bluuebunny/crossref_metadata_2025 huggingface dataset:
A first assessment of the Ticolichen biodiversity inventory in Costa Rica: the genus Gyalideopsis and its segregates (Ostropales: Gomphillaceae), with a world-wide key and name status checklist
Authors: LÜCKING Robert , APTROOT André , UMAÑA Loengrin , CHAVES José Luis , SIPMAN Harrie J. M. , NELSEN Matthew P.
Published: March 2006

Thirty-one taxa of Gyalideopsis and its recent segregates (Ferraroa, Jamesiella, Lithogyalideopsis) are reported in a first assessment of the group from Costa Rica. Six species are described as new, all originating from perhumid montane rainforest: Gyalideopsis altamirensis Lücking & Umaña, with broadly sessile, brown apothecia and single-spored asci with muriform ascospores (hyphophores unknown); G. macarthurii Lücking, Umaña & Aptroot, with dark greyish brown apothecia having their margin covered by triangular thalline lobules and shortly bristle-shaped hyphophores producing moniliform diahyphae; G. pseudoactinoplaca Lücking & Chaves, with sessile, globose diahyphal bunches similar to those of Actinoplaca and resembling isidia (apothecia unknown), G. wesselsii Lücking, Sipman & Chaves, with verrucose thallus, dark greyish brown, applanate apothecia with single-spored asci producing muriform ascospores, and minutely spathulate, dark brown hyphophores with moniliform diahyphae, and Jamesiella chaverriae Chaves, Umaña & Lücking, with isidioid hyphophores (‘thlasidia’) which are flask-shaped and apically thinly ciliate (apothecia unknown). A further species, Gyalideopsis sp., with distinctly stipitate, umbelliform hyphophores, is not formally described due to the lack of apothecia and hyphophores. A further nine species are reported as new to Costa Rica: G. buckei, G. capitata, G. confluens, G. giganteoides, G. megalospora, G. napoensis, G. nepalensis, G. palmata, and Lithogyalideopsis zeylandica. A world-wide key to all presently known 94 taxa of Gyalideopsis and its recent segregates is presented, as well as a checklist.
New or previously misunderstood species of Lithographa and Rimularia (Agyriaceae) from the southern subpolar region and western Canada
Authors: COPPINS Brian J. , FRYDAY Alan M.
Published: March 2006

Lithographa serpentina Coppins & Fryday is newly described from Campbell Island (New Zealand), and is unique within its genus and order for having submuriform ascospores. Also newly described are Lithographa opegraphoides Coppins & Fryday from the Falkland Isles, Rimularia actinostoma Coppins & Fryday from Canada (British Columbia), and Rimularia austrolimborina Coppins & Fryday from southern Chile. The new combination Lithographa graphidioides (Cromb.) Imshaug ex Coppins & Fryday is made to accommodate the previously misunderstood Stigmatidium graphidioides Cromb. and the more recently described Lithographa subantarctica Hertel & Rambold. A key to all described species of Lithographa is provided.
Nyungwea, a new genus of lichen with goniocyst-producing stipes from Rwanda and Uganda (East Africa)
Authors: SÉRUSIAUX Emmanuël , FISCHER Eberhard , KILLMANN Dorothee
Published: March 2006

The new genus and species Nyungwea pallida is characterized by a thin thallus lichenized with Trentepohlia and pale stipes producing goniocysts. Its taxonomic position is uncertain. It is named after the large pristine montane forest of Nyungwe in Rwanda (East Africa) where it was first found; it is also known from the Mabira forest in Uganda.
A new saxicolous Arthothelium from the Namib desert
Authors: APTROOT André , WIRTH Volkmar
Published: March 2006

The new species Arthothelium desertorum is described from the Namib desert. It is characterized by its saxicolous habit and areolate thallus containing divaricatic acid.
A new species of Chrysothrix (Arthoniales: Arthoniaceae) from India
Authors: JAGADEESH RAM T. A. M. , SINHA G. P. , LÜCKING Robert , LUMBSCH H. Thorsten
Published: March 2006

A new species, Chrysothrix septemseptata, is described from India. It is characterized by 7-septate ascospores, which is unique within the genus. It is currently known only from the Sundarbans Biosphere Reserve, where it grows in mangrove plantations.
New Late Cretaceous and Paleogene paleomagnetic results from south China and their geodynamic implications
Authors: Sun Zhiming , Yang Zhenyu , Yang Tienshui , Pei Junling , Yu Qinfan
Published: March 2006

