bluuebunny/crossref_metadata_2025 huggingface dataset:
Regional TEC mapping with Random Field Priors and Kriging
Authors: Sayin I. , Arikan F. , Arikan O.
Published: October 2008

Total Electron Content (TEC) is an important parameter in monitoring ionospheric variability. In a given region, TEC can be obtained only by interpolation of measurements due to sparsity of the useful data. The lack of a complete ionospheric model hinders the choice of the optimum interpolation algorithm. A plausible solution to this problem can be found by investigating the performance of alternative interpolation algorithms on synthetically generated TEC surfaces for various sampling scenarios. The synthetic TEC data should represent the possible trends and variations of ionosphere. In this study, the performance of Random Field Priors (RFP) and Kriging interpolation algorithms are investigated over the parameter set of spatially correlated synthetic TEC data for various variance, range and trend options. Synthetic TEC data are sampled with regular and random sampling patterns, for number of samples from sparse to dense samplings. Interpolation scenarios are generated to investigate the improvement of the interpolation accuracy of the methods for each parameter. It is observed that for the random sampling patterns, when the trend is not modeled correctly, the errors of the algorithms increase and when the trend is modeled correctly, the reconstruction errors decrease. For the regular sampling patterns, the trend model does not affect the accuracy of the methods, and the reconstruction errors are close to lower bound error values. An example reconstruction is also provided over GPS‐derived TEC, and error variances are compared over Kriging and Random Field Prior algorithms.
Exploring wind energy potential off the California coast
Authors: Jiang Qingfang , Doyle James D. , Haack Tracy , Dvorak Michael J. , Archer Cristina L. , Jacobson Mark Z.
Published: October 2008

Wind energy represents the nearest term cost‐effective renewable energy source. While efforts have been made to assess wind energy potential over land around the world, offshore wind energy resources are largely unexplored, in part because these regions have relatively sparse wind observations. In this study, the wind energy potential offshore of the California coast is evaluated using a well‐tested high‐resolution numerical model dataset. We found that along the coastline, the low‐level winds exhibit strong spatial variation and are characterized by alternating windspeed maxima and minima near coastal promontories associated with the interaction between the marine boundary layer and coastal topography. Further analysis highlights the enormous and reliable wind energy development potential in these persistent offshore windspeed maxima.
Contemporary vertical velocity of the central Basin and Range and uplift of the southern Sierra Nevada
Authors: Fay Noah P. , Bennett Richard A. , Hreinsdóttir Sigrún
Published: October 2008

We estimate the present‐day vertical velocity field in the central Basin and Range and Sierra Nevada mountains and consider causative processes. We analyzed data from 16 continuously operating Global Positioning System stations finding that relative vertical velocity precision (WRMS scatter) is ∼0.25 mm/yr. These data demonstrate that the southern Sierra Nevada is experiencing uplift of ∼0.5 mm/yr relative to a local reference frame in southern Nevada, and ∼1 mm/yr relative to stations in the northern Mojave Desert. As a possible source of these motions, we explored post‐seismic viscoelastic relaxation following the 1872 Owens Valley and 1952 Kern County earthquakes with models of laterally homogeneous lower crust and upper mantle viscosity. Post‐seismic deformation from these two earthquakes cannot entirely explain the data set. Lateral variations in viscosity or contributions from other regional earthquakes may also be important.
The Conrad Rise as an obstruction to the Antarctic Circumpolar Current
Authors: Durgadoo J. V. , Lutjeharms J. R. E. , Biastoch A. , Ansorge I. J.
Published: October 2008

The Antarctic Circumpolar Current (ACC) carries water freely around the whole continent of Antarctica, but not without obstructions. Some, such as the Drake Passage, constrict its path, while others, such as mid‐ocean ridges, may induce meandering in the current's cores and may cause the genesis of mesoscale turbulence. It has recently been demonstrated that some regions that are only relatively shallow may also have a major effect on the flow patterns of the ACC. This is here shown to be particularly true for the Conrad Rise. Using the trajectories of surface drifters, altimetry and the simulated velocities from a numerical model, we show that the ACC bifurcates at the western side of this Rise. In this process it forms two intense jets at the two meridional extremities of the Rise with a relatively stagnant water body over the Rise itself. Preliminary results from a recent cruise provide compelling support for this portrayal.
Anomalous geomagnetic storm of 21–22 January 2005: A storm main phase during northward IMFs
Authors: Du A. M. , Tsurutani B. T. , Sun W.
Published: October 2008

