IPG Photonics

About: IPG Photonics is a based out in . It is known for research contribution in the topics: Laser & Fiber laser. The organization has 903 authors who have published 1241 publications receiving 63339 citations.

Ab initio study of gap opening and screening effects in gated bilayer graphene

Abstract: The electronic properties of doped bilayer graphene in presence of bottom and top gates have been studied and characterized by means of density-functional theory (DFT) calculations. Varying independently the bottom and top gates it is possible to control separately the total doping charge on the sample and the average external electric field acting on the bilayer. We show that, at fixed doping level, the band gap at the $K$ point in the Brillouin zone depends linearly on the average electric field, whereas the corresponding proportionality coefficient has a nonmonotonic dependence on doping. We find that the DFT-calculated band gap at $K$, for small doping levels, is roughly half of the band gap obtained with standard tight-binding (TB) approach. We show that this discrepancy arises from an underestimate, in the TB model, of the screening of the system to the external electric field. In particular, on the basis of our DFT results we observe that, when bilayer graphene is in presence of an external electric field, both interlayer and intralayer screenings occur. Only the interlayer screening is included in TB calculations, while both screenings are fundamental for the description of the band-gap opening. We finally provide a general scheme to obtain the full band structure of gated bilayer graphene for an arbitrary value of the external electric field and of doping.

Insights into Magma Genesis at Convergent Margins from U-series Isotopes

Abstract: Convergent margins (oceanic and continental arcs) form one of the Earth’s key mass transfer locations, being sites where melting and transfer of new material to the Earth’s crust occurs and also where crustal materials, including water, are recycled back into the mantle. Volcanism in this tectonic setting constitutes ~15% (0.4–0.6 km3/yr) of the total global output (Crisp 1984) and the composition of the erupted magmas is, on average, similar to that of the continental crust (Taylor and McLennan 1981). Moreover, many arc volcanoes have been responsible for the most hazardous, historic volcanic eruptions. Yet, despite their importance, many fundamental aspects of convergent margin magmatism remain poorly understood. Key among these are the rates of processes of fluid addition from the subducting plate. Furthermore, in stark contrast to the ocean ridges, where adiabatic decompression provides a simple and robust physical model for partial melting, no consensus has yet been reached about the physics of the partial melting process and the mechanism of melt extraction beneath arcs. Preceding chapters concerned with partial melting in this volume (Lundstrom 2003; Bourdon and Sims 2003) have discussed how the differing half-lives and distribution coefficients of the various U-series nuclides result in disequilibria through in-growth. This provides important information on the nature and timing of mantle partial melting processes. In convergent margin settings the differential fluid mobility of U and Ra relative to Th and Pa provides an additional source of fractionation leading to in-growth and this is crucial to understanding the timing and mechanisms of fluid addition. Here we review the role that the proliferation of high quality U-series isotope data, over the last decade, have had in obtaining precise information on time scales and the development of quantitative physical models for convergent margin magmatism. Our approach is to use trace element …

Aluminium X-ray absorption Near Edge Structure in model compounds and Earth’s surface minerals

Abstract: Aluminium K-edge X-ray absorption near edge spectra (XANES) of a suite of silicate and oxides minerals consist of electronic excitations occurring in the edge region, and multiple scattering resonances at higher energies. The main XANES feature for four-fold Al is at around 2 eV lower energy than the main XANES feature for six-fold Al. This provides a useful probe for coordination numbers in clay minerals, gels, glasses or material with unknown Al-coordination number. Six-fold aluminium yields a large variety of XANES features which can be correlated with octahedral point symmetry, number of aluminium sites and distribution of Al-O distances. These three parameters may act together, and the quantitative interpretation of XANES spectra is difficult. For a low point symmetry (1), variations are mainly related to the number of Al sites and distribution of Al-O distances: pyrophyllite, one Al site, is clearly distinguished from kaolinite and gibbsite presenting two Al sites. For a given number of Al-site (1), variations are controlled by changes in point symmetry, the number of XANES features being increased as point symmetry decreases. For a given point symmetry (1) and a given number of Al site (1), variations are related to second nearest neighbours (gibbsite versus kaolinite). The amplitude of the XANES feature at about 1566 eV is a useful probe for the assessment of AlIV/Altotal ratios in 2/1 phyllosilicates. Al-K XANES has been performed on synthetic Al-bearing goethites which cannot be studied by 27Al NMR. At low Al content, Al-K XANES is very different from that of α-AlOOH but at the highest level, XANES spectrum tends to that of diaspore. Al-K XAS is thus a promising tool for the structural study of poorly ordered materials such as clay minerals and natural alumino-silicate gels together with Al-subsituted Fe-oxyhydroxides.

The accretion, composition and early differentiation of Mars

Abstract: The early development of Mars is of enormous interest, not just in its own right, but also because it provides unique insights into the earliest history of the Earth, a planet whose origins have been all but obliterated. Mars is not as depleted in moderately volatile elements as are other terrestrial planets. Judging by the data for Martian meteorites it has Rb/Sr ≈ 0.07 and K/U ≈ 19,000, both of which are roughly twice as high as the values for the Earth. The mantle of Mars is also twice as rich in Fe as the mantle of the Earth, the Martian core being small (~20% by mass). This is thought to be because conditions were more oxidizing during core formation. For the same reason a number of elements that are moderately siderophile on Earth such as P, Mn, Cr and W, are more lithophile on Mars. The very different apparent behavior of high field strength (HFS) elements in Martian magmas compared to terrestrial basalts and eucrites may be related to this higher phosphorus content. The highly siderophile element abundance patterns have been interpreted as reflecting strong partitioning during core formation in a magma ocean environment with little if any late veneer. Oxygen isotope data provide evidence for the relative proportions of chondritic components that were accreted to form Mars. However, the amount of volatile element depletion predicted from these models does not match that observed — Mars would be expected to be more depleted in volatiles than the Earth. The easiest way to reconcile these data is for the Earth to have lost a fraction of its moderately volatile elements during late accretionary events, such as giant impacts. This might also explain the nonchondritic Si/Mg ratio of the silicate portion of the Earth. The lower density of Mars is consistent with this interpretation, as are isotopic data. 87Rb-87Sr, 129I-129Xe, 146Sm-142Nd, 182Hf-182W, 187Re-187Os, 235U-207Pb and 238U-206Pb isotopic data for Martian meteorites all provide evidence that Mars accreted rapidly and at an early stage differentiated into atmosphere, mantle and core. Variations in heavy xenon isotopes have proved complicated to interpret in terms of 244Pu decay and timing because of fractionation thought to be caused by hydrodynamic escape. There are, as yet, no resolvable isotopic heterogeneities identified in Martian meteorites resulting from 92Nb decay to 92Zr, consistent with the paucity of perovskite in the martian interior and its probable absence from any Martian magma ocean. Similarly the longer-lived 176Lu-176Hf system also preserves little record of early differentiation. In contrast W isotope data, Ba/W and time-integrated Re/Os ratios of Martian meteorites provide powerful evidence that the mantle retains remarkably early heterogeneities that are vestiges of core metal segregation processes that occurred within the first 20 Myr of the Solar System. Despite this evidence for rapid accretion and differentiation, there is no evidence that Mars grew more quickly than the Earth at an equivalent size. Mars appears to have just stopped growing earlier because it did not undergo late stage (>20 Myr) impacts on the scale of the Moon-forming Giant Impact that affected the Earth.