To better understand the tectonic evolution of South China Block (SCB) in response to the India‐Asia collision, we present new paleomagnetic results from Late Cretaceous and Paleogene red bed formations of the Hengyang basin of Hunan province, in the interior part of the SCB. Stepwise thermal demagnetization of the rocks isolated a high‐temperature component. The tilt‐corrected mean direction from the Late Cretaceous rocks is D = 15.6°, I = 29.9° with a95 = 5.7°, N = 26 sites, corresponding to a paleopole at 71.9°N, 236.3°E with A95 = 4.7°, which passes reversal tests. The tilt‐corrected mean direction (D = 358.9°, I = 35.4° with a95 = 5.0°, N = 22 sites) from the Paleogene rock, which passes the reversal test, gives a pole at 82.6°N, 300.6°E with A95 = 4.4°. The low‐field anisotropy of magnetic susceptibility results suggest that the red beds have not experienced significant strain due to compaction or tectonic stress, while the anisotropy of isothermal remanence results suggest that postdepositional compaction in these red beds produced no more than 3°–4° of inclination shallowing. These paleomagnetic and rock magnetic tests imply that the remanence is primary. On the basis of paleomagnetic results obtained from coeval basalts in Mongolia and Siberia, we suggest that no significant latitudinal motion has taken place between the SCB and Siberia since the Late Cretaceous. The significant latitudinal discrepancy between the SCB and European block could be due to nonrigid behavior of the Eurasian plate since the Late Cretaceous. A 16.7 ± 5.0° clockwise rotation of the SCB during the Late Cretaceous to the Paleogene could be related to the collision of India and Asia in the west and the subduction of the circum‐Pacific plate to eastern Asia in the east.
Contribution of giant icebergs to the Southern Ocean freshwater flux
Authors: Silva T. A. M. , Bigg G. R. , Nicholls K. W.
Published: March 2006

In the period 1979–2003 the mass of “giant” icebergs (icebergs larger than 18.5 km in length) calving from Antarctica averaged 1089 ± 300 Gt yr−1 of ice, under half the snow accumulation over the continent given by the Intergovernmental Panel on Climate Change (2246 ± 86 Gt yr−1). Here we combine a database of iceberg tracks from the National Ice Center and a model of iceberg thermodynamics in order to estimate the amount and distribution of meltwater attributable to giant icebergs. By comparing with published modeled meltwater distribution for smaller bergs we show that giant icebergs have a different melting pattern: An estimated 35% of giant icebergs' mass is exported north of 63°S versus 3% for smaller bergs, although giant bergs spend more of the earlier part of their history nearer to the coast. We combine both estimates to produce the first iceberg meltwater map that takes into account giant icebergs. The average meltwater input is shown to exceed precipitation minus evaporation (P − E) in certain areas and is a nonnegligible term in the balance of freshwater fluxes in the Southern Ocean. The calving of giant icebergs is, however, episodic; this might have implications for their impact on the freshwater budget of the ocean. It is estimated that over the period 1987–2003 the meltwater flux in the Weddell and Ross seas has varied by at least 15,000 m3 s−1 over a month. Because of the potential sensitivity of the production of deep waters to abrupt changes in the freshwater budget, variations in iceberg melt rates of this magnitude might be climatologically significant.
Mantle wedge anisotropy in the Hikurangi subduction zone, central North Island, New Zealand
Authors: Morley A. M. , Stuart G. W. , Kendall J.‐M. , Reyners M.
Published: March 2006

The anisotropic characteristics of the Hikurangi subduction zone in central North Island, New Zealand are studied using shear‐wave splitting measurements from 55 local earthquakes recorded on a 200 km profile of 19 stations. The eastern fore‐arc shows trench‐parallel fast directions and a low average delay time of 0.2 s. The source of anisotropy is confined to the upper 60 km and is likely to be deformation induced within the overriding Australian plate. Central western North Island shows N‐S trending fast anisotropy which may signify the presence of a viscous blanket of mantle material being entrained by the absolute motion of the Australian plate. Between these regions a dramatic switch to extension‐parallel fast directions and delay times of 0.35 s are seen across the back‐arc in the Taupo Volcanic Zone (TVZ). These results suggest asthenospheric flow beneath the overriding plate with the olivine a‐axes oriented in the extension direction down to a maximum depth of 100 km.
Complexity of D″ in the presence of slab‐debris and phase changes
Authors: Sun Daoyuan , Song Teh‐Ru Alex , Helmberger Don
Published: March 2006

A lower mantle S‐wave triplication with a Scd branch is easily observed beneath Central American and its lateral variations has been studied extensively. Attempts to model Scd depth variations with a phase boundary based on tomography predict smooth variations [e.g., Sidorin et al., 1999a]. Recently, Hutko et al. [2005] presents evidence of a sharper feature with a vertical step‐like jump of over 50 km, for paths sampling the western edge of a fast lower mantle structure beneath the Cocos Plate. We present a similar type of very anomalous Scd waveforms sampling the edge further to the east. We are able to model these waveforms by adopting the phase‐boundary model but enhancing the existing tomographic anomaly just above the velocity jump, which appears to be a folded‐slab.
On the relative importance of self‐gravitation and elasticity in modeling volcanic ground deformation and gravity changes
Authors: Charco M. , Fernández J. , Luzón F. , Rundle J. B.
Published: March 2006

