186 research outputs found
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.
Removal of anionic surfactant from aqueous solutions by adsorption onto biochars:characterisation, kinetics, and mechanism
Biochar, a waste biomass-derived adsorbent, holds promise for decentralised wastewater treatment. However, limited research exists on its efficacy in adsorbing anionic surfactants in wastewater. To address this, the adsorption of sodium dodecyl sulphate (SDS), a common anionic surfactant, was studied using various biochar types: rice husk biochar (RH-550 and RH-700), wheat straw biochar (WS-550 and WS-700) produced at 550°C and 700°C, wood-based biochar (OB), and activated carbon (AC) as a control. The study investigated the impact of pH (3–9), adsorbent loading (1–10 g/L), adsorbent size (<0.5–2.5 mm), contact time (5–180 min), and initial concentration (50–200 mg/L) on SDS removal. Under optimised conditions (100 mg/L SDS, 4 g/L adsorbent, 1–2 mm particle size, pH 8.3, and 180 min contact time), maximum SDS removals were RH-550 (78%), RH-700 (82.4%), WS-550 (89.5%), WS-700 (90.4%), AC (97%), and OB (88.4%). Among the tested adsorbent materials, WS-550 exhibited the highest SDS adsorption capacity at 66.23 mg/g compared to AC (80.65 mg/g), followed by RH-550 (49.75 mg/g), OB (45.87 mg/g), RH-700 (43.67 mg/g), and WS-700 (42.74 mg/g). SDS adsorption followed a pseudo-second-order kinetic model, indicating chemisorption on the adsorbent surface. The Freundlich isotherm model exhibited a better fit for the experimental data on SDS adsorption using all tested adsorbents except for RH-550. This study showed that biochars produced from agricultural and forestry residues are effective adsorbents for SDS in aqueous solutions and can be a promising sustainable and low-cost material for the treatment of greywater containing anionic surfactants (e.g. handwashing, laundry, kitchen, and bathroom greywaters)
Microscopic analysis of the valence band and impurity band theories of (Ga,Mn)As
We analyze microscopically the valence and impurity band models of
ferromagnetic (Ga,Mn)As. We find that the tight-binding Anderson approach with
conventional parameterization and the full potential LDA+U calculations give a
very similar picture of states near the Fermi energy which reside in an
exchange-split sp-d hybridized valence band with dominant orbital character of
the host semiconductor; this microscopic spectral character is consistent with
the physical premise of the k.p kinetic-exchange model. On the other hand, the
various models with a band structure comprising an impurity band detached from
the valence band assume mutually incompatible microscopic spectral character.
By adapting the tight-binding Anderson calculations individually to each of the
impurity band pictures in the single Mn impurity limit and then by exploring
the entire doping range we find that a detached impurity band does not persist
in any of these models in ferromagnetic (Ga,Mn)As.Comment: 29 pages, 25 figure
The period-gap cataclysmic variable CzeV404 Her: A link between SW Sex and SU UMa systems
Context: We present a new study of the eclipsing cataclysmic variable CzeV404
Her (Porb = 0.098 d) that is located in the period gap. Aims: This report
determines the origin of the object and the system parameters and probes the
accretion flow structure of the system. Methods: We conducted simultaneous
time-resolved photometric and spectroscopic observations of CzeV404 Her. We
applied our light-curve modelling techniques and the Doppler tomography method
to determine the system parameters and analyse the structure of the accretion
disk. Results: We found that the system has a massive white dwarf M_WD =
1.00(2) M_sun a mass ratio of q = 0.16, and a relatively hot secondary with an
effective temperature T_2 = 4100(50) K. The system inclination is i =
78.8{\deg}. The accretion disk spreads out to the tidal limitation radius and
has an extended hot spot or line region. The hot spot or line is hotter than
the remaining outer part of the disk in quiescence or in intermediate state,
but does not stand out completely from the disk flux in (super)outbursts.