The major (minimum Dst = −105 nT) magnetic storm which occurred on 21–22 January 2005 is highly anomalous because the storm main phase (identified by the SYM‐H indices) developed during northward interplanetary magnetic fields (IMFs). We believe this to be the first event of its type to be reported in the literature. Interplanetary ACE and Cluster C1 data are used for solar wind diagnostics, and LANL 90 and 97, GOES 10 and 12 and GEOTAIL data are used for magnetospheric diagnostics. An unusually strong (magnetosonic Mach number equal to 5.4) shock detected at ∼1647 UT, 21 January 2005 by ACE causes a SI+ (of 57 nT) at ∼1712 UT at Earth. Southward magnetic fields in the sheath following the shock caused a decrease of SYM‐H with a peak value ∼−41 nT. A dynamic pressure jump across a double discontinuity in the solar wind at 1823 UT observed by ACE induced a second SI+ (of 25 nT) at 1847 UT at Earth. Southward magnetic fields following this event led to a second SYM‐H decrease with peak intensity −2 nT. However, when the storm main phase developed starting at 1946 UT, the IMF Bz turned northward. The IMF was northward from the portion of the main phase from ∼1946 UT to 0124 UT (almost 6 h). By comparing solar wind energy input (represented by integrated interplanetary Ey) with accumulated energy in the ring current (represented by integrated SYM‐H), we arrive at a possible explanation that there is first energy storage in the magnetotail and then a delayed energy injection (after storage in the magnetotail) into the magnetosphere. Other interpretations/mechanisms are possible.
Galactic cosmic ray H and He nuclei energy spectra measured by Voyagers 1 and 2 near the heliospheric termination shock in positive and negative solar magnetic polarity cycles
Authors: Webber W. R. , Cummings A. C. , McDonald F. B. , Stone E. C. , Heikkila B. , Lal N.
Published: October 2008

Using data from the Voyager 1 and 2 spacecraft, we have followed the intensity variations of H, He and C + O nuclei between 1998 and 2008 and determined the spectra for H and He at the time of minimum modulation in 1998, when the solar magnetic polarity was positive and again in 2008 when the solar magnetic polarity was negative. At these times these data are representative of conditions near a heliospheric termination shock assumed to be located at ∼90 AU. Above ∼400 MeV/nuc for He nuclei the 11‐year solar modulation cycle observed at the Earth is not seen; instead there is a 22‐year variation. The negative polarity cycle intensities above ∼150 MeV/nuc are higher than those in the positive polarity cycle by a factor of 1.4–1.7 times for both H and He nuclei. Below ∼100 MeV/nuc the C nuclei intensities are similar in the two cycles to within ±10%. These observations are compared with theoretical calculations which also show a negative to positive polarity cycle intensity difference at higher energies, most likely associated with energy changes due to drifts near the termination shock, but the comparison suggests that improved estimates of the local interstellar spectra are required.
Modeling of Saturn kilometric radiation arcs and equatorial shadow zone
Authors: Lamy L. , Zarka P. , Cecconi B. , Hess S. , Prangé R.
Published: October 2008

Accelerated electrons gyrating around Saturn's auroral (high latitude) magnetic field lines generate the intense Saturnian kilometric radiation (SKR). This radio emission is thought to be generated via the cyclotron maser instability (CMI) that predicts a strong anisotropy of the beaming pattern of the emission. Resulting visibility effects were suspected to be at the origin of characteristic features observed in Cassini's dynamic spectra as arc‐shaped structures and shadow zones. By using the Planetary Radio Emissions Simulator (PRES) code, we model these visibility effects and their consequence on Cassini's observations of SKR: we compute the dynamic spectra resulting from the beaming pattern of CMI‐generated SKR and its intersection with Cassini's trajectory. The SKR beaming pattern and its dependence on the frequency is computed for two typical electron distributions observed in auroral regions: a loss cone (favoring oblique emission) or a shell (favoring perpendicular emission). We successfully fit observed arc‐shaped structures and shadow zones. Although oblique and perpendicular emissions both allow to produce radio arcs, the fit is better in the oblique case. Similarities and differences between observations and model results are discussed and perspectives are outlined.
Finite frequency tomography in southeastern Tibet: Evidence for the causal relationship between mantle lithosphere delamination and the north–south trending rifts
Authors: Ren Yong , Shen Yang
Published: October 2008

While several mechanisms have been suggested to explain the evolution of the Tibetan Plateau, observational constraints on the deep lithospheric processes have been sparse, and previous seismic studies were mostly along profiles perpendicular to the collision front of the Indian and Eurasian plates. In this study, we show tomographic evidence for the delamination of the mantle lithosphere beneath southeastern Tibet, a process in which the entire mantle lithosphere peels away from the crust along the Moho and thus is a mechanism for rapid thinning of the lithosphere. Our P and S wave velocity models show the presence of a low‐velocity anomaly in the crust and upper mantle down to ∼300 km depth beneath a north–south trending rift zone in southeastern Tibet. This low‐velocity anomaly is situated above a tabular, high‐dipping‐angle, high‐velocity anomaly that extends into the upper mantle transition zone. The VP/VS ratio of this high‐velocity anomaly suggests that temperature variations are not the only cause of the anomaly and a highly melt‐depleted mantle is required. These observations suggest a causal relationship between the delamination of mantle lithosphere and the formation of the north–south trending rift in southeastern Tibet.
Postearthquake relaxation after the 2004 M6 Parkfield, California, earthquake and rate‐and‐state friction
Authors: Savage J. C. , Langbein J.
Published: October 2008