Elastic half‐space models, widely used to interpret displacements and gravity data in active volcanic areas, usually compute the displacement response to dilatational sources that simulate a change in pressure of the magma chamber. Elastic‐gravitational models allow the computation of gravity, deformation, and gravitational potential changes due to pressurized magma cavities and intruding masses together. This type of model takes into account the mass interaction with the self‐gravitation of the Earth through coupling between model equations. We perform a dimensional analysis of the elastic‐gravitational model estimating the magnitude of intrusion mass and coupling effects at the space scale associated with volcano monitoring. We show that the intrusion mass cannot be neglected in the interpretation of gravity changes while displacements are primarily caused by pressurization. Therefore the intrusion of mass, together with the associated pressurization of the magma chamber, produces distinctive changes in gravity that could be used to interpret gravity changes without ground deformation and vice versa, depending on what type of source plays the main role in the intrusion process. Theoretical experiments indicate that mass and self‐gravitation could produce changes in the magnitude and pattern of predicted gravity that may be above microgravity accuracy. Application of the elastic‐gravitational model to interpret geodetic precursors observed at Mayon volcano (Philippines) prior to the eruption of 2001 shows that inversions increase in precision by using this model. Therefore our elastic‐gravitational model is a refinement of purely elastic models and can better interpret gravity and deformation changes in active volcanic zones.
Evidence of instantaneous dynamic triggering during the seismic sequence of year 2000 in south Iceland
Authors: Antonioli A. , Belardinelli M. E. , Bizzarri A. , Vogfjord K. S.
Published: March 2006

We analyze the coseismic stress perturbation during the 17 June 2000 south Iceland seismic sequence; the main shock (Ms 6.6) was followed by three large events within a few tens of seconds (8, 26, and 30 s) located within 80 km. The aim of this paper is to investigate short‐term fault interaction and instantaneous triggering. This happens when a fault perturbed by a stress change fails before the end of the transient stress perturbation. We compute the shear, normal, and Coulomb stress changes as functions of time in a stratified elastic half‐space by using discrete wave number and reflectivity methods. We calculate dynamic stresses caused by the main shock at the hypocenters of these three subsequent events. Our numerical results show that the onset of the last two events is slightly delayed with respect to the arrival time of the second positive peak of Coulomb stress variation, while the first event occurred after the first positive stress peak. We have also analyzed the response of a spring slider system representing a fault governed by a rate‐ and state‐dependent friction law, perturbed by shear and normal stress variations caused by the main shock. The fault response to the computed stress perturbations is always clock advanced. We have found suitable constitutive parameters of the modeled fault that allow the instantaneous dynamic triggering of these three earthquakes. If the initial sliding velocity is comparable with the tectonic loading velocity, we obtained failure times close to the observed origin times for low values of the initial effective normal stress.
Intrathermocline eddies in the Southern Indian Ocean
Authors: Nauw J. J. , van Aken H. M. , Lutjeharms J. R. E. , de Ruijter W. P. M.
Published: March 2006

In 2001, two relatively saline intrathermocline eddies (ITEs) were observed southeast of Madagascar at 200 m depth. They are characterized by a subsurface salinity maximum of over 35.8 at potential temperatures between 18° and 22°C. The oxygen concentrations within the high salinity cores are slightly elevated compared with those of the surrounding water. Their horizontal extent is about 180 km, several times the Rossby deformation radius, while their thickness is about 150 m. The observed circulation around the ITEs is anticyclonic and maximum velocities of 20 to 30 cm/s are observed at 200 m depth. In these cores the potential density anomaly (25.0 < γ < 25.9 kg/m3) has a relatively low vertical gradient and therefore a low planetary potential vorticity. The hydrographic properties of these ITEs are distinctly different from those of the surrounding thermocline water, and especially from the much fresher water mass in the East Madagascar Current. Strong evidence has been found that the distant formation area of the water mass in the ITEs is the subtropical Southern Indian Ocean east of 90°E and south of 25°S, where Subtropical Underwater (STUW) is formed with similar characteristics. Similar high‐salinity cores as the ITEs are also found in the thermocline around 200 m depth along an almost zonal section between Madagascar and 100°E. Differences between the hydrographic properties of these cores and the ITEs near Madagascar may partly be explained by interannual variations in the temperature and salinity of the surface mixed layer water in the possible formation area.
Effect of impurities on grain growth in cold ice sheets
Authors: Durand G. , Weiss J. , Lipenkov V. , Barnola J. M. , Krinner G. , Parrenin F. , Delmonte B. , Ritz C. , Duval P. , Röthlisberger R. , Bigler M.
Published: March 2006

On the basis of a detailed study of the ice microstructure of the European Project for Ice Coring in Antarctica (EPICA) ice core at Dome Concordia, Antarctica, we analyze the effect of impurities (solubles, and insolubles, that is, dust particles) on the grain growth process in cold ice sheets. As a general trend, the average grain size increases with depth. This global increase, induced by the normal grain growth process, is punctuated by several sharp decreases that can be associated with glacial‐interglacial climatic transitions. To explain the modifications of the microstructure with climatic changes, we discuss the role of soluble and insoluble impurities on the grain growth process, coupled with an analysis of the pinning of grain boundaries by microparticles. Our data indicate that high soluble impurity content does not necessarily imply a slowdown of grain growth kinetics, whereas the pinning of grain boundaries by dust explains all the observed modifications of the microstructure. We propose a numerical model of the evolution of the average grain size in deep ice cores that takes into account recrystallization processes such as normal grain growth and rotation recrystallization as well as the pinning effect induced by dust particles, bubbles, and clathrates on the grain boundaries. Applied to the first 2135 m of the Dome Concordia core, the model reproduces accurately the measured mean grain radius. This indicates a major role of dust in the modification of polar ice microstructure and shows that the average grain size is not a true paleothermometer, as it is correlated with climatic transitions through the dust content of the ice.
Assemblages of submarine landforms produced by tidewater glaciers in Svalbard
Authors: Ottesen D. , Dowdeswell J. A.
Published: March 2006

