13 research outputs found
Tunable Band Gap in Graphene with a Non-Centrosymmetric Superlattice Potential
We show that, if graphene is subjected to the potential from an external
superlattice, a band gap develops at the Dirac point provided the superlattice
potential has broken inversion symmetry. As a numerical example, we calculate
the band structure of graphene in the presence of an external potential due to
periodically patterned gates arranged in a triangular graphene superlattice
(TGS) with broken inversion symmetry, and find that a band gap is created at
both the original and "second generation" Dirac point. The gap can be
controlled, in principle, by changing the external potential and the lattice
constant of the TGS.Comment: 6 figures, Phys. Rev. B 79, 20543
Magnonic Crystal with Two-Dimensional Periodicity as a Waveguide for Spin Waves
We describe a simple method of including dissipation in the spin wave band
structure of a periodic ferromagnetic composite, by solving the Landau-Lifshitz
equation for the magnetization with the Gilbert damping term. We use this
approach to calculate the band structure of square and triangular arrays of Ni
nanocylinders embedded in an Fe host. The results show that there are certain
bands and special directions in the Brillouin zone where the spin wave lifetime
is increased by more than an order of magnitude above its average value. Thus,
it may be possible to generate spin waves in such composites decay especially
slowly, and propagate especially large distances, for certain frequencies and
directions in -space.Comment: 13 pages, 4 figures, submitted to Phys Rev
Theory of band gap bowing of disordered substitutional II-VI and III-V semiconductor alloys
For a wide class of technologically relevant compound III-V and II-VI
semiconductor materials AC and BC mixed crystals (alloys) of the type
A(x)B(1-x)C can be realized. As the electronic properties like the bulk band
gap vary continuously with x, any band gap in between that of the pure AC and
BC systems can be obtained by choosing the appropriate concentration x, granted
that the respective ratio is miscible and thermodynamically stable. In most
cases the band gap does not vary linearly with x, but a pronounced bowing
behavior as a function of the concentration is observed. In this paper we show
that the electronic properties of such A(x)B(1-x)C semiconductors and, in
particular, the band gap bowing can well be described and understood starting
from empirical tight binding models for the pure AC and BC systems. The
electronic properties of the A(x)B(1-x)C system can be described by choosing
the tight-binding parameters of the AC or BC system with probabilities x and
1-x, respectively. We demonstrate this by exact diagonalization of finite but
large supercells and by means of calculations within the established coherent
potential approximation (CPA). We apply this treatment to the II-VI system
Cd(x)Zn(1-x)Se, to the III-V system In(x)Ga(1-x)As and to the III-nitride
system Ga(x)Al(1-x)N.Comment: 14 pages, 10 figure
Dirac Spectrum in Piecewise Constant One-Dimensional Potentials
We study the electronic states of graphene in piecewise constant potentials
using the continuum Dirac equation appropriate at low energies, and a transfer
matrix method. For superlattice potentials, we identify patterns of induced
Dirac points which are present throughout the band structure, and verify for
the special case of a particle-hole symmetric potential their presence at zero
energy. We also consider the cases of a single trench and a p-n junction
embedded in neutral graphene, which are shown to support confined states. An
analysis of conductance across these structures demonstrates that these
confined states create quantum interference effects which evidence their
presence.Comment: 10 pages, 12 figures, additional references adde
Genotypic and environmental effects on wheat technological and nutritional quality
International audienceTechnological (processing performance and end-product) and nutritional quality of wheat is in principle determined by a number of compounds within the wheat grain, including proteins, polysaccharides, lipids, minerals, heavy metals, vitamins and phytochemicals, effecting these characters. The genotype and environment is of similar importance for the determination of the content and composition of these compounds. Furthermore, the interaction between genotypes and the cultivation environment may play a significant role. Many studies have evaluated whether the genotype or the environment plays the major role in determining the content of the mentioned compounds. An overall conclusion of these studies is that except for compounds encoded by single major genes, importance of certain factors mainly depend on how wide environments and how diverse cultivars are within these comparative studies. Comparing environments all over, e.g. across Latin America, ends up with a high significance of the environment while large studies including genotypes