bluuebunny/crossref_metadata_2025 huggingface dataset:
Opinion: Projecting the effects of environmental change on Antarctic seals
Authors: Siniff Donald B. , Garrott Robert A. , Rotella Jay J. , Fraser William R. , Ainley David G.
Published: May 2008

We consider how Antarctic seals may respond to changes in climate, realizing that anthropogenic alteration of food webs will influence these responses. The species considered include the ice-obligate - crabeater (Lobodon carcinophaga), Weddell (Leptonychotes weddellii), Ross (Ommataphoca rossii) and leopard (Hydrurga leptonyx) seal - and the ice-tolerant Antarctic fur seal (Arctocephalus gazella) and southern elephant seal (Mirounga leonina). The data analysed are from long-term censuses of Weddell seals in McMurdo Sound (1997–2006), and of Weddell, fur and elephant seals at Arthur Harbour, Antarctic Peninsula (1974–2005). After considering their responses to recent changes in environmental features, as well as projected and current changes to their habitat our conclusions are that the distribution and abundance of 1) crabeater and Weddell seals will be negatively affected by changes in the extent, persistence and type of annual sea ice, 2) Ross and leopard seal will be the least negatively influenced by changes in pack ice characteristics, although, as may be the case for crabeater and Weddell, population size and distribution may be altered through changes in food web dynamics, and 3) southern elephant and fur seals will respond in ways opposite to the pack ice species, but could also be influenced most immediately by changes in their food resources due to factors other than climate.
Global distribution and interannual variations of mesospheric and lower thermospheric neutral wind diurnal tide: 1. Migrating tide
Authors: Wu Q. , Ortland D. A. , Killeen T. L. , Roble R. G. , Hagan M. E. , Liu H.‐L. , Solomon S. C. , Xu Jiyao , Skinner W. R. , Niciejewski R. J.
Published: May 2008

Using the TIMED Doppler interferometer (TIDI) mesospheric and lower thermospheric neutral‐wind multiyear data set (2002–2007) and NCAR TIME General Circulation Models (GCM) 1.2 annual run results (2002–2005) at the TIDI sampling points, we study the migrating diurnal tide's global distribution, interannual, and seasonal variations in connection with the mean zonal wind interannual variations. A strong quasi‐biennial oscillation (QBO) effect on the diurnal tide was observed in the TIDI data and reproduced to a lesser degree in the TIME‐GCM run. The migrating diurnal tide amplitude is larger during the eastward phase of the stratospheric QBO and weaker during the westward phase. Westward mesospheric equatorial mean zonal winds appeared during the eastward phase of the stratospheric QBO (in 2002, 2004, and 2006). The strongest QBO effect on both the migrating diurnal tide and mean zonal winds was observed during the March equinox. The stronger tides may be related to the weaker gravity wave filtering in the stratosphere during the eastward phase stratospheric QBO. The TIDI data also exhibit large interhemispheric asymmetry. The westward mean zonal winds in the mesosphere appeared to be associated with the enhanced diurnal tide. The TIME‐GCM 1.2 diurnal tide amplitudes are in general smaller than those observed by the TIDI instrument. Limited vertical spatial resolution for the TIME‐CGM 1.2 is suggested as the cause. Future improvements are expected with a higher spatial resolution in the model.
Prompt penetration electric fields (PPEFs) and their ionospheric effects during the great magnetic storm of 30–31 October 2003
Authors: Tsurutani B. T. , Verkhoglyadova O. P. , Mannucci A. J. , Saito A. , Araki T. , Yumoto K. , Tsuda T. , Abdu M. A. , Sobral J. H. A. , Gonzalez W. D. , McCreadie H. , Lakhina G. S. , Vasyliūnas V. M.
Published: May 2008

We explore the ionospheric effects of prompt penetration electric fields (PPEFs) for a variety of interplanetary magnetic field directions. We use the great magnetic storm of 30–31 October as an example of PPEF effects. For intense southward interplanetary magnetic fields (IMFs), inward plasma sheet convection occurs with the result of magnetospheric ring current formation and an intense magnetic storm. Concurrent with the above, positive phase ionospheric storms occur in the dayside, and negative phase ionospheric storms occur on the nightside, the topics of this paper. The dayside ionospheric storms due to PPEFs are characterized by transport of near‐equatorial plasma to higher altitudes and latitudes, forming a giant plasma fountain. These features are part of what is called the dayside ionospheric superfountain (DIS). For these southward IMFs, dusk and dawn plasma are predicted to be transported toward the dayside. For northward IMFs, negative phase ionospheric storms are expected on the dayside if the PPEFs indeed reach that region of space. IMF By components are expected to have weak or neglible ionospheric effects. On the basis of PPEF arguments, intervals of IMF By should not be related to geomagnetic storms (they are not). IMF By intervals should, however, cause a shearing of the magnetotail, a feature that has been previously reported in the literature.
An event study to provide validation of TING and CMIT geomagnetic middle‐latitude electron densities at the F2 peak
Authors: Burns A. G. , Wang W. , Wiltberger M. , Solomon S. C. , Spence H. , Killeen T. L. , Lopez R. E. , Landivar J. E.
Published: May 2008

