bluuebunny/crossref_metadata_2025 huggingface dataset:
TWINS energetic neutral atom observations of local‐time‐dependent ring current anisotropy
Authors: Goldstein J. , Valek P. , McComas D. J. , Redfern J.
Published: October 2012

Stereo energetic neutral atom (ENA) images from Two Wide‐angle Imaging Neutral‐atom Spectrometers (TWINS) are analyzed using a simple line‐of‐sight (LOS) mapping approximation, for two case studies: 6 April 2010 and 29 May 2010. A previous study first reported a global dawn‐dusk asymmetry favoring anisotropy on the duskside, observed by TWINS on 6 April. This paper extends and refines the previous analysis, and applies it to ENA data from 29 May, to find a second event containing the dawn‐dusk anisotropy asymmetry. Ion pitch angle distributions are retrieved using analytical expressions for anisotropy indices and their associated errors. The analysis is limited to intervals for which TWINS 1 and 2 have similar viewing geometries; i.e., the two spacecraft are at similar magnetic local time (MLT) but different latitudes, to sample two different but overlapping ranges of pitch angle. Application of the method is restricted to portions of the image for which the length of intersection of the LOS with the ENA source distribution is small. For each event, deduced pitch angle anisotropy is higher on the duskside than on the dawnside, a global asymmetry that persists for the hour‐long interval of favorable viewing geometry. The anisotropy generally decreases with energy from 1–30 keV. For 6 April the duskside anisotropy spectrum has a local peak at 12 keV, but exhibits an overall decrease over the measured energy range. For 29 May the duskside anisotropy falls monotonically with energy. Finding this previously unreported system‐level ring current attribute in two different events demonstrates that the global anisotropy is not a one‐time, unique occurrence, and also raises the question of how often and why this new phenomenon occurs. Further work is needed to determine the cause of the dawn‐dusk asymmetry in ion anisotropy. It is well known that multiple mechanisms, such as self‐consistent adiabatic ion drift, Coulomb collisions and charge exchange, influence the global ion pitch angle distribution. However, the degree to which these multiple mechanisms contribute specifically to the observed dawn‐dusk asymmetry will likely require direct comparison between the TWINS observations and simulations of these two events, including self‐consistent fields and realistic densities for the plasmasphere and neutral exosphere.
An application of the finite length Cowling channel model to auroral arcs with longitudinal variations
Authors: Fujii R. , Amm O. , Vanhamäki H. , Yoshikawa A. , Ieda A.
Published: October 2012

A physical process for the latitudinal motion of an auroral arc based on the four‐side bound Cowling channel model is proposed. Assuming that an upward field‐aligned current (FAC) is associated with the auroral arc that forms a Cowling channel with finite lengths not only latitudinally but also longitudinally and that the upward FAC region is primarily embedded in a purely northward electric field, the primary Hall current driven by the northward electric field accumulates positive excess charges at the eastern edge of the channel and negative charges at the western edge for a perfect or partial Cowling channel with a nonzero Cowling efficiency. The charges produce a westward secondary electric field, indicating that a westward electric field can thus be produced by a purely northward primary electric field. This secondary electric field moves the arc with its magnetospheric source drifting together with the magnetospheric plasmas equatorward and simultaneously produces the electric field outside the channel that moves the downward FAC equatorward of the upward FAC region equatorward together with the upward FAC. Thus, the whole 3‐D current system is expected to move equatorward as often observed in the afternoon auroral zone.
Reflection height of daytime tweek atmospherics during the solar eclipse of 22 July 2009
Authors: Ohya Hiroyo , Tsuchiya Fuminori , Nakata Hiroyuki , Shiokawa Kazuo , Miyoshi Yoshizumi , Yamashita Kozo , Takahashi Yukihiro
Published: October 2012

We report multipoint observations of daytime tweek atmospherics during the solar eclipse of 22 July 2009. Sixteen and sixty‐three tweek atmospherics were observed at Moshiri and Kagoshima, Japan, where the magnitudes of the solar eclipse were 0.458 and 0.966, respectively. This was the first observation of tweek atmospherics during a low‐magnitude eclipse (0.458). The average and standard deviation of the reflection height were 94.9 ± 13.7 km at Moshiri and 87.2 ± 12.9 km at Kagoshima. The reflection height at Moshiri was almost the same as that for normal nighttime conditions in July (96.7 ± 12.6 km) in spite of the low magnitude of the eclipse. The reflection height at Kagoshima seems be divided into two parts: propagation across the total solar eclipse path and propagation in the partial solar eclipse path. During the eclipse, we also observed the phase variation in the LF transmitter signals. The average change in the phase delay of the LF signals was 109° for the paths that crossed the eclipse path and 27° for the paths that did not cross the eclipse path. Assuming a normal daytime height for LF waves of 65 km, a ray tracing analysis indicates that the variations in phase correspond to a height increase of 5–6 km for the paths across the eclipse and 1–2 km for partial eclipse paths. The wide range of estimated tweek reflection heights at Kagoshima also suggests a difference in electron density in the lower ionosphere between total and partial solar eclipses.
Assessment of the amount of cesium‐137 released into the Pacific Ocean after the Fukushima accident and analysis of its dispersion in Japanese coastal waters
Authors: Estournel C. , Bosc E. , Bocquet M. , Ulses C. , Marsaleix P. , Winiarek V. , Osvath I. , Nguyen C. , Duhaut T. , Lyard F. , Michaud H. , Auclair F.
Published: October 2012

