Showing papers by "IPG Photonics published in 2006"

Raman spectrum of graphene and graphene layers.

Abstract: Graphene is the two-dimensional building block for carbon allotropes of every other dimensionality We show that its electronic structure is captured in its Raman spectrum that clearly evolves with the number of layers The D peak second order changes in shape, width, and position for an increasing number of layers, reflecting the change in the electron bands via a double resonant Raman process The G peak slightly down-shifts This allows unambiguous, high-throughput, nondestructive identification of graphene layers, which is critically lacking in this emerging research area

Nonadiabatic kohn anomaly in a doped graphene monolayer

Abstract: We compute, from first principles, the frequency of the E(2g), Gamma phonon (Raman G band) of graphene, as a function of the charge doping. Calculations are done using (i) the adiabatic Born-Oppenheimer approximation and (ii) time-dependent perturbation theory to explore dynamic effects beyond this approximation. The two approaches provide very different results. While the adiabatic phonon frequency weakly depends on the doping, the dynamic one rapidly varies because of a Kohn anomaly. The adiabatic approximation is considered valid in most materials. Here, we show that doped graphene is a spectacular example where this approximation miserably fails.

Widespread active detachment faulting and core complex formation near 13° N on the Mid-Atlantic Ridge

Abstract: Oceanic core complexes are massifs in which lower-crustal and upper-mantle rocks are exposed at the sea floor1,2,3. They form at mid-ocean ridges through slip on detachment faults rooted below the spreading axis2,3,4,5,6. To date, most studies of core complexes have been based on isolated inactive massifs that have spread away from ridge axes. Here we present a survey of the Mid-Atlantic Ridge near 13° N containing a segment in which a number of linked detachment faults extend for 75 km along one flank of the spreading axis. The detachment faults are apparently all currently active and at various stages of development. A field of extinct core complexes extends away from the axis for at least 100 km. Our observations reveal the topographic characteristics of actively forming core complexes and their evolution from initiation within the axial valley floor to maturity and eventual inactivity. Within the surrounding region there is a strong correlation between detachment fault morphology at the ridge axis and high rates of hydroacoustically recorded earthquake seismicity. Preliminary examination of seismicity and seafloor morphology farther north along the Mid-Atlantic Ridge suggests that active detachment faulting is occurring in many segments and that detachment faulting is more important in the generation of ocean crust at this slow-spreading ridge than previously suspected.

First-principles study of the OH-stretching modes of gibbsite

Abstract: The theoretical infrared (IR) and Raman spectra of gibbsite [α-Al(OH) 3 ] were computed using ab initio quantum mechanical calculations. The low-frequency dielectric tensor and the Raman tensors of gibbsite were determined using linear response theory. The transmission powder IR spectrum was found to strongly depend on the shape of the gibbsite particles. In the region of the OH-stretching bands, an excellent agreement between theory and experiment was obtained, providing an unambiguous interpretation of the OH bands in terms of vibrational modes. In contrast, the assignment of the bands observed at lower frequency is complicated by the significant overlap between neighboring bands together with their sensitivity to particle shape.

Cr(VI) detoxification by Desulfovibrio vulgaris strain Hildenborough: microbe-metal interactions studies

Abstract: Toxic heavy metals constitute a worldwide environmental pollution problem. Bioremediation technologies represent efficient alternatives to the classic cleaning-up of contaminated soil and ground water. Most toxic heavy metals such as chromium are less soluble and toxic when reduced than when oxidized. Sulfate-reducing bacteria (SRB) are able to reduce heavy metals by a chemical reduction via the production of H2S and by a direct enzymatic process involving hydrogenases and c3 cytochromes. We have previously reported the effects of chromate [Cr(VI)] on SRB bioenergetic metabolism and the molecular mechanism of the metal reduction by polyhemic cytochromes. In the current work, we pinpoint the bacteria–metal interactions using Desulfovibrio vulgaris strain Hildenborough as a model. The bacteria were grown in the presence of high Cr(VI) concentration, where they accumulated precipitates of a reduced form of chromium, trivalent chromium [Cr(III)], on their cell surfaces. Moreover, the inner and outer membranes exhibited precipitates that shared the spectroscopic signature of trivalent chromium. This subcellular localization is consistent with enzymatic metal reduction by cytochromes and hydrogenases. Regarding environmental significance, our findings point out the Cr(VI) immobilization mechanisms of SRB; suggesting that SRB are highly important in metal biogeochemistry.

Tectonics and human evolution

Abstract: The authors propose a new model for the origins of humans and their ecological adaptation. The evolutionary stimulus lies not in the savannah but in broken, hilly rough country where the early hominins could hunt and hide. Such "roughness", generated by tectonic and volcanic movement characterises not only the African rift valley but probably the whole route of early hominin dispersal.

