bluuebunny/crossref_metadata_2025 huggingface dataset:
Calcium isotope (δ44/40Ca) variations of Neogene planktonic foraminifera
Authors: Heuser Alexander , Eisenhauer Anton , Böhm Florian , Wallmann Klaus , Gussone Nikolaus , Pearson Paul N. , Nägler Thomas F. , Dullo Wolf‐Christian
Published: June 2005

Measurements of the calcium isotopic composition (δ44/40Ca) of planktonic foraminifera from the western equatorial Pacific and the Indian sector of the Southern Ocean show variations of about 0.6‰ over the past 24 Myr. The stacked δ44/40Ca record of Globigerinoides trilobus and Globigerina bulloides indicates a minimum in δ44/40Casw (seawater calcium) at 15 to 16 Ma and a subsequent general increase toward the present, interrupted by a second minimum at 3 to 5 Ma. Applying a coupled calcium/carbon cycle model, we find two scenarios that can explain a large portion of the observed δ44/40Casw variations. In both cases, variations in the Ca input flux to the ocean without proportional changes in the carbonate flux are invoked. The first scenario increases the riverine calcium input to the ocean without a proportional increase of the carbonate flux. The second scenario generates an additional calcium flux from the exchange of Ca by Mg during dolomitization. In both cases the calcium flux variations lead to drastic changes in the seawater Ca concentrations on million year timescales. Our δ44/40Casw record therefore indicates that the global calcium cycle may be much more dynamic than previously assumed.
Submillennial variations in ocean conditions during deglaciation based on diatom assemblages from the southwest Atlantic
Authors: Allen Claire S. , Pike Jennifer , Pudsey Carol J. , Leventer Amy
Published: June 2005

We present a high‐resolution paleoceanographic record of deglaciation based on diatom assemblages from a core located just south of the Polar Front in the southwest Atlantic. Core KC073 is from a sediment drift at the mouth of the Falkland Trough and contains sediments from the Last Glacial Maximum (LGM) to present, dated using radiocarbon dates on bulk organic matter and radiolarian stratigraphy. The site lies along the path of the Antarctic Circumpolar Current (ACC) and immediately downstream of where North Atlantic Deep Water (NADW) is entrained into the ACC. Significant variations in ocean conditions are reflected in high‐amplitude changes in diatom concentrations and assemblage composition. The diatom assemblage at the LGM indicates that winter sea ice extent was at least 5° farther north than present until at least 19.0 ka (calendar years) and summer sea ice may have occasionally extended over the site, but for the most part it lay to the south. During deglaciation, Chaetoceros resting spores (CRS) dominate the diatom assemblage with valve concentrations in excess of 500 × 106 valves per gram. Submillennial‐scale variations in the numbers of CRS and Thalassiosira antarctica occur throughout the late deglacial and dominate the changes in diatom concentration. We propose that the influx of CRS is controlled by the flow of NADW over the Falkland Plateau. As such our data provide unique evidence that NADW impacted on this sector of the Southern Ocean during deglaciation. During the Holocene the sedimentation rate dramatically reduced. We suggest that the ACC flow increased over the site and inhibited settling and winnowed the surface sediments.
Calcareous nannoplankton response to late Albian oceanic anoxic event 1d in the western North Atlantic
Authors: Watkins David K. , Cooper Matthew J. , Wilson Paul A.
Published: June 2005

Well‐preserved nannofossil and stable isotope records from the mid‐Cretaceous of Ocean Drilling Project Leg 171B (western North Atlantic) indicate cyclical, productivity‐based variations in surface water characteristics, suggesting orbitally paced changes in upwelling intensity and the strength of deep mixing associated with oceanic anoxic event (OAE)1d. Paleontologic and isotopic evidence suggest that collapse of upper water column stratification associated with OAE1d was preceded by approximately 1 m.y. of progressively decreasing water column stability and increasing surface water fertility. Thirteen species went extinct during a short (ca. 200 k.y.) interval associated with OAE1d. Nine of these have morphological characters suggesting adaptation to specific depths in the photic zone. Simultaneous extinction of these depth‐zoned species suggests that stratification collapse disrupted the habitat space for these specialized forms, resulting in extinction. A cyclostratigraphic model provides age estimates of several important nannofossil datums including the first appearances of Eiffellithusturriseiffelii (100.95 Ma) and Corollithionkennedyi (99.55 Ma).
Early Jurassic schizosphaerellid crisis in Cantabria, Spain: Implications for calcification rates and phytoplankton evolution across the Toarcian oceanic anoxic event
Authors: Tremolada Fabrizio , Van de Schootbrugge Bas , Erba Elisabetta
Published: June 2005