of wide genetic background result in a significant role for the genotype. In addition, for some technological properties and components, genotype has a higher effect (e.g. grain hardness and gluten proteins), while environment influences stronger on others (e.g. protein and mineral content).Content and concentration of proteins, but also to some extent of starch, some non-starch polysaccharides and lipids, are essential in determining the technological quality of a wheat flour. For nutritional quality of the flour, the majority of the compounds are together the important determinant. Thus an increased understanding of environmental effects is essential. As to how the environment is influencing the content of the compounds, there are some differences. The protein content and composition is strongly affected by environmental factors influencing nitrogen availability and cultivar development time. However, these two factors are impacted by a range of environmental (temperature, precipitation, humidity/sun hours, etc.) and agronomic (soil properties, crop management practices such as seeding density, nitrogen fertilizer application timing and amount, etc.) components. Thus, to understand the interplay between the various environmental and agronomic factors impacting the technological quality of a wheat flour, modeling is a useful tool. Several other compounds, including minerals and heavy metals, are to a higher extent determined by site specific variation, resulting in similar rankings of entries across locations, although the total content is varying among years. The bioactive compounds and vitamins are a part of the defense mechanisms of plants and thus there is a variation in these compounds depending on prevailing biotic and abiotic stresses (heat, drought, excess rainfall, nutrition, diseases and pests). Thus, even for nutritional quality of wheat, incorporating all compounds of relevance in the evaluation would benefit from modeling tools
Vascular Stenosis: An Introduction
International audienceAn arterial stenosis is a narrowing of the lumen that disturbs the local blood flow and precludes the adequate irrigation of perfused organs. A vascular stenosis can be extrinsic, which is caused by external compression (e.g., aneurysms and tumors), or intrinsic, currently related to atherosclerosis.Atherosclerosis is defined by an intramural retention of lipids coupled to inflammation and dyslipidemia. Atherosclerosis scatters throughout large and medium thick-walled systemic arteries, especially near and in branching regions. (Pulmonary arterial stenosis is a congenital defect.)Atherosclerosis is characterized by migration from the media, proliferation, and dedifferentiation of vascular smooth myocytes in the subendothelial layer, in addition to monocyte diapedesis and differentiation into macrophages. Both smooth myocytes and macrophages scavenge accumulated oxidized low-density lipoproteins (oxLDL) and transform into foam cells. Atherosclerosis produces symptoms when the arterial lumen is severely narrowed.Advanced atherosclerotic plaques can be destabilized, thereby being a source of clotting and subsequent emboli. Emboli block tissue perfusion in a smaller downstream artery, thereby causing ischemia and infarction.The treatment of stenotic arterial segments relies on surgical grafting or medical minimally invasive procedures such as stenting. However, both methods often lead to intimal hyperplasia resulting from uncontrolled proliferation of vascular smooth myocytes. Whereas atheroma evolves during a time magnitude order of 10 years, posttherapeutic intimal hyperplasia develops in a period of order 1 month.Successful stenting can be assumed as a procedure without strong endothelial injury. In other words, both delayed thrombosis and intimal hyperplasia result from stent deployment that more or less severely damages the vascular endothelium. To eliminate these complications, drug-eluting stents have been designed and fabricated. However, the antiproliferative drug not only blocks vascular smooth myocyte division but also precludes endothelium healing. In the absence of proper endothelial interface between blood and arterial wall, that is, when the local controller of blood coagulation and cell proliferation is missing, thrombosis and restenosis occur.Arterial stenoses have stimulated biomechanicians and applied mathematicians. They carried out flow visualization and pressure and velocity measurements in experimental models of stenoses with idealized, symmetrical or not, geometry. In parallel to technological improvements of medical imaging techniques, computational fluid dynamics, due to new numerical schemes and high-performance computing, enables to perform numerical tests on subject-specific compartments of the blood circulation, after 3D reconstruction, rather than focusing on more or less short arterial, branched or not, segments. In addition, the drug release from drug-eluting stents is investigated using mathematical models