bluuebunny/crossref_metadata_2025 huggingface dataset:
Electromagnetic momentum and the energy–momentum tensor in a linear medium with magnetic and dielectric properties
Authors: Crenshaw Michael E.
Published: April 2014

In a continuum setting, the energy–momentum tensor embodies the relations between conservation of energy, conservation of linear momentum, and conservation of angular momentum. The well-defined total energy and the well-defined total momentum in a thermodynamically closed system with complete equations of motion are used to construct the total energy–momentum tensor for a stationary simple linear material with both magnetic and dielectric properties illuminated by a quasimonochromatic pulse of light through a gradient-index antireflection coating. The perplexing issues surrounding the Abraham and Minkowski momentums are bypassed by working entirely with conservation principles, the total energy, and the total momentum. We derive electromagnetic continuity equations and equations of motion for the macroscopic fields based on the material four-divergence of the traceless, symmetric total energy–momentum tensor. We identify contradictions between the macroscopic Maxwell equations and the continuum form of the conservation principles. We resolve the contradictions, which are the actual fundamental issues underlying the Abraham–Minkowski controversy, by constructing a unified version of continuum electrodynamics that is based on establishing consistency between the three-dimensional Maxwell equations for macroscopic fields, the electromagnetic continuity equations, the four-divergence of the total energy–momentum tensor, and a four-dimensional tensor formulation of electrodynamics for macroscopic fields in a simple linear medium.
Atom chip apparatus for experiments with ultracold rubidium and potassium gases
Authors: Ivory M. K. , Ziltz A. R. , Fancher C. T. , Pyle A. J. , Sensharma A. , Chase B. , Field J. P. , Garcia A. , Jervis D. , Aubin S.
Published: April 2014

We present a dual chamber atom chip apparatus for generating ultracold 87Rb and 39K atomic gases. The apparatus produces quasi-pure Bose-Einstein condensates of 104 87Rb atoms in an atom chip trap that features a dimple and good optical access. We have also demonstrated production of ultracold 39K and subsequent loading into the chip trap. We describe the details of the dual chamber vacuum system, the cooling lasers, the magnetic trap, the multicoil magnetic transport system, the atom chip, and two optical dipole traps. Due in part to the use of light-induced atom desorption, the laser cooling chamber features a sufficiently good vacuum to also support optical dipole trap-based experiments. The apparatus is well suited for studies of atom-surface forces, quantum pumping and transport experiments, atom interferometry, novel chip-based traps, and studies of one-dimensional many-body systems.
Triplet state photochemistry and the three-state crossing of acetophenone within time-dependent density-functional theory
Authors: Huix-Rotllant Miquel , Ferré Nicolas
Published: April 2014

Even though time-dependent density-functional theory (TDDFT) works generally well for describing excited states energies and properties in the Franck-Condon region, it can dramatically fail in predicting photochemistry, notably when electronic state crossings occur. Here, we assess the ability of TDDFT to describe the photochemistry of an important class of triplet sensitizers, namely, aromatic ketones. We take acetophenone as a test molecule, for which accurate ab initio results exist in the literature. Triplet acetophenone is generated thanks to an exotic three-state crossing involving one singlet and two triplets states (i.e., a simultaneous intersystem crossing and triplet conical intersection), thus being a stringent test for approximate TDDFT. We show that most exchange-correlation functionals can only give a semi-qualitative picture of the overall photochemistry, in which the three-state crossing is rather represented as a triplet conical intersection separated from the intersystem crossing. The best result overall is given by the double hybrid functional mPW2PLYP, which is even able to reproduce quantitatively the three-state crossing region. We rationalize this results by noting that double hybrid functionals include a larger portion of double excitation character to the excited states.
Structural ordering and glass forming of soft spherical particles with harmonic repulsions
Authors: Sun Bin , Sun Zhiwei , Ouyang Wenze , Xu Shenghua
Published: April 2014

We carry out dissipative particle dynamics simulations to investigate the dynamic process of phase transformation in the system with harmonic repulsion particles. Just below the melting point, the system undergoes liquid state, face-centered cubic crystallization, body-centered cubic crystallization, and reentrant melting phase transition upon compression, which is in good agreement with the phase diagram constructed previously via thermodynamic integration. However, when the temperature is decreased sufficiently, the system is trapped into an amorphous and frustrated glass state in the region of intermediate density, where the solid phase and crystal structure should be thermodynamically most stable.
A multiscale variational approach to the kinetics of viscous classical liquids: The coarse-grained mean field approximation
Authors: Sereda Yuriy V. , Ortoleva Peter J.
Published: April 2014

A closed kinetic equation for the single-particle density of a viscous simple liquid is derived using a variational method for the Liouville equation and a coarse-grained mean-field (CGMF) ansatz. The CGMF ansatz is based on the notion that during the characteristic time of deformation a given particle interacts with many others so that it experiences an average interaction. A trial function for the N-particle probability density is constructed using a multiscale perturbation method and the CGMF ansatz is applied to it. The multiscale perturbation scheme is based on the ratio of the average nearest-neighbor atom distance to the total size of the assembly. A constraint on the initial condition is discovered which guarantees that the kinetic equation is mass-conserving and closed in the single-particle density. The kinetic equation has much of the character of the Vlasov equation except that true viscous, and not Landau, damping is accounted for. The theory captures condensation kinetics and takes much of the character of the Gross-Pitaevskii equation in the weak-gradient short-range force limit.
Simulation of high-density water: Its glass transition for various water models
Authors: Jehser Martin , Seidl Markus , Rauer Clemens , Loerting Thomas , Zifferer Gerhard
Published: April 2014

