Mixing Scenarios for Lattice String Breaking

We present some simple scenarios for string breaking on the lattice based on a crude strong coupling model introduced previously. We review the dependence of the model on lattice spacing and extend it to include degenerate dynamical quarks and also meson exchange diagrams. A comparison is made between quenched and unquenched calculations. We examine string breaking in the presence of a static quark-diquark system, a situation that is specific to SU(3).Comment: 15 pages 5 fig

Electrically Tunable Band Gap in Silicene

We report calculations of the electronic structure of silicene and the stability of its weakly buckled honeycomb lattice in an external electric field oriented perpendicular to the monolayer of Si atoms. We find that the electric field produces a tunable band gap in the Dirac-type electronic spectrum, the gap being suppressed by a factor of about eight by the high polarizability of the system. At low electric fields, the interplay between this tunable band gap, which is specific to electrons on a honeycomb lattice, and the Kane-Mele spin-orbit coupling induces a transition from a topological to a band insulator, whereas at much higher electric fields silicene becomes a semimetal

Quantum Monte Carlo Calculation of the Binding Energy of Bilayer Graphene

We report diffusion quantum Monte Carlo calculations of the interlayer binding energy of bilayer graphene. We find the binding energies of the AA- and AB-stacked structures at the equilibrium separation to be 11.5(9) and 17.7(9) meV/atom, respectively. The out-of-plane zone-center optical phonon frequency predicted by our binding-energy curve is consistent with available experimental results. As well as assisting the modeling of interactions between graphene layers, our results will facilitate the development of van der Waals exchange-correlation functionals for density functional theory calculations.Comment: 5 pages and 3 figures, submitted to Phys. Rev. Lett.; supplemental material is available on arXiv via the ancillary files attached to this submissio

Electrons and phonons in single layers of hexagonal indium chalcogenides from ab initio calculations

We use density functional theory to calculate the electronic band structures, cohesive energies, phonon dispersions, and optical absorption spectra of two-dimensional In$_2$X$_2$ crystals, where X is S, Se, or Te. We identify two crystalline phases (alpha and beta) of monolayers of hexagonal In$_2$X$_2$, and show that they are characterized by different sets of Raman-active phonon modes. We find that these materials are indirect-band-gap semiconductors with a sombrero-shaped dispersion of holes near the valence-band edge. The latter feature results in a Lifshitz transition (a change in the Fermi-surface topology of hole-doped In$_2$X$_2$) at hole concentrations $n_{\rm S}=6.86\times 10^{13}$ cm$^{-2}$, $n_{\rm Se}=6.20\times 10^{13}$ cm$^{-2}$, and $n_{\rm Te}=2.86\times 10^{13}$ cm$^{-2}$ for X=S, Se, and Te, respectively, for alpha-In$_2$X$_2$ and $n_{\rm S}=8.32\times 10^{13}$ cm$^{-2}$, $n_{\rm Se}=6.00\times 10^{13}$ cm$^{-2}$, and $n_{\rm Te}=8.14\times 10^{13}$ cm$^{-2}$ for beta-In$_2$X$_2$.Comment: 9 pages. arXiv admin note: text overlap with arXiv:1302.606

Land-cover change monitoring in Obuasi, Ghana: an integration of earth observation, geoinformation systems and stochastic modelling

For over twenty years, Obuasi Municipality, Ghana, has experienced land-cover change arising from gold mining and urbanisation. This project quantified the land-cover changes that have taken place and projected likely future land-cover. An integration of Earth Observation (or EO), Geographical Information Science (or GIS) and Stochastic Modelling was examined. Post-Classification Change Detection employed Landsat TM or ETM+ images from 1986, 2002 and 2008. Subsequently, Markov Chain Analysis projected the land-cover distribution for 2020. Seven broad land-use and land-cover classes were identified and mapped, namely: built-up areas; mine sites; tailing ponds; barren land; forestland; farmland; and, rangeland. The results obtained for the 2008 to 2020 projection revealed a continuous expansion of built-up areas (1.63%), mine sites (0.89%) and farmland (3.4%), and a reduction of forestland (4.17%) and rangeland (2.59%). Despite the advent of very high resolution satellite imagery, this use of EO and GIS technology focussed on low-cost and lower resolution satellite imagery, coupled with Markov Modelling and was found to be beneficial in describing and analysing land-cover change processes in the study area, and was hence potentially useful for strategic planning purposes