The Toarcian oceanic anoxic event (∼183 Myr ago) represents a global perturbation marked by increasing organic carbon burial and a general decrease in calcium carbonate production likely triggered by elevated carbon dioxide levels in the atmosphere. Here we present quantitative analyses of calcareous nannofossil diversity and abundance from the Castillo de Pedroso section in Cantabria, northern Spain. We compare these data with geochemical data (C and O isotopes) obtained from biogenic and bulk carbonate records in order to highlight the response of calcareous phytoplankton to major climatic and paleoceanographic changes. The Pliensbachian/Toarcian boundary is characterized by an abrupt decrease in abundance of Schizosphaerella punctulata, the most important lithogenic contributor to (hemi) pelagic carbonates in the Early Jurassic. The early Toarcian nannofloral assemblages show an increase in abundance of Mitrolithus jansae and small‐sized r‐selected taxa and a progressive decrease in S. punctulata percentages. The deep dwellers M. jansae and S. punctulata experienced a major crisis slightly prior to the deposition of the Toarcian black shales that are characterized by high abundances of eutrophic taxa such as Lotharingius spp. and Biscutum spp. The return of S. punctulata associated with lower percentages of eutrophic taxa was observed just above the Toarcian black shales. The Toarcian episode reveals that high CO2 levels and increasing primary productivity probably triggered a shift in abundance from highly calcified nannoliths such as S. punctulata and M. jansae to small‐sized r‐selected coccoliths that overall record a biocalcification crisis at the onset and during the Toarcian episode.
Atmospheric Chemistry Experiment (ACE): Mission overview
Authors: Bernath P. F. , McElroy C. T. , Abrams M. C. , Boone C. D. , Butler M. , Camy‐Peyret C. , Carleer M. , Clerbaux C. , Coheur P.‐F. , Colin R. , DeCola P. , DeMazière M. , Drummond J. R. , Dufour D. , Evans W. F. J. , Fast H. , Fussen D. , Gilbert K. , Jennings D. E. , Llewellyn E. J. , Lowe R. P. , Mahieu E. , McConnell J. C. , McHugh M. , McLeod S. D. , Michaud R. , Midwinter C. , Nassar R. , Nichitiu F. , Nowlan C. , Rinsland C. P. , Rochon Y. J. , Rowlands N. , Semeniuk K. , Simon P. , Skelton R. , Sloan J. J. , Soucy M.‐A. , Strong K. , Tremblay P. , Turnbull D. , Walker K. A. , Walkty I. , Wardle D. A. , Wehrle V. , Zander R. , Zou J.
Published: June 2005

SCISAT‐1, also known as the Atmospheric Chemistry Experiment (ACE), is a Canadian satellite mission for remote sensing of the Earth's atmosphere. It was launched into low Earth circular orbit (altitude 650 km, inclination 74°) on 12 Aug. 2003. The primary ACE instrument is a high spectral resolution (0.02 cm−1) Fourier Transform Spectrometer (FTS) operating from 2.2 to 13.3 μm (750–4400 cm−1). The satellite also features a dual spectrophotometer known as MAESTRO with wavelength coverage of 285–1030 nm and spectral resolution of 1–2 nm. A pair of filtered CMOS detector arrays records images of the Sun at 0.525 and 1.02 μm. Working primarily in solar occultation, the satellite provides altitude profile information (typically 10–100 km) for temperature, pressure, and the volume mixing ratios for several dozen molecules of atmospheric interest, as well as atmospheric extinction profiles over the latitudes 85°N to 85°S. This paper presents a mission overview and some of the first scientific results.
Comparisons between ACE‐FTS and ground‐based measurements of stratospheric HCl and ClONO2 loadings at northern latitudes
Authors: Mahieu E. , Zander R. , Duchatelet P. , Hannigan J. W. , Coffey M. T. , Mikuteit S. , Hase F. , Blumenstock T. , Wiacek A. , Strong K. , Taylor J. R. , Mittermeier R. L. , Fast H. , Boone C. D. , McLeod S. D. , Walker K. A. , Bernath P. F. , Rinsland C. P.
Published: June 2005

We report first comparisons of stratospheric column abundances of hydrogen chloride (HCl) and chlorine nitrate (ClONO2) derived from infrared solar spectra recorded in 2004 at selected northern latitudes by the spaceborne Atmospheric Chemistry Experiment‐Fourier Transform Spectrometer (ACE‐FTS) and by Fourier transform infrared (FTIR) instruments at the NDSC (Network for Detection of Stratospheric Change)‐affiliated sites of Thule (Greenland), Kiruna (Sweden), Jungfraujoch (Switzerland), and Egbert and Toronto (Canada). Overall, and within the respective uncertainties of the independent measurement approaches, the comparisons show that the ACE‐FTS measurements produce very good stratospheric volume mixing ratio profiles. Their internal precision allows to identify characteristic distribution features associated with latitudinal, dynamical, seasonal and chemical changes occurring in the atmosphere.
Comparison of atmospheric retrievals from ACE and HALOE
Authors: McHugh Martin , Magill Brian , Walker Kaley A. , Boone Chris D. , Bernath Peter F. , Russell James M.
Published: June 2005

Atmospheric retrievals from ACE are compared to nearly coincident measurements from HALOE. Comparisons are done for O3, HF, HCl, H2O, CH4, NO, NO2, and temperature. Statistics from 32 profile pairs that were reasonably close in space and time are presented. The temperature retrievals agree to within ±2 K, an important result as all other retrievals depend on this. ACE gas retrievals are typically higher than HALOE, with the exception of NO and NO2 where HALOE is higher. This comparison helps provide traceability between two important complementary remote sensing missions, lends confidence to the initial ACE dataset, and provides a baseline for future retrieval improvements.
First measurements of CFC‐113 and HCFC‐142b from space using ACE‐FTS infrared spectra
Authors: Dufour Gaëlle , Boone Chris D. , Bernath Peter F.
Published: June 2005

The first measurements of CFC‐113 (CCl2FCClF2) and HCFC‐142b (CH3CClF2) from space have been performed by the Atmospheric Chemistry Experiment (ACE), a satellite‐based mission for remote sensing of the Earth's atmosphere. Retrievals for the volume mixing ratio profiles of these two molecules have been performed for ACE‐FTS measurements at mid‐latitudes and in the tropics, and the results are compared with AGAGE surface measurements. The agreement between ACE and AGAGE mixing ratios is within 15% for both CFC‐113 and HCFC‐142b.
Properties of high‐altitude tropical cirrus clouds determined from ACE FTS observations
Authors: Eremenko M. N. , Zasetsky A. Y. , Boone C. D. , Sloan J. J.
Published: June 2005

