bluuebunny/crossref_metadata_2025 huggingface dataset:
On rainfall seasonality using a hidden semi‐Markov model
Authors: Sansom John , Thomson Peter
Published: August 2007

A hidden semi‐Markov model (HSMM) has previously been used successfully to model breakpoint rainfall data which provide a continuous record of steady rainfall rates and the times of rain rate changes. These data contain more information than the commoner fixed period rain accumulation data. Although the HSMM is a homogeneous model which does not explicitly account for seasonality, it does yield firm state classifications in the case of New Zealand breakpoint data and these were used to explore the nature of seasonality in the breakpoint data. These data and the state classifications were analyzed using simple graphical and numerical procedures. It was found that seasonality resides in the frequency of rainfall events rather than their intensity and that seasons start at varying rather than fixed times of the year and have varying length. Such a seasonal pattern is not well modeled by conventional periodic functions and further developments to the HSMM to include these features are discussed.
A simple method to estimate actual evapotranspiration from a combination of net radiation, vegetation index, and temperature
Authors: Wang Kaicun , Wang Pucai , Li Zhanqing , Cribb M. , Sparrow Michael
Published: August 2007

Satellite remote sensing is a promising technique for estimating global or regional evapotranspiration (ET). A simple and accurate method is essential when estimating ET using remote sensing data. Such a method is investigated by taking advantage of satellite measurements and the extensive ground‐based measurements available at eight enhanced surface facility sites located throughout the Southern Great Plains (SGP) area of the United States from January 2002 to May 2005. Data analysis shows that correlation coefficients between ET and surface net radiation are the highest, followed by temperatures (air temperature or land surface temperature, Ts), and vegetation indices (enhanced vegetation index (EVI) or normalized difference vegetation index (NDVI)). A simple regression equation is proposed to estimate ET using surface net radiation, air or land surface temperatures and vegetation indices. ET can be estimated using daytime‐averaged air temperature and EVI with a root mean square error (RMSE) of ∼30 W m−2 and a correlation coefficient of 0.91 across all sites and years. ET can also be estimated with comparable accuracy using NDVI and Ts. More importantly, the daytime‐averaged ET can also be estimated using only one measurement per day of temperatures (the daytime maximum air temperature or Ts) with comparable accuracy. A sensitivity analysis shows that the proposed method is only slightly sensitive to errors of temperatures, vegetation indices and net surface radiation. An independent validation was made using the measurements colleted by the eddy covariance method at six AmeriFlux sites throughout the United States from 2001 to 2006. The land cover associated with the AmeriFlux sites varies from grassland, to cropland and forest. The results show that ET can be reasonably predicted with a correlation coefficient that varies from 0.84 to 0.95 and a bias that varies from 3 W m−2 to 15 W m−2 and RMSE varying from ∼30 W m−2 to ∼40 W m−2. The positive bias partly comes from the energy imbalance problem encountered in the eddy covariance method. The proposed method can predict ET under a wide range of soil moisture contents and land cover types.
Magnetosheath plasma expansion: Hybrid simulations
Authors: Trávníček Pavel , Hellinger Petr , Taylor Matthew G. G. T. , Escoubet C. Philippe , Dandouras Iannis , Lucek Elizabeth
Published: August 2007

We investigate the effect of slow expansion on a magnetosheath plasma and low‐frequency waves using a two‐dimensional hybrid expanding box simulation. We start our simulation with a homogeneous high beta plasma, which is marginally stable to the mirror and proton cyclotron instabilities. The expansion is imposed as an external force: the physical size of the simulation box increases in two dimensions: one parallel and one perpendicular with respect to the ambient magnetic field. This expansion leads to a continuous decrease of proton beta and drives an increase of the proton temperature anisotropy. In the early stages of the simulation, both mirror and proton cyclotron waves appear. The system establishes a marginally stable state with respect to both mirror and proton cyclotron instabilities. Initially, the mirror waves dominate the proton cyclotron waves, even when the system is below the linear mirror threshold, but as time increases the proton cyclotron waves become dominant in the low beta region. We also include an initial comparison of the simulated data with Cluster observations.
Incursions and radiative impact of Asian dust in northern Alaska
Authors: Stone R. S. , Anderson G. P. , Andrews E. , Dutton E. G. , Shettle E. P. , Berk A.
Published: August 2007

The Arctic region is sensitive to incursions of aerosols that affect its radiation balance, directly through interactions with solar and terrestrial radiation and indirectly as cloud condensation nuclei. During spring 2002 dust was transported from the Gobi desert passing over instrumented field sites near Barrow, Alaska, providing the opportunity to measure the dust properties. Empirical determinations of the direct radiative forcing by dust were used to corroborate simulations made using the Moderate Resolution Transmittance radiative transfer code, MODTRAN™5. During sunlit periods, dust cools the surface while warming those layers in which it resides, increasing atmospheric stability. At night, dust layers tend to cool while the surface warms slightly due to infrared emissions from the dust layer.
Atmospheric bromine flux from the coastal Abu Dhabi sabkhat: A ground‐water mass‐balance investigation
Authors: Wood Warren W. , Sanford Ward E.
Published: August 2007

