bluuebunny/crossref_metadata_2025 huggingface dataset:
Eulerian and Lagrangian statistics in the Bluelink numerical model and AVISO altimetry: Validation of model eddy kinetics
Authors: Chiswell Stephen M. , Rickard Graham J.
Published: October 2008

We compare Eulerian and Lagrangian timescales, velocity autocorrelations, and spectra from 14 years of hindcast ocean model and satellite velocities with direct observations. Our main aim is to determine how well the model (Bluelink, version Bran 2.1) and satellite‐derived currents (Archiving, Validation and Interpretation of Satellite Oceanographic data (AVISO)) simulate mesoscale eddy variability, which is important in mixing heat, salinity, and momentum and in controlling dispersion. Ocean Eulerian observations are from Topex‐Poseidon (T/P) Crossover velocities, and Lagrangian observations are from Global Drifter Program (GDP) surface drifters. Eulerian spectra from Bluelink are similar to Eulerian spectra from Crossover velocities over the range of common frequencies (the ∼10‐day sampling from T/P limits the comparison to low frequencies), and Eulerian timescales from Bluelink are slightly shorter than from the observations. Part of this difference may be an effect of the T/P sampling rate. In contrast, Lagrangian spectra from Bluelink are considerably steeper than spectra from GDP drifters, and consequently, Lagrangian timescales from Bluelink are longer than from the observations. Both Eulerian and Lagrangian spectra from AVISO are steeper than from observations so that both Eulerian and Lagrangian AVISO timescales are longer than observed. The real ocean is an ocean where the Eulerian‐Lagrangian relationship described by Middleton (1985) is followed; the Tasman Sea averaged ratio of Lagrangian to Eulerian timescales is about 0.1 zonally and 0.15 meridionally. However, the Eulerian‐Lagrangian relationship described by Middleton (1985) is not obeyed by Bluelink, and we believe that this behavior in Bluelink reflects a loss of short‐wavelength eddies in the model.
A three‐dimensional analytic model for the scattering of a spherical bush
Authors: Dickinson Robert E. , Zhou Liming , Tian Yuhong , Liu Qing , Lavergne Thomas , Pinty Bernard , Schaaf Crystal B. , Knyazikhin Yuri
Published: October 2008

Advanced climate models require a more realistic description of canopy radiation with reasonable computational efficiency. This paper develops the mathematics of scattering from a spherical object conceptualized to be a spherical bush to provide a building block that helps to address this need of climate models. It is composed of a homogeneous distribution of individual smaller objects that scatter isotropically. In the limit of small optical depth, incident radiation will scatter isotropically as the sum of that scattered by all the individual scatterers, but at large optical depth the radiation leaving the spherical bush in a given direction is reduced by mutual shadowing of the smaller objects. In the single scattering limit, the scattering phase function and so the albedo are obtained by simple but accurate analytic expressions derived from analytic integration and numerical evaluation. Except in the limit of thin canopies, the scattering and hence albedos are qualitatively and quantitatively different than those derived from 1‐D modeling.
A simple surface conductance model to estimate regional evaporation using MODIS leaf area index and the Penman‐Monteith equation
Authors: Leuning Ray , Zhang Y. Q. , Rajaud Amelie , Cleugh Helen , Tu Kevin
Published: October 2008

We introduce a simple biophysical model for surface conductance, Gs, for use with remotely sensed leaf area index (Lai) data and the Penman‐Monteith (PM) equation to calculate daily average evaporation, E, at kilometer spatial resolution. The model for Gs has six parameters that represent canopy physiological processes and soil evaporation: gsx, maximum stomatal conductance; Q50 and D50, the values of solar radiation and atmospheric humidity deficit when the stomatal conductance is half its maximum; kQ and kA, extinction coefficients for visible radiation and available energy; and f, the ratio of soil evaporation to the equilibrium rate corresponding to the energy absorbed at the soil surface. Model parameters were estimated using 2–3 years of data from 15 flux station sites covering a wide range of climate and vegetation types globally. The PM estimates of E are best when all six parameters in the Gs model are optimized at each site, but there is no significant reduction in model performance when Q50, D50, kQ, and kA are held constant across sites and gsx and f are optimized (linear regression of modeled mean daily evaporation versus measurements: slope = 0.83, intercept = 0.22 mm/d, R2 = 0.80, and N = 10623). The average systematic root‐mean‐square error in daytime mean evaporation was 0.27 mm/d (range 0.09–0.50 mm/d) for the 15 sites. The average unsystematic component was 0.48 mm/d (range 0.28–0.71 mm/d). The new model for Gs with two parameters yields better estimates of E than an earlier, simple model Gs = cLLai, where cL is an optimized parameter. Our study confirms that the PM equation provides reliable estimates of evaporation rates from land surfaces at daily time scales and kilometer space scales when remotely sensed leaf area indices are incorporated into a simple biophysical model for surface conductance. Developing remote sensing techniques to measure the temporal and spatial variation in f is expected to enhance the utility of the model proposed in this paper.
Nonlinear attenuation and rock damage during strong seismic ground motions
Authors: Sleep Norman H. , Hagin Paul
Published: October 2008

