3,060 research outputs found
Excitonic Effects and Optical Spectra of Single-Walled Carbon Nanotubes
Many-electron effects often dramatically modify the properties of reduced
dimensional systems. We report calculations, based on an many-electron Green's
function approach, of electron-hole interaction effects on the optical spectra
of small-diameter single-walled carbon nanotubes. Excitonic effects
qualitatively alter the optical spectra of both semiconducting and metallic
tubes. Excitons are bound by ~ 1 eV in the semiconducting (8,0) tube and by ~
100 meV in the metallic (3,3) tube. These large many-electron effects explain
the discrepancies between previous theories and experiments.Comment: 6 pages, 3 figures, 2 table
Transport and Use of a Centaur Second Stage in Space
As nations continue to explore space, the desire to reduce costs will continue to grow. As a method of cost reduction, transporting and/or use of launch system components as integral components of missions may become more commonplace in the future. There have been numerous scenarios written for using launch vehicle components (primarily space shuttle used external tanks) as part of flight missions or future habitats. Future studies for possible uses of launch vehicle upper stages might include asteroid diverter using gravity orbital perturbation, orbiting station component, raw material at an outpost, and kinetic impactor. The LCROSS (Lunar CRater Observation and Sensing Satellite) mission was conceived as a low-cost means of determining whether water exists at the polar regions of the moon. Manifested as a secondary payload with the LRO (Lunar Reconnaissance Orbiter) spacecraft aboard an Atlas V launch vehicle, LCROSS guided its spent Centaur Earth Departure Upper Stage (EDUS) into the lunar crater Cabeu's, as a kinetic impactor. This paper describes some of the challenges that the LCROSS project encountered in planning, designing, launching with and carrying the Centaur upper stage to the moon
Tight--binding description of the quasiparticle dispersion of graphite and few--layer graphene
A universal set of third--nearest neighbour tight--binding (TB) parameters is
presented for calculation of the quasiparticle (QP) dispersion of stacked
graphene layers () with stacking sequence. The QP
bands are strongly renormalized by electron--electron interactions which
results in a 20% increase of the nearest neighbour in--plane and out--of--plane
TB parameters when compared to band structure from density functional theory.
With the new set of TB parameters we determine the Fermi surface and evaluate
exciton energies, charge carrier plasmon frequencies and the conductivities
which are relevant for recent angle--resolved photoemission, optical, electron
energy loss and transport measurements. A comparision of these quantitities to
experiments yields an excellent agreement. Furthermore we discuss the
transition from few layer graphene to graphite and a semimetal to metal
transition in a TB framework.Comment: Corresponding author: A. Gr\"uneis Tel.: +49 351 4659 519 e--mail:
[email protected]
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A ribose-functionalized NAD+ with unexpected high activity and selectivity for protein poly-ADP-ribosylation.
Nicotinamide adenine dinucleotide (NAD+)-dependent ADP-ribosylation plays important roles in physiology and pathophysiology. It has been challenging to study this key type of enzymatic post-translational modification in particular for protein poly-ADP-ribosylation (PARylation). Here we explore chemical and chemoenzymatic synthesis of NAD+ analogues with ribose functionalized by terminal alkyne and azido groups. Our results demonstrate that azido substitution at 3'-OH of nicotinamide riboside enables enzymatic synthesis of an NAD+ analogue with high efficiency and yields. Notably, the generated 3'-azido NAD+ exhibits unexpected high activity and specificity for protein PARylation catalyzed by human poly-ADP-ribose polymerase 1 (PARP1) and PARP2. And its derived poly-ADP-ribose polymers show increased resistance to human poly(ADP-ribose) glycohydrolase-mediated degradation. These unique properties lead to enhanced labeling of protein PARylation by 3'-azido NAD+ in the cellular contexts and facilitate direct visualization and labeling of mitochondrial protein PARylation. The 3'-azido NAD+ provides an important tool for studying cellular PARylation
Anomalous insulator metal transition in boron nitride-graphene hybrid atomic layers
The study of two-dimensional (2D) electronic systems is of great fundamental
significance in physics. Atomic layers containing hybridized domains of
graphene and hexagonal boron nitride (h-BNC) constitute a new kind of
disordered 2D electronic system. Magneto-electric transport measurements
performed at low temperature in vapor phase synthesized h-BNC atomic layers
show a clear and anomalous transition from an insulating to a metallic behavior
upon cooling. The observed insulator to metal transition can be modulated by
electron and hole doping and by the application of an external magnetic field.
