bluuebunny/crossref_metadata_2025 huggingface dataset:
Interannual variability of Greenland winter precipitation sources: 2. Effects of North Atlantic Oscillation variability on stable isotopes in precipitation
Authors: Sodemann H. , Masson‐Delmotte V. , Schwierz C. , Vinther B. M. , Wernli H.
Published: June 2008

A new Lagrangian moisture source diagnostic is applied to identify the atmospheric conditions relevant for the fractionation of stable water isotopes during evaporation over the ocean and subsequent transport to Greenland. Northern Hemisphere winter months with positive and negative North Atlantic Oscillation (NAO) index are studied on the basis of ERA‐40 reanalysis data. Diagnosed moisture transport conditions are supplied to a Rayleigh‐type isotope fractionation model to derive estimates for the isotopic composition of stable isotopes in winter precipitation on the Greenland plateau for the two NAO phases. Because of changes in atmospheric circulation, moisture source locations for precipitation in Greenland vary strongly for different phases of the NAO. The mean source SST is ∼5.0 K warmer during negative NAO months compared to the positive phase. This signal is considerably stronger than what would result from interannual SST variability at a spatially fixed moisture source. Furthermore, moisture transport takes place at warmer temperatures during NAO negative conditions. Simulated average isotopic depletion of Greenland precipitation is less negative by 3.8 ± 6.8‰ for δ18O during the negative compared to the positive NAO phase. Comparison with ice core data from central Greenland for three winters shows good agreement between observed and simulated variability. The synoptic interplay of the initial conditions at the moisture sources and of the atmospheric transport conditions leads to enhanced NAO‐related interannual variability of stable isotopes. This could be important for understanding rapid shifts in stable isotopes during past climates. The isotope modeling applied here, however, considerably underestimates the absolute level of isotopic depletion. The offset is attributed to approximations in the model and uncertainties in the comparison with observational data. The high spatial resolution of the Lagrangian method reveals the nonhomogeneous structure of isotope NAO variability over the Greenland ice sheet. The results are therefore potentially useful for selecting new ice core drilling sites with maximum NAO variability.
Multi‐layer arctic mixed‐phase clouds simulated by a cloud‐resolving model: Comparison with ARM observations and sensitivity experiments
Authors: Luo Yali , Xu Kuan‐Man , Morrison Hugh , McFarquhar Greg M. , Wang Zhien , Zhang Gong
Published: June 2008

A cloud‐resolving model (CRM) is used to simulate the multiple‐layer mixed‐phase stratiform (MPS) clouds that occurred during a three‐and‐a‐half day subperiod of the Department of Energy‐Atmospheric Radiation Measurement Program's Mixed‐Phase Arctic Cloud Experiment (M‐PACE) and to examine physical processes responsible for multilayer production and evolution. The CRM with a two‐moment cloud microphysics is initialized with concurrent meteorological, aerosol, and ice nucleus measurements and is driven by time‐varying large‐scale advective tendencies of temperature and moisture and surface sensible and latent heat fluxes. The CRM reproduces the dominant occurrences of the single‐ and double‐layer MPS clouds as revealed by the M‐PACE observations although the simulated first cloud layer is lower and the second cloud layer is thicker compared to observations. The aircraft measurements suggest that the CRM qualitatively captures the major characteristics in the vertical distribution and interperiod variation of liquid water content (LWC), droplet number concentration, total ice water content (IWC), and ice crystal number concentration (nis). However, the magnitude of LWC is overestimated and those of IWC and nis are underestimated. In particular, the simulated nis is one order of magnitude smaller than the observed. Sensitivity experiments suggest that both the surface fluxes and large‐scale advection control the formation of the lower cloud layer while the large‐scale advection initiates the formation of the upper cloud layer but the maintenance of multilayer structures relies on the longwave (LW) radiative effect. The LW cooling near cloud top produces a more saturated environment and a stronger dynamical circulation while cloud base radiative warming of the upper layer creates the stability gap between the two cloud layers. Both cloud layers are sensitive to ice‐forming nuclei number concentration since ice‐phase microphysics provides a strong sink of cloud liquid water mass.
Evidence for the impact of aerosols on the onset and microphysical properties of rainfall from a combination of satellite observations and cloud‐resolving model simulations
Authors: Berg Wesley , L'Ecuyer Tristan , van den Heever Susan
Published: June 2008

