bluuebunny/crossref_metadata_2025 huggingface dataset:
Spatio-temporal evolution of the L → I → H transition
Authors: Miki K. , Diamond P. H. , Gürcan Ö. D. , Tynan G. R. , Estrada T. , Schmitz L. , Xu G. S.
Published: September 2012

We investigate the dynamics of the low(L) → high(H) transition using a time-dependent, one dimensional (in radius) model which self-consistently describes the time evolution of zonal flows (ZFs), mean flows (MFs), poloidal spin-up, and density and pressure profiles. The model represents the physics of ZF and MF competition, turbulence suppression via E×B shearing, and poloidal flows driven by turbulence. Numerical solutions of this model show that the L→H transition can occur via an intermediate phase (I-phase) which involves oscillations of profiles due to ZF and MF competition. The I-phase appears as a nonlinear transition wave originating at the edge boundary and propagates inward. Locally, I-phase exhibits the characteristics of a limit-cycle oscillation. All these observations are consistent with recent experimental results. We examine the trigger of the L→H transition, by defining a ratio of the rate of energy transfer from the turbulence to the zonal flow to the rate of energy input into the turbulence. When the ratio exceeds order unity, ZF shear gains energy, and a net decay of the turbulence is possible, thus triggering the L→H transition. Numerical calculations indicate that the L→H transition is triggered by this peak of the normalized ZF shearing. Zonal flows act as “reservoir,” in which to store increasing fluctuation energy without increasing transport, thus allowing the mean flow shear to increase and lock in the transition. A counterpart of the L → I→H transition, i.e., an L→H transition without I-phase, is obtained in a fast power ramp, for which I-phase is compressed into a single burst of ZF, which triggers the transition. Effects of neutral charge exchange on the L→H transition are studied by varying ZF damping and neoclassical viscosity. Results show that the predicted L→H transition power increases when either ZF damping or viscosity increase, suggesting a link between recycling, ZF damping, and the L→H threshold. Studies of fueling effects on the transition and pedestal structure with an emphasis on the particle pinch are reported.
Multi-dimensional instability of dust-acoustic solitary waves in a magnetized plasma with opposite polarity dust
Authors: Akhter T. , Hossain M. M. , Mamun A. A.
Published: September 2012

Dust-acoustic (DA) solitary structures and their multi-dimensional instability in a magnetized dusty plasma (containing inertial negatively and positively charged dust particles, and Boltzmann electrons and ions) have been theoretically investigated by the reductive perturbation method, and the small-k perturbation expansion technique. It has been found that the basic features (polarity, speed, height, thickness, etc.) of such DA solitary structures, and their multi-dimensional instability criterion or growth rate are significantly modified by the presence of opposite polarity dust particles and external magnetic field. The implications of our results in space and laboratory dusty plasma systems have been briefly discussed.
Aluminium doped Zn1−xMgxO—A transparent conducting oxide with tunable optical and electrical properties
Authors: Fleischer K. , Arca E. , Smith C. , Shvets I. V.
Published: September 2012

A ternary mixed oxide Zn1−xMgxO has been doped with aluminium to create a range of transparent conducting oxides with tunable refractive index as well as work function. Conductive material was synthesised up to a magnesium concentration of x = 0.45, although the conductivity is reduced compared to standard ZnO:Al. The changes in band gap, work function, and conductivity have been attributed to a modified band structure and energetic position of the aluminium induced donor state.
Effect of toroidal rotation on the geodesic acoustic mode in magnetohydrodynamics
Authors: Ren Haijun
Published: September 2012

Theoretical research on the geodesic acoustic mode (GAM) induced by equilibrium toroidal rotation flow in the tokamak plasmas is approached by using ideal magnetohydrodynamic model. The dispersion relation of the GAM is presented by taking into account magnetic field perturbations. It is shown that β can decrease the frequency of the GAM.
A shallow-water theory of river bedforms in supercritical conditions
Authors: Vesipa Riccardo , Camporeale Carlo , Ridolfi Luca
Published: September 2012

A supercritical free-surface turbulent stream flowing over an erodible bottom can generate a characteristic pattern of upstream migrating bedforms known as antidunes. This morphological instability, which is quite common in fluvial environments, has attracted speculative and applicative interests, and has always been modelled in 2D or 3D mathematical frameworks. However, in this work we demonstrate that antidune instability can be described by means of a suitable one-dimensional model that couples the Dressler equations to a mechanistic model of the sediment particle deposition/entrainment. The results of the linear stability analysis match the experimental data very well, both for the instability region and the dominant wavelength. The analytical tractability of the 1D modeling allows us (1) to elucidate the key physical processes which drive antidune instability, (2) to show the secondary role played by sediment inertia, (3) to obtain the dispersion relation in explicit form, and (4) to demonstrate the absolute nature of antidune instability.
Application of dynamic global-coefficient subgrid-scale models to turbulent natural convection in an enclosed tall cavity
Authors: Lau G. E. , Yeoh G. H. , Timchenko V. , Reizes J. A.
Published: September 2012