Strong seismic waves cause nonlinear behavior in the shallow subsurface in fractured rocks. Seismologists use low‐amplitude signals from small repeating earthquakes to measure S wave velocity decrease after strong motion. The 2004 Parkfield, California, earthquake provides examples of such velocity changes in fractured sandstone with an S wave velocity of ∼300 m s−1. This nonlinear behavior occurred around the wave number depth of the incident waves, ∼30 m, for the ∼10 s−1 dominant angular frequency on a velocity seismogram. The low‐amplitude S wave velocity gradually recovered with the logarithm of time. The attenuation of strong waves in general depends nonlinearly on their amplitude. High dynamic stress triggered small, very shallow earthquakes, at sites including Parkfield. The theoretical frictional behavior of a fractured medium with heterogeneous prestress relates these phenomena. Failure occurs in the highly prestressed domains causing small earthquakes and opening‐mode cracks. The energy to dilate the cracks dissipates a significant fraction of the incoming seismic energy. The local high‐porosity domains close with the logarithm of time, as expected from the aging law of rate and state friction, increasing the S wave velocity. The domain model indicates that nonlinear effects increase gradually over a range of dynamic Coulomb stresses as observed and as included in the widely used Masing rules. The Linker and Dieterich (1992) relationship provides the maximum sustainable dynamic coefficient of friction needed to utilize the Masing rules. This parameter is the coefficient of friction at a laboratory normal traction plus a constant ∼0.15 times the logarithm of the ratio of field normal traction to the laboratory normal traction. It is helpful to relate S wave velocity to starting frictional strength, as coefficient of friction near the quarter‐wavelength depth determines nonlinear behavior. Then the dynamic coefficient of friction and equivalently the maximum sustainable acceleration at the dominant frequency depend weakly on S wave velocity. For example, the coefficient of friction at an angular frequency of 10 s−1 is less than 1.5 for rocks ranging from Parkfield sandstone to intact granite.
Trends in Saharan dust and tropical Atlantic climate during 1980–2006
Authors: Foltz Gregory R. , McPhaden Michael J.
Published: October 2008

Trends in tropical Atlantic sea surface temperature (SST), Sahel rainfall, and Saharan dust are investigated during 1980–2006. This period is characterized by a significant increase in tropical North Atlantic SST and the transition from a negative to a positive phase of the Atlantic multidecadal oscillation (AMO). It is found that dust concentrations over western Africa and the tropical North Atlantic Ocean decreased significantly between 1980 and 2006 in association with an increase in Sahel rainfall. The decrease in dust in the tropical North Atlantic tended to increase the surface radiative heat flux by 0.7 W m−2 which, if unbalanced, would lead to an increase in SST of 3°C. Coupled models significantly underestimate the amplitude of the AMO in the tropical North Atlantic possibly because they do not account for changes in Saharan dust concentration.
Simulated impact of altered Southern Hemisphere winds on the Atlantic Meridional Overturning Circulation
Authors: Delworth Thomas L. , Zeng Fanrong
Published: October 2008

Previous work has suggested that the strength and latitudinal position of the Southern Hemisphere (SH) mid‐latitude westerly winds has an important impact on climate and the Atlantic Meridional Overturning Circulation (AMOC). We probe this hypothesis by conducting ensembles of experiments using the GFDL CM2.1 coupled ocean‐atmosphere model with altered SH wind stress. We find, consistent with previous work, that enhanced (reduced) and poleward (equatorward) displaced SH westerly winds lead to an AMOC intensification (weakening). While the AMOC takes more than a century to respond fully to the altered SH winds, initial effects in the North Atlantic can occur within a few decades. The AMOC changes generate SST and surface air temperature responses in the North Atlantic and adjacent continental regions. In the Southern Hemisphere, the atmosphere responds to the altered ocean circulation with a further strengthening and poleward movement of the SH winds, thereby constituting a modest positive feedback.
Relationship between substorm activity and magnetic disturbances in two polar caps
Authors: Lyatskaya Sonya , Lyatsky W. , Khazanov G. V.
Published: October 2008

We examined the effect of magnetic disturbances in two polar caps on the generation of magnetospheric substorms. For this purpose we investigated the correlation between the AL index (showing substorm activity in the Northern hemisphere) and two geomagnetic activity indices, the Polar Cap (PC) index and Polar Magnetic (PM) index showing the magnetic disturbances in the Northern and Southern polar caps. For the analysis we used the data for four years when geomagnetic activity indices were available in both hemispheres. We obtained an unexpected yet important result: while in northern winter the correlation between AL index and northern PC/PM indices is very good, in northern summer the AL index correlates much better with southern PC/PM indices. Thus, substorm activity in summer months correlates much better with geomagnetic activity not in the nearby polar cap but in the opposite polar cap. This effect may be caused by the interhemispheric field‐aligned currents flowing from the summer high‐latitude ionosphere and closing through the ionosphere in the opposite auroral zone. An interesting feature of these interhemispheric currents is that they are directed opposite to the substorm field‐aligned currents in the summer hemisphere but along the substorm field‐aligned currents in the winter hemisphere. This leads to decreasing the total field‐aligned currents and their contribution to magnetic disturbances in the summer hemisphere but increasing these currents and related magnetic disturbances in winter hemisphere, which explains the experimental results obtained in our study.
AC magnetic susceptibility studies of Chinese red clay sediments between 4.8 and 4.1 Ma: Paleoceanographic and paleoclimatic implications
Authors: Nie Junsheng , King John , Jackson Mike , Fang Xiaomin , Song Yougui
Published: October 2008