High-density amorphous water is simulated by use of isothermal-isobaric molecular dynamics at a pressure of 0.3 GPa making use of several water models (SPC/E, TIP3P, TIP4P variants, and TIP5P). Heating/cooling cycles are performed in the temperature range 80–280 K and quantities like density, total energy, and mobility are analysed. Raw data as well as the glass transition temperatures Tg observed in our studies depend on the water model used as well as on the treatment of intramolecular bonds and angles. However, a clear-cut evidence for the occurrence of a glass-to-liquid transition is found in all cases. Thus, all models indicate that high-density amorphous ice found experimentally may be a low-temperature proxy of an ultraviscous high-density liquid.
Exploring the vibrational fingerprint of the electronic excitation energy via molecular dynamics
Authors: Deyne Andy Van Yperen-De , De Meyer Thierry , Pauwels Ewald , Ghysels An , De Clerck Karen , Waroquier Michel , Van Speybroeck Veronique , Hemelsoet Karen
Published: April 2014

A Fourier-based method is presented to relate changes of the molecular structure during a molecular dynamics simulation with fluctuations in the electronic excitation energy. The method implies sampling of the ground state potential energy surface. Subsequently, the power spectrum of the velocities is compared with the power spectrum of the excitation energy computed using time-dependent density functional theory. Peaks in both spectra are compared, and motions exhibiting a linear or quadratic behavior can be distinguished. The quadratically active motions are mainly responsible for the changes in the excitation energy and hence cause shifts between the dynamic and static values of the spectral property. Moreover, information about the potential energy surface of various excited states can be obtained. The procedure is illustrated with three case studies. The first electronic excitation is explored in detail and dominant vibrational motions responsible for changes in the excitation energy are identified for ethylene, biphenyl, and hexamethylbenzene. The proposed method is also extended to other low-energy excitations. Finally, the vibrational fingerprint of the excitation energy of a more complex molecule, in particular the azo dye ethyl orange in a water environment, is analyzed.
Study of microscopic origin of segregation for FexCu1−x and CuxCo1−x liquid binary alloys
Authors: Faruk Mir Mehedi , Bhuiyan G. M. , Biswas Amitabh , Hossain Md. Sazzad
Published: April 2014

The segregating properties for FexCu1−x and CuxCo1−x liquid-liquid binary alloys are investigated theoretically. Here, the free energy of mixing is calculated by using the electronic theory of metals within the framework of the perturbative approach. The calculated results such as the critical temperature and the critical concentration agree well with the available experimental data. Most importantly, the present work confirms our previous finding [M. Mehedi Faruk and G. M. Bhuiyan, Physica B 422, 56 (2013)] that the volume dependent part of the energy of mixing is mostly responsible for segregation of metallic alloys.
Reduced quantum dynamics with arbitrary bath spectral densities: Hierarchical equations of motion based on several different bath decomposition schemes
Authors: Liu Hao , Zhu Lili , Bai Shuming , Shi Qiang
Published: April 2014

We investigated applications of the hierarchical equation of motion (HEOM) method to perform high order perturbation calculations of reduced quantum dynamics for a harmonic bath with arbitrary spectral densities. Three different schemes are used to decompose the bath spectral density into analytical forms that are suitable to the HEOM treatment: (1) The multiple Lorentzian mode model that can be obtained by numerically fitting the model spectral density. (2) The combined Debye and oscillatory Debye modes model that can be constructed by fitting the corresponding classical bath correlation function. (3) A new method that uses undamped harmonic oscillator modes explicitly in the HEOM formalism. Methods to extract system-bath correlations were investigated for the above bath decomposition schemes. We also show that HEOM in the undamped harmonic oscillator modes can give detailed information on the partial Wigner transform of the total density operator. Theoretical analysis and numerical simulations of the spin-Boson dynamics and the absorption line shape of molecular dimers show that the HEOM formalism for high order perturbations can serve as an important tool in studying the quantum dissipative dynamics in the intermediate coupling regime.
An analysis of quantum effects on the thermodynamic properties of cryogenic hydrogen using the path integral method
Authors: Nagashima H. , Tsuda S. , Tsuboi N. , Koshi M. , Hayashi K. A. , Tokumasu T.
Published: April 2014

In this paper, we describe the analysis of the thermodynamic properties of cryogenic hydrogen using classical molecular dynamics (MD) and path integral MD (PIMD) method to understand the effects of the quantum nature of hydrogen molecules. We performed constant NVE MD simulations across a wide density–temperature region to establish an equation of state (EOS). Moreover, the quantum effect on the difference of molecular mechanism of pressure–volume–temperature relationship was addressed. The EOS was derived based on the classical mechanism idea only using the MD simulation results. Simulation results were compared with each MD method and experimental data. As a result, it was confirmed that although the EOS on the basis of classical MD cannot reproduce the experimental data of saturation property of hydrogen in the high-density region, the EOS on the basis of PIMD well reproduces those thermodynamic properties of hydrogen. Moreover, it was clarified that taking quantum effects into account makes the repulsion force larger and the potential well shallower. Because of this mechanism, the intermolecular interaction of hydrogen molecules diminishes and the virial pressure increases.
Crossing the dividing surface of transition state theory. I. Underlying symmetries and motion coordination in multidimensional systems
Authors: Lorquet J. C.
Published: April 2014

