4,461 research outputs found
Ab-initio calculation of the effect of stress on the chemical activity of graphene
Graphene layers are stable, hard, and relatively inert. We study how tensile
stress affects and bonds and the resulting change in the
chemical activity. Stress affects more strongly bonds that can become
chemically active and bind to adsorbed species more strongly. Upon stretch,
single C bonds are activated in a geometry mixing and ; an
intermediate state between and bonding. We use ab-initio
density functional theory to study the adsorption of hydrogen on large clusters
and 2D periodic models for graphene. The influence of the exchange-correlation
functional on the adsorption energy is discussed
Prolycopene, a Naturally Occurring Stereoisomer of Lycopene
In this paper we record the observation that there occurs in the variety of tomato called "tangerine tomato" a carotenoid, prolycopene, which is an isomer of lycopene; the isomeric relationship is similar to that between lycopene and neolycopene,I and in our opinion prolycopene is to be classed as a naturally occurring neolycopene, being the first observed natural neo form of a C40-carotenoid
Strongly correlated fermions on a kagome lattice
We study a model of strongly correlated spinless fermions on a kagome lattice
at 1/3 filling, with interactions described by an extended Hubbard Hamiltonian.
An effective Hamiltonian in the desired strong correlation regime is derived,
from which the spectral functions are calculated by means of exact
diagonalization techniques. We present our numerical results with a view to
discussion of possible signatures of confinement/deconfinement of fractional
charges.Comment: 10 pages, 10 figure
X-ray photoelectron spectroscopy studies of non-stoichiometric superconducting NbB2+x
Polycrystalline samples of NbB2+x with nominal composition (B/Nb) = 2.0, 2.1,
2.2, 2.3, 2.4 and 2.5 were studied by X-ray photoelectron spectroscopy (XPS).
The spectra revealed Nb and B oxides on the surface of the samples, mainly B2O3
and Nb2O5. After Ar ion etching the intensity of Nb and B oxides decreased. The
Nb 3d5/2 and B 1s core levels associated with the chemical states (B/Nb) were
identified and they do not change with etching time. The Binding Energy of the
Nb 3d5/2 and B 1s core levels increase as boron content increases, suggesting a
positive chemical shift in the core levels. On the other hand, analysis of
Valence Band spectra showed that the contribution of the Nb 4d states slightly
decreased while the contribution of the B 2p(pi) states increased as the boron
content increased. As a consequence, the electronic and superconducting
properties were substantially modified, in good agreement with band-structure
calculations.Comment: 10 pages, 7 figures, 1 tabl
Dielectric properties and lattice dynamics of alpha-PbO2-type TiO2: The role of soft phonon modes in pressure-induced phase transition to baddeleyite-type TiO2
Dielectric tensor and lattice dynamics of alpha-PbO2-type TiO2 have been
investigated using the density functional perturbation theory, with a focus on
responses of the vibrational frequencies to pressure. The calculated Raman
spectra under different pressures are in good agreement with available
experimental results and the symmetry assignments of the Raman peaks of
alpha-PbO2-type TiO2 are given for the first time. In addition, we identified
two anomalously IR-active soft phonon modes, B1u and B3u, respectively, around
200 cm-1 which have not been observed in high pressure experiments. Comparison
of the phonon dispersions at 0 and 10 GPa reveals that softening of phonon
modes also occurs for the zone-boundary modes. The B1u and B3u modes play an
important role in transformation from the alpha-PbO2-type phase to baddeleyite
phase. The significant relaxations of the oxygen atoms from the Ti4 plane in
the Ti2O2Ti2 complex of the baddeleyite phase are directly correlated to the
oxygen displacements along the directions given by the eigenvectors of the soft
B1u and B3u modes in the alpha-PbO2-type phase.Comment: 8 pages, 9 figure
Structural motifs of biomolecules
Biomolecular structures are assemblies of emergent anisotropic building
modules such as uniaxial helices or biaxial strands. We provide an approach to
understanding a marginally compact phase of matter that is occupied by proteins
and DNA. This phase, which is in some respects analogous to the liquid crystal
phase for chain molecules, stabilizes a range of shapes that can be obtained by
sequence-independent interactions occurring intra- and intermolecularly between
polymeric molecules. We present a singularityfree self-interaction for a tube
in the continuum limit and show that this results in the tube being positioned
in the marginally compact phase. Our work provides a unified framework for
understanding the building blocks of biomolecules.Comment: 13 pages, 5 figure
On the variability of return periods of European winter precipitation extremes over the last five centuries
International audienceWe investigate the changes of extreme European winter (December?February) precipitation over the last half millennium and show for various European regions that return periods of extremely wet and dry winters are subject to significant changes both before and after the onset of anthropogenic influences. Additionally, we examine the spatial pattern of the changes of the extremes covering the last 300 years where data quality is sufficient. Over central and eastern Europe dry winters occurred more frequently during the 18th and the second part of the 19th century relative to 1951?2000. Dry winters were less frequent during both the 18th and 19th century over the British Isles and the Mediterranean. Wet winters have been less abundant during the last three centuries compared to 1951?2000 except during the early 18th century in central Europe. Although winter precipitation extremes are affected by climate change, no obvious connection of these changes was found to solar, volcanic or anthropogenic forcing. However, physically meaningful interpretation with atmospheric circulation changes was possible
Realizing Colloidal Artificial Ice on Arrays of Optical Traps
We demonstrate how a colloidal version of artificial ice can be realized on
optical trap lattices. Using numerical simulations, we show that this system
obeys the ice rules and that for strong colloid-colloid interactions, an
ordered ground state appears. We show that the ice rule ordering can occur for
systems with as few as twenty-four traps and that the ordering transition can
be observed at constant temperature by varying the barrier strength of the
traps.Comment: 4 pages, 3 postscript figures; version to appear in Phys. Rev. Let
Impurity induced spin-orbit coupling in graphene
We study the effect of impurities in inducing spin-orbit coupling in
graphene. We show that the sp3 distortion induced by an impurity can lead to a
large increase in the spin-orbit coupling with a value comparable to the one
found in diamond and other zinc-blende semiconductors. The spin-flip scattering
produced by the impurity leads to spin scattering lengths of the order found in
recent experiments. Our results indicate that the spin-orbit coupling can be
controlled via the impurity coverage.Comment: 4 pages, 6 figure
Hydrogen on graphene: Electronic structure, total energy, structural distortions, and magnetism from first-principles calculations
Density functional calculations of electronic structure, total energy,
structural distortions, and magnetism for hydrogenated single-layer, bilayer,
and multi-layer graphene are performed. It is found that hydrogen-induced
magnetism can survives only at very low concentrations of hydrogen (single-atom
regime) whereas hydrogen pairs with optimized structure are usually
nonmagnetic. Chemisorption energy as a function of hydrogen concentration is
calculated, as well as energy barriers for hydrogen binding and release. The
results confirm that graphene can be perspective material for hydrogen storage.
Difference between hydrogenation of graphene, nanotubes, and bulk graphite is
discussed.Comment: 8 pages 8 figures (accepted to Phys. Rev. B
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