An unusually complete set of measurements (including rapid rate GPS over the first 10 days) of postseismic deformation is available at 12 continuous GPS stations located close to the epicenter of the 2004 M6.0 Parkfield earthquake. The principal component modes for the relaxation of the ensemble of those 12 GPS stations were determined. The first mode alone furnishes an adequate approximation to the data. Thus, the relaxation at all stations can be represented by the product of a common temporal function and distinct amplitudes for each component (north or east) of relaxation at each station. The distribution in space of the amplitudes indicates that the relaxation is dominantly strike slip. The temporal function, which spans times from about 5 min to 900 days postearthquake, can be fit by a superposition of three creep terms, each of the form βl loge(1 + t/τl), with characteristic times τl = 4.06, 0.11, and 0.0001 days. It seems likely that what is actually involved is a broad spectrum of characteristic times, the individual components of which arise from afterslip on different fault patches. Perfettini and Avouac (2004) have shown that an individual creep term can be explained by the spring‐slider model with rate‐dependent (no state variable) friction. The observed temporal function can also be explained using a single spring‐slider model (i.e., single fault patch) that includes rate‐and‐state‐dependent friction, a single‐state variable, and either of the two commonly used (aging and slip) state evolution laws. In the latter fits, the rate‐and‐state friction parameter b is negative.
Nonmethane hydrocarbons at Pico Mountain, Azores: 2. Event‐specific analyses of the impacts of mixing and photochemistry on hydrocarbon ratios
Authors: Honrath R. E. , Helmig D. , Owen R. C. , Parrish D. D. , Tanner D. M.
Published: October 2008

Nonmethane hydrocarbon (NMHC) measurements at Pico Mountain observatory are analyzed in combination with FLEXPART transport simulations. The results are used to assess the utility of NMHC as indicators for the occurrence of atmospheric transport events and photochemical processing on an event‐specific basis. Continuous in situ observations were made in lower free tropospheric air that had experienced a wide range of photochemical aging and traveled 3–10 days since last encountering significant emissions. They therefore provide a data set suitable for investigating the interaction of emissions, transport, and mixing in aged air parcels. Measurements of ethane, propane, and n‐butane during April 2005 are analyzed in detail to assess the degree to which the magnitude and variability of hydrocarbon ratios can be quantitatively understood as resulting from the mixing of emissions that have experienced varying degrees of photochemical oxidation. The age distribution of emissions was estimated with FLEXPART; resulting hydrocarbon ratios were simulated by applying first‐order decay to these emissions using the method of Parrish et al. (2007). These simulations capture the magnitude and large variations of observed hydrocarbon ratios. The relationship between the distribution of the timing of upwind emissions and measures of photochemical age calculated using these hydrocarbon ratios is discussed using the FLEXPART age distributions. The results demonstrate that continuous NMHC observations can be used to derive measures of photochemical age that can be quantitatively interpreted without the need for assumptions regarding mixing. Such analyses are valuable for studying OH levels and the distribution of upwind emission timing during pollution transport events.
Trends in wintertime climate in the northeastern United States: 1965–2005
Authors: Burakowski Elizabeth A. , Wake Cameron P. , Braswell Bobby , Brown David P.
Published: October 2008

Humans experience climate variability and climate change primarily through changes in weather at local and regional scales. One of the most effective means to track these changes is through detailed analysis of meteorological data. In this work, monthly and seasonal trends in recent winter climate of the northeastern United States (NE‐US) are documented. Snow cover and snowfall are important components of the region's hydrological systems, ecosystems, infrastructure, travel safety, and winter tourism and recreation. Temperature, snowfall, and snow depth data were collected from the merged United States Historical Climate Network (USHCN) and National Climatic Data Center Cooperative Network (COOP) data set for the months of December through March, 1965–2005. Monthly and seasonal time series of snow‐covered days (snow depth >2.54 cm) are constructed from daily snow depth data. Spatial coherence analysis is used to address data quality issues with daily snowfall and snow depth data, and to remove stations with nonclimatic influences from the regional analysis. Monthly and seasonal trends in mean, minimum, and maximum temperature, total snowfall, and snow‐covered days are evaluated over the period 1965–2005, a period during which global temperature records and regional indicators exhibit a shift to warmer climate conditions. NE‐US regional winter mean, minimum, and maximum temperatures are all increasing at a rate ranging from 0.42° to 0.46°C/decade with the greatest warming in all three variables occurring in the coldest months of winter (January and February). The regional average reduction in number of snow‐covered days in winter (−8.9 d/decade) is also greatest during the months of January and February. Further analysis with additional regional climate modeling is required to better investigate the causal link between the increases in temperature and reduction in snow cover during the coldest winter months of January and February. In addition, regionally averaged winter snowfall has decreased by about 4.6 cm/decade, with the greatest decreases in snowfall occurring in December and February. These results have important implications for the impacts of regional climate change on the northeastern United States hydrology, natural ecosystems, and economy.
Enhancement of land surface information and its impact on atmospheric modeling in the Heihe River Basin, northwest China
Authors: Gao Yanhong , Chen Fei , Barlage Michael , Liu Wei , Cheng Guodong , Li Xin , Yu Ye , Ran Youhua , Li Haiying , Peng Hongchun , Ma Mingguo
Published: October 2008