Satellite rainfall estimates from the Tropical Rainfall Measuring Mission's (TRMM) precipitation radar (PR) and microwave imager (TMI) exhibit large differences off the coast of China and extending east across the North Pacific storm track. These differences suggest the modification of precipitating clouds on a large scale with potentially important implications for seasonal rainfall estimates. Coincident TRMM/CloudSat observations for a case from 3 April 2007 show striking differences in both rain area and rainfall intensity from the TMI, PR, and CloudSat retrievals. Observations from the 94‐GHz CloudSat radar, which is highly sensitive to the onset of rain, confirm the presence of widespread light rain/drizzle containing relatively small drops below the ∼17 dBZ PR detection threshold. For pixels with reflectivities above the PR detection threshold, large differences are present in the satellite rain intensity estimates, which are consistent with either a decrease in the mean drop size, an increase in ratio of cloud water to rainwater, or both. To explore the potential link between aerosols and the observed changes in the observed cloud microphysics, idealized cloud‐resolving model (CRM) simulations initialized for the 3 April 2007 case are performed. The model results are generally consistent with the observations indicating high aerosol concentrations leading to an overall increase in the ratio of cloud water to rainwater for developed systems, as well as a delay in the onset of warm rain. The simulations also show an initial decrease in the mean raindrop size, although larger drops develop later leading to an overall increase in the total rainfall accumulation. On the basis of the combination of observations and CRM simulations, therefore, it is hypothesized that the observed differences may be due to an increase in the ratio of cloud water to rainwater leading to an overestimate in rain intensity by the CloudSat/TMI retrievals and/or a decrease in the mean drop size leading to an underestimate by the PR retrieval.
Martian South Polar Layered Deposit stratigraphy and implications for accumulation history
Authors: Milkovich Sarah M. , Plaut Jeffrey J.
Published: June 2008

We examine the stratigraphy of the Martian South Polar Layered Deposits (SPLD) exposed on scarp walls throughout the deposit using THEMIS visible (17 m/pxl) images from the Mars Odyssey spacecraft. By correlating marker layers and packets of layers among multiple locations, we are able to identify three sequences of layers within the SPLD, each with a different areal extent. While certain layers are correlatable over hundreds of km, implying widespread deposition of polar material, the absence of several identified layer sequences in some regions, notably the Promethei Lingula region, as well as the presence of several unconformities, indicates that deposition and sublimation or erosion of these deposits has not been uniform around the SPLD. This is further supported by the fact that individual layers display variations in thickness greater than a factor of 4 over hundreds of kilometers. Using these observations we propose an internal structure for the SPLD, with individual layers roughly following a dome‐like shape toward the center of the deposit, and becoming more planar toward the margins. We infer that the SPLD underwent at least three major periods of layer accumulation; each period is distinguished by a change in areal extent of net accumulation. The changes in areal extent of the layer sequences may be due to changes in Martian climate causing episodes of significant erosion of the SPLD between the deposition of each sequence or causing wide variations in accumulation zones of the SPLD over the history of these deposits.
Learning from model improvement: On the contribution of complementary data to process understanding
Authors: Fenicia Fabrizio , McDonnell Jeffrey J. , Savenije Hubert H. G.
Published: June 2008

A priori determined model structures are common in catchment rainfall‐runoff modeling. While this has resulted in many ready‐to‐use modeling tools, there are several shortcomings of a one‐size‐fits‐all model structure. The uniqueness of catchments with respect to their hydrological behavior and the need to adapt model complexity to data availability challenge this status quo. We present a flexible approach to model development where the model structure is adapted progressively based on catchment characteristics and the data described by the experimentalist. We demonstrate this approach with the Maimai catchment in New Zealand, a location with a large availability of data, including stream discharge, groundwater levels, and stream isotope measurements. Different types of data are introduced progressively, and the architecture of the model is adjusted in a stepwise fashion to better describe the processes suggested by the new data sources. The revised models are developed in a way to strike a balance between model complexity and data availability, by keeping models as simple as possible, but complex enough to explain the dynamics of the data. Our work suggests that (1) discharge data provides information on the dynamics of storage (represented by the “free” water in the reservoirs) subject to pressure wave propagation generated by rainfall into the catchment, (2) groundwater data provides information on thresholds and on the contribution of different portions of the catchment to stream discharge, and (3) isotope data provides information on particle transport and mixing of the rainfall with the storage present in the catchment. Moreover, while groundwater data appear to be correlated with discharge data, and only a marginal improvement could be obtained adding this information to the model development process, isotope data appear to provide an orthogonal view on catchment behavior. This result contributes to understanding the value of data for modeling, which may serve as a guidance in the process of gauging ungauged catchments.
“Sunshade World”: A fully coupled GCM evaluation of the climatic impacts of geoengineering
Authors: Lunt D. J. , Ridgwell A. , Valdes P. J. , Seale A.
Published: June 2008

Sunshade geoengineering ‐ the installation of reflective mirrors between the Earth and the Sun to reduce incoming solar radiation, has been proposed as a mitigative measure to counteract anthropogenic global warming. Although the popular conception is that geoengineering can re‐establish a ‘natural’ pre‐industrial climate, such a scheme would itself inevitably lead to climate change, due to the different temporal and spatial forcing of increased CO2 compared to reduced solar radiation. We investigate the magnitude and nature of this climate change for the first time within a fully coupled General Circulation Model. We find significant cooling of the tropics, warming of high latitudes and related sea ice reduction, a reduction in intensity of the hydrological cycle, reduced ENSO variability, and an increase in Atlantic overturning. However, the changes are small relative to those associated with an unmitigated rise in CO2 emissions. Other problems such as ocean acidification remain unsolved by sunshade geoengineering.
Experimental reconstruction of monsoon drought variability for Australasia using tree rings and corals
Authors: D'Arrigo Rosanne , Baker Patrick , Palmer Jonathan , Anchukaitis Kevin , Cook Garry
Published: June 2008

