bluuebunny/crossref_metadata_2025 huggingface dataset:
Planning the Next Generation of Arctic Ecosystem Experiments
Authors: Wullschleger Stan D. , Hinzman Larry D. , Wilson Cathy J.
Published: August 2011

Climate Change Experiments in High‐Latitude Ecosystems; Fairbanks, Alaska, 13–14 October 2010; A 2‐day climate change workshop was held at the International Arctic Research Center, University of Alaska Fairbanks. The workshop, sponsored by Biological and Environmental Research, Office of Science, U.S. Department of Energy (DOE), was attended by 45 subject matter experts from universities, DOE national laboratories, and other federal and nongovernmental organizations. The workshop sought to engage the Arctic science community in planning for a proposed Next‐Generation Ecosystem Experiments (NGEE‐Arctic) project in Alaska (http://ngee.ornl.gov/). The goal of this activity is to provide data, theory, and models to improve representations of high‐latitude terrestrial processes in Earth system models. In particular, there is a need to better understand the processes by which warming may drive increased plant productivity and atmospheric carbon uptake and storage in biomass and soils, as well as those processes that may drive an increase in the release of methane (CH4) and carbon dioxide (CO2) through microbial decomposition of soil carbon stored in thawing permafrost. This understanding is required to quantify the important feedback mechanisms that define the role of terrestrial processes in regional and global climate.
Research Spotlight: Comparing different land surface heat flux estimates
Authors: Tretkoff Ernie
Published: August 2011

Land surface heat fluxes are an important component of Earth's energy and water cycle, and quantifying these fluxes can help scientists better understand climate change. These heat fluxes are affected by factors such as cloud cover, precipitation, surface radiation, air temperature, and humidity. Different methods are used to estimate monthly mean land surface heat flux. To determine how well these different methods agree with one other, Jiménez et al. present a detailed global intercomparison of 12 such products for the period 1993–1995. Some of these products are based on combining global satellite‐based data and physical formulations, while others come from atmospheric reanalysis and land surface models. The authors found that although there were some differences among the products, the products all captured the seasonality of the heat fluxes as well as the expected spatial distributions related to major climatic regimes and geographical features. Furthermore, the products correlate well with each other in general, in part due to large seasonable variability and the fact that some of the products use the same forcing data. (Journal of Geophysical Research‐Atmospheres, doi:10.1029/2010JD014545, 2011)
Pearn Peter Niiler (1937–2010)
Authors: Schmitz William J. , Large William G.
Published: August 2011

Peter Niiler, great friend, valued colleague, and AGU Fellow since 1986, will be remembered for his scientific achievements and as one of the most colorful physical oceanographers of his generation. Indeed, the Tropic Heat experiment's colors of mauve on pink, complete with Peter's matching ensemble, remain a vivid memory of that otherwise orthodox 1980s program championed by Peter to target equatorial Pacific mixing. Peter always sought understanding: to get things right for the right reason. Therefore, he held a healthy skepticism of numerical modeling and remote sensing, while utilizing both satellites and his own brand of diagnostic modeling. Peter's science motives were noble and would lead him to fight the good fight when necessary, especially when advocating his views on the Gulf Stream path, trapped inertial currents, heat and vorticity balances, and other subjects evoking his passion.
Recovery of Global Surface Weather Observations for Historical Reanalyses and International Users
Authors: Allan Rob , Compo Gil , Carton Jim
Published: August 2011

Third International Atmospheric Circulation Reconstructions Over the Earth Initiative Workshop: Reanalysis and Applications; Baltimore, Maryland, 3–5 November 2010; The third Atmospheric Circulation Reconstructions over the Earth (ACRE) workshop advanced the goals of the international ACRE initiative (http://www.met‐acre.org/) to undertake and facilitate the recovery of instrumental terrestrial and marine global surface weather observations underpinning global weather reconstructions and reanalyses spanning the past 200–250 years (http://reanalyses.org). The workshop improved integration of the 35–40 ACRE‐linked international scientific projects, institutions, and organizations working toward these ends. The meeting highlighted the broad array and international usage of ACRE‐facilitated data sets and reanalysis: the International Surface Pressure Databank (ISPD; http://dss.ucar.edu/datasets/ds132.0/), the International Comprehensive Ocean‐Atmosphere Data Set (http:/icoads.noaa.gov/ICOADS;), and the 20th Century Reanalysis (20CR; http://www.esrl.noaa.gov/psd/data/20thC_Rean/). The need for more data recovery for all regions of the globe during the nineteenth and early twentieth centuries was emphasized. Many regional efforts for such recovery are under way. The Arctic and maritime regions were highlighted as particular areas needing attention. As a result of the meeting, connections with existing projects were made and new efforts were started to address these needs.
Research Spotlight: New emissions scenarios say 2°C warming may be unavoidable
Authors: Schultz Colin
Published: August 2011

