bluuebunny/crossref_metadata_2025 huggingface dataset:
TESTING FOR A UNIT ROOT IN THE PRESENCE OF A POSSIBLE BREAK IN TREND
Authors: Harris David , Harvey David I. , Leybourne Stephen J. , Taylor A.M. Robert
Published: October 2009

We consider the issue of testing a time series for a unit root in the possible presence of a break in a linear deterministic trend at an unknown point in the series. We propose a new break fraction estimator which, where a break in trend occurs, is consistent for the true break fraction at rateOp(T−1). Unlike other available estimators, however, when there is no trend break, our estimator converges to zero at rateOp(T−1/2). Used in conjunction with a quasi difference (QD) detrended unit root test that incorporates a trend break regressor, we show that these rates of convergence ensure that known break fraction null critical values are asymptotically valid. Unlike available procedures in the literature, this holds even if there is no break in trend (the break fraction is zero). Here the trend break regressor is dropped from the deterministic component, and standard QD detrended unit root test critical values then apply. We also propose a second procedure that makes use of a formal pretest for a trend break in the series, including a trend break regressor only where the pretest rejects the null of no break. Both procedures ensure that the correctly sized (near-) efficient unit root test that allows (does not allow) for a break in trend is applied in the limit when a trend break does (does not) occur.
ROBUST INFERENCE IN AUTOREGRESSIONS WITH MULTIPLE OUTLIERS
Authors: Cavaliere Giuseppe , Georgiev Iliyan
Published: October 2009

We consider robust methods for estimation and unit root (UR) testing in autoregressions with infrequent outliers whose number, size, and location can be random and unknown. We show that in this setting standard inference based on ordinary least squares estimation of an augumented Dickey–Fuller (ADF) regression may not be reliable, because (a) clusters of outliers may lead to inconsistent estimation of the autoregressive parameters and (b) large outliers induce a jump component in the asymptotic distribution of UR test statistics. In the benchmark case of known outlier location, we discuss why the augmentation of the ADF regression with appropriate dummy variables not only ensures consistent parameter estimation but also gives rise to UR tests with significant power gains, growing with the number and the size of the outliers. In the case of unknown outlier location, the dummy-based approach is compared with a robust, mixed Gaussian, quasi maximum likelihood (QML) approach, novel in this context. It is proved that, when the ordinary innovations are Gaussian, the QML and the dummy-based approach are asymptotically equivalent, yielding UR tests with the same asymptotic size and power. Moreover, as a by-product of QML the outlier dates can be consistently estimated. When the innovations display tails fatter than Gaussian, the QML approach ensures further power gains over the dummy-based method. Simulations show that the QML ADF-typet-test, in conjunction with standard Dickey–Fuller critical values, yields the best combination of finite-sample size and power.
HETEROSKEDASTICITY-ROBUST TESTING FOR A FRACTIONAL UNIT ROOT
Authors: Kew Hsein , Harris David
Published: October 2009

This paper shows how fractional unit root tests originally derived under stationarity can be made robust to heteroskedasticity. This is done by using existing tests nested in a regression framework and then implementing these tests using White’s heteroskedasticity consistent standard errors (White, 1980). We show this approach is effective both asymptotically and in finite samples. We also provide some evidence on the asymptotic local power of different implementations of the tests, under both homoskedasticity and heteroskedasticity.
GLS-BASED UNIT ROOT TESTS WITH MULTIPLE STRUCTURAL BREAKS UNDER BOTH THE NULL AND THE ALTERNATIVE HYPOTHESES
Authors: Carrion-i-Silvestre Josep Lluís , Kim Dukpa , Perron Pierre
Published: October 2009

Perron (1989,Econometrica57, 1361–1401) introduced unit root tests valid when a break at a known date in the trend function of a time series is present. In particular, they allow a break under both the null and alternative hypotheses and are invariant to the magnitude of the shift in level and/or slope. The subsequent literature devised procedures valid in the case of an unknown break date. However, in doing so most research, in particular the commonly used test of Zivot and Andrews (1992,Journal of Business & Economic Statistics10, 251–270), assumed that if a break occurs it does so only under the alternative hypothesis of stationarity. This is undesirable for several reasons. Kim and Perron (2009,Journal of Econometrics148, 1–13) developed a methodology that allows a break at an unknown time under both the null and alternative hypotheses. When a break is present, the limit distribution of the test is the same as in the case of a known break date, allowing increased power while maintaining the correct size. We extend their work in several directions: (1) we allow for an arbitrary number of changes in both the level and slope of the trend function; (2) we adopt the quasi–generalized least squares detrending method advocated by Elliott, Rothenberg, and Stock (1996,Econometrica64, 813–836) that permits tests that have local asymptotic power functions close to the local asymptotic Gaussian power envelope; (3) we consider a variety of tests, in particular the class ofM-tests introduced in Stock (1999,Cointegration, Causality, and Forecasting: A Festschrift for Clive W.J. Granger) and analyzed in Ng and Perron (2001,Econometrica69, 1519–1554).
TESTING THE NULL OF NO COINTEGRATION WHEN COVARIATES ARE KNOWN TO HAVE A UNIT ROOT
Authors: Elliott Graham , Pesavento Elena
Published: October 2009

