2,010 research outputs found
Statistics on parallelogram polyominoes and a q,t-analogue of the Narayana numbers
We study the statistics area, bounce and dinv on the set of parallelogram
polyominoes having a rectangular m times n bounding box. We show that the
bi-statistics (area, bounce) and (area, dinv) give rise to the same
q,t-analogue of Narayana numbers which was introduced by two of the authors in
[arXiv:1208.0024]. We prove the main conjectures of that paper: the
q,t-Narayana polynomials are symmetric in both q and t, and m and n. This is
accomplished by providing a symmetric functions interpretation of the
q,t-Narayana polynomials which relates them to the famous diagonal harmonics
Composition of Near-Earth Asteroid 2008 EV5: Potential target for Robotic and Human Exploration
We observed potentially hazardous asteroid (PHA) 2008 EV5 in the visible
(0.30-0.92 microns) and near-IR (0.75-2.5 microns) wavelengths to determine its
surface composition. This asteroid is especially interesting because it is a
potential target for two sample return mission proposals (Marco Polo-R and
Hayabusa-2) and human exploration due to its low delta-v for rendezvous. The
spectrum of 2008 EV5 is essentially featureless with exception of a weak
0.48-microns spin-forbidden Fe3+ absorption band. The spectrum also has an
overall blue slope. The albedo of 2008 EV5 remains uncertain with a lower limit
at 0.05 and a higher end at 0.20 based on thermal modeling. The Busch et al.
(2011) albedo estimate of 0.12 is consistent with our thermal modeling results.
The albedo and composition of 2008 EV5 are also consistent with a C-type
taxonomic classification (Somers et al. 2008). The best spectral match is with
CI carbonaceous chondrites similar to Orgueil, which also have a weak
0.48-microns feature and an overall blue slope. This 0.48-microns feature is
also seen in the spectrum of magnetite. The albedo of CI chondrites is at the
lower limit of our estimated range for the albedo of 2008 EV5.Comment: Pages: 19 Figures: 6 Tables:
Symmetry breaking in commensurate graphene rotational stacking; a comparison of theory and experiment
Graphene stacked in a Bernal configuration (60 degrees relative rotations
between sheets) differs electronically from isolated graphene due to the broken
symmetry introduced by interlayer bonds forming between only one of the two
graphene unit cell atoms. A variety of experiments have shown that non-Bernal
rotations restore this broken symmetry; consequently, these stacking varieties
have been the subject of intensive theoretical interest. Most theories predict
substantial changes in the band structure ranging from the development of a Van
Hove singularity and an angle dependent electron localization that causes the
Fermi velocity to go to zero as the relative rotation angle between sheets goes
to zero. In this work we show by direct measurement that non-Bernal rotations
preserve the graphene symmetry with only a small perturbation due to weak
effective interlayer coupling. We detect neither a Van Hove singularity nor any
significant change in the Fermi velocity. These results suggest significant
problems in our current theoretical understanding of the origins of the band
structure of this material.Comment: 7 pages, 6 figures, submitted to PR
A wide band gap metal-semiconductor-metal nanostructure made entirely from graphene
A blueprint for producing scalable digital graphene electronics has remained
elusive. Current methods to produce semiconducting-metallic graphene networks
all suffer from either stringent lithographic demands that prevent
reproducibility, process-induced disorder in the graphene, or scalability
issues. Using angle resolved photoemission, we have discovered a unique one
dimensional metallic-semiconducting-metallic junction made entirely from
graphene, and produced without chemical functionalization or finite size
patterning. The junction is produced by taking advantage of the inherent,
atomically ordered, substrate-graphene interaction when it is grown on SiC, in
this case when graphene is forced to grow over patterned SiC steps. This
scalable bottomup approach allows us to produce a semiconducting graphene strip
whose width is precisely defined within a few graphene lattice constants, a
level of precision entirely outside modern lithographic limits. The
architecture demonstrated in this work is so robust that variations in the
average electronic band structure of thousands of these patterned ribbons have
little variation over length scales tens of microns long. The semiconducting
graphene has a topologically defined few nanometer wide region with an energy
gap greater than 0.5 eV in an otherwise continuous metallic graphene sheet.
This work demonstrates how the graphene-substrate interaction can be used as a
powerful tool to scalably modify graphene's electronic structure and opens a
new direction in graphene electronics research.Comment: 11 pages, 7 figure
Social drivers forewarn of marine regime shifts
Some ecosystems can undergo regime shifts to alternative compositions of species. Although ecological indicators can identify approaching regime shifts, we propose that rapid changes in the social drivers underlying ecosystem change may provide additional and potentially earlier indicators of impending shifts. We demonstrate this by reconstructing the underlying social drivers of four iconic marine regime shifts: Pacific kelp forests, Northwest Atlantic continental shelf, Jamaican coral reefs, and the Chesapeake Bay estuary. In all cases, a range of social drivers – including opening of lucrative markets, technological innovations, and policies that enhanced the driver – ultimately prompted these ecosystem shifts. Drawing on examples emerging from environmental management practice, we present three practical recommendations for using social drivers as early indicators: monitor social change, determine social trigger points, and identify policy responses. We argue that accounting for the underlying social drivers of ecosystem change could improve decision making
Silicon intercalation into the graphene-SiC interface
In this work we use LEEM, XPEEM and XPS to study how the excess Si at the
graphene-vacuum interface reorders itself at high temperatures. We show that
silicon deposited at room temperature onto multilayer graphene films grown on
the SiC(000[`1]) rapidly diffuses to the graphene-SiC interface when heated to
temperatures above 1020. In a sequence of depositions, we have been able to
intercalate ~ 6 ML of Si into the graphene-SiC interface.Comment: 6 pages, 8 figures, submitted to PR
First direct observation of a nearly ideal graphene band structure
Angle-resolved photoemission and X-ray diffraction experiments show that
multilayer epitaxial graphene grown on the SiC(000-1) surface is a new form of
carbon that is composed of effectively isolated graphene sheets. The unique
rotational stacking of these films cause adjacent graphene layers to
electronically decouple leading to a set of nearly independent linearly
dispersing bands (Dirac cones) at the graphene K-point. Each cone corresponds
to an individual macro-scale graphene sheet in a multilayer stack where
AB-stacked sheets can be considered as low density faults.Comment: 5 pages, 4 figure
- …