When it comes to modeling climate change, researchers rely on the specification of plausible emissions scenarios to explore how climate will change over the coming century. Using a standardized set of carbon dioxide and other greenhouse gas scenarios, researchers from different modeling centers can compare results and more methodically assess uncertainty in the Intergovernmental Panel on Climate Change (IPCC) reports. The emissions scenarios used in the past two IPCC reports were published in 2001 and need to be updated to take into account more recent socioeconomic modeling results. In a new study, Arora et al. use a completely new set of scenarios, referred to as representative concentration pathways (RCPs). These will form the basis for new climate projections to be assessed in the IPCC's Fifth Assessment Report (due out in 2014). Using an upgraded Earth system model—which takes into account carbon dioxide and other greenhouse gases, aerosols, land use change, and the flow of carbon between the atmosphere and the underlying ocean and land surface—the researchers are able to calculate the carbon dioxide emissions compatible with each RCP and, in particular, the emissions reductions required to meet certain levels of global warming. (Geophysical Research Letters, doi:10.1029/2010GL046270, 2011)
Geoscience Initiative Develops Sustainable Science in Africa
Authors: Nyblade Andrew A. , Durrheim Ray , Dirks Paul , Graham Gerhard , Gibson Roger , Webb Susan
Published: August 2011

AfricaArray (http://www.AfricaArray.org) is a 20‐year initiative in the geosciences to meet the African Union's New Partnership for Africa's Development (NEPAD) requirements for continent‐wide cooperation in human resources development and capacity building. The name AfricaArray refers to arrays of scientists working on linked projects across the continent, arrays of shared training programs and recording stations, and, above all, a shared vision that Africa will retain capacity in an array of technical and scientific fields vital to its sustainable development. AfricaArray officially launched in January 2005 and, with support from many public and private partners, has become multifaceted, promoting a broad range of educational and research activities and supporting a multiuser sensor network (Figure 1). Though fostering geophysics education and research in South Africa was its initial focus, AfricaArray has expanded to 17 countries and is now branching out into all areas of the geosciences (Earth, atmosphere, and space).
Research Spotlight: Could the cold winter of 2009–2010 have been predicted?
Authors: Tretkoff Ernie
Published: August 2011

In parts of North America and Europe, the winter of 2009–2010 was one of the coldest and snowiest in recent history. These extreme weather patterns were likely the result of the strongly negative phase of the North Atlantic Oscillation (NAO). Researchers wondered whether this negative NAO could have been predicted. Jung et al. explored the origin and predictability of the unusually cold winter using numerical simulations and the European Centre for Medium‐Range Weather Forecasts monthly forecasting system. They analyzed the possible role of various forcing mechanisms, including anomalies in sea surface temperature and sea ice, tropical atmospheric circulation, incoming solar radiation, and near‐surface temperatures. The researchers found that none of these factors could account for the observed NAO anomaly, which could explain why most operational forecasting systems had trouble predicting the strongly negative NAO winter. The researchers suggest that internal atmospheric variability was primarily responsible for the negative NAO during the 2009–2010 winter. (Geophysical Research Letters, doi:10.1029/2011GL046786, 2011)
Currents can be driven in the polar ionosphere
Authors: Balcerak Ernie
Published: August 2011

The study of ionospheric heaters that induce controlled modifications in the ionosphere allows better understanding of effects driven naturally by solar activity in the ionosphere and the radiation belts. Previous ionospheric heaters at the Earth's poles have generated ultralow‐frequency, extremely low frequency, and very low frequency waves in the ionosphere's D andE regions by modulating the auroral electrojet, the strong horizontal currents that naturally flow in the D and E regions at high latitudes. Now Papadopoulos et al. present theoretical and computational results that indicate that using high‐frequency heating, low‐frequency ionospheric currents can also be generated at F region altitudes, independent of the presence or absence of electrojet currents. The new technique, whose validity has been confirmed tentatively in recent polar experiments, allows generation of low‐frequency waves by midlatitude heaters, such as the one under construction in Arecibo, Puerto Rico, and their subsequent injection in the inner radiation belt. It will permit for the first time the study of the interaction of artificially generated low‐frequency waves with multi‐MeV protons trapped in the inner belt and their precipitation rates in the South Atlantic anomaly region. (Geophysical Research Letters, doi:10.1029/2011GL047368, 2011)
Validation of a Mental Health Assessment in an African Conflict Population
Authors: Ertl Verena , Pfeiffer Anett , Saile Regina , Schauer Elisabeth , Elbert Thomas , Neuner Frank
Published: August 2011

We studied the validity of the assessment of posttraumatic stress disorder (PTSD) and depression within the context of an epidemiological mental health survey among war-affected adolescents and young adults in northern Uganda. Local language versions of the Posttraumatic Diagnostic Scale (PDS) and the Depression section of the Hopkins Symptom Checklist (DHSCL) were administered by trained local interviewers. Correlations with probable predictor variables (i.e., trauma exposure), outcomes (e.g., impaired functioning), and local idioms of distress (i.e., spirit possession) were determined to estimate criterion-related construct validity. To assess convergent validity, expert clinicians reinterviewed a subsample using structured interviews (the Clinician Administered PTSD Scale [CAPS] and the Mini International Neuropsychiatric Interview [MINI]). Depression and PTSD symptoms as assessed by the local interviewers correlated with the context variables as predicted. After optimizing the scoring algorithm, we found good agreement between the PDS-based diagnoses and expert diagnoses. However, the concordance for depression diagnoses was not satisfactory. Results show that mental health assessments in African languages can produce reliable and valid data but that caution is warranted in the unevaluated transfer of cutoff scores and scoring algorithms.
Sustainable Well-Being: A Potential Synergy between Sustainability and Well-Being Research
Authors: Kjell Oscar N. E.
Published: August 2011

