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 ${\bf k}$-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