Lattice Expansion of (Ga,Mn)As: The Role of Substitutional Mn and of the Compensating Defects

We apply the density-functional technique to determine the lattice constant of GaAs supercells containing Mn_Ga, Mn_int, and As_Ga impurities, and use a linear interpolation to describe the dependence of the lattice constant a of Ga_{1-x}Mn_xAs on the concentrations of these impurities. The results of the supercell calculations confirm that Mn_Ga does not contribute to the lattice expansion. The increase of a is due to both Mn_int and As_Ga, that are both created in the as-grown (Ga,Mn)As in proportion to x, and that are most probably present in a remarkable amount also in the best annealed materials.Comment: 3 pages, 2 figures, presented at XXXIV Int. School. on the Physics of Semiconducting Compounds - Jaszowiec 2005, Ustron-Jaszowiec, Poland, June 4-10, 200

Disorder-Induced Effects in III-V Semiconductors with Mn

The substitution of Mn in the III-V diluted magnetic semiconductors leads to a strong electron scattering on impurities. Besides the features induced in the valence band by the hybridization with the Mn d-states, also the conduction band is affected by the absence of the Mn s-states at its edge. Also the high concentration of compensating donors modifies the band structure. This is shown on the absorption coefficient epsilon2(omega) of GaP doped with Mn and Se. The absorption evaluated by ab initio density functional calculations starts with a smooth tail and does not show the structure typical for III-V materials. We analyze these features and the role of the donors on model systems using the tight-binding coherent potential approach.Comment: 8 pages, 3 figures, presented at XXXI Int. School of Semiconducting Compounds, Jaszowiec 2002, Polan

Measurement and correlation of aerodynamic heating to surface corrugation stiffened structures in thick turbulent boundary layers

The flow conditions for which heating distributions were measured on corrugated surfaces and wavy walls in turbulent boundary layers are shown, along with the ratio of the displacement thickness to the roughness height versus the local edge Mach number for an equivalent smooth surface. The present data are seen to greatly extend the range of data available on corrugated surfaces in turbulent boundary layers. These data were obtained by testing fullscale corrugation roughened panels in the wall boundary layer of a supersonic and hypersonic wind tunnel. The experimental program used to obtain the data is described. The data are analyzed and correlated in terms of the pertinent flow and geometric parameters. The developed correlations are compared with the available thin boundary layer data, as well as with previously published correlation techniques

Correlated Doping in Semiconductors: The Role of Donors in III-V Diluted Magnetic Semiconductors

We investigate the compositional dependence of the total energy of the mixed crystals (Ga,Mn)As co-doped with As, Sn, and Zn. Using the ab initio LMTO-CPA method we find a correlation between the incorporation of acceptors (Mn, Zn) and donors (Sn, antisite As). In particular, the formation energy of As_Ga is reduced by approx. 0.1 eV in the presence of Mn, and vice versa. This leads to the self-compensating behavior of (Ga,Mn)As.Comment: 8 pages, 2 figures, presented at the XXXI Int. School of Semiconducting Compounds, Jaszowiec 2002, Polan

Analytical comparison of hypersonic flight and wind tunnel viscous/inviscid flow fields

Flow fields were computed about blunted, 0.524 and 0.698 radians, cone configurations to assess the effects of nonequilibrium chemistry on the flow field geometry, boundary layer edge conditions, boundary layer profiles, and heat transfer and skin friction. Analyses were conducted at typical space shuttle entry conditions for both laminar and turbulent boundary layer flow. In these calculations, a wall temperature of 1365 K (2000 F) was assumed. The viscous computer program used in this investigation was a modification of the Blottner non-similar viscous code which incorporated a turbulent eddy viscosity model after Cebeci. The results were compared with equivalent calculations for similar (scaled) configurations at typical wind tunnel conditions. Wind tunnel test gases included air, nitrogen, CF4 and helium. The viscous computer program used for wind tunnel conditions was the Cebeci turbulent non-similar computer code

The Use of Landsat 8 and Sentinel-2 Data and Meterological Observations for Winter Wheat Yield Assessment

This study focuses on winter wheat yield assessment from NASA's Harmonized Landsat Sentinel-2 (HLS) product and meteorological observations through phenological fitting. Vegetation indices (VIs), namely difference vegetation index (DVI), normalized difference vegetation index (NDVI) and enhanced vegetation index (EVI2), extracted from satellite optical data, are fitted per pixel against accumulated growing degree days (AGDD) using a quadratic function. Accumulated VIs are correlated against winter wheat yields. Results show a better performance from DVI compared to NDVI and EVI2