High‐resolution swath bathymetry from the marine margins of several Svalbard tidewater glaciers shows an assemblage of submarine landforms that is probably linked to glacier surging. These landforms are essentially unmodified since their initial deposition over the past hundred years or so because they have not been subjected to subaerial erosion or periglacial activity. Swath images comprise an assemblage of superimposed landforms, allowing reconstruction of relative age of deposition: (1) large transverse ridges, interpreted as recessional moraines overridden by a subsequent ice advance; (2) a series of curvilinear streamlined bedforms orientated parallel to former ice flow, interpreted as lineations formed subglacially during rapid advance; (3) large terminal ridges, marking the farthest extent of ice at the last advance, with flow lobes immediately beyond interpreted as submarine debris flows; (4) a series of interconnected rhombohedral ridges, interpreted as a product of soft sediment squeezing into crevasses formed at the glacier bed, probably formed during immediate post‐surge stagnation; and (5) a series of fairly evenly spaced small transverse ridges, interpreted as push moraines produced annually at tidewater glacier termini during retreat. A simple descriptive landsystem model for tidewater glaciers of probable surge type is derived from these observations. We also show that megascale glacial lineations can form not only beneath large ice streams, but are also produced beneath surging tidewater glaciers lying on deforming sedimentary beds.
A quantitative framework for interpretation of basal ice facies formed by ice accretion over subglacial sediment
Authors: Christoffersen Poul , Tulaczyk Slawek , Carsey Frank D. , Behar Alberto E.
Published: March 2006

We have constructed a numerical model of basal ice formation for glacier ice in contact with subglacial sediment. The model predicts four different ice facies whose formation is controlled by availability of subglacial water to satisfy the basal freeze‐on rate. Clean (or clotted) facies may result from congelation (or frazil) ice growth occurring when (supercooled) meltwater separates ice base from substrate or if a groundwater source can supply water to the ice base at a velocity equal to the freezing rate. Laminated facies develops when the supply of subglacial water is sufficiently constrained for cryostatic suction to raise the subglacial effective stress above a threshold for intrusion of ice into sediment by regelation. Debris laminas (∼1 mm) are entrained by short, periodic regelation events (of a few hours) separated by longer periods (days to months) of congelation. Further meltwater limitation produces a massive dirty ice facies due to stacking of debris laminas. The model predicts growth of solid dirty ice facies when the bed is meltwater‐depleted with fast freezing (5 mm yr−1) causing enhanced erosion (30 mm yr−1). We find that basal ice facies and sediment entrainment are controlled mainly by the ratio of freezing rate to water supply rate. The predicted ice facies compare favorably with borehole camera imagery of the basal ice layer in Kamb Ice Stream, West Antarctica. Facies variability in this layer suggests complex hydrologic history for the West Antarctic Ice Sheet with significant changes occurring over a period of several thousand years.
Ethanol inhibits palmitoylation of G protein Gαs
Authors: Hallak Hazem , Rubin Raphael
Published: March 2006

Neurobiological actions of ethanol have been linked to perturbations in cyclic AMP (cAMP)‐dependent signaling processes. Chronic ethanol exposure leads to desensitization of cAMP production in response to physiological ligands (heterologous desensitization). Ethanol‐induced alterations in neuronal expression of G proteins Gs and Gi have been invoked as a cause of heterologous desensitization. However, effects of ethanol on G protein expression vary considerably among different experimental protocols, various brain regions and diverse neuronal cell types. Dynamic palmitoylation of G protein α subunits is critical for membrane localization and protein–protein interactions, and represents a regulatory feature of G protein function. We studied the effect of ethanol on Gαs palmitoylation. In NG108‐15 rat neuroblastoma × glioma hybrid cells, acute exposure to pharmacologically relevant concentrations of ethanol (25–100 mm) inhibited basal and prostaglandin E1‐stimulated incorporation of palmitate into Gαs. Exposure of NG108‐15 cells to ethanol for 72 h induced a shift in Gαs to its non‐palmitoylated state, coincident with an inhibition of prostaglandin E1‐induced cAMP production. Both parameters were restored following 24 h of ethanol withdrawal. Chronic ethanol exposure also induced the depalmitoylation of Gαs in human embryonic kidney (HEK)293 cells that overexpress wild‐type Gαs and caused heterologous desensitization of adenylyl cyclase. By contrast, HEK293 cells that express a non‐palmitoylated mutant of Gαs were insensitive to heterologous desensitization after chronic ethanol exposure. In summary, the findings identify a novel effect of ethanol on post‐translational lipid modification of Gαs, and represent a mechanism by which ethanol might affect adenylyl cyclase activity.
Time evolution and mixing characteristics of hydrogen and ethylene transverse jets in supersonic crossflows
Authors: Ben-Yakar A. , Mungal M. G. , Hanson R. K.
Published: March 2006