This article outlines a potential synergy between sustainability and well-being research. Currently aims within well-being and sustainability research focus on increasing well-being. First, sustainability is defined and important concepts within it highlighted, that is, aims, interdependencies, constraints, values and balanced adaptive processes. It is suggested that positioning well-being more clearly within the sustainability framework can enhance the role of sustainability; for example, in terms of aims and monitoring progress. In turn, the sustainability framework outlined, guides the second part of the article, illustrating how it can reciprocally enhance well-being research. That is, comprehensive empirical, evolutionary, cross-cultural, and self-conceptual evidence illustrate individuals’ interdependencies with other people and nature. Despite this, contemporary hedonic and eudaimonic well-being approaches and accompanying measures are demonstrated to be isolating; investigating well-being individualistically and in a decontextualized manner. This is in line with the individualistic and independent values of Western cultures. Therefore, it is suggested that employing the sustainability framework emphasizing interdependencies within well-being research can be beneficial; perhaps even resulting in an all-inclusive increase in well-being. Limitations are also raised and future research directions suggested. The author concludes that both sustainability and well-being research can benefit from the synergy toward sustainable well-being.
A Model of Meaning-Making Coping and Growth in Combat Veterans
Authors: Larner Brad , Blow Adrian
Published: August 2011

More than 1.6 million military men and women have deployed to fight the global war on terror. Although studies have suggested that approximately one third of these service men and women return with a mental health condition or a brain injury, a gap remains in our understanding about how these individuals cope with and grow from their experiences. In this article, we review the existing body of research related to growth and recovery from trauma and then propose an empirically informed and contextually sensitive model to guide future research with combat veterans. We draw from research focused on resilience, posttraumatic growth, and decline (negative or pathological) change trajectories, and we propose that meaning-making coping is a core mechanism of the posttraumatic growth process for combat veterans. Implications for practitioners and the next steps for future research are presented.
F1000 and data publishing
Authors: Lawrence Rebecca
Published: August 2011

Utility of the BioSharing standards and the BioDBCore registry in F1000 journal publishing.
The Effectiveness of Parent-Implemented Language Interventions: A Meta-Analysis
Authors: Roberts Megan Y. , Kaiser Ann P.
Published: August 2011

Purpose

The purpose of this meta-analysis was to systematically evaluate the effects of parent-implemented language interventions on the language skills of children between 18 and 60 months of age with primary and secondary language impairments.

Method

A systematic literature search yielded 18 studies that met the predetermined inclusion and exclusion criteria. Effect sizes for each study were calculated for 7 language outcome variables and analyzed using a random effects model. Separate analyses were conducted for each language outcome and for each comparison group. Outcomes were compared for children with and without intellectual disabilities and for parent report and direct observational language measures.

Results

The results indicate that parent-implemented language interventions have a significant, positive impact on receptive and expressive language skills of children with and without intellectual disabilities. Effect sizes (
g
) for child measures ranged from −0.15 to 0.82 depending on the outcome measure and comparison group.

Conclusion

The results of this review indicate that parent-implemented language interventions are an effective approach to early language intervention for young children with language impairments. Critical features of parent-implemented interventions are discussed in terms of implications for practice and future research.
Next Generation Utterance-Based Systems: What Do Pragmatic Studies Tell Us About System Design?
Authors: McCoy Kathleen F. , Hoag Linda , Bedrosian Jan
Published: August 2011

Utterance-based AAC systems hold the promise of faster/easier communication. Over the past 10 years, we have conducted a number of investigations into the kinds of pragmatic choices that a user of such a system will have to make and the effects of these choices on public attitudes and conversational behaviors. In this paper, we discuss some of the technological implications of the findings from these experiments.
Metal/graphene sheets as p-type transparent conducting electrodes in GaN light emitting diodes
Authors: Min Lee Jung , Yong Jeong Hae , Jin Choi Kyoung , Il Park Won
Published: August 2011

We demonstrate the use of graphene based transparent sheets as a p-type current spreading layer in GaN light emitting diodes (LEDs). Very thin Ni/Au was inserted between graphene and p-type GaN to reduce contact resistance, which reduced contact resistance from ∼5.5 to ∼0.6Ω/cm2, with no critical optical loss. As a result, LEDs with metal-graphene provided current spreading and injection into the p-type GaN layer, enabling three times enhanced electroluminescent intensity compared with those with graphene alone. We confirmed very strong blue light emission in a large area of the metal-graphene layer by analyzing image brightness.
Electronic structure of δ-Ta2O5 with oxygen vacancy: ab initio calculations and comparison with experiment
Authors: Ivanov Maxim V. , Perevalov Timofey V. , Aliev Vladimir S. , Gritsenko Vladimir A. , Kaichev Vasily V.
Published: August 2011

Electronic structure of oxygen vacancies in Ta2O5 have been studied theoretically by first-principles calculations and experimentally by x-ray photoelectron spectroscopy. Calculations of δ-Ta2O5 were performed using density functional theory within gradient-corrected approximation with the +U approach. Results indicate that the oxygen vacancy causes a defect level in the energy gap at 1.2 eV above the top of the valence band. To produce oxygen vacancies, amorphous films of Ta2O5 were bombarded with Ar+ ions. XPS results indicate that the Ar-ion bombardment leads to the generation of the oxygen vacancies in Ta2O5 that characterize the peak at 2 eV above the valence band. The calculated spectrum of crystalline δ-Ta2O5 demonstrates qualitative correspondence with the XPS spectrum of the amorphous Ta2O5 film after Ar-ion bombardment.
Controllable adhesion using field-activated fluids
Authors: Ewoldt Randy H. , Tourkine Piotr , McKinley Gareth H. , Hosoi A. E.
Published: August 2011

