bluuebunny/crossref_metadata_2025 huggingface dataset:
Deepwater mantle 3He plumes over the northern Mid‐Atlantic Ridge (36°N–40°N) and the Azores Platform
Authors: Jean‐Baptiste P. , Fourré E. , Dapoigny A. , Charlou J. L. , Donval J.‐P.
Published: March 2008

As part of a multidisciplinary project aimed at studying mid‐ocean ridge processes near the Azores, fifty water column profiles were analyzed for 3He/4He ratios in dissolved helium (a well‐known hydrothermal tracer) from 36°N to 40°N along the Mid‐Atlantic Ridge (MAR) and over the Azores Plateau. As expected, large δ3He anomalies could be observed over the Rainbow, Lucky Strike, and Menez Gwen hydrothermal sites. The main finding of the present study is the discovery of a large hydrothermal 3He plume north of the Açor Fracture Zone (north AFZ site), with a CH4/3He ratio indicative of a basaltic‐hosted hydrothermal system. Clear 3He and CH4 anomalies, likely corresponding to unknown venting sites too, were also detected in the Amar Minor segment and south of the Kurchatov Fracture Zone. Evidence for substantial mantle helium degassing was also observed in the deep nodal basins along the Terceira Rift. On the basis of 3He plumes over the total length of the surveyed segments, the distribution of hydrothermal sites corresponds to a site frequency of 1.3 ± 0.2 site/100 km, in good agreement with the global vent field statistics of Baker and German (2004). For the Rainbow, Lucky Strike, and Menez Gwen sites, the application of a plume model based on the conservation of mass, heat, and momentum shows that the heat output computed by the model is only an estimation of the heat released by the focused part of the flow imputable to one single vent. Applied to the north AFZ venting site for which the height of the plume is not known precisely, the model does not allow us to discriminate between a Menez Gwen/Rainbow type of venting or a more focused vent complex such as the one observed at the TAG site (26°N).
Climate informed flood frequency analysis and prediction in Montana using hierarchical Bayesian modeling
Authors: Kwon Hyun‐Han , Brown Casey , Lall Upmanu
Published: March 2008

It is widely acknowledged that climate variability modifies the frequency spectrum of extreme hydrologic events. Traditional hydrological frequency analysis methods do not account for year to year shifts in flood risk distributions that arise due to changes in exogenous factors that affect the causal structure of flood risk. We use Hierarchical Bayesian Analysis to evaluate several factors that influence the frequency of extreme floods for a basin in Montana. Sea surface temperatures, predicted GCM precipitation, climate indices and snow pack depth are considered as potential predictors of flood risk. The parameters of the flood risk prediction model are estimated using a Markov Chain Monte Carlo algorithm. The predictors are compared in terms of the resulting posterior distributions of the parameters that are used to estimate flood frequency distributions. The analysis shows an approach for exploiting the link between climate scale indicators and annual maximum flood, providing impetus for developing seasonal forecasting of flood risk applications and dynamic flood risk management strategies
Ice core record of rising lead pollution in the North Pacific atmosphere
Authors: Osterberg E. , Mayewski P. , Kreutz K. , Fisher D. , Handley M. , Sneed S. , Zdanowicz C. , Zheng J. , Demuth M. , Waskiewicz M. , Bourgeois J.
Published: March 2008

A high‐resolution, 8000 year‐long ice core record from the Mt. Logan summit plateau (5300 m asl) reveals the initiation of trans‐Pacific lead (Pb) pollution by ca. 1730, and a >10‐fold increase in Pb concentration (1981–1998 mean = 68.9 ng/l) above natural background (5.6 ng/l) attributed to rising anthropogenic Pb emissions from Asia. The largest rise in North Pacific Pb pollution from 1970–1998 (end of record) is contemporaneous with a decrease in Eurasian and North American Pb pollution as documented in ice core records from Greenland, Devon Island, and the European Alps. The distinct Pb pollution history in the North Pacific is interpreted to result from the later industrialization and less stringent abatement measures in Asia compared to North America and Eurasia. The Mt. Logan record shows evidence for both a rising Pb emissions signal from Asia and a trans‐Pacific transport efficiency signal related to the strength of the Aleutian Low.
Hydrogen isotope evidence for loss of water from Mars through time
Authors: Greenwood James P. , Itoh Shoichi , Sakamoto Naoya , Vicenzi Edward P. , Yurimoto Hisayoshi
Published: March 2008

The high D/H of the Martian atmosphere (∼5–6 × terrestrial) is considered strong evidence for the loss of Martian water to space. The timing and magnitude of the loss of water from Mars can be constrained by measurements of D/H in Martian meteorites. Previous studies of Martian meteorites have shown a large range in D/H, from terrestrial values to as high as the current Martian atmosphere. Here we show that the ancient (∼4 Ga) Mars meteorite ALH84001 has a D/H 4 × terrestrial and that the young (∼0.17 Ga) Shergotty meteorite has a D/H 5.6 × terrestrial. We also find that the young Los Angeles shergottite has zoning in D/H that can be correlated to igneous growth zoning, strongly suggesting assimilation of D‐enriched water during igneous crystallization near the Martian surface. In contrast to previous studies, we find higher and less variable D/H ratios in these three meteorites. Our results suggest a two‐stage evolution for Martian water—a significant early loss of water to space (prior to 3.9 Ga) followed by only modest loss to space in the last 4 billion years. The current Martian atmospheric D/H has remained essentially unchanged for the last 165 Ma.
Role of centennial geomagnetic changes in local atmospheric ionization
Authors: Usoskin I. G. , Korte M. , Kovaltsov G. A.
Published: March 2008