A solute mass‐balance study of ground water of the 3000 km2 coastal sabkhat (salt flats) of the Emirate of Abu Dhabi, United Arab Emirates, documents an annual bromide loss of approximately 255 metric tons (0.0032 Gmoles), or 85 kg/km2. This value is an order of magnitude greater than previously published direct measurements from the atmosphere over an evaporative environment of a salar in Bolivia. Laboratory evidence, consistent with published reports, suggests that this loss is by vapor transport to the atmosphere. If this bromine flux to the atmosphere is representative of the total earth area of active salt flats then it is a significant, and generally under recognized, input to the global atmospheric bromide flux.
Observations of changes in surface water over the western Siberia lowland
Authors: Grippa M. , Mognard N. M. , Le Toan T. , Biancamaria S.
Published: August 2007

We analyse the evolution of the fraction of water surface (FWS), derived by SSM/I between 1988 and 2002 over the Western Siberia Lowland. This whole region exhibits an increase in the amount of FWS that is in agreement with the observed trends of the Ob river discharge and precipitation. However, a similar increasing trend is not found over the most important wetland area of Sibirskie Uvaly Hills located in a region of sporadic permafrost. These observations support the hypothesis that climate warming in discontinuous permafrost environments may lead to a reduction of small lakes and wetland areas.
Frequency sensitive moment tensor inversion for light to moderate magnitude earthquakes in eastern Africa
Authors: Barth A. , Wenzel F. , Giardini D.
Published: August 2007

We provide a procedure for the routine determination of moment tensors from earthquakes with magnitudes as low as MW 4.4 using data recorded by only a few permanent seismic stations at regional to teleseismic distances. Waveforms are inverted for automatically determined frequency pass‐bands that depend on source‐receiver locations as well as the earthquake magnitude. Inversion results are stable against small variations in the frequency band and provide low data variances, i.e., a good fit between observed and modelled waveform traces. The total frequency band used for our procedure ranges from 10 mHz to 29 mHz (periods of 35 s to 100 s). This enables us to determine focal mechanisms for earthquakes that were not derived previously by routine procedures of CMT or other agencies. As a case study, we determine focal mechanism solutions of 38 light to moderate magnitude earthquakes in eastern Africa between 1995 and 2002.
Anthropogenic aerosol forcing and the structure of temperature trends in the southern Indian Ocean
Authors: Cai Wenju , Cowan Tim , Dix Martin , Rotstayn Leon , Ribbe Joachim , Shi Ge , Wijffels Susan
Published: August 2007

Over the past decades surface warming in the southern subtropical Indian Ocean (IO) has been greater than that in other oceans. The warming penetrates to a depth of 800 m, in contrast to the off‐equatorial surface warming which co‐exists with subsurface cooling. We examine the dynamics for this rich structure. Results from the 20th century experiments of the Intergovernmental Panel on Climate Change (IPCC) confirm that the southern subtropical IO surface‐to‐800 m warming is greater than that in the Pacific and Atlantic Oceans. Outputs from two targeted ensemble sets of coupled model experiments, one with and one without increasing anthropogenic aerosols, show that increasing aerosols strengthen the global Conveyor, and generate a greater poleward shift and intensification of the Agulhas outflow and its retroflection; the process increases the warming rate in the subtropics, and takes heat out of the off‐equatorial region generating a cooling.
Extended interpretations in Q vector analyses and applications in a torrential rain event
Authors: Cao Jie , Gao Shouting
Published: August 2007

Theoretical derivation shows that the curl of Q vector is associated with vertical variations of the vertical vorticity of ageostrophic wind. It may provide more information about distributions and evolutions of convective system along with the conventional usage of the divergence of Q vector in operational weather forecast. Therefore, a new phrase—extended interpretations in Q vector analyses—is defined in this paper, which represents the usages of the vertical component of the curl of Q vector as well as its divergence. Through dynamic identifications of both spatial distributions and temporal variations of heavy rain centers of a torrential rain event, advantages in the application of the vertical component of the curl of Q vector over its divergence are evident. In addition, the potential usage of the curl of Q vector in quantitative analysis of ageostrophic motions is pointed out.
Factors influencing new particle formation at the rural site, Harwell, United Kingdom
Authors: Charron Aurélie , Birmili Wolfram , Harrison Roy M.
Published: August 2007

A study of large‐scale episodes of new particle formation observed at a rural site in southern England which is a receptor site for air masses with varying degrees of pollution is presented. Large‐scale episodes of new particle formation are infrequent at Harwell (26 episodes for 1999–2001; data capture of 55%), and it is shown that the most favorable situation for new particle formation in southern England is also infrequent. All episodes but three occurred when clean cool arctic or polar maritime air masses arrived in the UK bringing days of high solar radiation and thin cloud cover. The incursion of these cool air masses may be associated with passage of a cold front and leads to stronger winds and probably strong turbulent mixing; all favor new particle production. More exceptionally, nucleation occurred in warm polluted continental or tropical maritime air masses which shows that different conditions are also possible but much less probable. Higher concentrations of SO2 and higher relative humidity favor higher particle number in agreement with nucleation processes involving H2SO4 and H2O. Taken together, these parameters are insufficient to predict when a burst of particle formation due to nucleation will occur at Harwell, and other important variables must be influential.
Frequency of tropical precipitating clouds as observed by the Tropical Rainfall Measuring Mission Precipitation Radar and ICESat/Geoscience Laser Altimeter System
Authors: Casey Sean P. F. , Dessler Andrew E. , Schumacher Courtney
Published: August 2007