Numerical modeling was used to provide a new estimate of the amount of 137Cs released directly into the ocean from the Fukushima Daiichi nuclear power plant (NPP) after the accident in March 2011 and to gain insights into the physical processes that led to its dispersion in the marine environment during the months following the accident. An inverse method was used to determine the time‐dependent137Cs input responsible for the concentrations observed at the NPP's two liquid discharge outlets. The method was then validated through comparisons of the simulated concentrations with concentrations measured in seawater at different points in the neighborhood of the plant. An underestimation was noticed for stations located 30 km offshore. The resulting bias in the release inventory was estimated. Finally, the maximum 137Cs activity released directly to the ocean was estimated to lie between 5.1 and 5.5 PBq (Peta Becquerel = 1015 Bq) but uncertainties remain on the amount of radionuclides released during the first few days after the accident. This estimate was compared to previous ones and differences were analyzed further. The temporal and spatial variations of the 137Cs concentration present in the coastal waters were shown to be strongly related to the wind intensity and direction. During the first month after the accident, winds blowing toward the south confined the radionuclides directly released into the ocean to a narrow coastal band. Afterwards, frequent northward wind events increased the dispersion over the whole continental shelf, leading to strongly reduced concentrations.
Water mass transformation and the North Atlantic Current in three multicentury climate model simulations
Authors: Langehaug H. R. , Rhines P. B. , Eldevik T. , Mignot J. , Lohmann K.
Published: October 2012

The warm and saline Subtropical Water carried by the North Atlantic Current undergoes substantial transformation on its way to higher latitudes, predominantly from oceanic heat loss to the atmosphere. The geographical distribution of the surface forced water mass transformation is assessed in multicentury climate simulations from three different climate models (BCM, IPSLCM4, and MPI‐M ESM), with a particular focus on the eastern subpolar North Atlantic Ocean. A diagnosis, originally introduced by Walin (1982), estimates the surface water mass transformation from buoyancy forcing. While the depth structure of the Atlantic Meridional Overturning Circulation (AMOC) is similar in all models, their climatological heat and freshwater fluxes are very different. Consistently, the models differ in their mean pathways of the North Atlantic Current, location of upper ocean low salinity waters, as well as in sea ice cover. In the two models with an excessive sea ice extent in the Labrador Sea, most of the water mass transformation in the subpolar region occurs in the eastern part (east of 35°W). The variability of the eastern water mass transformation on decadal time scales is related to the variable warm northward flow into the subpolar region, the upper branch of AMOC, where a strengthened flow leads an intensified transformation. This relationship seems to disappear with a weak connection between the Subtropical and Subpolar gyres.
Sensitivity of biogenic isoprene emissions to past, present, and future environmental conditions and implications for atmospheric chemistry
Authors: Pacifico F. , Folberth G. A. , Jones C. D. , Harrison S. P. , Collins W. J.
Published: October 2012

We have implemented a process‐based isoprene emission model in the HadGEM2 Earth‐system model with coupled atmospheric chemistry in order to examine the feedback between isoprene emission and climate. Isoprene emissions and their impact on atmospheric chemistry and climate are estimated for preindustrial (1860–1869), present‐day (2000–2009), and future (2100–2109) climate conditions. The estimate of 460 TgC/yr for present‐day global total isoprene emission is consistent with previous estimates. Preindustrial isoprene emissions are estimated to be 26% higher than present‐day. Future isoprene emissions using the RCP8.5 scenario are similar to present‐day because increased emissions resulting from climate warming are countered by CO2 inhibition of isoprene emissions. The impact of biogenic isoprene emissions on the global O3 burden and CH4 lifetime is small but locally significant, and the impact of changes in isoprene emissions on atmospheric chemistry depends strongly on the state of climate and chemistry.
Large‐scale environmental variables and transition to deep convection in cloud resolving model simulations: A vector representation
Authors: Hagos Samson , Leung L. Ruby
Published: October 2012

Cloud resolving model simulations and vector analysis are used to develop a quantitative method of assessing regional variations in the relationships between various large‐scale environmental variables and the transition to deep convection. Results of the CRM simulations from three tropical regions are used to cluster environmental conditions under which transition to deep convection does and does not take place. Projections of the large‐scale environmental variables on the difference between these two clusters are used to quantify the roles of these variables in the transition to deep convection. While the transition to deep convection is most sensitive to moisture and vertical velocity perturbations, the details of the profiles of the anomalies vary from region to region. In comparison, the transition to deep convection is found to be much less sensitive to temperature anomalies over all three regions. The vector formulation presented in this study represents a simple general framework for quantifying various aspects of how the transition to deep convection is sensitive to environmental conditions.
Intracalibration of particle detectors on a three‐axis stabilized geostationary platform
Authors: Rowland W. , Weigel R. S.
Published: October 2012