Many studies of solar‐terrestrial relation are based on globally (or hemispherically) averaged quantities, including the average cosmic ray flux. However, regional effects of cosmic ray induced ionization due to geomagnetic changes may be comparable to or even dominate over the solar signal at mid‐latitudes on centennial‐to‐millennial time scales. We show that local changes of the tropospheric ionization due to fast migration of the geomagnetic axis are crucial on centennial time scale, and the use of global averages may smear an important effect. We conclude that changes of the regional tropospheric ionization at mid‐latitudes are defined by both geomagnetic changes and solar activity, and none of the two processes can be neglected. This substantiates a necessity for a careful analysis of the regional, not global, indices at mid‐latitudes and offers a new possibility to disentangle direct (solar radiation) and indirect (via cosmic rays) effects in the solar‐terrestrial relations.
Postcollisional exhumation history of the Tanzawa Tonalite Complex, inferred from (U‐Th)/He thermochronology and fission track analysis
Authors: Yamada Kunimi , Tagami Takahiro
Published: March 2008

The Tanzawa Mountains in the South Fossa Magna region, in the middle of the Japan Arc, were formed due to collision of the Izu‐Bonin Arc against the Japan Arc during the last several million years. The Tanzawa Tonalite Complex is exposed in the Tanzawa Mountains and is inferred to represent middle crust of the Izu‐Bonin Arc. We performed new thermochronometry, consisting of (U‐Th)/He dating, fission track dating, and fission track length analysis for the complex to investigate the exhumation history of the Tanzawa Tonalite Complex caused by arc‐arc collision. Typical (U‐Th)/He ages were 2.0 ± 0.2 Ma on apatite, 3.3 ± 0.2 Ma on zircon, and 4.5 to 6.9 Ma on zircon using the fission track method. The mean fission track length was 9.7 ± 0.6 μm (2 SE). The cooling rate of the tonalite has been calculated to be in the range of 20°–60°C Ma−1 from ∼7 to 3.3 Ma, ∼80°C Ma−1 from 3.3 to 2.0 Ma, and ∼30°C Ma−1 from 2.0 to 0 Ma assuming a surface temperature of 10°C. The exhumation rate has been calculated to be 0.5 to 1.5 mm a−1 from ∼7 to 3.3 Ma, ∼2 mm a−1 from 3.3 to 2.0 Ma, and ∼0.8 mm a−1 from 2.0 to 0 Ma with an assumed geothermal gradient of 40°C km−1. The exhumation rate has not changed significantly despite the simultaneous tectonic changes around the Tanzawa Mountains after ∼7 Ma.
The long‐term and interannual variability of summer fresh water storage over the eastern Siberian shelf: Implication for climatic change
Authors: Dmitrenko Igor A. , Kirillov Sergey A. , Tremblay L. Bruno
Published: March 2008

A time series of summer fresh water content anomalies (FWCA) over the Laptev and East Siberian sea shelves was constructed from historical hydrographic records for the period from 1920 to 2005. Results from a multiple regression between FCWA and various atmospheric and oceanic indices show that the fresh water content on the shelves is mainly controlled by atmospheric vorticity on quasi‐decadal timescales. When the vorticity of the atmosphere on the shelves is antycyclonic, approximately 500 km3 of fresh water migrates from the eastern Siberian shelf to the Arctic Ocean through the northeastern Laptev Sea. When the vorticity of the atmosphere is cyclonic, this fresh water remains on the southern Laptev and East Siberian sea shelves. This FWCA represents approximately 35% of the total fresh water inflow provided by river discharge and local sea‐ice melt, and is about ten times larger than the standard deviation of the Lena River summer long‐term mean discharge. However, the large interannual and spatial variability in the fresh water content of the shelves, as well as the spatial coverage of the hydrographic data, makes it difficult to detect the long‐term tendency of fresh water storage associated with climate change.
Lightning return stroke current radiation in presence of a conducting ground: 2. Validity assessment of simplified approaches
Authors: Delfino Federico , Procopio Renato , Rossi Mansueto , Rachidi Farhad , Nucci Carlo Alberto
Published: March 2008

In this paper, the developed formulation, which we shall call the “reference” one, is used to assess the validity of the most popular simplified approach for the calculation of the lightning electromagnetic field over a conducting earth, namely, the Cooray‐Rubinstein (CR) approximation. This formula provides a simple method to evaluate the radial component of the electric field which is the component most affected by the finite ground conductivity and which plays an important role within the Agrawal et al. (1980) field‐to‐transmission line‐coupling model. Several configurations are examined, with different values for the ground conductivity and different field observation points. A thorough analysis of all the simulated field components is carried out and comparisons are also made with the “ideal” field, namely, the field that would be present under the assumption of perfectly conducting ground. It is shown that for channel base current typical of subsequent strokes and for very low conductivities, the CR formula exhibits some deviations from the reference one but it still represents a conservative estimation of the radial field component, since it behaves as un upper bound for the exact curve. The developed algorithm is characterized by fast performances in terms of CPU time, lending itself to be used for several applications, including a coupling code for lightning induced overvoltages calculations.
Contribution of isoprene to chemical budgets: A model tracer study with the NCAR CTM MOZART‐4
Authors: Pfister G. G. , Emmons L. K. , Hess P. G. , Lamarque J.‐F. , Orlando J. J. , Walters S. , Guenther A. , Palmer P. I. , Lawrence P. J.
Published: March 2008

