bluuebunny/crossref_metadata_2025 huggingface dataset:
Neurogenic hyperadrenergic orthostatic hypotension: a newly recognized variant of orthostatic hypotension in older adults with elevated norepinephrine (noradrenaline)
Authors: Mar Philip L. , Shibao Cyndya A. , Garland Emily M. , Black Bonnie K. , Biaggioni Italo , Diedrich André , Paranjape Sachin Y. , Robertson David , Raj Satish R.
Published: February 2015

Patients with neurogenic orthostatic hypotension (OH) typically have impaired sympathetic nervous system tone and therefore low levels of upright plasma norepinephrine (NE) (noradrenaline). We report a subset of patients who clinically have typical neurogenic OH but who paradoxically have elevated upright levels of plasma NE. We retrospectively studied 83 OH patients evaluated at the Vanderbilt Autonomic Dysfunction Center between August 2007 and May 2013. Based on standing NE, patients were dichotomized into a hyperadrenergic OH group [hyperOH: upright NE ≥ 3.55 nmol/l (600 pg/ml), n=19] or a non-hyperadrenergic OH group [nOH: upright NE < 3.55 nmol/l (600 pg/ml), n=64]. Medical history and data from autonomic testing, including the Valsalva manoeuvre (VM), were analysed. HyperOH patients had profound orthostatic falls in blood pressure (BP), but less severe than in nOH [change in SBP (systolic blood pressure): −53±31 mmHg compared with −68±33 mmHg, P=0.050; change in DBP (diastolic blood pressure): −18±23 mmHg compared with −30±17 mmHg, P=0.01]. The expected compensatory increase in standing heart rate (HR) was similarly blunted in both hyperOH and nOH groups [84±15 beats per minute (bpm) compared with 82±14 bpm; P=0.6]. HyperOH patients had less severe sympathetic failure as evidenced by smaller falls in DBP during phase 2 of VM and a shorter VM phase 4 BP recovery time (16.5±8.9 s compared with 31.6±16.6 s; P<0.001) than nOH patients. Neurogenic hyperOH patients have severe neurogenic OH, but have less severe adrenergic dysfunction than nOH patients. Further work is required to understand whether hyperOH patients will progress to nOH or whether this represents a different disorder.
Clinical Swallow Examination (CSE): Can We Talk?
Authors: Riquelme Luis F.
Published: February 2015

The clinical swallow examination (CSE) is widely used by clinicians evaluating patients with dysphagia, yet this method remains controversial among many. The composition and purpose of the CSE are highly variable by region, facility, age group and individual clinician. This article will review literature on the topic and present clinical practice suggestions to further its usefulness and ongoing investigation.
Robust blind carrier frequency synchronisation for direct sequence spread spectrum systems
Authors: Le Nir V. , Scheers B.
Published: February 2015

A robust blind carrier frequency synchronisation technique applicable to direct sequence spread spectrum (DSSS) systems is presented. The proposed method extends a blind timing synchronisation technique based on an autocorrelation matrix by applying open‐loop single‐frequency estimation algorithms to the eigenvector of the time synchronised autocorrelation matrix of the largest eigenvalue. A comparison of different single‐frequency estimation algorithms is given. Simulation results show that some algorithms attain the Cramer–Rao bound (CRB) even at low signal‐to‐noise ratios (SNRs).
Gaussian process inference approximation for indoor pedestrian localisation
Authors: Medvesek J. , Symington A. , Trost A. , Hailes S.
Published: February 2015

Clutter has a complex effect on radio propagation, and limits the effectiveness of deterministic methods in wireless indoor positioning. In contrast, a Gaussian process () can be used to learn the spatially correlated measurement error directly from training samples, and build a representation from which a position can be inferred. A method of exploiting 𝕆ℬ inference to obtain measurement predictions from within a pose graph optimisation framework is presented. However, inference has a run‐time complexity of 𝕆(N3) in the number of training samples N, which precludes it from being called in each optimiser iteration. The novel contributions of this work are a method for building an approximate inference map and an 𝕆(1) bi‐cubic interpolation strategy for sampling this map during optimisation. Using inertial, magnetic, signal strength and time‐of‐flight measurements between four anchors and a single mobile sensor, it is shown empirically that the presented approach leads to decimetre precision indoor pedestrian localisation.
Voltage sag source location technology based on corresponding sequence components
Authors: Zhu Ke , Wang Yixuan , Yin Peng , Ni Jian
Published: February 2015

Determining the relative region where voltage sags originates is useful to identify the responsible party causing the power quality problem and monitor system or equipment operating condition. As different kinds of voltage sag will cause different sequence components to change according to the neutral point operation mode in the distribution system, a modified incremental impedance‐based method for determining voltage sag source (VSS) location is proposed in this study. This study starts with the VSS direction theory based on incremental impedance, and derives location criteria for different types of VSSs according to the neutral point operation mode of the Chinese distribution network. The VSS location strategy is finally accomplished through the analyses of voltage sag's capacity for passing through the service transformer and the impact of system transient on incremental impedance calculation. The method locates various VSSs based on existing PQ measure points and has clear physics concept. Theoretical analysis, simulation and lab experiments have verified its effectiveness.
Low computational burden grid voltage estimation for grid connected voltage source converter‐based power applications
Authors: Gholami‐Khesht Hosein , Monfared Mohammad , Golestan Saeed
Published: February 2015