With increasing computational resources, atmospheric/environmental models continue to run at finer‐grid spacing that can resolve land surface characteristics, such as topography, land use/land cover, and soil texture. This paper assesses the improvement in land surface information data sets and its impact on atmospheric modeling. The study focuses on the Heihe River Basin (HRB) in northwestern China. Fine‐scale, remotely sensed, and in situ land surface data in HRB are derived and compared with the global data sets used in most mesoscale models. The incorporation of these fine‐scale land surface data, compared to those currently used in MM5, yields substantially improved HRB land surface data sets. HRB local and regional data sets and the global land data set are used in a nonhydrostatic mesoscale model (MM5) to investigate the influences of land surface uncertainty on meteorological modeling in the lower atmosphere. Main results suggest the following: (1) enhanced land data sets have a stronger impact on atmospheric water vapor fields in the lower boundary layer than other meteorological fields. Soil texture data greatly impacts the local precipitation simulation and landuse data improves the air temperature simulation in the lower atmosphere; (2) generally, the average land surface temperature biases are reduced using the enhanced land surface information, but the low bias in zones with higher elevation and high bias in zones with lower elevation still persist; (3) the wet bias over rugged terrain and dry bias in the simulated water vapor in the flat plains are both reduced. Area mean bias of simulated accumulated monthly precipitation is greatly reduced using the enhanced soil data. Convective available potential energy was larger in the HRB mountain regions using the default land data, while it was decreased using the enhanced ones; (4) analysis of the correlation coefficient between simulation bias and the geographic features shows that there are some patterns in the simulation bias distribution. Generally, larger bias still exists in the foothills of the mountains.
Continued evolution of Jakobshavn Isbrae following its rapid speedup
Authors: Joughin Ian , Howat Ian M. , Fahnestock Mark , Smith Ben , Krabill William , Alley Richard B. , Stern Harry , Truffer Martin
Published: October 2008

Several new data sets reveal that thinning and speedup of Jakobshavn Isbrae continue, following its recent rapid increase in speed as its floating ice tongue disintegrated. The present speedup rate of ∼5% a−1 over much of the fast‐moving region appears to be a diffusive response to the initial much larger speedup near the front. There is strong seasonality in speed over much of the fast‐flowing main trunk that shows a good inverse correlation with the seasonally varying length of a short (typically ∼6 km) floating ice tongue. This modulation of speed with ice front position supports the hypothesis that the major speedup was caused by loss of the larger floating ice tongue from 1998 to 2003. Analysis of image time series suggests that the transient winter ice tongue is formed when sea ice bonds glacier ice in the fjord to produce a nearly rigid mass that almost entirely suppresses calving. Major calving only resumes in late winter when much of this ice clears from the fjord. The collapse of the ice tongue in the late 1990s followed almost immediately after a sharp decline in winter sea‐ice concentration in Disko Bay. This decline may have extended the length of the calving season for several consecutive years, leading to the ice tongue's collapse.
Continuous estimates of CO2 efflux from arctic and boreal soils during the snow‐covered season in Alaska
Authors: Sullivan Patrick F. , Welker Jeffrey M. , Arens Seth J. T. , Sveinbjörnsson Bjartmar
Published: October 2008

Soils in arctic and boreal ecosystems hold large stocks of soil carbon (C), which may be sensitive to changes in climate. Recent studies suggest that winter CO2 efflux from the subnivean environment may be an important component of annual C budgets in arctic and boreal ecosystems. The present study was designed to examine the seasonal patterns and magnitudes of winter CO2 efflux from arctic tussock tundra near Toolik Lake, Alaska, and upland boreal forest in Anchorage, Alaska. The seasonal pattern of winter CO2 efflux differed strongly between the two sites. Tussock tundra showed a prolonged period with very low flux rates between late December and mid‐April. In contrast, rates of CO2 efflux were relatively high, but variable, throughout the winter at the boreal site. Estimates of C efflux for the snow‐covered season were within the range of previous studies in tussock tundra (24 g C m−2) and upland boreal forest (75 g C m−2). A simple exponential model, using shallow soil temperatures, explained much of the variation in CO2 efflux from tussock tundra. In contrast, air temperature was an important secondary control on winter CO2 efflux from the boreal forest, suggesting that trees became active during prolonged winter thaws. Results of the study imply that warmer winters may increase rates of C efflux from high‐latitude ecosystems. Changes in the depth or duration of snow cover will, however, modulate the relationship between air and soil temperatures and could amplify or dampen the effects of climate warming.
Assisting natural forest regeneration in Northern Ethiopia: one measure is not enough
Authors: Aerts Raf , November Eva , Hermy Martin , Deckers Jozef , Haile Mitiku , Muys Bart
Published: October 2008