Convective clouds in the tropics can be grouped into three categories: shallow clouds with cloud top heights near 2 km above the surface, midlevel congestus clouds with tops near the 0°C level, and deep convective clouds capped by the tropopause. This trimodal distribution is visible in cloud data from the Geoscience Laser Altimeter System (GLAS), carried aboard the Ice, Cloud, and land Elevation Satellite (ICESat), as well as in precipitation data from the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR). Fractional areal coverage (FAC) data is calculated at each of the three levels to describe how often optically thick clouds or precipitation are seen at each level. By dividing the FAC of TRMM PR‐observed precipitation by the FAC of thick GLAS/ICESat‐observed clouds, we derive the fraction of clouds that are precipitating. We find that the tropical mean precipitating cloud fraction is low: 3.7% for shallow clouds, 6.5% for midlevel clouds, and 24.1% for deep clouds.
Estimating lightning channel characteristics of positive narrow bipolar events using intrachannel current reflection signatures
Authors: Hamlin T. , Light T. E. , Shao X. M. , Eack K. B. , Harlin J. D.
Published: August 2007

A set of 133 positive narrow bipolar lightning events located by the Los Alamos Sferic Array (LASA) is examined. The events were selected because their fast electric field change (EFC) records show a secondary positive peak, following the primary positive peak, which is consistent with what would be expected for a propagating current pulse reflecting off an impedance discontinuity along a transmission line. With an assumed propagation speed, the time difference between the primary and secondary (reflected) peak in the field change record offers a direct technique for estimating the length of the discharge channel. If the signal amplitude is assumed to decay exponentially with time and that the reflection coefficient is near unity, the time difference between peaks and the ratio of their amplitudes yields an estimate of the temporal decay rate constant. Statistics on the 133 located events examined show an average peak‐to‐reflected‐peak time of 6.7 μs, a mean amplitude ratio estimate of 6.1, and an amplitude decay rate constant of 4.0 μs. Bound by the speed of light, the upper limit on channel length for the typical positive narrow bipolar event (NBE) in this study is shown to be approximately 2 km.
Inhibition of the cellular uptake of anandamide by genistein and its analogue daidzein in cells with different levels of fatty acid amide hydrolase‐driven uptake
Authors: Thors L , Eriksson J , Fowler C J
Published: August 2007

Background and purpose:

Genistein, a tyrosine kinase inhibitor used to block caveolae dependent endocytosis, reduces the cellular uptake of anandamide in RBL2H3 basophilic leukaemia cells. However, genistein is also a competitive inhibitor of fatty acid amide hydrolase, the enzyme responsible for anandamide hydrolysis. Here we have investigated whether inhibition of fatty acid amide hydrolase rather than inhibition of endocytosis is the primary determinant of genistein actions upon anandamide uptake.

Experimental approach:

Cellular uptake of anandamide, labelled in the arachidonoyl part of the molecule was assessed in four different cell lines using a standard method. Fatty acid amide hydrolase activity in homogenates and intact cells was measured using anandamide labelled in the ethanolamine part of the molecule.

Key results:

The fatty acid amide hydrolase inhibitor URB597 inhibited anandamide uptake into RBL2H3 cells and R3327 AT‐1 prostate cancer cells, but not into 3T3‐L1 preadipocytes or PC‐3 prostate cancer cells. An identical pattern was seen with genistein. The related compound daidzein inhibited anandamide hydrolysis in homogenates and intact cells, and reduced its uptake into RBL2H3 and R3327 AT‐1, but not PC‐3 cells. Anandamide hydrolysis by cell homogenates was in the order RBL2H3 > R3327 AT‐1 > PC‐3 ≈3T3‐L1.

Conclusions and implications:

The ability of genistein to inhibit anandamide uptake is mimicked by daidzein (which does not affect tyrosine kinase), and is only seen in cells that show sensitivity to URB597. This indicates that blockade of fatty acid amide hydrolase is the primary determinant of the effects of genistein on cellular anandamide uptake.

British Journal of Pharmacology(2007)152, 744–750; doi:10.1038/sj.bjp.0707401; published online 6 August 2007
Giant domain wall contribution to the dielectric susceptibility in BaTiO3 single crystals
Authors: Wang Y. L. , Tagantsev A. K. , Damjanovic D. , Setter N.
Published: August 2007

Giant domain wall contribution to the dielectric susceptibility was observed in the rhombohedral phase of BaTiO3 single crystals. The unusual contribution can be two orders of magnitude larger than the thermodynamically estimated lattice susceptibility, even at driving fields that are much smaller than the coercive field. Nonlinearity measurements suggest a local sliding motion of depinned domain walls being responsible for the ultrahigh susceptibility. Pretreatments, particularly those performed in high temperature ferroelectric phases, can strongly affect the contribution of this local sliding mechanism.
Density dependent potentials: Structure and thermodynamics
Authors: Merabia Samy , Pagonabarraga Ignacio
Published: August 2007

Local density dependent potentials constitute a family of many body potentials which have been recently introduced in mesoscopic modeling of simple and complex fluids. We construct a field theory for these potentials and calculate the structure factor of the fluid through a saddle point expansion. We propose also an integral equation for local density potentials which shows quantitative agreement both for the correlation functions and thermodynamic properties of such potentials, even close to binodals where the simpler saddle point approximation fails. Contrary to the limitations of global density dependent potentials, there is no ambiguity in the expression of thermodynamics quantities such as the pressure.
Diffusion and segmental dynamics of rodlike molecules by fluorescence correlation spectroscopy
Authors: Winkler Roland G.
Published: August 2007