An experimental reconstruction uses three well‐dated, annually‐resolved proxies from Australasia (0–40°S, 95–155°E) to provide large‐scale information on Sep–Jan Australasian monsoon variability based on the Palmer Drought Severity Index (PDSI) for 1787–2002. The proxies are: (1) a ring width chronology of Callitris intratropica for northern Australia (1847–2006); (2) a tree‐ring and coral–based reconstruction of the Oct–Nov PDSI (1787–2003) for Java, Indonesia; and (3) a rainfall reconstruction for northeastern Australia (1631–2002) based on Great Barrier Reef coral luminescence. All three proxies show considerable explanatory value for reconstructing monsoon rainfall variability over much of Australia and environs, which will improve as additional records become available. The success of this “proof of concept” experiment largely reflects the highly significant, spatially‐coherent correlations between austral spring and summer PDSI, Australasian climate and ENSO.
Observations and simulations of quasiperiodic ionospheric oscillations and large‐scale traveling ionospheric disturbances during the December 2006 geomagnetic storm
Authors: Lei Jiuhou , Burns Alan G. , Tsugawa Takuya , Wang Wenbin , Solomon Stanley C. , Wiltberger Michael
Published: June 2008

A numerical simulation was performed to investigate quasiperiodic ionospheric oscillations that were observed with periods of 4–5 h by the ionosonde network (Okinawa, Yamagawa, Kokubunji, and Wakkanai) in Japan during the 15 December 2006 magnetic storm. This simulation used the Coupled Magnetosphere Ionosphere Thermosphere (CMIT) 2.0 model. The CMIT model reproduced the main characteristics of the observed ionospheric oscillations, although it remains a challenging task to simulate the observations in a quantitative sense. Term analysis of the ion continuity equation demonstrated that the ionospheric oscillations in this event were mainly induced by the disturbed neutral winds, which were associated with the large scale thermospheric circulation and traveling atmospheric disturbances (TADs) during the storm. The TADs simulated from the model were then compared with those observed by the GPS Earth Observation Network (GEONET) in Japan to validate the simulation results. A prominent northward propagating large‐scale traveling ionospheric disturbance (LSTID) during daytime, seen by the GEONET total electron content (TEC) data, was captured by the CMIT model. Two southward LSTIDs observed by GEONET GPS network were also reproduced by the CMIT model. However, the model gave faster phase speeds for the southward propagating LSTID occurred during 0620–0800 UT and the northward propagating LSTID; furthermore, the model missed the LSTID seen in the TEC perturbation data during 0140–0220 UT. Finally, both observations and simulations showed a strong hemispheric asymmetry for the TAD propagation that occurred during 0000–0400 UT, which may be associated with the hemispheric asymmetry of the change of Joule heating at high latitude.
Cross sections for the O II (83.4 nm) emission from electron impact on O2
Authors: Malone C. P. , Johnson P. V. , McConkey J. W. , Kanik I.
Published: June 2008

Vacuum ultraviolet emissions resulting from electron impact on O2 have been studied at 100, 200, and 300 eV. The ratio of the 83.4 nm and 130.4 nm emissions were used in combination with the recently revised recommendation of the O I (130.4 nm) cross section to determine the O II (83.4 nm) cross sections. The present results are compared with existing measurements and resolve the controversy that has existed regarding the magnitude and shape of the 83.4 nm emission cross section. We recommend an O II (83.4 nm) emission cross section of (1.50 ± 0.17) × 10−18 cm2 at 100 eV.
Using Student Article Presentations for Evidence-Based Practice and Evidence-Based Education
Authors: Hester Eva Jackson
Published: June 2008

Increased attention to evidence-based practice (EBP) within the profession has required more emphasis on research studies used in graduate courses. Instructors are now faced with determining effective methods for incorporating and documenting the inclusion of research aspects of EBP. This paper discusses the importance of incorporating EBP research aspects within the framework of evidence-based education (EBE) by using student article presentations.
“Two First Authors”: Defining and Evaluating Collaborative Partnership
Authors: Jackson Catherine , Seymour Patricia J.
Published: June 2008

Models of academic collaboration typically assume roles dictated by experience or seniority. Our experience as two new faculty members engaged in a long-term curriculum revision project led us to a partnership-type of collaboration, the “two first authors” model. This model required us to assume flexible roles with regard to workload and participation at various points in the project. Although this model of collaboration does not meet traditional expectations for evaluating faculty roles in academic projects, the intellectual and practical benefits became clear throughout the progress of our work. We encourage instructors, researchers and reviewers to explicitly identify their assumptions and expectations regarding collaboration, and to explore options related to establishing fluid roles for members of a collaborative team.
Evidence-Based Analysis of Formative Assessment
Authors: Levey Sandra
Published: June 2008