Large-eddy simulations examining natural convection in an enclosed cavity with the simultaneous presence of laminar, transitional, and turbulent flow regimes were conducted. The Rayleigh number based on height of the cavity is 4.6 × 1010. Different dynamic global-coefficient procedures to compute the Vreman [A. W. Vreman, “An eddy-viscosity subgrid-scale model for turbulent shear flow: Algebraic theory and applications,” Phys. Fluids 16, 3670 (2004)] model coefficient were implemented for the subgrid-scale tensors in both the momentum and energy equations. Based on comparison with experimental and existing numerical data, it is shown that the dynamic model derived from the “global equilibrium” hypothesis gives favorable results in the mean flow and turbulence quantities. Nevertheless, because of higher subgrid-scale dissipation, transition to a turbulent flow is postponed when the Vreman model coefficient is either uniform or determined dynamically using the Germano identity approach. This suggests that much finer grid is desired when using these models in order to better capture the weak transitional boundary layer. Further, by exploring the instantaneous flow dynamics, it is demonstrated that characteristics of the coherent structures which resemble streaks in forced convection boundary layers are somewhat dissimilar in the different models.
Symmetry analysis and self-similar forms of fluid flow and heat-mass transfer in turbulent boundary layer flow of a nanofluid
Authors: Avramenko A. A. , Blinov D. G. , Shevchuk I. V. , Kuznetsov A. V.
Published: September 2012

Heat, momentum, and mass transport in turbulent boundary layer nanofluid flow over a flat plate were investigated. Boundary layer equations were reduced to self-similar forms and solved numerically. The Lie group technique, which is based on the symmetry properties of governing equations, was used to derive self-similar forms of these equations. Turbulent viscosity was predicted using the mixing-length model. Also, dependences of physical properties (viscosity, thermal conductivity, and diffusion coefficients) on the nanofluid concentration and temperature were accounted for. Influences of different dimensionless parameters and nanoparticle concentration on the velocity and temperature profiles, as well as on the relative Nusselt number and skin-friction coefficient, were investigated.
Vortex ring deformation, capture, and entrainment by a columnar vortex
Authors: Candon S. , Marshall J. S.
Published: September 2012

A vortex ring placed outside of a columnar vortex is stretched by the local straining flow induced by the vortex. This straining flow is known to lead to wrapping of the ring about the columnar vortex, which is a primary feature of multiscale vortex interactions. In this paper we show that continuous stretching of the ring and wrapping about the columnar vortex is only one of the several possible outcomes of vortex ring interaction with a columnar vortex. For cases where the local straining rate of the flow induced by the columnar vortex is sufficiently small, the shape of the vortex ring is found to oscillate between stretching in the principal direction of stretching of the induced flow and stretching in the direction of compression, all while exhibiting an increase in the average ring radius. This oscillatory behavior allows the ring to escape capture and entrainment into the columnar vortex core. In other cases the vortex ring pinches off on itself as it is strained by the columnar vortex. The paper examines the different types of behavior observed during vortex ring interaction with a columnar vortex using a vortex filament approach together with an approximate analytical theory. The focus of the paper is on understanding the fundamental physics underlying the different types of vortex ring behaviors observed.
Assembly of uniaxially aligned rare-earth-free nanomagnets
Authors: Balamurugan B. , Das B. , Shah V. R. , Skomski R. , Li X. Z. , Sellmyer D. J.
Published: September 2012

We report HfCo7 nanoparticles with appreciable permanent-magnet properties (magnetocrystalline anisotropy K1 ≈ 10 Mergs/cm3, coercivity Hc ≈ 4.4 kOe, and magnetic polarization Js ≈ 10.9 kG at 300 K) deposited by a single-step cluster-deposition method. The direct crystalline-ordering of nanoparticles during the gas-aggregation process, without the requirement of a high-temperature thermal annealing, provides an unique opportunity to align their easy axes uniaxially by applying a magnetic field of about 5 kOe prior to deposition, and subsequently to fabricate exchange-coupled nanocomposites having Js as high as 16.6 kG by co-depositing soft magnetic Fe-Co. This study suggests HfCo7 as a promising rare-earth-free permanent-magnet alloy, which is important for mitigating the critical-materials aspects of rare-earth elements.
Efficient computation of adiabatic electronic populations in multi-mode vibronic systems: Theory, implementation, and application
Authors: Nikoobakht Behnam , Köppel Horst , Gindensperger Etienne , Cederbaum Lorenz S.
Published: September 2012

The effective-mode formalism developed earlier is applied to efficiently compute adiabatic electronic populations of multi-mode vibronic systems. Two different versions of the formalism are utilized. In the first one, the effective modes are used as new vibrational basis, and the time-dependent wave function as well as populations are calculated numerically exactly. In the second variant, the hierarchy-of-modes formalism is applied as an approximation scheme which leads to accurate results when including typically 7–10 members of the hierarchy. While in the first version, the propagation of the wave packet becomes numerically tedious, the computation of the adiabatic populations is rendered possible at all. Through the hierarchy-of-modes formalism, both the propagation as well as computation of adiabatic populations are speeded up by 1–3 orders of magnitude in typical cases. The formalism is applied to study the dynamics of pyrazine-type models for varying vibronic coupling strength and a (very) large number of bath modes.
Efficient broadband energy transfer via momentum matching at hybrid junctions of guided-waves
Authors: Lin Charles , Wong Herman M. K. , Lau Benedict , Swillam Mohamed A. , Helmy Amr S.
Published: September 2012