This study presents a simple and efficient approach for grid voltage estimation based on the converter equations in the stationary reference frame. The proposed technique is adopted for the sensorless operation of the voltage source converter‐based power conditioners. In this study, the effectiveness of the proposed estimation method when it is combined with a proportional‐resonant (PR)‐based current controller is investigated. When the PR controller is combined with the proposed voltage estimation technique, it offers an additional benefit, such that the grid voltage synchronisation is done without using any phase‐locked loop or band‐pass filter. The proposed sensorless control method shows an excellent steady‐state and transient performance. It is proven through extensive simulations and experiments that the grid voltage estimation with a very low computational burden is achieved even when the grid voltage is distorted.
Theoretical modelling of the storage energy envelope of high frequency AC reactive components to predict chaos boundary
Authors: Xu Cui Dong , Cheng Ka Wai Eric , Liu Jun Feng , Ding Kai
Published: February 2015

The storage energy of high frequency reactive component for the switched mode power converter is a new factor to assess the performance. The instability of the switched‐mode converter under the peak current control mode is examined for its storage energy. Bifurcation and chaos of the storage energy in the inductor and capacitor are observed based on the stroboscopic map. The boundary of maximum storage energy is given to be an alternative to study the instability of the system in this study. Zero energy is obtained in stable period 1 and the storage energy with the same value and opposite direction is obtained in period 2. The simulation results of bifurcation and chaos of energy are obtained under different parameters and verified by experiment. The boundary of instability of energy stored in the reactive component under various parameters is formulated and a single equation can be used to indicate the factors. These results can provide a new guideline to design a stable system and provide a new control algorithm for power electronic system.
Double tapped‐inductor boost converter
Authors: Vázquez Nimrod , Medina Fernando , Hernández Claudia , Arau Jaime , Vázquez Esli
Published: February 2015

High boost converters, because of its ability of having high gain level, have become important in applications for energy generation known as renewable sources. High efficiency, few components and low voltage and current stress on switches are desired features in these kinds of converters. Double tapped boost converter with just a single switch is proposed in this study for high gain; two pairs of coupled magnetically coils are charged in parallel and discharged in series with the source, this let us to obtain not only a higher gain with very few components but also an acceptable efficiency. Not only analysis for the topology is shown, but also simulation and experimental results are presented in order to illustrate the system behaviour for an output power of 200 W. The system efficiency is closer to 90% at hard switching.
Low cost wind energy conversion system based on the discontinuous conduction mode three‐phase semi‐controlled rectifier
Authors: Oliveira Eduardo Façanha , Tofoli Fernando Lessa , Antunes Fernando Marcelo , Barreto Luiz Henrique Silva Colado , Praça Paulo Peixoto , Olivaira Demercil de Sousa
Published: February 2015

This study introduces a low‐cost topology used in a wind energy conversion system with efficient power control in order to meet battery charge requirements. The proposed structure is based on a high‐frequency three‐phase semi‐controlled rectifier operating in the discontinuous conduction mode associated with a buck converter. The system comprises three control loops, which limit the current and voltage ratings to the maximum values allowed by the batteries and also regulate the voltage across the intermediate dc link. In addition, a novel maximum power point tracking algorithm that intends to maximise the power extracted from the wind turbine/generator/converter association is proposed. The principle of the proposed technique is based on a modified version of power signal feedback algorithm, where a given power level can be extracted from the wind turbine for each value of the mechanical speed. However, since the optimal power curve is not exactly known and temperature causes considerable variations in the optimal power curve, a method to search the best match is proposed. Experimental results from a 300‐W prototype are presented and discussed.
Optimisation of LCL filter based on closed‐loop total harmonic distortion calculation model of the grid‐connected inverter
Authors: Zheng Xinxin , Xiao Lan , Lei Yun , Wang Zilong
Published: February 2015

A novel optimisation method of LCL filter based on closed‐loop total harmonic distortion (THD) calculation model of the grid‐connected inverter is proposed in this study. If the parameters of the LCL filter are large enough, the current harmonics can be significantly inhibited. However, the size, weight and price of the filter may increase. If the parameters of the LCL filter are too small, the attenuation of the current harmonics may be not enough. In the proposed method, the target of optimisation is to obtain the minimum value of the inductors on the premise of the allowed current harmonics. The harmonics should be analysed and the control‐loop should be taken into consideration. According to this, a closed‐loop THD calculation model is established. Generalised reduced gradient (GRG) method is applied to do the optimisation based on the model. With the optimised parameters of LCL filter, the size, weight and price of the filter can be reduced. The relationship between THD and the parameters of the LCL filter is analysed. The principle of GRG is discussed. Finally an 18 kVA three‐phase grid‐connected inverter with the optimised LCL filter has been established and tested. Simulation and experimental results verify the theoretical analysis.
Efficient time‐varying interference suppression method for synthetic aperture radar imaging based on time‐frequency reconstruction and mask technique
Authors: Li Dong , Liu Hongqing , Yang Lisheng
Published: February 2015

