6,382 research outputs found
Quantum Hall effect in polycrystalline graphene: The role of grain boundaries
We use numerical simulations to predict peculiar magnetotransport
fingerprints in polycrystalline graphene, driven by the presence of grain
boundaries of varying size and orientation. The formation of Landau levels is
shown to be restricted by the polycrystalline morphology, requiring the
magnetic length to be smaller than the average grain radius. The nature of
localization is also found to be unusual, with strongly localized states at the
center of Landau levels (including the usually highly robust zero-energy state)
and extended electronic states lying between Landau levels. These extended
states percolate along the network of grain boundaries, resulting in a finite
value for the bulk dissipative conductivity and suppression of the quantized
Hall conductance. Such breakdown of the quantum Hall regime provoked by
extended structural defects is also illustrated through two-terminal
Landauer-B\"uttiker conductance calculations, indicating how a single grain
boundary induces cross-linking between edge states lying at opposite sides of a
ribbon geometry
Airborne observations of methane in Comet Kohoutek
The experiment is described for airborne observations of Comet Kohoutek using an infrared tilting-filter photometer. Preliminary analysis of the data established an upper limit to the Comet's fluorescence radiation in methane lines at 3.3 microns
Spin Hall effect and Weak Antilocalization in Graphene/Transition Metal Dichalcogenide Heterostructures
We report on a theoretical study of the spin Hall Effect (SHE) and weak
antilocal-ization (WAL) in graphene/transition metal dichalcogenide (TMDC)
heterostructures, computed through efficient real-space quantum transport
methods, and using realistic tight-binding models parametrized from ab initio
calculations. The graphene/WS 2 system is found to maximize spin proximity
effects compared to graphene on MoS 2 , WSe 2 , or MoSe 2 , with a crucial role
played by disorder, given the disappearance of SHE signals in the presence of
strong intervalley scattering. Notably, we found that stronger WAL effects are
concomitant with weaker charge-to-spin conversion efficiency. For further
experimental studies of graphene/TMDC heterostructures, our findings provide
guidelines for reaching the upper limit of spin current formation and for fully
harvesting the potential of two-dimensional materials for spintronic
applications.Comment: This document is the unedited Author's version of a Submitted Work
that was subsequently accepted for publication in Nano Letters,
copyright\c{opyright}American Chemical Society after peer review. To access
the final edited and published work see
http://pubs.acs.org/articlesonrequest/AOR-c2pZ8WnmG7pcF4MIivj
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Evapotranspiration Mapping for Forest Management in California's Sierra Nevada
We assessed the response of densely forested watersheds with little apparent annual water limitation to forest disturbance
and climate variability, by studying how past wildfires changed forest evapotranspiration, and what past evapotranspiration
patterns imply for the availability of subsurface water storage for drought resistance. We determined annual spatial patterns
of evapotranspiration using a top-down statistical model, correlating measured annual evapotranspiration from eddycovariance
towers across California with NDVI (Normalized Difference Vegetation Index) measured by satellite, and with
annual precipitation. The study area was the Yuba and American River watersheds, two densely forested watersheds in the
northern Sierra Nevada. Wildfires in the 1985-2015 period resulted in significant post-fire reductions in evapotranspiration
for at least 5 years, and in some cases for more than 20 years. The levels of biomass removed in medium-intensity fires (25-
75% basal area loss), similar to magnitudes expected from forest treatments for fuels reduction and forest health, reduced
evapotranspiration by as much 150-200 mm yr-1 for the first 5 years. Rates of recovery in post-wildfire evapotranspiration
confirm the need for follow-up forest treatments at intervals of 5-20 years to sustain lower evapotranspiration, depending
on local landscape attributes and interannual climate. Using the metric of cumulative precipitation minus evapotranspiration
(P-ET) during multi-year dry periods, we found that forests in the study area showed little evidence of moisture stress
during the 1985-2018 period of our analysis, owing to relatively small reliance on interannual subsurface water storage to
meet dry-year evapotranspiration needs of vegetation. However, more-severe or sustained drought periods will push some
lower-elevation forests in the area studied toward the cumulative P-ET thresholds previously associated with widespread
forest mortality in the southern Sierra Nevada
Spin transport in graphene/transition metal dichalcogenide heterostructures
Since its discovery, graphene has been a promising material for spintronics:
its low spin-orbit coupling, negligible hyperfine interaction, and high
electron mobility are obvious advantages for transporting spin information over
long distances. However, such outstanding transport properties also limit the
capability to engineer active spintronics, where strong spin-orbit coupling is
crucial for creating and manipulating spin currents. To this end, transition
metal dichalcogenides, which have larger spin-orbit coupling and good interface
matching, appear to be highly complementary materials for enhancing the
spin-dependent features of graphene while maintaining its superior charge
transport properties. In this review, we present the theoretical framework and
the experiments performed to detect and characterize the spin-orbit coupling
and spin currents in graphene/transition metal dichalcogenide heterostructures.