We present a study of the sensitivity of isoprene emission calculations in a global chemistry transport model (CTM) to input land cover characteristics and analyze the impacts of changes in isoprene on the tropospheric budgets of atmospheric key species. The CTM Model for Ozone and Related Chemical Species, version 4 (MOZART‐4) includes the online calculation of isoprene emissions based on the Model of Emissions of Gases and Aerosols from Nature (MEGAN), which is driven by three different land parameter inputs. We also included a tagging scheme in the CTM, which keeps track of the production of carbon containing species from isoprene oxidation. It is found that the amount of tropospheric carbon monoxide (CO), formaldehyde (HCHO) and peroxyacetylnitrate (PAN) explained by isoprene oxidation ranges from 9–16%, 15–27%, and 22–32%, depending on the isoprene emissions scenario. Changes in the global tropospheric burden with different land cover inputs can reach up to 10% for CO, 15% for HCHO, and 20% for PAN. Changes for ozone are small on a global scale, but regionally differences are as large as 3DU in the tropospheric column and as large as 5 ppbv in the surface concentrations. Our results demonstrate that a careful integration of isoprene emissions and chemistry in CTMs is very important for simulating the budgets of a number of atmospheric trace gases. We further demonstrate that the model tagging scheme has the capability of improving conventional methods of constraining isoprene emissions from space‐borne HCHO column observations, especially in regions where a considerable part of the variability in the HCHO column is not related to isoprene.
HOx chemistry during INTEX‐A 2004: Observation, model calculation, and comparison with previous studies
Authors: Ren Xinrong , Olson Jennifer R. , Crawford James H. , Brune William H. , Mao Jingqiu , Long Robert B. , Chen Zhong , Chen Gao , Avery Melody A. , Sachse Glen W. , Barrick John D. , Diskin Glenn S. , Huey L. Greg , Fried Alan , Cohen Ronald C. , Heikes Brian , Wennberg Paul O. , Singh Hanwant B. , Blake Donald R. , Shetter Richard E.
Published: March 2008

OH and HO2 were measured with the Airborne Tropospheric Hydrogen Oxides Sensor (ATHOS) as part of a large measurement suite from the NASA DC‐8 aircraft during the Intercontinental Chemical Transport Experiment‐A (INTEX‐A). This mission, which was conducted mainly over North America and the western Atlantic Ocean in summer 2004, was an excellent test of atmospheric oxidation chemistry. The HOx results from INTEX‐A are compared to those from previous campaigns and to results for other related measurements from INTEX‐A. Throughout the troposphere, observed OH was generally 0.95 of modeled OH; below 8 km, observed HO2 was generally 1.20 of modeled HO2. This observed‐to‐modeled comparison is similar to that for TRACE‐P, another midlatitude study for which the median observed‐to‐modeled ratio was 1.08 for OH and 1.34 for HO2, and to that for PEM‐TB, a tropical study for which the median observed‐to‐modeled ratio was 1.17 for OH and 0.97 for HO2. HO2 behavior above 8 km was markedly different. The observed‐to‐modeled HO2 ratio increased from ∼1.2 at 8 km to ∼3 at 11 km with the observed‐to‐modeled ratio correlating with NO. Above 8 km, the observed‐to‐modeled HO2 and observed NO were both considerably greater than observations from previous campaigns. In addition, the observed‐to‐modeled HO2/OH, which is sensitive to cycling reactions between OH and HO2, increased from ∼1.5 at 8 km to almost 3.5 at 11 km. These discrepancies suggest a large unknown HOx source and additional reactants that cycle HOx from OH to HO2. In the continental planetary boundary layer, the observed‐to‐modeled OH ratio increased from 1 when isoprene was less than 0.1 ppbv to over 4 when isoprene was greater than 2 ppbv, suggesting that forests throughout the United States are emitting unknown HOx sources. Progress in resolving these discrepancies requires a focused research activity devoted to further examination of possible unknown OH sinks and HOx sources.
Biological indicators of sedimentary dynamics in the central Gulf of Papua: Seasonal and decadal perspectives
Authors: Aller Josephine Y. , Alongi Daniel M. , Aller Robert C.
Published: March 2008

Benthic and pelagic communities in the central Gulf of Papua, a major deltaic clinoform located on the south coast of Papua New Guinea, reflect the spatial and seasonal patterns in physical sedimentation conditions and the availability of food resources across this system. The distribution of terrigenous and marine organic substrates, the transport of riverine‐derived sediment, and seafloor dynamics are largely controlled by two seasons and wind patterns: the relatively quiescent NW Monsoon and more energetic SE Trades. The physically disturbed inner‐topset deposits (<20 m) are dominated at all times by bacterial biomass and have depressed numbers of macrofauna, with evidence of periodic recolonization recorded in sedimentary structures. The inner‐topset region also shows strong seasonality in the production and availability of labile organic matter and in bacterial activity. As measured by the concentrations of photopigments and microbial biomass in the upper 2 cm of surface sediments and in the water column, the comparatively quiescent NW Monsoon period is associated with net delivery of reactive substrates as labile organic matter to the bottom and net growth of the benthic microbial community, whereas the Transition period between the NW Monsoonal‐Winds (NW Monsoon) and SE Trade‐Winds (SE Trades), and the SE Trades, are periods of reactive substrate depletion in bottom deposits and decreased benthic bacterial and macroinfaunal biomass. In contrast to the inner‐topset, the less physically disturbed outer‐topset and foreset show relatively enhanced, but still low, abundances of large macrobenthos and possible evidence of variation related to food supply. The greatest observed seasonal variation in macrofauna occurs in the Umuda Valley off the Fly River, an apparent mobile sediment conduit.
Coupled barrier island–resort model: 1. Emergent instabilities induced by strong human‐landscape interactions
Authors: McNamara D. E. , Werner B. T.
Published: March 2008