A number of tests have been suggested for the test of the null of no cointegration. Under this null, correlations are spurious in the sense of Granger and Newbold (1974) and Phillips (1986). We examine a set of models local to the null of no cointegration and derive tests with optimality properties in order to examine the efficiency of available tests. We find that, for a sufficiently tight weighting over potential cointegrating vectors, commonly employed full system tests have power that can, in some situations, be quite far from the power bounds for the models examined.
A NOTE ON THE POOLING OF INDIVIDUAL PANIC UNIT ROOT TESTS
Authors: Westerlund Joakim , Larsson Rolf
Published: October 2009

One of the most cited studies in recent years within the field of nonstationary panel data analysis is that of Bai and Ng (2004), in which the authors propose PANIC, a new framework for analyzing the nonstationarity of panels with idiosyncratic and common components. The problem is that the asymptotic validity of PANIC as a platform for constructing pooled panel unit root tests based on averaging is not fully proven. This paper provides the required results, whose usefulness is verified through simulations.
VAGUENESS, INTERPRETATION, AND THE LAW
Authors: Jónsson Ólafur Páll
Published: October 2009

It is widely accepted that vagueness in law calls for a specific interpretation of the law—interpretation that changes the meaning of the law and makes it more precise. According to this view, vagueness causes gaps in the law, and the role of legal interpretation in the case of vagueness is to fill such gaps. I argue that this view is mistaken and defend the thesis that vagueness in law calls only for an application of the law to the case at hand, leaving the meaning of the law intact.
Different Components of Metacognition and their Relationship to Psychotic-Like Experiences
Authors: Reeder Clare , Rexhepi-Johansson Teuta , Wykes Til
Published: October 2009

Background: Theories of the development of psychotic symptoms have suggested that metacognitive beliefs might play a part. However, studies offering supporting evidence have failed to distinguish between metacognitive beliefs about the consequences of having certain thoughts, and metacognitive beliefs about one's own cognitive skills. Aims: To distinguish metacognitive beliefs and investigate the extent of their association with psychotic-like experiences. Method: Participants were 60 healthy adults recruited primarily from two university campuses. Three measures of metacognition were administered: (i) Metacognitions Questionnaire (MCQ-30); (ii) Metacognitive Assessment Inventory; and (iii) Koriat General Questions Test; and two schizotypy questionnaires: O-Life and SPQ-B and data were analysed using an exploratory principal components analysis of the metacognition measures. Results: Three principal components were identified: (i) Beliefs about thoughts; (ii) Cognitive confidence; and (iii) Beliefs about cognitive regulation. Only the “beliefs about thoughts” component was significantly associated with the “psychotic-like experiences” factor, extracted from the measures of schizotypy. Conclusions: The finding supports theories suggesting that psychotic symptoms may be caused in part by negative metacognitive beliefs about thoughts. However, metacognition is a complex construct that is currently poorly understood.
The regression hypothesis as a framework for first language attrition
Authors: KEIJZER MEREL
Published: October 2009

In an attempt to explain first language attrition in emigrant populations, this paper investigates the explanatory power of a framework that has – until now – received little attention: the regression hypothesis (Jakobson, 1941). This hypothesis predicts that the order of attrition is the reverse of the order of acquisition. The regression hypothesis was tested in relation to the loss of morphology and syntax in Dutch immigrants in Anglophone Canada. Evidence in favor of regression was found, but mainly in the morphological domain. Syntax, on the other hand, was mostly characterized by L2 influences from English. As it is problematic to treat regression as a theory in its own right, these findings are then explained in the light of both generative and usage-based approaches, as well as the more recent Dynamic Systems Theory.
Languages at play: The relevance of L1 attrition to the study of bilingualism
Authors: SCHMID MONIKA S.
Published: October 2009

Speakers who routinely use more than one language may not use any of their languages in ways which are exactly like that of a monolingual speaker. In sequential bilingualism, for example, there is often evidence of interference from the L1 in the L2 system. Describing these interference phenomena and accounting for them on the basis of theoretical models of linguistic knowledge has long been a focus of interest of Applied Linguistics. More recently, research has started to investigate linguistic traffic which goes the other way: L2 interferences and contact phenomena evident in the L1. Such phenomena are probably experienced to some extent by all bilinguals. They are, however, most evident among speakers for whom a language other than the L1 has started to play an important, if not dominant, role in everyday life (Schmid and Köpke, 2007). This is the case for migrants who move to a country where a language is spoken which, for them, is a second or foreign language. We refer to the phenomena of L1 change and L2 interference which can be observed in such situations aslanguage attrition.
Mental control of the bilingual lexico-semantic system
Authors: GREEN DAVID W.
Published: October 2009

This paper aims to foster discussion of the means by which bilinguals control their two language systems. It proposes an inhibitory control (IC) model that embodies the principle that there are multiple levels of control. In the model a language task schema (modulated by a higher level of control) “reactively” inhibits potential competitors for production at the lemma level by virtue of their language tags. The IC model is used to expand the explanation of the effect of category blocking in translation proposed by Kroll and Stewart (1994), and predictions of the model are tested against other data. Its relationship to other proposals and models is considered and future directions proposed.
Drainage through subglacial water sheets
Authors: Creyts Timothy T. , Schoof Christian G.
Published: October 2009