The coupled thermosphere‐ionosphere magnetosphere (CMIT) model and the Thermosphere Ionosphere Nested Grid (TING) model have been run to simulate the 15 May 1997 interplanetary coronal mass ejection's (ICME) effects on the Earth's ionosphere and thermosphere. Comparisons were made between these model runs, the IRI‐2007 model, and geomagnetic middle‐latitude ionosonde data (NmF2) from the World Data Center to determine how well the models simulated the event and to understand the causes of model‐data disagreement. The following conclusions were drawn from this study: (1) skill scores were more often negative than positive on average; (2) the best and the worst skill scores occurred on the recovery day; (3) the line plots comparing models to data look better than the skill scores might suggest; (4) skill scores are significantly affected by timing issues and large, short‐duration variability; (5) skill scores give an indication of the relative ability of one model relative to another, rather than an absolute statement of model accuracy; (6) the models capture negative storm effects better than they capture positive storm effects; (7) the TING model captured many short duration features seen in the data at high middle latitude stations that result from changes in the size of the auroral oval; (8) CMIT overestimates the energy driving changes in NmF2, whereas TING provides approximately the correct energy input as a result of the saturation effects on potential that are included in TING; and (9) both TING and CMIT electron densities decreased too rapidly after sunset.
Insights on the March 1998 eruption at Piton de la Fournaise volcano (La Réunion) from microgravity monitoring
Authors: Bonvalot Sylvain , Remy Dominique , Deplus Christine , Diament Michel , Gabalda Germinal
Published: May 2008

We investigate the temporal gravity changes associated with one of the major recent eruptions at Piton de la Fournaise volcano (March 1998) that occurred after an unusual five‐year quiet period and initiated a new eruptive cycle. Repeated microgravity surveys allowed us to measure residual gravity changes up to 100 μGal within the Enclos Fouqué caldera for four months before the start of the eruption. We first analyzed the temporal gravity changes and the height changes, also measured at the gravity benchmarks, on the basis of an intrusive dyke model previously proposed for this eruption from InSAR and tilt data. This analysis reveals that such simple model (finite rectangular shaped tensile dislocation) cannot fit both gravity and geodetic data and leads us to propose a dual source gravity model for this eruption. The gravity data inversion using a genetic algorithm search method allows us to quantify the mass change produced by the intrusive dyke (3.9 to 8.7 × 109kg) and to suggest an additional mass increase at sea level depth (4.6 to 7.2 × 1010kg). We interpret the latter as the effect of magma ascent into a reservoir that occurred within the four months before the eruption. We suggest that high concentrations of gas content in the magma reservoir account for a higher compressibility of the intruded magma and produce significant mass change with moderate surface deformation. The 1996 seismic crisis, interpreted as an upward migration of magmatic fluids into a reservoir located at sea level, might support such a hypothesis.
Toward a warmer Arctic Ocean: Spreading of the early 21st century Atlantic Water warm anomaly along the Eurasian Basin margins
Authors: Dmitrenko Igor A. , Polyakov Igor V. , Kirillov Sergey A. , Timokhov Leonid A. , Frolov Ivan E. , Sokolov Vladimir T. , Simmons Harper L. , Ivanov Vladimir V. , Walsh David
Published: May 2008

We document through the analysis of 2002–2005 observational data the recent Atlantic Water (AW) warming along the Siberian continental margin due to several AW warm impulses that penetrated into the Arctic Ocean through Fram Strait in 1999–2000. The AW temperature record from our long‐term monitoring site in the northern Laptev Sea shows several events of rapid AW temperature increase totaling 0.8°C in February–August 2004. We hypothesize the along‐margin spreading of this warmer anomaly has disrupted the downstream thermal equilibrium of the late 1990s to earlier 2000s. The anomaly mean velocity of 2.4–2.5 ± 0.2 cm/s was obtained on the basis of travel time required between the northern Laptev Sea and two anomaly fronts delineated over the Eurasian flank of the Lomonosov Ridge by comparing the 2005 snapshot along‐margin data with the AW pre‐1990 mean. The magnitude of delineated anomalies exceeds the level of pre‐1990 mean along‐margin cooling and rises above the level of noise attributed to shifting of the AW jet across the basin margins. The anomaly mean velocity estimation is confirmed by comparing mooring‐derived AW temperature time series from 2002 to 2005 with the downstream along‐margin AW temperature distribution from 2005. Our mooring current meter data corroborate these estimations.
Soil sedimentology at Gusev Crater from Columbia Memorial Station to Winter Haven
Authors: Cabrol Nathalie A. , Herkenhoff Kenneth E. , Greeley Ronald , Grin Edmond A. , Schröder Christian , d'Uston Claude , Weitz Catherine , Yingst R. Aileen , Cohen Barbara A. , Moore Jeffrey , Knudson Amy , Franklin Brenda , Anderson Robert C. , Li Ron
Published: May 2008

A total of 3140 individual particles were examined in 31 soils along Spirit's traverse. Their size, shape, and texture were quantified and classified. They represent a unique record of 3 years of sedimentologic exploration from landing to sol 1085 covering the Plains Unit to Winter Haven where Spirit spent the Martian winter of 2006. Samples in the Plains Unit and Columbia Hills appear as reflecting contrasting textural domains. One is heterogeneous, with a continuum of angular‐to‐round particles of fine sand to pebble sizes that are generally dust covered and locally cemented in place. The second shows the effect of a dominant and ongoing dynamic aeolian process that redistributes a uniform population of medium‐size sand. The texture of particles observed in the samples at Gusev Crater results from volcanic, aeolian, impact, and water‐related processes.
Diffraction corrections to GO backscattering from a perfectly conducting 3‐D rough surface
Authors: Fuks Iosif
Published: May 2008