We demonstrate that field-responsive magnetorheological fluids can be used for variable-strength controllable adhesion. The adhesive performance is measured experimentally in tensile tests (a.k.a. probe-tack experiments) in which the magnetic field is provided by a cylindrical permanent magnet. Increasing the magnetic field strength induces higher peak adhesive forces. We hypothesize that the adhesion mechanism arises from the shear resistance of a yield stress fluid in a thin gap. This hypothesis is supported by comparing the experimentally measured adhesive performance to the response predicted by a lubrication model for a non-Newtonian fluid with a field-dependent yield stress. The model predictions are in agreement with experimental data up to moderate field strengths. Above a critical magnetic field strength the model over-predicts the experimentally measured values indicating non-ideal conditions such as local fluid dewetting from the surface.
Towards highly accurate ab initio thermochemistry of larger systems: Benzene
Authors: Harding Michael E. , Vázquez Juana , Gauss Jürgen , Stanton John F. , Kállay Mihály
Published: August 2011

The high accuracy extrapolated ab initio thermochemistry (HEAT) protocol is applied to compute the total atomization energy (TAE) and the heat of formation of benzene. Large-scale coupled-cluster calculations with more than 1500 basis functions and 42 correlated electrons as well as zero-point energies based on full cubic and (semi)diagonal quartic force fields obtained with the coupled-cluster singles and doubles with perturbative treatment of the triples method and atomic natural orbital (ANO) triple- and quadruple-zeta basis sets are presented. The performance of modifications to the HEAT scheme and the scaling properties of its contributions with respect to the system size are investigated. A purely quantum-chemical TAE and associated conservative error bar of 5463.0 ± 3.1 kJ mol−1 are obtained, while the corresponding 95% confidence interval, based on a statistical analysis of HEAT results for other and related molecules, is ± 1.8 kJ mol−1. The heat of formation of benzene is determined to be 101.5 ± 2.0 kJ mol−1 and 83.9 ± 2.1 kJ mol−1 at 0 K and 298.15 K, respectively.
Development of a 3-body:many-body integrated fragmentation method for weakly bound clusters and application to water clusters (H2O)n= 3 − 10, 16, 17
Authors: Bates Desiree M. , Smith Joshua R. , Janowski Tomasz , Tschumper Gregory S.
Published: August 2011

A 3-body:many-body integrated quantum mechanical (QM) fragmentation method for non-covalent clusters is introduced within the ONIOM formalism. The technique captures all 1-, 2-, and 3-body interactions with a high-level electronic structure method, while a less demanding low-level method is employed to recover 4-body and higher-order interactions. When systematically applied to 40 low-lying (H2O)n isomers ranging in size from n = 3 to 10, the CCSD(T):MP2 3-body:many-body fragmentation scheme deviates from the full CCSD(T) interaction energy by no more than 0.07 kcal mol−1 (or <0.01 kcal mol−1 per water). The errors for this QM:QM method increase only slightly for various low-lying isomers of (H2O)16 and (H2O)17 (always within 0.13 kcal mol−1 of the recently reported canonical CCSD(T)/aug-cc-pVTZ energies). The 3-body:many-body CCSD(T):MP2 procedure is also very efficient because the CCSD(T) computations only need to be performed on subsets of the cluster containing 1, 2, or 3 monomers, which in the current context means the largest CCSD(T) calculations are for 3 water molecules, regardless of the cluster size.
Controlled growth of SiGe nanowires by addition of HCl in the gas phase
Authors: Potié Alexis , Baron Thierry , Latu-Romain Laurence , Rosaz Guillaume , Salem Bassem , Montès Laurent , Gentile Pascal , Kreisel Jens , Roussel Hervé
Published: August 2011

Growth of Si, Ge, and, thus, SiGe nanowires (NWs) by catalyzed chemical vapor deposition proceeds at different process conditions, preventing easy realization of axial multijunctions interesting for device realization. In this paper, we propose a common process to obtain both Si, Ge, and alloyed NWs simply by adding HCl in the gas phase. It is demonstrated that addition of HCl during the growth improves the structural quality of the SiGe NWs, avoids the tapering of NWs by decreasing the uncatalyzed growth, increases the Ge fraction of the SiGe alloy NWs, and decreases the growth rate. A qualitative model based on the experimental results is proposed to explain the role of HCl during the growth. This model can be more generally applied to explain the tendency observed in the literature concerning the growth of SiGe alloyed NWs without HCl. It is based on a competition between adsorption, decomposition, and incorporation of Si and Ge in the catalyst. This competition is mainly regulated by the gas phase composition and by the reaction between the reactive species and the catalyst surface.
Experimental validation of a radio frequency photogun as external electron injector for a laser wakefield accelerator
Authors: Stragier X. F. D. , Luiten O. J. , van der Geer S. B. , van der Wiel M. J. , Brussaard G. J. H.
Published: August 2011