Management strategies aimed at rehabilitating degraded and cleared forests often rely on temporary or permanent exclusion of herbivores (wild animals, livestock or both). But in many cases, this simple management technique is not sufficient to induce ecosystem restoration: many negative effects keep the ecosystem in a suboptimal, low biomass state. The presence of such stable states requires restoration measures to act on multiple stress factors simultaneously.Compensating for all limiting factors is neither practically nor economically feasible. But detailed knowledge about the autoecology of tree species – i.e. their site requirements, regeneration strategies and recruitment dynamics – may be used to tailor management to the most pertinent problems. Here we illustrate this approach with results from forest restoration experiments in grazing exclosures in northern Ethiopia using African wild olive (Olea europaea ssp. cuspidata) as a representative Afromontane climax species.The recruitment of African wild olive is affected by seed limitation, restricted seed dispersal and germination and survival limitation. The exclusion of grazing animals as a single measure to restore forest is not enough. Degraded grazing land moves into a state dominated by persistent shrubs, arresting forest succession and discouraging local stakeholders. Direct sowing or planting of seedlings in fertile patches under selected pioneer shrubs, however, may help to overcome this form of bush encroachment, in particular during years with an above-average rainfall.
Measurement of anisotropic fracture energies in periodic templated silica/polymer composite coatings
Authors: Chen X. , Richman E. K. , Kirsch B. L. , Senter R. , Tolbert S. H. , Gupta V.
Published: October 2008

We report measurements of the fracture energies of hexagonal honeycomb structured silica/polymer composite films that were produced through an evaporation induced self-assembly process. These films exhibit large anisotropy with their hexagonal pore axes aligned with the dip-coating direction. The experimental strategy included depositing films onto a flexible Kapton substrate and then straining them, in situ, under a microscope. To study the effect of the anisotropic microstructure on the fracture energy, cracks were propagated both parallel and perpendicular to the cylindrical pore axis directions. For both cases, the geometries of the evolving crack patterns with loading were micrographically recorded and the desired energy release rates were calculated using a two-dimensional steady-state channeling crack model. The model was implemented using the ANSYS finite element program. The experimental observations showed significant inelastic film deformation prior to crack propagation. These deformations were fully captured in the model, with properties obtained directly from the experiments. The calculated energy release rates were 12.3±0.5 J/m2 for the parallel direction and 6.7±0.5 J/m2 for the perpendicular direction. These numbers are significantly larger than the bulk silica value of roughly 4 J/m2, indicating the role of the local nanostructure in blunting and deflecting the crack tips. Experimental validation of the highly anisotropic energy release rates was obtained through transmission electron microscopy images of fractured films.
Growth of unsteady Hopf bifurcation modes and their swapping in a transitional coupled wake
Authors: Sau Amalendu
Published: October 2008

The present work is devoted to unfolding the flow physics and the growth of different higher order Hopf bifurcation modes in the supercritical transitional coupled wake of two square cylinders in side-by-side arrangement. Depending on gap to diameter ratio g∗ between the cylinders, the coupled growths of the wakes (and therefore the corresponding Benard von Karman streets) through of each of the antiphase, in-phase, and biased patterns were found to be essentially governed by several dominant unsteady modes of Hopf bifurcations, occurring along the spanwise extended corelines of the shedded vortices. During the transient process of synchronized/biased wake evolution there noted significant spanwise pressure, velocity, and vorticity oscillations along the vortex corelines, which considerably amplified, as the flows reached transitional state. Such an unsteady spanwise oscillation of the wake facilitated the growth of higher order Hopf bifurcation modes and their frequent/random swapping. Instantaneous wakes (synchronized/biased) thereby evolved either through a particular mode of Hopf bifurcation or there remained simultaneous presence of multiple bifurcation modes. The growth of a number of instantaneous local pressure maxima over the spanwise extended corelines of the shedded vortices and the gradual decrease in pressure along two sides of the maximum pressure regions, together with the developed azimuthal pressure fluctuation around the vortex corelines, are found to be responsible for initiating the bifurcations. Notably, the present findings, while remaining closely consistent with recent experimental measurements (performed with a pair of circular cylinders) and the existing theoretical predictions (made on the basis of Landau/Ginzburg–Landau equations), clearly demonstrate the existence of five different transient modes of Hopf bifurcating solutions of the Navier–Stokes equations in the wake of two side-by-side square cylinders. Flow transition in the wake is noted to occur essentially through random swapping of these unsteady Hopf bifurcation modes with arbitrarily varying length scales.
Preserving the Boltzmann ensemble in replica-exchange molecular dynamics
Authors: Cooke Ben , Schmidler Scott C.
Published: October 2008

We consider the convergence behavior of replica-exchange molecular dynamics (REMD) [Sugita and Okamoto, Chem. Phys. Lett. 314, 141 (1999)] based on properties of the numerical integrators in the underlying isothermal molecular dynamics (MD) simulations. We show that a variety of deterministic algorithms favored by molecular dynamics practitioners for constant-temperature simulation of biomolecules fail either to be measure invariant or irreducible, and are therefore not ergodic. We then show that REMD using these algorithms also fails to be ergodic. As a result, the entire configuration space may not be explored even in an infinitely long simulation, and the simulation may not converge to the desired equilibrium Boltzmann ensemble. Moreover, our analysis shows that for initial configurations with unfavorable energy, it may be impossible for the system to reach a region surrounding the minimum energy configuration. We demonstrate these failures of REMD algorithms for three small systems: a Gaussian distribution (simple harmonic oscillator dynamics), a bimodal mixture of Gaussians distribution, and the alanine dipeptide. Examination of the resulting phase plots and equilibrium configuration densities indicates significant errors in the ensemble generated by REMD simulation. We describe a simple modification to address these failures based on a stochastic hybrid Monte Carlo correction, and prove that this is ergodic.
Evolution of magnetization configuration phase diagram with increasing exchange coupling in synthetic antiferromagnets
Authors: Fujiwara Hideo
Published: October 2008