The objective of the present paper is to show the existence of motion coordination among a bundle of trajectories crossing a saddle point region in the forward direction. For zero total angular momentum, no matter how complicated the anharmonic part of the potential energy function, classical dynamics in the vicinity of a transition state is constrained by symmetry properties. Trajectories that all cross the plane R = R* at time t = 0 (where R* denotes the position of the saddle point) with the same positive translational momentum $P_{R_* }$PR* can be partitioned into two sets, denoted “gerade” and “ungerade,” which coordinate their motions. Both sets have very close average equations of motion. This coordination improves tremendously rapidly as the number of degrees of freedom increases. This property can be traced back to the existence of time-dependent constants of the motion.
Crossing the dividing surface of transition state theory. II. Recrossing times for the atom–diatom interaction
Authors: Lorquet J. C.
Published: April 2014

We consider a triatomic system with zero total angular momentum and demonstrate that, no matter how complicated the anharmonic part of the potential energy function, classical dynamics in the vicinity of a saddle point is constrained by symmetry properties. At short times and at not too high energies, recrossing dynamics is largely determined by elementary local structural parameters and thus can be described in configuration space only. Conditions for recrossing are given in the form of inequalities involving structural parameters only. Explicit expressions for recrossing times, valid for microcanonical ensembles, are shown to obey interesting regularities. In a forward reaction, when the transition state is nonlinear and tight enough, one-fourth of the trajectories are expected to recross the plane R = R* (where R* denotes the position of the saddle point) within a short time. Another fourth of them are expected to have previously recrossed at a short negative time, i.e., close to the saddle point. These trajectories do not contribute to the reaction rate. The reactive trajectories that obey the transition state model are to be found in the remaining half. However, no conclusion can be derived for them, except that if recrossings occur, then they must either take place in the distant future or already have taken place in the remote past, i.e., far away from the saddle point. Trajectories that all cross the plane R = R* at time t = 0, with the same positive translational momentum $P_{R_* }$PR* can be partitioned into two sets, distinguished by the parity of their initial conditions; both sets have the same average equation of motion up to and including terms cubic in time. Coordination is excellent in the vicinity of the saddle point but fades out at long (positive or negative) times, i.e., far away from the transition state.
Comparison of particle trajectories and collision operators for collisional transport in nonaxisymmetric plasmas
Authors: Landreman M. , Smith H. M. , Mollén A. , Helander P.
Published: April 2014

In this work, we examine the validity of several common simplifying assumptions used in numerical neoclassical calculations for nonaxisymmetric plasmas, both by using a new continuum drift-kinetic code and by considering analytic properties of the kinetic equation. First, neoclassical phenomena are computed for the LHD and W7-X stellarators using several versions of the drift-kinetic equation, including the commonly used incompressible-E × B-drift approximation and two other variants, corresponding to different effective particle trajectories. It is found that for electric fields below roughly one third of the resonant value, the different formulations give nearly identical results, demonstrating the incompressible E × B-drift approximation is quite accurate in this regime. However, near the electric field resonance, the models yield substantially different results. We also compare results for various collision operators, including the full linearized Fokker-Planck operator. At low collisionality, the radial transport driven by radial gradients is nearly identical for the different operators; while in other cases, it is found to be important that collisions conserve momentum.
Models for twistable elastic polymers in Brownian dynamics, and their implementation for LAMMPS
Authors: Brackley C. A. , Morozov A. N. , Marenduzzo D.
Published: April 2014

An elastic rod model for semi-flexible polymers is presented. Theory for a continuum rod is reviewed, and it is shown that a popular discretised model used in numerical simulations gives the correct continuum limit. Correlation functions relating to both bending and twisting of the rod are derived for both continuous and discrete cases, and results are compared with numerical simulations. Finally, two possible implementations of the discretised model in the multi-purpose molecular dynamics software package LAMMPS are described.
Interlayer coupling enhancement in graphene/hexagonal boron nitride heterostructures by intercalated defects or vacancies
Authors: Park Sohee , Park Changwon , Kim Gunn
Published: April 2014

Hexagonal boron nitride (hBN), a remarkable material with a two-dimensional atomic crystal structure, has the potential to fabricate heterostructures with unusual properties. We perform first-principles calculations to determine whether intercalated metal atoms and vacancies can mediate interfacial coupling and influence the structural and electronic properties of the graphene/hBN heterostructure. Metal impurity atoms (Li, K, Cr, Mn, Co, and Cu), acting as extrinsic defects between the graphene and hBN sheets, produce n-doped graphene. We also consider intrinsic vacancy defects and find that a boron monovacancy in hBN acts as a magnetic dopant for graphene, whereas a nitrogen monovacancy in hBN serves as a nonmagnetic dopant for graphene. In contrast, the smallest triangular vacancy defects in hBN are unlikely to result in significant changes in the electronic transport of graphene. Our findings reveal that a hBN layer with some vacancies or metal impurities enhances the interlayer coupling in the graphene/hBN heterostructure with respect to charge doping and electron scattering.
Development of turbulence behind the single square grid
Authors: Zhou Yi , Nagata Kouji , Sakai Yasuhiko , Suzuki Hiroki , Ito Yasumasa , Terashima Osamu , Hayase Toshiyuki
Published: April 2014