A purpose-built RF-photogun as external electron injector for a laser wakefield accelerator has been thoroughly tested. Different properties of the RF-photogun have been measured such as energy, energy spread and transverse emittance. The focus of this study is the investigation of the smallest possible focus spot and focus stability at the entrance of the plasma channel. For an electron bunch with 10 pC charge and 3.7 MeV kinetic energy, the energy spread was 0.5% with a shot-to-shot stability of 0.05%. After focusing the bunch by a pulsed solenoid lens at 140 mm from the middle of the lens, the focal spot was 40 μm with a shot-to-shot stability of 5 μm. Higher charge leads to higher energy spread and to a larger spot size, due to space charge effects. All properties were found to be close to design values. Given the limited energy of 3.7 MeV, the properties are sufficient for this gun to serve as injector for one particular version of laser wakefield acceleration, i.e., injection ahead of the laser pulse. These measured electron bunch properties were then used as input parameters for simulations of electron bunch injection in a laser wakefield accelerator. The arrival time jitter was deduced from measurements of the energy fluctuation, in combination with earlier measurements using THz coherent transition radiation, and is around 150 fs in the present setup. The bunch length in the focus, simulated using particle tracking, depends on the accelerated charge and goes from 100 fs at 0.1 pC to 1 ps at 50 pC. When simulating the injection of the 3.7 MeV electron bunch of 10 pC in front of a 25 TW laser pulse with a waist of 30 μm in a plasma with a density of 0.7 × 1024 m−3, the maximum accelerated charge was found to be 1.2 pC with a kinetic energy of ∼900 MeV and an energy spread of ∼5%. The experiments combined with the simulations show the feasibility of external injection and give a prediction of the output parameters that can be expected from a laser wakefield accelerator with external injection of electrons.
The effect of surface conductance on lateral gated quantum devices in Si/SiGe heterostructures
Authors: Lin Xi , Hu Jingshi , Lai Andrew P. , Zhang Zhenning , MacLean Kenneth , Dillard Colin , Xie Ya-Hong , Kastner Marc A.
Published: August 2011

Quantum dots in Si/SiGe heterostructures are expected to have relatively long electron spin decoherence times, because of the low density of nuclear spins and the weak coupling between nuclear and electron spins. We provide experimental evidence suggesting that electron motion in a conductive layer parallel to the two-dimensional electron gas, possibly resulting from the donors used to dope the Si quantum well, is responsible for the well-known difficulty in achieving well-controlled dots in this system. Charge motion in the conductive layer can cause depletion on large length scales, making electron confinement in the dot impossible, and can give rise to noise that can overwhelm the single-electron charging signal. Results of capacitance versus gate bias measurements to characterize this conductive layer are presented.
Direct thrust measurements and modelling of a radio-frequency expanding plasma thruster
Authors: Lafleur T. , Takahashi K. , Charles C. , Boswell R. W.
Published: August 2011

It is shown analytically that the thrust from a simple plasma thruster (in the absence of a magnetic field) is given by the maximum upstream electron pressure, even if the plasma diverges downstream. Direct thrust measurements of a thruster are then performed using a pendulum thrust balance and a laser displacement sensor. A maximum thrust of about 2 mN is obtained at 700 W for a thruster length of 17.5 cm and a flow rate of 0.9 mg s−1, while a larger thrust of 4 mN is obtained at a similar power for a length of 9.5 cm and a flow rate of 1.65 mg s−1. The measured thrusts are in good agreement with the maximum upstream electron pressure found from measurements of the plasma parameters and in fair agreement with a simple global approach used to model the thruster.
Chirped pulse interferometry for time resolved density and velocity measurements of laser produced plasmas
Authors: Prasad Y. B. S. R. , Barnwal S. , Naik P. A. , Chakera J. A. , Gupta P. D.
Published: August 2011

An optical diagnostic method to record the evolution of plasma density on picosecond time scale with ∼8 ps resolution in a single shot is presented. A chirped laser pulse of 200 ps is used for obtaining the time resolved interferograms to provide the temporal evolution of plasma density (1017/cm3) during the probe beam duration. It is shown that the plasma expansion velocity can also be measured from the diffraction pattern superimposed on the interferograms. Plasma velocity of ∼1.2 × 107 cm/s was estimated along with its temporal profile. The details of the method and the results are presented.
Annealing effects on CoFeB-MgO magnetic tunnel junctions with perpendicular anisotropy
Authors: Meng H. , Lum W. H. , Sbiaa R. , Lua S. Y. H. , Tan H. K.
Published: August 2011

We studied annealing effects on perpendicular anisotropy in CoFeB-MgO magnetic tunnel junctions. The results show that annealing is an effective method to improve the perpendicular anisotropy of a CoFeB-MgO system. It is found that a thicker CoFeB layer requires a higher annealing temperature to buildup its perpendicular anisotropy. However, perpendicular anisotropy could be seriously degraded if the annealing temperature is more than 350 °C. Our study suggests that CoFeB thickness should be optimized so that the required annealing temperature window for perpendicular anisotropy could match the annealing temperature for high magnetoresistance. In this work, the perpendicular anisotropy energy density of 2.5 × 106 erg/cm3 was achieved with tunnel magnetoresistive value exceeding 70%. The use of CoFeB films will enable the development of high density nonvolatile memory with size down to 30 nm.
Three-dimensional electronic spectroscopy of excitons in asymmetric double quantum wells
Authors: Davis J. A. , Hall C. R. , Dao L. V. , Nugent K. A. , Quiney H. M. , Tan H. H. , Jagadish C.
Published: August 2011