Comprehensive analysis of the magnetization response to an in-plane field for a balanced synthetic antiferromagnet has been performed. Evolution of the critical field pseudoastroids and the magnetization configuration phase diagram with increasing exchange coupling are presented. It is found that the nonparallel magnetization configurations are classified into two, one existing for all exchange coupling strengths and the other starting to appear at a critical value. The analytic formulas for the off-anisotropy-axis cusp of the pseudoastroid are also presented.
Electrostatic guidelines and molecular tailoring for density functional investigation of structures and energetics of (Li)n clusters
Authors: K. V. Jovan Jose , Gadre Shridhar R.
Published: October 2008

A molecular electrostatic potential (MESP)-guided method for building metal aggregates is proposed and tested on prototype lithium (Li)n clusters from n=4 to 58. The smaller clusters are subsequently subjected to direct density functional theory based geometry optimization, while the larger ones are optimized via molecular tailoring approach (MTA). The calculations are performed using PW91-PW91 as well as B3LYP functionals, and the trends in the interaction energies are found to be similar. The MESP-guided model for building metal clusters is validated by comparing the resulting cluster geometries with the ones reported in the literature up to n=20. A comparison of the ionization potential and polarizability (up to n=22) with their experimental counterparts shows a fairly good agreement. A new MTA-based scheme for calculating the ionization potential and polarizability values of large metal clusters is proposed and tested on Li40 and Li58 clusters. Further, the existence of “magic numbered clusters” up to n=22 is justified in terms of “maximum hardness principle” as well based on molecular electron density topography and distance descriptors.
Broadband dielectric spectroscopy of a nematic liquid crystal in benzene
Authors: Kundu Shyamal Kumar , Okudaira Shun , Kosuge Masanori , Shinyashiki Naoki , Yagihara Shin
Published: October 2008

Broadband dielectric spectroscopy has been used to analyze the temperature, frequency, and concentration dependences of the molecular dynamics of a nematic liquid crystal (5CB) mixed with the nonpolar solvent benzene. Differential scanning calorimetry measurement has been also performed to confirm the phase transitions of 5CB/benzene mixtures. The phase transition temperatures (crystalline to isotropic phases) thus obtained have been described very accurately from the temperature-dependent relaxation strength, the relaxation time, and the symmetric shape parameter of the relaxation function obtained from the fitting procedure. Two relaxation processes reflecting overall rotations around the short and long molecular axes are observed in both the nematic and isotropic phases. In the crystalline phase, the former process with the longer relaxation time disappeared, and latter process with shorter relaxation time shows a discontinuity at the freezing temperature. The relaxation process with shorter relaxation time obtained in the crystalline phase is larger than that obtained in the nematic phase because of the large restrictions in the crystalline phase. For the first time, we have precisely explained the molecular mechanism and structure of liquid crystalline materials as a function of concentration, temperature, and frequency.
Analytical calculations of frequency-dependent hypermagnetizabilities and Cotton–Mouton constants using London atomic orbitals
Authors: Thorvaldsen Andreas J. , Ruud Kenneth , Rizzo Antonio , Coriani Sonia
Published: October 2008

We present the first gauge-origin-independent, frequency-dependent calculations of the hypermagnetizability anisotropy, which determines the temperature-independent contribution to magnetic-field-induced linear birefringence, the so-called Cotton–Mouton effect. A density-matrix-based scheme for analytical calculations of frequency-dependent molecular properties for self-consistent field models has recently been developed, which is also valid with frequency- and field-dependent basis sets. Applying this scheme to Hartree–Fock wave functions and using London atomic orbitals in order to obtain gauge-origin-independent results, we have calculated the hypermagnetizability anisotropy. Our results show that the use of London orbitals leads to somewhat better basis-set convergence for the hypermagnetizability compared to conventional basis sets and that London orbitals are mandatory in order to obtain reliable magnetizability anisotropies.
Chemical probing spectroscopy of H3+ above the barrier to linearity
Authors: Kreckel Holger , Bing Dennis , Reinhardt Sascha , Petrignani Annemieke , Berg Max , Wolf Andreas
Published: October 2008

We have performed chemical probing spectroscopy of H3+ ions trapped in a cryogenic 22-pole ion trap. The ions were buffer gas cooled to ∼55 K by collisions with helium and argon. Excitation to states above the barrier to linearity was achieved by a Ti:sapphire laser operated between 11 300 and 13 300 cm−1. Subsequent collisions of the excited H3+ ions with argon lead to the formation of ArH+ ions that were detected by a quadrupole mass spectrometer with high sensitivity. We report the observation of 17 previously unobserved transitions to states above the barrier to linearity. Comparison to theoretical calculations suggests that the transition strengths of some of these lines are more than five orders of magnitude smaller than those of the fundamental band, which renders them—to the best of our knowledge—the weakest H3+ transitions observed to date.
Alkali-promoted CO dissociation on Cu(111) and Ni(111) at room temperature
Authors: Politano A. , Formoso V. , Chiarello G.
Published: October 2008