Recent studies show that measuring AC magnetic susceptibility (with in‐phase χ′ and quadrature χ″ components) versus temperature provides an effective way to map the grain‐size distribution of pedogenic maghemite. Applying this method, we mapped the pedogenic maghemite grain‐size distribution of samples of the red clay sediments deposited on the Chinese Loess Plateau (CLP) during the interval 4.8–4.1 Ma, during which the validity of magnetic susceptibility as an East Asian summer monsoon precipitation intensity proxy has been questioned. The results show that the pedogenic maghemite grain‐size distribution is the same as in the overlying loess‐paleosol sequence and in the other intervals of the red clay sequence, suggesting similar pedogenesis mechanisms and marked precipitation seasonality on the CLP between 4.8 and 4.1 Ma compared with the other intervals. We argue that the seasonal position shift of the polar westerlies produces the required precipitation seasonality and thus produces a uniform pedogenic grain‐size distribution during this time interval. Reconstructing paleo‐rainfall on the central CLP on the basis of surface calibration results suggests that the intensification of the East Asian summer monsoon might be a key trigger for the subsequent significant ocean and cryosphere adjustments during the Pliocene.
Anomalous sulfur isotope compositions of volcanic sulfate over the last millennium in Antarctic ice cores
Authors: Baroni Mélanie , Savarino Joël , Cole‐Dai Jihong , Rai Vinai K. , Thiemens Mark H.
Published: October 2008

The reconstruction of past volcanism from glaciological archives is based on the measurement of sulfate concentrations in ice. This method does not allow a proper evaluation of the climatic impact of an eruption owing to the uncertainty in classifying an event between stratospheric or tropospheric. This work develops a new method, using anomalous sulfur isotope composition of volcanic sulfate in order to identify stratospheric eruptions over the last millennium. The advantages and limits of this new method are established with the examination of the 10 largest volcanic signals in ice cores from Dome C and South Pole, Antarctica. Of the 10, seven are identified as stratospheric eruptions. Among them, three have been known to be stratospheric (Tambora, Kuwae, the 1259 Unknown Event) and they exhibit anomalous sulfur isotope compositions. Three unknown events (circa 1277, 1230, 1170 A.D.) and the Serua eruption have been identified as stratospheric eruptions, which suggests for the first time that they could have had significant climatic impact. However, the Kuwae and the 1259 Unknown Event stratospheric eruptions exhibit different anomalous sulfur isotope compositions between South Pole and Dome C samples. Differences in sulfate deposition and preservation patterns between the two sites can help explain these discrepancies. This study shows that the presence of an anomalous sulfur isotope composition of volcanic sulfate in ice core indicates a stratospheric eruption, but the absence of such composition does not necessarily lead to the conclusion of a tropospheric process because of differences in the sulfate deposition on the ice sheet.
β‐Arrestin and casein kinase 1/2 define distinct branches of non‐canonical WNT signalling pathways
Authors: Bryja Vítězslav , Schambony Alexandra , Čajánek Lukáš , Dominguez Isabel , Arenas Ernest , Schulte Gunnar
Published: October 2008

Recent advances in understanding β‐catenin‐independent WNT (non‐canonical) signalling suggest an increasing complexity, raising the question of how individual non‐canonical pathways are induced and regulated. Here, we examine whether intracellular signalling components such as β‐arrestin (β‐arr) and casein kinases 1 and 2 (CK1 and CK2) can contribute to determining signalling specificity in β‐catenin‐independent WNT signalling to the small GTPase RAC‐1. Our findings indicate that β‐arr is sufficient and required for WNT/RAC‐1 signalling, and that casein kinases act as a switch that prevents the activation of RAC‐1 and promotes other non‐canonical WNT pathways through the phosphorylation of dishevelled (DVL, xDSH in Xenopus). Thus, our results indicate that the balance between β‐arr and CK1/2 determines whether WNT/RAC‐1 or other non‐canonical WNT pathways are activated.
Three-dimensional analysis of the structure and ecology of a novel, ultra-small archaeon
Authors: Comolli Luis R , Baker Brett J , Downing Kenneth H , Siegerist Cristina E , Banfield Jillian F
Published: October 2008

Fully understanding the biology of acid mine drainage (AMD) is central to our ability to control and manipulate its environmental impact. Although genomics and biogeochemical methods are relatively well established in the field, their combination with high-resolution imaging of intact members of microbial biofilm communities has not yet reached its full potential. Here, we used three-dimensional (3D) cryogenic electron tomography to determine the size and ultrastructure of intact ARMAN cells, a novel ultra-small archaeon, and sought evidence for their interactions with other members of its community. Within acid mine drainage biofilms, apparently free-living ARMAN cells from a deeply branched archaeal lineage have volumes of 0.009–0.04 μm3 (mean ∼0.03±0.01 μm3), only ∼92 ribosomes, yet are frequent hosts for replicating viruses. Organization within the periplasm and partitioning of ribosomes to the inner surface of the cytoplasmic membrane may be factors in size minimization. Most cells contain enigmatic tubular structures of unknown function. The low ribosome copy number per unit volume, indicative of slow growth rates and targeting of cells by diverse viruses may account for the low abundance of ARMAN cells compared with other biofilm community members. Our results provide the first 3D analysis of structural features of these novel and enigmatic cells and their interactions with at least two types of viruses. Our findings also emphasize that new biological phenomena remain to be discovered among lower abundance organisms from novel uncultivated lineages.
Grand canonical Monte Carlo investigations of electrical double layer in molten salts
Authors: Lamperski Stanisław , Kłos Jacek
Published: October 2008