We demonstrate three-dimensional (3D) electronic spectroscopy of excitons in a double quantum well system using a three-dimensional phase retrieval algorithm to obtain the phase information that is lost in the measurement of intensities. By extending the analysis of two-dimensional spectroscopy to three dimensions, contributions from different quantum mechanical pathways can be further separated allowing greater insight into the mechanisms responsible for the observed peaks. By examining different slices of the complete three-dimensional spectrum, not only can the relative amplitudes be determined, but the peak shapes can also be analysed to reveal further details of the interactions with the environment and inhomogeneous broadening. We apply this technique to study the coupling between two coupled quantum wells, 5.7 nm and 8 nm wide, separated by a 4 nm barrier. Coupling between the heavy-hole excitons of each well results in a circular cross-peak indicating no correlation of the inhomogeneous broadening. An additional cross-peak is isolated in the 3D spectrum which is elongated in the diagonal direction indicating correlated inhomogeneous broadening. This is attributed to coupling of the excitons involving the two delocalised light-hole states and the electron state localised on the wide well. The attribution of this peak and the analysis of the peak shapes is supported by numerical simulations of the electron and hole wavefunctions and the three-dimensional spectrum based on a density matrix approach. An additional benefit of extending the phase retrieval algorithm from two to three dimensions is that it becomes substantially more reliable and less susceptible to noise as a result of the more extensive use of a priori information.
Ferromagnetic resonance investigation of Py/Cr multilayer system
Authors: Erkovan M. , Öztürk S. T. , Topkaya R. , Özdemir M. , Aktaş B. , Öztürk O.
Published: August 2011

The multilayer thin films consisting of alternating stacks of ferromagnetic Permalloy (Py) and ultra thin non-magnetic (sub-nanometer) spacer (Cr) layers have been investigated by using ferromagnetic resonance (FMR) and dc magnetization measurement techniques. The non-magnetic spacer layer thickness changes from 0.5 Å to 2.5 Å by 0.5 Å steps. Polycrystalline composite metallic films were grown on Si substrate by usual magnetron sputtering techniques at UHV conditions. The magnetic hysteresis curves were recorded by conventional dc magnetization. The ac and dc magnetic properties were investigated by using ferromagnetic resonance. FMR measurements were carried out for different directions of external dc magnetic field in order to search magnetic anisotropy. Two strong and well defined peaks (acoustic and optic mode) were observed in FMR spectrum as the magnetic field direction approaches to the film normal. Their positions and relative intensities helped to characterize coupling species. The magnetic parameters have been deduced by using a theoretical model. It has been found that the exchange coupling parameter between ferromagnetic layers through non-magnetic Cr spacer is ferromagnetic in nature and strongly decreases with increasing spacer layer thickness. The magnetic anisotropy parameters strictly depend on magnetic layer thickness while the dc magnetization is almost constant for a few nanometer thick Permalloy as well. We observed only ferromagnetic coupling between ferromagnetic layers in all samples.
Seniority and orbital symmetry as tools for establishing a full configuration interaction hierarchy
Authors: Bytautas Laimutis , Henderson Thomas M. , Jiménez-Hoyos Carlos A. , Ellis Jason K. , Scuseria Gustavo E.
Published: August 2011

We explore the concept of seniority number (defined as the number of unpaired electrons in a determinant) when applied to the problem of electron correlation in atomic and molecular systems. Although seniority is a good quantum number only for certain model Hamiltonians (such as the pairing Hamiltonian), we show that it provides a useful partitioning of the electronic full configuration interaction (FCI) wave function into rapidly convergent Hilbert subspaces whose weight diminishes as its seniority number increases. The primary focus of this study is the adequate description of static correlation effects. The examples considered are the ground states of the helium, beryllium, and neon atoms, the symmetric dissociation of the N2 and CO2 molecules, as well as the symmetric dissociation of an H8 hydrogen chain. It is found that the symmetry constraints that are normally placed on the spatial orbitals greatly affect the convergence rate of the FCI expansion. The energy relevance of the seniority zero sector (determinants with all paired electrons) increases dramatically if orbitals of broken spatial symmetry (as those commonly used for Hubbard Hamiltonian studies) are allowed in the wave function construction.
Non-contact and all-electrical method for monitoring the motion of semiconducting nanowires
Authors: Hoch S. W. , Montague J. R. , Bright V. M. , Rogers C. T. , Bertness K. A. , Teufel J. D. , Lehnert K. W.
Published: August 2011

The authors demonstrate an all-electric, non-contact method of monitoring the motion of semiconducting nanowires. This technique uses a microwave resonant circuit whose resonance is modulated by the varying capacitance between GaN nanowires and a metal tip. They estimate that the method is capable of detecting motion with a sensitivity of 1pm/Hz. They verify their technique by driving a nanowire and comparing the electrical signal to the nanowire motion determined from a scanning electron microscope image. Finally, they use this technique to measure the pressure dependence of a nanowire’s mechanical damping to be 8.37(0.08) Hz/Pa in the molecular flow regime.
Polymer-electrolyte gated graphene transistors for analog and digital phase detection
Authors: Sagar Adarsh , Balasubramanian Kannan , Burghard Marko , Kern Klaus , Sordan Roman
Published: August 2011

