Trace element and isotope constraints on crustal anatexis by upwelling mantle melts in the North Atlantic Igneous Province: an example form the Isle of Rum, NW Scotland

Sr and Nd isotope ratios, together with lithophile trace elements, have been measured in a representative set of igneous rocks and Lewisian gneisses from the Isle of Rum in order to unravel the petrogenesis of the felsic rocks that erupted in the early stages of Palaeogene magmatism in the North Atlantic Igneous Province (NAIP). The Rum rhyodacites appear to be the products of large amounts of melting of Lewisian amphibolite gneiss. The Sr and Nd isotopic composition of the magmas can be explained without invoking an additional granulitic crustal component. Concentrations of the trace element Cs in the rhyodacites strongly suggests that the gneiss parent rock had experienced Cs and Rb loss prior to Palaeogene times, possibly during a Caledonian event. This depletion caused heterogeneity with respect to 87Sr/86Sr in the crustal source of silicic melts. Other igneous rock types on Rum (dacites, early gabbros) are mixtures of crustalmelts and and primarymantle melts. Forward Rare Earth Element modelling shows that late stage picritic melts on Rum are close analogues for the parent melts of the Rum Layered Suite, and for the mantle melts that caused crustal anatexis of the Lewisian gneiss. These primary mantle melts have close affinities to Mid-Oceanic Ridge Basalts (MORB), whose trace element content varies from slightly depleted to slightly enriched. Crustal anatexis is a common process in the rift-to-drift evolution during continental break-up and the formation of Volcanic Rifted Margins systems. The ‘early felsic–later mafic’ volcanic rock associations from Rum are compared to similar associations recovered from the now-drowned seaward-dipping wedges on the shelf of SE Greenland and on the Vøring Plateau (Norwegian Sea). These three regions show geochemical differences that result from variations in the regional crustal composition and the depth at which crustal anatexis took place

Localisation and mass generation for non-Abelian gauge fields

It has been suggested recently that in the presence of suitably "warped" extra dimensions, the low-energy limit of pure gauge field theory may contain massive elementary vector bosons localised on a "brane", but no elementary Higgs scalars. We provide non-perturbative evidence in favour of this conjecture through numerical lattice measurements of the static quark-antiquark force of pure SU(2) gauge theory in three dimensions, of which one is warped. We consider also warpings leading to massless localised vector bosons, and again find evidence supporting the perturbative prediction, even though the gauge coupling diverges far from the brane in this case.Comment: 27 pages; small clarifications adde

Non-invasive laminar inference with MEG: comparison of methods and source inversion algorithms

Magnetoencephalography (MEG) is a direct measure of neuronal current flow; its anatomical resolution is therefore not constrained by physiology but rather by data quality and the models used to explain these data. Recent simulation work has shown that it is possible to distinguish between signals arising in the deep and superficial cortical laminae given accurate knowledge of these surfaces with respect to the MEG sensors. This previous work has focused around a single inversion scheme (multiple sparse priors) and a single global parametric fit metric (free energy). In this paper we use several different source inversion algorithms and both local and global, as well as parametric and non-parametric fit metrics in order to demonstrate the robustness of the discrimination between layers. We find that only algorithms with some sparsity constraint can successfully be used to make laminar discrimination. Importantly, local t-statistics, global cross-validation and free energy all provide robust and mutually corroborating metrics of fit. We show that discrimination accuracy is affected by patch size estimates, cortical surface features, and lead field strength, which suggests several possible future improvements to this technique. This study demonstrates the possibility of determining the laminar origin of MEG sensor activity, and thus directly testing theories of human cognition that involve laminar- and frequency-specific mechanisms. This possibility can now be achieved using recent developments in high precision MEG, most notably the use of subject-specific head-casts, which allow for significant increases in data quality and therefore anatomically precise MEG recordings

A dedicated haem lyase is required for the maturation of a novel bacterial cytochrome c with unconventional covalent haem binding

In bacterial c-type cytochromes, the haem cofactor is covalently attached via two cysteine residues organized in a haem c-binding motif. Here, a novel octa-haem c protein, MccA, is described that contains only seven conventional haem c-binding motifs (CXXCH), in addition to several single cysteine residues and a conserved CH signature. Mass spectrometric analysis of purified MccA from Wolinella succinogenes suggests that two of the single cysteine residues are actually part of an unprecedented CX15CH sequence involved in haem c binding. Spectroscopic characterization of MccA identified an unusual high-potential haem c with a red-shifted absorption maximum, not unlike that of certain eukaryotic cytochromes c that exceptionally bind haem via only one thioether bridge. A haem lyase gene was found to be specifically required for the maturation of MccA in W. succinogenes. Equivalent haem lyase-encoding genes belonging to either the bacterial cytochrome c biogenesis system I or II are present in the vicinity of every known mccA gene suggesting a dedicated cytochrome c maturation pathway. The results necessitate reconsideration of computer-based prediction of putative haem c-binding motifs in bacterial proteomes

Ab initio study of ferroelectric domain walls in PbTiO3

We have investigated the atomistic structure of the 180-degree and 90-degree domain boundaries in the ferroelectric perovskite compound PbTiO3 using a first-principles ultrasoft-pseudopotential approach. For each case we have computed the position, thickness and creation energy of the domain walls, and an estimate of the barrier height for their motion has been obtained. We find both kinds of domain walls to be very narrow with a similar width of the order of one to two lattice constants. The energy of the 90-dergree domain wall is calculated to be 35 mJ/m^2, about a factor of four lower than the energy of its 180-degree counterpart, and only a miniscule barrier for its motion is found. As a surprising feature we detected a small offset of 0.15-0.2 eV in the electrostatic potential across the 90-degree domain wall.Comment: 12 pages, with 9 postscript figures embedded. Uses REVTEX and epsf macros. Also available at http://www.physics.rutgers.edu/~dhv/preprints/bm_dw/index.htm

