Charge disproportionation and Jahn-Teller distortion in LiNiO2 and NaNiO2: A density functional theory study

Density functional theory calculations have been performed on three potential ground-state configurations of LiNiO2 and NaNiO2. These calculations show that, whereas NaNiO2 shows the expected cooperative Jahn-Teller distortion (and therefore a crystal structure with C2/m symmetry), LiNiO2 shows at least two possible crystal structures very close in energy (within 3 meV/f.u.): P21/c and P2/c. Moreover, one of them (P2/c) shows charge disproportionation of the (expected) Ni3+ cations into Ni2+ and Ni4+. We discuss the implications of this complex ground state for the interpretation of the available electron and neutron structure data, its electronic and complex magnetic behaviour

Measuring sigma(e^+e^- \to hadrons) using tagged photon

We propose to use events with radiated photons in e^+e^- collisions to measure the total cross section of e^+ e^- \to hadrons as a function of the center of mass energy. The Monte Carlo simulation for the collider DAPHNE shows that a competitive accuracy can be achieved with this method.Comment: revtex, 13 page

Large eddy simulation of a lifted ethylene flame using a dynamic nonequilibrium model for subfilter scalar variance and dissipation rate

Accurate prediction of nonpremixed turbulent combustion using large eddy simulation(LES) requires detailed modeling of the mixing between fuel and oxidizer at scales finer than the LES filter resolution. In conserved scalar combustion models, the small scale mixing process is quantified by two parameters, the subfilter scalar variance and the subfilter scalar dissipation rate. The most commonly used models for these quantities assume a local equilibrium exists between production and dissipation of variance. Such an assumption has limited validity in realistic, technically relevant flow configurations. However, nonequilibrium models for variance and dissipation rate typically contain a model coefficient whose optimal value is unknown a priori for a given simulation. Furthermore, conventional dynamic procedures are not useful for estimating the value of this coefficient. In this work, an alternative dynamic procedure based on the transport equation for subfilter scalar variance is presented, along with a robust conditional averaging approach for evaluation of themodel coefficient. This dynamic nonequilibrium modeling approach is used for simulation of a turbulent lifted ethylene flame, previously studied using DNS by Yoo et al. (Proc. Comb. Inst., 2011). The predictions of the new model are compared to those of a static nonequilibrium modeling approach using an assumed model coefficient, as well as those of the equilibrium modeling approach. The equilibrium models are found to systematically underpredict both subfilter scalar variance and dissipation rate. Use of the dynamic procedure is shown to increase the accuracy of the nonequilibrium modeling approach. However, numerical errors that arise as a consequence of grid-based implicit filtering appear to degrade the accuracy of all three modeling options. Thus, while these results confirm the usefulness of the new dynamic model, they also show that the quality of subfilter model predictions depends on several factors extrinsic to the formulation of the subfilter model itself

Analysis of turbulent flame propagation in equivalence ratio-stratified flow

Equivalence ratio-stratified combustion is an important technology for achieving stable low-emission operation in internal combustion engines and gas turbines. This study examines how equivalence ratio stratification affects the physics of turbulent flame propagation using Direct Numerical Simulation. Three-dimensional simulations of a turbulent slot-Bunsen flame configuration are performed with accurate multi-step kinetic modelling for methane-air combustion. We compare one perfectly-premixed and three equivalence ratio-stratified cases with the mean equivalence ratio gradient aligned with, tangential to or opposed to the mean flame brush. The simulation results are analysed in terms of flame surface area and the burning intensity. The local effects of stratification are then investigated further by examining statistics of the displacement speed conditioned on the flame-normal equivalence ratio gradient. The local burning intensity is found to depend on the orientation of the stratification with respect to the flame front, so that burning intensity is enhanced when the flame speed in the products is faster than in the reactants. This effect of alignment between equivalence ratio gradients and flame fronts has been observed previously in laminar flames and it is found here that it also affects the global behaviour of turbulent flames. The flame surface area is also influenced by equivalence ratio stratification and this may be explained by differences in the surface-averaged consumption speed and differential propagation effects due to flame speed variations associated with equivalence ratio fluctuations

Line-integral representations of the displacement and stress fields due to an arbitrary Volterra dislocation loop in a transversely isotropic elastic full space