As humans increasingly occupy and modify marginal landscapes, previously unobserved long timescale, emergent behaviors related to interactions between natural processes and human agency are possible. Barrier islands, which are low‐lying strips of sand separated from a coast by lagoons, cut by inlets and topped by sand dunes, have been significantly modified through the development of tourist resorts. Resorts and barrier islands are dynamically coupled through storm damage and beach erosion, and measures taken to prevent or mitigate them. In response to rising sea level, a natural barrier island migrates steadily up the continental shelf. In contrast, we show that in a novel numerical model‐coupling barrier island processes with resort development, storm damage, and hazard mitigation, policy decisions driven by market dynamics destabilize barrier island response to rising sea level, giving rise to emergent, episodic boom and bust cycles, which alternate in phase alongshore, and less frequent, regionally extensive resort destruction events. Developed barrier islands are precariously maintained at lower elevations and further offshore than their natural counterparts, a situation exacerbated by insurance, which can lead to island inundation. Our results suggest that coastal areas that have recently instituted protection measures eventually will experience a widespread upsurge in damage if these practices are sustained, even in the absence of climate‐change‐induced increased storminess.
Modeling of nonaqueous phase liquid (NAPL) migration in heterogeneous saturated media: Effects of hysteresis and fluid immobility in constitutive relations
Authors: Fagerlund F. , Niemi A. , Illangasekare T. H.
Published: March 2008

The confidence in model predictions for nonaqueous phase liquid (NAPL) transport in stochastically heterogeneous systems is limited. The fundamental approaches as well as the constitutive models have not been sufficiently validated, mainly because of the lack of appropriate experimental data. Recently, Fagerlund et al. (2007a, 2007b) presented a set of well‐controlled laboratory data that are used here (1) to analyze the overall performance of the continuum‐based approach for predicting two‐phase NAPL‐water flow in stochastically heterogeneous media and (2) to compare the predictions from different constitutive models. The five models tested were the nonhysteretic Brooks‐Corey‐Burdine (BCB) and van Genuchten‐Mualem (VGM) models, the hysteretic versions of these models (HBCB and HVGM), and the HVGBCB model, a model combining the hysteretic van Genuchten (HVG) Pc‐S relation and the hysteretic Brooks‐Corey‐Burdine kr‐S relation. Two cases of NAPL migration were considered: a layered system of two homogeneous sands separated by a dipping interface and a system where one of the layers was stochastically heterogeneous. The results showed that the best models could indeed capture the main characteristics of the spreading and immobilization well, demonstrating the validity of the continuum‐based approach for this level of stochastic heterogeneity. Implementation of hysteresis was necessary for correct prediction of the observed speed of NAPL migration as well as the amount of immobilized NAPL. The three hysteretic models were similar in their overall prediction error‐based performance. The HVGM model produced less overestimation of NAPL saturations but instead underestimated the entrapment at capillary barriers in comparison to the HBCB and HVGBCB models. The HVGM model also overestimated the speed of NAPL migration, which is attributed to its closed‐form kr‐S function, for which the VG parameter m has to be fitted under the constraint m = 1 − 1/n. The HVG (and VG) Pc‐S function, in contrast, used a different set of VG parameters produced with no constraint on m, which better represented the pore size distributions of the sands. A relation for partial nonwetting phase immobility during drainage is also presented.
Spatio‐temporal combination of MODIS images – potential for snow cover mapping
Authors: Parajka J. , Blöschl G.
Published: March 2008

MODIS snow cover products are appealing for hydrological applications because of their good accuracy and daily availability. Their main limitation, however, is cloud obscuration. In this study we evaluate simple mapping methods, termed temporal and spatial filters, that reduce cloud coverage by using information from neighboring non‐cloud covered pixels in time or space, and by combining MODIS data from the Terra and Aqua satellites. The accuracy of the filter methods is evaluated over Austria, using daily snow depth observations at 754 climate stations and daily MODIS images in the period 2003–2005. The results indicate that the filtering techniques are remarkably efficient in cloud reduction, and the resulting snow maps are still in good agreement with the ground snow observations. There exists a clear, seasonally dependent, trade off between accuracy and cloud coverage for the various filtering methods. An average of 63% cloud coverage of the Aqua images is reduced to 52% for combined Aqua‐Terra images, 46% for the spatial filter, 34% for the 1‐day temporal filter and 4% for the 7‐day temporal filter, and the corresponding overall accuracies are 95.5%, 94.9%, 94.2%, 94.4% and 92.1%, respectively.
The C2 domain of SynGAP is essential for stimulation of the Rap GTPase reaction
Authors: Pena Vladimir , Hothorn Michael , Eberth Alexander , Kaschau Nikolai , Parret Annabel , Gremer Lothar , Bonneau Fabien , Ahmadian Mohammad Reza , Scheffzek Klaus
Published: March 2008