This study examined evidence for the effectiveness of one approach to formative assessment based on students' anonymous reports of material that was “best learned” and “least learned” at the end of each lecture. Responses to “least learned” information were covered in subsequent lectures or were placed on Blackboard. Two graduate classes' judgments of the effectiveness of this approach to formative assessment were examined, along with the specific topics that were reported to present the most difficulty according to their reports of “least learned” information. Two classes participated in this study: 20 students in the first class and 21 in the second. Students were presented with a questionnaire that contained a Likert Scale that asked if this approach led to (a) better understanding of course material, (b) better coverage by the instructor, (c) appreciation of anonymity, (d) asking more questions, and (e) more confidence in asking questions. There was no significant difference between the responses to the questionnaire for the first class and the second class. Students reported benefits of formative assessment which provides an ongoing and evidence-based analysis of learning skills.
Student Self-Directed Professional Development as a Formative Assessment Skill
Authors: Cascella Paul W. , Vogel Donald A.
Published: June 2008

While professional development is most typically thought of as continuing education beyond the master’s degree, we have formulated a process for master’s degree students in speech-language pathology to identify and self-evaluate their own professional development. Current standards for preparing graduate students in speech-language pathology focus primarily on introducing students to an extensive array of academic and clinical topics (e.g., disorders, conditions, service delivery models, evidence-based practice). However, indirect support for self-directed professional development can be found in two CAA standards, Standard 3.1B (i.e., “breadth and depth of the scope of practice”) and Standard 4.2 (i.e., reasonable accommodations). The authors describe a series of self-reflection activities that are integrated within speech-language pathology course work for the purpose of fostering professional development at the preservice level. Student comments and outcomes to date are described.
The theory of the kink mode during the vertical plasma disruption events in tokamaks
Authors: Zakharov Leonid E.
Published: June 2008

This paper explains the existence of the m∕n=1∕1 kink mode during the vertical disruption event when the plasma has an electrical contact with the plasma facing conducting surfaces. It is shown that the kink perturbation can be in an equilibrium state even with a stable safety factor q>1, if the surface currents, excited by the kink mode, can flow through the conducting structure. The present theory suggests an explanation of the so-called sideway forces on the tokamak in-vessel components during the disruption event. For the general case of such wall touching kink modes, the energy principle is derived, which includes the current sharing effect.
Exponential stability of synchronization in asymmetrically coupled dynamical networks
Authors: Li Zhi
Published: June 2008

Based on the original definition of the synchronization stability, a general framework is presented for investigating the exponential stability of synchronization in asymmetrically coupled networks. By choosing an appropriate Lyapunov function, we prove that the mechanism of the exponential synchronization stability is the asymmetrical coupling matrix with diffusive condition. We deduce the second largest eigenvalue of a symmetric matrix to govern the exponential stability of synchronization in asymmetrically coupled networks. Moreover, we have given the threshold value which can guarantee that the states of the asymmetrically coupled network achieve the exponential stability of synchronization.
Stochastic dynamics of bionanosystems: Multiscale analysis and specialized ensembles
Authors: Pankavich S. , Miao Y. , Ortoleva J. , Shreif Z. , Ortoleva P.
Published: June 2008

An approach for simulating bionanosystems such as viruses and ribosomes is presented. This calibration-free approach is based on an all-atom description for bionanosystems, a universal interatomic force field, and a multiscale perspective. The supramillion-atom nature of these bionanosystems prohibits the use of a direct molecular dynamics approach for phenomena such as viral structural transitions or self-assembly that develop over milliseconds or longer. A key element of these multiscale systems is the cross-talk between, and consequent strong coupling of processes over many scales in space and time. Thus, overall nanoscale features of these systems control the relative probability of atomistic fluctuations, while the latter mediate the average forces and diffusion coefficients that induce the dynamics of these nanoscale features. This feedback loop is overlooked in typical coarse-grained methods. We elucidate the role of interscale cross-talk and overcome bionanosystem simulation difficulties with (1) automated construction of order parameters (OPs) describing suprananometer scale structural features, (2) construction of OP-dependent ensembles describing the statistical properties of atomistic variables that ultimately contribute to the entropies driving the dynamics of the OPs, and (3) the derivation of a rigorous equation for the stochastic dynamics of the OPs. As the OPs capture hydrodynamic modes in the host medium, “long-time tails” in the correlation functions yielding the generalized diffusion coefficients do not emerge. Since the atomic-scale features of the system are treated statistically, several ensembles are constructed that reflect various experimental conditions. Attention is paid to the proper use of the Gibbs hypothesized equivalence of long-time and ensemble averages to accommodate the varying experimental conditions. The theory provides a basis for a practical, quantitative bionanosystem modeling approach that preserves the cross-talk between the atomic and nanoscale features. A method for integrating information from nanotechnical experimental data in the derivation of equations of stochastic OP dynamics is also introduced.
Water and ice in contact with octadecyl-trichlorosilane functionalized surfaces: A high resolution x-ray reflectivity study
Authors: Mezger Markus , Schöder Sebastian , Reichert Harald , Schröder Heiko , Okasinski John , Honkimäki Veijo , Ralston John , Bilgram Jörg , Roth Roland , Dosch Helmut
Published: June 2008

