1,370 research outputs found
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
PbSn 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 PbSnSe 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.
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