The brain‐specific synaptic guanosine triphosphatase (GTPase)‐activating protein (SynGAP) is important in synaptic plasticity. It shows dual specificity for the small guanine nucleotide‐binding proteins Rap and Ras. Here, we show that RapGAP activity of SynGAP requires its C2 domain. In contrast to the isolated GAP domain, which does not show any detectable RapGAP activity, a fragment comprising the C2 and GAP domains (C2–GAP) stimulates the intrinsic GTPase reaction of Rap by approximately 1 × 104. The C2–GAP crystal structure, complemented by modelling and biochemical analyses, favours a concerted movement of the C2 domain towards the switch II region of Rap to assist in GTPase stimulation. Our data support a catalytic mechanism similar to that of canonical RasGAPs and distinct from the canonical RapGAPs. SynGAP presents the first example, to our knowledge, of a GAP that uses a second domain for catalytic activity, thus pointing to a new function of C2 domains.
Concentration gradients in mixed magnetic and nonmagnetic colloidal suspensions
Authors: Erb Randall M. , Yellen Benjamin B.
Published: March 2008

An approximate analytic model is derived to describe the equilibrium concentration profile within mixed suspensions of magnetic and nonmagnetic colloidal particles, and a strong qualitative agreement is shown between theory and experiment. This work extends previous theoretical and experimental investigations of density profiles within magnetic nanoparticle fluids, known as ferrofluid.
Polarizable interaction potential for water from coupled cluster calculations. II. Applications to dimer spectra, virial coefficients, and simulations of liquid water
Authors: Bukowski Robert , Szalewicz Krzysztof , Groenenboom Gerrit C. , van der Avoird Ad
Published: March 2008

The six-dimensional CC-pol interaction potential for the water dimer was used to predict properties of the dimer and of liquid water, in the latter case after being supplemented by a nonadditive potential. All the results were obtained purely from first principles, i.e., without any fitting to experimental data. Calculations of the vibration-rotation-tunneling levels of (H2O)2 and (D2O)2, a very sensitive test of the potential surface, gave results in good agreement with experimental high-resolution spectra. Also the virial coefficients and properties of liquid water agree well with measured values. The present model performs better than published force fields for water in a simultaneous reproduction of experimental data for dimer spectra, virials, and properties of the liquid.
The study of the D′Πu1 state of H2: Transition probabilities from the ground state, predissociation yields, and natural linewidths
Authors: Glass-Maujean M. , Klumpp S. , Werner L. , Ehresmann A. , Schmoranzer H.
Published: March 2008

The absorption spectrum of the H2 molecule was studied at high resolution in the 81–72nm spectral range. A detailed analysis of the D′Πu1→XΣg+1 electronic band system is reported. In the spectrum, more than 70 new lines were assigned. For wavelengths longer than 75nm, the D′Πu+1 and Πu−1 components show a clearly different behavior: Τhe Πu+1 one dissociates into H(1s)+H(n=2) whereas the Πu−1 one leads to molecular fluorescence. For shorter wavelengths, both components are predissociated into H(1s)+H(n=3). The predissociation yields, the dissociation widths, and the absolute values of the transition probabilities were measured over the vibrational progression from v′=3 to 17, i.e., up to the dissociation limit. The comparison between these absolute transition probabilities and the values calculated in the adiabatic and nonadiabatic approximations demonstrates clearly the importance of nonadiabatic couplings.
Room temperature ferromagnetism in Mn- and Fe-doped indium tin oxide thin films
Authors: Venkatesan M. , Gunning R. D. , Stamenov P. , Coey J. M. D.
Published: March 2008

Undoped and transition-metal doped indium tin oxide films have been grown by pulsed laser deposition technique, on single crystalline c-plane (0001) and r-plane (1102) sapphire substrates maintained at 500–850°C. Magnetization measurements of films deposited at different temperatures indicate that ferromagnetism appears for deposition temperatures, Tdep>600°C, with the highest moment for films deposited around 750°C. Qualitative different ferromagnetic behavior has been observed at room temperature in Fe- and Mn-doped thin films. The stable, hysteretic ferromagnetism of the Fe-doped films is due to the presence of magnetite, as seen in transmission Mössbauer spectra. The Mn-doped films show anhysteretic ferromagnetism which decays over time. It is somehow intrinsic, but not due to the Mn ions, which remains paramagnetic down to 4K. No anomalous Hall effect is observed.
Nucleation and wall motion in graded media
Authors: Skomski R. , George T. A. , Sellmyer D. J.
Published: March 2008

Magnetization reversal in graded magnetic-recording media and its effect on the areal density are investigated by model calculations. By choosing suitable solid-solution materials it is conceptually straightforward, though practically challenging, to achieve arbitrarily low write fields. The writing process involves both the nucleation of reverse domains and their propagation along elongated particles. The performance of the medium is optimized for pinning and nucleation fields of comparable size, and the two fields can be tuned by adjusting the length of the elongated particles (pillars) and the anisotropies of the hard and soft ends. However, the write-field reduction negatively affects the areal density, and there remains a trade-off between write field and bit size.
Solitary wave dynamics in time-dependent potentials
Authors: Abou Salem Walid K.
Published: March 2008

The long time dynamics of solitary wave solutions of the nonlinear Schrödinger equation in time-dependent external potentials is rigorously studied. To set the stage, the well-posedness of the Cauchy problem for a generalized nonautonomous nonlinear Schrödinger equation with time-dependent nonlinearities and potential is established. Afterward, the dynamics of NLS solitary waves in time-dependent potentials is studied. It is shown that in the space-adiabatic regime where the external potential varies slowly in space compared to the size of the soliton, the dynamics of the center of the soliton is described by Hamilton’s equations, plus terms due to radiation damping. Finally, two physical applications are discussed: the first is adiabatic transportation of solitons and the second is the Mathieu instability of trapped solitons due to time-periodic perturbations.
Hysteresis of ultrasmall Fe–Pt particles
Authors: Skomski Ralph , Liu J. P. , Rong C. B. , Sellmyer D. J.
Published: March 2008