Subglacial drainage plays an important role in controlling coupling between glacial ice and underlying bed. Here, we study the flow of water in thin, macroporous sheets between ice and bed. Previous work shows that small perturbations in depth of a nearly parallel‐sided water film grow unstably because these areas have enhanced viscous dissipation that leads to enhanced melting of an ice roof. We argue that in the presence of bed protrusions bridging a water sheet, downward motion of the ice roof can stabilize this sheet. Stability results when the rate of roof closure increases faster with water depth than the rate of viscous dissipation. We therefore modify existing theory to include protrusions that partially support the overlying glacier. Differences in the pressure on protrusions relative to water pressure drive roof closure. The mechanisms of both regelation and creep normal to the bed close the overlying ice roof and decrease the ice‐bed gap. In order to account for multiple protrusion sizes along the bed (for instance, resulting from an assortment of various‐sized sediment grains), we incorporate a method of partitioning overburden pressure among different protrusion size classes and the water sheet. Partitioning is dependent on the amount of ice protrusion contact and, therefore, water depth. This method allows prediction of roof closure rates. We then investigate stable, steady sheet configurations for reasonable parameter choices and find that these steady states can occur for modest water depths at very low effective pressures, as is appropriate for ice streams. Moreover, we find that multiple steady sheet thicknesses exist, raising the possibility of switches between low and high hydraulic conductivity regimes for the subglacial water system.
Frictional and hydrologic properties of a major splay fault system, Nankai subduction zone
Authors: Ikari Matt J. , Saffer Demian M. , Marone Chris
Published: October 2009

We report on laboratory experiments examining the frictional and hydrologic properties of fault gouge and wall rock along a borehole transect that crosses a major out‐of‐sequence thrust splay fault within the Nankai accretionary complex. At 25 MPa effective normal stress, the fault zone material is frictionally weak (μ ≤ 0.44) and exhibits low permeability after shearing (k < 5.5 × 10−20 m2). Fault zone samples are weaker and less permeable than the surrounding wall rocks, consistent with their slightly higher clay abundance. All samples exhibit velocity‐strengthening frictional behavior over most of the experimental conditions we explored, consistent with aseismic slip at shallow depths. The fault and wall rock both exhibit prominent minima in the friction rate parameter (a − b) for sliding velocities of 1–10 μm/s (∼0.1–1.0 m/day), comparable to rates for slow slip events. This suggests that the frictional properties of fault zone material play a central role in governing the mode of slip on subduction megathrusts.
Are Atlantic Niños enhancing Pacific ENSO events in recent decades?
Authors: Rodríguez‐Fonseca Belén , Polo Irene , García‐Serrano Javier , Losada Teresa , Mohino Elsa , Mechoso Carlos Roberto , Kucharski Fred
Published: October 2009

This work presents observational evidence of a change in Atlantic‐Pacific Niños connection since the late 60's. Accordingly, summer Atlantic Niños (Niñas) alter the tropical circulation favoring the development of following‐winter Pacific Niñas (Niños). The same change is obtained in an ensemble of AGCM integrations in which SSTs in the Atlantic are the observed in 1949–2002 and those in the tropical Indo‐Pacific are from a coupled OGCM. The mechanism (for the positive Atlantic phase) involves the strengthening of the Walker circulation with ascending branch over the Atlantic and descending branch over the central Pacific. The enhanced surface divergence in the latter region shallows the equatorial thermocline triggering coupled processes, and favoring the development of a Pacific La Niña. Results could be linked to the reported 60's and 70's climate shifts; emphasizing the importance of tropical Atlantic for the success of seasonal forecast skill.
A 2 year comparison of AMSR‐E and MODIS cloud liquid water path observations
Authors: Greenwald Thomas J.
Published: October 2009

An extensive comparison of cloud liquid water path (LWP) observations from solar reflectance and passive microwave satellite measurements over the global oceans is presented. Two years of MODerate resolution Imaging Spectroradiometer (MODIS) and Advanced Microwave Scanning Radiometer‐EOS (AMSR‐E) LWP observations were collocated at full resolution and compared under special conditions to minimize errors, i.e., overcast, low‐level liquid clouds with no detectable precipitation. Best agreement occurred in stratus regions off the west coasts of North/South America, southern Africa, and Australia. Large differences found in the winter hemispheres at higher latitudes were caused by a strong dependence of MODIS optical depth retrievals on solar zenith angle. Other differences, notably in the tropics and parts of the subtropics, were due to a dependence of AMSR‐E LWP observations on surface wind speed and total precipitable water. Results highlight the need to provide more accurate cloud LWP observations for climate studies and model verification.
A simplified dissolution‐precipitation model of the smectite to illite transformation and its application
Authors: Shi G. R.
Published: October 2009

