Interstitial Mn in (Ga,Mn)As: Binding energy and exchange coupling

We present ab initio calculations of total energies of Mn atoms in various interstitial positions. The calculations are performed by the full-potential linearized plane-wave method. The minimum energy is found for tetrahedral T(As4) position, but the energy of the T(Ga4) site differs by only a few meV. The T(Ga4) position becomes preferable in the p-type materials. In samples with one substitutional and one interstitial Mn the Mn atoms tend to form close pair with antiparallel magnetic moments. We also use the spin-splitting of the valence band to estimate the exchange coupling Jpd for various positions of Mn. It is the same for the substitutional and T(As4) position and it is only slightly reduced for the T(Ga4) position. The hybridization of Mn d-states with six next-nearest neighbors of the interstitial Mn explains the insensitivity of Jpd to the position of Mn.Comment: 6 pages, 3 figures, 3 tables, submitted to the Physical Review

Prospects of high temperature ferromagnetism in (Ga,Mn)As semiconductors

We report on a comprehensive combined experimental and theoretical study of Curie temperature trends in (Ga,Mn)As ferromagnetic semiconductors. Broad agreement between theoretical expectations and measured data allows us to conclude that T_c in high-quality metallic samples increases linearly with the number of uncompensated local moments on Mn_Ga acceptors, with no sign of saturation. Room temperature ferromagnetism is expected for a 10% concentration of these local moments. Our magnetotransport and magnetization data are consistnent with the picture in which Mn impurities incorporated during growth at interstitial Mn_I positions act as double-donors and compensate neighboring Mn_Ga local moments because of strong near-neighbor Mn_Ga-Mn_I antiferromagnetic coupling. These defects can be efficiently removed by post-growth annealing. Our analysis suggests that there is no fundamental obstacle to substitutional Mn_Ga doping in high-quality materials beyond our current maximum level of 6.2%, although this achievement will require further advances in growth condition control. Modest charge compensation does not limit the maximum Curie temperature possible in ferromagnetic semiconductors based on (Ga,Mn)As.Comment: 13 pages, 12 figures, submitted to Phys. Rev.

Influence of soil tillage on oats yield in Central Bohemia Region

ArticleThe paper describes results of the field experiment with two technologies of oats crop stand establishment. First technology is conventional technology and second is no - tillage technology. The arti cle describes an experiment which was conducted in years 2013 – 2017. The experiments were located in Nesperská Lhota at Central Bohemia region. Altitude of selected field is 460 m. The experimental field is covered by a light cambi - soil. The differences bet ween the two technologies stand establishment are discussed. Statistical evaluation was performed on both methods in the parameter number of tillers per m 2 and yield. Crop yield is the basic indicator of agricultural production and usually affected by qual ity of soil tillage. One of the parameters affecting the yield is tillage (technology, working tools, depth, turning of soil etc.). Tillage depends mainly on the depth of processing and other quality parameters. An examination of the crop yield is necessar y in terms of sensitivity, depending on agro - technical conditions. Among those can be included processing plant soil nutrition and plant protection. It's always necessary commemorate, in these experiments can't be excluded some parameters like locality or meteorological parameter influences. Field trial was conceived as multi - year experiment with minimal changes of agro - technical conditions. During the experiment, the positive effect of conventional technology on crop yields was found. This difference was r eflected in the number of tillers and in total yield. The measurement shows the beneficial effect of the loosening of soil on the state of the oat crops. The results of experiment show that, the average yield in 5 years observation was 2.11 t ha - 1 for no - t ill technology and 4.22 t ha - 1 for conv entional technology of tillage

Mn incorporation in as-grown and annealed (Ga,Mn)As layers studied by x-ray diffraction and standing-wave uorescence

A combination of high-resolution x-ray diffraction and a new technique of x-ray standing wave uorescence at grazing incidence is employed to study the structure of (Ga,Mn)As diluted magnetic semiconductor and its changes during post-growth annealing steps. We find that the film is formed by a uniform, single crystallographic phase epilayer covered by a thin surface layer with enhanced Mn concentration due to Mn atoms at random non-crystallographic positions. In the epilayer, Mn incorporated at interstitial position has a dominant effect on lattice expansion as compared to substitutional Mn. The expansion coeffcient of interstitial Mn estimated from our data is consistent with theory predictions. The concentration of interstitial Mn and the corresponding lattice expansion of the epilayer are reduced by annealing, accompanied by an increase of the density of randomly distributed Mn atoms in the disordered surface layer. Substitutional Mn atoms remain stable during the low-temperature annealing.Comment: 9 pages, 9 figure

Comparison of tillage systems in terms of water infiltration into the soil during the autumn season