We report an experimental investigation that reveals significant differences in the near-flowfield properties of hydrogen and ethylene jets injected into a supersonic crossflow at a similar jet-to-freestream momentum flux ratio. Previously, the momentum flux ratio was found to be the main controlling parameter of the jet’s penetration. Current experiments, however, demonstrate that the transverse penetration of the ethylene jet was altered, penetrating deeper into the freestream than the hydrogen jet even for similar jet-to-freestream momentum flux ratios. Increased penetration depths of ethylene jets were attributed to the significant differences in the development of large-scale coherent structures present in the jet shear layer. In the hydrogen case, the periodically formed eddies persist long distances downstream, while for ethylene injection, these eddies lose their coherence as the jet bends downstream. The large velocity difference between the ethylene jet and the freestream induces enhanced mixing at the jet shear layer as a result of the velocity induced stretching-tilting-tearing mechanism. These new observations became possible by the realization of high velocity and high temperature freestream conditions which could not be achieved in conventional facilities as have been widely used in previous studies. The freestream flow replicates a realistic supersonic combustor environment associated with a hypersonic airbreathing engine flying at Mach 10. The temporal evolution, the penetration, and the convection characteristics of both jets were observed using a fast-framing-rate (up to 100 MHz) camera acquiring eight consecutive schlieren images, while OH planar laser-induced fluorescence was performed to verify the molecular mixing.
Nonlinear mechanics of wavy instability of steady longitudinal vortices and its effect on skin friction rise in boundary layer flow
Authors: Girgis I. G. , Liu J. T. C.
Published: March 2006

Wavy secondary instability of steady longitudinal vortices in boundary layer flow is studied. The nonlinear interaction problem is parabolized through scaling guided by observations. Emphasis is placed on the nonlinear modification of the steady problem by the Reynolds stresses of the wavy disturbance. It is found that the skin friction in such a modification process increases well above the local turbulent boundary layer value.
Performance evaluation of picosecond high-voltage power switches based on propagation of superfast impact ionization fronts in SiC structures
Authors: Rodin Pavel , Ivanov Pavel , Grekhov Igor
Published: March 2006

We employ a simple analytical model of superfast impact ionization front in a reversely biased p+-n-n+ structure to evaluate the performance of prospective 4H-SiC closing switches based on propagation of ionization fronts. The model allows to relate the order of magnitude values of the front velocity and the electron-hole plasma concentration behind the front to the basic material and structural parameters. We show that high avalanche breakdown field and impact ionization rate of the wide-band-gap 4H-SiC lead to dramatic improvement of switching characteristics with respect to Si structures currently used in pulse power applications. The concentration of electron-hole plasma generated by the front passage is of the order of 1018 versus 1016cm−3 in Si. The velocity of ionization front in 4H-SiC is several times larger than in Si. Finally, we discuss possible triggering mechanisms for the ionization front in SiC.
Small particles in homogeneous turbulence: Settling velocity enhancement by two-way coupling
Authors: Bosse Thorsten , Kleiser Leonhard , Meiburg Eckart
Published: March 2006

The gravitational settling of an initially random suspension of small solid particles in homogeneous turbulence is investigated numerically. The simulations are based on a pseudospectral method to solve the fluid equations combined with a Lagrangian point-particle model for the particulate phase (Eulerian-Lagrangian approach). The focus is on the enhancement of the mean particle settling velocity in a turbulent carrier fluid, as compared to the settling velocity of a single particle in quiescent fluid. Results are presented for both one-way coupling, when the fluid flow is not affected by the presence of the particles, and two-way coupling, when the particles exert a feedback force on the fluid. The first case serves primarily for validation purposes. In the case with two-way coupling, it is shown that the effect of the particles on the carrier fluid involves an additional increase in their mean settling velocity compared to one-way coupling. The underlying physical mechanism is analyzed, revealing that the settling velocity enhancement depends on the particle loading, the Reynolds number, and the dimensionless Stokes settling velocity if the particle Stokes number is about unity. Also, for particle volume fractions Φv≳10−5, a turbulence modification is observed. Furthermore, a direct comparison with recent experimental studies by Aliseda et al. [J. Fluid Mech. 468, 77 (2002)] and Yang and Shy [J. Fluid Mech. 526, 171 (2005)] is performed for a microscale Reynolds number Reλ≈75 of the turbulent carrier flow.
The viscous decay of progressive interfacial waves
Authors: Troy C. D. , Koseff J. R.
Published: March 2006

The viscous damping of progressive, two-layer interfacial waves is examined theoretically and experimentally. Traditional water wave theory is modified to derive the damping rates associated with interfacial wave propagation in a rectangular channel. The individual wave damping contributions are considered from the bottom, side, and interfacial boundary layers, as well as the damping associated with the wave-induced velocities within the homogenous fluid layers. These results show that for most laboratory-scale experiments, sidewall friction plays the dominant role in wave damping. Laboratory experiments are conducted to verify the damping rates for progressive two-layer internal waves in a rectangular channel. Experiments are conducted on both monochromatic and polychromatic wave trains. The results of these experiments are in good agreement with the derived damping rates, but show poorer agreement for large-amplitude waves when the sidewall boundary layers become turbulent. More work is necessary to quantify the damping associated with nonlinear internal waves in order to allow for accurate interpretation of the results from laboratory experiments.
Modeling and experimental investigation of spot dynamics on graphite cathodes in dc plasma arcs at high pressure
Authors: Muñoz-Serrano E. , Casado E. , Colomer V. , Baronnet J.-M.
Published: March 2006