We describe an algorithm for intracalibration of measurements from plasma or energetic particle detectors on a three‐axis stabilized platform. Modeling and forecasting of Earth's radiation belt environment requires data from particle instruments, and these data depend on measurements which have an inherent calibration uncertainty. Pre‐launch calibration is typically performed, but on‐orbit changes in the instrument often necessitate adjustment of calibration parameters to mitigate the effect of these changes on the measurements. On‐orbit calibration practices for particle detectors aboard spin‐stabilized spacecraft are well established. Three‐axis stabilized platforms, however, pose unique challenges even when comparisons are being performed between multiple telescopes measuring the same energy ranges aboard the same satellite. This algorithm identifies time intervals when different telescopes are measuring particles with the same pitch angles. These measurements are used to compute scale factors which can be multiplied by the pre‐launch geometric factor to correct any changes. The approach is first tested using measurements from GOES‐13 MAGED particle detectors over a 5‐month time period in 2010. We find statistically significant variations which are generally on the order of 5% or less. These results do not appear to be dependent on Poisson statistics nor upon whether a dead time correction was performed. When applied to data from a 5‐month interval in 2011, one telescope shows a 10% shift from the 2010 scale factors. This technique has potential for operational use to help maintain relative calibration between multiple telescopes aboard a single satellite. It should also be extensible to inter‐calibration between multiple satellites.
An examination of the sensitivity of the Great Salt Lake to changes in inputs
Authors: Mohammed Ibrahim Nourein , Tarboton David G.
Published: October 2012

The Great Salt Lake is a closed basin lake in which level and volume fluctuate due to differences between inflows and outflows. The only outflow is evaporation, which depends directly on lake area and salinity, both of which depend on lake volume. The lake's level, volume, and area adjust to balance, on average, precipitation and streamflow inflows by evaporation. In this paper, we examine the sensitivity of lake volume changes to precipitation, streamflow, and evaporation and the interactions among these processes and lake area and salinity related to volume. A mass balance model is developed to generate representative realizations of future lake level from climate and streamflow inputs simulated using the k‐nearest‐neighbor method. Climate and salinity are used to estimate evaporation from the lake using a Penman model adjusted for the salinity‐dependent saturation vapor pressure. Our results show that fluctuation in streamflow is the dominant factor in lake level fluctuations, but fluctuations in lake area that modulate evaporation and precipitation directly on the lake are also important. The results also quantify the sensitivity of lake level to changes in streamflow and air temperature inputs. They predict that a 25% decrease in streamflow would reduce lake level by about 66 cm (2.2 feet), while a +4°C air temperature increase would reduce lake level by about 34 cm (1.1 feet) on average. This sensitivity is important in evaluating the impacts of climate change or streamflow change due to increased consumptive water use on the level of the lake.
Quantifying the change in soil moisture modeling uncertainty from remote sensing observations using Bayesian inference techniques
Authors: Harrison Kenneth W. , Kumar Sujay V. , Peters‐Lidard Christa D. , Santanello Joseph A.
Published: October 2012

Operational land surface models (LSMs) compute hydrologic states such as soil moisture that are needed for a range of important applications (e.g., drought, flood, and weather prediction). The uncertainty in LSM parameters is sufficiently great that several researchers have proposed conducting parameter estimation using globally available remote sensing data to identify best fit local parameter sets. However, even with in situ data at fine modeling scales, there can be significant remaining uncertainty in LSM parameters and outputs. Here, using a new uncertainty estimation subsystem of the NASA Land Information System (LIS) (described herein), a Markov chain Monte Carlo (MCMC) technique is applied to conduct Bayesian analysis for the accounting of parameter uncertainties. The Differential Evolution Markov Chain (DE‐MC) MCMC algorithm was applied, for which a new parallel implementation was developed. A case study is examined that builds on previous work in which the Noah LSM was calibrated to passive (L‐band) microwave remote sensing estimates of soil moisture for the Walnut Gulch Experimental Watershed. In keeping with prior related studies, the parameters subjected to the analysis were restricted to the soil hydraulic properties (SHPs). The main goal is to estimate SHPs and soil moisture simulation uncertainty before and after consideration of the remote sensing data. The prior SHP uncertainty is based on the original source of the standard SHP lookup tables for the Noah LSM. Conclusions are drawn regarding the value and viability of Bayesian analysis over alternative approaches (e.g., parameter estimation, lookup tables) and further research needs are identified.
Crystallographic and magnetic identification of secondary phase in orientated Bi5Fe0.5Co0.5Ti3O15 ceramics
Authors: Palizdar Meghdad , Comyn Tim P. , Ward Michael B. , Brown Andrew P. , Harrington John P. , Kulkarni Santosh , Keeney Lynette , Roy Saibal , Pemble Martyn , Whatmore Roger , Quinn Christopher , Kilcoyne Susan H. , Bell Andrew J.
Published: October 2012

The fabrication of highly-oriented polycrystalline ceramics of Bi5Fe0.5Co0.5Ti3O15, prepared via molten salt synthesis and uniaxial pressing of high aspect ratio platelets is reported. Electron backscatter images show a secondary phase within the ceramic which is rich in cobalt and iron. The concentration of the secondary phase obtained from scanning electron microscopy is estimated at less than 2% by volume, below the detection limit of x-ray diffraction (XRD). The samples were characterized by x-ray diffraction, polarization-electric field measurements, superconducting quantum interference device as a function of sample orientation and vibrating sample magnetometry as a function of temperature. It is inferred from the data that the observed ferromagnetic response is dominated by the secondary phase. This work highlights the importance of rigorous materials characterisation in the study of multiferroics as small amounts of secondary phase, below the limit of XRD, can lead to false conclusions.
Electron transport properties of carbon nanotube–graphene contacts
Authors: Cook Brandon G. , French William R. , Varga Kálmán
Published: October 2012