The dynamics of weakly bending polymers is analyzed on the basis of a Gaussian semiflexible chain model and the fluorescence correlation spectroscopy (FCS) correlation function is determined. Particular attention is paid to the influence of the rotational motion on the decay of the FCS correlation function. An analytical expression for the correlation function is derived, from which the averaged segmental mean square displacement can be determined independent of any specific model for the polymer dynamcis. The theoretical analysis exhibits a strong dependence of the correlation function on the rotational motion for semiflexible polymers with typical lengths and persistence lengths of actin filaments or fd viruses. Hence, FCS allows for a measurement of the rotational motion of such semiflexible polymers. The theoretical results agree well with experimental measurements on actin filaments and confirm the importance of large relaxation times.
Invited Review Article: A review of techniques for attaching micro- and nanoparticles to a probe’s tip for surface force and near-field optical measurements
Authors: Gan Yang
Published: August 2007

Cantilevers with single micro- or nanoparticle probes have been widely used for atomic force microscopy surface force measurements and apertureless scanning near-field optical microscopy applications. In this article, I critically review the particle attachment and modification techniques currently available, to help researchers choose the appropriate techniques for specific applications.
Efficient and accurate three-dimensional Poisson solver for surface problems
Authors: Genovese Luigi , Deutsch Thierry , Goedecker Stefan
Published: August 2007

We present a method that gives highly accurate electrostatic potentials for systems where we have periodic boundary conditions in two spatial directions but free boundary conditions in the third direction. These boundary conditions are needed for all kinds of surface problems. Our method has an O(NlogN) computational cost, where N is the number of grid points, with a very small prefactor. This Poisson solver is primarily intended for real space methods where the charge density and the potential are given on a uniform grid.
Transport properties of microstructured ultrathin films of La0.67Ca0.33MnO3 on SrTiO3
Authors: Beekman C. , Komissarov I. , Hesselberth M. , Aarts J.
Published: August 2007

The authors have investigated the electrical transport properties of 8nm thick La0.67Ca0.33MnO3 (LCMO) films, sputter deposited on SrTiO3 (STO), and etched into 5μm wide bridges by Ar-ion etching. The authors find that even slight overetching of the film leads to conductance of the STO substrate, and asymmetric and nonlinear current-voltage (I-V) characteristics. However, a brief oxygen plasma etch allows full recovery of the insulating character of the substrate. The I-V characteristics of the bridges are then fully linear over a large range of current densities. The authors find colossal magnetoresistance properties typical for strained LCMO on STO but no signature of nonlinear effects (the so-called electroresistance) connected to electronic inhomogeneites. In the metallic state below 150K, the highest current densities lead to heating effects and nonlinear I-V characteristics.
Efficient spatial redistribution of quantum dot spontaneous emission from two-dimensional photonic crystals
Authors: Kaniber M. , Kress A. , Laucht A. , Bichler M. , Meyer R. , Amann M.-C. , Finley J. J.
Published: August 2007

The authors investigate the modification of the spontaneous emission dynamics and the external quantum efficiency for self-assembled InGaAs quantum dots coupled to extended and localized photonic states in GaAs two-dimensional photonic crystals. The photonic band gap is shown to give rise to a five to ten times enhancement of the external quantum efficiency, while the spontaneous emission rate is simultaneously reduced by a comparable factor. The findings are quantitatively explained by a modal redistribution of spontaneous emission due to the modified local density of photonic states. The results suggest that quantum dots embedded within photonic crystals are suitable for practical single photon sources with high external efficiency.
Scheme of detecting microscopic inhomogeneity in binary liquid mixtures utilizing resonantly coupled vibrational modes
Authors: Torii Hajime
Published: August 2007

Influence of microscopic inhomogeneity in binary liquid mixtures on their vibrational spectra is studied by doing calculations on a model liquid system. The concentration dependence of the noncoincidence effect (NCE), which is a feature of vibrational bands related to the intermolecular resonant coupling of vibrational modes, is analyzed. It is suggested that observation of convex behavior of the NCEs for the vibrational bands of both species, especially that of the less polar species, in a binary liquid mixture is an indication of the occurrence of microscopic inhomogeneity.
Growth and pyroelectric properties of [001]-oriented (1−x)Pb(Zn1∕3Nb2∕3)O3–xPbTiO3 single crystals
Authors: Fang Bijun , Li Jinhua , Xu Haiqing , Luo Haosu
Published: August 2007

Large-size and high-quality (1−x)Pb(Zn1∕3Nb2∕3)O3–xPbTiO3 [PZNT(1−x)∕x] single crystals (PZNT95/5, PZNT91/9, and PZNT85/15) were grown by a modified Bridgman method using an allomeric 0.69Pb(Mg1∕3Nb2∕3)O3–0.31PbTiO3 (PMNT69/31) single crystal as a seed. At room temperature, dielectric constant and loss tangent are about 1890 and 0.0692, 4710 and 0.0191, and 673 and 0.0320 for PZNT95/5, PZNT91/9, and PZNT85/15 crystals measured at 100Hz, respectively. These crystals exhibit high pyroelectric coefficients of 458, 1713, and 460μC∕Km2 for PZNT95/5, PZNT91/9, and PZNT85/15 crystals, respectively. The calculated detectivity and voltage responsivity figures of merit Fd and Fv exceed 5.27, 23.63, and 13.14μPa−1∕2, and 0.0106, 0.0162, and 0.0302m2∕C for PZNT95/5, PZNT91/9, and PZNT85/15 crystals, respectively, over the frequency range of 40–2000Hz. However, Fd and Fv increase greatly over the temperature range measured. It is needed to improve temperature stability and voltage responsivity to meet the requirements of practical applications. Such investigation reveals that superior pyroelectric performance makes PZNT single crystals promising candidates for infrared detectors and other pyroelectric applications.
Determination of all syzygies for the dependent polynomial invariants of the Riemann tensor. III. Mixed invariants of arbitrary degree in the Ricci spinor
Authors: Lim A. E. K. , Carminati J.
Published: August 2007