We present a high energy x-ray reflectivity study of the density profiles of water and ice at hydrophobic and hydrophilic substrates. At the hydrophobic water/octadecyl-trichlorosilane (water-OTS) interface, we find clear evidence for a thin density depletion layer with an integrated density deficit corresponding to approximately 40% of a monolayer of water molecules. We discuss the experimental results in terms of a simple model of hydrophobic/hydrophilic solid-liquid interfaces. Our results also exclude the presence of nanobubbles. A detailed study of possible radiation damage induced by the intense x-ray beam at the dry OTS surface and at the ice-OTS, as well as at water-OTS interfaces, discloses that noticeable damage is only induced at the water-OTS interface, and thus points to the dominant role of highly mobile radicals formed in bulk water close to the interface.
Nonlinear interactions in electrophoresis of ideally polarizable particles
Authors: Saintillan David
Published: June 2008

In the classical analysis of electrophoresis, particle motion is a consequence of the interfacial fluid slip that arises inside the ionic charge cloud (or Debye screening layer) surrounding the particle surface when an external field is applied. Under the assumptions of thin Debye layers, weak applied fields, and zero polarizability, it can be shown that the electrophoretic velocity of a collection of particles with identical zeta potential is the same as that of an isolated particle, unchanged by interactions [L. D. Reed and F. A. Morrison, “Hydrodynamic interaction in electrophoresis,” J. Colloid Interface Sci. 54, 117 (1976)]. When some of these assumptions are relaxed, nonlinear effects may also arise and result in relative motions. First, the perturbation of the external field around the particles creates field gradients, which may result in nonzero dielectrophoretic forces due to Maxwell stresses in the fluid. In addition, if the particles are able to polarize, they can acquire a nonuniform surface charge, and the action of the field on the dipolar charge clouds surrounding them drives disturbance flows in the fluid, causing relative motions by induced-charge electrophoresis. These two nonlinear effects are analyzed in detail in the prototypical case of two equal-sized ideally polarizable spheres carrying no net charge, using accurate boundary-element simulations, along with asymptotic calculations by the method of twin multipole expansions and the method of reflections. It is found that both types of interactions result in significant relative motions and can be either attractive or repulsive depending on the configuration of the spheres.
On breathing in temperature in flows of a vapor due to evaporation from its spherical condensed phase
Authors: Onishi Yoshimoto , Fuji Takahiro
Published: June 2008

One of the features in evaporation flows from a condensed phase of finite size placed in an infinite expanse of its vapor in three-dimensional space would be the existence of the so-called breathing phenomenon of temperature in its course to approach the state at infinity. The pressure of the gas, as one might expect, approaches uniformly the state specified at infinity, whereas the temperature does not but seems to take some rest for a while on its way and then starts again to approach slowly and monotonically the specified state at infinity. This is what we call breathing in temperature. This phenomenon was pointed out by Takeuchi and Onishi [Rarefied Gas Dynamics, edited by C. Shen (Pekin University Press, Beijing, 1996), pp. 887–882] in their weakly nonlinear asymptotic analysis based on the kinetic equation on steady evaporation flows from a spherical condensed phase placed in a mixture of its vapor and a noncondensable gas. The detailed discussion on this phenomenon, however, is not yet given; particularly, the reason why the temperature seems to take some rest and what originates this are not known. The present study aims to clarify the phenomenon, paying particular attention to its origin and pursuing what it is. In so doing, we have made a certain conjecture on the origin and, to show this to be true, we have taken up a simple initial and boundary-value problem of evaporation from a spherical condensed phase immersed in an infinite expanse of its vapor. Starting from a simple initial state, we followed in a numerical simulation how the flow field developed in time, getting finally a conclusion that the contact region produced initially by the evaporation process which occurs at the interface of the condensed phase is the cause; the contact region remains left undiffused at a finite distance from the interface in the flow field even at the steady state due to the counterbalance between the convection ability of the flow in the radial direction and the diffusion ability of the region itself in the opposite direction.
A functional form for the energy spectrum parametrizing bottleneck and intermittency effects
Authors: Meyers Johan , Meneveau Charles
Published: June 2008

A functional form for the energy spectrum is proposed, which includes a predissipative bottleneck effect and an intermittency correction. The form contains five parameters for which five constraints are formulated. Three of them are related to the condition that the integrals of the spectrum, E(k), k2E(k), and k4E(k), should yield appropriate values for total energy, enstrophy, and derivative skewness. Two extra conditions are inferred from the location and height of the intermittency corrected dissipation peak, which is found to be independent of Reynolds number (70<Reλ<1200) in high-resolution direct numerical simulations. Good representation of Direct Numerical Simulation (DNS) and experimental data is found for a range of Reynolds numbers Reλ between 250 and 1200.
Generalized-gradient exchange-correlation hole obtained from a correlation factor ansatz
Authors: Bahmann Hilke , Ernzerhof Matthias
Published: June 2008