ArticleThe soil belongs to the most valuable parts of the planet Earth. It is, endangered by water erosion, which causes huge destruction every year, or damage to farmland. More than half of the agricultural land in the Czech Republic is threatened by water erosion. The measurement was performed in the location Nesperská Lhota. The trial has been established on loamy sand Cambisol. In the field experiment, there were created 6 different variants which wad differed by soil tillage and crop. In the individual variants maize and oats were located. The field trial has been existing for a long time, as it was founded in 2009. Two measuring methods of water infiltration were used for the measurements: a mini disk infiltrometer and a single ring. The measurement was performed in the period of September 2016 before the harvest of maize. The soil aggregates were already stabilized at that time after all tillage operations. The measurement result showed the difference between the methods of soil tillage. The greatest ability of infiltration had a variant of maize with inter-row oats. Surprisingly, it was followed by maize, which was processed by ploughing technology. The lowest infiltration capacity was showed by oats reduced by soil tillage. A variant without vegetation had the second lowest infiltration. Our results obtained at rate of water infiltration into the soil affirm the need to control measures in the late vegetative stages. It is important for most of the rainfall to be quickly infiltrated so that it prevents the formation of massive surface runoff

Systematic study of Mn-doping trends in optical properties of (Ga,Mn)As

We report on a systematic study of optical properties of (Ga,Mn)As epilayers spanning the wide range of accessible substitutional Mn_Ga dopings. The growth and post-growth annealing procedures were optimized for each nominal Mn doping in order to obtain films which are as close as possible to uniform uncompensated (Ga,Mn)As mixed crystals. We observe a broad maximum in the mid-infrared absorption spectra whose position exhibits a prevailing blue-shift for increasing Mn-doping. In the visible range, a peak in the magnetic circular dichroism blue shifts with increasing Mn-doping. These observed trends confirm that disorder-broadened valence band states provide a better one-particle representation for the electronic structure of high-doped (Ga,Mn)As with metallic conduction than an energy spectrum assuming the Fermi level pinned in a narrow impurity band.Comment: 22 pages, 14 figure

Winter wheat, winter rape and poppy crop growth evaluation with the help of remote and proximal sensing measurements

Monitoring of agricultural crops with the help of remote and proximal sensors during the growing season plays important role for site-specific management decisions. Winter wheat, winter rape and poppy are representatives of typical agricultural crops from the family Poacea, Brassicaceae and Papaveraceae, growing in relative dry area of Rakovník district in the Czech Republic. Ten Sentinel 2 satellite images acquired during vegetation season of the crops were downloaded and processed. Crops were monitored with the help of unmanned aerial vehicles (UAV) equipped with consumer grade Red Green Blue (RGB) camera and multispectral (MS) MicaSense RedEdge MX camera. In-field variability was assessed by computing RGB-based vegetation indices Triangular Greenness Index (TGI), Green Leaf Index (GLI) and Visible Atmospherically Resistant Index (VARI) and commonly used vegetation indices as Normalised Difference Vegetation Index (NDVI) and Green NDVI (GNDVI). The results derived from satellite and UAV images were supported with in-situ measurements of hand-held GreenSeeker and Chlorophyll Meter Content sensors. The study showed the usability of individual vegetation indices, especially the TGI index for chlorophyll content estimation, and VARI index for green vegetation fraction detection and leaf area index estimation, in comparison with selected handheld devices. The results showed as well that leaf properties and canopy structure of typical characteristics of selected families can significantly influence the spectral response of the crops detected in different phenological stages

Prospect for room temperature tunneling anisotropic magnetoresistance effect: density of states anisotropies in CoPt systems

Tunneling anisotropic magnetoresistance (TAMR) effect, discovered recently in (Ga,Mn)As ferromagnetic semiconductors, arises from spin-orbit coupling and reflects the dependence of the tunneling density of states in a ferromagnetic layer on orientation of the magnetic moment. Based on ab initio relativistic calculations of the anisotropy in the density of states we predict sizable TAMR effects in room-temperature metallic ferromagnets. This opens prospect for new spintronic devices with a simpler geometry as these do not require antiferromagnetically coupled contacts on either side of the tunnel junction. We focus on several model systems ranging from simple hcp-Co to more complex ferromagnetic structures with enhanced spin-orbit coupling, namely bulk and thin film L1$_0$-CoPt ordered alloys and a monatomic-Co chain at a Pt surface step edge. Reliability of the predicted density of states anisotropies is confirmed by comparing quantitatively our ab initio results for the magnetocrystalline anisotropies in these systems with experimental data.Comment: 4 pages, 2 figure