In this paper, we rigorously prove that the complete set of Riemann tensor invariants given by Sneddon [J. Math. Phys. 40, 5905 (1999)] is both minimal and complete. Furthermore, we provide a two-stage algorithm for the explicit construction of polynomial syzygies relating any dependent Riemann tensor invariant to members of the complete set.
Autoresonant control of the many-electron dynamics in nonparabolic quantum wells
Authors: Manfredi G. , Hervieux P.-A.
Published: August 2007

The optical response of nonparabolic quantum wells is dominated by a strong peak at the plasmon frequency. When the electrons reach the anharmonic regions, resonant absorption becomes inefficient. This limitation is overcome by using a chirped laser pulse in the autoresonant regime. By direct simulations using the Wigner phase-space approach, the authors prove that, with a sequence of just a few pulses, electrons can be efficiently detrapped from a nonparabolic well. For an array of multiple quantum wells, they can create and control an electronic current by suitably applying an autoresonant laser pulse and a slowly varying dc electric field.
Detection of higher order modulation harmonics in magnetic resonance force microscopy
Authors: Mewes T. , Mewes C. K. A. , Nazaretski E. , Kim J. , Fong K. C. , Obukhov Y. , Pelekhov D. V. , Wigen P. E. , Hammel P. C.
Published: August 2007

Magnetic resonance force microscopy measurements of the electron spin resonance of a thin film of 2,2-diphenyl-1-picrylhydrazyl were performed using a low doped silicon cantilever with a high coercivity SmCo particle glued to its end. The low doping level enables amplitude modulation of the microwave field with only small spurious driving of the cantilever. Besides amplitude modulation we use frequency modulation of the microwave field at integer fractions of the cantilever resonance frequency leading to derivative signals up to the fourth derivative of the amplitude modulation response signal. The influence of the modulation depth on the line shape of the first derivative response is also presented.
Improved surface morphology and texture of Ag films on indium tin oxide via Cu additions
Authors: Han H. , Zoo Yeongseok , Mayer J. W. , Alford T. L.
Published: August 2007

The surface and texture properties of Ag(Cu) alloy thin films on indium tin oxide (ITO) has been investigated and compared to pure Ag thin films on ITO. Atomic force microscopy (AFM) and x-ray diffraction results of annealed films show differences in the evolution of surface morphology and texture with annealing. The presence of Cu atoms in the silver alters the alloy’s surface energy and surface diffusion. This results in Ag and Ag(Cu) alloy having the very different surface morphology and crystallographic texture. The (111) texture of the Ag(Cu) is enhanced when compared to that of pure Ag films. The resistivity of Ag(Cu) films annealed at high temperatures (∼up to 600°C for 1h in vacuum) remained constant due to absence of agglomeration.
Atomic-scale study of TbCo2.5/Fe multilayers by laser-assisted tomographic atom probe
Authors: Grenier A. , Lardé R. , Cadel E. , Vurpillot F. , Juraszek J. , Teillet J. , Tiercelin N.
Published: August 2007

Sputtered (TbCo2.5 25 nm/Fe 20 nm) multilayers have been analyzed by laser-assisted tomographic atom probe. It allowed us to perform three-dimensional reconstructions of the layers and to determine their composition at the atomic scale. From the concentration profiles inside the multilayer, we show that the diffused interfaces are not symmetric and that a stronger Fe-Co mixing is present at the top of the crystalline iron layers as compared to the top of amorphous TbCo layers.
The fabrication and material properties of compositionally multilayered Ba1−xSrxTiO3 thin films for realization of temperature insensitive tunable phase shifter devices
Authors: Cole M. W. , Ngo E. , Hirsch S. , Demaree J. D. , Zhong S. , Alpay S. P.
Published: August 2007

Compositionally layered BaxSr1−xTiO3 (Ba0.60Sr0.40TiO3–Ba0.75Sr0.25TiO3–Ba0.90Sr0.10TiO3) 220nm thin film heterostructures were fabricated on Pt coated high resistivity Si substrates via the metal organic solution deposition technique (MOSD). Optimization of the material design was achieved by evaluating two integration schemes, namely, the single- and multianneal process protocols. Materials characterization demonstrated that both film process protocols resulted in smooth, dense, crack-free films with a single phase perovskite structure. Rutherford backscattering spectroscopy revealed compositionally distinct layers and severe elemental interdiffusion for the films fabricated via the multianneal and single-anneal process protocols, respectively. The retention of the compositional layering subsequent to film crystallization deemed the multianneal processed BaxSr1−xTiO3 (BST) film suitable for evaluation of dielectric properties. The dielectric properties were compared to both paraelectric uniform composition BST and to the relevant compositionally graded BST films reported in the technical literature. Our results made evident that the multiannealed compositionally layered BST films possessed higher permittivity (εr=360) and lower dissipation factor (tanδ=0.012) with respect to both uniform composition paraelectric Ba0.60Sr0.40TiO3 film fabricated via the same MOSD processing method and the relevant literature values for compositionally graded BST films. The multilayered BST material design exhibited minimal dielectric dispersion in the range of 90to−10°C, showing a 6.4% decrease in permittivity (corresponding to a temperature coefficient of capacitance TCC20–90=−0.921) as the temperature was elevated from 20to90°C and only a 2.1 increase in permittivity (TCC20–(−10)=−0.716) as the temperature was lowered from 20to−10°C. Additionally, the dielectric tunability of the multilayered BST structures over the temperature range of −10–90°C was temperature independent. Our results show that the multilayered BST design has excellent dielectric properties and the enhanced tunability and dielectric loss are stable over a relatively broad temperature range (−10–90°C), thereby making them excellent candidates for the next generation of enhanced performance temperature stable tunable devices.
Influence of nonholonomic constraints on variations, symplectic structure, and dynamics of mechanical systems
Authors: Guo Yong-Xin , Liu Shi-Xing , Liu Chang , Luo Shao-Kai , Wang Yong
Published: August 2007