Owing to the vulnerability of synthetic aperture radar (SAR) system to interferences, effective interference suppression has been an urgent task for SAR imaging. In this work, a novel time‐varying narrowband interference and wideband interference suppression method is presented based on time‐frequency reconstruction and mask technique. This method consists of four key stages. First, the authors present an efficient signal reconstruction method (ESRM) based on eigenvalue decomposition from the Wigner distribution. Next, echoes contaminated by the interference are identified in the frequency domain and the jamming components are obtained by using the mask technique in time‐frequency plane. Then, the ESRM is applied to quickly and accurately synthesise interference signals by incorporating the principle of energy minimum without the need of the prior knowledge of the interference. Finally, the interference components are subtracted from the echoes in data domain, and the well‐focused SAR image is obtained by conventional imaging algorithm. Results of simulated and measured data prove the validity of the proposed method.
Robust training samples selection algorithm based on spectral similarity for space–time adaptive processing in heterogeneous interference environments
Authors: Yifeng Wu , Tong Wang , Jianxin Wu , Jia Duan
Published: February 2015

In space‐time adaptive processing (STAP), the required interference covariance matrix is commonly estimated from homogeneous samples. However, the real interference environments are often heterogeneous, which is induced by the fact that the training samples do not share the same interference property with the cell under test (CUT), and thus the interference covariance matrix estimated from the training samples is mismatched with the real one. To improve the performance of STAP in heterogeneous interference environments, this paper proposes a robust training samples selection algorithm which is based on the spectral similarity of the interference. The spectrums of samples are estimated firstly, which are then utilised to calculate the similarity between the interference of the CUT and those of the training samples. Then, the samples whose interference spectrums are similar to that of the CUT are selected as the final training samples, such that the performance of the interference covariance matrix estimated from these samples can be improved significantly. Besides, to avoid the target self‐nulling effect, a simple technique is adopted to exclude the samples which are contaminated by target signal. The proposed method is applied to real radar data, and experimental results demonstrate the effectiveness of the proposed method.
Experimental study of polarisation technique on multi‐FM‐based passive radar
Authors: You Jun , Wan Xianrong , Fu Yan , Fang Gao
Published: February 2015

The receiver and transmitter polarisations (RTPs) have great influence on detection performance in multi‐FM‐based passive radar. As the transmitter polarisation is uncontrollable in passive radar, the purpose of this study is to determine the appropriate receiver polarisation of the surveillance channel. Firstly, the impact of polarisation on radar detection performance is discussed from three aspects. Then, the probability of detection is used as a metric to select the better RTPs. Theoretical results show that, when most of the transmitted signals in the working band are vertically polarised, the horizontally polarised receiver can obtain better detection performance than the vertically polarised one. Finally, theoretical results are confirmed by field experiments.
Density modulation experiment to determine transport coefficients on Joint-TEXT Tokamak
Authors: Chen W. , Zhuang G. , Gao L. , Gentle K. W. , Chen J. , Shi P. , Liu Y. , Li Q. , Wang Z. J.
Published: February 2015

Density modulation experiments have been conducted on Joint-TEXT (J-TEXT) Tokamak Ohmic discharge to investigate particle transport based on a model with constant diffusion plus inward convection. Like the HCN interferometer, the newly developed three-wave polarimeter-interferometer system (POLARIS) is used to measure the perturbed density. The comparison of results between the HCN interferometer and POLARIS is given. The consistent results indicate the validity of the analysis scheme. At lower densities, the typical particle confinement time τp is found to increase with electron density, while it saturates at higher densities.
Improvement of electric and magnetic properties of patterned magnetic tunnel junctions by recovery of damaged layer using oxygen showering post-treatment process
Authors: Jeong J. H. , Endoh T.
Published: February 2015

In order to recover the patterning damage and improve the electric and magnetic properties of the patterned magnetic tunnel junctions (MTJs), the novel post-treatment process using oxygen ions has been studied. Generally, the oxygen is known as an unsuitable gas for the MTJs patterning because it causes degradation of the patterned MTJs by over-oxidation of MgO. By the way, if the damaged layer could be oxidized selectively without over-oxidation of the damage-less area, oxygen can be the most effective gas to recover the patterning damage of the MTJs. In this study, for the selective oxidation, we proposed the non-reactive oxygen treatment scheme called the oxygen showering post-treatment process (OSP) using an ozone diffusion chamber. By the specific OSP conditions, 8 l/min of the flow rate, 250 °C of the temperature, and 30 s of the time, the magneto-resistance (MR) was increased from 103% to 110%, and the switching current was decreased from 41.1 μA to 31.6 μA when compared with reference data at the same resistance level. These results show that the electric and magnetic properties of the patterned MTJs by the OSP treatment have been improved compared to the reference sample. The improvement in electric and magnetic properties by the OSP treatment is assumed because the reference sample already contains slight patterning damages at the edge of the MTJs despite the optimized patterning process, and these damages have been oxidized by the OSP treatment. Moreover, by the OSP treatment, the proportion of the electric short fail was dramatically decreased from 1.51% to 0%, which is a remarkable improvement in terms of a successful commercialization of spintronic devices.
Rényi generalizations of the conditional quantum mutual information
Authors: Berta Mario , Seshadreesan Kaushik P. , Wilde Mark M.
Published: February 2015