Momentum matching at hybrid junctions is examined for efficient broadband energy transfer between internal reflection guided waves and evanescence-based plasmonic-gap guided waves. We demonstrate a nanoscale orthogonal junction coupler between 50 nm air-filled plasmonic slot waveguides (PSWs) and 450 nm silicon rib waveguides. Non-resonant junction coupling efficiency of 50 ± 2 % between 1450 nm and 1650 nm is achieved experimentally and PSW propagation loss is directly measured to be only 2.5 dB/μm. This taperless hybrid junction reduces PSW-based device footprint and enhances device tolerance to temperature and fabrication process variations, serving as a potential platform for hybrid silicon-plasmonic interconnects.
The structure of ionic aqueous solutions at interfaces: An intrinsic structure analysis
Authors: Bresme Fernando , Chacón Enrique , Tarazona Pedro , Wynveen Aaron
Published: September 2012

We investigate the interfacial structure of ionic solutions consisting of alkali halide ions in water at concentrations in the range 0.2–1.0 molal and at 300 K. Combining molecular dynamics simulations of point charge ion models and a recently introduced computational approach that removes the averaging effect of interfacial capillary waves, we compute the intrinsic structure of the aqueous interface. The interfacial structure is more complex than previously inferred from the analysis of mean profiles. We find a strong alternating double layer structure near the interface, which depends on the cation and anion size. Relatively small changes in the ion diameter disrupt the double layer structure, promoting the adsorption of anions or inducing the density enhancement of small cations with diameters used in simulation studies of lithium solutions. The density enhancement of the small cations is mediated by their strong water solvation shell, with one or more water molecules “anchoring” the ion to the outermost water layer. We find that the intrinsic interfacial electrostatic potential features very strong oscillations with a minimum at the liquid surface that is ∼4 times stronger than the electrostatic potential in the bulk. For the water model employed in this work, SPC/E, the electrostatic potential at the water surface is ∼−2 V, equivalent to ∼80 kBT (for T = 300 K), much stronger than previously considered. Furthermore, we show that the utilization of the intrinsic surface technique provides a route to extract ionic potentials of mean force that are not affected by the thermal fluctuations, which limits the accuracy of most past approaches including the popular umbrella sampling technique.
The inelastic Enskog equation with external force
Authors: Wei Jinbo , Zhang Xianwen
Published: September 2012

In this paper, the Cauchy problem for the inelastic Enskog equation with external force is considered for near vacuum data. Under the assumptions on the bi-characteristic, we proved the global existence of mild solution.
Perpendicular magnetization of Co20Fe50Ge30 films induced by MgO interface
Authors: Ding Manli , Joseph Poon S.
Published: September 2012

Epitaxial growth of Co20Fe50Ge30 films on single crystal MgO (001) substrate is reported. Structure characterization revealed (001)-oriented B2 order of Co20Fe50Ge30, well lattice matched with the MgO barrier. Perpendicular magnetic anisotropy was achieved in the MgO/Co20Fe50Ge30/MgO structure with an optimized magnetic anisotropy energy density of 2 × 106 erg/cm3. The magnetic anisotropy is found to depend strongly on the thickness of the MgO and Co20Fe50Ge30 layers, indicating that the perpendicular magnetic anisotropy of Co20Fe50Ge30 is contributed by the interfacial anisotropy between Co20Fe50Ge30 and MgO. With reported low damping constant, Co20Fe50Ge30 films are promising spintronic materials for achieving low switching current.
Magneto-modulational instability in Kappa distributed plasmas with self-generated magnetic fields
Authors: Liu San-Qiu , Chen Hui , Li XiaoQing
Published: September 2012

The behavior of magnetic fields generated by high-frequency transverse plasmons in Kappa distributed plasmas can be described by generalized Zakharov equations, which have considered the nonlinear wave-wave, wave-particle interactions in the kinetic regime and the effects of superthermal electrons. Modulational instability of the spontaneous magnetic fields is investigated on the basis of the equations. The effect of the superthermal electrons on the modulational instability is discussed and a comparison with previous results is presented. It is shown that the characteristic scale and maximum growth rate of the magnetic fields depend on the energy density of transverse plasmons and the superthermal index κ.
Epitaxial (111) films of Cu, Ni, and CuxNiy on α−Al2O3 (0001) for graphene growth by chemical vapor deposition
Authors: Miller David L. , Keller Mark W. , Shaw Justin M. , Chiaramonti Ann N. , Keller Robert R.
Published: September 2012

Films of (111)-textured Cu, Ni, and CuxNiy were evaluated as substrates for chemical vapor deposition of graphene. A metal thickness of 400 nm to 700 nm was sputtered onto a substrate of α−Al2O3(0001) at temperatures of 250 °C to 650 °C. The films were then annealed at 1000 °C in a tube furnace. X-ray and electron backscatter diffraction measurements showed all films have (111) texture but have grains with in-plane orientations differing by 60°. The in-plane epitaxial relationship for all films was [110]metal||[101¯0]Al2O3. Reactive sputtering of Al in O2 before metal deposition resulted in a single in-plane orientation over 97% of the Ni film but had no significant effect on the Cu grain structure. Transmission electron microscopy showed a clean Ni/Al2O3 interface, confirmed the epitaxial relationship, and showed that formation of the 60° twin grains was associated with features on the Al2O3 surface. Increasing total pressure and Cu vapor pressure during annealing decreased the roughness of Cu and CuxNiy films. Graphene grown on the Ni(111) films was more uniform than that grown on polycrystalline Ni/SiO2 films, but still showed thickness variations on a much smaller length scale than the distance between grains.
Structure and optical band gap of ZnO1−xSx thin films synthesized by chemical spray pyrolysis for application in solar cells
Authors: Thankalekshmi Ratheesh R. , Rastogi A. C.
Published: September 2012