By combining the compaction effect EC from a dissolution‐precipitation model (DPM) and the thermal effect ET from a kinetic model (KM), a simplified DPM is presented. A pair of apparent activation energy E and frequency factor A to be used is proposed in the model, which is chosen to cause the predicted smectite to fit X‐ray diffraction (XRD) data. The proposed model is then applied to the illite‐smectite interstratifications of mudstone in well Tazhong4 of the Tarim Basin in western China. A KM is also applied for the sake of comparison. This case study shows that the most sensitive parameter governing smectite to illite (S‐I) transformation is the pair of E and A. To calculate an accurate S‐I transformation history, it is vital, therefore, to calibrate the E and A by using XRD data as a constraint. In this specific case study, the calibrated E ranges between 5 and 10.28 kcal mol−1 while A is between 9.0623 × 10−11 and 3 × 10−7 s−1 L mol−1 for the simplified DPM, and E ranges between 3 and 5 while A is between 1.3888 × 10−11 and 3.5052 × 10−9 for the KM. The relation of E versus ln(A) appears nonlinear. By using these E and A, the present‐day values of the calculated smectite history agree well with the XRD‐determined data, with mean absolute relative residuals of 4.45% for the simplified DPM and 5.67% for the KM. Moreover, the case study indicates that EC and ET have the same importance for S‐I transformation as contributions of EC of 43.5% and of ET of 56.5%.
Influence of tropical cyclones on seasonal ocean circulation in the South China Sea
Authors: Wang Guihua , Ling Zheng , Wang Chunzai
Published: October 2009

The seasonal variability of South China Sea (SCS) ocean circulation influenced by tropical cyclones (TCs) is studied by using satellite QuikSCAT wind data, Sverdrup theory, and a reduced gravity model. TCs can induce a positive (negative) wind stress curl in the northwestern (southeastern) SCS in summer and a positive wind stress curl for the whole SCS in winter. With these wind stress curls induced by TCs, the cyclonic gyre in the northern SCS and the anticyclonic gyre in the southern SCS are intensified in summer. In winter, the cyclonic gyre in the northern SCS is intensified and the gyre in the southern SCS is weakened except in November and December when both gyres are enhanced. The model results show that the dipole structure off central Vietnam in summer is intensified and the eddy off northwestern Luzon Island in winter is weakened by TCs. The present paper shows that TCs can affect both large‐scale and mesoscale SCS ocean circulation, suggesting that studies including the effect of TCs are necessary to help improve our understanding of SCS ocean circulation dynamics.
Are mesoscale eddies in shelf seas formed by baroclinic instability of tidal fronts?
Authors: Badin G. , Williams R. G. , Holt J. T. , Fernand L. J.
Published: October 2009

The formation of eddies along tidal fronts in the shelf seas is investigated through both the analysis of high‐resolution conductivity‐temperature‐depth (CTD) data and drifters for the North Sea as well as experiments with an eddy‐permitting model. Baroclinic eddies are difficult to observe in the shelf seas due to their small size, and their surface signals are dampened by air‐sea interaction and eroded by wind and tidal mixing. High‐resolution CTD profiles reveal possible eddy signals along the thermocline where there are intrusions of waters of reduced salinity on the eddy scale. These freshwater intrusions carry low potential vorticity, indicating their origin from the surface mixed layer. An instability analysis of these frontal systems suggests that these eddy signals can be formed by baroclinic instability. Numerical model integrations reveal that mesoscale eddy variability along the thermocline increases in strength in late summer, in accord with the increase in stratification. The stability analysis of the modeled fronts and the scaling of the eddy kinetic energy to the available potential energy are also consistent with baroclinic instability theory. While influencing the eddy variability through the passage of synoptic weather systems, wind energy input is too small to explain the high values of eddy kinetic energy during late summer. These eddy circulations provide localized regions of high vorticity and high stirring, which are important for the transfer of tracers at middepth, such as potential vorticity, salinity, and nutrients.
Evaluation of surface and near‐surface melt characteristics on the Greenland ice sheet using MODIS and QuikSCAT data
Authors: Hall Dorothy K. , Nghiem Son V. , Schaaf Crystal B. , DiGirolamo Nicolo E. , Neumann Gregory
Published: October 2009

The Greenland ice sheet has been the focus of much attention recently because of increasing melt in response to regional climate warming and can be studied using Moderate Resolution Imaging Spectroradiometer (MODIS) and Quick Scatterometer (QuikSCAT) data. To improve our ability to measure surface melt, we use remote sensing data products to study surface and near‐surface melt characteristics of the Greenland ice sheet for the 2007 melt season when record melt extent and runoff occurred. MODIS daily land surface temperature (LST), MODIS daily snow albedo, and a special diurnal melt product derived from QuikSCAT (QS) scatterometer data, are all effective in measuring the evolution of melt on the ice sheet. These daily products, produced from different parts of the electromagnetic spectrum, are sensitive to different geophysical features, though QS‐ and MODIS‐derived melt generally show excellent correspondence when surface melt is present. Values derived from the MODIS daily snow albedo product drop in response to melt and change with apparent grain size changes. For the 2007 melt season, the MODIS LST and QS products detect 766,184 km2 ± 8% and 862,769 km2 ± 3% of melt, respectively. The QS product detects about 11% greater melt extent than is detected by the MODIS LST product probably because QS is more sensitive to surface melt and can also see subsurface melt. The consistency of the response of the different products demonstrates unequivocally that physically meaningful melt/freeze boundaries are detected. We have demonstrated that when these products are used together we can improve the precision in mapping surface and near‐surface melt extent on the Greenland ice sheet.
Middle‐late Pleistocene deep water circulation in the southwest subtropical Pacific
Authors: Russon T. , Elliot M. , Kissel C. , Cabioch G. , De Deckker P. , Corrège T.
Published: October 2009

