions of neon and argon, extracted with initial energies up to 4000 eV per unit charge, are captured and stored to study the confinement properties of a one-magnet trap and a two-magnet trap. Design considerations and some test results are discussed.
Simulations of ionization in a hot cavity surface ion source
Authors: Turek M. , Droździel A. , Pyszniak K. , Mączka D. , Słowiński B.
Published: February 2012

A new numerical Monte Carlo method based model of a hot cavity surface ionization ion source is presented in this paper. The model, intended to support the studies on ionization phenomena in a widely used class of ion sources, takes into account geometry of the ion source and extraction system, ionizer temperature and other features. The results of ion source efficiency calculations for various configurations of the extraction field are reviewed. The dominant role of the ionizer region near the extraction opening is described. Simulated dependences of ionization efficiency on the working parameters like ionizer length and temperature, ionization potential of the substance, and extraction voltage are discussed. A good agreement of the experimental data (e.g., influence of ionizer temperature, current-voltage curve) and the predictions of the model is found. It is also shown that the contribution to the ionization yield from impact of thermionic electrons accelerated by the extraction field may be significant, especially for the substances of small surface ionization coefficient. The simulation results are compared to the predictions of different theoretical models of the ion source—the obtained simulation data are in accordance both with a well-known Kirchner formula and the so called spherical ionizer model.
Threading dislocation reduction in transit region of GaN terahertz Gunn diodes
Authors: Li Liang , Yang Lin-An , Zhang Jin-Cheng , Xue Jun-Shuai , Xu Sheng-Rui , Lv Ling , Hao Yue , Niu Mu-Tong
Published: February 2012

An effect of the position of notch-doping layer in 1-μm GaN Gunn diode on threading dislocations (TDs) distribution is investigated by transmission electron microscopy. Compared with the top-notching-layer (TNL) structure, the bottom-notching-layer (BNL) structure can efficiently reduce the TDs density and improve the crystal quality in the transit region of GaN Gunn diode because it exhibits twice-transition of growth mode from atomic step flow to layer-by-layer nucleation and leads to a significant annihilation of TDs before penetrating into the transit region. X-ray diffraction and Raman spectroscopy reveal that the BNL structure has less compressive stress than the TNL structure.
Physical model for the laser induced forward transfer process
Authors: Röder Tobias C. , Köhler Jürgen R.
Published: February 2012

This paper presents a numerical model which describes the underlying physical processes during laser induced forward transfer. The laser induced forward transfer uses a pulsed laser to transfer thin layers from a transparent support to a substrate. The model predicts the threshold energies Eth as well as the blow-off time tblow, thus allowing a profound physical understanding of the transfer process. The good agreement of simulated with measured Eth and tblow of thin nickel layers demonstrates the accuracy of the model. The model shows that gasification of the soda-lime glass support is the main driving force of the transfer process.
Layer dependent mechanical responses of graphene composites to near-infrared light
Authors: Loomis James , King Ben , Panchapakesan Balaji
Published: February 2012

The authors report layer dependent physical responses of graphene-based polymer composites to near-infrared (NIR) light. Thin film polymer composites were fabricated and compared utilizing both single layer graphene and few layer graphene (∼5 layers). NIR-induced axial stress and kinetics were measured and found to exhibit three distinct pre-strain dependent response states. Composite stress responses, elastic moduli, and efficiencies were observed to depend on the number of graphene layers in the two-dimensional crystal.
Electronic transport in doped mixed-phase hydrogenated amorphous/nanocrystalline silicon thin films
Authors: Wienkes L. R. , Blackwell C. , Kakalios J.
Published: February 2012

We report observations of three distinct conduction mechanisms in n-type doped mixed-phase amorphous/nanocrystalline silicon thin films over a range of nanocrystallite concentrations and temperatures. As the temperature is varied from 470 to 10 K, we observe activated conduction, multiphonon hopping (MPH), and Mott variable range hopping (VRH) as the nanocrystal content is increased. The transition from MPH to Mott-VRH hopping around 50 K is tentatively ascribed to the freeze out of the phonon modes.
Influence of the renormalization plasma screening on the electron-atom collision in partially ionized plasmas
Authors: Hong Woo-Pyo , Jung Young-Dae
Published: February 2012

The renormalization plasma screening effects on the elastic electron-atom collision are investigated in partially ionized dense hydrogen plasmas using the eikonal method. It is found that the renormalization plasma screening suppresses the eikonal phase shift and cross section for the elastic electron-atom collision in partially ionized plasmas. It is also found that the renormalization plasma screening effect on the elastic electron-atom collision process increases with an increasing impact parameter. In addition, it is found that the maximum position of the differential cross section is receded from the center of the atom with an increase of the Debye length.
In0.53Ga0.47As metal-oxide-semiconductor field-effect transistors with self-aligned metal source/drain using Co-InGaAs alloys
Authors: Kim SangHyeon , Yokoyama Masafumi , Taoka Noriyuki , Nakane Ryosho , Yasuda Tetsuji , Ichikawa Osamu , Fukuhara Noboru , Hata Masahiko , Takenaka Mitsuru , Takagi Shinichi
Published: February 2012

We have demonstrated InGaAs metal-oxide-semiconductor field-effect transistors (MOSFETs) with self-aligned Co-InGaAs source/drain (S/D). The fabricated MOSEFETs exhibited excellent transistor operation and an on/off ratio of 104 without any S/D ion implantation. It was found that the Co-InGaAs alloys can be formed by direct reaction of Co and InGaAs during annealing at low temperature and that the unreacted Co is selectively etched from Co-InGaAs by an HCl solution without significant etching of Co-InGaAs. We also found that the Co-InGaAs alloys have low sheet resistance of less than 50 Ω/square and relatively low Schottky barrier height of 0.12 eV against electrons in InGaAs with high thermal stability.
Influence of laser irradiated spot size on energetic electron injection and proton acceleration in foil targets
Authors: Coury M. , Carroll D. C. , Robinson A. P. L. , Yuan X. H. , Brenner C. M. , Burza M. , Gray R. J. , Quinn M. N. , Lancaster K. L. , Li Y. T. , Lin X. X. , Tresca O. , Wahlström C.-G. , Neely D. , McKenna P.
Published: February 2012

The influence of irradiated spot size on laser energy coupling to electrons, and subsequently to protons, in the interaction of intense laser pulses with foil targets is investigated experimentally. Proton acceleration is characterized for laser intensities ranging from 2×1018-6×1020 W/cm2, by (1) variation of the laser energy for a fixed irradiated spot size, and (2) by variation of the spot size for a fixed energy. At a given laser pulse intensity, the maximum proton energy is higher under defocus illumination compared to tight focus and the results are explained in terms of geometrical changes to the hot electron injection.
Enhancement of thermoelectric properties in graphene nanoribbons modulated with stub structures
Authors: Xie Zhong-Xiang , Tang Li-Ming , Pan Chang-Ning , Li Ke-Min , Chen Ke-Qiu , Duan Wenhui
Published: February 2012

The thermoelectric properties in graphene nanoribbons modulated with stub structures are studied using atomistic simulation of electron and phonon transport. The results show that the phonon transport is dramatically suppressed by the elastic scattering of the stub structure; while the thermopower S can be enhanced by a few times of magnitude. This leads to a strong enhancement of the figure of merit (ZT). Moreover, it is found that the enhancement of ZT can be effectively tuned by modulating geometric parameters of the stub and edge shapes, which offers an effective way to improve the thermoelectric performance of graphene nanoribbons.