The conditional quantum mutual information I(A; B|C) of a tripartite state ρABC is an information quantity which lies at the center of many problems in quantum information theory. Three of its main properties are that it is non-negative for any tripartite state, that it decreases under local operations applied to systems A and B, and that it obeys the duality relation I(A; B|C) = I(A; B|D) for a four-party pure state on systems ABCD. The conditional mutual information also underlies the squashed entanglement, an entanglement measure that satisfies all of the axioms desired for an entanglement measure. As such, it has been an open question to find Rényi generalizations of the conditional mutual information, that would allow for a deeper understanding of the original quantity and find applications beyond the traditional memoryless setting of quantum information theory. The present paper addresses this question, by defining different α-Rényi generalizations Iα(A; B|C) of the conditional mutual information, some of which we can prove converge to the conditional mutual information in the limit α → 1. Furthermore, we prove that many of these generalizations satisfy non-negativity, duality, and monotonicity with respect to local operations on one of the systems A or B (with it being left as an open question to prove that monotonicity holds with respect to local operations on both systems). The quantities defined here should find applications in quantum information theory and perhaps even in other areas of physics, but we leave this for future work. We also state a conjecture regarding the monotonicity of the Rényi conditional mutual informations defined here with respect to the Rényi parameter α. We prove that this conjecture is true in some special cases and when α is in a neighborhood of one.
A versatile apparatus for the fine-tuned synthesis of cluster-based materials
Authors: Fischer A. , Kruk R. , Hahn H.
Published: February 2015

In this paper, a custom-designed experimental setup for the fine-tuned synthesis of various cluster–based materials is presented. Providing custom-designed deposition stages and special sample holders it offers a high degree of control over the sample characteristics such as the cluster size, cluster amounts, and sample homogeneity in combination with high sample purity. The system is capable of producing thin films of pure clusters and various cluster-matrix combinations with cluster sizes ranging from single atoms up to aggregates of several thousand atoms. Two custom-designed deposition stages are available, one utilizes the full cluster beam, yielding micrograms of pure clusters within a few hours and the second one uses mass-separated clusters (mass-resolution between 2% and 10%), yielding nanograms of pure clusters in the same timescale. Furthermore, at the second deposition stage, a variety of matrix materials can be co-deposited at a controlled sample temperature between 153 K and 673 K. In order to prove the capabilities of the apparatus, a series of experiments with Fe clusters embedded in Ag matrices with different volume fractions of clusters were carried out. Energy dispersive X-ray spectroscopy measurements revealed that the amount of deposited clusters as well as the sample homogeneity can be controlled with an outstanding accuracy of 10%. Additional measurements of the magnetic properties indicated the presence of separated clusters for volume fractions of clusters around 2 volume percent (vol. %), while above this concentration (10 vol. %) a partial aggregation of the clusters was observed. It was also shown that the resulting thin films are nearly oxygen free, which ascertains that oxygen sensitive materials can be safely handled in this newly developed apparatus.
Nanoscale pressure sensors realized from suspended graphene membrane devices
Authors: Aguilera-Servin Juan , Miao Tengfei , Bockrath Marc
Published: February 2015

We study the transport properties of graphene layers placed over ∼200 nm triangular holes via attached electrodes under applied pressure. We find that the injected current division between counter electrodes depends on pressure and can be used to realize a nanoscale pressure sensor. Estimating various potential contributions to the resistivity change of the deflected graphene membrane including piezoresistivity, changing gate capacitance, and the valley Hall effect due to the pressure-induced synthetic magnetic field, we find that the valley Hall effect yields the largest expected contribution to the longitudinal resistivity modulation for accessible device parameters. Such devices in the ballistic transport regime may enable the realization of tunable valley polarized electron sources.
Fog spontaneously folds mosquito wings
Authors: Dickerson Andrew K. , Liu Xing , Zhu Ting , Hu David L.
Published: February 2015

The flexibility of insect wings confers aerodynamic benefits, but can also present a hazard if exposed to fog or dew. Fog can cause water to accumulate on wings, bending them into tight taco shapes and rendering them useless for flight. In this combined experimental and theoretical study, we use high-speed video to film the spontaneous folding of isolated mosquito wings due to the evaporation of a water drop. We predict shapes of the deformed wing using two-dimensional elastica theory, considering both surface tension and Laplace pressure. We also recommend fold-resistant geometries for the wings of flapping micro-aerial vehicles. Our work reveals the mechanism of insect wing folding and provides a framework for further study of capillarity-driven folding in both natural and biomimetic systems at small scales.
Investigation of Compton profiles of molecular methane and ethane
Authors: Zhao Xiao-Li , Yang Ke , Xu Long-Quan , Kang Xu , Liu Ya-Wei , Ma Yong-Peng , Yan Shuai , Ni Dong-Dong , Zhu Lin-Fan
Published: February 2015