During tropical observations made in February and August of 2004, the Atmospheric Chemistry Experiment (ACE) FTIR instrument aboard the Canadian SciSat‐1 satellite recorded several extinction spectra in the range of 700–4400 cm−1 that exhibit strong signatures of cirrus clouds. We have analyzed these spectra in order to quantify the properties of these high‐altitude cirrus clouds. This Letter reports the vertical profiles of the average ice density in the clouds and the particle size distributions. We also discuss the effect of particle shape on the retrieval results.
Electronic transitions at defect states in Cz p-type silicon
Authors: Castaldini A. , Cavalcoli D. , Cavallini A. , Binetti S. , Pizzini S.
Published: June 2005

Point and extended defects introduced in p-type Cz Si by oxygen precipitation and plastic deformation have been investigated with electrical and optical methods. Different materials (oxygen precipitated and/or deformed Cz Si and Fz Si) were examined in order to separate the role of oxygen precipitation, plastic deformation, and metallic contamination on the radiative and nonradiative electronic transitions at defect centers. A deep hole trap, named T1, has been associated with dislocation-related impurity centers; additional deep traps have been related to contamination by grown-in transition metals and to clusters involving oxygen atoms.
Magnetostriction in ⟨110⟩ and ⟨112⟩ oriented crystals Tb0.36Dy0.64(Fe0.85Co0.15)2
Authors: Ma Tianyu , Jiang Chengbao , Xu Huibin
Published: June 2005

Magnetostriction of ⟨110⟩ and ⟨112⟩ oriented crystals Tb0.36Dy0.64(Fe0.85Co0.15)2 under various conditions of compressive prestress and applied fields was investigated at room temperature (30°C), elevated temperature (100°C) and cryogenic temperature (−80°C). Obvious magnetostriction jump effect was observed in both ⟨110⟩ and ⟨112⟩ oriented crystals at room temperature and elevated temperature, but less obvious at cryogenic temperature. When the compressive prestress is not higher than 10MPa, the ⟨112⟩ oriented crystal exhibits larger magnetostriction than that of ⟨110⟩ oriented crystal in low applied field at room temperature and cryogenic temperature, but smaller magnetostriction at elevated temperature.
Short pitch cholesteric electro-optical device stabilized by nonuniform polymer network
Authors: Kim Sang Hwa , Chien Liang-Chy , Komitov Lachezar
Published: June 2005

We have developed a method for stabilization of the uniform lying helix (ULH) texture of short pitch cholesterics in an electro-optical device, based on the flexoelectro-optic effect in such a texture. By using a small concentration of photoreactive liquid crystal monomer (less than 5wt.%) and selecting the illumination conditions, we were able to create a nonuniform polymeric network in the liquid crystal bulk (localized essentially at both substrate surfaces) which stabilized efficiently the amplitude and the phase modulation modes of the device. Most importantly, the effect of the residual birefringence of the polymeric network in the field-unwound state of the device was eliminated resulting thus in a substantial improvement of device performance.
Free and bound excitons in GaN∕AlGaN homoepitaxial quantum wells grown on bulk GaN substrate along the nonpolar (112¯0) direction
Authors: Teisseyre H. , Skierbiszewski C. , Łucznik B. , Kamler G. , Feduniewicz A. , Siekacz M. , Suski T. , Perlin P. , Grzegory I. , Porowski S.
Published: June 2005

Nonpolar multiple quantum wells (MQWs) have been grown by plasma assisted molecular beam epitaxy on bulk GaN crystals oriented along the (112¯0) direction. The photoluminescence intensity of the nonpolar MQWs was significantly higher than that found for the polar samples, both at low (10 K) and room temperature. This is a consequence of the lack of built-in electric field in samples grown along the (112¯0) direction. Clearly resolved spectra of the excitons have been observed in the studied MQWs. Studies of these excitonic structures, by means of polarization and temperature measurements enabled us to assign the observed lines to free and bound excitons in GaN quantum wells.
Stationary van Hove limit
Authors: Dereziński Jan , Früboes Rafał
Published: June 2005

The weak coupling (van Hove) limit of one parameter groups of contractions is studied by the stationary approach. We show that the resolvent of the properly renormalized and rescaled generator of a contractive semigroup has a limit as the coupling constant goes to zero. This limit is the resolvent of the generator of a certain contractive semigroup. Our results can be viewed as a stationary counterpart to the well known results about the weak coupling limit obtained by the time-dependent approach, due to Davies. We compare both approaches.
Strain relaxation, band-structure deformation, and optical absorption in free-hanging quantum-well microstructures
Authors: Stievater T. H. , Rabinovich W. S. , Park D. , Goetz Peter G. , Boos J. B. , Katzer D. S. , Biermann M. L. , Kanakaraju S. , Calhoun L. C.
Published: June 2005

We have measured changes to the band structure due to strain relaxation in micromachined multiple quantum-well microstructures that are released from the substrate. Analytic expressions for the strain tensor in the free-hanging heterostructure are presented and used in an eight-band k∙p model to predict the changes to the band structure in the micromachined heterostructure. The deformed band structure is measured optically using spatially resolved differential transmission spectroscopy and found to be in excellent agreement with predictions. The measurements indicate (i) that strained semiconductor heterostructures relax elastically upon release from the substrate, (ii) that k∙p theory combined with our strain relaxation expressions accurately model the optical properties of micromachined heterostructures, and (iii) that a significant asymmetry exists between the effects of compressive and tensile strains on optical absorption in these microstructures. Our results show that micromachining represents an important tool to modify the optical properties of quantum-well heterostructures.
Eliminating the hysteresis effect for reactive sputtering processes
Authors: Nyberg T. , Berg S. , Helmersson U. , Hartig K.
Published: June 2005