High‐frequency radio wave diffraction by a perfectly conducting arbitrarily shaped smooth surface is considered analytically. The results of solving the integral equation for surface currents at a gentle surface, when multiple scattering and surface shadowing can be neglected, are presented. The diffraction corrections to GO values of the surface currents were obtained (up to the second order of the small parameter ∼1/k, where k is the wave number of the incident EM wave) depending on the local angle of incidence, surface curvatures, and their space derivatives. It is shown that diffraction caused by the local surface curvature results not only in small corrections to the GO currents, but also gives rise to the surface current components orthogonal to those induced by the incident wave in the local GO limit. These general results are applied to the specific problem of backscattering from a Gaussian statistically rough surface at normal incidence. The diffraction corrections to the GO statistically averaged backscattering cross sections, HH/VV polarization ratio (which is equal to unity in the GO limit) and cross‐polarization coefficient are obtained as functions of the 2nd (surface slope variances) and 4th (surface curvature variances) moments of the surface roughness power spectra. It is shown that diffraction results in the appearance of the cross‐polarized component in the backscattered field, which is equal to zero in the GO limit.
Event to multidecadal persistence in rainfall and runoff in southeast Arizona
Authors: Goodrich D. C. , Unkrich C. L. , Keefer T. O. , Nichols M. H. , Stone J. J. , Levick L. R. , Scott R. L.
Published: May 2008

Spatial and temporal rainfall variability over watersheds directly impacts the hydrologic response over virtually all watershed scales. Changes in the precipitation regime over decades due to some combination of inherent local variability and climate change may contribute to changes in vegetation, water supply, and, over longer timescales, landscape evolution. Daily, seasonal, and annual precipitation volumes and intensities from the dense network of rain gauges on the Agricultural Research Service, U. S. Department of Agriculture Walnut Gulch Experimental Watershed (WGEW) in southeast Arizona are evaluated for multidecadal trends in amount and intensity over a range of watershed scales (1.5 ha to 149 km2) using observations from 1956 to 2006. Rainfall and runoff volume and rate variability are compared over the same spatial scales over a 40 year period (1966–2006). The major findings of this study are that spatial variability of cumulative precipitation decreases exponentially with time, and, on average, became spatially uniform after 20 years of precipitation accumulation. The spatial variability of high‐intensity, runoff‐producing precipitation also decreased exponentially, but the variability was still well above the measurement error after 51 years. There were no significant temporal trends in basin scale precipitation. A long‐term decrease in runoff from 1966 to 1998 from ephemeral tributaries like the WGEW may be a critical factor in decreasing summer flows in the larger San Pedro due to changes in higher‐intensity, runoff‐producing rainfall.
Observation of non‐developing and developing tropical disturbances over the South China Sea using SSM/I satellite
Authors: Wang Lei , Lau Kai‐Hon , Zhang Qing‐Hong , Fung Chi‐Hung
Published: May 2008

The columnar water vapor (CWV), columnar liquid water (CLW) and the total latent heat release (TLHR) derived from the Special Sensor Microwave/Imager (SSM/I) are examined for 13 developing tropical disturbances and 30 non‐developing tropical disturbances over the South China Sea (SCS) in 2000 and 2001. The mean TLHR within 500 km of the center of tropical disturbances in the day immediately prior to tropical cyclogenesis for 13 developing tropical disturbances is 4.671 × 1014 W, while the mean maximum daily TLHR within 500 km of the center of tropical disturbances during the lifetime of 30 non‐developing tropical disturbances is 2.760 × 1014 W. On average, the mean TLHR of developing tropical disturbances one‐day prior to cyclogenesis is 1.7 times the mean maximum daily TLHR of non‐developing tropical disturbances. Direct satellite observations from SSM/I reveal that persistent large amount of latent heat release is necessary for the occurrence of tropical cyclogenesis. These results suggest that observations from SSM/I are helpful in determining the potential of development for tropical disturbances.
Assessing the distribution and growth rates of NOx emission sources by inverting a 10‐year record of NO2 satellite columns
Authors: Stavrakou T. , Müller J.‐F. , Boersma K. F. , De Smedt I. , van der A R. J.
Published: May 2008

Tropospheric NO2 columns retrieved from the GOME and SCIAMACHY satellite instruments between January 1997 and December 2006 are used together with the IMAGES CTM and its adjoint to construct a top‐down inventory for NOx emissions, with a focus on anthropogenic sources. The influence of the emission updates on the chemical lifetime of NOx is taken into account, and found to have a significant impact on the results. Anthropogenic emission trends are inferred over industrialized regions of the Northern Hemisphere. The largest emission increases are found over eastern China, and in particular in the Beijing area (growth rate of 9.6%/yr), whereas important emission decreases are calculated over the United States (−4.3%/yr in the Ohio River Valley), and to a lesser extent over Europe (−1.4%/yr in Germany, −1.0%/yr in the Po Basin). The emission changes result in significant trends in surface ozone, amounting to more than 15%/decade over large parts of China in summertime.
The La‐related protein LARP7 is a component of the 7SK ribonucleoprotein and affects transcription of cellular and viral polymerase II genes
Authors: Markert Andreas , Grimm Michael , Martinez Javier , Wiesner Julia , Meyerhans Andreas , Meyuhas Oded , Sickmann Albert , Fischer Utz
Published: May 2008