The modern δ13CDIC distribution in southwest subtropical Pacific deep waters is consistent with a regional mixing regime between water masses of open Pacific Ocean and Tasman Sea origin. This mixing regime is reconstructed across the middle‐late Pleistocene using a record of benthic foraminiferal δ13C in a sediment core from the New Caledonia Trough. The relative influence on the mixing regime from open Pacific Ocean deep waters is seen to be significantly reduced during glacial in comparison to interglacial stages over the past 1.1 Ma. The spatial δ13C gradient in the Southern Ocean between deep waters entering the Tasman Sea and the open Pacific Ocean is shown to be consequently greater during glacial than interglacial stages but was generally reduced across the period of the Middle Pleistocene Transition. The existence of strong spatial chemical gradients in the glacial Southern Ocean limits its capacity to act as an enhanced sink for atmospheric carbon.
Plasmasphere effects for GPS TEC measurements in North America
Authors: Mazzella Andrew J.
Published: October 2009

Plasmasphere effects on total electron content (TEC) measurements conducted using Global Positioning System (GPS) receivers are typically neglected for the North American region, because of the relatively high magnetic latitudes there, but model and measurement cases for this region are presented here to demonstrate the magnitude of the effects for GPS TEC measurements away from vertical and for the associated equivalent vertical TEC values. For high solar flux conditions, the effects of high, distant electron content can range up to 25 TEC units for equivalent vertical TEC, or up to 65 TEC units for slant TEC determinations derived from vertical TEC measurements. The effects of the plasmasphere for TEC calibrations conducted in North America include systematic overestimation of the equivalent vertical TEC and excessive latitudinal gradients, especially at night. These may not be readily evident in the calibration results, and some alternatives for addressing these circumstances are considered.
Tissue factor: beyond coagulation in the cardiovascular system
Authors: Breitenstein Alexander , Camici Giovanni G. , Tanner Felix C.
Published: October 2009

TF (tissue factor) is the main trigger of the coagulation cascade; by binding Factor VIIa it activates Factor IX and Factor X, thereby resulting in fibrin formation. Various stimuli, such as cytokines, growth factors and biogenic amines, induce TF expression and activity in vascular cells. Downstream targets of these mediators include diverse signalling molecules such as MAPKs (mitogen-activated protein kinases), PI3K (phosphoinositide 3-kinase) and PKC (protein kinase C). In addition, TF can be detected in the bloodstream, known as circulating or blood-borne TF. Many cardiovascular risk factors, such as hypertension, diabetes, dyslipidaemia and smoking, are associated with increased expression of TF. Furthermore, in patients presenting with acute coronary syndromes, elevated levels of circulating TF are found. Apart from its role in thrombosis, TF has pro-atherogenic properties, as it is involved in neointima formation by inducing vascular smooth muscle cell migration. As inhibition of TF action appears to be an attractive target for the treatment of cardiovascular disease, therapeutic strategies are under investigation to specifically interfere with the action of TF or, alternatively, promote the effects of TFPI (TF pathway inhibitor).
Dining in with BCL-2: new guests at the autophagy table
Authors: Germain Marc , Slack Ruth S.
Published: October 2009

BCL-2 homologues are major regulators of apoptosis and, as such, play an active role in the survival of adult neurons following injury. In recent years, these proteins have also been associated with the regulation of autophagy, a catabolic process involved in the recycling of nutrients upon starvation. Basal levels of autophagy are also required to eliminate damaged proteins and organelles. This is illustrated by the accumulation of ubiquitin-positive aggregates in cells deficient in autophagy and, in the nervous system, this is associated with progressive cell loss and signs of neurodegeneration. Given the importance of both apoptosis and autophagy for neuronal survival in adult neurons, understanding how BCL-2 homologues co-ordinately regulate these processes will allow a better understanding of the cellular processes leading to neurodegeneration. In the present review, we will discuss the roles of BCL-2 homologues in the regulation of apoptosis and autophagy, focussing on their impact on adult neurons.
High-gain photoconductivity in semiconducting InN nanowires
Authors: Chen Reui-San , Yang Tsang-Ho , Chen Hsin-Yi , Chen Li-Chyong , Chen Kuei-Hsien , Yang Ying-Jay , Su Chun-Hsi , Lin Chii-Ruey
Published: October 2009

We report on the photoconductivity study of the individual infrared-absorbing indium nitride (InN) nanowires. Temperature-dependent dark conductivity measurement indicates the semiconducting transport behavior of these InN nanowires. An enhanced photosensitivity from 0.3 to 14 is observed by lowering the temperature from 300to10K. A calculated ultrahigh photoconductive gain at around 8×107 at room temperature is obtained from the low-bandgap nitride nanowire under 808nm excitation.
Development, simulation, and validation of NMR relaxation-based exchange measurements
Authors: Dortch R. D. , Horch R. A. , Does M. D.
Published: October 2009