From a model recently developed for refractory cathodes [Muñoz-Serrano et al., J. Appl. Phys.98, 093303 (2005)], the behavior of a graphite cathode spot in a dc plasma torch at atmospheric pressure was investigated. Furthermore, an experimental study of these cathodes was made guided by the results obtained from the model. The model includes the modeling of the cathode region, the solution of the heat conduction problem in the cathode, and the simulation of the cathode ablation process. As a result of the model, the values of the parameters which characterize the cathode region were obtained, and the evolution of the spot under different working conditions determined by the value of the initial voltage drop in the cathode region, U0, was investigated. The results obtained show that the maximum spot radius diminishes when U0 increases. Furthermore, two qualitatively different conditions for the spot dynamics were found. For U0⩾31V, the spot radius continually grows over time until reaching a maximum value rm, and the spot remains fixed on a point of the cathode surface. For values of U0 less than 31V the spot radius continues growing over time until reaching a maximum value with which it is not possible to satisfy the energy balance equation. This leads to spot extinction and to its appearance at another point of the cathode surface. Several graphite cathodes were experimentally studied, each one using different interelectrode voltage drop values Ua-c. Before and after arcing, the cathode surface was explored by an electron microscope, and the roughness profile of that surface was determined by a perthometer. This allowed measuring the average size of the craters produced on the cathode surface by the arc. The movement of the spot attachment for different values of interelectrode voltage was observed by a digital video camera. It was experimentally found that the average crater radius diminished when the Ua-c potential increased. Furthermore, it was seen that for Ua-c values less than 32V, the spot moved over the cathode surface and that this movement became slower when the Ua-c increased. The spot became immobile for Ua-c=32V. These experimental results corroborate the spot behavior obtained from the theoretical model.
Electronic structure and optical properties of (ZnSe)n∕(Si2)m (111) superlattices
Authors: Laref A. , Laref S. , Belgoumene B. , Bouhafs B. , Tadjer A. , Aourag H.
Published: March 2006

The electronic properties of (ZnSe)n∕(Si2)m (111) superlattices (SLs) are investigated theoretically in order to clarify the general features of the zone-folding and the band-mixing effects in superlattices composed of an indirect-band-gap semiconductor (Si). The detailed electronic structure of (ZnSe)n∕(Si2)m (111) SLs are studied with the range n=m=10–16, giving special attention to the role of the interface states at the Zn–Si and Se–Si polar interfaces. The presence of the electric field in the SL is totally ignored, i.e., “the zero-field model.” The degeneracy of the energy minima of the conduction band at the M point in the zinc-blende-type bulk material cannot be lifted by the zone-folding effects alone. The band-mixing effect through the interfaces between the two constituent materials plays an important role in determining the overall band lineup throughout the entire Brillouin zone. The states at the conduction- and valence-band edges are confined two dimensionally in the Si layers. Furthermore, we have found two interface bands in the lower and upper regions of the gap. The states of the lower interface band are located at the Zn–Si interface, while those of the upper interface band are located at the Se–Si interface. The energies of the interface states depend on the parameters representing the Zn–Si and Se–Si bond lengths and the valence-band discontinuity between ZnSe and Si, but the interface states do not disappear from the gap with reasonable choices of the parameters. The electronic structure of the superlattice turns out to be quite sensitive to the combination of the well and barrier layer thicknesses. This sensitivity suggests the possibility of designing suitable band structures for device applications. Furthermore, the absorption spectra of the superlattices are calculated and are found to be quite different from those of bulk ZnSe and Si but fairly close to their average. The electronic and optical properties suggest that superlattices composed of indirect-band-gap semiconductors offer great potential for application to optical devices.
The mechanical properties of polycrystalline 3C-SiC films grown on polysilicon substrates by atmospheric pressure chemical-vapor deposition
Authors: Roy Shuvo , Zorman Christian , Mehregany Mehran , DeAnna Russell , Deeb Chris
Published: March 2006

This paper presents the results of a study to determine Young’s modulus, residual stress, and burst strength of polycrystalline 3C silicon carbide (poly-SiC) films grown on as-deposited and annealed polysilicon substrate layers. The biaxial modulus and residual stress were determined for bulk micromachined poly-SiC diaphragms using an interferometric load-deflection measurement apparatus. The load-deflection data were analyzed using a least-squares fitting technique to extract the biaxial modulus and residual stress values, and Young’s modulus was calculated assuming a Poisson ratio of 0.15. Poly-SiC films comprised of equiaxed grains exhibited Young’s modulus values ranging from 452 to 494GPa, while columnar films with a high degree of (110) texture exhibited Young’s modulus values between 340 and 357GPa. The residual stress for these films did not exhibit a discernable relationship with microstructure; however, the values exhibited a general dependence on growth temperature. Poly-SiC films grown at 1280 °C had residual stress values ranging from 401 to 486MPa, while a film grown at 1160 °C had a residual stress value of 113MPa. Burst strength was determined using a combination of finite element analysis and burst pressure measurements of the suspended diaphragms. Poly-SiC films grown at 1280 °C exhibited an average burst strength value of 1718MPa, while the poly-SiC film grown at 1160 °C had an average burst strength value of 1321MPa.
Dissipative particle dynamics simulation of a colloidal micropump
Authors: De Palma Pietro , Valentini P. , Napolitano M.
Published: March 2006