Structural environments around molybdenum in silicate glasses and melts. i. influence of composition and oxygen fugacity on the local structure of molybdenum

Abstract: The coordination chemistry of molybdenum was investigated in nine series of synthetic silicate glasses, including sodium disilicate (NS2), sodium trisilicate (NS3), albite (Ab), anorthite (An), Ab50An50, Ab30An70, diopside (DI), rhyolite (RH), and basalt (BA), using electron paramagnetic resonance (EPR) and X-ray absorption fine structure (XAFS) spectroscopies. The Mo content of these glasses ranges from 300 ppm to 3 wt.%. On the basis of results derived from high-resolution X-ray absorption near-edge structure (XANES) spectroscopy, molybdenum is present primarily as molybdate moieties [Mo(VI)O42−] in most of the glass compositions prepared at f (O2) values ranging from 1 atm to 10−12 atm (temperatures ranging from 1100 to 1700°C, i.e., from air to IW+4). Analysis of extended XAFS (EXAFS) spectra of these glasses indicates an average Mo–O distance of ~1.76(1) A. No evidence for second-neighbor Si or Al around Mo was found in any of the glasses, confirming that molybdate moieties are not connected to the tetrahedral framework, in agreement with Pauling bond-valence predictions. The presence of molybdate moieties in regions of these glasses enriched in network modifiers helps explain why crystalline molybdates can nucleate easily in silicate glasses (and, by extension, in the corresponding melts). In the highly polymerized glass compositions (such as “Ab” or “RH”), Mo(VI)O66− moieties also exist, but at minor levels (<20% of the total Mo). In glasses prepared at low f (O2) (near IW), reduced species of Mo occur, such as molybdenyl [Mo(V) and Mo(IV)]. In glasses prepared at even lower f (O2) (near IW+4), Mo is present as a metallic precipitate. The prevalence of molybdate moieties in silicate glasses until relatively low oxygen fugacities (IW) are achieved appears to be at variance with the fact that molybdenite, Mo(IV)S2, is the dominant Mo-bearing mineral in the Earth’s crust. In a companion paper, we re-examine the speciation of molybdenum in more complex systems that are closer to geochemical reality, such as high-temperature melts, densified (high-pressure) glasses, and silicate glass compositions enriched in volatiles.

Ab initio Raman spectrum of the normal and disordered Mg Al 2 O 4 spinel

Abstract: Ab initio calculations, based on density functional theory, of the vibrational Raman spectra of both normal and cationic disordered $\mathrm{Mg}{\mathrm{Al}}_{2}{\mathrm{O}}_{4}$ spinel are reported. The vibrational properties of the disordered system are obtained in two steps. First, the interatomic force constants of small defective supercells are computed in a fully ab initio way. Later, these force constants are used to build the dynamical matrices of larger disordered systems. The calculated Raman spectrum presents the mode near $727\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ which is due to the presence of cationic disorder and which is commonly attributed to a breathing mode (BM) of $\mathrm{Al}{\mathrm{O}}_{4}$ tetrahedra. Contrary to previous interpretations, the $727\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ mode is due to phonons which are silent in the perfect crystal and become Raman active in the disordered structure because of the coupling with the BM of the $\mathrm{Mg}{\mathrm{O}}_{4}$ tetrahedra.

Structure and evolution of the eastern Gulf of Aden: insights from magnetic and gravity data (Encens-Sheba MD117 cruise)

Abstract: Magnetic and gravity data gathered during the Encens-Sheba cruise (2000 June) in the eastern Gulf of Aden provide insights on the structural evolution of segmentation from rifted margins to incipient seafloor spreading. In this study, we document the conjugate margins asymmetry, confirm the location of the ocean–continent transition (OCT) previously proposed by seismic data, and describe its deep structure and segmentation. In the OCT, gravity models indicate highly thinned crust while magnetic data indicate presence of non-oceanic high-amplitude magnetic anomalies where syn-rift sediments are not observed. Thus, the OCT could be made of ultra-stretched continental crust intruded by magmatic bodies. However, locally in the north, the nature of the OCT could be either an area of ultra-slow spreading oceanic crust or exhumed serpentinized mantle. Between the Alula-Fartak and Socotra fracture zones, the non-volcanic margins and the OCT are segmented by two N027°E-trending transfer fault zones. These transfer zones define three N110°E-trending segments that evolve through time. The first evidence of oceanic spreading corresponds to the magnetic anomaly A5d and is thus dated back to 17.6 Ma at least. Reconstruction of the spreading process suggests a complex non-uniform opening by an arc-like initiation of seafloor spreading in the OCT. The early segmentation appears to be directly related to the continental margin segmentation. The spreading axis segmentation evolved from three segments (17.6 to 10.95 Ma) to two segments (10.95 Ma to present). At the onset of the spreading process, the western segment propagated eastwards, thus reducing the size of the central segment. The presence of a propagator could explain the observed spreading asymmetry with the northern flank of the Sheba ridge being wider than the southern one.