The properties of carbon nanotube-graphene junctions are investigated with first-principles electronic structure and electron transport calculations. Contact properties are found to be key factors in determining the performance of nanotube based electronic devices. In a typical single-walled carbon nanotube-metal junction, there is a p-type Schottky barrier of up to ∼0.4 eV which depends on the nanotube diameter. Calculations of the Schottky barrier height in carbon nanotube-graphene contacts indicate that low barriers of 0.09 eV and 0.04 eV are present in nanotube-graphene contacts ((8,0) and (10,0) nanotubes, respectively). Junctions with a finite contact region are investigated with simulations of the current-voltage characteristics. The results suggest the suitability of the junctions for applications and provide insight to explain recent experimental findings.
Molecular dynamics simulation of rotational relaxation in nitrogen: Implications for rotational collision number models
Authors: Valentini Paolo , Zhang Chonglin , Schwartzentruber Thomas E.
Published: October 2012

We study the rotational relaxation process in nitrogen using all-atom molecular dynamics (MD) simulations and direct simulation Monte Carlo (DSMC). The intermolecular model used in the MD simulations is shown to (i) reproduce very well the shear viscosity of nitrogen over a wide range of temperatures, (ii) predict the near-equilibrium rotational collision number in good agreement with published trajectory calculations done on ab initio potential energy surfaces, and (iii) produce shock wave profiles in excellent accordance with the experimental measurements. We find that the rotational relaxation process is dependent not only on the near-equilibrium temperature (i.e., when systems relax to equilibrium after a small perturbation), but more importantly on both the magnitude and direction of the initial deviation from the equilibrium state. The comparison between MD and DSMC, based on the Borgnakke-Larsen model, for shock waves (both at low and high temperatures) and one-dimensional expansions shows that a judicious choice of a constant Zrot can produce DSMC results which are in relatively good agreement with MD. However, the selection of the rotational collision number is case-specific, depending not only on the temperature range, but more importantly on the type of flow (compression or expansion), with significant limitations for more complex simulations characterized both by expansion and compression zones. Parker's model, parametrized for nitrogen, overpredicts Zrot for temperatures above about 300 K. It is also unable to describe the dependence of the relaxation process on the direction to equilibrium. Finally, we present a demonstrative cell-based formulation of a rotational relaxation model to illustrate how, by including the key physics obtained from the MD data (dependence of the relaxation process on both the rotational and the translational state of the gas), the agreement between MD and DSMC solutions is drastically improved.
Why and how does collective red blood cells motion occur in the blood microcirculation?
Authors: Ghigliotti Giovanni , Selmi Hassib , Asmi Lassaad El , Misbah Chaouqi
Published: October 2012

The behaviour of red blood cells (RBCs), modelled as vesicles, in Poiseuille flow, mimicking the microvasculature, is studied with numerical simulations in two dimensions. RBCs moving in the centre of the Poiseuille flow (as in blood capillaries) are shown to attract each other and form clusters only due to hydrodynamic interactions, provided that their distance at a given time is below a certain critical value. This distance depends on physical parameters, such as the flow strength. Our simulations reveal that clusters are unstable above a threshold value in the number of forming RBCs, beyond which one or few cells escape the pack by a self-regulating mechanism that select the marginally stable size. This size selection depends on the flow strength as well as on the RBC swelling ratio. The results are interpreted via the analysis of the perturbation of the flow field induced by the vesicles and the interplay with bending and tension forces. This sheds a novel light on the process of collective motion of RBCs observed in vivo.
Resonant tunneling through double barrier graphene systems: A comparative study of Klein and non-Klein tunneling structures
Authors: Rodríguez-Vargas I. , Madrigal-Melchor J. , Oubram O.
Published: October 2012

We study the resonant tunneling effects through double barrier graphene systems (DBGSs). We have considered two types of DBGSs in order to take into account or rule out Klein tunneling effects: (1) the well-known and documented electrostatic-barrier structures (EBSs) created by means of electrostatic probes that act perpendicularly to the graphene sheet; and (2) substrate-barrier structures (SBSs) built sitting the graphene layer on alternating substrates, such as SiO2 and SiC, which are capable of non-open and open an energy bandgap on graphene. The transfer matrix approach is used to obtain the transmittance, linear-regime conductance, and electronic structure for different set of parameters such as electron energy, electron incident angle, barrier, and well widths. Particular attention is paid to the asymmetric characteristics of the DBGSs, as well as to the main differences between Klein and non-Klein tunneling structures. We find that: (1) the transmission properties can be modulated readily changing the energy and angle of the incident electrons, the widths of the well and barrier regions; (2) the linear-regime conductance is easily enhancing, diminishing, and shifted changing from symmetric to asymmetric DBGSs configuration overall in the case of non-Klein tunneling structures; (3) the conductance shows an oscillatory behavior as function of the well width, with peaks that are directly related to the opening and opening-closure of bound-state subbands for EBSs and SBSs, respectively. Finally, it is important to mention that electrostatic DBGSs or substrate DBGSs could be more suitable depending on a specific application, and in the case of non-Klein tunneling structures, they seem possible considering the sophistication of the current epitaxial growth techniques and whenever substrates that open an energy bandgap on graphene, without diminishing the carrier's mobility, be experimentally discovered.
Ultra-low power electrically reconfigurable magnetoelectric microwave devices
Authors: Wu Tao , Carman Gregory P.
Published: October 2012