Results of the Monte Carlo simulation of the electrode/molten salt interface are reported. The system investigated was modeled by the restricted primitive model of electrolyte being in contact with the charged hard wall (hard spheres of diameter d=400pm and relative permittivity εr=10). The temperature analysis of the mean activity coefficient γ±, heat capacity Cv and radial distribution function, g, indicated the range of temperatures of the study. Calculations for the electrode/electrolyte interface were carried out for temperatures 1300, 1400, and 1500K and in the range of the electrode charge densities σ from 0.025to0.5Cm−2. Singlet distribution functions showed a multilayer structure of the electrolyte in the vicinity of the electrode surface. The structure depended on the electrode charge, but not much on temperature. The capacitance curves had a parabolalike shape with the maximum located at σ=0. This result is not consistent with the Gouy–Chapman theory, but has been confirmed by the modified Poisson–Boltzmann theory, which includes the correlation and exclusion volume effects.
Some comments on the second virial coefficient of semiflexible polymers
Authors: Ida Daichi , Yoshizaki Takenao
Published: October 2008

A Monte Carlo study is made of the mean-square radius of gyration ⟨S2⟩ and second virial coefficient A2 for the two freely rotating chains with the Lennard-Jones (LJ) 6-12 potential and the hard-sphere (HS) one in the range of the bond angle θ from 109° (typical flexible chain) to 175° (typical semiflexible or stiff chain) and in the range of the number n of bonds from 6 to 1000. It is shown that a value may be properly assigned to the collision diameter of the HS potential so that ⟨S2⟩ of the chain with the HS potential agrees well with that of the chain with the LJ one whose parameter values correspond to a good-solvent condition irrespective of the chain stiffness. It is then found that A2 of the latter chain becomes remarkably smaller than that of the former as the chain stiffness is increased. The result implies that the binary-cluster approximation does not seem to work well for typical semiflexible and stiff polymers.
Density functional localized orbital corrections for transition metals
Authors: Rinaldo David , Tian Li , Harvey Jeremy N. , Friesner Richard A.
Published: October 2008

This paper describes the development of the B3LYP localized orbital correction model which improves the accuracy of the B3LYP thermochemical predictions for compounds containing transition metals. The development of this model employs a large data set containing 36 experimental atomic energies and 71 bond dissociation energies. B3LYP calculations were carried out on these systems with different basis sets. Based on an electronic structure analysis and physical arguments, we built a set of 10 parameters to correct atomic data and a set of 21 parameters to correct bond dissociation energies. Using the results from our biggest basis set, the model was shown to reduce the mean absolute deviation from 7.7to0.4kcal∕mol for the atomic data and from 5.3to1.7kcal∕mol for the bond dissociation energies. The model was also tested using a second basis set and was shown to give relatively accurate results too. The model was also able to predict an outlier in the experimental data that was further investigated with high level coupled-cluster calculations.
Zinc oxide-chitosan nanobiocomposite for urea sensor
Authors: Solanki Pratima R. , Kaushik Ajeet , Ansari Anees A. , Sumana G. , Malhotra B. D.
Published: October 2008

Zinc oxide (ZnO)-chitosan (CH) nanobiocomposite film onto indium-tin-oxide (ITO) coated glass has been used to immobilize urease (Urs) and glutamate dehydrogenase (GLDH) for urea detection. The presence of ZnO nanoparticles in CH results in its increased surface area and enhanced electron transfer kinetics. The Urs-GLDH/CH-ZnO/ITO bioelectrode characterized using electrochemical, Fourier transform infrared, and scanning electron microscopy studies exhibit linearity of 5–100mg∕dl, detection limit of 3mg∕dl, response time of 10s, reproducibility as 20 times, and shelf life of 3months. The low Michaelis–Menten constant (Km) value (4.92mg∕dl) indicates enhanced affinity of enzyme with nanobiocomposite.
Optical observation of the C, 3sσgF3, and 3pπuG3 Π3u states of N2
Authors: Lewis B. R. , Baldwin K. G. H. , Sprengers J. P. , Ubachs W. , Stark G. , Yoshino K.
Published: October 2008

High-resolution laser-based one extreme-ultraviolet (EUV)+one UV two-photon ionization spectroscopy and EUV photoabsorption spectroscopy have been employed to study spin-forbidden Π3u-X Σ1g+(v,0) transitions in N142 and N152. Levels of the C Π3u valence and 3sσgF3 and 3pπuG3 Π3u Rydberg states are characterized, either through their direct optical observation, or, indirectly, through their perturbative effects on the Π1u and Σ1u+ states, which are accessible in dipole-allowed transitions. Optical observation of the G3-X(0,0) and (1,0) transitions is reported for the first time, together with evidence for six new vibrational levels of the C state. Following the recent observation of the F3-X(0,0) transition at rotational resolution [J. P. Sprengers et al., J. Chem. Phys. 123, 144315 (2005)], the F3(v=1) level is found to be responsible for a local perturbation in the rotational predissociation pattern of the b′ Σ1u+(v=4) state. Despite their somewhat fragmentary nature, these new observations provide a valuable database on the Π3u states of N2 and their interactions which will help elucidate the predissociation mechanisms for the nitrogen molecule.
A coupled-channel model of the Π3u states of N2: Structure and interactions of the 3sσgF3 Π3u and 3pπuG3 Π3u Rydberg states
Authors: Lewis B. R. , Heays A. N. , Gibson S. T. , Lefebvre-Brion H. , Lefebvre R.
Published: October 2008