Reactive sputter processes frequently exhibit stability problems. The cause of this is that these processes normally exhibit hysteresis effects in the processing curves. Eliminating the hysteresis would significantly simplify the use of reactive sputtering processes. So far the only known way of eliminating the hysteresis is to increase the pumping speed to unrealistically high values. By an increased understanding of the process we have realized a fully realistic technique to eliminate the hysteresis for reactive sputtering processes. By simply reducing the size of the target sputter erosion zone below a critical value, simulations predicted that hysteresis should be eliminated. This has been experimentally verified for reactive sputtering of Al in an Ar∕O2 atmosphere. The fundamental explanation to this behavior as well as the experimental verification are presented.
The Ohmic heating of particulates in a lossless medium
Authors: Tang Wilkin , Bosman Herman , Lau Y. Y. , Gilgenbach R. M.
Published: June 2005

This paper provides a general theory on the Ohmic dissipation of electromagnetic energy by a spherical particulate that is embedded in a lossless medium. The particulate may possess an arbitrary electrical conductivity, and both the medium and the particulate may assume general values of permittivity and permeability. Under the assumption that the wavelength of the electromagnetic field in the medium is large compared with the particulate size, we provide an accurate account of the degree of Ohmic heating by the radio frequency (rf) electric field and by the rf magnetic field of the electromagnetic field. It is found that, in general, heating by the rf magnetic field is dominant whenever δ<a, where δ is the resistive skin depth and a is the radius of the particulate. Analytic scaling laws in the various regimes are derived, from the static case to very high frequency, and for ratios of δ∕a ranging from zero to infinity. The calculation is extended to a transient electromagnetic pulse. Also constructed is the loss tangent of the medium, resulting from a distribution of particulates.
Grooving of a grain boundary by evaporation–condensation below the roughening transition
Authors: Stone H. A. , Aziz M. J. , Margetis D.
Published: June 2005

The development of surface grooves at grain boundaries that intersect a planar surface is analyzed for the case that the evolution occurs below the thermodynamic roughening transition by evaporation–condensation processes. The dynamics are described by a nonlinear partial differential equation that has a similarity solution, so the resulting groove profile is described by a nonlinear ordinary differential equation. An approximate analytical solution to the nonlinear problem is obtained and is in excellent agreement with the numerical solution. The depth and width of the groove varies as t1∕2, where t is time, analogous to the classical results valid above the thermodynamic roughening temperature. In addition, the approximate analytical solution provides an explicit relation between the groove width and the dihedral angle, and is in sufficiently good agreement with the numerical results as to make such numerical solutions unnecessary for this problem. The results demonstrate explicitly how the groove shape depends on the functional form of the slope-dependent surface mobility.
Effect of grain boundaries on hydrocarbon sensing in Fe-doped p-type semiconducting perovskite SrTiO3 films
Authors: Mahesh Kumar M. , Post M. L.
Published: June 2005

Films of Fe-doped SrTiO3 deposited using pulsed laser deposition on sapphire and alumina, when exposed to propane, showed different sensor responses measured (as a resistance change) as a function of temperature and microstructure. The film deposited on alumina has a stronger response towards 3000-ppm propane than does the film deposited on sapphire. Films deposited on alumina exhibit higher dc conductivity than the films on sapphire. The activation energies indicate a mixed electronic/ionic conduction at low temperature with the high-temperature regime showing a temperature-independent conductivity. The origin of the difference in gas responses caused by varying temperature and morphology has been explored using ac impedance techniques and measured as a function of frequency (1Hz⩽f⩽1MHz) and temperature (200⩽T⩽480°C). A single relaxation (a single semicircle in the complex impedance plane) in the frequency domain was observed in addition to the relaxation due to the electrode-film interface. A model and a mechanism of conduction for the above are derived using equivalent circuits to fit the ac impedance data and dc conductivity. It is proposed that the reduction of Fe-doped SrTiO3, which is induced by propane, enhances the space-charge barrier near the grain boundaries and increases the sensitivity to propane.
Alternate and direct current conductivity of conducting polyaniline dispersed with poly vinyl alcohol and blended with methyl cellulose
Authors: Sarkar A. , Ghosh P. , Meikap A. K. , Chattopadhyay S. K. , Chatterjee S. K. , Ghosh M.
Published: June 2005

The electrical conductivity, both dc and ac, of conducting polyanilline dispersed with poly vinyl alcohol and blended with methyl cellulose has been investigated in the temperature range of 77⩽T⩽300K and in the frequency range 20 Hz to 1 MHz. The dc conductivity does not obey the standard variable range hopping model, but it satisfies hopping transport between the superlocalized states of polymer. The frequency dependence of conductivity has been described by the power law, σ(ω)αωs. The variation of “s” with temperature suggests that the ac conduction is due to the correlated barrier hopping. Temperature and frequency dependent dielectric constant indicates the Debye-type dispersion.
Enhanced photoconductivity of ZnO films Co-doped with nitrogen and tellurium
Authors: Porter H. L. , Cai A. L. , Muth J. F. , Narayan J.
Published: June 2005