The coadsorption of alkalis (K, Na) and CO on Cu(111) was investigated by high-resolution electron energy loss spectroscopy. Measurements performed at room temperature showed that CO adsorption is partially dissociative on a potassium-precovered Cu(111) surface and fully dissociative for Na/Cu(111). Carbon monoxide molecules occupy adsorption sites directly adjacent to those of alkali adatoms, as suggested by the absence of a threshold alkali precoverage for CO dissociation. On the contrary, for alkali+CO/Ni(111) a threshold alkali precoverage for CO dissociation was found to exist.
Shear mode bulk acoustic wave resonator based on c-axis oriented AlN thin film
Authors: Milyutin Evgeny , Gentil Sandrine , Muralt Paul
Published: October 2008

A shear mode resonator based on bulk waves trapped in c-axis oriented AlN thin films was fabricated, simulated, and tested. The active 1.55 μm thick AlN layer was deposited on top of an acoustic Bragg reflector composed of SiO2/AlN λ/4 layer pairs. The resonance was excited by means of interdigitated electrodes consisting of 150 nm thick Al lines. Analytical and simulation calculations show that the in-plane electric field excites bulk acoustic wave shear modes that are trapped in such an AlN film slab. The experimental frequency corresponds well to the theoretical one. The evaluated resonance of the fundamental shear mode at 1.86 GHz revealed a coupling of 0.15% and Q-factor of 870 in air and 260 in silicon oil.
Approximate analytical structure and thermodynamics for penetrable sphere fluids using effective hard spheres
Authors: Khanpour Mehrdad , Hashim Rauzah
Published: October 2008

A penetrable sphere fluid is modeled using the effective hard sphere fluid and its known structure and thermodynamics. It is observed that the obtained equation of state and the radial distribution function are in fairly good agreement with computer simulation data both at low temperatures and/or low densities.
Renner–Teller interactions in the dissociative recombination of HCO+
Authors: Jungen Ch. , Pratt S. T.
Published: October 2008

The formalism developed in the preceding paper for vibrational autoionization via Renner–Teller active vibrations is adapted to treat dissociative recombination and applied to the reaction of HCO++e−. Existing spectroscopic data on the rovibrational structure of the HCO+ Σ2+ ion and the HCO 3pπ 2Π Rydberg state are fitted by using the semirigid bender model to extract the parameters required to calculate the autoionization and electron capture widths. The results of this simple model are in good agreement with more detailed first principles calculations of the dissociative recombination cross section and confirm the earlier conclusion that coupling due to the Renner–Teller interaction is largely responsible for the observed dissociative recombination cross section at electron energies below ∼0.1 eV.
Diffusion of two particles with a finite interaction potential in one dimension
Authors: Ambjörnsson Tobias , Silbey Robert J.
Published: October 2008

We investigate the dynamics of two interacting diffusing particles in an infinite effectively one-dimensional system; the particles interact through a steplike potential of width b and height ϕ0 and are allowed to pass one another. By solving the corresponding 2+1-variate Fokker–Planck equation, an exact result for the two-particle conditional probability density function (PDF) is obtained for arbitrary initial particle positions. From the two-particle PDF, we obtain the overtake probability, i.e., the probability that the two particles have exchanged positions at time t compared to the initial configuration. In addition, we calculate the trapping probability, i.e., the probability that the two particles are trapped close to each other (within the barrier width b) at time t, which is mainly of interest for an attractive potential, ϕ0<0. We also investigate the tagged particle PDF, relevant for describing the dynamics of one particle which is fluorescently labeled. Our analytic results are in excellent agreement with the results of stochastic simulations, which are performed using the Gillespie algorithm.
Nonadiabatic simulation study of photoisomerization of azobenzene: Detailed mechanism and load-resisting capacity
Authors: Shao Junfeng , Lei Yibo , Wen Zhenyi , Dou Yusheng , Wang Zhisong
Published: October 2008

Nonadiabatic dynamical simulations were carried out to study cis-to-trans isomerization of azobenzene under laser irradiation and/or external mechanical loads. We used a semiclassical electron-radiation-ion dynamics method that is able to describe the coevolution of the structural dynamics and the underlying electronic dynamics in a real-time manner. It is found that azobenzene photoisomerization occurs predominantly by an out-of-plane rotation mechanism even under a nontrivial resisting force of several tens of piconewtons. We have repeated the simulations systematically for a broad range of parameters for laser pulses, but could not find any photoisomerization event by a previously suggested in-plane inversion mechanism. The simulations found that the photoisomerization process can be held back by an external resisting force of 90–200 pN depending on the frequency and intensity of the lasers. This study also found that a pure mechanical isomerization is possible from the cis-to-trans state if the azobenzene molecule is stretched by an external force of ∼1250–1650 pN. Remarkably, the mechanical isomerization first proceeds through a mechanically activated inversion, and then is diverted to an ultrafast downhill rotation that accomplishes the isomerization. Implications of these findings to azobenzene-based nanomechanical devices are discussed.
Numerical approach to determine the elastic modulus of sintered natural hydroxyapatite
Authors: Weinand W. R. , Bento A. C. , Baesso M. L. , Lima W. M.
Published: October 2008