These results supported by ab-initio calculations suggest that this transition
in h-BNC has distinctly different characteristics when compared to other 2D
electron systems and is the result of the coexistence between two distinct
mechanisms, namely, percolation through metallic graphene networks and hopping
conduction between edge states on randomly distributed insulating h-BN domains.Comment: 9 pages, 15 figure
Disorder, pseudospins, and backscattering in carbon nanotubes
We address the effects of disorder on the conducting properties of metal and
semiconducting carbon nanotubes. Experimentally, the mean free path is found to
be much larger in metallic tubes than in doped semiconducting tubes. We show
that this result can be understood theoretically if the disorder potential is
long-ranged. The effects of a pseudospin index that describes the internal
sublattice structure of the states lead to a suppression of scattering in
metallic tubes, but not in semiconducting tubes. This conclusion is supported
by tight-binding calculations.Comment: four page
Ab-initio theory of NMR chemical shifts in solids and liquids
We present a theory for the ab-initio computation of NMR chemical shifts
(sigma) in condensed matter systems, using periodic boundary conditions. Our
approach can be applied to periodic systems such as crystals, surfaces, or
polymers and, with a super-cell technique, to non-periodic systems such as
amorphous materials, liquids, or solids with defects. We have computed the
hydrogen sigma for a set of free molecules, for an ionic crystal, LiH, and for
a H-bonded crystal, HF, using density functional theory in the local density
approximation. The results are in excellent agreement with experimental data.Comment: to appear in Physical Review Letter
Anomalous Quasiparticle Lifetime in Graphite: Band Structure Effects
We report ab initio calculation of quasiparticle lifetimes in graphite, as
determined from the imaginary part of the self-energy operator within the GW
aproximation. The inverse lifetime in the energy range from 0.5 to 3.5 eV above
the Fermi level presents significant deviations from the quadratic behavior
naively expected from Fermi liquid theory. The deviations are explained in
terms of the unique features of the band structure of this material. We also
discuss the experimental results from different groups and make some
predictions for future experiments.Comment: 4 pages, 4 figures, submitted PR
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Strong correlations and orbital texture in single-layer 1T-TaSe2
Strong electron correlation can induce Mott insulating behaviour and produce intriguing states of matter such as unconventional superconductivity and quantum spin liquids. Recent advances in van der Waals material synthesis enable the exploration of Mott systems in the two-dimensional limit. Here we report characterization of the local electronic properties of single- and few-layer 1T-TaSe2 via spatial- and momentum-resolved spectroscopy involving scanning tunnelling microscopy and angle-resolved photoemission. Our results indicate that electron correlation induces a robust Mott insulator state in single-layer 1T-TaSe2 that is accompanied by unusual orbital texture. Interlayer coupling weakens the insulating phase, as shown by reduction of the energy gap and quenching of the correlation-driven orbital texture in bilayer and trilayer 1T-TaSe2. This establishes single-layer 1T-TaSe2 as a useful platform for investigating strong correlation physics in two dimensions
Interplay between electron-phonon and Coulomb interactions in cuprates
Evidence for strong electron-phonon coupling in high-Tc cuprates is reviewed,
with emphasis on the electron and phonon spectral functions. Effects due to the
interplay between the Coulomb and electron-phonon interactions are studied. For
weakly doped cuprates, the phonon self-energy is strongly reduced due to
correlation effects, while there is no corresponding strong reduction for the
electron self-energy. Polaron formation is studied, focusing on effects of
Coulomb interaction and antiferromagnetic correlations. It is argued that
experimental indications of polaron formation in undoped cuprates are due to a
strong electron-phonon interaction for these systems.Comment: 43 pages and 22 figure
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