Highly crystalline c-axis oriented and homogeneous ZnO1−xSx films with sulfur composition 0.05 ≤ x ≤ 0.9 without phase separation were deposited using spray pyrolysis of aqueous precursors. A mechanism of film growth is proposed which envisages a slower kinetics of ZnO precursor decomposition and its specific by-products combined with S-precursor decomposition which enables homogeneous ZnO1−xSx phase formation over a wide S-composition range 0.05 ≤ x ≤ 0.9. This is achieved by controlling the substrate temperature to ≤ 300 °C and the spray rate at ∼ 3 ml/min. The ZnO1−xSx films primarily form by S2− substitution at the O2− lattice sites which is confirmed by detailed analysis of the Zn2p, S2p and O1s x-ray photoelectron spectroscopy peaks. With the increasing of S-content, a structural transformation is observed in ZnO1−xSx films; for x < 0.3, the ZnO1−xSx films in the oxygen-rich phase are in ZnO wurtzite crystal structure and for x ≥ 0.44, the ZnO1−xSx films lie in the sulfur-rich phase with β-ZnS structure. At threshold x = 0.3, in the structural transition state, diffraction peaks corresponding to both structural phases are observed. The optical transmission spectra at the band gap transition energy position shift to the red side for 0.05 ≤ x < 0.52 and towards the blue side with the further increase in S-content in the 0.52 < x ≤ 0.9 range. Optical band gap energy determined for ZnO1−xSx films show strong band gap bowing over the entire S-composition range, 0.05 ≤ x ≤ 0.9. The band gap modulation with increased S-content is inconsistent with the use of a single bowing parameter. A bowing parameter of 2.5 eV shows a closer fit for 0.05 ≤ x < 0.52. By accounting for additional effect of the strain energy on the band gap due to addition of excess S-content, a closer fit for the observed variation in the band gap is explained in the 0.52 < x ≤ 0.9 composition range. Electrical resistivity variation with S-content is consistent with the compensating effect of S2− substitution at the O2− sites in ZnO1−xSx films.
Quantum-confined stark effect in localized luminescent centers within InGaN/GaN quantum-well based light emitting diodes
Authors: De Suman , Layek Arunasish , Bhattacharya Sukanya , Kumar Das Dibyendu , Kadir Abdul , Bhattacharya Arnab , Dhar Subhabrata , Chowdhury Arindam
Published: September 2012

The nature of the polarization-field in disorder induced nanoscale potential fluctuations (radiative traps) within (In,Ga)N based quantum-well (QW) heterostructures remains ambiguous. Spectrally resolved photoluminescence microscopy has been utilized to probe the local polarization field by monitoring the extent of quantum-confined Stark effect (QCSE) in radiative trap centers spontaneously formed within an (In,Ga)N QW based light emitting diode. Interestingly, two distinct categories of nanoscale radiative domains, which arise from indium compositional and interface-morphology related fluctuations of the active layers, are found to have very different degree of built-in polarization fields. Screening of QCSE in indium-rich emission centers results in blue-shift of transition energies by up to 400 meV, significantly higher than that reported previously for group III-nitride based semiconductor heterostructures. A lack of correlation between the extent of QCSE and local indium mole-fractions suggests that size, shape, and strain of individual localization centers play a crucial role in modulating the local polarization field.
High field carrier transport in graphene: Insights from fast current transient
Authors: Majumdar Kausik , Kallatt Sangeeth , Bhat Navakanta
Published: September 2012

In this work, we observe gate tunable negative differential conductance (NDC) and current saturation in single layer and bilayer graphene transistor at high source-drain field, which arise due to the interplay among (1) self-heating, (2) hot carrier injection, and (3) drain induced minority carrier injection. The magnitude of the NDC is found to be reduced for a bilayer, in agreement with its weaker carrier-optical phonon coupling and less efficient hot carrier injection. The contributions of different mechanisms to the observed results are decoupled through fast transient measurements with nanosecond resolution. The findings provide insights into high field transport in graphene.
Effect of Mg interlayer on perpendicular magnetic anisotropy of CoFeB films in MgO/Mg/CoFeB/Ta structure
Authors: Ma Q. L. , Iihama S. , Kubota T. , Zhang X. M. , Mizukami S. , Ando Y. , Miyazaki T.
Published: September 2012

The effects of Mg metallic interlayer on the magnetic properties of thin CoFeB films in MgO/Mg (tMg)/CoFeB (1.2 nm)/Ta structures were studied in this letter. Our experimental result shows that the CoFeB film exhibits perpendicular magnetic anisotropy (PMA) when the CoFeB and MgO layers are separated by a metallic Mg layer with a maximum thickness of 0.8 nm. The origin of PMA was discussed by considering the preferential transmission of the Δ1 symmetry preserved by the Mg interlayer in crystallized MgO/Mg/CoFeB/Ta. In addition, the thin Mg interlayer also contributes to enhancing the thermal stability and reducing the effective damping constant and coercivity of the CoFeB film.
Heat flow control in thermo-magnetic convective systems using engineered magnetic fields
Authors: Lee Jaewook , Nomura Tsuyoshi , Dede Ercan M.
Published: September 2012

