73 research outputs found
Giant inelastic tunneling in epitaxial graphene mediated by localized states
Local electronic structures of nanometer-sized patches of epitaxial graphene
and its interface layer with SiC(0001) have been studied by atomically resolved
scanning tunneling microscopy and spectroscopy. Localized states belonging to
the interface layer of a graphene/SiC system show to have an essential
influence on the electronic structure of graphene. Giant enhancement of
inelastic tunneling, reaching 50% of the total tunneling current, has been
observed at the localized states on a nanometer-sized graphene monolayer
surrounded by defects.Comment: 6 pages, 5 figures, accepted for publication in Phys. Rev.
Cones, pringles, and grain boundary landscapes in graphene topology
A polycrystalline graphene consists of perfect domains tilted at angle
{\alpha} to each other and separated by the grain boundaries (GB). These nearly
one-dimensional regions consist in turn of elementary topological defects,
5-pentagons and 7-heptagons, often paired up into 5-7 dislocations. Energy
G({\alpha}) of GB computed for all range 0<={\alpha}<=Pi/3, shows a slightly
asymmetric behavior, reaching ~5 eV/nm in the middle, where the 5's and 7's
qualitatively reorganize in transition from nearly armchair to zigzag
interfaces. Analysis shows that 2-dimensional nature permits the off-plane
relaxation, unavailable in 3-dimensional materials, qualitatively reducing the
energy of defects on one hand while forming stable 3D-landsapes on the other.
Interestingly, while the GB display small off-plane elevation, the random
distributions of 5's and 7's create roughness which scales inversely with
defect concentration, h ~ n^(-1/2)Comment: 9 pages, 4 figure
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Practical nonlinear analysis of unreinforced concrete tunnel linings
A comprehensive methodology for modelling, analyzing and assessing the structural response of unreinforced concrete tunnel linings is presented. Various modelling techniques are described, considering the plane finite element representation of the lining geometry, material constitutive laws, and boundary and interface conditions. Furthermore, all relevant external loading cases are studied, including gravity, environmental, fire, blast, and seismic loading. Potential pitfalls in the modelling and analysis procedures are identified and properly dealt with. The suggested methodology is finally applied to actual tunnel linings and the interpretation of the analysis results leads to important conclusions regarding the applicability of different analysis methods and the performance of unreinforced concrete linings
Grain Boundaries in Graphene on SiC(000) Substrate
Grain boundaries in epitaxial graphene on the SiC(000) substrate are
studied using scanning tunneling microscopy and spectroscopy. All investigated
small-angle grain boundaries show pronounced out-of-plane buckling induced by
the strain fields of constituent dislocations. The ensemble of observations
allows to determine the critical misorientation angle of buckling transition
. Periodic structures are found among the flat
large-angle grain boundaries. In particular, the observed highly ordered grain boundary is assigned to the previously
proposed lowest formation energy structural motif composed of a continuous
chain of edge-sharing alternating pentagons and heptagons. This periodic grain
boundary defect is predicted to exhibit strong valley filtering of charge
carriers thus promising the practical realization of all-electric valleytronic
devices
Room temperature magnetic order on zigzag edges of narrow graphene nanoribbons
Magnetic order emerging in otherwise non-magnetic materials as carbon is a
paradigmatic example of a novel type of s-p electron magnetism predicted to be
of exceptional high-temperature stability. It has been demonstrated that atomic
scale structural defects of graphene can host unpaired spins. However, it is
still unclear under which conditions long-range magnetic order can emerge from
such defect-bound magnetic moments. Here we propose that in contrast to random
defect distributions, atomic scale engineering of graphene edges with specific
crystallographic orientation, comprising edge atoms only from one sub-lattice
of the bipartite graphene lattice, can give rise to a robust magnetic order. We
employ a nanofabrication technique based on Scanning Tunneling Microscopy to
define graphene nanoribbons with nanometer precision and well-defined
crystallographic edge orientations. While armchair ribbons display quantum
confinement gap, zigzag ribbons narrower than 7 nm reveal a bandgap of about
0.2 - 0.3 eV, which can be identified as a signature of interaction induced
spin ordering along their edges. Moreover, a semiconductor to metal transition
is revealed upon increasing the ribbon width, indicating the switching of the
magnetic coupling between opposite ribbon edges from antiferromagnetic to
ferromagnetic configuration. We found that the magnetic order on graphene edges
of controlled zigzag orientation can be stable even at room temperature,
raising hope for graphene-based spintronic devices operating under ambient
conditions
Ferromagnetism in graphene nanoribbons: split versus oxidative unzipped ribbons
Two types of graphene nanoribbons: (a) potassium-split graphene nanoribbons
(GNRs), and (b) oxidative unzipped and chemically converted graphene
nanoribbons (CCGNRs) were investigated for their magnetic properties using the
combination of static magnetization and electron spin resonance measurements.
The two types of ribbons possess remarkably different magnetic properties.