The Perdew–Burke–Ernzerhof (PBE) approximation to the exchange-correlation energy is employed as reference point for the construction of an angle-averaged exchange-correlation hole. First, we develop a new model for the PBE exchange hole. In contrast to the previous model [Ernzerhof and Perdew, J. Chem. Phys. 109, 3313 (1998)], it contains an atomic exchange hole, similar to the Becke–Roussel model [Becke and Roussel, Phys. Rev. A 39, 3761 (1989)]. A correlation factor, i.e., a function multiplying the exchange hole, is proposed that turns the exchange into an exchange-correlation hole. The correlation factor has a simple form and is determined through a number of known conditions that should be satisfied by a generalized-gradient exchange-correlation hole.
Nonmixing vortex cores in wavy Taylor vortex flow
Authors: Rudman Murray , Metcalfe Guy , Graham Lachlan J. W.
Published: June 2008

When the symmetry of axisymmetric Taylor vortex flow is broken, time-periodic wavy vortex flow (WVF) appears and quite quickly becomes globally chaotic (in the Lagrangian sense) with increasing Reynolds number. Previously published simulations of WVF suggest that beyond a certain Re, nonmixing vortex cores reappear in the flow and grow in size with further increases in Re. This reappearance occurs well into the inertia-dominated flow regime and coincides with a decrease in axial fluid dispersion and an increase in flow symmetry as measured by certain Eulerian symmetry measures. In this brief paper, we present experimental dye-reaction visualization results from two WVF wave states in the region where vortex cores are predicted numerically. The experimental results show unambiguous visual evidence for the existence of vortex cores and provide visual agreement with the numerical results. They are significant in that experimental evidence for these structures in WVF has not been reported before. The results also suggest that vortex-to-vortex transport occurs via sheetlike structures that are pulled from one vortex to another and become wrapped around the vortex cores before being stretched to the point at which molecular diffusion dominates.
Electronic effects induced by single hydrogen atoms on the Ge(001) surface
Authors: Radny M. W. , Shah G. A. , Smith P. V. , Schofield S. R. , Curson N. J.
Published: June 2008

The properties of an isolated dangling bond formed by the chemisorption of a single hydrogen atom on a dimer of the Ge(001) surface are investigated by first-principles density functional theory (DFT) calculations, and scanning tunneling microscopy (STM) measurements. Two stable atomic configurations of the Ge–Ge–H hemihydride with respect to the neighboring bare Ge–Ge dimers are predicted by DFT. For both configurations, the unpaired electron of the H∕Ge(001) system is found to be delocalized over the surface, rendering the isolated dangling bond of the hemihydride unoccupied. However, local surface charge accumulation, such as may occur during STM imaging, leads to the localization of two electrons onto the hemihydride dangling bond. The calculated surface densities of states for one of the charged Ge–Ge–H hemihydride configurations are found to be in good agreement with atomic-resolution STM measurements on n-type Ge(001). Comparison with a Si–Si–H hemihydride of the Si(001) surface shows similarities in structural properties, but substantial differences in electronic properties.
Invited Article: VEDA: A web-based virtual environment for dynamic atomic force microscopy
Authors: Melcher John , Hu Shuiqing , Raman Arvind
Published: June 2008

We describe here the theory and applications of virtual environment dynamic atomic force microscopy (VEDA), a suite of state-of-the-art simulation tools deployed on nanoHUB (www.nanohub.org) for the accurate simulation of tip motion in dynamic atomic force microscopy (dAFM) over organic and inorganic samples. VEDA takes advantage of nanoHUB’s cyberinfrastructure to run high-fidelity dAFM tip dynamics computations on local clusters and the teragrid. Consequently, these tools are freely accessible and the dAFM simulations are run using standard web-based browsers without requiring additional software. A wide range of issues in dAFM ranging from optimal probe choice, probe stability, and tip-sample interaction forces, power dissipation, to material property extraction and scanning dynamics over hetereogeneous samples can be addressed.
Analysis of asymmetric Bragg–coupler based optical add-drop multiplexer operating on certain phase-matching conditions
Authors: Muldarisnur A. , Soehianie A. , Iskandar A. , Tjia M. O.
Published: June 2008

The operational characteristics of a two-dimensional optical add-drop multiplexer were investigated for coupled planar waveguides with embedded grating structure in one of them. The analytical formulation of the wave propagation property was carried out on the basis of coupled mode theory derived from the Lorentz reciprocity theorem. Simplified mathematical formulation and analytical solution for the device modeling were attained by means of well-defined phase-matching conditions. Further numerical analysis was performed to study the effects of various geometrical and material parameters on the device performance in terms of the drop reflectance, input insertion loss, the transmission bandwidth, and the midgap separation. The optimal performance characterized by 99% drop reflectance, 1% input insertion loss, and 0.2 nm full width at half maximum at λ=1.55 μm was achieved with realistically chosen device parameters consisting of refractive indices of the materials and geometrical parameters of the system. This result has demonstrated in principle the feasibility of developing the model for dense wavelength division multiplexing application.
Spin reorientation in the Er2−xFe14+2xSi3 single crystal studied by the Fe57 Mössbauer spectroscopy and magnetic measurements
Authors: Żukrowski J. , Błachowski A. , Ruebenbauer K. , Przewoźnik J. , Sitko D. , Kim-Ngan N.-T. H. , Tarnawski Z. , Andreev A. V.
Published: June 2008