We present the design of a magnetically controlled convective heat transfer system. The underlying thermo-magnetic instability phenomenon is described, and enhanced convective fluid flow patterns are determined using non-linear programming techniques plus a design sensitivity analysis. Specifically, the magnetic fluid body force is computed by finding the optimal distribution and magnetization direction of a magnetic field source, where the objective is to minimize the maximum temperature of a closed loop heat transfer system. Sizeable fluid recirculation zones are induced by arranging magnetic field generation elements in configurations similar to Halbach arrays. Applications include improved heat flow control for electromechanical systems.
Robust hydrophobic Fe-based amorphous coating by thermal spraying
Authors: Zhang C. , Wu Y. , Liu L.
Published: September 2012

Metallic surface is intrinsically hydrophilic due to its high surface energy. In this work, we present a different picture that highly hydrophobic metallic coatings could be directly fabricated by thermal spraying of Fe-based amorphous powders through the surface roughness control. These hydrophobic coatings are amorphous, exhibiting super-high hardness and excellent corrosion resistance. With low surface energy modification, the coatings become superhydrophobic and exhibit clearly self-cleaning effect. The present work opens a window for the applications of the amorphous coatings.
Dislocation-limited open circuit voltage in film crystal silicon solar cells
Authors: Alberi Kirstin , Branz Howard M. , Guthrey Harvey , Romero Manuel J. , Martin Ina T. , Teplin Charles W. , Stradins Paul , Young David L.
Published: September 2012

Carrier recombination at dislocations is a major source of efficiency loss in epitaxial film Si solar cells and significantly affects the open circuit voltage, VOC. We develop a simple empirical model that yields a logarithmic relationship between VOC and the dislocation density, which fits well to our data. Straightforward evaluation of device performance with this model provides qualitative information about the recombination activity at dislocations.
Large quantity ion beam generation by persistent Coulomb explosion in a near-critical density plasma channel
Authors: Gu Y. J. , Yu Q. , Kong Q. , Zhu Z. , Li X. F. , Chen C. Y. , Kawata S.
Published: September 2012

The mechanism of Coulomb explosion induced by the interactions of ultra-intense laser pulses with near-critical density plasmas was investigated using 2.5D particle-in-cell simulations. While the Coulomb explosion occurred continuously during pulse propagation inside the plasma, a large quantity of charge was generated and acquired in the backward direction. The accelerated ion beam had a peak energy of several tens of MeV, and the maximum energy was over hundreds MeV. A theoretical model has been proposed to estimate the total acquired charge quantity, the maximum ion energy, and their dependence on the initial plasma density.
Effect of spin drift on spin accumulation voltages in highly doped silicon
Authors: Kameno Makoto , Ando Yuichiro , Shikoh Eiji , Shinjo Teruya , Sasaki Tomoyuki , Oikawa Tohru , Suzuki Yoshishige , Suzuki Toshio , Shiraishi Masashi
Published: September 2012

We investigated the effect of spin drift on spin accumulation in highly doped silicon (Si) by using a non-local three-terminal (NL-3T) and four-terminal (NL-4T) methods, and have clarified that the spin accumulation voltages in a NL-3T device were modulated due to spin drift and that spin lifetime can be accurately extracted by employing a modified spin drift-diffusion equation. The extracted spin lifetime is 4-7 ns, which is slightly shorter than the intrinsic spin lifetime (8 ns) measured in the NL-4T method in the same Si device because of spin drift. It is elucidated that the spin drift effect should be considered for the precise estimation of spin lifetime in Si by NL-3T method.
Direct actuation of cantilever in aqueous solutions by electrostatic force using high-frequency electric fields
Authors: Umeda Ken-ichi , Kobayashi Kei , Matsushige Kazumi , Yamada Hirofumi
Published: September 2012

We recently developed a method to directly actuate a cantilever in aqueous solutions by electrostatic force [K.-I. Umeda et al., Appl. Phys. Express 3, 065205 (2010)]. However, the cantilever was actuated by surface stress in a low frequency regime. We solved this problem by applying amplitude-modulated high-frequency electric fields, which actuates the cantilever solely by electrostatic force. The time variations of the self-oscillation frequency of a cantilever and the Au(111) surface images by frequency-modulation atomic force microscopy using acoustic, photothermal, and the improved electrostatic actuation methods were compared, which demonstrates the advantages of the last method such as stability and simplicity in instrumentation.
High-power tunable two-wavelength generation in a two chip co-linear T-cavity vertical external-cavity surface-emitting laser
Authors: Hessenius Chris , Lukowski Michal , Fallahi Mahmoud
Published: September 2012

We report the design and experimental results of a two chip co-linear T-cavity vertical external cavity surface emitting laser (VECSEL) capable of generating two continuously tunable orthogonally polarized lasing wavelengths. A polarizing beamsplitter is used to overlap two VECSEL cavities thus generating high-power intra-cavity two-wavelength generation ideal for type-II nonlinear frequency conversion. Nearly equal power at each wavelength with a combined dual-wavelength CW output power in excess of 13 W was observed. In a high Q cavity, the wavelength separation was tuned from 35 nm to 52 nm by using birefringent filters in regions where each wavelength lases independently of the other.
Driving high-gain shock-ignited inertial confinement fusion targets by green laser light
Authors: Atzeni Stefano , Marocchino Alberto , Schiavi Angelo
Published: September 2012