The main problem in using hydroxyapatite as implants is its low mechanical resistance. The parameter that defines such characteristic is the elastic modulus, which can be used to evaluate the elasticity that a given sample may support. Elastic modulus is currently obtained by the material’s microhardness data and/or exhausting and indirect procedures, which may lead to low accuracy within the intended estimation range. The aim of this work is to develop a numerical model to predict the elastic modulus of natural hydroxyapatite based on experimental data of samples sintered between 900 and 1300 °C during 1 h in a 2 l/min air flow. The elastic modulus was obtained from nanoindentation measurements. Material consists of less than 37 μm hydroxyapatite powder obtained from fish bones. Samples measuring 10 mm in diameter and 2 mm in thickness were compacted at 350 MPa. The proposed model provides an evaluation of this characteristic with an error of approximately 3.5% when compared to the experimental data.
Controlling charge and current neutralization of an ion beam pulse in a background plasma by application of a solenoidal magnetic field: Weak magnetic field limit
Authors: Kaganovich I. D. , Startsev E. A. , Sefkow A. B. , Davidson R. C.
Published: October 2008

Propagation of an intense charged particle beam pulse through a background plasma is a common problem in astrophysics and plasma applications. The plasma can effectively neutralize the charge and current of the beam pulse, and thus provides a convenient medium for beam transport. The application of a small solenoidal magnetic field can drastically change the self-magnetic and self-electric fields of the beam pulse, thus allowing effective control of the beam transport through the background plasma. An analytic model is developed to describe the self-magnetic field of a finite-length ion beam pulse propagating in a cold background plasma in a solenoidal magnetic field. The analytic studies show that the solenoidal magnetic field starts to influence the self-electric and self-magnetic fields when ωce≳ωpeβb, where ωce=eB∕mec is the electron gyrofrequency, ωpe is the electron plasma frequency, and βb=Vb∕c is the ion beam velocity relative to the speed of light. This condition typically holds for relatively small magnetic fields (about 100G). Analytical formulas are derived for the effective radial force acting on the beam ions, which can be used to minimize beam pinching. The results of analytic theory have been verified by comparison with the simulation results obtained from two particle-in-cell codes, which show good agreement.
Instrument and methods for surface dilatational rheology measurements
Authors: Russev Stoyan C. , Alexandrov Nikola , Marinova Krastanka G. , Danov Krassimir D. , Denkov Nikolai D. , Lyutov Lyudmil , Vulchev Vassil , Bilke-Krause Christine
Published: October 2008

We describe an instrument combining the advantages of two methods, axisymmetric drop shape analysis for well-deformed drops and capillary pressure tensiometry for spherical drops, both used for measuring the interfacial tension and interfacial rheological parameters. The rheological parameters are the complex interfacial elasticity, and the surface elasticity and viscosity of Kelvin–Voigt and Maxwell rheological models. The instrument is applicable for investigation of the effect of different types of surfactants (nonionic, ionic, proteins, and polymers) on the interfacial rheological properties both of air/water and oil/water interfaces, and of interfaces between liquids with equal mass densities. A piezodriven system and a specially designed interface unit, implemented in the instrument, ensure precise control for standard periodic waveforms of surface deformation (sine, square, triangle, and sawtooth) at a fixed frequency, or produce surface deformation at constant rate. The interface unit ensures accurate synchronization between the pressure measurement and the surface control, which is used for real-time data processing and feedback control of drop area in some of the applications.
Biaxial stress-dependent optical band gap, crystalline, and electronic structure in wurtzite ZnO: Experimental and ab initio study
Authors: Li Y. F. , Yao B. , Lu Y. M. , Gai Y. Q. , Cong C. X. , Zhang Z. Z. , Zhao D. X. , Zhang J. Y. , Li B. H. , Shen D. Z. , Fan X. W. , Tang Z. K.
Published: October 2008

The relationship between band gap and biaxial stress in wurtzite ZnO thin films has been investigated by side-inclination x-ray diffraction technique and optical absorbance spectrum as well as ab initio calculation. The experimental result shows that differing from other semiconductor thin films with hexagonal structure, such as GaN, the band gap of ZnO thin films increases with the increase in biaxial tensile stress. For explaining the difference, ab initio calculation is performed to simulate the relationship between band gap and biaxial stress of wurtzite ZnO and GaN. The calculated result indicates that the band gap of ZnO increases under biaxial tensile stress but GaN is opposite, supporting our experimental result. The band offset calculation shows that the conduction-band minimum (CBM) and the valence-band maximum (VBM) of ZnO and GaN offset to low energy under biaxial tensile stress. The VBM offset of ZnO is larger than the CBM, responsible for the increase in band gap. The VBM offset of GaN is smaller than the CBM, responsible for the decrease in band gap. The difference between ZnO and GaN is attributed to the strong p-d coupling in valence band of ZnO, but strong p-p coupling in valence band of GaN.