The magnetization reversal in very small FePt particles is investigated by analytical and numerical calculations. The modeling focuses on particles with diameters from 3to15nm, as produced by a salt-matrix annealing technique. Experiment shows that the particles exhibit a certain degree of structural inhomogenity, which has a far-reaching effect on the magnetic hysteresis. In particles larger than about 10nm, the magnetization-reversal mode is strongly inhomogeneous, and there are several scenarios that depend on the symmetry of inhomogenity. Small particles reverse nearly coherently, and the coercivity is essentially equal to the volume-averaged anisotropy. In this case, nonrectangular hysteresis loops reflect factors such as grain misalignment, particle-size distribution, and different degrees of L10 order in different particles.
Alternating target laser ablation deposition of high quality barium hexaferrite thin films from barium monoferrite and hematite targets
Authors: Geiler A. L. , Yoon S. D. , Chen Y. , Yang A. , Chinnasamy C. N. , Geiler M. , Harris V. G. , Vittoria C.
Published: March 2008

An optimized alternating target laser ablation deposition (ATLAD) technique has been developed to grow high quality barium hexaferrite (BaFe12O19) thin films on basal plane oriented sapphire (Al2O3) substrates from barium monoferrite (BaFe2O4) and hematite (α-Fe2O3) targets. Crystallographic and structural characterization results show that the films possess low c-axis dispersion of Δω=0.259° and hexagonal terraced surface morphology. Saturation magnetization and uniaxial magnetic anisotropy field were determined to be consistent with reference data on high quality barium hexaferrite films and bulk single crystals grown by other techniques. Ferromagnetic resonance linewidth of 42Oe was measured at 52GHz by the shorted waveguide technique. We conclude that the ATLAD technique is capable of growing high quality barium hexaferrite films while providing unique opportunities to control the ionic distribution in the hexagonal unit cell by allowing different species of ions to be introduced from the respective targets in any order and at any time during film growth.
Speed control of the 12-step sensorless drive for a brushless dc motor
Authors: Fang Chih-Hsing , Lin Shir-Kuan , Wu Yu-Jheng , Lo Chen-Kuo
Published: March 2008

The 12-step sensorless drive scheme requires an interval to detect the zero crossing of the back emf. The voltage regulating pulse-width modulation (PWM) is commonly used for the speed control of a motor. However, it induces the fluctuation of the back emf and makes the zero-crossing detection difficult. This paper proposes two digital approaches to the zero-crossing detection for the presence of the PWM. One approach is to vary the zero-crossing detecting period linearly with respect to the speed of the motor. The other is to detect the back emf only on the positive pulses of the PWM in order to avoid the fluctuation of the back emf. A digital filter is also incorporated in order to eliminate the noise generated by the PWM. Experiments show that both approaches have good performance for the speed control of the brushless dc motor.
Electron spin resonance of Gd3+ in GdmMnIn3m+2n (M=Rh,Ir; n=,1; m=1,2) antiferromagnets
Authors: Duque J. G. S. , Adriano C. , Lora-Serrano R. , Rettori C. , Urbano R. R. , Sarrao J. L. , Oseroff S. B. , Pagliuso P. G.
Published: March 2008

We report electron spin resonance experiments of Gd3+ in the GdmMnIn3m+2n (M=Rh,Ir; n=0,1; m=1,2) intermetallic compounds. For T>TN∼45K, all compounds present a single Dysonian resonance and show a Korringa-like temperature dependence of the linewidth, ΔH=a+bT. The residual linewidth a is strongly affected by the transition metal M=Rh or Ir and/or by the layering (m=1 or 2) or change in structure (n=0,1). The residual linewidth is associated with an unresolved crystalline electrical field (CEF) fine structure. Consequently, a systematic evolution of the CEF in the GdmMnIn3m+2n compounds is inferred. A discussion to what extent our results can explain to the CEF effects observed in isostructural R-based compounds will be given.
Spin-polarized tunneling with Au impurity layers
Authors: Gabureac M. S. , Dempsey K. J. , Porter N. A. , Marrows C. H. , Rajauria S. , Courtois H.
Published: March 2008

We have inserted nonmagnetic impurity layers of Au into sputtered AlOx-based magnetic tunnel junctions (F/I/F) and Meservey–Tedrow junctions (S/I/F) in order to study their effect on the tunneling magnetoresistance (TMR) and spin polarization (TSP). Both room temperature TMR and the TSP at 250mK decay exponentially as an interfacial Au layer is introduced between the barrier and one Co electrode, with 1∕e decay lengths λTMR=11±3Å and λTSP=14±2Å. We also inserted a 1Å thick Au layer at a variable distance from the barrier/Co interface and find that both the TMR and TSP recover to the undoped value with the shorter exponential length scales of λTMR=7±4Å and λTSP=6±2Å.
Modification of charge compensation in semi-insulating semiconductors by high energy light ion irradiation
Authors: Kabiraj D. , Grötzschel Rainer , Ghosh Subhasis
Published: March 2008