Standard direct-drive inertial confinement fusion requires UV light irradiation in order to achieve ignition at total laser energy of the order of 1 MJ. The shock-ignition approach opens up the possibility of igniting fusion targets using green light by reducing the implosion velocity and laser-driven ablation pressure. An analytical model is derived, allowing to rescale UV-driven targets to green light. Gain in the range 100–200 is obtained for total laser energy in the range 1.5–3 MJ. With respect to the original UV design, the rescaled targets are less sensitive to irradiation asymmetries and hydrodynamic instabilities, while operating in the same laser-plasma interaction regime.
Relationship between structure, magnetism, and thermoelectricity in CePd3Mx alloys
Authors: Boona Stephen R. , Morelli Donald T.
Published: September 2012

A series of CePd3Mx compounds (M = Na, Mg, Ca, B, Al, Ga, Si, Ge, S, Te, In, and Sn) has been synthesized and their structural, magnetic, and thermoelectric properties measured. The formation of an ordered CePd3M0.125 phase appears to be a gradual transition for elements from rows 2-4 of the periodic table while a miscibility gap appears to exist in the pseudo binary CePd3-CePd3M0.125 phase diagram for M elements from row 5. We report for the first time the existence of alloys with M = Te and S synthesized through a novel precursor approach, with the S-filled compounds showing an unusual contraction of the unit cell. The average valence of the Ce ions in CePd3Mx appears to be strongly correlated with both the electron count and filling fraction of the M elements, while the thermal conductivity reduction achieved through partial filling appears to saturate for M elements with masses above ∼25 amu and at filling levels above ∼5%. The combination of these effects results in a modest enhancement in the thermoelectric figure of merit from 0.07 to 0.11 at 100 K for samples filled with Mg and Ca at a nominal filling level of x = 0.02. Further modest increases may be possible using this approach if improved synthesis procedures can be developed.
Buckling instability of carbon nanoscrolls
Authors: Zhang Zhao , Huang Yinjun , Li Teng
Published: September 2012

Formed by rolling up a monolayer graphene into a spiral structure, a carbon nanoscroll (CNS) is topologically open and has two free edges along its axial direction, distinct from a multi-walled carbon nanotube (MWCNT). Through systematic molecular mechanics simulations, we show that the unique structure of a CNS produces distinct features of its buckling instability under axial compression, twisting, and bending from those of a MWCNT. The results should be instrumental in future structural design of CNS-based applications. As an example, we demonstrate molecular mass transport through a CNS enabled by its torsional buckling instability. The understanding of reversible buckling instability of CNSs could potentially enable the design of novel nano-devices.
Numerical study on coupling effects among multiple Savonius turbines
Authors: Sun Xiaojing , Luo Daihai , Huang Diangui , Wu Guoqing
Published: September 2012

A Savonius rotor can be used as a wind or water current energy conversion device that produces electricity. In spite of their simple structure and assembly, Savonius turbines have less commercial appeal than other types of turbines due to their relatively low energy conversion efficiency. In order to increase the output power of a Savonius turbine, most studies have only focused on optimization of the rotor configuration or installation of ancillary equipment around the rotor. However, previous research has found that a beneficial interaction that existed between two parallel Savonius turbines can also augment the power output of each rotor if they are rotating side by side. This paper numerically examines the interactions among multiple Savonius turbines with the help of the commercial computational fluid dynamics software fluent and finds that these coupling effects can effectively increase the overall power output of a Savonius turbine farm, especially when Savonius turbines are arranged relatively close together. Numerical results also indicate that the separation distance and relative phase angle between the two adjacent Savonius rotors can greatly influence this positive interaction between them.
Improvement of the oxidative stability of used-oil biodiesel by epoxidation reaction
Authors: Kongyai Chainarong , Hunsom Mali
Published: September 2012

Improvement in the oxidative stability (OS) of biodiesel derived from used-oil (UO) was carried out by epoxidation of biodiesel. A 2k factorial design was used to identify the effects of temperature, mole ratio of hydrogen peroxide (H2O2):formic acid (HCOOH), stirring rate, and reaction time. The results indicated that the H2O2:HCOOH mole ratio, stirring rate, and their interaction had significant effect on the OS of biodiesel. At the derived optimum condition, greater than 99.4% of unsaturated carbon bonds in the used-oil biodiesel were converted to epoxy used-oil biodiesel with a selectivity of 69.0%. Only one species of by-product, hydroxyl-biodiesel, was observed in this system. Asides the cold flow properties and methyl ester content, all other key properties of biodiesel were within the specifications of the European Standard EN14214. The OS for the biodiesel was higher than the original.
Magnetically doped semiconducting topological insulators
Authors: Kou X. F. , Jiang W. J. , Lang M. R. , Xiu F. X. , He L. , Wang Y. , Wang Y. , Yu X. X. , Fedorov A. V. , Zhang P. , Wang K. L.
Published: September 2012

The time invariant behaviors of topological insulators are expected to be changed with magnetic doping, which motivate the present study. Here, we show that for Bi2−xCrxSe3 (0.01 ≤ x ≤ 0.3) thin films grown on Si, the non-trivial topological surface state is weakened by the Cr dopants. The band gap of surface is opened and monotonically increased with Cr concentration up to ∼100 meV at 10 K. Meanwhile, the semiconducting behavior is well-maintained in the bulk owing to the reduction of background doping by means of a modified growth strategy and an in situ passivation method. Besides, we also observe the existence of unconventional ferromagnetic ordering below 35 K, for which the Curie-Weiss Law and conventional/modified Arrott equations do not apply. These observations may further help us investigate extraordinary magneto-electric effect in topological insulators, and the result will also pave the way for realizing the quantized anomalous Hall effect.
The Fourier analysis of magnetic force microscopy imaging
Authors: Wu Dongping , Lou Yuanfu , Zheng Fu , Liu Liwang , Wei Dan , Wei Fulin
Published: September 2012