While the low temperature ferromagnet-like feature is observed in both types of
ribbons, such room temperature feature persists only in potassium-split
ribbons. The GNRs show negative exchange bias, but the CCGNRs exhibit a
'positive exchange bias'. Electron spin resonance measurements infer that the
carbon related defects may responsible for the observed magnetic behaviour in
both types of ribbons. Furthermore, proton hyperfine coupling strength has been
obtained from hyperfine sublevel correlation experiments performed on the GNRs.
Electron spin resonance provides no indications for the presence of potassium
(cluster) related signals, emphasizing the intrinsic magnetic nature of the
ribbons. Our combined experimental results may infer the coexistence of
ferromagnetic clusters with anti-ferromagnetic regions leading to disordered
magnetic phase. We discuss the origin of the observed contrast in the magnetic
behaviours of these two types of ribbons
sp-Electron Magnetic Clusters with a Large Spin in Graphene
Motivated by recent experimental data (Sepioni, M. et al. Phys. Rev. Lett.
2010, 105, 207205), we have studied the possibility of forming magnetic
clusters with spin S> 1/2 on graphene by adsorption of hydrogen atoms or
hydroxyl groups. Migration of hydrogen atoms and hydroxyl groups on the surface
of graphene during the delamination of HOPG led to the formation of seven-atom
or seven-OH-group clusters with S=5/2 that were of a special interest. The
coincidence of symmetry of the clusters with the graphene lattice strengthens
the stability of the cluster. For (OH)7 clusters that were situated greater
than 3 nm from one another, the reconstruction barrier to a nonmagnetic
configuration was approximately 0.4 eV, whereas for H7 clusters, there was no
barrier and the high-spin state was unstable. Stability of the high-spin
clusters increased if they were formed on top of ripples. Exchange interactions
between the clusters were studied and we have shown that the ferromagnetic
state is improbable. The role of the chemical composition of the solvent used
for the delamination of graphite is discussed.Comment: 22 pages, 1 table, 4 figures. Minor changes, few refs added. Accepted
to ACS Nan
Inhibition of Melanogenesis by the Pyridinyl Imidazole Class of Compounds: Possible Involvement of the Wnt/β-Catenin Signaling Pathway
While investigating the role of p38 MAPK in regulating melanogenesis, we found that pyridinyl imidazole inhibitors class compounds as well as the analog compound SB202474, which does not inhibit p38 MAPK, suppressed both α-MSH-induced melanogenesis and spontaneous melanin synthesis. In this study, we demonstrated that the inhibitory activity of the pyridinyl imidazoles correlates with inhibition of the canonical Wnt/β-catenin pathway activity. Imidazole-treated cells showed a reduction in the level of Tcf/Lef target genes involved in the β-catenin signaling network, including ubiquitous genes such as Axin2, Lef1, and Wisp1 as well as cell lineage-restricted genes such as microphthalmia-associated transcription factor and dopachrome tautomerase. Although over-expression of the Wnt signaling pathway effector β-catenin slightly restored the melanogenic program, the lack of complete reversion suggested that the imidazoles interfered with β-catenin-dependent transcriptional activity rather than with β-catenin expression. Accordingly, we did not observe any significant change in β-catenin protein expression. The independence of p38 MAPK activity from the repression of Wnt/β-catenin signaling pathway was confirmed by small interfering RNA knockdown of p38 MAPK expression, which by contrast, stimulated β-catenin-driven gene expression. Our data demonstrate that the small molecule pyridinyl imidazoles possess two distinct and opposite mechanisms that modulate β-catenin dependent transcription: a p38 inhibition-dependent effect that stimulates the Wnt pathway by increasing β-catenin protein expression and an off-target mechanism that inhibits the pathway by repressing β-catenin protein functionality. The p38-independent effect seems to be dominant and, at least in B16-F0 cells, results in a strong block of the Wnt/β-catenin signaling pathway
Mapping cortical brain asymmetry in 17,141 healthy individuals worldwide via the ENIGMA Consortium.
Hemispheric asymmetry is a cardinal feature of human brain organization. Altered brain asymmetry has also been linked to some cognitive and neuropsychiatric disorders. Here, the ENIGMA (Enhancing NeuroImaging Genetics through Meta-Analysis) Consortium presents the largest-ever analysis of cerebral cortical asymmetry and its variability across individuals. Cortical thickness and surface area were assessed in MRI scans of 17,141 healthy individuals from 99 datasets worldwide. Results revealed widespread asymmetries at both hemispheric and regional levels, with a generally thicker cortex but smaller surface area in the left hemisphere relative to the right. Regionally, asymmetries of cortical thickness and/or surface area were found in the inferior frontal gyrus, transverse temporal gyrus, parahippocampal gyrus, and entorhinal cortex. These regions are involved in lateralized functions, including language and visuospatial processing. In addition to population-level asymmetries, variability in brain asymmetry was related to sex, age, and intracranial volume. Interestingly, we did not find significant associations between asymmetries and handedness. Finally, with two independent pedigree datasets (n = 1,443 and 1,113, respectively), we found several asymmetries showing significant, replicable heritability. The structural asymmetries identified and their variabilities and heritability provide a reference resource for future studies on the genetic basis of brain asymmetry and altered laterality in cognitive, neurological, and psychiatric disorders
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