High energy light ions have been used to engineer the electrical properties of semi-insulating InP and GaAs by creation and annihilation of native defects. Due to light mass, high energy ions lose most of their energy during flight inside the material by electronic excitation, which also modifies the charge states of the defect. Imbalance in charge compensation caused by irradiation induced defect engineering results in the modification of resistivity of the semi-insulating samples. The activation energies of the defect levels responsible for the modified resistivity have been estimated from the measurement of thermally activated currents. It is shown that the modified charge compensation is due to annihilation and/or creation of native defects depending on the material (GaAs or InP) and the fluence of irradiated ions. It has also been observed that annealing does not have any effect on modified compensation in InP, but in the case of GaAs, compensation recovers after thermal annealing.
On the origin of nitrogen-induced retardation of boron diffusion in amorphous silica
Authors: Kuo Chin-Lung , Hwang Gyeong S.
Published: March 2008

The effect of N incorporation on B diffusion in amorphous SiO2 is presented based on spin-polarized density functional theory calculations. Our results show that N incorporation leads to the decrease of O vacancy concentration, which is largely responsible for the retarded B diffusion by reducing diffusion mediators such as E′ and S centers. We also determine the ground state structure of the BN complex, along with possible formation routes. The direct BN bonding interaction appears to only slightly increase the activation energy of B diffusion.
Thermal stability of graphene edge structure and graphene nanoflakes
Authors: Barnard Amanda S. , Snook Ian K.
Published: March 2008

One of the most exciting recent developments in nanoscience was the discovery of graphene (single sheets of carbon atoms, a two-dimensional “(2D) crystal”) and the subsequent discovery of the fascinating properties of this new material, e.g., electrons behaving as massless relativistic particles and an anomalous quantum Hall effect [A. K. Geim and K. S. Novoselov, Nat. Mater. 6, 183 (2007)]. It is also surprising that large sheets of graphene exist as it was widely believed that 2D crystals are unstable. Furthermore, because of the stability of folded graphene sheets, i.e., carbon nanotubes (CNTs), a fascinating question is why does not graphene spontaneously transform into CNTs? In this paper, we explore the thermal stability of small pieces of graphene, i.e., graphene nanoflakes by ab initio quantum mechanical techniques. We find that indeed nanoflakes are stable to being heated and do not under any conditions used here transform to CNTs. They do not, however, remain strictly 2D as at finite temperatures, they undergo extensive vibrational motion and remain buckled if annealed and then quenched to room temperature.
Surface recombination in GaAs thin films with two-dimensional photonic crystals
Authors: Xu Xingsheng , Yamada Toshiki , Otomo Akira
Published: March 2008

The dynamics of spontaneous emission from GaAs slabs with photonic crystals etched into them are investigated both theoretically and experimentally. It is found that the intensity of spontaneous emission decreases significantly and that photonic crystals significantly shorten the lifetime of emission. The mechanics of enhancement and the reduction of emission from photonic crystals are analyzed by considering the surface recombination of GaAs. The measured and calculated lifetimes agree at a surface recombination velocity of 1.88×105cm∕s.
Quantum confinement effect in high quality nanostructured CeO2 thin films
Authors: Nie Jia Cai , Hua Zhen Yong , Dou Rui Fen , Tu Qing Yun
Published: March 2008

High quality crystalline nanostructured CeO2 thin films were fabricated by pulsed laser deposition on R-cut sapphire substrates. As-grown and postannealed (in oxygen) films were investigated by atomic force microscopy, x-ray diffraction, and optical transmission and reflectance spectroscopy. The optical spectra changed as the microstructure varied from small grains to large quasi-two-dimensional islands. Hence, the band gap energy (Eg) was found to increase systematically with the decrease in the grain size and ranged from 3.58to3.71eV for the direct and from 3.01to3.26eV for the indirect transitions. This is consistent with the quantum confinement model and, up to now, not observed in a convincing manner in the CeO2 thin films. Results indicate that Eg can be controlled in nanostructured thin films through thickness control and postannealing.
Optical properties of nearly stacking-fault-free m-plane GaN homoepitaxial films grown by metal organic vapor phase epitaxy on low defect density freestanding GaN substrates
Authors: Chichibu S. F. , Yamaguchi H. , Zhao L. , Kubota M. , Okamoto K. , Ohta H.
Published: March 2008

Prospective optical properties were demonstrated for nearly stacking fault-free m-plane GaN homoepitaxial films grown by metal organic vapor phase epitaxy on the m-plane freestanding GaN substrates. Values of full width at half maximum of x-ray rocking curves were close to the substrate values being 31arcsec for the (101¯0) diffraction with ⟨0001⟩ azimuth and 48arcsec for the (101¯2) diffraction. Threading dislocation densities were lower than 5×106cm−2. The film surfaces exhibited atomically flat morphology with well-aligned monolayer steps. Low-temperature photoluminescence (PL) spectra exhibited polarization-dependent well-resolved bound and free exciton emission lines, and a characteristic π(k⊥c,E∥c)-polarized PL line was also observed. Room-temperature effective PL lifetime of the free exciton peak increased with increasing supply ratio of ammonia to trimethylgallium, and a record long value for m-plane GaN (268ps) was obtained.
Negative helium generation upon surface scattering: Application in space science
Authors: Wurz Peter , Saul Lukas , Scheer Jürgen A. , Möbius Eberhard , Kucharek Harald , Fuselier Stephen A.
Published: March 2008