New and existing spectroscopic data on N2, obtained using a wide variety of experimental techniques, are interpreted using a coupled-channel Schrödinger-equation (CSE) model of the structure and predissociation dynamics for the interacting Rydberg and valence states of Π3u symmetry. As a result, v>0 levels of the 3pπuG3 Π3u Rydberg state are assigned correctly for the first time, leading to the identification of very strong perturbations in the G3-state vibrational structure. A four-channel CSE model, which includes the 3sσgF3 Π3u and 3pπuG3 Π3u Rydberg states and the C′ Π3u and C Π3u valence states, indicates strong Rydberg–Rydberg coupling between the F3 and G3 states, strong Rydberg-valence coupling between the G3 and C′ states, and weaker coupling between the F3 and C′ states.
Sign reversal of the spin-orbit constant for the C Π3u state of N2
Authors: Ndome H. , Hochlaf M. , Lewis B. R. , Heays A. N. , Gibson S. T. , Lefebvre-Brion H.
Published: October 2008

Ab initio calculations are performed at the multireference configuration-interaction level of theory on the diagonal spin-orbit functions for the lowest non-Rydberg states of Π3u symmetry in molecular nitrogen. Spin-orbit constants deduced from the ab initio results confirm the recent suggestion, based on new experimental results, that the C Π3u state of N2, long known to be regular in the region of its potential-energy curve minimum, becomes inverted at higher energies. By removing the effects of the crossing C′ Π3u state, it is shown that Av for the C state changes sign from positive to negative near v=8, corresponding to a change in principal molecular-orbital configuration from (1σg)2(1σu)2(2σg)2(2σu)(3σg)2(1πu)4(1πg) to (1σg)2(1σu)2(2σg)2(2σu)2(3σg)(1πu)3(1πg)2 at an internuclear distance near 1.4 Å.
Energy landscapes for diffusion: Analysis of cage-breaking processes
Authors: de Souza Vanessa K. , Wales David J.
Published: October 2008

A wide spectrum of potential energy barriers exists for binary Lennard-Jones systems. Here we examine the barriers and cage-breaking rearrangements that are pertinent to long-term diffusion. Single-step cage-breaking processes, which follow high-barrier routes, are identified, and different methods and criteria for defining a cage-breaking process are considered. We examine the extent to which a description of cage-breaking within the energy landscape is a description of long-term diffusion. This description includes the identification of cage-breaks that are reversed, and those that are productive towards long-term diffusion. At low temperatures, diffusion is adequately described by productive cage-breaks, or by considering all cage-breaks and accounting for the effect of reversals. To estimate the diffusion constant we require only the mean square displacement of a cage-break, the average waiting time for a cage-break, and a measure of the number of reversed cage-breaks. Cage-breaks can be visualized within the potential energy landscape using disconnectivity graphs, and we compare the use of productive cage-breaks with previous definitions of “megabasins” or “metabasins.”
Dependence of the photoluminescence of annealed III-V semiconductor quantum dots on their shape and dimension
Authors: Kumar Subindu , Kabi Sanjib , Biswas Dipankar
Published: October 2008

Interdiffusion in III-V semiconductor quantum dots (QDs) may occur during growth and subsequent device processing steps. The photoluminescence (PL) spectra of InXGa1−XAs/GaAs and InXGa1−XN/GaN QDs change significantly on annealing. The size and shape of a QD dot are important parameters, which govern this change of the PL spectra. In this communication, we have investigated the effects of interdiffusion in realistic InXGa1−XAs/GaAs and InXGa1−XN/GaN QDs with various geometries which are of theoretical and practical interest such as pyramidal, truncated pyramidal, and lens shaped, through quantum mechanical computations.
Experimental study of domain wall motion in long nanostrips under the influence of a transverse field
Authors: Glathe S. , Berkov I. , Mikolajick T. , Mattheis R.
Published: October 2008

We report the real time study of field driven domain wall (DW) motion in giant magnetoresistance nanostrips under the influence of both longitudinal and transverse fields. We have found clear evidence that transverse fields influence the DW dynamics below and above the Walker field [N. L. Schryer and L. R. Walker, J. Appl. Phys. 45, 5406 (1974)] drastically. We show the suppression of the Walker breakdown process and a huge enhancement in DW velocity below the Walker field. The maximum velocity reported here is 4500 m/s.
Nonadiabatic orientation, toroidal current, and induced magnetic field in BeO molecules
Authors: Barth Ingo , Serrano-Andrés Luis , Seideman Tamar
Published: October 2008

It is predicted that oriented BeO molecules would give rise to unprecedentedly strong, unidirectional electric ring current and an associated magnetic field upon excitation by a right or left circularly polarized laser pulse into the first excited degenerate singlet state. The strong toroidal electric ring current of this state is dominated by the ring current of the 1π± orbital about the molecular axis. Our predictions are based on the analysis of the orbital composition of the states involved and are substantiated by high level electronic structure calculations and wavepacket simulations of the laser-driven orientation and excitation dynamics.
Growth of ZnO nanostructures on Au-coated Si: Influence of growth temperature on growth mechanism and morphology
Authors: Kumar R. T. Rajendra , McGlynn E. , Biswas M. , Saunders R. , Trolliard G. , Soulestin B. , Duclere J.-R. , Mosnier J. P. , Henry M. O.
Published: October 2008