In this paper, direct numerical simulations are carried out to study single-square grid-generated turbulence at a Reynolds number \documentclass[12pt]{minimal}\begin{document}$Re_{L_0}$\end{document}ReL0 = 20 000 (based on the inlet velocity Uin and the length of grid bar L0). Different from the regular grid and the multiscale/fractal grid, here only single large square grid is placed at the center near the inlet. First, we investigate the evolutions of turbulence characteristics (e.g., mean streamwise velocity, turbulence intensity, Taylor microscale, etc.) along the centerline. The common characteristics possessed by turbulent flows generated by the single square grid and by the fractal square grid are presented. We confirm the hypothesis proposed by Mazellier and Vassilicos [“Turbulence without Richardson-Kolmogorov cascade,” Phys. Fluids 22, 075101 (2010)] that for the fractal square grid, the location of turbulence intensity peak along the centerline is mainly determined by large-scale wake interactions. Current numerical results show that in turbulence generated by the single square grid, wake interactions occur close to the grid and cause extreme/intense events. Then, the spatial development of invariants of the velocity gradient tensor is studied. For example, the (QW, −QS) maps are analyzed to show how turbulence generated by a single square grid obtains large scale vortices along the centerline.
Jet orientation of a collapsing bubble near a solid wall with an attached air bubble
Authors: Goh B. H. T. , Ohl S. W. , Klaseboer E. , Khoo B. C.
Published: April 2014

The interaction between a cavitation bubble and a non-oscillating air bubble attached to a horizontal polyvinyl chloride plate submerged in de-ionized water is investigated using a low-voltage spark-discharge setup. The attached air bubble is approximately hemi-spherical in shape, and its proximity to a spark-induced oscillating bubble (represented by the dimensionless stand-off distance H′) determines whether or not a jet is formed in the oscillating bubble during its collapse. When the oscillating bubble is created close to the plate, it jets towards or away from the plate. The ratio of oscillating bubble oscillation time and the wall-attached bubble oscillation time (T ′) is found to be an important parameter for determining the jet direction. This is validated with numerical simulations using an axial-symmetrical boundary element model. Our study highlights prospects in reducing cavitation damage with a stationary bubble, and in utilizing a cavitation collapse jet by controlling the jet's direction.
Numerical simulation of fluid flow through random packs of cylinders using immersed boundary method
Authors: Matsumura Y. , Jackson T. L.
Published: April 2014

The macroscopic properties of two-dimensional random periodic packs of monomodal and bimodal cylinders are investigated by means of numerical methods. We solve the unsteady, two-dimensional Navier-Stokes equations on a staggered Cartesian grid and use the immersed boundary method to treat internal flow boundaries. A number of verification problems for the numerical method are presented. The effects of porosity, diameter ratio, large-to-total particle ratio, and Reynolds number on the macroscopic permeability are studied. For small Reynolds numbers, we show that the permeability can be correlated to the underlying microstructure by means of a suitably defined statistical descriptor, the mean shortest Delaunay edge. With proper scaling, the results for bimodal cylinders collapse onto the data for monomodal cylinders, which can then be fitted with a universal curve. For larger Reynolds numbers we show that a modified Forchheimer equation can characterize the flow.
Efficient self-consistent quantum transport simulator for quantum devices
Authors: Gao X. , Mamaluy D. , Nielsen E. , Young R. W. , Shirkhorshidian A. , Lilly M. P. , Bishop N. C. , Carroll M. S. , Muller R. P.
Published: April 2014

We present a self-consistent one-dimensional (1D) quantum transport simulator based on the Contact Block Reduction (CBR) method, aiming for very fast and robust transport simulation of 1D quantum devices. Applying the general CBR approach to 1D open systems results in a set of very simple equations that are derived and given in detail for the first time. The charge self-consistency of the coupled CBR-Poisson equations is achieved by using the predictor-corrector iteration scheme with the optional Anderson acceleration. In addition, we introduce a new way to convert an equilibrium electrostatic barrier potential calculated from an external simulator to an effective doping profile, which is then used by the CBR-Poisson code for transport simulation of the barrier under non-zero biases. The code has been applied to simulate the quantum transport in a double barrier structure and across a tunnel barrier in a silicon double quantum dot. Extremely fast self-consistent 1D simulations of the differential conductance across a tunnel barrier in the quantum dot show better qualitative agreement with experiment than non-self-consistent simulations.
Electroplated Fe films prepared from a deep eutectic solvent
Authors: Yanai T. , Shiraishi K. , Shimokawa T. , Watanabe Y. , Ohgai T. , Nakano M. , Suzuki K. , Fukunaga H.
Published: April 2014