Two-dimensional (2D) nuclear magnetic resonance correlation experiments have recently been proposed as a means for studying exchange in porous media. Most notable of these is the T2−T2 relaxation exchange spectroscopy (REXSY) experiment. Unfortunately, quantifying exchange with this method requires a relatively long, three-dimensional acquisition. To reduce acquisition times, novel 2D methods for quantifying exchange were developed. For each method, model equations were derived (for an arbitrary N-pool system), tested via simulation studies, and validated via experimental studies in an aqueous urea model system. Results indicate that the novel methods outperform REXSY—in terms of uncertainty per unit time for the fitted exchange rate—for certain model systems. The relative merits of each method are discussed in the text.
Structural properties and site specific interactions of Pt with the graphene/Ru(0001) moiré overlayer
Authors: Donner Kerstin , Jakob Peter
Published: October 2009

The coherence of graphene layers on Ru(0001) over extended distances has been employed to identify fcc and hcp regions of the associated moiré superstructure. These findings can be used as a straightforward method to discriminate between fcc and hcp hollow sites of Ru(0001). Our approach thereby makes use of the “magnifying lens” characteristics of the graphene/Ru(0001) overlayer and its coherence across several monatomic steps of the substrate. We demonstrate that the individual regions of the graphene/Ru(0001) overlayer exhibit pronounced variations in interaction strengths with deposited metal atoms. Specifically, Pt clusters have been grown at 140–180 K and they are found to organize in a well-ordered periodic array defined by the moiré superlattice. Their preferred location within the graphene/Ru(0001) moiré unit cell is identified to be the fcc region.
On the definition of discrete hydrodynamic variables
Authors: Español Pep , Zúñiga Ignacio
Published: October 2009

The Green–Kubo formula for discrete hydrodynamic variables involves information about not only the fluid transport coefficients but also about discrete versions of the differential operators that govern the evolution of the discrete variables. This gives an intimate connection between discretization procedures in fluid dynamics and coarse-graining procedures used to obtain hydrodynamic behavior of molecular fluids. We observed that a natural definition of discrete hydrodynamic variables in terms of Voronoi cells leads to a Green–Kubo formula which is divergent, rendering the full coarse-graining strategy useless. In order to understand this subtle issue, in the present paper we consider the coarse graining of noninteracting Brownian particles. The discrete hydrodynamic variable for this problem is the number of particles within Voronoi cells. Thanks to the simplicity of the model we spot the origin of the singular behavior of the correlation functions. We offer an alternative definition, based on the concept of a Delaunay cell that behaves properly, suggesting the use of the Delaunay construction for the coarse graining of molecular fluids at the discrete hydrodynamic level.
First-principles methodology for quantum transport in multiterminal junctions
Authors: Saha Kamal K. , Lu Wenchang , Bernholc J. , Meunier Vincent
Published: October 2009

We present a generalized approach for computing electron conductance and I-V characteristics in multiterminal junctions from first-principles. Within the framework of Keldysh theory, electron transmission is evaluated employing an O(N) method for electronic-structure calculations. The nonequilibrium Green function for the nonequilibrium electron density of the multiterminal junction is computed self-consistently by solving Poisson equation after applying a realistic bias. We illustrate the suitability of the method on two examples of four-terminal systems, a radialene molecule connected to carbon chains and two crossed-carbon chains brought together closer and closer. We describe charge density, potential profile, and transmission of electrons between any two terminals. Finally, we discuss the applicability of this technique to study complex electronic devices.
Experimental and theoretical study on effects of magnetic field topology on near wall conductivity in a Hall thruster
Authors: Yu Daren , Li Hong , Wu Zhiwen , Ning Zhongxi , Yan Guojun
Published: October 2009

An experiment has been made to investigate the effect of curved magnetic field topology on near wall conductivity in the ion acceleration region of Hall thrusters. The experimental results show that the electron current due to near wall conductivity is of the minimum in the case of focused topology and increases in the cases of both less-focus and over-focus topologies. This finding cannot be explained properly by the magnetic mirror effect, which is the one and only reported effect related to the magnetic field curvature so far. Based on the analysis of interaction between the plasma and the wall, a new physical effect is proposed. The difference of magnetic field topology causes different electric potential distribution, leads to different ion flux to the wall, results in the change of sheath property and secondary electron emission, and finally affects the electron current due to near wall conductivity. This effect is further justified by the agreement between the experiment and simulation which is performed with a particle-in-cell model. Therefore, we conclude that the ion flow injection is a significant effect to near wall conductivity in the scope of curved magnetic field topology besides the magnetic mirror effect. Moreover, we find that the focus topology of magnetic field is favorable to obtain a high thruster performance from both the ion acceleration aspect and the electron conduction aspect and so is useful practically for thruster optimization.
Fluid-acoustic interactions in self-sustained oscillations in turbulent cavity flows. I. Fluid-dynamic oscillations
Authors: Yokoyama Hiroshi , Kato Chisachi
Published: October 2009