We present an electrical tunable spiral inductor design and simulation results. By integrating (011) oriented single crystal ferroelectric substrate [Pb(Mg1/3Nb2/3)O3](1−x)-[PbTiO3]x (PMN-PT, x ≈ 0.32) with tunable and switchable remanent strain property, the tunability of such magnetoelectric microwave inductor can retain after releasing the electric actuation, i.e., electrically reconfigurable. A series of reconfigurable passive magnetoelectric microwave devices based on this concept are proposed and discussed. Those electrically reconfigurable magnetoelectric microwave devices have the advantage of low power consumption and fast and wide tunability, which can be potentially used for many applications.
Enhancement of the ultraviolet absorption and Raman efficiencies of a few nanometer thick Si-on-insulator
Authors: Poborchii Vladimir , Tada Tetsuya , Morita Yukinori , Migita Shinji , Kanayama Toshihiko , Geshev Pavel I.
Published: October 2012

We show that a 3–7 nm thick Si-on-insulator (SOI) film displays UV absorption and Raman enhancement compared to bulk Si at a proper thickness of the buried oxide layer between the film and Si substrate. Experiment shows ∼fivefold SOI Raman enhancement for 3–4 nm thick films while theory predicts enhancement up to a factor of ∼20 for 2 nm thick SOI. This discrepancy is attributed to non-uniformity of SOI thickness.
Optimal selection and sizing of distributed energy resources for distributed power systems
Authors: Logenthiran Thillainathan , Srinivasan Dipti
Published: October 2012

Optimal selection and sizing of distributed energy resources is an important research problem in the development of distributed power systems. This paper presents a methodology for optimal selection and sizing of distributed energy resources in integrated microgrids using the evolutionary strategy. Integrated microgrid is an innovative architecture in distributed power systems, wherein several microgrids are interconnected with each other for control and management of the distributed power systems. Right coordination between distributed energy resources and proper harmony between microgrids and the main distribution grid are some of the critical challenges in this research problem. Selection of distributed energy resources and their capacities are optimized such that the integrated microgrid provides reliable energy at a cheap cost. In this research, the problem is formulated as a nonlinear mixed-integer minimization problem, which minimizes the sum of capital and annual operational cost of distributed energy resources subject to various system and unit constraints. A novel methodology based on flexible evolutionary strategy was developed in this research and used to design an integrated microgrid for AStar IEDS project. The design results have shown that the proposed methodology provides excellent convergence and feasible optimum solution.
Broadband and ultrathin screen with magnetic substrate for microwave reflectivity reduction
Authors: Huang Ruifeng , Li Zheng-Wen
Published: October 2012

This paper presents an ultrathin screen with magnetic substrate for broadband microwave reflectivity reduction. The magnetic substrate was fabricated by mixing a MnTi substituted M-type barium ferrite and a Co substituted W-type barium ferrite. The screen with the magnetic substrate thickness of 1.1 mm (λ0/25) can achieve the fractional bandwidth (Δf/f0) of 46% at X band, which almost increases by 150%, as compared to the corresponding screen with dielectric substrate.
Efficient Cherenkov-type terahertz generation in Si-prism-LiNbO3-slab structure pumped by nanojoule-level ultrashort laser pulses
Authors: Bakunov M. I. , Mashkovich E. A. , Tsarev M. V. , Gorelov S. D.
Published: October 2012

We demonstrate, both theoretically and experimentally, that a sandwich-type structure consisting of a thin LiNbO3 slab and Si prism outcoupler can be an efficient convertor of unamplified laser pulses into broadband terahertz radiation. Pumping a 1 cm long sandwich structure with a 35 μm thick LiNbO3 slab by 8 nJ, 100 fs optical pulses from Ti:sapphire oscillator we achieved the conversion efficiency of 0.8×10−4, i.e., two orders of magnitude higher than in the conventional generation scheme with ZnTe crystal. Using laser oscillator as a pump has an advantage of high repetition rate and, therefore, potentially high signal-to-noise ratio (∼50 dB in our experiment).
Tuning the direction of exchange bias in ferromagnetic/antiferromagnetic bilayer by angular-dependent spin-polarized current
Authors: Tang XiaoLi , Su Hua , Zhang Huai-Wu , Jing Yu-Lan , Zhong Zhi-Yong
Published: October 2012