Serpentinites in an Alpine convergent setting: Effects of metamorphic grade and deformation on microstructures

Abstract: Alpine antigorite serpentinites associated with eclogites were investigated to determine if they can be used as indicators of the tectono-metamorphic conditions during subduction and exhumation processes. The detailed petrology of serpentinites sampled in the Monviso massif (Western Alps, Italy) was combined with a transmission electron microscopy study. Alpine serpentinites display a degree of serpentinization close to 100%. Antigorite is the main mineral present, forming non-pseudomorphic textures in the various studied samples and exhibiting a homogeneous chemical composition with limited cationic substitutions. Considering its oceanic origin, the Alpine serpentinite in the Monviso massif formed a lizardite + chrysotile assemblage that recrystallized under greenschist-facies conditions into poorly ordered antigorite, with a modulation wavelength showing significant variations at the crystal scale. Under blueschist-facies conditions, the modulation wavelength of antigorite becomes regular. Thus, periodic antigorites can be related to high-grade conditions, while poorly ordered antigorites characterize lower metamorphic grade. In the present study, we failed to observe any elimination of structural defects with increasing metamorphic grade. While around 50% of the antigorite crystals are highly ordered, it seems that this ordering is at least partly obliterated by retrogressive deformation. Antigorite displays strong evidence of deformation-sensitivity, and the observed microstructures can be directly related to the mechanical behaviour of serpentinites in subduction zones. We investigated the deformation-induced microstructures in serpentinites collected in the Erro-Tobbio eclogitic unit (Ligurian Alps, Italy), which appear to preserve prograde and retrograde structures formed during subduction. According to the microstructural evidence, shearing is accommodated by brittle and/or ductile deformation mechanisms. Collected samples were fractured at different scales (cm to nm) and have a well-developed schistosity characterized by a strong crystallographic fabric. With increasing metamorphic grade, the brittle behaviour gives way to pressure-solution, which persists up to eclogite-facies conditions. The common obliteration of high-grade microstructures in antigorite, as observed in the Monviso serpentinites, results from continuous recrystallization of this mineral during retrogressive deformation.

Continental France and Belgium during the early Cretaceous: paleoweatherings and paleolandforms

Abstract: During the early Cretaceous, successive tectonic phases and several sea level falls resulted in the emersion of the main part of western Europe and the development of thick "lateritic" weathering. This long period of continental evolution ended with the Upper Cretaceous transgressions. During this period, the exposed lands displayed a mosaic of diverse morphologies and weathered landscapes. Bauxites are the most spectacular paleoweathering features, known for long in southern France. Recently, new residual outcrops have been identified, trapped in the karstic depressions of the Grands Causses. Other bauxitic formations, containing gibbsite, have also been recognised, occurring with the Clay-with-Jurassic-cherts in the southeastern border of the Paris Basin. These bauxitic formations overlay Jurassic limestone and are buried beneath Upper Cretaceous marine deposits. The recognition of bauxites up north into the southern Paris Basin significantly widens the extension of the Lower Cretaceous bauxitic paleolandscapes. On the Hercynian basements thick kaolinitic weathering mantles occur. They have been classically ascribed to the Tertiary. The first datings of these in situ paleosoils, by means of paleomagnetism and/or radiogenic isotopes, record especially early Cretaceous ages. This is the case for the "Siderolithic" formations on the edges of the French Massif Central, but also for the kaolinitic profiles in the Belgian Ardennes. In the Flanders, the Brabant basement is deeply kaolinised beneath the Upper Cretaceous cover. These paleosoils show polygenetic evolutions. The relief of these basement paleolandscapes may have been significant. There where probably high scarps (often of tectonic origin) reaching 200 m in elevation or beyond, as well as wide surfaces with inselbergs, as in the present day landscapes of tropical Africa and South America. On the Jurassic limestone platforms occur diverse kaolinitic and ferruginous weathering products. Around the Paris Basin they show various facies, ranging from kaolinitic saprolites to ferricretes. Due to the lack of sedimentary cover, the age of these ferruginous and kaolinitic weathering products has been debated for long, most often allocated to the Siderolithic sensu lato (Eocene-Oligocene). Recent datings by paleomagnetism have enabled to date them (Borne de Fer in eastern Paris Basin) back also to the early Cretaceous (130 ± 10 Ma). These wide limestone plateaus show karstified paleolandforms, such as vast closed and flat depressions broken by conical buttes, but also deep sinkholes in the higher areas of the plateaus and piedmonts. The depth of the karst hollows may be indicative of the range of relative paleoelevations. Dissolution holes display seldom contemporaneous karst fillings, thus implying that the karstland had not a thick weathering cover or that this cover had been stripped off before or by the late Cretaceous transgression. Nevertheless, some areas, especially above chert-bearing Jurassic limestone or marl, show weathering products trapped in the karst features or as a thick weathering mantle. In the Paris Basin, the Wealden gutter looked like a wide floodplain in which fluvio-deltaic sands and clays were deposited and on which paleosoils developed during times of non-deposition. The edges of the gutter were shaped as piedmonts linked up with the upstream basement areas. The rivers flowing down to the plain deposited lobes of coarse fluvial sands and conglomerates. The intensity of the weathering, the thickness of the profiles and their maturation are directly dependent on the duration of the emersion and the topographic location relative to the gutter. Near the axis of the gutter, where emersion was of limited duration, the paleoweathering features are restricted to rubefaction and argillization of the Lower Cretaceous marine formations. On the other hand, on the borders of the basin and on the Hercynian basement, where emersion was of longer duration, the weathering profiles are thicker and more intensively developed. The inventory of the Lower Cretaceous paleoweathering features shows the complexity of the continental history of this period. Moreover, the preserved weathering products are only a part of this long lasting period, all the aspects relative to erosion phases are still more difficult to prove and to quantify. In this domain, apatite fission tracks thermochronology (AFTT) can be helpful to estimate the order of magnitude of denudation. Residual testimonies and subsequent transgressions may enable to estimate relative elevations, but in return, we presently have no reliable tool to estimate absolute paleoelevations. In the work presented here, the inventory enabled to draw a continental paleogeographic map showing the nature of the weathering mantles and the paleolandscape features, just as paleoenvironments and paleobathymetry presently appear on marine paleogeographic maps. For the future, the challenge is to make progress in dating the paleoweathering profiles and especially in the resolution of these datings, in order to correlate precisely the continental records with the different events which trigger them (eustatism, climate, regional and global geodynamics). The final goal will be to build up a stratigraphic scale of the "continental geodynamic and climatic events" in parallel with "sequential stratigraphy" in the marine realm.