Dissipative particle dynamics (DPD) is a recently developed model for computing complex fluid flows at mesoscopic scales. This article provides a novel DPD simulation of complex microfluidic devices involving the momentum exchange between a body moving with a prescribed law of motion and the surrounding fluid. To this purpose, a DPD computational method is developed and equipped with an elastic collision model between the moving body and the DPD fluid particles surrounding it. The method is first validated versus well known theoretical, numerical, and experimental results, providing a sensitivity analysis of the dependence of continuum-flow properties on DPD parameters, as well as verifying its reliability for well known continuum-flow test cases. The method is then applied to its main goal, namely, the simulation of the flow driven by a peristaltic micropump, constructed by assembling several colloidal spheres. The DPD fluid model provides quite accurate results with respect to the experimental data and gives a detailed description of local flow properties. It is found that a careful choice of the DPD parameters is needed to avoid spurious compressibility effects and to match the real fluid characteristics; furthermore, due to the very coarse graining used in the present simulation, the thermal kinetic energy of the DPD particles needs to be reduced, in order to correctly evaluate their displacement, which is determined mainly by the momentum driving the flow. Finally, thanks to such a very coarse graining, the proposed DPD method provides an accurate prediction of local mesoscale flow properties with a dramatic reduction of the computational cost with respect to molecular dynamics simulations.
Enhancement and depression in second-order optical nonlinearity of Ba2TiGe2O8 in crystallized glass prepared in a high magnetic field
Authors: Toyohara N. , Benino Y. , Fujiwara T. , Tanaka S. , Uematsu K. , Komatsu T. , Takahashi Y.
Published: March 2006

A magnetic field of H=10T was applied perpendicularly or parallel to the surface of the 30BaO.15TiO2.55GeO2 (BTG) glass during its crystallization, and the effect of the magnetic field on the crystal orientation and second-order optical nonlinearity of the Ba2TiGe2O8 crystals formed at the glass surface was examined. Compared with the sample prepared by conventional crystallization in zero magnetic field, the c-axis (polarization axis) orientation and second-harmonic intensity of the Ba2TiGe2O8 crystals at the surface were enhanced in the sample fabricated with the field perpendicular to the surface and, by contrast, were depressed in the sample fabricated with the field parallel to the surface. The present study indicates that the application of high magnetic fields during crystallization is a technique to control the crystal orientations and optical properties of BTG crystallized glasses.
Spatial variation of plasma parameters and ion acceleration in an inductive plasma system
Authors: Volynets V. N. , Park Wontaek , Tolmachev Yu. N. , Pashkovsky V. G. , Yoo Jinwoo
Published: March 2006

Plasma parameters of inductively coupled plasma system with an annular plasma source have been studied experimentally. At low pressures (about 1mTorr), electron temperature inside the plasma source is rather high (8–13eV) and is much greater than in the diffusion (main) chamber (4–5eV). The plasma potential inside the source is also much higher than in the main chamber. There is a rapid drop of the electron temperature and plasma potential at the boundary between the plasma source and the main chamber. The drop of the plasma potential at the boundary (about 20V) means the existence of a strong axial electric field, which retards the electrons inside the plasma source and accelerates the ions from the source into the main chamber. Measurements of ion energy distributions in the main chamber volume reveal the existence of ions with kinetic energies about 15eV.
Thickness-dependent properties of FeTaN thin films deposited on flexible substrate
Authors: Liu Z. W. , Liu Y. , Yan L. , Tan C. Y. , Ong C. K.
Published: March 2006

The FeTaN thin films were deposited on the flexible plastic substrates by rf magnetron sputtering. Good soft magnetic properties with coercivity <10Oe have been obtained in the films with thickness <250nm. Relatively high values of the complex permeability and ferromagnetic resonance frequency were obtained and they can be adjusted by changing the film thickness. However, compared to the films on the rigid substrate, a decrease of the permeability level and a shift of the resonance peak in the permeability spectra have been noticed for the films on the flexible substrate. The electric properties, including sheet resistance, resistivity, and magnetoresistance for the films with various thicknesses, have also been investigated.
Spin-polarized transport in one-dimensional waveguide structures with spatially periodic electric fields
Authors: Wang L. G. , Chang Kai , Chan K. S.
Published: March 2006

We investigate theoretically spin-polarized transport in a one-dimensional waveguide structure under spatially periodic electric fields. Strong spin-polarized current can be obtained by tuning the external electric fields. It is interesting to find that the spin-dependent transmissions exhibit gaps at various electron momenta and/or gate lengths, and the gap width increases with increasing the strength of the Rashba effect. The strong spin-polarized current arises from the different transmission gaps of the spin-up and spin-down electrons.
Influence of alignment layer thickness on ferroelectric liquid-crystal structure
Authors: Petkovšek Rok , Čopič Martin , Pirš Janez
Published: March 2006

We have investigated the influence of the thickness of polyvinyl alcohol alignment layer on azimuthal anchoring of ferroelectric liquid crystals by measuring the azimuthal angle of the chevron C2 structure. By varying not only the alignment but also the ferroelectric liquid-crystal layer thickness, we also studied the influence of the ions screening the polarization charge on the chevron layer structure. A simple model describing the free enthalpy of the system was used to calculate the anchoring coefficient from the azimuthal angle of the structure.
Nonequilibrium radiation of long-wavelength InAs∕GaSb superlattice photodiodes
Authors: Hoffman Darin , Hood Andrew , Fuchs Frank , Razeghi Manijeh
Published: March 2006

