The Ruled Vertex and Nontoric del Pezzo Surfaces

We construct the topological partition function of local nontoric del Pezzo surfaces using the ruled vertex formalism.Comment: 16 pages, 4 figure

Model of hopping dc conductivity via nearest neighbor boron atoms in moderately compensated diamond crystals

Expressions for dependences of the pre-exponential factor \sigma_3 and the thermal activation energy \epsilon_3 of hopping electric conductivity of holes via boron atoms on the boron atom concentration N and the compensation ratio K are obtained in the quasiclassical approximation. It is assumed that the acceptors (boron atoms) in charge states (0) and (-1) and the donors that compensate them in the charge state (+1) form a nonstoichiometric simple cubic lattice with translational period R_h = [(1 + K)N]^{-1/3} within the crystalline matrix. A hopping event occurs only over the distance R_h at a thermally activated accidental coincidence of the acceptor levels in charge states (0) and (-1). Donors block the fraction K/(1 - K) of impurity lattice sites. The hole hopping conductivity is averaged over all possible orientations of the lattice with respect to the external electric field direction. It is supposed that an acceptor band is formed by Gaussian fluctuations of the potential energy of boron atoms in charge state (-1) due to Coulomb interaction only between the ions at distance R_h. The shift of the acceptor band towards the top of the valence band with increasing N due to screening (in the Debye--H\"uckel approximation) of the impurity ions by holes hopping via acceptor states was taken into account. The calculated values of \sigma_3(N) and \epsilon_3(N) for K \approx 0.25 agree well with known experimental data at the insulator side of the insulator--metal phase transition. The calculation is carried out at a temperature two times lower than the transition temperature from hole transport in v-band of diamond to hopping conductance via boron atoms.Comment: 6 pages, 2 figure

Perspectives on safety culture

Overviewing selected elements from the literature, this paper locates the notion of safety culture within its parent concept of organisational culture. A distinction is drawn between functionalist and interpretive perspectives on organisational culture. The terms ‘culture’ and ‘climate’ are clarified as they are typically applied to organisations and to safety. A contrast is drawn between strategic top down and data-driven bottom up approaches to human factors as an illustrative aspect of safety. A safety case study is used to illustrate two measurement approaches. Key issues for future study include valid measurement of safety culture and developing methods to adequately represent mechanisms through which safety culture might influence, and be influenced by, other safety factors

Discrete cilia modelling with singularity distributions

We discuss in detail techniques for modelling flows due to finite and infinite arrays of beating cilia. An efficient technique, based on concepts from previous ‘singularity models’ is described, that is accurate in both near and far-fields. Cilia are modelled as curved slender ellipsoidal bodies by distributing Stokeslet and potential source dipole singularities along their centrelines, leading to an integral equation that can be solved using a simple and efficient discretisation. The computed velocity on the cilium surface is found to compare favourably with the boundary condition. We then present results for two topics of current interest in biology. 1) We present the first theoretical results showing the mechanism by which rotating embryonic nodal cilia produce a leftward flow by a ‘posterior tilt,’ and track particle motion in an array of three simulated nodal cilia. We find that, contrary to recent suggestions, there is no continuous layer of negative fluid transport close to the ciliated boundary. The mean leftward particle transport is found to be just over 1 μm/s, within experimentally measured ranges. We also discuss the accuracy of models that represent the action of cilia by steady rotlet arrays, in particular, confirming the importance of image systems in the boundary in establishing the far-field fluid transport. Future modelling may lead to understanding of the mechanisms by which morphogen gradients or mechanosensing cilia convert a directional flow to asymmetric gene expression. 2) We develop a more complex and detailed model of flow patterns in the periciliary layer of the airway surface liquid. Our results confirm that shear flow of the mucous layer drives a significant volume of periciliary liquid in the direction of mucus transport even during the recovery stroke of the cilia. Finally, we discuss the advantages and disadvantages of the singularity technique and outline future theoretical and experimental developments required to apply this technique to various other biological problems, particularly in the reproductive system

Binding Energy of Charged Excitons in ZnSe-based Quantum Wells

Excitons and charged excitons (trions) are investigated in ZnSe-based quantum well structures with (Zn,Be,Mg)Se and (Zn,Mg)(S,Se) barriers by means of magneto-optical spectroscopy. Binding energies of negatively () and positively (X+) charged excitons are measured as functions of quantum well width, free carrier density and in external magnetic fields up to 47 T. The binding energy of shows a strong increase from 1.4 to 8.9 meV with decreasing quantum well width from 190 to 29 A. The binding energies of X+ are about 25% smaller than the binding energy in the same structures. The magnetic field behavior of and X+ binding energies differ qualitatively. With growing magnetic field strength, increases its binding energy by 35-150%, while for X+ it decreases by 25%. Zeeman spin splittings and oscillator strengths of excitons and trions are measured and discussed

Heterogeneity of discontinuous carbon fibre composites: damage initiation captured by Digital Image Correlation

This paper aims to identify architectural features which lead to damage initiation and failure in discontinuous carbon fibre composites formed from randomly orientated bundles. A novel multi-camera digital image correlation system was used to simultaneously view strain fields from opposing surfaces of coupons, in order to map progression of failure. The highest strain concentrations were found to occur when the ends of fibre bundles aligned in the direction of loading coincided with underlying transverse bundles. The failure plane was observed to grow between a number of strain concentrations at critical features, coalescing sites of damage to create the final fracture surface. Although potential failure sites can be detected at low global strains in the form of strain concentrations, the strain field observed at low applied loads cannot be extrapolated to reliably predict final failure