Zinc oxide films are typically found to be n type, and conductive under most growth conditions and growth methods. Co-doping with multiple elements is one strategy for improving the electrical and optical properties of zinc oxide materials for optoelectronic device applications. Using pulsed-laser deposition, thin ZnO films were grown on c-axis oriented sapphire. The films were co-doped with nitrogen and tellurium. Depending on the relative concentrations of the dopants, the resistivity of the films was observed to increase by several orders of magnitude, significantly improving the photoconductive response. The enhancement of the photosensitivity reached a maximum at a tellurium concentration of around 1020cm−3.
The effect of ion drift on the sheath, presheath, and ion-current collection for cylinders in a collisionless plasma
Authors: McMahon J. C. , Xu G. Z. , Laframboise J. G.
Published: June 2005

A calculation is presented of the behavior of the sheath and presheath surrounding an infinite cylindrical conducting object, representing a spacecraft or electrostatic probe, which is moving transversely through a collisionless plasma, such as is encountered in the ionosphere. The calculation is done by solving the coupled Vlasov (collisionless Boltzmann) and Poisson equations in an iterative manner. The results show that for some ratios of probe radius to electron Debye length, the ion current collected by the probe in a drifting plasma can be less than that collected in a nondrifting plasma. These changes in the current-collection behavior can be linked to changes that occur in the sheath and presheath with plasma drift, including at large enough drift speeds the disappearance or “collapse” of the presheath.
Direct measurement of the energy thresholds to conformational isomerization. II. 3-indole-propionic acid and its water-containing complex
Authors: Clarkson Jasper R. , Baquero Esteban , Zwier Timothy S.
Published: June 2005

The methods of stimulated emission pumping-hole-filling spectroscopy (SEP-HFS) and population transfer spectroscopy (SEP-PTS) were used to place direct experimental bounds on the energetic barriers to conformational isomerization in 3-indole-propionic acid (IPA) and its water-containing complex. By contrast with tryptamine (Paper I), IPA has only two conformations with significant population in them. The structures of the two conformers are known from previous work [P. M. Felker, J. Phys. Chem. 96, 7844 (1992)]. The energy thresholds for A→B and B→A isomerizations are placed at 854 and 754cm−1, respectively. Lower bounds on the isomerization barrier in the two directions are determined from the last transition not observed in the SEP-PT spectra. These are placed at 800 and 644cm−1 for A→B and B→A, respectively. The combined results place bounds on the relative energies of the A and B minima, with E(B)−E(A)=46–210cm−1. Like the IPA monomer, the IPA-H2O complex forms two conformational isomers. Both these isomers incorporate the water molecule as a bridge between the carbonyl and OH groups of the carboxylic acid. Previous rotational coherence measurements (L. L. Connell, Ph.D. thesis, UCLA, 1991) have determined that these complexes retain the same IPA conformational structure as the monomers. SEP-PTS and SEP-HFS were carried out on the IPA-H2O complexes. It was demonstrated that it is possible to use SEP to drive conformational isomerization between the two conformational isomers of IPA-H2O. Bounds on the energy barriers to conformational isomerization are not effected greatly by the presence of the water molecule, with Ebarrier(A→B)=771–830cm−1 and Ebarrier(B→A)=583–750cm−1. This is a simple consequence of the fact that the barrier is an intramolecular barrier, and the water molecule is held fixed in the COOH pocket, where it interacts with the ring only peripherally during the isomerization process. Finally, changes in the SEP-PT spectral intensity in transitions near the top of the barrier to isomerization as a function of the position of SEP excitation relative to the pulsed valve exit provide some insight to the competition between vibrational relaxation and isomerization in a molecule the size of IPA.
Classical trajectory study of collisions of Ar with alkanethiolate self-assembled monolayers: Potential-energy surface effects on dynamics
Authors: Scott Day B. , Morris John R. , Troya Diego
Published: June 2005

We have investigated collisions between Ar and alkanethiolate self-assembled monolayers (SAMs) using classical trajectory calculations with several potential-energy surfaces. The legitimacy of the potential-energy surfaces is established through comparison with molecular-beam data and ab initio calculations. Potential-energy surfaces used in previous work overestimate the binding of Ar to the SAM, leading to larger energy transfer than found in the experiments. New calculations, based on empirical force fields that better reproduce ab initio calculations, exhibit improved agreement with the experiments. In particular, polar-angle-dependent average energies calculated with explicit-atom potential-energy surfaces are in excellent agreement with the experiments. Polar- and azimuthal-angle-dependent product translational energies are examined to gain deeper insight into the dynamics of Ar+SAM collisions.
Towards SiC surface functionalization: An ab initio study
Authors: Cicero Giancarlo , Catellani Alessandra
Published: June 2005

We present a microscopic model of the interaction and adsorption mechanism of simple organic molecules on SiC surfaces as obtained from ab initio molecular-dynamics simulations. Our results open the way to functionalization of silicon carbide, a leading candidate material for biocompatible devices.
Magnetically induced current densities in Al42− and Al44− species studied at the coupled-cluster level
Authors: Lin Ying-Chan , Jusélius Jonas , Sundholm Dage , Gauss Jürgen
Published: June 2005