The Compton profiles of methane and ethane molecules have been determined at an incident photon energy of 20 keV based on the third generation synchrotron radiation, and the statistical accuracy of 0.2% is achieved near pz = 0. The density functional theory with aug-cc-pVTZ basis set was used to calculate the Compton profiles of methane and ethane. The present experimental Compton profiles are in better agreement with the theoretical calculations in the whole pz region than the previous experimental results, which indicates that the present experimental Compton profiles are accurate enough to serve as the benchmark data for methane and ethane molecules.
Verification of a magnetic island in gyro-kinetics by comparison with analytic theory
Authors: Zarzoso D. , Casson F. J. , Hornsby W. A. , Poli E. , Peeters A. G.
Published: February 2015

A rotating magnetic island is imposed in the gyrokinetic code GKW, when finite differences are used for the radial direction, in order to develop the predictions of analytic tearing mode theory and understand its limitations. The implementation is verified against analytics in sheared slab geometry with three numerical tests that are suggested as benchmark cases for every code that imposes a magnetic island. The convergence requirements to properly resolve physics around the island separatrix are investigated. In the slab geometry, at low magnetic shear, binormal flows inside the island can drive Kelvin-Helmholtz instabilities which prevent the formation of the steady state for which the analytic theory is formulated.
Mechanical confinement for tuning ferroelectric response in PMN-PT single crystal
Authors: Patel Satyanarayan , Chauhan Aditya , Vaish Rahul
Published: February 2015

Ferroelectrics form an important class of materials and are employed for a variety of applications. However, specific applications dictate the need of tailored ferroelectric response. This creates a requirement to obtain ferroelectric materials with tunable properties. Generally, chemical modifications or domain engineering are employed to this effect. This study attempts to shed light on the use of compressive pre-stresses for tuning and enhancing the ferroelectric properties. For the purpose, polarization versus electric field hysteresis data for 68Pb(Mn1/3Nb2/3)O3-32PbTiO3 (PMN-PT) single crystals were obtained as a function of uniaxial compressive stresses and operating temperatures. These data were utilized to investigate the effects of mechanical confinement for four individual case studies of electrocaloric effect, electrical energy storage, pyroelectric, and piezoelectric effect. A significant improvement was obtained for all case studies. The adiabatic temperature change was improved by ≈80% (28 MPa, 353 K); energy storage density increased by a factor of five (28 MPa, 353 K); pyroelectric figure of merits improved by an order of magnitude (21 MPa) and the piezoelectric coefficient was tailored (variable stress). The results offer promising insight into the use of directional confinement for improving application specific ferroelectric properties in PMN-PT single crystal.
A zero dead-time multi-particle time and position sensitive detector based on correlation between brightness and amplitude
Authors: Urbain X. , Bech D. , Van Roy J.-P. , Géléoc M. , Weber S. J. , Huetz A. , Picard Y. J.
Published: February 2015

A new multi-particle time and position sensitive detector using only a set of microchannel plates, a waveform digitizer, a phosphor screen, and a CMOS camera is described. The assignment of the timing information, as taken from the microchannel plates by fast digitizing, to the positions, as recorded by the camera, is based on the COrrelation between the BRightness of the phosphor screen spots, defined as their integrated intensity and the Amplitude of the electrical signals (COBRA). Tests performed by observing the dissociation of HeH, the fragmentation of H3 into two or three fragments, and the photo-double-ionization of Xenon atoms are presented, which illustrate the performances of the COBRA detection scheme.
Time-resolved phase-sensitive second harmonic generation spectroscopy
Authors: Nowakowski Paweł J. , Woods David A. , Bain Colin D. , Verlet Jan R. R.
Published: February 2015

A methodology based on time-resolved, phase-sensitive second harmonic generation (SHG) for probing the excited state dynamics of species at interfaces is presented. It is based on an interference measurement between the SHG from the sample and a local oscillator generated at a reference together with a lock-in measurement to remove the large constant offset from the interference. The technique is characterized by measuring the phase and excited state dynamics of the dye malachite green at the water/air interface. The key attributes of the technique are that the observed signal is directly proportional to sample concentration, in contrast to the quadratic dependence from non-phase sensitive SHG, and that the real and imaginary parts of the 2nd order non-linear susceptibility can be determined independently. We show that the method is highly sensitive and can provide high quality excited state dynamics in short data acquisition times.
Higher-derivative mechanics with N=2l-conformal Galilei supersymmetry
Authors: Masterov Ivan
Published: February 2015

The analysis previously developed in [J. Math. Phys. 55 102901 (2014)] is used to construct systems which hold invariant under N=2l-conformal Galilei superalgebra. The models describe two different supersymmetric extensions of a free higher-derivative particle. Their Newton-Hooke counterparts are derived by applying appropriate coordinate transformations.
Fluid simulation and experimental validation of plasma radial uniformity in 60 MHz capacitively coupled nitrogen discharges
Authors: Liang Ying-Shuang , Liu Yong-Xin , Zhang Yu-Ru , Wang You-Nian
Published: February 2015