Electroplating of Fe films was carried out from choline chloride-ethylene glycol types of DES (Deep Eutectic Solvent). We investigated magnetic properties of the plated Fe films and evaluated the productivity for the electroplating process. Consequently, we found that surface morphology and current efficiency of the plated films were affected by the bath temperature. We obtained the Fe films with relatively smooth surface and high current efficiency in the bath temperature range from 70 to 110 °C. The deposition rate for our process depended on the current density, and we obtained high deposition rate value of approximately 120 μm/h. We also obtained high current efficiency values of approximately 90% in the wide range of plating time. These results indicate that the DES-based bath has industrial advantages for mass-producing Fe films. Therefore, we conclude that the DES-based bath is an attractive plating bath for Fe films.
A quantitative quantum-chemical analysis tool for the distribution of mechanical force in molecules
Authors: Stauch Tim , Dreuw Andreas
Published: April 2014

The promising field of mechanochemistry suffers from a general lack of understanding of the distribution and propagation of force in a stretched molecule, which limits its applicability up to the present day. In this article, we introduce the JEDI (Judgement of Energy DIstribution) analysis, which is the first quantum chemical method that provides a quantitative understanding of the distribution of mechanical stress energy among all degrees of freedom in a molecule. The method is carried out on the basis of static or dynamic calculations under the influence of an external force and makes use of a Hessian matrix in redundant internal coordinates (bond lengths, bond angles, and dihedral angles), so that all relevant degrees of freedom of a molecule are included and mechanochemical processes can be interpreted in a chemically intuitive way. The JEDI method is characterized by its modest computational effort, with the calculation of the Hessian being the rate-determining step, and delivers, except for the harmonic approximation, exact ab initio results. We apply the JEDI analysis to several example molecules in both static quantum chemical calculations and Born-Oppenheimer Molecular Dynamics simulations in which molecules are subject to an external force, thus studying not only the distribution and the propagation of strain in mechanically deformed systems, but also gaining valuable insights into the mechanochemically induced isomerization of trans-3,4-dimethylcyclobutene to trans,trans-2,4-hexadiene. The JEDI analysis can potentially be used in the discussion of sonochemical reactions, molecular motors, mechanophores, and photoswitches as well as in the development of molecular force probes.
A Gaussian graphical model approach to climate networks
Authors: Zerenner Tanja , Friederichs Petra , Lehnertz Klaus , Hense Andreas
Published: April 2014

Distinguishing between direct and indirect connections is essential when interpreting network structures in terms of dynamical interactions and stability. When constructing networks from climate data the nodes are usually defined on a spatial grid. The edges are usually derived from a bivariate dependency measure, such as Pearson correlation coefficients or mutual information. Thus, the edges indistinguishably represent direct and indirect dependencies. Interpreting climate data fields as realizations of Gaussian Random Fields (GRFs), we have constructed networks according to the Gaussian Graphical Model (GGM) approach. In contrast to the widely used method, the edges of GGM networks are based on partial correlations denoting direct dependencies. Furthermore, GRFs can be represented not only on points in space, but also by expansion coefficients of orthogonal basis functions, such as spherical harmonics. This leads to a modified definition of network nodes and edges in spectral space, which is motivated from an atmospheric dynamics perspective. We construct and analyze networks from climate data in grid point space as well as in spectral space, and derive the edges from both Pearson and partial correlations. Network characteristics, such as mean degree, average shortest path length, and clustering coefficient, reveal that the networks posses an ordered and strongly locally interconnected structure rather than small-world properties. Despite this, the network structures differ strongly depending on the construction method. Straightforward approaches to infer networks from climate data while not regarding any physical processes may contain too strong simplifications to describe the dynamics of the climate system appropriately.
Note: Efficient generation of optical sidebands at GHz with a high-power tapered amplifier
Authors: Zappala J. C. , Bailey K. , Lu Z.-T. , O’Connor T. P. , Jiang W.
Published: April 2014

Two methods using a laser-diode tapered amplifier to produce high-power, high-efficiency optical frequency sidebands over a wide tunable frequency range are studied and compared. For a total output of 500 mW at 811 nm, 20% of the power can be placed in each of the first-order sidebands. Functionality and characterization are presented within the sideband frequency region of 0.8–2.3 GHz, and it is shown that both methods can be applied beyond this frequency range. These methods provide a versatile and effective tool for atomic physics experiments.
Analytical compensation of axisymmetric equilibrium fluxes picked up by locked mode detectors in tokamaks
Authors: Ding Y. H. , Wang N. C. , Rao B. , Jin X. S. , Chen Z. P. , Hu Q. M. , Jin H. , Jin W. , Li J. C. , Xie S. J. , Yi B. , Zhuang G. , Pan Y.
Published: April 2014

In the detection of locked modes using saddle loops, the problem of how to remove the axisymmetric equilibrium flux picked up by the loops has still to be solved. The problem becomes more difficult when there are conductive structures located near the saddle loops. In this paper, we present an analytical model based on lumped eddy current circuits and use it to interpret the measured equilibrium flux and the corresponding eddy current fluxes. Using this model, precise compensation for fluxes induced by the horizontal field coils and the toroidal field coils, with relative errors of less than 1%, has been realized for the saddle loops in the Joint Texas Experimental Tokamak. This paper also presents a new method to compensate for the detection of equilibrium flux by the locked mode detector.
Note: Deep ultraviolet Raman spectrograph with the laser excitation line down to 177.3 nm and its application
Authors: Jin Shaoqing , Fan Fengtao , Guo Meiling , Zhang Ying , Feng Zhaochi , Li Can
Published: April 2014