Sequence effects in donor-acceptor oligomeric semiconductors comprising benzothiadiazole and phenylenevinylene monomers

To understand the influence of monomer sequence on the properties and performance of conjugated oligomers, a series of dimers, trimers, and tetramers were prepared from phenylene (P) and benzothiadiazole (B) monomers linked by vinylene groups. Optical and electrochemical studies established the influence of sequence on both the λmax and redox potentials of this series of structurally related oligomers. For tetramers with bromo end groups (PBBP, BPPB, PBPB, PPBB), the λmax ranged from 493 to 512 nm (Δ = 19 nm), the electrochemical oxidation potential from 0.65 to 0.82 (Δ = 0.17 V) and the reduction potential from-1.45 to-1.31 (Δ = 0.14 V), all of which are sequence-dependent. The effect of end groups (cyano, bromo, and alkyl) was also demonstrated to be important for the properties of these oligomers. DFT calculations of the tetramers were performed and the energy levels were correlated well with the experimentally determined spectroscopic data. Bulk heterojunction (BHJ) solar cells fabricated with selected tetramers as the donor and PC61BM as the acceptor exhibited power conversion efficiencies that varied by a factor of 3 as a function of sequence (0.47-1.85%). These results suggest that sequence control is important for tuning optoelectronic properties and photovoltaic performance of these structurally related conjugated oligomers

Discrete cilia modelling with singularity distributions

We discuss in detail techniques for modelling flows due to finite and infinite arrays of beating cilia. An efficient technique, based on concepts from previous ‘singularity models’ is described, that is accurate in both near and far-fields. Cilia are modelled as curved slender ellipsoidal bodies by distributing Stokeslet and potential source dipole singularities along their centrelines, leading to an integral equation that can be solved using a simple and efficient discretisation. The computed velocity on the cilium surface is found to compare favourably with the boundary condition. We then present results for two topics of current interest in biology. 1) We present the first theoretical results showing the mechanism by which rotating embryonic nodal cilia produce a leftward flow by a ‘posterior tilt,’ and track particle motion in an array of three simulated nodal cilia. We find that, contrary to recent suggestions, there is no continuous layer of negative fluid transport close to the ciliated boundary. The mean leftward particle transport is found to be just over 1 μm/s, within experimentally measured ranges. We also discuss the accuracy of models that represent the action of cilia by steady rotlet arrays, in particular, confirming the importance of image systems in the boundary in establishing the far-field fluid transport. Future modelling may lead to understanding of the mechanisms by which morphogen gradients or mechanosensing cilia convert a directional flow to asymmetric gene expression. 2) We develop a more complex and detailed model of flow patterns in the periciliary layer of the airway surface liquid. Our results confirm that shear flow of the mucous layer drives a significant volume of periciliary liquid in the direction of mucus transport even during the recovery stroke of the cilia. Finally, we discuss the advantages and disadvantages of the singularity technique and outline future theoretical and experimental developments required to apply this technique to various other biological problems, particularly in the reproductive system

Statistical Survey of Type III Radio Bursts at Long Wavelengths Observed by the Solar TErrestrial RElations Observatory (STEREO)/Waves Instruments: Radio Flux Density Variations with Frequency

We have performed a statistical study of $152$ Type III radio bursts observed by Solar TErrestrial RElations Observatory (STEREO)/Waves between May 2007 and February 2013. We have investigated the flux density between $125$kHz and $16$MHz. Both high- and low-frequency cutoffs have been observed in $60\,$ of events suggesting an important role of propagation. As already reported by previous authors, we observed that the maximum flux density occurs at $1$MHz on both spacecraft. We have developed a simplified analytical model of the flux density as a function of radial distance and compared it to the STEREO/Waves data.Comment: published in Solar Physic

Photoproduction of mesons in nuclei at GeV energies

In a transport model that combines initial state interactions of the photon with final state interactions of the produced particles we present a calculation of inclusive photoproduction of mesons in nuclei in the energy range from 1 to 7 GeV. We give predictions for the photoproduction cross sections of pions, etas, kaons, antikaons, and $\pi^+\pi^-$ invariant mass spectra in ^{12}C and ^{208}Pb. The effects of nuclear shadowing and final state interaction of the produced particles are discussed in detail.Comment: Text added in summary in general reliability of the method, references updated. Phys. Rev. C (2000) in pres

Doping-dependent study of the periodic Anderson model in three dimensions

We study a simple model for $f$-electron systems, the three-dimensional periodic Anderson model, in which localized $f$ states hybridize with neighboring $d$ states. The $f$ states have a strong on-site repulsion which suppresses the double occupancy and can lead to the formation of a Mott-Hubbard insulator. When the hybridization between the $f$ and $d$ states increases, the effects of these strong electron correlations gradually diminish, giving rise to interesting phenomena on the way. We use the exact quantum Monte-Carlo, approximate diagrammatic fluctuation-exchange approximation, and mean-field Hartree-Fock methods to calculate the local moment, entropy, antiferromagnetic structure factor, singlet-correlator, and internal energy as a function of the $f-d$ hybridization for various dopings. Finally, we discuss the relevance of this work to the volume-collapse phenomenon experimentally observed in f-electron systems.Comment: 12 pages, 8 figure