A two-dimensional self-consistent electrostatic fluid model and the experimental diagnostic method are employed to investigate the plasma radial uniformity in capacitively coupled nitrogen discharges driven at 60 MHz. The effects of the rf power and electrode gap on the spatial profiles of the N2+ ion density and the radial profiles of the ion flux to the lower electrode are demonstrated. It is found in the simulation that with the increase of rf power or the decrease of electrode gap, the electrostatic edge effect becomes remarkable, which gives rise to an increase in the positive ion density at the electrode edge and thus the radial uniformity of plasma becomes worse. Moreover, the radial profiles of the N2+ ion flux to the lower electrode show a similar behavior to that of the ion density. These results are further understood by the calculated axial and radial components of the power deposition, which exhibit pronounced peaks at the electrode edge at high rf power or small electrode gap. In order to validate the simulation results, the radial profiles of the N2+ ion density were measured by a floating double probe. A general qualitative agreement between the experimental and calculated results is achieved.
Effect of the radio frequency discharge on the dust charging process in a weakly collisional and fully ionized plasma
Authors: Motie Iman , Bokaeeyan Mahyar
Published: February 2015

A close analysis of dust charging process in the presence of radio frequency (RF) discharge on low pressure and fully ionized plasma for both weak and strong discharge's electric field is considered. When the electromagnetic waves pass throughout fully ionized plasma, the collision frequency of the plasma is derived. Moreover, the disturbed distribution function of plasma particles in the presence of the RF discharge is obtained. In this article, by using the Krook model, we separate the distribution function in two parts, the Maxwellian part and the perturbed part. The perturbed part of distribution can make an extra current, so-called the accretion rate of electron (or ion) current, towards a dust particle as a function of the average electron-ion collision frequency. It is proven that when the potential of dust grains increases, the accretion rate of electron current experiences an exponential reduction. Furthermore, the accretion rate of electron current for a strong electric field is relatively smaller than that for a weak electric field. The reasons are elaborated.
Simulated annealing applied to two-dimensional low-beta reduced magnetohydrodynamics
Authors: Chikasue Y. , Furukawa M.
Published: February 2015

The simulated annealing (SA) method is applied to two-dimensional (2D) low-beta reduced magnetohydrodynamics (R-MHD). We have successfully obtained stationary states of the system numerically by the SA method with Casimir invariants preserved. Since the 2D low-beta R-MHD has two fields, the relaxation process becomes complex compared to a single field system such as 2D Euler flow. The obtained stationary state can have fine structure. We have found that the fine structure appears because the relaxation processes are different between kinetic energy and magnetic energy.
Ion transport and structural dynamics in homologous ammonium and phosphonium-based room temperature ionic liquids
Authors: Griffin Philip J. , Holt Adam P. , Tsunashima Katsuhiko , Sangoro Joshua R. , Kremer Friedrich , Sokolov Alexei P.
Published: February 2015

Charge transport and structural dynamics in a homologous pair of ammonium and phosphonium based room temperature ionic liquids (ILs) have been characterized over a wide temperature range using broadband dielectric spectroscopy and quasi-elastic light scattering spectroscopy. We have found that the ionic conductivity of the phosphonium based IL is significantly enhanced relative to the ammonium homolog, and this increase is primarily a result of a lower glass transition temperature and higher ion mobility. Additionally, these ILs exhibit pronounced secondary relaxations which are strongly influenced by the atomic identity of the cation charge center. While the secondary relaxation in the phosphonium IL has the expected Arrhenius temperature dependence characteristic of local beta relaxations, the corresponding relaxation process in the ammonium IL was found to exhibit a mildly non-Arrhenius temperature dependence in the measured temperature range—indicative of molecular cooperativity. These differences in both local and long-range molecular dynamics are a direct reflection of the subtly different inter-ionic interactions and mesoscale structures found in these homologous ILs.
Temperature dependence of anisotropic magnetoresistance in antiferromagnetic Sr2IrO4
Authors: Wang C. , Seinige H. , Cao G. , Zhou J.-S. , Goodenough J. B. , Tsoi M.
Published: February 2015

Temperature-dependent magnetotransport properties of the antiferromagnetic semiconductor Sr2IrO4 are investigated with point-contact devices. The point-contact technique allows to probe very small volumes and, therefore, to look for electronic transport on a microscopic scale. Point-contact measurements with single crystals of Sr2IrO4 were intended to see whether the additional local resistance associated with a small contact area between a sharpened Cu tip and the antiferromagnet shows magnetoresistance (MR) such as that seen in bulk crystals. Point-contact measurements at liquid nitrogen temperature revealed large MRs (up to 28%) for modest magnetic fields (250 mT) applied within an IrO2 (ab) plane with angular dependence showing a crossover from four-fold to two-fold symmetry with an increasing magnetic field. Point contact measurement exhibits distinctive anisotropic magnetoresistance (AMR) in comparison to a bulk experiment, imposing intriguing questions about the mechanism of AMR in this material. Temperature-dependent MR measurements show that the MR falls to zero at the Neel temperature, but the temperature dependence of the MR ratio differs qualitatively from that of the resistivity. This AMR study helps to unveil the entanglement between electronic transport and magnetism in Sr2IrO4 while the observed magnetoresistive phenomena can be potentially used to sense the antiferromagnetic order parameter in spintronic applications.
Magnetic ordering in Ho2Fe2Si2C
Authors: Susilo R. A. , Cadogan J. M. , Cobas R. , Hutchison W. D. , Avdeev M. , Campbell S. J.
Published: February 2015