Spin reorientation in the single crystal of Er2−xFe14+2xSi3 with x=0.25(5) has been studied in the temperature range of 4–300K by means of the magnetic measurements and Mössbauer spectroscopy on Fe57 by using the 14.41keV resonant transition. The bulk magnetic moment has been measured versus applied field up to ±8.9T along the c axis of the P63∕mmc cell at 4K. The hysteresis loop has been measured at 300K for the external field applied along the c axis. The bulk moment has been measured versus temperature in the moderate external field of 0.1T applied along the c axis. The ac susceptibility has been measured for several frequencies and amplitudes of the ac field applied along the c axis versus temperature either in the null external field or in the external field of 0.1T along the c axis. Mössbauer measurements were performed versus temperature on the powder sample and single crystal with the radiation beam oriented along the c axis. The spin reorientation from the [a−b] plane onto the c axis occurs for all iron sublattices except for iron dumbbells substituting erbium (2b) in the temperature range of 130–80K due to the domain flip mechanism. The reorientation is less perfect for the sublattices containing silicon, i.e., (12j) and (12k). A gradual second reorientation of the above sublattices occurs below 50K leading to the partial recovery of the high temperature spin structure. Iron dumbbells substituting erbium do not participate in this recovery. For temperatures below 20K, some dipolar contribution to the iron field on (4f) dumbbell sites is seen. It is probably induced by the reorientation of the erbium magnetic moments.
Adhesion induced mesoscale instability patterns in thin PDMS-metal bilayers
Authors: Pangule Ravindra C. , Banerjee Indrani , Sharma Ashutosh
Published: June 2008

We show that the surface of a thin elastomer-metal (aluminum) hybrid bilayer becomes spontaneously patterned when brought in adhesive contact with a rigid surface. The self-organized surface patterns show three distinct morphological phases—columns, labyrinths, and holes—depending on the area of contact. The characteristic wavelength of these patterns is found to be 2.94±0.20 times the total film thickness, independent of the morphological phase and the surface properties of the contacting surface. Interestingly, the metal films 60–120nm thick showed the same scaling, but the bilayers with thicker metal films were completely stable. This observation demonstrates for the first time a “hard” transition to the instability as the elastic stiffness of the film is varied. We also report a protocol for alignment of the instability patterns and for transferring the metal patterns to another surface.
Analysis of operation mechanism of semi-insulating GaAs photoconductive semiconductor switches
Authors: Tian Liqiang , Shi Wei
Published: June 2008

This paper reports that the quenched-domain mode of luminous charge domain has been observed in semi-insulating (SI) GaAs photoconductive semiconductor switches (PCSSs) and the typical characteristics of lock-on effect have been explained based on the model of luminous charge domain. It is shown that the domain is formed by photogenerated carriers and the quenched domain is due to the interaction of the circuit self-excitation and transferred-electron oscillation in the bulk of switch. During the transit of the domain, the bias electric field (larger than Gunn threshold) across the switch is modulated by the ac electric field, when the instantaneous bias electric field is swinging below the sustaining field (the minimum electric field required to support the domain), and then the quenched-domain mode is obtained. When PCSSs operate in nonlinear mode, the ultrahigh electric field of domain induced by photogenerated carriers leads to strong impact ionization accompanied by electron-hole recombination radiation in the switch. Therefore new avalanche domains can be nucleated uninterruptedly by the carriers generated by absorption of recombination radiation which causes the effective carrier velocities to be larger than the saturation velocity. Lock-on field resulted from the length proportional number of domains and steadfast electric fields inside and outside the domains, and the recovery of lock-on effect is caused by the domain quenching. The calculations agree with the experimental results, and the analysis also indicates that SI-GaAs PCSS is a photoactivated charge domain device.
Steady-state and time-resolved photoluminescence from relaxed and strained GaN nanowires grown by catalyst-free molecular-beam epitaxy
Authors: Schlager John B. , Bertness Kris A. , Blanchard Paul T. , Robins Lawrence H. , Roshko Alexana , Sanford Norman A.
Published: June 2008

We report steady-state and time-resolved photoluminescence (TRPL) measurements on individual GaN nanowires (6–20 μm in length, 30–940 nm in diameter) grown by a nitrogen-plasma-assisted, catalyst-free molecular-beam epitaxy on Si(111) and dispersed onto fused quartz substrates. Induced tensile strain for nanowires bonded to fused silica and compressive strain for nanowires coated with atomic-layer-deposition alumina led to redshifts and blueshifts of the dominant steady-state PL emission peak, respectively. Unperturbed nanowires exhibited spectra associated with high-quality, strain-free material. The TRPL lifetimes, which were similar for both relaxed and strained nanowires of similar size, ranged from 200 ps to over 2 ns, compared well with those of low-defect bulk GaN, and depended linearly on nanowire diameter. The diameter-dependent lifetimes yielded a room-temperature surface recombination velocity S of 9×103 cm/s for our silicon-doped GaN nanowires.
Integrated computational approach to vibrationally resolved electronic spectra: Anisole as a test case
Authors: Bloino Julien , Biczysko Malgorzata , Crescenzi Orlando , Barone Vincenzo
Published: June 2008