Deep UV Raman spectrograph with the laser excitation line down to 177.3 nm was developed in this laboratory. An ellipsoidal mirror and a dispersed-subtractive triple monochromator were used to collect and disperse Raman light, respectively. The triple monochromator was arranged in a triangular configuration with only six mirrors used. 177.3 nm laser excited Raman spectrum with cut-off wavenumber down to 200 cm−1 and spectral resolution of 8.0 cm−1 can be obtained under the condition of high purity N2 purging. With the C–C σ bond in Teflon selectively excited by the 177.3 nm laser, resonance Raman spectrum of Teflon with good quality was recorded on the home-built instrument and the σ-σ* transition of C–C bond was studied. The result demonstrates that deep UV Raman spectrograph is powerful for studying the systems with electronic transition located in the deep UV region.
Size dependence of magnetization switching and its dispersion of Co/Pt nanodots under the assistance of radio frequency fields
Authors: Furuta Masaki , Okamoto Satoshi , Kikuchi Nobuaki , Kitakami Osamu , Shimatsu Takehito
Published: April 2014

We have studied the dot size dependence of microwave assisted magnetization switching (MAS) on perpendicular magnetic Co/Pt multilayer dot array. The significant microwave assistance effect has been observed over the entire dot size D ranging from 50 nm to 330 nm examined in the present study. The MAS behavior, however, critically depends on D. The excitation frequency dependence of the switching field is well consistent with the spin wave theory, indicating that the magnetization precession in MAS is in accordance with the well defined eigenmodes depending on the dot diameter. The lowest order spin wave is only excited for D ≤ 100 nm, and then the MAS effect is well consistent with that of the single macrospin prediction. On the other hand, higher order spin waves are excited for D > 100 nm, giving rise to the significant enhancement of the MAS effect. The dispersion of MAS effect also depends on D and is significantly reduced for the region of D > 100 nm. This significant reduction of the dispersion is attributed to the essential feature of the MAS effect which is insensitive to the local fluctuation of anisotropy field, such as defect, damaged layer, and so on.
The core structure of dislocations and their relationship to the material γ-surface
Authors: Hunter A. , Zhang R. F. , Beyerlein I. J.
Published: April 2014

Using a density functional theory-phase field dislocation dynamics model, we reveal a strong inverse relationship between the dislocation equilibrium core width and the normalized intrinsic stacking fault energy for nine face centered cubic (fcc) metals, in quantitative agreement with experiments but not with conventional continuum models. In addition, we show that due to an anomalous feature in its γ-surface, platinum has a fundamentally different core structure and a much wider equilibrium core width than expected. Based on ab initio electronic structure calculations, we attribute this anomaly to distinct differences in the directionality of charge transfer in platinum.
Photoconductive response of strained silicon nanowires: A Monte Carlo study
Authors: Shiri Daryoush , Verma Amit , Khader Mahmoud M.
Published: April 2014

Using Ensemble Monte Carlo simulations, the photocurrent in a 500 nm long strained [110] silicon nanowire with diameter of 3.1 nm is investigated. It was observed that a phototransistor based on this nanowire can have responsivities in the order of 21.3 mA/W for an input light wavelength of 532 nm and intensity of 0.25–2.5 kW/cm2. The super-unity slope of 1.2 in photo conductance versus input light intensity suggests that the nanowire has a photoconductive gain and highlights its advantage over germanium nanowires with sub-unity slope (0.77). The generated photocurrents are in the 0.1 nA–1 nA range. Density Functional Theory and Tight Binding methods were used for strain application and band structure calculation, respectively. Both longitudinal acoustic and optical phonons were included in the calculation of the carrier-phonon scattering events, which showed a two-order of magnitude stronger role for longitudinal optical phonons.
TiO2 nanospheres and spiny nanospheres for high conversion efficiency in dye-sensitized solar cells with gel electrolyte
Authors: Wang Juangang , Chen Tie-dan
Published: April 2014

In this paper, the TiO2 nanospheres and spiny nanospheres were synthesized as the photoanode films of dye-sensitized solar cells (DSSCs) with quasi-solid electrolyte for the improvement of solar conversion efficiency. For a given film thickness, the spherical photoanode films have shorter electron transport time and several times slower recombination than those of nanoparticles, indicating that the charge-collection efficiencies of the spherical photoanodes were markedly improved. DSSCs containing the TiO2 spherical photoelectrode exhibited higher short circuit current because of this specific morphology.
Schottky contacts to In2O3
Authors: von Wenckstern H. , Splith D. , Schmidt F. , Grundmann M. , Bierwagen O. , Speck J. S.
Published: April 2014