Magnetically induced current densities in the four-membered rings of Al42− and Al44− species have been calculated at the coupled-cluster singles and doubles (CCSD) level by applying the recently developed gauge-including magnetically induced current (GIMIC) method. The strength of the ring-current susceptibilities were obtained by numerical integration of the current densities passing through a cross section perpendicular to the Al4 ring. The GIMIC calculations support the earlier notion that Al42− with formally two π electrons sustains a net diatropic ring current. The diatropic contribution to the ring-current susceptibility is carried by the electrons in both the σ(16.7nA∕T) and the π(11.3nA∕T) orbitals. The induced ring current in the Al44− compounds, with four π electrons, consists of about equally strong diatropic σ and paratropic π currents of about 14 and −17nA∕T, respectively. The net current susceptibilities obtained for Al4Li−, Al4Li2, Al4Li3−, and Al4Li4 at the CCSD level using a triple-zeta basis set augmented with polarization functions are 28.1, 28.1, −5.9, and −3.1nA∕T, respectively. The corresponding diatropic (paratropic) contributions to the ring-current susceptibilities are 32.4 (0.0), 36.7 (0.0), 18.9 (−19.9), and 18.6 (−16.8)nA∕T, respectively. For the Al42− and Al44− species, the net currents circling each Li+ cation is estimated to 4.3 and 2.4nA∕T, respectively.
Extension of renormalized coupled-cluster methods including triple excitations to excited electronic states of open-shell molecules
Authors: Włoch Marta , Gour Jeffrey R. , Kowalski Karol , Piecuch Piotr
Published: June 2005

The general-purpose open-shell implementation of the completely renormalized equation-of-motion coupled-cluster approach with singles, doubles, and noniterative triples [CR-EOMCCSD(T)] is reported. Benchmark calculations for the low-lying doublet and quartet states of the CH radical show that the CR-EOMCCSD(T) method is capable of providing a highly accurate description of ground and excited states of open-shell molecules. This includes states with strong double excitation character, for which the conventional EOMCCSD approach fails.
An optimized molecular potential for carbon dioxide
Authors: Zhang Zhigang , Duan Zhenhao
Published: June 2005

An optimized molecular potential model for carbon dioxide is presented in this paper. Utilizing the established techniques of molecular-dynamics and histogram reweighting grand canonical Monte Carlo simulations, this model is demonstrated to show excellent predictability for thermodynamic, transport, and liquid structural properties in a wide temperature-pressure range with remarkable accuracies. The average deviations of this new model from experimental data for the saturated liquid densities, vapor densities, vapor pressures, and heats of vaporization are around 0.1%, 2.3%, 0.7%, and 1.9%, respectively. The calculated critical point is almost pinpointed by the new model. The experimental radial distribution functions ranging from 240.0 to 473.0 K are well reproduced as compared to neutron-diffraction measurements. The predicted self-diffusion coefficients are in good agreement with the nuclear-magnetic-resonance measurements. The previously published potential models for CO2 are also systematically evaluated, and our proposed new model is found to be superior to the previous models in general.
Interaction between zigzag single-wall carbon nanotubes and polymers: A density-functional study
Authors: Simeoni Mirko , De Luca Cinzia , Picozzi Silvia , Santucci Sandro , Delley Bernard
Published: June 2005

Density-functional all-electrons calculations within local-density approximation show that the two isoelectronic polymers poly(para-phenylene) and poly(para-borazylene) weakly interact with zigzag single-walled carbon nanotubes. The analysis of the electronic properties of the joint systems, both with the polymer inside and outside the nanotubes, reveals a physisorption process with small changes in band structures and densities of states with respect to the constituents. We evaluate the potential barrier arising between polymers and nanotubes. Finally, we remark a generic selectivity of poly(para-phenylene) with respect to the electronic behavior of nanotubes, leading to a change in the density of states of metallic tubules.
Magnetite Schottky barriers on GaAs substrates
Authors: Watts Steven M. , Boothman Catherine , van Dijken Sebastiaan , Coey J. M. D.
Published: June 2005

Carrier transport across Fe3O4∕GaAs interfaces has been studied for n- and p-type GaAs(001) substrates with medium (7.7×1017cm−3) to high (3.5×1018cm−3) carrier concentrations. Current–voltage (I-V) measurements on medium-doped substrates show a rectifying behavior that is characteristic for thermionic emission/diffusion across a Schottky barrier. The n-type structure exhibits a low ideality factor of 1.3 and a Schottky barrier height of 0.58–0.63eV. The Schottky barrier height of the p-type sample is 0.51eV. For Fe3O4∕GaAs structures with higher doping levels the I–V dependence is nearly symmetric. In this case, tunneling of electrons and holes through the Schottky barrier dominates transport between the Fe3O4 layer and the GaAs substrate.
Development of accelerator-based γ-ray-induced positron annihilation spectroscopy technique
Authors: Selim F. A. , Wells D. P. , Harmon J. F. , Williams J.
Published: June 2005

Accelerator-based γ-ray-induced positron annihilation spectroscopy performs positron annihilation spectroscopy by utilizing MeV bremsstrahlung radiation generated from an accelerator (We have named the technique “accelerator-based γ-ray-induced PAS,” even though “bremsstrahlung” is more correct here than “γ rays”. The reason for that is to make the name of the technique more general, since PAS may be performed by utilizing MeV γ rays emitted from nuclei through the use of accelerators as described later in this article and as in the case of positron lifetime spectroscopy [F.A. Selim, D.P. Wells, and J.F. Harmon, Rev. Sci. Instrum. 76, 033905 (2005)].) instead of using positrons from radioactive sources or positron beams. MeV γ rays create positrons inside the materials by pair production. The induced positrons annihilate with the material electrons emitting a 511-keV annihilation radiation. Doppler broadening spectroscopy of the 511-keV radiation provides information about open-volume defects and plastic deformation in solids. The high penetration of MeV γ rays allows probing of defects at high depths in thick materials up to several centimeters, which is not possible with most of the current nondestructive techniques. In this article, a detailed description of the technique will be presented, including its benefits and limitations relative to the other nondestructive methods. Its application on the investigation of plastic deformation in thick steel alloys will be shown.
Charging and emission effects of multiwalled carbon nanotubes probed by electric force microscopy
Authors: Zdrojek M. , Mélin T. , Boyaval C. , Stiévenard D. , Jouault B. , Wozniak M. , Huczko A. , Gebicki W. , Adamowicz L.
Published: June 2005