AbstractTransversely isotropic materials or hexagonal crystals are commonly utilized in various engineering fields; however, dislocation solutions for such special materials have not been fully developed. In this paper, we present a comprehensive study on this important topic, where only Volterra dislocations of the translational type are considered. Based on the potential theory of linear elasticity, we extend the well-known Burgers displacement equation for an arbitrarily shaped dislocation loop in an isotropic elastic full space to the transversely isotropic case. Both the induced displacements and stresses are expressed uniformly in terms of simple and explicit line integrals along the dislocation loop. We introduce three quasi solid angles to describe the displacement discontinuities over the dislocation surface and extract a simple step function out of these angles to characterize the dependence of the displacements on the configuration of the dislocation surface. We also give a new explicit formula for calculating accurately and efficiently the traditional solid angle of an arbitrary polygonal dislocation loop. From the present line-integral representations, exact closed-form solutions in terms of elementary functions are further obtained in a unified way for the displacement and stress fields due to a straight dislocation segment of arbitrary orientation. The non-uniqueness of the elastic field solution due to an open dislocation segment is rigorously discussed and demonstrated. For a circular dislocation loop parallel to the plane of isotropy, a new explicit expression of the induced elastic field is presented in terms of complete elliptic integrals. Several numerical examples are also provided as illustration and verification of the derived dislocation solutions, which further show the importance of material anisotropy on the dislocation-induced elastic field, and reveal the non-uniqueness feature of the elastic field due to a straight dislocation segment

Line-integral representations for the elastic displacements, stresses and interaction energy of arbitrary dislocation loops in transversely isotropic bimaterials

AbstractThe elastic displacements, stresses and interaction energy of arbitrarily shaped dislocation loops with general Burgers vectors in transversely isotropic bimaterials (i.e. joined half-spaces) are expressed in terms of simple line integrals for the first time. These expressions are very similar to their isotropic full-space counterparts in the literature and can be easily incorporated into three-dimensional (3D) dislocation dynamics (DD) simulations for hexagonal crystals with interfaces/surfaces. All possible degenerate cases, e.g. isotropic bimaterials and isotropic half-space, are considered in detail. The singularities intrinsic to the classical continuum theory of dislocations are removed by spreading the Burgers vector anisotropically around every point on the dislocation line according to three particular spreading functions. This non-singular treatment guarantees the equivalence among different versions of the energy formulae and their consistency with the stress formula presented in this paper. Several numerical examples are provided as verification of the derived dislocation solutions, which further show significant influence of material anisotropy and bimaterial interface on the elastic fields and interaction energy of dislocation loops

System-focused risk identification and assessment for disaster preparedness: Dynamic threat analysis

AbstractCurrent approaches to risk management stress the need for dynamic (i.e. continuous, ongoing) approaches to risk identification as part of a planned resource application aimed at reducing the expected consequences of undesired outcomes for the object of the assessment. We contend that these approaches place insufficient emphasis on the system knowledge available to the assessor, particularly in respect of three factors, namely the dynamic behavior of the system under threat, the role of human agents and the knowledge availability to those agents.In this paper we address the first of these shortcomings, namely the mobilization of explicit system knowledge in the identification of risks. We present a procedure for mobilizing quantitative and qualitative dynamic system knowledge using the case of flood threat to an electricity substation as a worked example. We assert that the approach described offers the potential of improving risk cognition by mobilizing system knowledge

Observation of hypertritons in Au+Au collisions at \sqrt{s_{NN}} = 200 GeV

We report preliminary results of hypertriton observation in heavy-ion collisions at RHIC. We have identified 157 +- 30 candidates in the current sample containing ~10^8 Au+Au events at \sqrt{s_{NN}} = 200 GeV. The production rate of hypertriton is close to that of helium 3. No extra penalty factor is observed for hypertriton, in contrast to results observed at the AGS.Comment: Turn off the line number package for QM09 proceeding publicatio

Center or Limit Cycle: Renormalization Group as a Probe

Based on our studies done on two-dimensional autonomous systems, forced non-autonomous systems and time-delayed systems, we propose a unified methodology - that uses renormalization group theory - for finding out existence of periodic solutions in a plethora of nonlinear dynamical systems appearing across disciplines. The technique will be shown to have a non-trivial ability of classifying the solutions into limit cycles and periodic orbits surrounding a center. Moreover, the methodology has a definite advantage over linear stability analysis in analyzing centers

Self-replication and evolution of DNA crystals

Is it possible to create a simple physical system that is capable of replicating itself? Can such a system evolve interesting behaviors, thus allowing it to adapt to a wide range of environments? This paper presents a design for such a replicator constructed exclusively from synthetic DNA. The basis for the replicator is crystal growth: information is stored in the spatial arrangement of monomers and copied from layer to layer by templating. Replication is achieved by fragmentation of crystals, which produces new crystals that carry the same information. Crystal replication avoids intrinsic problems associated with template-directed mechanisms for replication of one-dimensional polymers. A key innovation of our work is that by using programmable DNA tiles as the crystal monomers, we can design crystal growth processes that apply interesting selective pressures to the evolving sequences. While evolution requires that copying occur with high accuracy, we show how to adapt error-correction techniques from algorithmic self-assembly to lower the replication error rate as much as is required