n-type binary compound semiconductors such as InN, InAs, or In2O3 are especial because the branch-point energy or charge neutrality level lies within the conduction band. Their tendency to form a surface electron accumulation layer prevents the formation of rectifying Schottky contacts. Utilizing a reactive sputtering process in an oxygen-containing atmosphere, we demonstrate Schottky barrier diodes on indium oxide thin films with rectifying properties being sufficient for space charge layer spectroscopy. Conventional non-reactive sputtering resulted in ohmic contacts. We compare the rectification of Pt, Pd, and Au Schottky contacts on In2O3 and discuss temperature-dependent current-voltage characteristics of Pt/In2O3 in detail. The results substantiate the picture of oxygen vacancies being the source of electrons accumulating at the surface, however, the position of the charge neutrality level and/or the prediction of Schottky barrier heights from it are questioned.
Growth and magnetic property of antiperovskite manganese nitride films doped with Cu by molecular beam epitaxy
Authors: Yu Fengmei , Ren Lizhu , Meng Meng , Wang Yunjia , Yang Mei , Wu Shuxiang , Li Shuwei
Published: April 2014

Manganese nitrides thin films on MgO (100) substrates with and without Cu-doping have been fabricated by plasma assisted molecular beam epitaxy. Antiperovskite compounds Mn3.6Cu0.4N have been grown in the case of Cu-doping, and the pure Mn3N2 single crystal has been obtained without Cu-doping. The Mn3.6Cu0.4N exhibits ferrimagnetism, and the magnetization of Mn3.6Cu0.4N increases upon the temperature decreasing from 300 K to 5 K, similar to Mn4N. The exchange bias (EB) effects emerge in the Mn3.6Cu0.4N films. The EB behavior is originated from the interfaces between ferrimagnetic Mn3.6Cu0.4N and antiferromagnetic metal Mn, which is verified to be formed by the data of x-ray photoelectron spectroscopy. The present results not only provide a strategy for producing functional antiperovskite manganese nitrides, but also shed promising light on fabricating the exchange bias part of spintronic devices.
Barrier height enhancement of Ni/GaN Schottky diode using Ru based passivation scheme
Authors: Kumar Ashish , Kumar Mukesh , Kaur Riajeet , Joshi Amish G. , Vinayak Seema , Singh R.
Published: April 2014

Wet chemical passivation of n-GaN surface using Ru based solution has been reported. X-ray photoelectron spectroscopy characterization of the GaN surface revealed removal of surface oxides by the introduction of Ru complex species. Ni/n-GaN Schottky barrier diodes were fabricated on passivated GaN and a remarkable improvement in Schottky barrier height from 0.76 eV to 0.92 eV was observed.
Photoconductivity and photo-detection response of multiferroic bismuth iron oxide
Authors: Anshul Avneesh , Borkar Hitesh , Singh Paritosh , Pal Prabir , Kushvaha Sunil S. , Kumar Ashok
Published: April 2014

We report visible light detection with in-plane BiFeO3 (BFO) thin films grown on pre-patterned inter-digital electrodes. In-plane configured BFO film displayed photocurrents with a 40:1 photo-to-dark-current ratio and improved photo-sensing ability for >15 000 s (4 h) under small bias voltage (42 V). Nearly, sixty percent of the photo-induced charge carriers decay in 1.0 s and follow a double-exponential decay model. At 373 K, the effect of light does not significantly increase the dark current, probably due to reduced mobility. Sub-bandgap weak monochromatic light (1 mw/cm2) shows one fold increase in photo-charge carriers.
Designing one dimensional Co-Ni/Co3O4-NiO core/shell nano-heterostructure electrodes for high-performance pseudocapacitor
Authors: Singh Ashutosh K. , Sarkar Debasish , Gopal Khan Gobinda , Mandal Kalyan
Published: April 2014

A high-performance supercapacitor electrode based on unique 1D Co-Ni/Co3O4-NiO core/shell nano-heterostructures is designed and fabricated. The nano-heterostructures exhibit high specific capacitance (2013 F g−1 at 2.5 A g−1), high energy and power density (23 Wh kg−1 and 5.5 kW kg−1, at the discharge current density of 20.8 A g−1), good capacitance retention and long cyclicality. The remarkable electrochemical property of the large surface area nano-heterostructures is demonstrated based on the effective nano-architectural design of the electrode with the coexistence of the two highly redox active materials at the surface supported by highly conducting metal alloy channel at the core for faster charge transport.
Helium gas bubble trapped in liquid helium in high magnetic field
Authors: Bai H. , Hannahs S. T. , Markiewicz W. D. , Weijers H. W.
Published: April 2014

High magnetic field magnets are used widely in the area of the condensed matter physics, material science, chemistry, geochemistry, and biology at the National High Magnetic Field Laboratory. New high field magnets of state-of-the-art are being pursued and developed at the lab, such as the current developing 32 T, 32 mm bore fully superconducting magnet. Liquid Helium (LHe) is used as the coolant for superconducting magnets or samples tested in a high magnetic field. When the magnetic field reaches a relatively high value the boil-off helium gas bubble generated by heat losses in the cryostat can be trapped in the LHe bath in the region where BzdBz/dz is less than negative 2100 T2/m, instead of floating up to the top of LHe. Then the magnet or sample in the trapped bubble region may lose efficient cooling. In the development of the 32 T magnet, a prototype Yttrium Barium Copper Oxide coil of 6 double pancakes with an inner diameter of 40 mm and an outer diameter of 140 mm was fabricated and tested in a resistive magnet providing a background field of 15 T. The trapped gas bubble was observed in the tests when the prototype coil was ramped up to 7.5 T at a current of 200 A. This letter reports the test results on the trapped gas bubble and the comparison with the analytical results which shows they are in a good agreement.
Comment on “Dimpling in critical current density vs. magnetic field angle in YBa2Cu3O7 films irradiated with 3-MeV gold ions” [J. Appl. Phys. 114, 233911 (2013)]
Authors: Long N. J. , Wimbush S. C.
Published: April 2014