Electrostatic properties of single-separated multiwalled carbon nanotubes (MWCNTs) deposited on a dielectric layer have been investigated by charge injection and electric force microscopy (EFM) experiments. We found that upon local injection from the biased EFM tip, charges delocalize over the whole nanotube length (i.e., 1–10μm), consistent with a capacitive charging of the MWCNT-substrate capacitance. In addition, the insulating layer supporting the nanotubes is shown to act as a charge-sensitive plate for electrons emitted from the MWCNTs at low electric fields, thus allowing the spatial mapping of MWCNT field-emission patterns.
Addition of water, methanol, and ammonia to Al3O3− clusters: Reaction products, transition states, and electron detachment energies
Authors: Guevara-García Alfredo , Martínez Ana , Ortiz J. V.
Published: June 2005

Products of reactions between the book and kite isomers of Al3O3− and three important molecules are studied with electronic structure calculations. Dissociative adsorption of H2O or CH3OH is highly exothermic and proton-transfer barriers between anion-molecule complexes and the products of these reactions are low. For NH3, the reaction energies are less exothermic and the corresponding barriers are higher. Depending on experimental conditions, Al3O3− (NH3) coordination complexes or products of dissociative adsorption may be prepared. Vertical electron detachment energies of stable anions are predicted with ab initio electron propagator calculations and are in close agreement with experiments on Al3O3− and its products with H2O and CH3OH. Changes in the localization properties of two Al-centered Dyson orbitals account for the differences between the photoelectron spectra of Al3O3− and those of the product anions.
Ion-beam-induced amorphization and order-disorder transition in the murataite structure
Authors: Lian Jie , Wang L. M. , Ewing Rodney C. , Yudintsev Sergey V. , Stefanovsky Sergey V.
Published: June 2005

Murataite (A3B6C2O22−x∕2,F4¯3m), a derivative of an anion-deficient fluorite structure, has been synthesized as different polytypes as a result of cation ordering. Ion-beam-induced amorphization has been investigated by 1-MeV Kr2+ ion irradiation with in situ transmission electron microscopy. The critical amorphization dose was determined as a function of temperature and the degree of structural disordering. A lower critical amorphization temperature (∼860K) was obtained for the disordered murataite as compared with that of the murataite superstructure (930to1060K). An ion-beam-induced ordered murataite to a disordered fluorite transition occurred in the murataite superstructure, similar to that observed in the closely related pyrochlore structure-type, A2B2O7. The ion-beam-induced defect fluorite structure is more energetically stable in the murataite structure with a higher degree of structural disordering, as compared with the murataite superstructure. This suggests that the degree of intrinsic structural disorder has a significant effect on the energetics of structural disordering process; this affects the tendency toward the order-disorder structural transition for fluorite-related compounds and their response to ion-beam-induced amorphization.
Stable single-atom contacts of zinc whiskers
Authors: Konrad P. , Bacca C. , Scheer E. , Brenner P. , Mayer-Gindner A. , Löhneysen H. v.
Published: June 2005

We report low-temperature measurements of the electrical transport properties of single-atom and tunnel contacts of Zn. We present a variant of the mechanically controllable break-junction technique that comprises a whisker as the central part. This technique combines the possibility to fabricate reproducibly single-atom contacts and well-characterized electronic and lattice properties of the electrodes. From tunneling current-voltage characteristics, we deduce a well-pronounced quasiparticle spectrum for whiskers, while for standard “notched-wire” and thin-film break junctions the spectrum is smeared out.
Temperature- and electric-field-dependent domain structures and phase transformations in (001)-cut tetragonal Pb(Mg1∕3Nb2∕3)1−xTixO3 (x=0.40) single crystal
Authors: Chien R. R. , Hugo Schmidt V. , Hung L.-W. , Tu Chi-Shun
Published: June 2005

Temperature- and electric (E)-field-dependent domain structures and phase transformations in a (001)-cut Pb(Mg1∕3Nb2∕3)0.6Ti0.4O3 (PMNT40%) single crystal have been investigated by polarizing microscopy and dielectric permittivity. The unpoled crystal has tetragonal phase domains (15–60μm wide) with polarizations along [100] and [010], which are separated by 90° domain walls. They coexist with a small fraction of the monoclinic phase domains near 200K. The tetragonal domains increase rapidly at the expense of the monoclinic domains as the temperature increases toward room temperature. This may account for the steplike dielectric anomaly in ε′ from around 280K. The whole crystal reaches the cubic phase near 464K. The E-field-dependent domain structures at room temperature show a significant change near 11kV∕cm when the polarizations rotate to the tetragonal [001] (T001) direction through the monoclinic phase. Microcrackings begin to develop near 12.5kV∕cm. The entire crystal becomes T001 monodomain near 33kV∕cm. After the E field is removed, the crystal does not reestablish the T100 and T010 macrodomains, but instead breaks up into tetragonal microdomains.
Supersonic propagation of ionization waves in an underdense, laser-produced plasma
Authors: Constantin C. , Back C. A. , Fournier K. B. , Gregori G. , Landen O. L. , Glenzer S. H. , Dewald E. L. , Miller M. C.
Published: June 2005

