High-pressure structural, elastic and electronic properties of the scintillator host material, KMgF_3

The high-pressure structural behaviour of the fluoroperovskite KMgF_3 is investigated by theory and experiment. Density functional calculations were performed within the local density approximation and the generalized gradient approximation for exchange and correlation effects, as implemented within the full-potential linear muffin-tin orbital method. In situ high-pressure powder x-ray diffraction experiments were performed up to a maximum pressure of 40 GPa using synchrotron radiation. We find that the cubic Pm\bar{3}m crystal symmetry persists throughout the pressure range studied. The calculated ground state properties -- the equilibrium lattice constant, bulk modulus and elastic constants -- are in good agreement with experimental results. By analyzing the ratio between the bulk and shear modulii, we conclude that KMgF_3 is brittle in nature. Under ambient conditions, KMgF_3 is found to be an indirect gap insulator with the gap increasing under pressure.Comment: 4 figure

Circularly polarized colour reflection from helicoidal structures in the beetle Plusiotis boucardi

Copyright © 2007 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft. This is the published version of an article published in New Journal of Physics Vol. 9, article 99. DOI: 10.1088/1367-2630/9/4/099A detailed optical study of the iridescent outer-shell of the beetle Plusiotis boucardi has revealed a novel microstructure which controls both the polarization and wavelength of reflected light. A previously unreported hexagonal array across the integument of the beetle exhibits highly localized regions of reflection of only red and green left-handed circularly-polarized light. Optical and transmission electron microscopy (TEM) imaging reveals the origin of this effect as an array of 'bowl-shaped' recesses on the elytra that are formed from a dual-pitch helicoidal layer. Reflectivity spectra collected from the beetle are compared to theoretical data produced using a multi-layer optics model for modelling chiral, optically anisotropic media such as cholesteric liquid crystals. Excellent agreement is obtained between data and theory produced using a model that incorporates an upper isotropic layer (of cuticular wax), followed by a short pitch (310 (± 1) nm) overlying a longer pitch (370 (±1) nm) helicoidal layer of optically anisotropic material. These layers are backed by an absorbing underlayer. Synthetic replication of this form of structure may provide a route to the fabrication of tuneable micro-mirrors for optical applications

A Three-Dimensional Atlas of the Honeybee Neck

Three-dimensional digital atlases are rapidly becoming indispensible in modern biology. We used serial sectioning combined with manual registration and segmentation of images to develop a comprehensive and detailed three-dimensional atlas of the honeybee head-neck system. This interactive atlas includes skeletal structures of the head and prothorax, the neck musculature, and the nervous system. The scope and resolution of the model exceeds atlases previously developed on similar sized animals, and the interactive nature of the model provides a far more accessible means of interpreting and comprehending insect anatomy and neuroanatomy

Arcus as a tensegrity structure in the arolium of wasps (Hymenoptera : Vespidae)

The unfolding of the hymenopteran attachment pad (arolium) may be achieved in two ways, hydraulic and mechanical. The first was confirmed in experiments with pressure applied to more proximal leg parts and on immersion in hypotonic solutions. Presumably, this way of unfolding does not play an important role for a living hornet. Mechanical unfolding was studied experimentally with the aid of a micromanipulator pulling the tendon of the musculus retractor unguis. Ablation experiments on different parts of the arolium indicated that the arcus is the most crucial element for mechanical unfolding. The shape of the arcus in closed and open conditions was measured using a 3D measurement microscope and reconstructed by means of 3D computer graphics. The arcus coils up upon being freed from the arolium tissues, and coils up even more after immersion into a 10% aqueous solution of NaOH. Geometrical models of the arcus are proposed, from which the rotational moment of elasticity is derived. Conformations and deformations of the arcus are quantified in order to explain its role in the folding and unfolding processes of the arolium. The diversity of approaches supports the idea that the arcus is a prestressed (tensegrity) structure providing immediate, soft, and graded transmission of forces during folding and unfolding action of the arolium