We have used neutron diffraction and 57Fe Mössbauer spectroscopy, complemented by magnetisation and specific heat measurements, to examine the magnetic ordering of Ho2Fe2Si2C. We have established that Ho2Fe2Si2C orders antiferromagnetically below TN = 16(1) K with a magnetic structure involving ordering of the Ho sublattice along the b-axis with a propagation vector k=[0 0 12]. 57Fe Mössbauer spectra collected below TN show no evidence of a magnetic splitting, demonstrating the absence of long range magnetic ordering of the Fe sublattice. A small line broadening is observed in the 57Fe spectra below TN, which is due to a transferred hyperfine field—estimated to be around 0.3 T at 10 K—from the Ho sublattice.
Optical probing of MgZnO/ZnO heterointerface confinement potential energy levels
Authors: Solovyev V. V. , Van'kov A. B. , Kukushkin I. V. , Falson J. , Zhang D. , Maryenko D. , Kozuka Y. , Tsukazaki A. , Smet J. H. , Kawasaki M.
Published: February 2015

Low-temperature photoluminescence and reflectance measurements were employed to study the optical transitions present in two-dimensional electron systems confined at MgxZn1–xO/ZnO heterojunctions. Transitions involving A- and B-holes and electrons from the two lowest subbands formed within the confinement potential are detected. In the studied density range of 2.0–6.5 × 1011 cm−2, the inter-subband splitting is measured and the first excited electron subband is shown to be empty of electrons.
Numerical study on the stabilization of neoclassical tearing modes by electron cyclotron current drive
Authors: Wang Xiaoguang , Zhang Xiaodong , Wu Bin , Zhu Sizheng , Hu Yemin
Published: February 2015

It is well known that electron cyclotron current drive (ECCD) around the o-point of magnetic island along the plasma current direction can stabilize neoclassical tearing modes (NTMs) in tokamak devices. The effects of the radial misalignment between the island and the driven current, the phase misalignment, and the on-duty ratio for modulated current drive on NTM stabilization are studied numerically in this paper. A small radial misalignment is found to significantly decrease the stabilizing effect. When a sufficiently large phase misalignment occurs for the modulated ECCD, the stabilization effect is also reduced a lot. The optimal on-duty ratio of modulated ECCD to stabilize NTMs is found to be in the range of 60%–70%. A larger on-duty ratio than 50% could also mitigate the effect of phase misalignment if it is not too large. There is no benefit from modulation if the phase misalignment is larger than a threshold.
Beltrami–Bernoulli equilibria in plasmas with degenerate electrons
Authors: Berezhiani V. I. , Shatashvili N. L. , Mahajan S. M.
Published: February 2015

A new class of Double Beltrami–Bernoulli equilibria, sustained by electron degeneracy pressure, is investigated. It is shown that due to electron degeneracy, a nontrivial Beltrami–Bernoulli equilibrium state is possible even for a zero temperature plasma. These states are, conceptually, studied to show the existence of new energy transformation pathways converting, for instance, the degeneracy energy into fluid kinetic energy. Such states may be of relevance to compact astrophysical objects like white dwarfs, neutron stars, etc.
Random lasing in Nd3+ doped potassium gadolinium tungstate crystal powder
Authors: Moura André L. , Fewo Serge I. , Carvalho Mariana T. , Kuzmin Andrey N. , Prasad Paras N. , Gomes Anderson S. L. , de Araújo Cid B.
Published: February 2015

Random laser (RL) emission in Nd3+ doped potassium gadolinium tungstate—KGd(WO4)2:Nd3+—crystal powder is demonstrated. The powder was excited at 813 nm in resonance with the Nd3+ transition 4I9/2→4F5/2. RL emission at 1067 nm due to the 4F3/2→4I11/2 transition was observed and characterized. An intensity threshold dependent on the laser spot area and bandwidth narrowing from ≈2.20 nm to ≈0.40 nm were observed and measured. For a beam spot area of 0.4 mm2, a RL threshold of 6.5 mJ/mm2 (90 MW/cm2) was determined. For excitation intensity smaller than the RL threshold, only spontaneous emission from level 4F3/2 with decay time in the tens microsecond range was observed, but for excitation above the RL threshold, significant shortening of excited level lifetime, characteristic of a stimulated process was found. The overall characteristics measured show that KGd(WO4)2:Nd3+ is an efficient material for operation of solid state RLs in the near-infrared.
The microscopic Z-pinch process of current-carrying rarefied deuterium plasma shell
Authors: Ning Cheng , Feng Zhixing , Xue Chuang , Li Baiwen
Published: February 2015