ZnO nanostructures were grown on Au-catalyzed Si silicon substrates using vapor phase transport at growth temperatures from 800 to 1150 °C. The sample location ensured a low Zn vapor supersaturation during growth. Nanostructures grown at 800 and 850 °C showed a faceted rodlike morphology with mainly one-dimensional (1D) growth along the nanorod axis. Samples grown at intermediate temperatures (900, 950, and 1050 °C) in all cases showed significant three dimensional (3D) growth at the base of 1D nanostructures. At higher growth temperatures (1100 and 1150 °C) 3D growth tended to dominate resulting in the formation of a porous, nanostructured morphology. In all cases growth was seen only on the Au-coated region. Our results show that the majority of the nanostructures grow via a vapor-solid mechanism at low growth temperatures with no evidence of Au nanoparticles at their tip, in sharp contrast to the morphology expected for the vapor-liquid-solid (VLS) process often reported as the growth mechanism on Au-catalyzed Si. We see VLS growth only at 900 and 950 °C. Transmission electron microscopy data indicate that the nanorods are single crystalline without gross structural defects. Luminescence data reveal strong ultraviolet emission in all samples and weak defect emission in the visible region. We discuss the growth mechanisms with reference to various models in the literature and suggest reasons for VLS growth only in a narrow temperature range. We also discuss the potential effects of the Zn oxidation reaction on the growth morphologies, aspects largely ignored in the general literature on this subject.
A density matrix functional with occupation number driven treatment of dynamical and nondynamical correlation
Authors: Rohr Daniel R. , Pernal Katarzyna , Gritsenko Oleg V. , Baerends Evert Jan
Published: October 2008

A recently proposed series of corrections to the earliest JK-only functionals has considerably improved the prospects of density matrix functional theory (DMFT). Still, the most advanced of these functionals (correction C3) requires a preselection of the terms in the pair density Γ(r1,r2) involving the bonding and antibonding natural orbitals (NOs) belonging to an electron pair bond. Ideally, a DMFT functional should only depend on the NOs and their occupation numbers, and we propose a functional with an occupation number driven weighing of terms in the pair density. These are formulated as “damping” for certain ranges of occupation numbers of the two-electron cumulant that arises in the expansion of the two-particle density matrix of the paradigmatic two-electron system. This automatic version of C3, which we denote AC3, provides the correct dissociation limit for electron pair bonds and it excellently reproduces the potential energy curves of the multireference configuration interaction (MRCI) method for the dissociation of the electron pair bond in the series of the ten-electron hydrides CH4, NH3, H2O, and HF. AC3 reproduces closely the experimental equilibrium distances and at Re it yields correlation energies of the ten-electron systems with an average error in the absolute values of only 3.3% compared to the MRCI values. We stress the importance of treatment of strong correlation cases (NO occupation numbers differing significantly from 2.0 and 0.0) by appropriate terms in the cumulant.
Simulation study of the effects of grain shape and size on the performance of Cu(In,Ga)Se2 solar cells
Authors: Nerat M. , Černivec G. , Smole F. , Topič M.
Published: October 2008

A study of the effects of grain shape, size, and recombination activity in the thin film polycrystalline Cu(In,Ga)Se2 absorber layer on a solar cell’s performance by means of computer-aided simulations in two dimensions is presented. This paper addresses the influence of the grain boundary area with its spatial position and electronic activity on the output performance. Simulation results indicate the highest conversion efficiency of a solar cell when grains are large enough to extend beyond the entire absorber thickness and the surface recombination velocity at the grain boundaries is below 103 cm s−1. The influence of the grain boundaries was found to be most detrimental when placed in the space charge region close to the CdS/Cu(In,Ga)Se2 heterointerface. Simulations indicate that this also holds true for differently shaped grains. The output performance depends primarily on the amount and electronic activity of grain boundaries within the space charge region regardless of the shape of grains.
Dynamics of the reaction C(P3)+SiH4: Experiments and calculations
Authors: Lu I-Chung , Chen Wei-Kan , Huang Wen-Jian , Lee Shih-Huang
Published: October 2008

We conducted the reaction C(P3)+SiH4 at a collision energy of 4.0 kcal mol−1 in a crossed molecular-beam apparatus measuring time-of-flight mass spectra and selective photoionization. Product ions with m/z=41–43 are associated with two product channels, H2SiCH/HSiCH2/SiCH3+H and H2SiC/HSiCH/SiCH2+H2. Apart from daughter ions and isotopic variants of reaction products, the species observed at m/z=43 is assigned to product H2SiCH/HSiCH2/SiCH3 and that at m/z=42 to product H2SiC/HSiCH/SiCH2. The signals observed at m/z=41 are due to dissociative ionization of silicon-carbon hydrides of these two types. We report time-of-flight spectra of products at specific laboratory angles and theoretical simulations, from which both kinetic-energy and angular distributions of products in the center-of-mass frame were derived. The release of kinetic energy is weakly dependent on the scattering angle for these two reactions. The channels for loss of H and H2 release average translational energies of 10.5 and 16.7 kcal mol−1, respectively. As hydrogen transfer before decomposition is facile, products H2SiCH/HSiCH2/SiCH3 and H2SiC/HSiCH/SiCH2 exhibit mildly forward/backward preferred and isotropic angular distributions, respectively. We estimate the branching ratios of these channels for loss of H and H2 to be roughly 6:4. The measurements of release of kinetic energy and ionization thresholds of products indicate that SiCH3(A2″) and SiCH2(A32) are dominant among isomeric products. To explore the reaction mechanism, we computed the potential-energy surfaces for the reaction C(P3)+SiH4. The most likely mechanism is that atom C P3 inserts into bond Si–H of SiH4 in the entrance channel, and the reaction complex H3SiCH subsequently isomerizes to HSiCH3 followed by decomposition to SiCH3(A2″)+H and SiCH2(A32)+H2. We observed no significant evidence for the reaction C(D1)+SiH4.
Electrostatic drift modes in quantum pair plasmas
Authors: Ren Hai Jun , Cao Jintao , Wu Zhengwei
Published: October 2008