We report the formation of negatively charged helium ions upon scattering of neutral helium atoms from silicon wafers coated with diamondlike carbon (DLC). The energy of the primary He atoms ranges from 10–1500eV. The negative ion fraction in the scattered He beam ranges from 7×10−5 down to 10−5 with increasing energy. The decreasing negative ion fraction with increasing energy we find for the DLC surfaces is in contrast with earlier results obtained for low work function metallic surfaces. The observed energy dependence cannot be explained by theory because at present there is no theory available that explains the electron transfer between diamond surfaces and scattered atoms. Although the negative ion fraction is low, this ionization technique allows the direct detection of interstellar neutral He with species identification for the first time. This measurement will be performed with the IBEX-Lo sensor on the Interstellar Boundary Explorer mission of NASA.
Superhydrophobic behavior on transparency and conductivity controllable ZnO∕Zn films
Authors: Liu Yi , Tan Tian , Wang Bo , Song Xuemei , Li Er , Wang Hao , Yan Hui
Published: March 2008

This study employs a facile method to selectively fabricate a pair of ZnO films on glass slide with controllable transparency and conductivity. The resultant surfaces both show static superhydrophobicity with alike contact angles of 155° and 158° but exhibit a significant difference under dynamic conditions. The sliding angle of the clustered ZnO film is 31° smaller than that of the one randomly covered with nanorods, due to its more discrete wetting structure of multiscaled hierachical architecture of the clusters. The kinetic mechanism is briefly discussed and a further modification for nanorod film to achieve real superhydrophobicity is suggested.
Stray-field nuclear magnetic resonance imaging in microgravity conditions
Authors: Garrido Leoncio , Sampayo José
Published: March 2008

Magnetic levitation has been proposed as an alternative approach to simulate on Earth microgravity conditions encountered in space, allowing the investigation of weightlessness on materials and biological systems. In general, very strong magnetic fields, 15T or higher, are required to achieve levitation for a majority of diamagnetic substances. Here, we show that it is possible to achieve levitation of these substances in a commercial superconductive magnet operating with a nuclear magnetic resonance (NMR) spectrometer at 9.4T at ambient conditions. Furthermore, stray-field proton NMR imaging is performed in situ at the location where a sample is levitating, showing that it is feasible to obtain the corresponding one-dimensional profile. Considering that water is a diamagnetic substance and the main constituent of living systems, the outlined approach could be useful to investigate alterations in water proton NMR properties induced by low gravity and magnetic forces upon levitating, e.g., seeds, cells, etc. In addition to protons, it would also be possible to observe other nuclei (e.g., F19, P31, etc.) that may be of interest in metabolic and therapeutic investigations.
Capacitance-voltage characteristics of heterostructures with high leakage currents
Authors: Goldenblum A. , Stancu V. , Buda M. , Iordache G. , Pintilie I. , Negrila C. , Botila T.
Published: March 2008

Leakage currents determine bumps in the capacitance-voltage characteristics of metal-oxide-semiconductor-type heterostructures with capacitance values larger than the insulator capacitance. A nearly parallel shift with frequency is also observed. These aspects are connected with the presence of a high density of interface states. We illustrate these effects for the case of nano-PbS∕SiO2∕Si heterostructures. The phenomena were simulated and we propose a method to quickly estimate the interface state density level of a heterostructure when leakage channels are present.
On the ion- and electron-phonon interaction effects on impurity states in polar cylindrical quantum wires
Authors: Zhao Zeng-Ru , Liang X. X.
Published: March 2008

The phonon effect on the hydrogenic impurity states in a polar cylindrical quantum wire is studied by taking both the confined bulk longitudinal-optical and interface-optical (IO) phonons into account. The binding energies are calculated as functions of the transverse dimension of the quantum wire and the donor-impurity position. The numerical results confirm that screening effect for the Coulomb potential caused by the ion-phonon coupling is generally dominant in various phonon contributions to the impurity states in quantum wires. It is also shown that the IO phonon modes play an important role when the impurity shifts toward the surface.
Upconversion in Er3+-doped ZrO2 nanocrystals pumped at 1.426μm
Authors: Gómez Luis A. , Maciel Glauco S. , de Araújo Cid B. , Patra Amitava
Published: March 2008

The effect of frequency upconversion (UC) was investigated in 0.05 and 2.00mol% Er2O3 doped in ZrO2 nanocrystals prepared by a sol-emulsion-gel process. Laser excitation at 1.426μm produced UC luminescence at ∼0.800, ∼0.525, ∼0.550, and ∼0.660μm corresponding to Er3+ transitions I9∕24→I15∕24, H11∕22→I15∕24, S3∕24→I15∕24, and F9∕24→I15∕24, respectively. The role played by excited state absorption and energy transfer mechanisms in the UC process was analyzed using rate equations for Er3+ energy level populations. The energy transfer rate between pairs of Er3+ ions (I9∕24+I13∕24→I15∕24+S3∕24) was determined from comparison between theoretical predictions and experimental data.
Incorporation of nitrogen in Cr∕Sc multilayers giving improved soft x-ray reflectivity
Authors: Ghafoor Naureen , Eriksson Fredrik , Gullikson Eric , Hultman Lars , Birch Jens
Published: March 2008

Soft x-ray reflectivity (SXR) of Cr∕Sc multilayer with bilayer thickness of Λ=1.56nm was increased by 100% by an intentional introduction of nitrogen during magnetron sputtering. Multilayers deposited at background pressures of ⩽2×10−6Torr exhibited amorphous layers with flat interfaces. At 2×10−5Torr, understoichiometric CrNx∕ScNy multilayer with a nitrogen content of ∼34at.% was formed. CrNx∕ScNy multilayer comprising of only 100 periods exhibited a SXR of 11.5%. X-ray and electron microscopy analyses showed that the improvement in performance is a result of reduced interfacial diffusion yielding interface widths of ⩽0.29nm. The CrNx∕ScNy multilayer exhibited thermal stability up to >380°C.