The emission behavior of binary-binary type-II InAs∕GaSb superlattice photodiodes has been studied in the spectral range between 8 and 13μm. With a radiometric calibration of the experimental setup the internal and external quantum efficiencies have been determined in the temperature range between 80 and 300K for both the negative and positive luminescences. The negative luminescence efficiency approaches values as high as 35% without antireflection coating. An analytic description of the temperature dependence of the internal quantum efficiency around a zero-bias voltage allows for the determination of the coefficient for electron-hole-electron Auger recombination Γn=1×1024cm6s−1. For an n-type material, the minority-carrier lifetime is provided as a function of band gap and temperature, explaining the strong decrease of the minority-carrier lifetime in the case of an n-type residual background exceeding 1×1016cm−3. Furthermore, an analytic expression of the quantum efficiency for the radiation upon forward-bias conditions is given.
Irreversible increase of the low-temperature paramagnetism in GaAs substrates
Authors: Ney A. , Jan G. , Parkin S. S. P.
Published: March 2006

The magnetic susceptibility of undoped GaAs substrates is measured from 2to300K. It comprises two types of temperature-dependent net-paramagnetic contributions. One is a Van Vleck-type paramagnetic contribution which scales inversely with the band gap of the semiconductor and thus increases up to room temperature. The other resembles a Langevin-type paramagnetism which sets in only at low temperatures after certain thermal cycling procedures.
Thermal transport in sheared electro- and magnetorheological fluids
Authors: Heine Martin C. , de Vicente Juan , Klingenberg D. J.
Published: March 2006

Thermal energy transport in sheared electrorheological and magnetorheological (ER and MR) fluids is analyzed. Although energy production by viscous dissipation can be significant, energy transport on the particle length scale can be analyzed by ignoring viscous dissipation. For typical situations, energy transport normal to the flow direction is dominated by conduction. Particle-level simulations were employed to determine the suspension structure as a function of Mason number and volume fraction. A self-consistent mean-field dipole model is used to estimate the effective thermal conductivities for these simulated structures. The field-induced chain-like aggregates that form at small Mason number result in a larger effective thermal conductivity at small Mason number than at large Mason number. Effects of higher-order multipoles are estimated by analyzing effective thermal conductivities of model structures. For highly conducting particles, the effective thermal conductivity of a sheared ER or MR suspension is predicted to roughly double as the Mason number is decreased from the large to the small Mason number limits.
Pulsed laser-induced silicidation on TiN-capped Co∕Si bilayers
Authors: Chow F. L. , Lee P. S. , Pey K. L. , Tang L. J. , Tung C. H. , Wang X. C. , Lim G. C.
Published: March 2006

This paper studies the effects of pulsed laser-induced annealing of TiN-capped Co∕Si bilayers with and without preamorphized Si substrate. For a low fluence of 0.2J∕cm2, nonstoichiometry Co silicide with triple-layered structure is formed. On the other hand, highly textured CoSi2 grains in (111) direction are formed for a high fluence of 0.7J∕cm2. The highly textured CoSi2 layer is monocrystalline and fully coherent with the (111) plane of the Si substrate. However, it has a large amount of microstructural defects throughout the layer. Competitive growth mechanisms between crystallization of homogenous intermixed layer and the nucleation from the melt boundary are discussed.
Structural and transport properties of Ba8Ga16SixGe30−x clathrates
Authors: Martin J. , Erickson S. , Nolas G. S. , Alboni P. , Tritt T. M. , Yang J.
Published: March 2006

We report on the structural, chemical, electrical resistivity, Seebeck coefficient, thermal conductivity, and Hall measurements of polycrystalline Si–Ge type-I clathrates with nominal composition of Ba8Ga16SixGe30−x such that a constant Ga-to-group-IV element ratio is maintained while varying the Si-to-Ge ratio. Electrical transport measurements on the n-type specimens show a modest increase in the absolute Seebeck coefficient and a decrease in electrical resistivity with increasing Si content. This may imply a modification in the band structure with Si substitution and reveals a possible approach for optimizing these materials for thermoelectric applications. These data are compared with those of Ba8Ga16Ge30 and Sr8Ga16SixGe30−x.
In situ energy dispersive x-ray reflectometry to investigate the (RuPc)2∕NOx interaction process evidenced by ex situ measurements
Authors: Generosi Amanda , Paci Barbara , Rossi Albertini Valerio , Perfetti Paolo , Paoletti Anna M. , Pennesi Gianna , Rossi Gentilina , Caminiti Ruggero
Published: March 2006

A systematic energy dispersive x-ray reflectometry study of different ruthenium phthalocyanine (RuPc)2 thin films was performed in order to investigate their reactivity with the oxidizing NOx gas. A preliminary ex situ analysis, consisting of the comparison between the morphological parameters of different films (before and after the exposure to the gas), was performed. It suggests that a reaction involving two different mechanisms takes place. The following in situ (while fluxing the gas) reflectometry analysis confirms this hypothesis, and clarifies the temporal evolution, also revealing that the first mechanism is limited to the film surface, while the second is a bulk diffusion process.