The positive transcription elongation factor b (P‐TEFb) is a heterodimeric complex composed of cyclin‐dependent kinase 9 and its regulator cyclin T1/2. It stimulates transcription elongation by phosphorylation of serine 2 residues in the carboxy‐terminal domain of polymerase II. 7SK RNA and HEXIM proteins can antagonize transcriptional stimulation by sequestering P‐TEFb in a catalytically inactive ribonucleoprotein (RNP). Here, we show that the previously uncharacterized La‐related protein 7 (LARP7) has a role in 7SK‐mediated regulation of transcription. LARP7 binds to the highly conserved 3′‐terminal U‐rich stretch of 7SK RNA and is an integral part of the 7SK RNP. On stimulation, LARP7 remains associated with 7SK RNA, whereas P‐TEFb is released. Interestingly, reduction of LARP7 by RNA interference enhances transcription from cellular polymerase II promoters, as well as a TAT‐dependent HIV‐1 promoter. Thus, LARP7 is a negative transcriptional regulator of polymerase II genes, acting by means of the 7SK RNP system.
Separation of long-range and short-range interactions in Rydberg states of diatomic molecules
Authors: Kay Jeffrey J. , Coy Stephen L. , Petrović Vladimir S. , Wong Bryan M. , Field Robert W.
Published: May 2008

Observation and analysis of the f(ℓ=3), g(ℓ=4), and h(ℓ=5) Rydberg series of CaF in the range 13⩽n⩽17 is presented. Simultaneous analysis of the f, g, and h Rydberg series of CaF, combined with significant improvements to the long-range model for nonpenetrating Rydberg states, provides a generally applicable scheme for separating the effects of the long-range (electric multipole and polarization) and short-range (core penetration) interactions between the Rydberg electron and ion core in diatomic molecules. Techniques for rigorous assignment of nonpenetrating states, as well as extensions of the long-range model, are discussed. Explicit formulas for the first- and second-order matrix elements of the first four anisotropic electric multipole interactions (electric dipole, quadrupole, octupole, and hexadecapole) are given. The discrepancies between the observed behavior and that predicted by the long-range model, which are particularly significant for the f series, are shown to be due to penetration of the Rydberg electron wavefunction within the ion core. We show that these penetration effects can be understood within the framework of ligand field theory and conclude with a discussion of the relative contributions of the long- and short-range interactions to the quantum defects of the core-penetrating “s,” “p,” and “d” series of CaF.
Collective modes in superfluid helium when there is a relative velocity between the normal and superfluid components
Authors: Adamenko I. N. , Nemchenko K. E. , Slipko V. A. , Wyatt A. F. G.
Published: May 2008

Collective modes are studied in superfluid helium when the normal and superfluid components have a relative velocity w. In this paper the general dispersion relation for first and second sound modes is obtained for arbitrary values of w, and we have found the relationship between the amplitudes of the oscillating variables for first sound. It is shown that in a first sound wave, both temperature and pressure can oscillate, and moreover, the normal fluid velocity can exceed the superfluid velocity in the wave. In the general case of first sound, the normal fluid not only has a velocity component parallel to the wave vector, but also a transverse velocity component. It is shown that when there is only a phonon system in the helium, the amplitude of the temperature oscillation in a first sound wave in an anisotropic phonon system can exceed that in a second sound wave in an isotropic phonon system, for similar values of the normal fluid density.
Neutron reflection from a liquid helium surface
Authors: Charlton T. R. , Dalgliesh R. M. , Kirichek O. , Langridge S. , Ganshin A. , McClintock P. V. E.
Published: May 2008

The reflection of neutrons from a helium surface is observed for the first time. The He4 surface is smoother in the superfluid state at 1.54K than in the case of the normal liquid at 2.3K. In the superfluid state we also observe a surface layer ∼200Å thick which has a subtly different neutron scattering cross section, which may be explained by an enhanced Bose-Einstein condensate fraction close to the helium surface. The application of neutron reflectometry described in this paper creates new and exciting opportunities for the surface and interfacial study of quantum fluids.
Bose–Einstein condensation in a decorated lattice: an application to the problem of supersolid He
Authors: Fil D. V. , Shevchenko S. I.
Published: May 2008

The Bose–Einstein condensation of vacancies in a three-dimensional decorated lattice is considered. The model describes possible scenario of superfluidity of solid helium, caused by the presence of zero-point vacancies in a dislocation network. It is shown that the temperature of Bose–Einstein condensation decreases under increase of the length of the network segments, and the law of decrease depends essentially on the properties of the vertices of the network. If the vertices correspond to barriers with a small transparency, the critical temperature varies inversely as the square of the length of the segment. On the contrary, if the vertices correspond to traps for the vacancies (it is energetically preferable for the vacancies to be localized at the vertices), an exponential lowering of the temperature of transition takes place. The highest temperature of Bose–Einstein condensation is reached in the intermediate case of vertices with large transparency, but in the absence of tendency of localization at them. In the latter case the critical temperature is inversely as the length of the segment.
Electronic contributions to infrared spectra of adsorbate molecules on metal surfaces: Ethene on Cu(1 1 1)
Authors: Skibbe O. , Binder M. , Otto A. , Pucci A.
Published: May 2008

