A note on classes of structured matrices with elliptical type numerical range

Czechoslovak Mathematical Journal - We identify new classes of structured matrices whose numerical range is of the elliptical type, that is, an elliptical disk or the convex hull of elliptical disks.     

Thermochemistry of diphenic anhydride. A combined experimental and theoretical study

The standard (p°?=?0.1?MPa) molar enthalpy of formation for solid and gaseous diphenic anhydride (2,2′-biphenyldicarboxylic anhydride, dibenz[c,e]oxepin-5,7-dione) was derived from the standard molar enthalpy of combustion, in oxygen, at T?=?298.15?K, measured by static bomb combustion calorimetry, and the standard molar enthalpy of sublimation, at T?=?298.15?K, measured by Calvet microcalorimetry: ?(258.4?±?4.9) kJ?mol^?1. In addition, ab initio and density functional theory calculations have been performed at a variety of levels. The degree of aromaticity of diphenic anhydride is discussed in the context of other oxygen-containing (ring and keto) heterocycles and related carbocycles: this species is surprisingly destabilized.     

Symmetric differentiation on time scales

We define a symmetric derivative on an arbitrary nonempty closed subset of the real numbers and derive some of its properties. It is shown that real-valued functions defined on time scales that are neither delta nor nabla differentiable can be symmetric differentiable.     

Evolutionary snowdrift game incorporating costly punishment in structured populations

The role of punishment and the effects of a structured population in promoting cooperation are important issues. Within a recent model of snowdrift game (SG) incorporating a costly punishing strategy (P), we study the effects of a population connected through a square lattice. The punishers, who carry basically a cooperative (C) character, are willing to pay a cost α$\alpha$ so as to punish a non-cooperative (D) opponent by β$\beta$. Depending on α$\alpha$, β$\beta$, the cost-to-benefit ratio r$r$ in SG, and the initial conditions, the system evolves into different phases that could be homogeneous or inhomogeneous. The spatial structure imposes geometrical constraint on how one agent is affected by neighboring agents. Results of extensive numerical simulations, both for the steady state and the dynamics, are presented. Possible phases are identified and discussed, and isolated phases in the r–β$r\text{–}\beta$ space are identified as special local structures of strategies that are stable due to the lattice structure. In contrast to a well-mixed population where punishers are suppressed due to the cost of punishment, the altruistic punishing strategy can flourish and prevail for appropriate values of the parameters, implying an enhancement in cooperation by imposing punishments in a structured population. The system could evolve to a phase corresponding to the coexistence of C, D, and P strategies at some particular payoff parameters, and such a phase is absent in a well-mixed population. The pair approximation, a commonly used analytic approach, is extended from a two-strategy system to a three-strategy system. We show that the pair approximation can, at best, capture the numerical results only qualitatively. Due to the improper way of including spatial correlation imposed by the lattice structure, the approximation does not give the frequencies of C, D, and P accurately and fails to give the homogeneous AllD and AllP phases.     

The influence of a weak magnetic field in the Renormalization-Group functions of (2 + 1)-dimensional Dirac systems

The experimental observation of the renormalization of the Fermi velocity v _F as a function ofdoping has been a landmark for confirming the importance of electronic interactions ingraphene. Although the experiments were performed in the presence of a perpendicularmagnetic field B, the measurements are well described by arenormalization-group (RG) theory that did not include it. Here we clarify this issue, forboth massive and massless Dirac systems, and show that for the weak magnetic fields atwhich the experiments are performed, there is no change in the renormalization-groupfunctions. Our calculations are carried out in the framework of the Pseudo-quantumelectrodynamics (PQED) formalism, which accounts for dynamical interactions. We includeonly the linear dependence in B, and solve the problem using two differentparametrizations, the Feynman and the Schwinger one. We confirm the results obtainedearlier within the RG procedure and show that, within linear order in the magnetic field,the only contribution to the renormalization of the Fermi velocity for the massive casearises due to electronic interactions. In addition, for gapped systems, we observe arunning of the mass parameter.     

Boron nitride nanotubes as templates for half-metal nanowires

We investigate by means of a GGA + U implementation of density functional theory the electronic and structural properties of magnetic nanotubes composed of an iron oxide monolayer and (n,0) boron nitride (BN) nanotubes, with n ranging from 6 to 14. The formation energy per FeO molecule of FeO covered tubes is smaller than the formation energy of small FeO nanoparticles, which suggests that the FeO molecules may cover the BN nanotubes rather than aggregating locally. Both GGA (PBE) and Van der Waals functionals predict an optimal FeO-BN interlayer distance of 2.94 ?. Depending on the diameter of the tube, novel electronic properties for the FeO covered BN nanotubes were found. They can be semiconductors, intrinsic half-metals or semi-half-metals that can become half-metals if charged with either electrons or holes. Such results are important in the spintronics context.     

Ionic and radical adsorption on the Au(hkl) surfaces: A DFT study

Adsorption of several ionic and non-ionic species (OH^?, O, O^?, O^2?, H, H⁺ and H^?) on the low-index Miller Au(hkl) surfaces has been investigated by means of density functional theory based methods. The stability order for adsorptions on the three surfaces decreases with the increasing of the coordination number of the outermost gold atoms in each surface, i.e., Au(110) > Au(100) > Au(111), which is in agreement with the experimental evidences. The detailed COOPs analysis of the various adsorption sites for all adsorbates in the surface with the most stable adsorption(s), Au(110), evidenced that adsorption in the gold surfaces may be a function of particle size and charge and substantiates the variability in the order of preferences sites for the adsorption of the different species found in these low-index Miller Au(hkl) surfaces. This variability increases with the increasing of the stability of the adsorptions on the gold surfaces, i.e., the Au(110) presents more variability in the order of preferences sites for the adsorption of different species.     

Turbulent Flow with Embedded Vortical Structures Induced by Vortex Generators in a Cascade

Presented work is the next step after several experimental examinations of vortex generator influence on a flow separation occurring on a model of the NACA 63A421 airfoil with deflected simple flap. In this stage of research the vortices produced by vortex generators (VGs) were studied using Particle Image Velocimetry technique (PIV) and numerical simulations. Vane type VGs with two spacings among VGs pairs in straight channel with turbulent flow were tested. The average velocity flow field, peak of vorticity and circulation decay downstream of VGs were evaluated. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)