The oxidation state of vanadium in titanomagnetite from layered basic intrusions

Abstract: The redox conditions prevailing during the formation of vanadiferous titanomagnetites from three layered intrusions (Bushveld; Koillismaa; Skaergaard) have been estimated from the valence state of vanadium using synchrotron X-ray absorption near edge structure spectroscopy (XANES). Using a high energy-resolution X-ray emission spectrometer, we show that vanadium occurs mostly as V 3+ , with minor V 4+ . The most concentrated samples (up to 2.4 wt% V 2 O 3 ) contain approximately 10% of vanadium as V 4+ . Both V 3+ and V 4+ occur in the octahedral site of the spinel structure. Considering the low magnetite/melt V 4+ partition coefficients, this suggests that vanadium ores crystallized under specific oxidizing conditions.

Study of damage in ion-irradiated α-SiC by optical spectroscopy.

Abstract: UV–visible absorption and Raman scattering spectroscopy were used to investigate the effects of 4 MeV Xe-ion and 4 MeV Au-ion irradiations on α-SiC single crystals The evolution of transmission spectra upon irradiation evidences an increase of the optical absorption The optical band-gap energy decreases versus fluence, which is linked to band-gap closure attributed to the creation of localized states into the forbidden energy band A strong effect of the irradiation temperature is observed as a result of dynamic annealing enhanced by the temperature increase The Urbach energy increases versus fluence due to disorder accumulation in the damaged layer Comparison of Urbach energy and disorder parameters extracted from Raman spectra shows that the Urbach energy is sensitive to the disorder induced by the accumulation of point defects

Stokes flow through a channel with wavy walls

Abstract: Stokes flow is solved through a channel with three-dimensional wavy walls enclosed by two wavy walls whose amplitude is proportional to the mean clearance of the channel multiplied by the small dimensionless parameter ɛ. The application of an analytical-numerical algorithm yields efficient formulas for the velocities and permeability. These formulas include ɛ in symbolic form. When ɛ increases, the Poiseuille flow (ɛ=0) is disturbed and eddies can arise above a critical value ɛ =ɛ e . These results are also successfully compared to the ones derived by a fully numerical solution.

Evidence for symmetric cationic sites in zirconium-bearing oxide glasses

Abstract: The local environment of Zr in a borosilicate glass has been investigated by combining molecular dynamics (MD) simulations with Zr K-edge x-ray absorption spectroscopy (XAS) measurements. Two- and three-body distribution functions in agreement with the experimental data were obtained. The oxygen first neighbors around Zr are found to be narrowly distributed, revealing almost perfectly regular ZrO6 octahedra. Significant three-body contributions (Zr-O-O) to the XAS signal are observed, probing the existence of highly symmetric cationic sites in a glassy network. The gZrO(r) first peak derived from the experimental data is narrower than the one given by the MD model, thus providing elements for further improvements of the MD potentials.