Conclusions: We claim that this object represents a link between two distinct
classes of SU UMa-type and SW Sex-type cataclysmic variables. The accretion
flow structure in the disk corresponds to the SW Sex systems, but the physical
conditions inside the disk fit the behaviour of SU UMa-type objects
Theory of ferromagnetic (III,Mn)V semiconductors
The body of research on (III,Mn)V diluted magnetic semiconductors initiated
during the 1990's has concentrated on three major fronts: i) the microscopic
origins and fundamental physics of the ferromagnetism that occurs in these
systems, ii) the materials science of growth and defects and iii) the
development of spintronic devices with new functionalities. This article
reviews the current status of the field, concentrating on the first two, more
mature research directions. From the fundamental point of view, (Ga,Mn)As and
several other (III,Mn)V DMSs are now regarded as textbook examples of a rare
class of robust ferromagnets with dilute magnetic moments coupled by
delocalized charge carriers. Both local moments and itinerant holes are
provided by Mn, which makes the systems particularly favorable for realizing
this unusual ordered state. Advances in growth and post-growth treatment
techniques have played a central role in the field, often pushing the limits of
dilute Mn moment densities and the uniformity and purity of materials far
beyond those allowed by equilibrium thermodynamics. In (III,Mn)V compounds,
material quality and magnetic properties are intimately connected. In the
review we focus on the theoretical understanding of the origins of
ferromagnetism and basic structural, magnetic, magneto-transport, and
magneto-optical characteristics of simple (III,Mn)V epilayers, with the main
emphasis on (Ga,Mn)As. The conclusions we arrive at are based on an extensive
literature covering results of complementary ab initio and effective
Hamiltonian computational techniques, and on comparisons between theory and
experiment.Comment: 58 pages, 49 figures Version accepted for publication in Rev. Mod.
Phys. Related webpage: http://unix12.fzu.cz/ms
Diet during early life defines testicular lipid content and sperm quality in adulthood
Childhood obesity is a serious concern associated with ill health later in life. Emerging data suggest that obesity has long-term adverse effects upon male sexual and reproductive health but few studies addressed this issue. We hypothesized that exposure to high-fat diet during early life alters testicular lipid content and metabolism leading to permanent damage to sperm parameters. After weaning (day 21 after birth), 36 male mice were randomly divided into 3 groups and fed with different diet regimen for 200 days: CTRL-standard chow; HFD-high-fat diet (Carbohydrate: 35.7%, Protein: 20.5%, Fat: 36.0%); HFDt-high-fat diet for 60 days then replaced by standard chow. Biometric and metabolic data were monitored. Animals were then sacrificed, and tissues collected. Epididymal sperm parameters and endocrine parameters were evaluated. Testicular metabolites were extracted and characterized by 1H-NMR and GC-MS. Testicular mitochondrial and antioxidant activity were evaluated. Our results show that mice fed with high-fat diet, even if only until early adulthood, had lower sperm viability and motility, and higher incidence of head and tail defects. Although diet reversion with weight loss during adulthood prevents the progression of metabolic syndrome, testicular content in fatty acids is irreversibly affected. Excessive fat intake promoted an over-accumulation of pro-inflammatory n-6 polyunsaturated fatty acids in testis, which are strongly correlated with negative effects upon sperm quality. Therefore, the adoption of high-fat diets during early life correlates to irreversible changes in testicular lipid content and metabolism, which are related to permanent damage to sperm quality later in life
Resonances in a two-dimensional electron waveguide with a single delta-function scatterer
We study the conductance properties of a straight two-dimensional electron
waveguide with an s-like scatterer modeled by a single delta-function potential
with a finite number of modes. Even such a simple system exhibits interesting
resonance phenomena. These resonances are explained in terms of quasi-bound
states both by using a direct solution of the Schroedinger equation and by
studying the Green's function of the system. Using the Green's function we
calculate the survival probability as well as the power absorption and show the
influence of the quasi-bound states on these two quantities.Comment: 5 pages, 6 figures, to be published in Physical Review