The stray field near the magnetic thin film sample surface has been calculated from the measured signal of the magnetic force microscopy (MFM) image by the method of discrete Fourier transform. It is found that the domain structure of the film cannot uniquely be determined by the MFM image because the MFM image just reflects the magnetic charges distribution at sample surface. In fact, the magnetic charges at sample surface have two sources: magnetic domain or domain wall. For the same MFM image, the different sources of the magnetic charges will result in very different domain structure. A proper way to determine the domain structure is to know the source of the magnetic charges and use the different components of the stray field to redraw the image.
Phase transformation and nanograin refinement of silicon by processing through high-pressure torsion
Authors: Ikoma Yoshifumi , Hayano Kazunori , Edalati Kaveh , Saito Katsuhiko , Guo Qixin , Horita Zenji
Published: September 2012

Si(100) wafers were subjected to severe plastic deformation under a pressure of 24 GPa using high-pressure torsion (HPT). Si wafers were plastically deformed at room temperature. HPT-processed samples were composed of metastable body centered cubic Si-III and rhombohedral Si-XII phases in the initial cubic diamond Si-I. The volume fraction of metastable phases increased with increasing plastic strain. Successive annealing at 873 K led to the reverse transformation of metastable phases. A broad photoluminescence peak centered at about 650 nm appears due to the reverse transformation of Si-III/Si-XII nanograins and the reduction of number of defects in Si-I nanograins.
Ambipolar charge transport in polymer:fullerene bulk heterojunctions for different polymer side-chains
Authors: Fall S. , Biniek L. , Leclerc N. , Lévêque P. , Heiser T.
Published: September 2012

We use field-effect transistors to investigate electron and hole mobilities in polymer:fullerene blends. Low-band-gap polymers with a common conjugated backbone and differing side-chains are utilized in order to clarify the link between the side-chain molecular structure and grafting position, and the power-conversion efficiency of related bulk heterojunction solar cells. The results show that, at a fixed polymer:fullerene weight ratio, the electron mobility increases by more than four orders of magnitude when changing from linear to branched side-chains. As a consequence, the photovoltaic performances are highest at low fullerene contents for branched chains while the opposite is true for linear chains.
Exchange bias coupling of Co with ultrathin La2/3Sr1/3MnO3 films
Authors: Shi Y. J. , Zhou Y. , Ding H. F. , Zhang F. M. , Pi L. , Zhang Y. H. , Wu D.
Published: September 2012

Magnetic properties of epitaxially grown ultrathin La2/3Sr1/3MnO3 (LSMO) films down to a thickness of one unit cell (u.c.) have been systematically investigated by studying their magnetic behaviors with Co capping layers. For LSMO thickness below 3 u.c., the Co/LSMO bilayers exhibit strong exchange bias (EB) effects after field cooling, suggesting the existence of antiferromagnetic (AFM) phase at the interfaces in ultrathin LSMO. The presence of exchange bias effect for the bilayer with 1 u.c. thick LSMO further demonstrates that the AFM ordering of the LSMO is C-type AFM ordering structure. For 10 u.c. LSMO, the magnetic properties are clearly not altered by the capping Co film, suggesting that the observed phenomena are caused by the intrinsic properties of LSMO.
Electro-optic polymer/TiO2 multilayer slot waveguide modulators
Authors: Enami Y. , Yuan B. , Tanaka M. , Luo J. , Jen A. K.-Y.
Published: September 2012

We report an all-dielectric electro-optic (EO) polymer/TiO2 multilayer slot waveguide modulator with low optical insertion loss for high-speed operations. The EO polymer is sandwiched between thin TiO2 slot waveguide films to improve mode confinement in the EO polymer. The structure increased the mode confinement in the TiO2 and EO polymer slot layers and reduced the electrode distance between the Au electrodes without introducing optical loss from the metal electrodes. The half-wave voltage of the modulator was 6.5 V for a 5-mm-long electrode at a wavelength of 1550 nm. The half-wave voltage and length product was 3.25 V·cm.
Influence of deep defects on device performance of thin-film polycrystalline silicon solar cells
Authors: Fehr M. , Simon P. , Sontheimer T. , Leendertz C. , Gorka B. , Schnegg A. , Rech B. , Lips K.
Published: September 2012

Employing quantitative electron-paramagnetic resonance analysis and numerical simulations, we investigate the performance of thin-film polycrystalline silicon solar cells as a function of defect density. We find that the open-circuit voltage is correlated to the density of defects, which we assign to coordination defects at grain boundaries and in dislocation cores. Numerical device simulations confirm the observed correlation and indicate that the device performance is limited by deep defects in the absorber bulk. Analyzing the defect density as a function of grain size indicates a high concentration of intra-grain defects. For large grains (>2 μm), we find that intra-grain defects dominate over grain boundary defects and limit the solar cell performance.
Pronounced ductility in CuZrAl ternary bulk metallic glass composites with optimized microstructure through melt adjustment
Authors: Liu Zengqian , Li Ran , Liu Gang , Song Kaikai , Pauly Simon , Zhang Tao , Eckert Jürgen
Published: September 2012