Specifically, we will concentrate on recent measurements of Hanle precession,
weak antilocalization and the spin Hall effect, and provide a comprehensive
theoretical description of the interconnection between these phenomena.Comment: 21 pages, 11 figures. This document is the unedited Author's version
of a Submitted Work that was subsequently accepted for publication in Nano
Letters, copyright\c{opyright}American Chemical Society after peer review. To
access the final edited and published work see
http://pubs.rsc.org/en/Content/ArticleLanding/2018/CS/C7CS00864
Extracting nucleon strange and anapole form factors from world data
The complete world set of parity violating electron scattering data up to
Q^2~0.3 GeV^2 is analysed. We extract the current experimental determination of
the strange electric and magnetic form factors of the proton, as well as the
weak axial form factors of the proton and neutron, at Q^2 = 0.1 GeV^2. Within
experimental uncertainties, we find that the strange form factors are
consistent with zero, as are the anapole contributions to the axial form
factors. Nevertheless, the correlation between the strange and anapole
contributions suggest that there is only a small probability that these form
factors all vanish simultaneously.Comment: 4 pages, 3 figs; v2: version to appear in PR
Testing the Standard Model by precision measurement of the weak charges of quarks
In a global analysis of the latest parity-violating electron scattering
measurements on nuclear targets, we demonstrate a significant improvement in
the experimental knowledge of the weak neutral-current lepton-quark
interactions at low energy. The precision of this new result, combined with
earlier atomic parity-violation measurements, places tight constraints on the
size of possible contributions from physics beyond the Standard Model.
Consequently, this result improves the lower-bound on the scale of relevant new
physics to ~1 TeV.Comment: 4 pages, 3 figures; v2: further details on extraction of electroweak
parameters, new figur
Association between body condition score and live weight in pasture-based Holstein-Friesian dairy cows
peer-reviewedThe objective was to quantify the strength of the relationship between body condition score (BCS) and live weight (LW) in pasture-based Holstein-Friesian dairy cattle, and to determine the kg LW per unit BCS. A total of 26021 test-day records with information on both BCS (1–10 scale, where 1 is emaciated and 10 is obese) and LW across 1110 lactations from one research farm were used in the analysis. Correlation and regression analyses were used to determine the degree of association between BCS and LW in different parities, stages of the inter-calving interval and years. Correlations between BCS and LW were relatively consistent, with the mean correlation between BCS and LW across all data of 0·55 implying that differences in BCS explain approximately 30% of the variation in LW. Significantly different regressions of LW on BCS were present within stage of inter-calving interval by parity subclasses. Excluding calving, LW per unit BCS varied from 17 kg (early to mid lactation in parity 1) to 36 kg (early lactation in parity 4 and 5). However, LW per unit BCS was greatest at calving varying from 44 kg in first parity animals to 62 kg in second parity animals. On average, 1 BCS unit equated to 31 kg LW across all data
Inducing and Optimizing Magnetism in Graphene Nanomesh
Using first-principles calculations, we explore the electronic and magnetic
properties of graphene nanomesh (GNM), a regular network of large vacancies,
produced either by lithography or nanoimprint. When removing an equal number of
A and B sites of the graphene bipartite lattice, the nanomesh made mostly of
zigzag (armchair) type edges exhibit antiferromagnetic (spin unpolarized)
states. In contrast, in situation of sublattice symmetry breaking, stable
ferri(o)magnetic states are obtained. For hydrogen-passivated nanomesh, the
formation energy is dramatically decreased, and ground state is found to
strongly depend on the vacancies shape and size. For triangular shaped holes,
the obtained net magnetic moments increase with the number difference of
removed A and B sites in agreement with Lieb's theorem for even A+B. For odd
A+B triangular meshes and all cases of non-triangular nanomeshes including the
one with even A+B, Lieb's theorem does not hold anymore which can be partially
attributed to introduction of armchair edges. In addition, large triangular
shaped GNM could be as robust as non-triangular GNMs, providing possible
solution to overcome one of crucial challenges for the sp-magnetism. Finally,
significant exchange splitting values as large as eV can be obtained
for highly asymmetric structures evidencing the potential of GNM for room
temperature carbon based spintronics. These results demonstrate that a turn
from 0-dimensional graphene nanoflakes throughout 1-dimensional graphene
nanoribbons with zigzag edges to GNM breaks localization of unpaired electrons
and provides deviation from the rules based on Lieb's theorem. Such
delocalization of the electrons leads the switch of the ground state of system
from antiferromagnetic narrow gap insulator discussed for graphene nanoribons
to ferromagnetic or nonmagnetic metal.Comment: 7 pages, 5 figures, 1 tabl
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