Electrostatic drift waves in a nonuniform quantum magnetized electron-positron (pair) plasma are investigated. An explicit and straightforward analytical expression of the fluctuation frequency is presented. The effects induced by quantum fluctuations, density gradients, and magnetic field inhomogeneity on the wave frequencies are discussed and a purely quantum drift mode appears. The present analytical investigations are relevant to dense astrophysical objects as well as laboratory ultracold plasmas.
Terahertz transmission lines based on surface waves in plasmonic waveguides
Authors: Ghamsari Behnood G. , Majedi A. Hamed
Published: October 2008

This paper studies the guided-wave transmission of terahertz electromagnetic waves along the surface waveguides and parallel-plate waveguides (PPWGs). Field analysis is presented to show that low-loss and quasi-TEM propagation is only possible when the waveguides are operating in their plasmonic modes. It is shown that the observed low-loss and quasi-TEM propagation of terahertz waves in the PPWGs is due to resonant coupling of surface plasmons between the two plates. Transmission line models based on the peculiar characteristics of the surface waves associated with the plasmonic modes of the guides are developed and the distinctions with the conventional transmission line theory are highlighted.
The formation and electronic structures of 3d transition-metal atoms doped in silicon nanowires
Authors: Xu Qiang , Li Jingbo , Li Shu-Shen , Xia Jian-Bai
Published: October 2008

Using first-principles methods, we systematically study the mechanism of defect formation and electronic structures for 3d transition-metal impurities (V, Cr, Mn, Fe, and Co) doped in silicon nanowires. We find that the formation energies of 3d transition-metal impurities with electrons or holes at the defect levels always increase as the diameters of silicon nanowires decrease, which suggests that self-purification, i.e., the difficulty of doping in silicon nanowires, should be an intrinsic effect. The calculated results show that the defect formation energies of Mn and Fe impurities are lower than those of V, Cr, and Co impurities in silicon nanowires. It indicates that Mn and Fe can easily occupy substitutional site in the interior of silicon nanowires. Moreover, they have larger localized moments, which means that they are good candidates for Si-based dilute magnetic semiconductor nanowires. The doping of Mn and Fe atom in silicon nanowires introduces a pair of energy levels with t2 symmetry. One of which is dominated by 3d electrons of Mn or Fe, and the other by neighboring dangling bonds of Si vacancies. In addition, a set of nonbonding states localized on the transition-metal atom with e symmetry is also introduced.
Direct imaging of current paths in multiwalled carbon nanofiber polymer nanocomposites using conducting-tip atomic force microscopy
Authors: Trionfi A. , Scrymgeour D. A. , Hsu J. W. P. , Arlen M. J. , Tomlin D. , Jacobs J. D. , Wang D. H. , Tan L.-S. , Vaia R. A.
Published: October 2008

Using conducting-tip atomic force microscopy (C-AFM), we study the spatial distribution of current paths and local electrical properties in carbon nanofiber/polymer nanocomposites. Previous studies of similar systems were hindered by a polymer-rich skin layer that exists at the nanocomposite surfaces. We present an experimental technique using oxygen plasma etching to controllably remove this polymer skin layer. After this treatment, we can directly probe the microscopic transport characteristics of the nanocomposite using C-AFM. The C-AFM results show that the electrical transport is solely carried by the carbon nanofiber (CNF) networks in the nanocomposites. In addition, high-resolution C-AFM maps show nonuniform distribution of current along the length of some CNFs, suggesting the presence of a heterogeneously distributed adsorbed polymer layer around nanofibers. Finally, two probe conductivity measurements in which one electrode (the C-AFM tip) is contacting a single constituent conducting particle were performed to study local conductivity. Results indicate that Ohmic pathways exist in the conducting network of the nanocomposite to the lowest measured nanofiber concentrations. However, non-Ohmic behavior indicating tunneling transport may also be present, especially near the percolation threshold.
Shock response of a gamma titanium aluminide
Authors: Shazly Mostafa , Prakash Vikas
Published: October 2008

Potential use of γ-TiAl alloys in aerospace and other structural applications require knowledge of their impact behavior for better evaluation and modeling. In the present study plate impact experiments are conducted using a single-stage gas gun to better understand the shock behavior of the recently developed class of gamma titanium aluminide alloys—the Gamma-Met PX. The Gamma-Met PX showed superior shock properties when compared to the conventional titanium aluminide alloys. The spall strength of Gamma-Met PX is 1.8±0.09 GPa, which is four to six times higher than those reported for other gamma titanium aluminide alloys. Moreover, it has a Hugoniot elastic limit of 1.88 GPa at a target thickness of 3.86 mm, which drops to 1.15 GPa at target thickness of 15.8 mm. The decay in the elastic precursor is continuous without showing an asymptote to a constant level within the range of target thicknesses studied.
High coercivity below percolation threshold in polymer nanocomposite
Authors: Shekhar S. , Sajitha E. P. , Prasad V. , Subramanyam S. V.
Published: October 2008