Already at coverages well below 1 ML copper ad-atoms on Cu(1 1 1) significantly modify infrared reflection-absorption spectra of ethene (C2H4) on Cu(1 1 1). Raman modes of the centrosymmetric molecule appear in the spectra. Their lines do not involve significant shifts of vibration frequencies as they would be expected for strong distortions of the molecule. High-resolution electron-energy-loss spectra of adsorbed ethene for various Cu-adatom precoverage show the same vibration lines. Since the molecules are obviously unchanged, the Raman lines in the infrared spectra must have obtained dynamic dipole moment from transient electron transfer favored by atomic disorder on the metal surface.
Two-dimensional propagation of magnetocardiac T wave signals for characterizing myocardial ischemia
Authors: Wu C. C. , Huang H. C. , Liu Y. B. , Lin L. C. , Lin L. Y. , Chen M. F. , Tsai M. C. , Gao Y. L. , Yang S. Y. , Horng H. E. , Yang H. C. , Tseng W. K. , Lee T. L. , Hsuan C. F. , Pan Y. F. , Lee Y. H.
Published: May 2008

Instead of detecting electrical signals for diagnosing cardiac abnormalities, a promising alternative is to detect the magnetic signals generated from cardiac electrical currents. The system utilizing 64 low-transition-temperature superconducting quantum interference devices was tested to detect the time-dependent magnetocardiac signals that are spatially distributed over the heart. To achieve efficient acquisition and analysis, we propose a method to detect two-dimensionally, the T wave propagation of electromagnetic signals of beating hearts. In addition to characterizing the propagating behaviors, the differences between normal hearts and those with coronary artery disease were investigated.
Coarse-grained lattice kinetic Monte Carlo simulation of systems of strongly interacting particles
Authors: Dai Jianguo , Seider Warren D. , Sinno Talid
Published: May 2008

A general approach is presented for spatially coarse-graining lattice kinetic Monte Carlo (LKMC) simulations of systems containing strongly interacting particles. While previous work has relied on approximations that are valid in the limit of weak interactions, here we show that it is possible to compute coarse-grained transition rates for strongly interacting systems without a large computational burden. A two-dimensional square lattice is employed on which a collection of (supersaturated) strongly interacting particles is allowed to reversibly evolve into clusters. A detailed analysis is presented of the various approximations applied in LKMC coarse graining, and a number of numerical closure rules are contrasted and compared. In each case, the overall cluster size distribution and individual cluster structures are used to assess the accuracy of the coarse-graining approach. The resulting closure approach is shown to provide an excellent coarse-grained representation of the systems considered in this study.
Electro-optical effect in films of azobenzene polycomplexes with cobalt
Authors: Davidenko N. A. , Davidenko I. I. , Savchenko I. A. , Popenaka A. N. , Baath L. B.
Published: May 2008

Polymeric films containing azobenzene fragments with donor and acceptor groups were produced as well as films of polycomplexes of these fragments with cobalt. Absorption spectra of these films before and during application of an external electric field were investigated. Electro-optical properties of the films are conditioned by spatial reorientation of dipole moments of azobenzene groups induced by polarized light in an external electric field. Increase in dipole moments of azobenzene groups reduces the influence of cobalt ions on the electro-optical properties of the polycomplex films. In the proposed phenomenological model, the electro-optical effect is explained by effect of induced forces upon azobenzene isomers and metallic ions.
Micellar crystals in solution from molecular dynamics simulations
Authors: Anderson J. A. , Lorenz C. D. , Travesset A.
Published: May 2008

Polymers with both soluble and insoluble blocks typically self-assemble into micelles, which are aggregates of a finite number of polymers where the soluble blocks shield the insoluble ones from contact with the solvent. Upon increasing concentration, these micelles often form gels that exhibit crystalline order in many systems. In this paper, we present a study of both the dynamics and the equilibrium properties of micellar crystals of triblock polymers using molecular dynamics simulations. Our results show that equilibration of single micelle degrees of freedom and crystal formation occur by polymer transfer between micelles, a process that is described by transition state theory. Near the disordered (or melting) transition, bcc lattices are favored for all triblocks studied. Lattices with fcc ordering are also found but only at lower kinetic temperatures and for triblocks with short hydrophilic blocks. Our results lead to a number of theoretical considerations and suggest a range of implications to experimental systems with a particular emphasis on Pluronic polymers.
A spline for your saddle
Authors: Granot Rebecca , Baer Roi
Published: May 2008

Pinpointing extrema on a multidimensional hypersurface is an important generic problem with a broad scope of application in statistical mechanics, biophysics, chemical reaction dynamics, and quantum chemistry. Local minima of the hypersurface correspond to metastable structures and are usually the most important points to look for. They are relatively easy to find using standard minimizing algorithms. A considerably more difficult task is the location of saddle points. The saddle points most sought for are those which form the lowest barriers between given minima and are usually required for determining rates of rare events. We formulate a path functional minimum principle for the saddle point. We then develop a cubic spline method for applying this principle and locating the saddle point(s) separating two local minima on a potential hypersurface. A quasi-Newton algorithm is used for minimization. The algorithm does not involve second derivatives of the hypersurface and the number of potential gradients evaluated is usually less than 10% of the number of potential evaluations. We demonstrate the performance of the method on several standard examples and on a concerted exchange mechanism for self-diffusion in diamond. Finally, we show that the method may be used for solving large constrained minimization problems which are relevant for self-consistent field iterations in large systems.
Numerical simulation of carrier collection and recombination at grain boundaries in Cu(In,Ga)Se2 solar cells
Authors: Taretto K. , Rau U.
Published: May 2008