A laser-driven supersonic ionization wave propagating through a millimeter-scale plasma of subcritical density up to 2–3keV electron temperatures was observed. Propagation velocities initially ten times the sound speed were measured by means of time-resolved x-ray imaging diagnostics. The measured ionization wave trajectory is modeled analytically and by a two-dimensional radiation-hydrodynamics code. The comparison to the modeling suggests that nonlocal heat transport effects may contribute to the attenuation of the heat-wave propagation.
A numerical study of the supply mode effects on high-pressure mercury discharge lamp dynamic thermal behavior
Authors: Kaziz S. , Ahmed R. Ben , Araoud Z. , Gazzah M. H. , Charrada K. , Said R.
Published: June 2005

This paper presents a numerical study of the dynamic behavior of high-pressure mercury discharge lamp as a function of supply mode. Bidimensional time-dependent equations have been solved by using a semi-implicit finite-element code. The model has been developed in the frame of the local thermodynamic equilibrium hypothesis. After validation by using experimental data from the literature, this model was applied to analyze the effects of some key parameters such as frequency for an ac arc current and the ratio cycle parameter for square arc-current wave form on the convective process. The results obtained have been used to analyze the dynamic thermal behavior of high-pressure mercury plasma (pressure equal to 0.3 MPa) working under currents widely different in frequency and wave form.
Oxidation-enhanced diffusion of boron in very low-energy N2+-implanted silicon
Authors: Skarlatos D. , Tsamis C. , Perego M. , Fanciulli M.
Published: June 2005

In this article we study the interstitial injection during oxidation of very low-energy nitrogen-implanted silicon. Buried boron δ layers are used to monitor the interstitial supersaturation during the oxidation of nitrogen-implanted silicon. No difference in boron diffusivity enhancement was observed compared to dry oxidation of nonimplanted samples. This result is different from our experience from N2O oxynitridation study, during which a boron diffusivity enhancement of the order of 20% was observed, revealing the influence of interfacial nitrogen on interstitial kinetics. A possible explanation may be that implanted nitrogen acts as an excess interstitial sink in order to diffuse towards the surface via a non-Fickian mechanism. This work completes a wide study of oxidation of very low-energy nitrogen-implanted silicon related phenomena we performed within the last two years [D. Skarlatos, C. Tsamis, and D. Tsoukalas, J. Appl. Phys. 93, 1832 (2003); D. Skarlatos, E. Kapetanakis, P. Normand, C. Tsamis, M. Perego, S. Ferrari, M. Fanciulli, and D. Tsoukalas, J. Appl. Phys. 96, 300 (2004)].
Parameters of the plasma produced at the surface of a ferroelectric cathode by different driving pulses
Authors: Peleg O. , Chirko K. , Gurovich V. , Felsteiner J. , Krasik Ya. E. , Bernshtam V.
Published: June 2005

Spectroscopic investigations of the properties of a plasma produced by a ferroelectric-plasma source are presented. The electron plasma density, the electron and ion temperature, and the density of desorbed neutrals near the ferroelectric surface are determined from spectral line intensities and profiles. Three different methods of surface plasma formation are analyzed using a simplified model for the plasma production. The model predicts the total amount of charge in the plasma to be proportional to the dielectric constant of the ferroelectric material. Also, the model shows a strong dependence of the plasma parameters on the resistivity of the plasma transition layer. A maximal plasma density of ∼1015cm−3 is achieved when the electrons that were attached by the driving field to the ferroelectric surface are released from the surface owing to driving pulse sharp decay and ionized heavy atoms desorbed from the ferroelectric.
Stress development kinetics in plasma-enhanced chemical-vapor-deposited silicon nitride films
Authors: Hughey Michael P. , Cook Robert F.
Published: June 2005

The kinetics of post-deposition irreversible tensile stress development on annealing in plasma-enhanced chemical-vapor-deposited silicon nitride films are described. Films were deposited at 150, 250, and 300 °C, and in situ stress measurements were performed during thermal cycling and annealing at various temperatures up to 500 °C and for annealing times up to 36 h. A constitutive relationship between stress development and hydrogen concentration change based on a modified first-order rate law that assumes a spread in activation energies for hydrogen reaction and exsolution is proposed and found to adequately describe stress development kinetics. The long-standing belief that stress development is generated by the evolution of bonded hydrogen is validated on comparison of film stress and hydrogen concentration, as measured by forward recoil spectrometry and infrared spectroscopy. Stress development is found to be proportional to the reduction in hydrogen concentration throughout annealing with a proportionality constant that depends on deposition temperature.
Effect of a cap layer on morphological stability of a strained epitaxial film
Authors: Liu Hai , Huang Rui
Published: June 2005

A strained epitaxial film often undergoes surface roughening during growth and subsequent processes. One possible means to reduce roughening so as to produce an epitaxial film with a flat surface is to deposit an oxide cap layer on the film to suppress the kinetic process of roughening. This paper analyzes the effect of a cap layer on the stability of an epitaxial film and the kinectics of roughening, assuming the interface diffusion between the film and the cap layer as the dominant mechanism of mass transport. A variational principle is formulated, which leads to a nonlinear evolution equation coupled with a boundary-value problem of elasticity. A linear perturbation analysis is then performed, from which the critical wavelength and the fastest growing mode of roughening are obtained. It is found that both the thickness and the residual stress of the cap layer play important roles in controlling the morphological stability and the roughening kinetics.