We provide an alternative explanation for the results of Matsui et al. [J. Appl. Phys. 114, 233911 (2013)] by considering the effects of disorder on flux pinning.
Electric field control of multiferroic domain wall motion
Authors: Chen Hong-Bo , Liu Ye-Hua , Li You-Quan
Published: April 2014

The dynamics of a multiferroic domain wall in which an electric field can couple to the magnetization via inhomogeneous magnetoelectric interaction is investigated by the collective-coordinate framework. We show how the electric field is capable of delaying the onset of the Walker breakdown of the domain wall motion, leading to a significant enhancement of the maximum wall velocity. Moreover, we show that in the stationary regime the chirality of the domain wall can be efficiently reversed when the electric field is applied along the direction of the magnetic field. These characteristics suggest that the multiferroic domain wall may provide a new prospective means to design faster and low-power-consumption domain wall devices.
Simulation analysis of GaN microdomes with broadband omnidirectional antireflection for concentrator photovoltaics
Authors: Han Lu , Zhao Hongping
Published: April 2014

Microdome structures are analyzed as surface topology to reduce surface reflection over a broad spectral range and wide light incidence angle for concentrator photovoltaics application. Three dimensional finite difference time domain method was used to accurately calculate the surface reflection and transmission for surface topologies with different feature sizes and aspect ratios. Studies show that the use of GaN microdomes will lead to a significant reduction of the surface reflection over a broad wavelength range and wide incidence angle range. The surface reflection significantly depends on the surface structure feature size and geometrical shape. The design of the GaN microdomes provides flexibility to tune the structure in order to obtain the minimum surface reflection for different designs of concentrator optical systems. The surface reflections of the GaN microdomes are compared with that of the conventional flat surface as well as the one with antireflection coating.
Electrically dependent bandgaps in graphene on hexagonal boron nitride
Authors: Kaplan D. , Recine G. , Swaminathan V.
Published: April 2014

We present first-principles calculations on the bandgap of graphene on a layer of hexagonal boron nitride in three different stacking configurations. Relative stability of the configurations is identified and bandgap tunability is demonstrated through the application of an external, perpendicularly applied electric field. We carefully examine the bandgap's sensitivity to both magnitude of the applied field as well as separation between the graphene and hexagonal boron nitride layers. Features of the band structure are examined and configuration-dependent relationships between the field and bandgap are revealed and elucidated through the atom-projected density of states. These findings suggest the potential for opening and modulating a bandgap in graphene as high as several hundred meV.
Performances of effective medium model in interpreting optical properties of polyvinylcarbazole:ZnSe nanocomposites
Authors: Benchaabane Aida , Ben Hamed Zied , Kouki Fayçal , Abderrahmane Sanhoury Mohamed , Zellama Kacem , Zeinert Andreas , Bouchriha Habib
Published: April 2014

The effective medium model is applied to investigate the optical properties of hybrid nanocomposite layers of Polyvinylcarbazole (PVK) and nanoparticles of Zinc Selenide (ZnSe). Thin films of PVK:ZnSe nanocomposites show a porous microstructure with pore diameters of 500 nm. Numerical calculations led to the determination of optical constants such as the refractive index n, the extinction coefficient k, the dielectric permittivity ε, and absorption coefficient α. Using common theoretical models, we have determined the Cauchy parameters of the refractive index, namely, static εs and lattice ε∞ dielectric constants as well as the plasma frequency ωp, carrier density to effective mass ratio Nme*, and the optical conductivity σoc. We show that the optical band gap energy Eg of the nanocomposite structure decreases slightly upon the increase of the nanoparticles volume fraction and is in good agreement with the Vegard law.
Antisite defects in n-type Bi2(Te,Se)3: Experimental and theoretical studies
Authors: Oh M. W. , Son J. H. , Kim B. S. , Park S. D. , Min B. K. , Lee H. W.
Published: April 2014

Intrinsic defects in Bi2Te3 were analyzed using first-principles density functional theory calculations. The results demonstrated that antisite defects were dominant in both p- and n-type Bi2Te3. To investigate the characteristics of these defects, n-type Bi2(Te,Se)3 compounds were fabricated using powder prepared by high-energy ball milling and their electrical properties were examined. Electrical resistivity and the Seebeck coefficient increased with ball milling time. These changes in both properties were attributed to a reduction in the carrier concentration, which was supported by the Hall coefficient measurement. The antisite defects were found to be responsible for the changes in carrier concentration. Lattice parameter analysis supported the postulate that variations in the concentration of antisite defects were responsible for the changes in carrier concentration. These findings suggest that optimization of carrier concentration should be considered in the high-energy ball milling process, in which reductions in grain size are expected to improve thermoelectric properties.