Microstructures and mechanical properties of as-cast Cu47.5Zr47.5Al5 bulk metallic glass composites are optimized by appropriate remelting treatment of master alloys. With increasing remelting time, the alloys exhibit homogenized size and distribution of in situ formed B2 CuZr crystals. Pronounced tensile ductility of ∼13.6% and work-hardening ability are obtained for the composite with optimized microstructure. The effect of remelting treatment is attributed to the suppressed heterogeneous nucleation and growth of the crystalline phase from undercooled liquid, which may originate from the dissolution of oxides and nitrides as well as from the micro-scale homogenization of the melt.
Two-color ghost interference with photon pairs generated in hot atoms
Authors: Ding Dong-Sheng , Zhou Zhi-Yuan , Shi Bao-Sen , Zou Xu-Bo , Guo Guang-Can
Published: September 2012

We report on an experimental observation of a two-photon ghost interference experiment. A distinguishing feature of our experiment is that the photons are generated via a non-degenerated spontaneous four-wave mixing process in a hot atomic ensemble; therefore the photon has narrow bandwidth. Besides, there is a large difference in frequency between two photons in a pair. Our works may be important to achieve more secure, large transmission capacity long-distance quantum communication.
Novel doping for synthesis monodispersed TiO2 grains filled into spindle-like hematite bi-component nanoparticles by ion implantation
Authors: Sun L. L. , Wu W. , Zhang S. F. , Zhou J. , Cai G. X. , Ren F. , Xiao X. H. , Dai Z. G. , Jiang C. Z.
Published: September 2012

The synthesis of multifunctional nanoparticles (NPs) that combine some of the unique physical and chemical characteristics of two or more classes of materials is still a key goal in modern materials chemistry and has attracted substantial interests in recent years. In that context, magnetic monodispersed TiO2 grains filled into spindle-like hematite bi-component NPs were synthesized by ion implantation. The α-Fe2O3/TiO2 bi-component NPs present superparamagnetism at room temperature, and its saturation magnetization was enhanced from 0.53 emu/g (α-Fe2O3 seeds) to 3.55 emu/g. The proposed doping synthesis method allows for fabricating the other bi- or multi-component NPs. The magnetic α-Fe2O3/TiO2 would be acted as “nanoreactors” and provide a promising composite photocatalyst with narrow and wide bandgap semiconductors in the future.
Carbonic acid revisited: Vibrational spectra, energetics and the possibility of detecting an elusive molecule
Authors: Huber Stefan E. , Dalnodar Silvia , Kausch Wolfgang , Kimeswenger Stefan , Probst Michael
Published: September 2012

We calculate harmonic frequencies of the three most abundant carbonic acid conformers. For this, different model chemistries are investigated with respect to their benefits and shortcomings. Based on these results we use perturbation theory to calculate anharmonic corrections at the ωB97XD/aug-cc-pVXZ, X = D, T, Q, level of theory and compare them with recent experimental data and theoretical predictions. A discrete variable representation method is used to predict the large anharmonic contributions to the frequencies of the stretching vibrations in the hydrogen bonds in the carbonic acid dimer. Moreover, we re-investigate the energetics of the formation of the carbonic acid dimer from its constituents water and carbon dioxide using a high-level extrapolation method. We find that the ωB97XD functional performs well in estimating the fundamental frequencies of the carbonic acid conformers. Concerning the reaction energetics, the accuracy of ωB97XD is even comparable to the high-level extrapolation method. We discuss possibilities to detect carbonic acid in various natural environments such as Earth's and Martian atmospheres.
Effect of thermal cycle on the interfacial antiferromagnetic spin configuration and exchange bias in Ni-Mn-Sb alloy
Authors: Wang R. L. , Lee M. K. , Xu L. S. , Sun Z. G. , Marchenkov V. V. , Tien C. , Huang J. C. A. , Yang C. P.
Published: September 2012

Effect of thermal cycle on the interfacial antiferromagnetic (AFM) spin configuration and exchange bias in Ni50Mn36Sb14 alloy has been investigated. The results indicate thermal cycle can induce further martensitic transition from part of arrested FM phase to AFM phase, leading to the reconstruction of interfacial antiferromagnetic spin configuration. The shape of hysteresis loops at 5 K after cooling back can be tuned from a single-shifted loop to a nearly symmetric double-shifted loop gradually accompanied with exchange bias field increasing to peak value and then decreasing. The evolutions can be illustrated intuitively by a simple AFM bidomain model.
Phylogenetics and taxonomy of Ventrops - the largest genus of Afrotropical Rhinophoridae (Diptera)
Authors: Cerretti Pierfilippo , Pape Thomas
Published: September 2012

Three new species of the Afrotropical genus Ventrops Crosskey, 1977 (V. aethiopicus, sp. nov., V. freidbergi, sp. nov. and V. stuckenbergi, sp. nov.) are described, and all seven known species are included in a morphology-based phylogenetic analysis both to provide a rigorous argumentation for the generic affiliation of the newly described species, and to provide a first explicit hypothesis of phylogenetic relationships between the included species. Also, by including exemplar species from several genera of the Rhinophoridae as outgroups, this analysis is the first explicitly phylogenetic definition of the genus Ventrops within a broader systematic context. The seven species of Ventrops are arranged in three species groups (milichioides-group, hannemariae-group and aethiopicus-group).