Based on a serious analysis of the Frobenius integrability condition for affine differential constraints that mechanical systems are subject to the necessary and sufficient conditions for coincidence of three kinds of unfree variations, the existence of simple symplectic structure of the constraint submanifold and equivalence of nonholonomic and vakonomic dynamics for the constrained systems are, respectively, obtained, which are all related with the Frobenius integrability condition in their special forms. Two illustrative examples are presented to verify the results.
Directed assembly of gold nanoparticle nanowires and networks for nanodevices
Authors: Xiong Xugang , Busnaina Ahmed , Selvarasah Selvapraba , Somu Sivasubramanian , Wei Ming , Mead Joey , Chen Chia-Ling , Aceros Juan , Makaram Prashanth , Dokmeci Mehmet R.
Published: August 2007

Alternating electric field is used to assemble gold nanoparticle nanowires from liquid suspensions. The effects of electrode geometry and the dielectrophoresis force on the chaining and branching of nanowire formation are investigated. The nanowire assembly processes are modeled using finite element calculations, and the particle trajectories under the combined influence of dielectrophoresis force and viscous drag are simulated. Nanoparticle nanowires with 10nm resolution are fabricated. The wires can be further oriented along an externally introduced flow. This work provides an approach towards rapid assembly and organization of ultrasmall nanoparticle networks.
Thermal and chemical vapor deposition of Si nanowires: Shape control, dispersion, and electrical properties
Authors: Colli A. , Fasoli A. , Beecher P. , Servati P. , Pisana S. , Fu Y. , Flewitt A. J. , Milne W. I. , Robertson J. , Ducati C. , De Franceschi S. , Hofmann S. , Ferrari A. C.
Published: August 2007

We investigate and compare complementary approaches to SiNW production in terms of yield, morphology control, and electrical properties. Vapor-phase techniques are considered, including chemical vapor deposition (with or without the assistance of a plasma) and thermal evaporation. We report Au-catalyzed nucleation of SiNWs at temperatures as low as 300°C using SiH4 as precursor. We get yields up to several milligrams by metal-free condensation of SiO powders. For all processes, we control the final nanostructure morphology. We then report concentrated and stable dispersions of SiNWs in solvents compatible with semiconducting organic polymers. Finally, we investigate the electrical response of intrinsic SiNWs grown by different methods. All our SiNWs exhibit p-type behavior and comparable performance, though in some cases ambipolar devices are observed. Thus, processing and morphology, rather than the growth technique, are key to achieve optimal samples for applications.
Dichotomy of photoinduced quasiparticle on CuO2 planes of YBa2Cu3O7 directly revealed by femtosecond polarization spectroscopy
Authors: Luo C. W. , Chen L. Y. , Lee Y. H. , Wu K. H. , Juang J. Y. , Uen T. M. , Lin J.-Y. , Gou Y. S. , Kobayashi T.
Published: August 2007

The dichotomy of photoinduced quasiparticle dynamics in the anisotropic high Tc superconductors (YBa2Cu3O7) is revealed by using the orientation-resolved femtosecond reflection spectroscopy in the specially designed thin films. The transient reflectivity ΔR∕R along the ab diagonal and b axis are totally different in the slow relaxation process which relates to the opening of the superconducting gap. In addition, the distribution of the temperature evolution of ΔR∕R on the CuO2 planes of YBa2Cu3O7 may indicate the d-wave symmetry of the superconducting gap in high Tc superconductors.
Optical cell with periodic resistive heating for the measurement of heat, mass, and thermal diffusions in liquid mixtures
Authors: Hartung M. , Köhler W.
Published: August 2007

A new technique for the measurement of heat, mass, and thermal diffusions in liquids has been developed. Similar to laser induced dynamic gratings, a temperature grating is created in the sample. Thermal expansion transforms the temperature into a refractive-index grating, which is read by diffraction of a readout laser beam. In a multicomponent mixture an additional concentration grating is formed by thermal diffusion driven by the temperature gradients of the temperature grating. Differently to laser induced dynamic grating experiments we use Joule heating instead of optical heating. For that purpose we have built cuvettes which have a grating of transparent conducting strips on the inner side of one of their windows. If heated by an electric current a temperature grating will build up in the sample. Both the heat equation and the extended diffusion equation have been solved in two dimensions to allow for quantitative data analysis. Our apparatus and method of analysis have been validated by measurements of heat, mass, and thermal diffusions in pure and binary liquids. Heat diffusion can be correctly determined as was shown for pure toluene, pure dodecane, and the symmetric mixture of isobutylbenzene dodecane. Mass and thermal diffusions were studied in the three symmetric mixtures of dodecane, isobutylbenzene, and tetralin. The obtained diffusion and Soret coefficients agree with the literature values within the experimental errors. Uncompensated transient heating effects limit the resolution of the experimental technique.
Impact of surface chemical treatment on capacitance-voltage characteristics of GaAs metal-oxide-semiconductor capacitors with Al2O3 gate dielectric
Authors: Shahrjerdi Davood , Tutuc Emanuel , Banerjee Sanjay K.
Published: August 2007

