Simulation study of pressure and temperature dependence of the negative thermal expansion in Zn(CN)(2)

12 pages, 16 figures12 pages, 16 figures12 pages, 16 figures12 pages, 16 figure

First and second order magnetic and structural transitions in BaFe2(1−x)_{2(1-x)}Co2x_{2x}As2_{2}

We present here high resolution magnetization measurements on high-quality BaFe$_{2(1-x)}$Co$_{2x}$As$_{2}$, 0$\leq$x$\leq$0.046 as-grown single crystals. The results confirm the existence of a magnetic tricritical point in the ($x$,$T$) plane at x$^{m}_{tr}$$\approx$0.022 and reveal the emergence of the heat capacity anomaly associated with the onset of the structural transition at x$^{s}$$\approx$0.0064. We show that the samples with doping near x$^{m}_{tr}$ do not show superconductivity, but rather superconductivity emerges at a slightly higher cobalt doping, x$\approx$0.0315Comment: 4 pages, 5 figure

Rigid unit modes in tetrahedral crystals

The 'rigid unit mode' (RUM) model requires unit blocks, in our case tetrahedra of SiO_4 groups, to be rigid within first order of the displacements of the O-ions. The wave-vectors of the lattice vibrations, which obey this rigidity, are determined analytically. Lattices with inversion symmetry yield generically surfaces of RUMs in reciprocal space, whereas lattices without this symmetry yield generically lines of RUMs. Only in exceptional cases as in beta-quartz a surface of RUMs appears, if inversion symmetry is lacking. The occurence of planes and bending surfaces, straight and bent lines is discussed. Explicit calculations are performed for five modifications of SiO_2 crystals.Comment: 18 pages, 6 figures, improved notatio

Dynamics from diffraction

A model-independent approach for the extraction of detailed lattice dynamical information from neutron powder diffraction data is described. The technique is based on a statistical analysis of atomistic configurations generated using reverse Monte Carlo structural refinement. Phonon dispersion curves extracted in this way are shown to reproduce many of the important features found in those determined independently using neutron triple-axis spectroscopy. The extent to which diffraction data are sensitive to lattice dynamics is explored in a range of materials. The prospect that such detailed dynamical information might be accessible using comparatively facile experiments such as neutron powder diffraction is incredibly valuable when studying systems for which established spectroscopic methods are prohibitive or inappropriate

CamGrid: Experiences in constructing a university-wide, Condor-based grid at the University of Cambridge

Proceedings of the 2004 UK e-Science All Hands Meeting, 31st August - 3rd September, Nottingham UKIn this article we describe recent work done in building a university-wide grid at the University of Cambridge based on the Condor middleware [1]. Once the issues of stakeholder concerns (e.g. security policies) and technical problems (e.g. firewalls and private IP addresses) have been taken into account, a solution based on two separate Condor environments was decided on. The first of these is a single large pool administered centrally by the University Computing Service (UCS) and the second a federated service of flocked Condor pools belonging to various departments and run over a Virtual Private Network (VPN). We report on the current status of this ongoing work

Failure mechanisms of graphene under tension

Recent experiments established pure graphene as the strongest material known to mankind, further invigorating the question of how graphene fails. Using density functional theory, we reveal the mechanisms of mechanical failure of pure graphene under a generic state of tension. One failure mechanism is a novel soft-mode phonon instability of the $K_1$-mode, whereby the graphene sheet undergoes a phase transition and is driven towards isolated benzene rings resulting in a reduction of strength. The other is the usual elastic instability corresponding to a maximum in the stress-strain curve. Our results indicate that finite wave vector soft modes can be the key factor in limiting the strength of monolayer materials

Pair distribution functions calculated from interatomic potential models using the General Utility Lattice Program.

A new module has been developed for the widely used General Utility Lattice\ud Program (GULP). The phonon-based theory developed by Chung & Thorpe\ud [Phys. Rev. B (1999), 59, 4807–4812] to calculate pair distribution function\ud (PDF) peak widths has been utilized to give a selection of commonly used\ud correlation functions. A numerical library of neutron scattering information is\ud now available within GULP, and is used to produce results that can be\ud compared with neutron scattering experimental data. The influence of different\ud phonon modes on the PDF can be assessed by excluding modes above or below\ud a cut-off frequency. Results are presented for sample crystallographic systems,\ud MgO, SrTiO3 and -cristobalite, as well as CaxSr1xTiO3 at x = 0.5, which makes\ud use of the capability to handle partial occupancies to compare different Ca/Sr\ud ordering arrangements with a disordered model in which every Ca/Sr site has\ud 50% occupancy of both species

Grid computing and molecular simulations: the vision of the eMinerals Project

This paper discusses a number of aspects of using grid computing methods in support of molecular simulations, with examples drawn from the eMinerals project. A number of components for a useful grid infrastructure are discussed, including the integration of compute and data grids, automatic metadata capture from simulation studies, interoperability of data between simulation codes, management of data and data accessibility, management of jobs and workflow, and tools to support collaboration. Use of a grid infrastructure also brings certain challenges, which are discussed. These include making use of boundless computing resources, the necessary changes, and the need to be able to manage experimentation