Reconstruction, rumpling, and Dirac states at the (001) surface of a topological crystalline insulator Pb1-xSnxSe

Equilibrium atomic configuration and electronic structure of the (001) surface of IV-VI semiconductors PbTe, PbSe, SnTe and SnSe, is studied using the density functional theory (DFT) methods. At surfaces of all those compounds, the displacements of ions from their perfect lattice sites reveal two features characteristic of the rock salt crystals. First, the ionic displacements occur only along the direction perpendicular to the surface, and they exhibit the rumpling effect, i.e., the vertical shifts of cations and anions differ. Second, the interlayer spacing of the first few monolayers at the surface oscillates. Our results are in good agreement with the previous X-ray experimental data and theoretical results where available. They also are consistent with the presence of two {110} mirror planes at the (001) surface of the rock salt. One the other hand, experiments preformed for the topological Pb$_{1-x}$Sn$_x$ Se alloy indicate breaking of the mirror symmetry due to a large 0.3 {\AA} relative displacement of the cation and anion sublattices at the surface, which induces the opening of the gap of the Dirac cones. Our results for Pb$_{1-x}$Sn$_x$Se including the simulated STM images, are in contradiction with these findings, since surface reconstructions with broken symmetry are never the ground state configurations. The impact of the theoretically determined surface configurations and of the chemical disorder on the surface states is analyzed.Comment: 9 pages, 11 figure

Assessment of sweep net and suction sampling for evaluating pest insect populations in hay alfalfa

Insect populations in alfalfa grown for hay can be sampled using several methods. However, in a pest management program a relatively easy, quick, and reliable method of sampling is essential for making effective pest control decisions. A study was conducted to determine if two different sampling methods, sweepnet sampling and suction sampling, led to similar pest control decisions. Differences between sweepnet and D-Vac insect population estimates varied over sampling dates and years and were dependent on the insect species, their developmental stages, and abiotic factors. Our results indicate that, for many sampling dates, decisions on control of some pest insects would be similar for the two sampling methods.Keywords: Insecta; Medicago sativa; alfalfa weevil; pea aph

Theory of Ferromagnetism in Diluted Magnetic Semiconductor Quantum Wells

We present a mean field theory of ferromagnetism in diluted magnetic semiconductor quantum wells. When subband mixing due to exchange interactions between quantum well free carriers and magnetic impurities is neglected, analytic result can be obtained for the dependence of the critical temperature and the spontaneous magnetization on the distribution of magnetic impurities and the quantum well width. The validity of this approximate theory has been tested by comparing its predictions with those from numerical self-consistent field calculations. Interactions among free carriers, accounted for using the local-spin-density approximation, substantially enhance the critical temperature. We demonstrate that an external bias potential can tune the critical temperature through a wide range.Comment: 4 pages, 3 figures, submitted to Phys. Rev.