Two-dimensional numerical device simulations investigate the influence of grain boundaries (GBs) on the performance of Cu(In,Ga)Se2 solar cells. We find that the electronic activity of grain boundaries can reduce the efficiency of Cu(In,Ga)Se2 solar cells from 20% to below 12% making proper passivation of GBs a primary requirement for high efficiency. Cell efficiencies larger than 19% require GB defect densities below 1011 cm−2. Also, an internal band offset in the valence band due to a Cu-poor region adjacent to the GBs could effectively passivate grain boundaries that are otherwise very recombination active. It is shown that such a barrier must be more than 300 meV high and at least 3 nm wide to virtually suppress all grain boundary recombination. Contrariwise, such a barrier represents an obstacle for hole transport reducing carrier collection across grain boundaries that are not perpendicular to the cell surface. We further find that inverted grain boundaries lead to an accumulation of the short circuit current along the grain boundary, which in certain situations enhances the total short circuit current. However, we do not find any beneficial effect of any type of grain boundaries on the overall cell efficiency.
The NO+O3 reaction: A triple oxygen isotope perspective on the reaction dynamics and atmospheric implications for the transfer of the ozone isotope anomaly
Authors: Savarino J. , Bhattacharya S. K. , Morin S. , Baroni M. , Doussin J.-F.
Published: May 2008

Atmospheric nitrate shows a large oxygen isotope anomaly (ΔO17), characterized by an excess enrichment of O17 over O18, similar to the ozone molecule. Modeling and observations assign this specific isotopic composition mainly to the photochemical steady state that exists in the atmosphere between ozone and nitrate precursors, namely, the nitrogen oxides (NOx=NO+NO2). However, this transfer is poorly quantified and is built on unverified assumptions about which oxygen atoms of ozone are transferred to NOx, greatly weakening any interpretation of the nitrate oxygen isotopic composition in terms of chemical reaction pathways and the oxidation state of the atmosphere. With the aim to improve our understanding and quantify how nitrate inherits this unusual isotopic composition, we have carried out a triple isotope study of the reaction NO+O3. Using ozone intramolecular isotope distributions available in the literature, we have found that the central atom of the ozone is abstracted by NO with a probability of (8±5)%(±2σ) at room temperature. This result is at least qualitatively supported by dynamical reaction experiments, the non-Arrhenius behavior of the kinetic rate of this reaction, and the kinetic isotope fractionation factor. Finally, we have established the transfer function of the isotope anomaly of O3 to NO2, which is described by the linear relationship ΔO17(NO2)=A×ΔO17(O3)+B, with A=1.18±0.07(±1σ) and B=(6.6±1.5)‰(±1σ). Such a relationship can be easily incorporated into models dealing with the propagation of the ozone isotope anomaly among oxygen-bearing species in the atmosphere and should help to better interpret the oxygen isotope anomaly of atmospheric nitrate in terms of its formation reaction pathways.
Characterization of an atmospheric double arc argon-nitrogen plasma source
Authors: Tu X. , Chéron B. G. , Yan J. H. , Yu L. , Cen K. F.
Published: May 2008

In the framework of studies devoted to hazardous waste destruction, an original dc double anode plasma torch has been designed and tested, which produces an elongated, weak fluctuation and reproducible plasma jet at atmospheric pressure. The arc instabilities and dynamic behavior of the double arc argon-nitrogen plasma jet are investigated through the oscillations of electrical signals by combined means of fast Fourier transform and Wigner distribution. In our experiment, the restrike mode is identified as the typical fluctuation behavior in an argon-nitrogen plasma jet. The Fourier spectra and Wigner distributions exhibit two characteristic frequencies of 150 Hz and 4.1 kHz, which reveals that the nature of fluctuations in the double arc argon-nitrogen plasma can be ascribed to the undulation of the power supply and both arc roots motion on the anode channels. In addition, the microscopic properties of the plasma jet inside and outside the arc chamber are investigated by means of optical emission spectroscopy, which yields excitation, electronic, rotational, and vibrational temperatures, as well as the electron number density. The results allow us to examine the validity criteria of a local thermodynamic equilibrium (LTE) state in the plasma arc. The measured electron densities are in good agreement with those calculated from the LTE model, which indicates that the atmospheric double arc argon-nitrogen plasma in the core region is close to the LTE state under our experimental conditions.
Bilayer processing for an enhanced organic-electrode contact in ultrathin bottom contact organic transistors
Authors: Park Jeongwon , Yang Richard D. , Colesniuc Corneliu N. , Sharoni Amos , Jin Sungho , Schuller Ivan K. , Trogler William C. , Kummel Andrew C.
Published: May 2008

A bilayer lift-off process has been employed to fabricate optimal electrode contact geometry for statistical characterization of ultrathin organic thin-film transistors (OTFTs). For over 100 p-channel ultrathin (12 ML) copper phthalocyanine (CuPc) OTFTs, the bilayer photoresist lift-off process increased the field effect mobility by two orders of magnitude, decreased the contact resistance by three orders of magnitude, increased the on/off ratio by one order of magnitude, and the threshold voltage was decreased by a factor of three compared to conventionally processed devices. The generality of the method was validated by fabricating OTFTs in four different phthalocynaines and CuPc OTFTs with eight different channel thicknesses.
Thermodynamically consistent integral equation for soft repulsive spheres
Authors: Carbajal-Tinoco Mauricio D.
Published: May 2008