The authors examine the impact of two different chemical surface treatment methods on capacitance-voltage characteristics of GaAs metal-oxide-semiconductor (MOS) capacitors using NH4OH and (NH4)2S prior to atomic layer deposition (ALD) of Al2O3. In both cases, x-ray photoelectron spectroscopy data confirm the removal of As2O3∕As2O6 upon Al2O3 deposition. However, Ga–O bonds appear to incorporate in the final gate stack at the Al2O3∕GaAs interface. MOS capacitors exhibit a steep transition from accumulation to depletion as well as very low leakage current density indicating high quality of ALD-Al2O3. The midgap interface trap density was evaluated to be (∼3–5)×1011∕cm2eV using the Terman method. In addition, quasistatic capacitance-voltage (C-V) measurement confirms the formation of true inversion layer in GaAs using both chemical treatment protocols. However, sulfur-passivated GaAs demonstrates better frequency dispersion behavior and slightly smaller capacitance equivalent thickness than hydroxylated GaAs. A statistical study substantiates the reproducibility of these results.
Operation of multiple 90° switching systems in barium titanate single crystals under electromechanical loading
Authors: Shieh J. , Yeh J. H. , Shu Y. C. , Yen J. H.
Published: August 2007

Hysteresis evolution of a 5×5×2mm3 barium titanate single crystal during a combined electromechanical loading sequence reveals incomplete switching characteristics and a considerable disproportion of slope gradients at zero electric field for the measured polarization and strain hysteresis curves. A likely cause for the disproportion of gradients is the cooperative operation of multiple 90° switching systems by which “polarization-free” strain changes are induced. In addition, a theoretical study indicates that the presence of depolarization fields generated from the unshielded boundaries and/or incompatible domains within the crystal can have a substantial influence on the polarization measurement in the loading direction.
High-flux characterization of ultrasmall multijunction concentrator solar cells
Authors: Korech Omer , Hirsch Baruch , Katz Eugene A. , Gordon Jeffrey M.
Published: August 2007

A characterization study of ultrasmall ultraefficient multijunction concentrator solar cells is presented, with emphasis on performance sensitivity to light intensity and distribution. Information of this type is essential in the design and optimization of the latest generations of high-flux photovoltaic systems. Cell miniaturization allows increasing the concentration at which efficiency peaks, facilitates passive heat rejection, and permits the use of all-glass optics. However, few device measurements have been published on ultrasmall cells. Extensive measurements, up to ∼5000 suns, on the 1.0mm2 active region within the busbars are reported and analyzed.
Electrostatic characteristics of tether atoms in connecting organic molecules to the surface of silicon
Authors: Zhao Peiji , Woolard Dwight
Published: August 2007

In this letter, the authors analyze the electrostatic characteristics of the tether atoms connecting organic molecules onto silicon (100)-2×1 surfaces, which is a key factor in the design of molecular devices for information processing and biomolecular sensing. Design principles for silicon surfaces with required electrostatic functionality are presented.
Lifetime modeling for stress-induced voiding in integrated circuit interconnections
Authors: Tan Cher Ming , Hou Yuejin
Published: August 2007

By considering the stress-induced voiding (SIV) as a result of strain energy relief in the presence of flaws, an analytical lifetime model for SIV is derived from the energy perspective. The SIV lifetime is strongly dependent on the passivation integrity of the cap layer, effective bulk modulus of the interconnect system, diffusivities of the interconnect atoms in the dominant diffusion paths, stress free temperature, and temperature of the interconnection. The calculated SIV lifetime and the critical temperature are found to be consistent with the experimental values.
Fine tuning of equilibrium distance of two-dimensional colloidal assembly under an alternating electric field
Authors: Liu Yu , Xie Rong-Guo , Liu Xiang-Yang
Published: August 2007

The authors study the effect of temperature on two-dimensional (2D) colloidal assembly subjected to an alternating electric field (AEF). At high frequencies (>500Hz), an increase in temperature leads to an increase in the upper cutoff frequency of the assembly, consistent with an electrohydrodynamic (EHD) mechanism. At low frequencies (<300Hz), the authors observe a crossover frequency at which the behavior of the temperature dependence changes, suggesting a change in the origin of the EHD mechanism. By controlling the temperature and the AEF, the authors develop a method to finely tune the equilibrium distance of the 2D colloidal assembly over a wide range.
Room-temperature ferromagnetism in Fe-doped PbTiO3 nanocrystals
Authors: Ren Zhaohui , Xu Gang , Wei Xiao , Liu Yong , Hou Xiaohong , Du Piyi , Weng Wenjian , Shen Ge , Han Gaorong
Published: August 2007

