288 research outputs found
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 RakovnÃk 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-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
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