A new general and effective procedure to compute Franck–Condon spectra from first principles is exploited to elucidate the subtle features of the vibrationally resolved optical spectra of anisole. Methods based on the density functional theory and its time-dependent extension for electronic excited states [B3LYP∕6-311+G(d,p) and TD-B3LYP∕6-311+G(d,p)] have been applied to geometry optimizations and harmonic frequency calculations. Perturbative anharmonic frequencies [J. Chem. Phys. 122, 014108 (2005)] have been calculated for the ground state, and the Duschinsky matrix elements have been used to evaluate the corresponding anharmonic corrections for the first excited electronic state. The relative energetics of both electronic states has been refined by single point calculations at the coupled clusters (CC) level with the aug-cc-pVDZ basis set. Theoretical spectra have been evaluated using a new optimized implementation for the effective computation of Franck–Condon factors. The remarkable agreement between theoretical and experimental spectra allowed for revision of some assignments of fundamental vibrations in the S1 state of anisole.
More transition amplitudes on the Riemann sphere
Authors: Dimock J.
Published: June 2008

We consider a conformal field theory for bosons on the Riemann sphere. Correlation functions are defined as singular limits of functional integrals. The main result is that these amplitudes define transition amplitudes, that is multilinear Hilbert–Schmidt functionals on a fixed Hilbert space.
Kirkwood–Buff theory of four and higher component mixtures
Authors: Kang Myungshim , Smith Paul E.
Published: June 2008

Explicit expressions are developed for the chemical potential derivatives, partial molar volumes, and isothermal compressibility of solution mixtures involving four components at finite concentrations using the Kirkwood–Buff theory of solutions. In addition, a general recursion relationship is provided which can be used to generate the chemical potential derivatives for higher component solutions.
Structures, energetics, vibrational spectra of NH4+(H2O)n=4,6 clusters:Ab initiocalculations and first principles molecular dynamics simulations
Authors: Karthikeyan S. , Singh Jiten N. , Park Mina , Kumar Rajesh , Kim Kwang S.
Published: June 2008

Important structural isomers of NH4+(H2O)n=4,6 have been studied by using density functional theory, Møller–Plesset second order perturbation theory, and coupled-cluster theory with single, double, and perturbative triple excitations [CCSD(T)]. The zero-point energy (ZPE) correction to the complete basis set limit of the CCSD(T) binding energies and free energies is necessary to identify the low energy structures for NH4+(H2O)n=4,6 because otherwise wrong structures could be assigned for the most probable structures. For NH4+(H2O)6, the cage-type structure, which is more stable than the previously reported open structure before the ZPE correction, turns out to be less stable after the ZPE correction. In first principles Car–Parrinello molecular dynamics simulations around 100K, the combined power spectrum of three lowest energy isomers of NH4+(H2O)4 and two lowest energy isomers of NH4+(H2O)6 explains each experimental IR spectrum.
Dynamic correlations with time-dependent quantum Monte Carlo
Authors: Christov Ivan P.
Published: June 2008

In this paper, we solve quantum many-body problem by propagating ensembles of trajectories and guiding waves in physical space. We introduce the “effective potential” correction within the recently proposed time-dependent quantum Monte Carlo methodology to incorporate the nonlocal quantum correlation effects between the electrons. The associated correlation length is calculated by adaptive kernel density estimation over the walker distribution. The general formalism is developed and tested on one-dimensional helium atom in laser field of different intensities and carrier frequencies. Good agreement with exact results for the atomic ionization is obtained.
Hard x-ray generation by low energy, tightly focused laser pulses
Authors: Takano K. , Nemoto K. , Nayuki T. , Oishi Y. , Fujii T. , Zhidkov A. , Hotta E. , Todoriki M. , Hasegawa S.
Published: June 2008

The interaction of λ2 laser pulses with plasma is studied to scale the high-field physics from high power laser systems to low-energy, high-repetition-rate lasers. For this purpose, 20mJ, 40fs Ti:sapphire laser pulses were focused in a spot of 2.2×2.3μm2 with a maximum intensity of I∼1019W∕cm2. Efficient generations of energetic electrons (∼1MeV) and hard x-rays with energies of over 100keV were obtained by irradiating the low-energy, tightly focused laser pulses on a massive Cu target.
A generalized Poisson equation and short-range self-interaction energies
Authors: Varganov Sergey A. , Gilbert Andrew T. B. , Gill Peter M. W.
Published: June 2008

We generalize the Poisson equation to attenuated Newtonian potentials. If the attenuation is at least exponential, the equation provides a local mapping between the density and its potential. We use this to derive several density functionals for the short-range self-interaction energy.
Contact photolithographic micropatterning for bistable nematic liquid crystal displays
Authors: Niitsuma Jun-ichi , Yoneya Makoto , Yokoyama Hiroshi
Published: June 2008

Orthogonal alignment patterns of 2×2μm2 size that induce nematic bistability were fabricated over 4×4cm2 by a double exposure process employing a rewritable photoalignment material. Bistable liquid crystal cells with patterned surfaces were fabricated, and reproducible switching between the bistable states driven by orthogonal in-plane electric fields at 8.2 and 3.9V∕μm was confirmed.