We present an alternating current (ac) circuit based on a misoriented bilayer graphene device for analog and digital phase detection. We exploit the ambipolar nature of the transfer characteristics of a misoriented bilayer graphene transistor. The transistor action here is realized using an electrochemical gate integrated into a solid polymer electrolyte layer. This unique combination provides a voltage gain close to unity under ambient conditions, which is one order of magnitude higher than that attainable in back-gated devices. The achieved gain provides sufficient sensitivity to detect phase differences between pairs of analog or digital signals.
Linewidth reduction in a spin-torque nano-oscillator caused by non-conservative current-induced coupling between magnetic layers
Authors: Gusakova D. , Quinsat M. , Sierra J. F. , Ebels U. , Dieny B. , Buda-Prejbeanu L. D. , Cyrille M.-C. , Tiberkevich V. , Slavin A. N.
Published: August 2011

We demonstrate by macrospin simulations that in a spin-torque nano-oscillator having synthetic antiferromagnet fixed layer, the non-conservative dynamic coupling between the free and fixed layers caused by spin-torque effect leads to a substantial reduction of the linewidth of the current-induced spin wave mode, involving oscillations in all three magnetic layers. By analysing the phase and amplitude noise extracted from the simulated signal, we prove that the obtained linewidth reduction is related to the reduction of the dimensionless non-linear amplitude-phase coupling parameter ν.
Linear and non-linear dynamics of entangled linear polymer melts by modified tunable coarse-grained level Dissipative Particle Dynamics
Authors: Yamanoi Mikio , Pozo Oliver , Maia Joao M.
Published: August 2011

Dissipative particle dynamics (DPD) is a well-known simulation method for soft materials and has been applied to a variety of systems. However, doubts have been cast recently on its adequacy because of upper coarse-graining limitations, which could prevent the method from being applicable to the whole mesoscopic range. This paper proposes a modified coarse-grained level tunable DPD method and demonstrates its performance for linear polymeric systems. The method can reproduce both static and dynamic properties of entangled linear polymer systems well. Linear and non-linear viscoelastic properties were predicted and despite being a mesoscale technique, the code is able to capture the transition from the plateau regime to the terminal zone with decreasing angular frequency, the transition from the Rouse to the entangled regime with increasing molecular weight and the overshoots in both shear stress and normal-stress differences upon start-up of steady shear.
Response to “Comment on ‘On the role of dissipation on the Casimir-Polder potential between molecules in dielectric media’” [J. Chem. Phys. 135, 047101 (2011)]
Authors: Rodriguez Justo J. , Salam A.
Published: August 2011

In this Response to the Comment by Dalvit and Milonni, we go into further details as to why the dispersion interaction potential appearing in each of our works differs, and why the form given by Rodriguez and Salam correctly accounts for absorption in the dielectric medium. We also point out and address a number of fallacies raised in the Comment.
Solubilization mechanism of vesicles by surfactants: Effect of hydrophobicity
Authors: Lin Chun-Min , Chang Geng-Pei , Tsao Heng-Kwong , Sheng Yu-Jane
Published: August 2011

Simulations based on dissipative particle dynamics are performed to investigate the solubilization mechanism of vesicles by surfactants. Surfactants tend to partition themselves between vesicle and the bulk solution. It is found that only surfactants with suitable hydrophobicity are able to solubilize vesicles by forming small mixed micelles. Surfactants with inadequate hydrophobicity tend to stay in the bulk solution and only a few of them enter into the vesicle. Consequently, the vesicle structure remains intact for all surfactant concentrations studied. On the contrary, surfactants with excessive hydrophobicity are inclined to incorporate with the vesicle and thus the vesicle size continues to grow as the surfactant concentration increases. Instead of forming discrete mixed micelles, lipid and surfactant are associated into large aggregates taking the shapes of cylinders, donuts, bilayers, etc. For addition of surfactant with moderate hydrophobicity, perforated vesicles are observed before the formation of mixed micelles and thus the solubilization mechanism is more intricate than the well-known three-stage hypothesis. As the apparent critical micellar concentration ($\varphi _{\rm s,v}^{\rm a,CMC}$ϕs,va, CMC ) is attained, pure surfactant micelles form and the vesicle deforms because the distribution of surfactant within the bilayer is no longer uniform. When the surfactant concentration reaches $\varphi _{\rm s,v}^{\rm p}$ϕs,vp, the vesicle perforates. The extent of perforation grows with increasing surfactant concentration. The solubilization process begins at $\varphi _{\rm s,v}^{\rm sol}$ϕs,v sol , and lipids leave the vesicle and join surfactant micelles to form mixed micelles. Eventually, total collapse of the vesicle is observed. In general, one has $\varphi _{\rm s,v}^{\rm a,CMC} \le \varphi _{\rm s,v}^{\rm p} \le \varphi _{\rm s,v}^{\rm sol}$ϕs,va, CMC ≤ϕs,vp≤ϕs,v sol .
Collateral evidence for an excellent radiative performance of AlxGa1−xN alloy films of high AlN mole fractions
Authors: Chichibu S. F. , Hazu K. , Onuma T. , Uedono A.
Published: August 2011