The fluid-acoustic interactions in a flow over a two-dimensional rectangular cavity are investigated by directly solving the compressible Navier–Stokes equations. The upstream boundary layer is turbulent. The depth-to-length ratio of the cavity is 0.5. Phase-averaged flow fields reveal the mechanism for the acoustic radiation. Large-scale vortices form in the shear layer that separates from the upstream edge of the cavity. When a large-scale vortex collides with the downstream wall, the low-pressure fluid in the vortex spreads along the downstream wall. As a result, a local downward velocity is induced by the local pressure gradient, causing the upstream fluid to expand. Finally, an expansion wave propagates to the outside of the cavity. The large-scale vortices originate from the convective disturbances that develop in the shear layer. The disturbances grow due to the Kelvin–Helmholtz instability, similar to the growth of those in a laminar cavity flow. To clarify the mechanism for the generation of the initial convective disturbances, computations for backward-facing step flows with an artificial acoustic source are also performed. As the artificial acoustic waves become more intense, the initial convective disturbances in the shear layer become more intense while the spatial growth rate of these disturbances does not change. This means that the initial convective disturbances in the shear layer are induced by the acoustic waves.
Multi-beam multi-wavelength semiconductor lasers
Authors: Yu Nanfang , Kats Mikhail A. , Pflügl Christian , Geiser Markus , Wang Qi Jie , Belkin Mikhail A. , Capasso Federico , Fischer Milan , Wittmann Andreas , Faist Jérôme , Edamura Tadataka , Furuta Shinichi , Yamanishi Masamichi , Kan Hirofumi
Published: October 2009

Multibeam emission and spatial wavelength demultiplexing in semiconductor lasers by patterning their facets with plasmonic structures is reported. Specifically, a single-wavelength laser was made to emit beams in two directions by defining on its facet two metallic gratings with different periods. The output of a dual-color laser was spatially separated according to wavelength by using a single metallic grating. The designs can be integrated with a broad range of active or passive optical components for applications such as interferometry and demultiplexing.
Enhanced three-photon absorption and nonlinear refraction in ZnS and Mn2+ doped ZnS quantum dots
Authors: Chattopadhyay M. , Kumbhakar P. , Sarkar R. , Mitra A. K.
Published: October 2009

Enhanced three-photon absorption (3PA) and self-focusing nonlinear refraction are reported for the first time in ZnS and (1%–2.5%) Mn2+ doped ZnS quantum dots (QDs) of average size of 1.5 nm by using z-scan technique at 532 nm laser radiations. At this wavelength, the obtained maximum value of the 3PA coefficient both in the ZnS and in the doped ZnS QDs is ∼107 times that of bulk ZnS. Also intensity dependent saturation of 3PA has been observed and the characteristic saturation intensity is estimated to be 0.85±0.09 GW/cm2 for ZnS QDs.
Enhancement of thermoelectric efficiency in oxygen-deficient Sr1−xLaxTiO3−δ ceramics
Authors: Liu J. , Wang C. L. , Su W. B. , Wang H. C. , Zheng P. , Li J. C. , Zhang J. L. , Mei L. M.
Published: October 2009

We report that the Seebeck coefficient (S) is remarkably enhanced in oxygen-deficient Sr1−xLaxTiO3−δ ceramics. The S values of all oxygen-deficient samples are larger than those of the near-stoichiometric ones and are temperature-independent at high temperatures, showing a narrow band behavior. This indicates that the introduction of oxygen vacancy changes the density of electronic states around the Fermi energy. The maximum for the figure of merit (ZT) of Sr0.9La0.1TiO3−δ ceramic reaches 0.21 at about 750 K, demonstrating enhancement by a factor of more than 1.3 over that of the near-stoichiometric materials.
Blueshift of intersubband transition wavelength in AlN/GaN multiple quantum wells by low temperature metal organic vapor phase epitaxy using pulse injection method
Authors: Yang Jung-Seung , Sodabanlu Hassanet , Sugiyama Masakazu , Nakano Yoshiaki , Shimogaki Yukihiro
Published: October 2009

AlN/GaN multiquantum wells (MQWs) were grown at different growth temperatures via a metal organic vapor phase epitaxy (MOVPE) system using a pulse injection method and their intersubband transition (ISBT) properties were investigated. Strong ISBT at 1.58 μm measured at room temperature was realized with MQWs grown at 770 °C and its absorption properties was the best reported in MOVPE system using GaN buffer layer. Clear blueshift of ISB absorption wavelength by lowering growth temperature was observed, which suggests that interdiffusion within MQWs was suppressed at lower growth temperatures.
Efficient hole transport in asymmetric coupled InGaN multiple quantum wells
Authors: Zhang Jiang-Yong , Cai Li-E , Zhang Bao-Ping , Hu Xiao-Long , Jiang Fang , Yu Jin-Zhong , Wang Qi-Ming
Published: October 2009