We present a new closure relation that is an extension of the thermodynamically consistent approximation of Rogers and Young [Phys. Rev. A 30, 999 (1984)]. In the proposed closure, we introduce a nonlinear term along with an additional thermodynamic consistency equation. We compare some thermodynamic and structural properties obtained within our scheme with the corresponding results emerging from other approaches that include numerical simulations and other closure approximations. For the systems considered here, the results derived from our scheme are in better agreement with the reference values of the simulations.
Two scale generalized model of polypeptide chains
Authors: Badasyan A. V. , Tonoyan Sh. A. , Tsarukyan A. V. , Mamasakhlisov Y. Sh. , Benight A. S. , Morozov V. F.
Published: May 2008

The generalized model of polypeptide chains (GMPC) is expanded to simultaneously consider two types of interactions occurring over different scales. This new two scale GMPC is applied in several specific cases to examine: The combined influence of stacking or antistacking and hydrogen bonding, or spatial restrictions on the length of helical segments, on the cooperativity and temperature interval of the helix-coil transition of duplex DNA. For the cases of stacking or antistacking in combination with hydrogen bonding the model reduces to the basic uniscale model with a redefined scaling parameter Δ̃. Antistacking increases the cooperativity, while stacking decreases it. In each case, explanations are given in terms of different lengths of helical segments. Restrictions on the length of helical regions result in the appearance of antiferromagnetic-type correlations where there is no apparent link between cooperativity and transition interval.
Effect of molecular beam epitaxy growth conditions on the Bi content of GaAs1−xBix
Authors: Lu X. , Beaton D. A. , Lewis R. B. , Tiedje T. , Whitwick M. B.
Published: May 2008

We describe how the Bi content of GaAs1−xBix epilayers grown on GaAs can be controlled by the growth conditions in molecular beam epitaxy. Nonstandard growth conditions are required because of the strong tendency for Bi to surface segregate under usual growth conditions for GaAs. A maximum Bi content of 10% is achieved at low substrate temperature and low arsenic pressure, as inferred from x-ray diffraction measurements. A model for bismuth incorporation is proposed that fits a large body of experimental data on Bi content for a wide range of growth conditions. Low growth rates are found to facilitate the growth of bismide alloys with a low density of Bi droplets.
Gibbs ensemble Monte Carlo simulation of adsorption for model surfactant solution in confined slit pores
Authors: Liu Lili , Yang Xiaoning , Xu Zhijun
Published: May 2008

An isobaric-isothermal Gibbs ensemble Monte Carlo simulation has been carried out to study the adsorption of a model surfactant/solvent mixture in slit nanopores. The adsorption isotherms, the density distributions, and the configuration snapshots were simulated to illustrate the adsorption and self-assembly behaviors of the surfactant in the confined pores. The adsorption isotherms are stepwise: a two-step curve for the smaller (30Å) pore and a three-step one for the larger (50Å) pore. The adsorption isotherms and the interfacial aggregate structure of the surfactants in the pores with various sizes show a qualitatively consistent performance with the previous experimental observation. The micelle size distributions of the adsorbed surfactant aggregates have been analyzed in order to understand the adsorption mechanism, which suggests that the step rise in the surfactant adsorption is associated with the considerable formation of the micelle aggregates in the confined pores. The effect of the interaction between the pore surface and the surfactant on the adsorption behavior has also been investigated. The simulation results indicate that a change in the interaction can modify the shape of adsorption isotherms. A nonlinear mathematical model was used to represent the multistep adsorption isotherms. A good agreement between the model fitting and the simulation data was obtained for both the amount of adsorption and the jump point concentration.
Antireflection coating for improved optical trapping
Authors: Hu Ying , Nieminen Timo A. , Heckenberg Norman R. , Rubinsztein-Dunlop Halina
Published: May 2008

An antireflection coating improves the trapping of high-index particles in optical tweezers by reducing the scattering force. This can allow the strong trapping of high-index particles that cannot normally be trapped, and the use of lower numerical aperture objectives while still obtaining strong trapping. The improvement is not overly sensitive to the refractive index or thickness of the coating.
Probing the band structure of InAs∕GaAs quantum dots by capacitance-voltage and photoluminescence spectroscopy
Authors: Lei W. , Offer M. , Lorke A. , Notthoff C. , Meier C. , Wibbelhoff O. , Wieck A. D.
Published: May 2008

The band structure of self-assembled InAs quantum dots, embedded in a GaAs matrix, is probed with capacitance-voltage spectroscopy and photoluminescence (PL) spectroscopy. The electron energy levels in the quantum dots with respect to the electron ground state of the wetting layer (WL) are determined from the capacitance-voltage measurements with a linear lever arm approximation. In the region where the linear lever arm approximation is not valid anymore (after the charging of the WL), the energetic distance from the electron ground state of the WL to the GaAs conduction band edge can be indirectly inferred from a numerical simulation of the conduction band under different gate voltages. In combination with PL measurements, the complete energy band diagram of the quantum dot sample is extracted.
Carrier multiplication in a PbSe nanocrystal and P3HT/PCBM tandem cell
Authors: Kim Sung Jin , Kim Won Jin , Cartwright Alexander N. , Prasad Paras N.
Published: May 2008

Photoactivation of multiple carrier charge extraction in a tandem cell configuration is demonstrated. An all-solution-processed tandem photovoltaic structure, consisting of a hydrazine treated PbSe nanocrystal film and a P3HT/PCBM bulk-heterojunction solar cell, was fabricated. The PbSe nanocrystal film was used as the top cell to predominantly absorb short wavelength light (UV/blue). The lower P3HT/PCBM cell absorbed the remaining solar energy and supplied photocurrent as well as an electric field that extracts the photogenerated charge from the PbSe layer. Carrier multiplication from excitation of the PbSe film with high energy photons was quantified by selective light bias activation measurements.
Size- and charge-dependent geometric and electronic structures of Bin (Bin−) clusters (n=2–13) by first-principles simulations
Authors: Gao Liming , Li Pinglin , Lu Heqiang , Li S. F. , Guo Z. X.
Published: May 2008