For insight into the microscopic mechanism of Z-pinch dynamic processes, a code of two-dimensional particle-in-cell (PIC) simulation has been developed in cylindrical coordinates. In principle, the Z-pinch of current-carrying rarefied deuterium plasma shell has been simulated by means of this code. Many results related to the microscopic processes of the Z-pinch are obtained. They include the spatio-temporal distributions of electromagnetic field, current density, forces experienced by the ions and electrons, positions and energy distributions of particles, and trailing mass and current. In radial direction, the electric and magnetic forces exerted on the electrons are comparable in magnitude, while the forces exerted on the ions are mainly the electric forces. So in the Z-pinch process, the electrons are first accelerated in Z direction and get higher velocities; then, they are driven inwards to the axis at the same time by the radial magnetic forces (i.e., Lorentz forces) of them. That causes the separations between the electrons and ions because the ion mass is much larger than the electron's, and in turn a strong electrostatic field is produced. The produced electrostatic field attracts the ions to move towards the electrons. When the electrons are driven along the radial direction to arrive at the axis, they shortly move inversely due to the static repellency among them and their tiny mass, while the ions continue to move inertially inwards, and later get into stagnation, and finally scatter outwards. Near the stagnation, the energies of the deuterium ions mostly range from 0.3 to 6 keV, while the electron energies are mostly from 5 to 35 keV. The radial components, which can contribute to the pinched plasma temperature, of the most probable energies of electron and ion at the stagnation are comparable to the Bennett equilibrium temperature (about 1 keV), and also to the highest temperatures of electron and ion obtained in one dimensional radiation magnetohydrodynamic simulation of the plasma shell Z-pinch. The trailing mass is about 20% of the total mass of the shell, and the maximum trailing current is about 7% of the driven current under our trailing definition. Our PIC simulation also demonstrates that the plasma shell first experiences a snow-plow like implosion process, which is relatively stable.
Strain engineering of quantum dots for long wavelength emission: Photoluminescence from self-assembled InAs quantum dots grown on GaAs(001) at wavelengths over 1.55 μm
Authors: Shimomura K. , Kamiya I.
Published: February 2015

Photoluminescence (PL) at wavelengths over 1.55 μm from self-assembled InAs quantum dots (QDs) grown on GaAs(001) is observed at room temperature (RT) and 4 K using a bilayer structure with thin cap. The PL peak has been known to redshift with decreasing cap layer thickness, although accompanying intensity decrease and peak broadening. With our strain-controlled bilayer structure, the PL intensity can be comparable to the ordinary QDs while realizing peak emission wavelength of 1.61 μm at 4 K and 1.73 μm at RT. The key issue lies in the control of strain not only in the QDs but also in the cap layer. By combining with underlying seed QD layer, we realize strain-driven bandgap engineering through control of strain in the QD and cap layers.
Magnetic spin structure and magnetoelectric coupling in BiFeO3-BaTiO3 multilayer
Authors: Lazenka Vera , Lorenz Michael , Modarresi Hiwa , Bisht Manisha , Rüffer Rudolf , Bonholzer Michael , Grundmann Marius , Van Bael Margriet J. , Vantomme André , Temst Kristiaan
Published: February 2015

Magnetic spin structures in epitaxial BiFeO3 single layer and an epitaxial BaTiO3/BiFeO3 multilayer thin film have been studied by means of nuclear resonant scattering of synchrotron radiation. We demonstrate a spin reorientation in the 15 × [BaTiO3/BiFeO3] multilayer compared to the single BiFeO3 thin film. Whereas in the BiFeO3 film, the net magnetic moment m→ lies in the (1–10) plane, identical to the bulk, m→ in the multilayer points to different polar and azimuthal directions. This spin reorientation indicates that strain and interfaces play a significant role in tuning the magnetic spin order. Furthermore, large difference in the magnetic field dependence of the magnetoelectric coefficient observed between the BiFeO3 single layer and multilayer can be associated with this magnetic spin reorientation.
The formation of blobs from a pure interchange process
Authors: Zhu P. , Sovinec C. R. , Hegna C. C.
Published: February 2015

In this work, we focus on examining a pure interchange process in a shear-less slab configuration as a prototype mechanism for blob formation. We employ full magnetohydrodynamic simulations to demonstrate that the blob-like structures can emerge through the nonlinear development of a pure interchange instability originating from a pedestal-like transition region. In the early nonlinear stage, filamentary structures develop and extend in the direction of the effective gravity. The blob-like structures appear when the radially extending filaments break off and disconnect from the core plasma. The morphology and the dynamics of these filaments and blobs vary dramatically with a sensitive dependence on the dissipation mechanisms in the system and the initial perturbation. Despite the complexity in morphology and dynamics, the nature of the entire blob formation process in the shear-less slab configuration remains strictly interchange without involving any change in magnetic topology.
Oxidation dynamics of aluminum nanorods
Authors: Li Ying , Kalia Rajiv K. , Nakano Aiichiro , Vashishta Priya
Published: February 2015

Aluminum nanorods (Al-NRs) are promising fuels for pyrotechnics due to the high contact areas with oxidizers, but their oxidation mechanisms are largely unknown. Here, reactive molecular dynamics simulations are performed to study thermally initiated burning of oxide-coated Al-NRs with different diameters (D = 26, 36, and 46 nm) in oxygen environment. We found that thinner Al-NRs burn faster due to the larger surface-to-volume ratio. The reaction initiates with the dissolution of the alumina shell into the molten Al core to generate heat. This is followed by the incorporation of environmental oxygen atoms into the resulting Al-rich shell, thereby accelerating the heat release. These results reveal an unexpectedly active role of the alumina shell as a “nanoreactor” for oxidation.