InGaN based light emitting devices (LEDs) with asymmetric coupled quantum wells (AS-QWs) and conventional symmetric coupled quantum wells (CS-QWs) active structures were grown by metal-organic chemical vapor deposition technique. The LEDs with AS-QWs active region show improved light emission intensity and reduced forward voltage compared with LEDs with CS-QWs active region. Based on the electroluminescence measurements and the devices structure analysis, it can be concluded that these improvements are mainly attributed to the efficient hole tunneling through barriers and consequently the uniform distribution of carriers in the AS-QWs.
Efficiency droop behavior of direct current aged GaN-based blue light-emitting diodes
Authors: Shao Xianjie , Lu Hai , Chen Dunjun , Xie Zili , Zhang Rong , Zheng Youdou
Published: October 2009

By direct current (dc) stressing, GaN-based blue light-emitting diodes (LEDs) with different density of nonradiative recombination centers in the active region of InGaN/GaN multiple quantum wells were obtained and studied for injection-current-induced efficiency droop. It is found that with increasing stressing time, the overall quantum efficiency of the aged LEDs drops while the peak-efficiency-current shifts toward higher magnitude. At selected injection current levels, the electroluminescence spectra of the aged LEDs show little change in peak position and shape. The shift in peak-efficiency-current, which follows the same trend as the degree of luminescence decay, is explained by a rate-equation model in which the newly created defects by dc stressing enlarge the dominant low-current region of nonradiative recombinations.
Ultraviolet and solar-blind spectral imaging with subwavelength transmission gratings
Authors: Lim S. H. , Yu E. T.
Published: October 2009

Aluminum gratings with subwavelength slit widths were designed and analyzed for spectral filtering of ultraviolet (UV) light. Although schemes for optical wavelength filtering have been thoroughly studied, options for UV wavelength filtering are far more limited. We analyze the unique requirements for UV based imaging and evaluate the suitability of our structures by electromagnetic simulations and experimental measurements. Rayleigh–Wood anomalies are shown to lead to sharp drops in transmission at resonance wavelengths, producing a high finesse band reject filter. Finally, we show that the structures are effective for both TE and TM polarizations and easily integrated onto semiconductor photodetectors.
Validation of FIRETEC wind-flows over a canopy and a fuel-break
Authors: Pimont François , Dupuy Jean-Luc , Linn Rodman R. , Dupont Sylvain
Published: October 2009

The wildfire model FIRETEC simulates the large coherent eddies of the wind-flows induced by the canopy. It has been qualitatively validated in its ability to simulate fire behavior, but there is still a need to validate physical submodels separately. In the present study, the dynamics and turbulence of the flow simulated by FIRETEC are validated in a manner similar to other air-flow models without fire, through comparison with measurements associated with flows within continuous and discontinuous forests captured through in situ and wind-tunnel experiments with neutral thermal stratification. The model is shown to be able to reproduce accurately all essential features of turbulent flow over both forests. Moreover, a short sensitivity study shows that the model is not very sensitive to uncertain parameters such as vegetation drag coefficient. Finally, it is shown in the discontinuous forest case that wind gusts on fuel-breaks can be very strong and significantly higher than in surrounding canopies, even if their directions are more stable. These results and others briefly reviewed in the present paper allow better understanding of wind-flow perturbations induced by fuel-breaks. This new validation added to previous ones confirms the ability of FIRETEC for investigating effects of fuel-break design on fire propagation.
Flame interactions and burning characteristics of two live leaf samples
Authors: Pickett Brent M. , Isackson Carl , Wunder Rebecca , Fletcher Thomas H. , Butler Bret W. , Weise David R.
Published: October 2009

Combustion experiments were performed over a flat-flame burner that provided the heat source for multiple leaf samples. Interactions of the combustion behavior between two leaf samples were studied. Two leaves were placed in the path of the flat-flame burner, with the top leaf 2.5 cm above the bottom leaf. Local gas and particle temperatures, as well as local oxygen concentrations, were measured along with burning characteristics of both leaves. Results showed that the time to ignition of the upper leaf was not significantly affected by the presence of the lower leaf. The major difference observed was that the time of flame duration of the upper leaf was significantly affected by the presence of the lower leaf. Causes for the prolonged flame were found to be the consumption of oxygen by the burning lower leaf and the obstruction provided by the lower leaf, causing a wake effect, thus altering the combustion behavior of the upper leaf.
Activity and behaviour of lactating echidnas (Tachyglossus aculeatus multiaculeatus) from hatching of egg to weaning of young
Authors: Rismiller Peggy D. , McKelvey Michael W.
Published: October 2009

Throughout their Australia-wide distribution, short-beaked echidnas breed during the winter months, i.e. June through early September. Actual duration of the female reproductive cycle, from attracting males and mating until weaning the young, can vary from 7 to 9 months depending on geographic location. Much of this variation is due to length of courtship and age of weaning the young. This paper presents data about the behaviour of free-ranging female echidnas on Kangaroo Island from egg laying through to weaning and compares it with findings from other areas. On Kangaroo Island, behaviour of lactating echidnas falls into two distinct phases: (1) while carrying the young in the pouch; and (2) after the young is placed in a nursery burrow. Females significantly increased both the number of hours active each day as well as the size of area utilised after the young was in the nursery. Although types of nursery burrows are similar throughout Australia, duration of use and frequency visited varies between geographic locations. A major difference is that Kangaroo Island echidnas are often active while carrying either the egg or the young in the pouch and echidnas in other regions remain in a burrow for extended periods.