The angular dependence of an external magnetic field applied with an in-plane alternating pulse that induces a new direction of exchange bias (EB) is observed in an NiFe/IrMn bilayer. Depending strongly on the direction of the external field, EB field gradually orients along the external field with the increase in current pulse, and the new direction of EB is finally established. Furthermore, the new direction of EB can also be induced along the external field at once when the applied pulse is larger than the critical current. Because the strength and direction of the EB is highly correlated with the micromagnetic state distribution of the antiferromagnet, the observations are explained by the spin torque exerted on the antiferromagnetic moments and provide evidence for the prediction of spin transfer and current-induced switching in antiferromagnets.
Critical current reduction in coated conductors when in-plane fields are applied
Authors: Lécrevisse Th. , Rey J.-M. , Thimont Y. , Bernstein P. , McLoughlin C.
Published: October 2012

We report measurements of the critical current of two commercial coated conductors at low temperatures when in-plane magnetic fields up to 15 T are applied. The comparison between the two samples suggests that the presence of a magnetic substrate may have a detrimental effect, at least at low and medium fields. Nevertheless, the critical current density at 30 K and 15 T is in the range of that measured in self field at 77 K. We ascribe its reduction in presence of the field to inter-vortex forces.
Surface potential analysis of AlN/GaN heterostructures by electrochemical capacitance-voltage measurements
Authors: Pietzka C. , Li G. , Alomari M. , Xing H. , Jena D. , Kohn E.
Published: October 2012

AlN/n+-GaN heterostructure samples with AlN barrier layer thickness between 1 nm and 4 nm have been analyzed by electrochemical capacitance-voltage measurements with a semiconductor-electrolyte contact to estimate the surface potential of this heterostructure. The combination of using a semiconductor-electrolyte interface for characterization and using an n+-doped GaN buffer layer enabled the extraction of the surface potential from the full range of date between the two flatband conditions, flatband in the AlN barrier and flatband in the GaN buffer. Such analysis is otherwise difficult to obtain due to the tunneling restriction. In the present case of an AlN/GaN heterostructure, the analysis leads to a surface potential of ∼1.9 eV, independent of the AlN barrier layer thickness.
The effect of electroactive interlayer on the ferroelectric properties in poly(vinylidene fluoride-trifluoroethylene) copolymer ultrathin films
Authors: Zhang Xiuli , Hou Ying , Zhang Yuan , Lv Zhaoyue , Xu Guoqiang , Xu Haisheng
Published: October 2012

A series of ferroelectric polarization reversal measurements have been conducted on spin-coated ultrathin poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer films sandwiched with different electroactive materials between P(VDF-TrFE) and metal electrodes. Three types of electroactive interlayers, including highly polarizable material copper-phthalocyanine (CuPc), electron conductive polyethylene dioxythiophene/poly(styrene sulfonate acid), and ion (proton) conductive poly (vinyl phosphonic) acid, have been employed in our experiment. In addition, the ferroelectric film with CuPc exhibits larger remnant polarization and the enhancement of fatigue resistance although higher drive voltage and more switching time can be observed, which differs from the samples with electron and proton conductors as interlayers. The further analysis shows that this may be attributed to higher dielectric constant of CuPc than that of P(VDF-TrFE) layer. These results suggest that three types of interlayers can lead to a large improvement of ferroelectric switching, which is promising for potential applications.
Thermoelectric effect in a graphene sheet connected to ferromagnetic leads
Authors: Zhou Benhu , Zhou Benliang , Liu Zongqiao , Zhou Guanghui
Published: October 2012

We theoretically investigate the thermoelectric property of a graphene sheet coupled to two ferromagnetic electrodes with noncollinear magnetic moments. By using the nonequilibrium Green's function combining with the tight-binding Hamiltonian, it is demonstrated that the thermopower weakly depends on the polarization strength as well as on magnetic configuration of the leads. On the contrary, the electronic contribution to the thermal conductance is sensitive to both the polarization strength and the relative alignment of magnetization directions. Furthermore, the thermopower shows a linear dependence on temperature and an inverse dependence on the Fermi energy in the graphene, which agrees well the recent experimental measurements.
Enhanced leakage current performance and conduction mechanisms of Bi1.5Zn1.0Nb1.5O7/Ba0.5Sr0.5TiO3 bilayered thin films
Authors: Li Ruguan , Jiang Shuwen , Gao Libin , Li Yanrong
Published: October 2012

The Bi1.5Zn1.0Nb1.5O7 (BZN)/Ba0.5Sr0.5TiO3 (BST) bilayered films and BST thin films were prepared via radio frequency (RF) magnetron sputtering, and the leakage current behaviors of the films were investigated. Lower leakage currents of BZN/BST bilayered films were achieved compared with that of BST thin films, especially in the high field region. The potential barrier height of top contact is increased by insertion of a BZN layer in between BST and top Pt-electrodes, which might be responsible for the current reduction of BZN/BST under negative biases (positive voltage applied to bottom electrodes). For the case of positive biases, BZN/BST showed a significant increase in the threshold electric field of Poole-Frenkel emission compared with that of BST, resulting in a current decrease in BZN/BST than that in BST. For the BZN/BST bilayered films at 300 K, since the Schottky emission, Fowler-Nordheim tunneling and Poole-Frenkel emission occur in an extremely high electric field, the leakage current is governed by Ohmic conductivity behavior in a wide applied field of up to ∼1 MV/cm.
Technology-compatible hot carrier solar cell with energy selective hot carrier absorber and carrier-selective contacts
Authors: König D. , Takeda Y. , Puthen-Veettil B.
Published: October 2012