The magnetic properties of nanocomposites of polymer and magnetic (carbon cased iron carbide) nanoparticles have been studied from 300 to 10 K. The nanocomposites show an enhanced coercivity and they retain the magnetic properties at very low concentration of filler. The high coercivity is produced due to dipolar interaction between nanoparticles. A low value of Mr/Ms is observed for nanocomposites, which increases at low temperatures. The dependence of saturation magnetization on temperature follows Bloch’s T3/2 law and a departure from that is observed at higher temperatures. The Mössabauer spectra of nanocomposites show a sextet, which correspond to a ferromagnetic behavior of nanocomposites.
Ferromagnetic behavior in La(Cu3−xMnx)Mn4O12 (x=1,2) perovskites
Authors: Muñoz A. , Martínez-Lope M. J. , Retuerto M. , Falcón H. , Alonso J. A.
Published: October 2008

High oxygen pressure techniques were used to prepare two ferromagnetic compounds of the series La(Cu3−xMnx)Mn4O12 (x=1 and x=2). In a previous work, the end member LaCu3Mn4O12 was reported to be cubic, space group Im3¯, showing a ferrimagnetic structure driven by antiferromagnetic coupling between Cu and Mn magnetic moments below TC=361 K. The partial replacement of Cu2+ by Mn3+ cations does not change the symmetry or space group of La(Cu3−xMnx)Mn4O12. However, the introduction of Mn3+ cations at the square-planar 6b site, occupied by Cu, increases the valence mixing of Mn3+ and Mn4+ ions at the octahedral 8c site. This drives to an increase in the bonding distances in the MnO6 octahedron and a decrease of the Cu–O distance in the CuO4 polyhedron. Susceptibility measurements indicate that La(Cu3−xMnx)Mn4O12 becomes magnetically ordered below TC=310 K for x=1 and below TC=150 K for x=2. Neutron powder diffraction (NPD) data reveal a parallel arrangement between the (Cu/Mn)6b and Mn8c moments, in contrast with the antiferromagnetic arrangement described in LaCu3Mn4O12. At low temperature, the ordered magnetic moments determined from NPD data on the 8c Mn site are 2.66(4)μB and 2.17(6)μB for x=1 and x=2, respectively. For the 6b site, the magnetic moments of the Cu/Mn atoms are 0.72(5)μB and 0.75(7)μB for x=1 and x=2, respectively.
Magnetotransport properties of inverse-spinel CrxFe3−xO4 thin films
Authors: Kim Kwang Joo , Lee Hee Jung , Lee Jung Han , Lee Seungho , Kim Chul Sung
Published: October 2008

Magnetotransport properties of CrxFe3−xO4 (x≤0.95) thin films prepared using a sol-gel method were measured and analyzed in comparison with their magnetic properties. The samples were polycrystalline and exhibited a slight decrease in lattice constant as the Cr composition (x) increases. The observed decreasing trend in the saturation magnetization (Ms) of CrxFe3−xO4 with increasing x can be explained in terms of the decrease in net spin magnetic moment due to the substitution of Cr3+(3 μB) for octahedral Fe3+(5 μB). The magnetoresistance (MR) of the CrxFe3−xO4 films was found to increase in a linear manner with increasing external field (H≤5 kOe) while the observed magnetization of the samples started to saturate near H=2 kOe. For a given H, MR decreases with increasing x. The observed increase in MR with increasing H in CrxFe3−xO4 is mainly attributable to the reduction in tunneling resistance of spin-polarized carriers through grain boundaries (GBs). Transition layers of magnetization formed around magnetic defects in the GBs are likely to be connected to the spin-dependent tunneling. The decrease in MR with increasing x is attributable to the disorder-induced scattering.
Electrical characterization of an inductively coupled gaseous electronics conference reference cell
Authors: Singh S. V. , Pargmann C.
Published: October 2008

Plasma parameters and particularly the external electrical operational parameters are examined with respect to mode transition and hysteresis. The external electrical parameters such as antenna current (Ic) and antenna voltage (Vc) amplitudes are measured after the matching network and discussed for an inductively coupled argon discharge. A wide range of discharge conditions by varying applied power (up to 150 W at 13.56 MHz), gas pressure (0.7–7.5 Pa), and electrostatic coupling strength are measured for a gaseous electronics conference radio frequency reference cell. The effect on the power coupling efficiency by varying electrostatic coupling strength is studied via implementing two distinct grounded Faraday shields, in addition to the original nonshielded condition. A brief discussion on the evolution of floating potential, plasma potential, electron density, and electron energy distribution function with power and pressure is also presented mainly in context to mode transitions and hysteresis. Relatively smooth transitions in the plasma parameters and in the external electrical parameters are measured close to H to E mode transition region. Contrary to plasma parameters, however, the reverse transition from E to H mode was found to be abrupt in external parameters. The plasma parameters are measured using a commercial Langmuir probe, whereas the antenna currents and voltages are measured using a homebuilt Rogowski coil and capacitor divider, respectively.
Picovoltmeter for probing vortex dynamics in a single weak-pinning Corbino channel
Authors: Heitmann T. W. , Yu K. , Song C. , DeFeo M. P. , Plourde B. L. T. , Hesselberth M. B. S. , Kes P. H.
Published: October 2008

We have developed a picovoltmeter using a Nb dc superconducting quantum interference device for measuring the flux-flow voltage from a small number of vortices moving through a submicron weak-pinning superconducting channel. We have applied this picovoltmeter to measure the vortex response in a single channel arranged in a circle on a Corbino disk geometry. The circular channel allows the vortices to follow closed orbits without encountering any sample edges, thus eliminating the influence of entry barriers.