Recombination dynamics for the deep-ultraviolet (DUV) near-band-edge emission of AlxGa1−xN epilayers of high AlN mole fractions (x) are studied using time-resolved spectroscopy. Their low-temperature radiative lifetime (τR) is longer than that for the epilayers of low-x AlxGa1−xN, AlN, or GaN due to the contribution of bound and localized tail-states. However, τR shows little change with temperature rise, and the value is a few ns at 300 K. The results essentially indicate an excellent radiative performance of AlxGa1−xN alloys of high x, although the luminescence efficiency of AlxGa1−xN DUV light-emitting-diodes reported so far is limited by the short nonradiative lifetime.
Small polaron hopping conduction mechanism in Fe doped LaMnO3
Authors: Khan Wasi , Naqvi Alim H. , Gupta Maneesha , Husain Shahid , Kumar Ravi
Published: August 2011

The structural and electrical transport properties of LaMn1−xFexO3 (0.1 ≤ x ≤ 0.6) bulk samples have been investigated. The powder x-ray diffraction patterns at room temperature show that all samples are formed in single phase. The temperature dependent resistivity data have been fitted with the Mott's variable-range hopping (VRH) model for an entire studied range of the temperature (77–300 K) to calculate the hopping distance (Rh) and the density of states at Fermi level (N(EF)). It is found that all parameters vary systematically with the increase in Fe concentration. Moreover, the resistivity data were also fitted in the small polaron hopping (SPH) model. The non-adiabatic SPH conduction mechanism is followed by all samples. This type conduction mechanism is far accompanied by subtle electronically induced structural changes involving in Fe–O–Fe and Fe–O–Mn bond angles and bond lengths. Thus we suggest that the transport properties can be explained according to the additional localization of charge carriers induced by Fe doping.
Molecular dynamics simulations of ion transport through carbon nanotubes. III. Influence of the nanotube radius, solute concentration, and applied electric fields on the transport properties
Authors: Beu Titus A.
Published: August 2011

The present investigations continue previous research on transport in aqueous ionic solutions through carbon nanotubes. Specifically, the effects of the nanotube radius, solute concentration, and applied external electric fields on the transport properties are investigated in terms of mobilities, currents, and pairing times of the solute ions. The simulated transport features are corroborated with general theoretical results of nanofluidics (such as the linear log-log regime of the nanochannel conductance as function of the solute concentration and the current-voltage curve of the channel). Discontinuities in the partial ionic currents are explained on the basis of a recent theoretical model of quantized ionic conductance in nanopores, developed by Zwolak et al. Correlations between the structural and dynamic properties are established, linking causally the highly structured spatial density profiles, the ion pairing phenomenon and the ionic currents.
Least-squares analysis of overlapped bound-free absorption spectra and predissociation data in diatomics: The C(1Πu) state of I2
Authors: Tellinghuisen Joel
Published: August 2011

Absorption spectra are recorded at low resolution but high quantitative precision for I2 vapor at 35 °C and 64 °C. These and literature spectra are analyzed by least-squares quantum spectral simulation of the overlapped A ← X, B ← X, and C(1Πu) ← X transitions, with the aid of a pseudocontinuum model for the discrete regions of the A ← X and B ← X spectra. The analysis yields improved descriptions of the small-R regions of the A- and B-state potentials, which are known precisely at larger R from discrete spectroscopy. The C potential is determined at small R from its C ← X absorption, at intermediate R from literature data for B → C predissociation, and at large R from its known van der Waals well. The estimates of the electronic transition moment function |μe(R)| for the B–X transition expand upon precise results from a recent determination by a different method. For the C–X and A–X transitions, the R-dependence of the transition moment functions resembles that found previously for these systems in Br2. Of the spectroscopic properties, the C ← X spectrum is most altered from the previous analysis, being now ∼20% weaker. For B → C predissociation, no derived C potential has yielded computed rates in adequate statistical agreement with the analyzed experimental data.
Highly depth-resolved chirped pulse photothermal radar for bone diagnostics
Authors: Kaiplavil Sreekumar , Mandelis Andreas
Published: August 2011

A novel chirped pulse photothermal (PT) radiometric radar with improved sensitivity over the conventional harmonically modulated thermal-wave radar technique and alternative pulsed laser photothermal radiometry is introduced for the diagnosis of biological samples, especially bones with tissue and skin overlayers. The constraints imposed by the laser safety (maximum permissible exposure) ceiling on pump laser energy and the strong attenuation of thermal-wave signals in tissues significantly limit the photothermally active depth in most biological specimens to a level which is normally insufficient for practical applications (a few mm below the skin surface). A theoretical approach for improvement of signal-to-noise ratio (SNR), minimizing the static (dc) component of the photothermal signal and making use of the photothermal radiometric nonlinearity has been introduced and verified by comparing the SNR of four distinct excitation wave forms (sine-wave, square-wave, constant-width and constant duty-cycle pulses) for chirping the pump laser, under constant exposure energy. At low frequencies fixed-pulsewidth chirps of large peak power were found to be superior to all other equal-energy modalities, with an SNR improvement up to two orders of magnitude. Distinct thickness-dependent characteristic delay times in a goat bone were obtained, establishing an active depth resolution range of ∼2.8 mm in a layered skin-fat-bone structure, a favorable result compared to the maximum reported pulsed photothermal radiometric depth resolution <1 mm in turbid biological media.