We propose a hot carrier solar cell based on epitaxial growth of a quantum well superlattice and adjacent contact barriers. The concept fulfills required electronic, optical, and several phononic criteria. The first superlattice miniband determines the absorption threshold. The second miniband with appropriate energy width and position provides energy selectivity in situ; contacts are optimized for carrier selectivity exclusively. Electronic transport properties were investigated including elastic random electron–electron scattering, random layer thickness deviation, and illumination as differential absorption per quantum well using a Monte-Carlo code. Carrier extraction probability and energy selectivity strongly suggest a practical implementation of the proposed concept.
Analytical description of nonlinear particle transport in slab turbulence: High particle energies and stochastic acceleration
Authors: Shalchi A.
Published: October 2012

Pitch-angle scattering, parallel spatial diffusion, and stochastic acceleration of cosmic rays are investigated analytically. Based on a second-order quasilinear theory, we derive analytical expressions for the aforementioned transport parameters for all possible magnetic field strengths and particle energies. This work complements previous work where only parallel diffusion for low energetic particles was considered. Furthermore, we compute the first time the momentum diffusion coefficient. It is also shown that the relation between the momentum diffusion coefficient and the parallel spatial diffusion coefficient is more complicated than assumed in previous work.
Generation of extreme state of water by spherical wire array underwater electrical explosion
Authors: Antonov O. , Gilburd L. , Efimov S. , Bazalitski G. , Gurovich V. Tz. , Krasik Ya. E.
Published: October 2012

The results of the first experiments on the underwater electrical explosion of a spherical wire array generating a converging strong shock wave are reported. Using a moderate pulse power generator with a stored energy of ≤6 kJ and discharge current of ≤500 kA with a rise-time of ∼300 ns, explosions of Cu and Al wire arrays of different diameters and with a different number and diameter of wires were tested. Electrical, optical, and destruction diagnostics were used to determine the energy deposited into the array, the time-of-flight of the shock wave to the origin of the implosion, and the parameters of water at that location. The experimental and numerical simulation results indicate that the convergence of the shock wave leads to the formation of an extreme state of water in the vicinity of the implosion origin that is characterized by pressure, temperature, and compression factors of (2 ± 0.2) × 1012 Pa, 8 ± 0.5 eV, and 7 ± 0.5, respectively.
Complex transport behavior accompanying domain switching in La0.1Bi0.9FeO3 sandwiched capacitors
Authors: Gao R. L. , Chen Y. S. , Sun J. R. , Zhao Y. G. , Li J. B. , Shen B. G.
Published: October 2012

Polarization-modulated resistive switching and fatigue behaviors of the Ag/La0.1Bi0.9FeO3/La0.7Sr0.3MnO3 capacitors have been investigated. The device resistance is found to show a V-shaped dependence on poling voltage, and the lowest resistance appears at the voltage corresponding to the coercive field of La0.1Bi0.9FeO3. Based on this relation, three distinct resistance states can be achieved by applying appropriate pulse trains, which manifests a potential application in high-density storage technology. The fatigue properties of the sample under repeated bipolar or unipolar pulses were further analyzed. Bipolar pulses enhance the rectifying characters of the current-voltage relation, whereas unipolar pulses produce a reverse effect. Based on impedance analysis, we propose the formation of leakage paths along conductive domain walls, and it is the domain reconstruction during repeated polarization flipping that results in the complex transport behavior observed.
Resonances in ferroelectric phononic superlattice
Authors: Ostrovskii Igor , Cremaldi Lucien
Published: October 2012

The periodically poled ferroelectric wafer is a two-dimensional phononic superlattice. The important applications of such a solid include ultrasonic transducers at the micro/nano-scale for low intensity ultra-sonography, ferroelectric data storage, and development of very high frequency chips for next generation communication and information technologies, and others. In this work, we show theoretically and experimentally that a ferroelectric phononic superlattice has two distinctive resonances in acousto-electric transformation. They are associated with a split acoustic mode at the boundary between the first and second acoustic Brillouin zone.
Gallium nitride-based light-emitting diodes with embedded air voids grown on Ar-implanted AlN/sapphire substrate
Authors: Sheu Jinn-Kong , Tu Shang-Ju , Yeh Yu-Hsiang , Lee Ming-Lun , Lai Wei-Chih
Published: October 2012

GaN-based light-emitting diodes (LEDs) grown on sapphire with ex situ AlN nucleation layer prepared by radio-frequency sputtering were investigated. GaN-based epitaxial layers grown on the Ar-implanted AlN/sapphire (AIAS) substrates exhibited selective growth and subsequent lateral growth due to the difference of lattice constants between the implanted and implantation-free regions. Consequently, air voids over the implanted regions were formed around the GaN/AlN/sapphire interfaces. We proposed the growth mechanisms of the GaN layer on the AIAS substrates and characterized the LEDs with embedded air voids. With a 20 mA current injection, experimental results indicate that the light output power of LEDs grown on the AIAS substrates was enhanced by 25% compared with those of conventional LEDs. This enhancement can be attributed to the light scattering at the GaN/air void interfaces to increase the light extraction efficiency of the LEDs.
Capacitive micromachined ultrasonic transducer based tilt sensing
Authors: Yu Hongbin , Guo Bin , Haridas Kuruveettil , Lin Tsu-Hui , Hao Cheong Jia , Lin Tsai Ming , Boon Yee Tack
Published: October 2012