Room-temperature ferromagnetism has been observed in Fe-doped PbTiO3 nanocrystals. The magnetism of the nanocrystals develops from diamagnetism to ferromagnetism and the paramagnetism on increasing nominal Fe doping concentration from 0to4mol%. Transmission electron microscope (TEM) and high-resolution TEM data indicate that Fe-doped PbTiO3 nanocrystals with the size of ∼100nm are organized to form a planarlike self-assembly via oriented aggregation. These assembled nanostructures effectively improve room-temperature ferromagnetism of the sample. The exchange interaction of ferric ions via an electron trapped in a bridging oxygen vacancy (F center) is employed to explain the ferromagnetism of Fe-doped PbTiO3 nanocrystals.
Nonreciprocal spin wave spectroscopy of thin Ni–Fe stripes
Authors: Khalili Amiri Pedram , Rejaei Behzad , Vroubel Marina , Zhuang Yan
Published: August 2007

The authors report on the observation of nonreciprocal spin wave propagation in a thin (∼200nm) patterned Ni–Fe stripe. The spin wave transmission spectrum is measured using a pair of microstrip lines as antennas. The nonreciprocity of surface wave dispersion brought about by an adjacent aluminum ground leads to a nonreciprocal coupling of the antennas. The effects of Ni–Fe film conductivity, thickness, and reflections caused by the lateral confinement of the magnetic stripe are discussed. The nonreciprocity observed in this structure can potentially be used to realize nonreciprocal microwave devices on silicon.
Nanoindentation initiated dislocations in barium titanate (BaTiO3)
Authors: Scholz T. , McLaughlin K. K. , Giuliani F. , Clegg W. J. , Espinoza-Beltrán F. J. , Swain M. V. , Schneider G. A.
Published: August 2007

Plastic behavior during loading of ferroelectric bulk materials is commonly explained by domain switching processes. In this study the authors show that dislocation induced deformation also occurs during nanoindentation of BaTiO3. Studies were conducted using a spheroconical shaped indenter together with observations using atomic force microscopy (AFM) and focussed ion beam cross sections for transmission electron microscopy (TEM) of the contact areas. Force/displacement measurements showed the sudden appearance of “pop-in” events, which AFM/TEM observations revealed were associated with plastic deformation and dislocation arrays at the surface. TEM cross sections of the indents showed paired parallel slip lines with 45° to the original surface.
Enhancement of polycrystalline silicon solar cells using ultrathin films of silicon nanoparticle
Authors: Stupca M. , Alsalhi M. , Al Saud T. , Almuhanna A. , Nayfeh M. H.
Published: August 2007

Ultrathin films of highly monodispersed luminescent Si nanoparticles are directly integrated on polycrystalline Si solar cells. The authors monitor the open-circuit voltage and the short circuit current. The results demonstrate that films of 1nm blue luminescent or 2.85nm red luminescent Si nanoparticles produce large voltage enhancements with improved power performance of 60% in the UV/blue range. In the visible, the enhancements are ∼10% for the red and ∼3% for the blue particles. The results point to a significant role for charge resonant transport across the nanofilm and Schottky-like rectification at nanoparticle-metal interface.
Gain degradation mechanisms in wafer fused AlGaAs∕GaAs∕GaN heterojunction bipolar transistors
Authors: Lian Chuanxin , Xing Huili Grace , Wang Chad S. , Brown David , McCarthy Lee
Published: August 2007

The authors have compared AlGaAs∕GaAs∕GaN heterojunction bipolar transistors (HBTs) formed by wafer fusion with AlGaAs∕GaAs∕GaAs as-grown HBTs subject to high temperature annealing conditions similar to those used in the wafer fusion process. The high temperature annealing alone is found to cause gain degradation by a factor of 2–6, a result of reduction in minority carrier lifetime in the base. Detailed analysis indicates that the fused HBTs also suffer from higher recombination in the emitter-base junction, exacerbated base degradation as well as effective potential barriers formed at the GaAs base/GaN collector junction.
Continuum generation and carving on a silicon chip
Authors: Koonath Prakash , Solli Daniel R. , Jalali Bahram
Published: August 2007

Self-phase modulation is utilized to generate a continuum of wavelengths on a silicon chip. The propagation of near transform-limited, picosecond pulses (3ps) through a micron sized silicon waveguide results in the generation of a spectral continuum. This spectral continuum is then carved by microdisk resonators that are integrated three dimensionally on the same chip, resulting in multiple wavelength channels, with the farthest channel being around 3.1nm away from the center wavelength of the seed pulse. This technique can potentially enable wavelength division multiplexing at the chip level using a single off-chip laser.
Blue-emitting AlN:Eu2+ nitride phosphor for field emission displays
Authors: Hirosaki N. , Xie R.-J. , Inoue K. , Sekiguchi T. , Dierre B. , Tamura K.
Published: August 2007

An Eu2+-activated AlN phosphor was synthesized by firing the powder mixture of AlN, α-Si3N4, and Eu2O3 at 2050°C for 4h under 1.0MPa N2. This nitride phosphor emits a strong blue color with the chromaticity coordinates of x=0.139 and y=0.106 at an accelerating voltage of 3kV. The cathodoluminescence properties of AlN:Eu2+ was evaluated by utilizing it in the Spindt-type field emission display panel. It shows that the nitride phosphor exhibits higher brightness, higher color purity, lower saturation, and longer lifetime than the currently used Y2SiO5:Ce3+, indicative of the suitability of the AlN:Eu2+ blue phosphor in field emission displays.