Neutral and negatively charged bismuth clusters, Bin and Bin− (n=2–13), are investigated by first-principles simulations with the scalar-relativistic projector-augmented wave potential and the spin-polarized generalized gradient approximation. Both types of clusters show size-dependent odd-even oscillations in stability, density of states, and vertical and adiabatic electron affinities, in close agreement with experiment. The negative charge thoroughly reverses the oscillations and considerably influences the geometric structures, particularly of the odd-sized clusters. We note that most atoms in the ground states and the low-lying isomers are three coordinated with a quasilayerlike growth mode based on pentagon units, due to a weak s-p hybridization. The Bi12 cluster is found to prefer a small elongated tubelike structure with the surface consists of six curved-pentagon rings and two triangular facets, which may be the basis for the formation of bismuth nanotubes experimentally reported.
Giant positive magnetoresistance in Co-doped ZnO nanocluster films
Authors: Tian Y. F. , Antony J. , Souza R. , Yan S. S. , Mei L. M. , Qiang Y.
Published: May 2008

We have studied nanostructures and magnetoresistance of 0.5%, 12%, and 30% Co-doped ZnO nanocluster films which were deposited by a third generation sputtering-gas-aggregation source on Si wafer. Microstructure analysis is performed by x-ray photoelectron spectrometer, transmission electron microscopy, and x-ray diffraction and shows a uniform mean nanocrystallite size of 20nm with perfect wurtzite ZnO structure. Magnetoresistance (MR) increases at 5K with Co doping concentration, that is, 0.5% Co-doped ZnO with 469% MR, while the other two samples have 744% and 811%. The large positive MR is explained by the suppression of spin-dependent hopping paths when localized states with onsite correlation undergo a relatively large spin (Zeeman) splitting in a magnetic field due to strong s,p-d interactions in Co-doped ZnO nanocluster films.
Inductively coupled probe for the measurement of partial discharges
Authors: Robles Guillermo , Martínez Juan Manuel , Rojas Mónica , Sanz Javier
Published: May 2008

Partial discharges are a transient phenomena whose measurement is remarkably important for electrical equipment diagnosis and maintenance. These discharges appear in the measurement circuit as very narrow current pulses of some nanoseconds. Therefore, discharge pulse detection is a particularly difficult problem, especially because they are superposed on the high voltage waveforms that cause them. In this paper, we present an inductively coupled probe able to measure this physical phenomenon by means of a very simple and inexpensive device that can be installed in the equipment under test. After modeling the probe, its response will be compared to that of commercial devices using both calibrated discharges and partial discharges occurring at real power.
Magnetic flux noise in MgB2 superconductor
Authors: Khare Neeraj , Singh D. P. , Gupta Ajai K.
Published: May 2008

Magnetic flux noise in MgB2 polycrystalline sample is measured using a high-TC rf-superconducting quantum interference device in the temperature range of 6–40K. A small magnetic field (∼200mG) was applied while cooling the sample. The flux noise exhibits 1∕fα type of behavior with α∼1.0–1.3 and shows enhanced noise around 24 and 37K. The flux noise seems to originate from thermally activated vortex hopping. The large magnetic noise at 24K indicates the presence of larger density of pinning sites with energies ∼0.061eV leading to enhanced magnetic fluctuations at temperatures much below TC.
ZnO metal-semiconductor field-effect transistors with Ag-Schottky gates
Authors: Frenzel H. , Lajn A. , Brandt M. , Wenckstern H. von , Biehne G. , Hochmuth H. , Lorenz M. , Grundmann M.
Published: May 2008

Metal-semiconductor field-effect transistors (MESFETs) were fabricated by reactive dc sputtering of Ag-Schottky gate contacts on ZnO thin-film channels grown by pulsed-laser deposition on sapphire. The n-type conductivity (normally on) of typical MESFETs is tunable over 8 decades in a voltage range of 2.5V with an off voltage of −1.5V and very low off-current density in the range of 10−6A∕cm2. Channel mobilities of up to 27cm2∕Vs have been achieved.
High quality GaN grown on silicon(111) using a SixNy interlayer by metal-organic vapor phase epitaxy
Authors: Cheng Kai , Leys M. , Degroote S. , Germain M. , Borghs G.
Published: May 2008

High quality GaN layers with dislocation density of (3.0±0.5)×108∕cm2 have been grown on silicon(111) substrates using a combination of AlGaN intermediate layers and a SixNy interlayer. A smooth and fully coalesced layer was obtained by virtue of a high temperature growth process which accelerates coalescence and improves at the same time the crystalline quality. This was confirmed by high resolution x-ray diffraction showing a full width of half maximum of 415arcsec for the asymmetric (−2201) rocking curve.
Collisionless phenomena in heteronuclear clusters
Authors: Jha J. , Krishnamurthy M.
Published: May 2008

Argon K-shell x-ray yields and electron fluxes are measured for water doped Ar clusters as a function of laser pulse width. The high energy electron and x-ray yields are significantly enhanced in doped clusters. We show that doping of the clusters can be used to enhance collisionless absorption.