In this paper, a tilt sensing mechanism based on the capacitive micromachined ultrasound transducers (CMUTs) is presented. By measuring the difference in the time of flight of various pulse-echo signals from different CMUT transmitting elements to one common receiving element in the oil bath, the tilt angle of the oil surface can be determined. With the proposed device, the maximum tilt angles of 20° and 28° have been measured in the clockwise and counterclockwise directions, respectively, and the difference between the measured and the theoretical values of the tilt angle was found to be within 0.05° during the whole test.
Analysis of electron and light scattering in a fluorescent thin film by combination of Monte Carlo simulation and finite-difference time-domain method
Authors: Inami Wataru , Fujiwara Jun , Masahiro Fukuta , Ono Atsushi , Kawata Yoshimasa
Published: October 2012

We analyzed light intensity distributions in a subwavelength fluorescent film, which was excited by a focused electron beam. We have developed an analyzing method using Monte Carlo simulation and the finite-difference time-domain (FDTD) method. Electron scattering and trajectories were calculated by Monte Carlo simulation. Propagation and scattering of light excited with the electrons was calculated by FDTD method. A nanometric light spot was formed on the fluorescent film surface and its light intensity and its full width at half maximum (FWHM) were evaluated. We discuss the intensity and the FWHM dependence on the thickness of the fluorescent thin film and the acceleration voltage of an incident electron beam.
Ultra low field electron emission of graphene exfoliated from carbon cloth
Authors: Liu Jianlong , Zeng Baoqing , Wang Xiangru , Zhu Jinfeng , Fan Ying
Published: October 2012

Graphene emitters with high enhancement factor and ultra low emission field were exfoliated from carbon fibers in carbon cloth. This method made the graphene sheets to stay on the carbon fiber and were used, in situ, as emitters. Due to their high multistage enhancement factor and intrinsic good contact, electron emission from these graphene sheets required ultra low emission field. The threshold-field corresponding to emission current density of 1 mA/cm2 was about 0.7 V/μm. This high enhancement factor was attributed to the effects of the high aspect ratio of graphene sheets and the geometry of carbon cloth.
High quality ultrathin Bi2Se3 films on CaF2 and CaF2/Si by molecular beam epitaxy with a radio frequency cracker cell
Authors: Zhang Li , Hammond Robert , Dolev Merav , Liu Min , Palevski Alexander , Kapitulnik Aharon
Published: October 2012

We report a method to fabricate high quality Bi2Se3 thin films using molecular beam epitaxy with a radio frequency cracker cell as an atomic selenium source. With Se-to-Bi ratios close to exact stoichiometry, optimal layer-by-layer growth of high quality Bi2Se3 thin films with smooth surfaces has been achieved on CaF2(111) substrates and Si(111) substrates with a thin epitaxial CaF2 buffer layer (CaF2/Si). Transport measurements show a characteristic weak-antilocalization magnetoresistance in all the films, with the emergence of a weak-localization contribution in the ultrathin film limit. Quantum oscillations, attributed to the topological surface states have been observed, including in films grown on CaF2/Si.
Hole mobility characterization of DNA biopolymer by time-of-flight technique
Authors: Lin Ting-Yu , Chen I-Ching , Hung Yu-Chueh
Published: October 2012

In this study, the hole mobility of DNA biopolymer is examined by time-of-flight measurement. DNA biopolymer film is characterized under different electric fields and temperatures. The hole mobility exhibits a negative field dependence at lower fields above 273 K and is found to be positively correlated with temperature. Analysis of the experimental results is carried out and the carrier transport mechanisms in DNA biopolymer based on the experimental results are proposed and discussed.
Improvement of resistive switching in NiO-based nanowires by inserting Pt layers
Authors: Huang Yen-Chun , Chen Po-Yuan , Chin Tsung-Shune , Liu Ru-Shi , Huang Chao-Yuan , Lai Chih-Huang
Published: October 2012

Nonpolar resistive switching is demonstrated in polycrystalline NiO-based nanowires. The lower switching voltages and narrower switching distributions are exhibited in multilayered NiO/Pt nanowires, compared to the monolithic NiO nanowires. The temperature dependence of resistance at low resistance state reveals the conduction is attributed to the hopping through percolation paths composed of oxygen-related defects. The inserted Pt layers behave as intermediate electrodes to reduce migration length of oxygen ions and to store the oxygen ions near the electrodes. Therefore, the localized formation/migration of oxygen ions confines the occurrence of percolation paths, leading to improvement of the switching parameters.
Iron-Catalyzed Enantioselective Reactions Through the Use of Chiral Bipyridine-Containing Ligands
Authors: Plancq Baptiste , Ollevier Thierry
Published: October 2012

Recent examples highlighted the use of iron salts in combination with chiral bipyridine-containing ligands for highly enantioselective reactions. Very high enantioselectivities were obtained for various organic transformations, such as oxidation, Mukaiyama aldol and epoxide-opening reactions. These examples represent new entries in the field of green chemistry and sustainable development.