On the character of states near the Fermi level in (Ga,Mn)As: impurity to valence band crossover
We discuss the character of states near the Fermi level in Mn doped GaAs, as
revealed by a survey of dc transport and optical studies over a wide range of
Mn concentrations. A thermally activated valence band contribution to dc
transport, a mid-infrared peak at energy hbar omega approx 200 meV in the ac-
conductivity, and the hot photoluminescence spectra indicate the presence of an
impurity band in low doped (<<1% Mn) insulating GaAs:Mn materials. Consistent
with the implications of this picture, both the impurity band ionization energy
inferred from the dc transport and the position of the mid-infrared peak move
to lower energies and the peak broadens with increasing Mn concentration. In
metallic materials with > 2% doping, no traces of Mn-related activated
contribution can be identified in dc-transport, suggesting that the impurity
band has merged with the valence band. No discrepancies with this perception
are found when analyzing optical measurements in the high-doped GaAs:Mn. A
higher energy (hbar omega approx 250 meV) mid-infrared feature which appears in
the metallic samples is associated with inter-valence band transitions. Its
red-shift with increased doping can be interpreted as a consequence of
increased screening which narrows the localized-state valence-band tails and
weakens higher energy transition amplitudes. Our examination of the dc and ac
transport characteristics of GaAs:Mn is accompanied by comparisons with its
shallow acceptor counterparts, confirming the disordered valence band picture
of high-doped metallic GaAs:Mn material.Comment: 10 pages, 12 figure
Magneto crystalline anisotropies in (Ga,Mn)As: A systematic theoretical study and comparison with experiment
We present a theoretical survey of magnetocrystalline anisotropies in
(Ga,Mn)As epilayers and compare the calculations to available experimental
data. Our model is based on an envelope function description of the valence
band holes and a spin representation for their kinetic-exchange interaction
with localised electrons on Mn ions, treated in the mean-field approximation.
For epilayers with growth induced lattice-matching strains we study in-plane to
out-of-plane easy-axis reorientations as a function of Mn local-moment
concentration, hole concentration, and temperature. Next we focus on the
competition of in-plane cubic and uniaxial anisotropies. We add an in-plane
shear strain to the effective Hamiltonian in order to capture measured data in
bare, unpatterned epilayers, and we provide microscopic justification for this
approach. The model is then extended by an in-plane uniaxial strain and used to
directly describe experiments with strains controlled by postgrowth lithography
or attaching a piezo stressor. The calculated easy-axis directions and
anisotropy fields are in semiquantitative agreement with experiment in a wide
parameter range
COâ‚‚ gasification of bio-char derived from conventional and microwave pyrolysis
Thermal-chemical processing of biomass is expected to provide renewable and clean energy and fuels in the future. Due to the nature of endothermic reactions, microwave and conventional heating have been applied to this technology. However, more studies need to be carried out to clarify the difference between these two heating technologies. In this work, we investigated two bio-char samples produced from conventional pyrolysis of wood biomass (yield of bio-char: 38.48 and 59.70 wt.%, respectively) and one bio-char produced from microwave pyrolysis with a yield of 45.16 wt.% from the same biomass sample at different process conditions. Various methodologies have been used to characterise the bio-chars. COâ‚‚ gasification of bio-char has also been studied using a thermogravimetric analyser (TGA) and a fixed-bed reaction system. The results show that volatile and carbon contents of the bio-char derived from microwave pyrolysis were between the two conventional bio-chars. However, the microwave bio-char is more reactive for COâ‚‚ gasification, as more CO was released during TGA experiments, and the CO release peak was narrower compared with the COâ‚‚ gasification of the conventional bio-chars. It is suggested that the conventional bio-char is less reactive due to the presence of more secondary chars which are produced from secondary reactions of volatiles during the conventional biomass pyrolysis. While the microwave pyrolysis generates more uniform bio-chars with less secondary char, and therefore, has advantages of producing bio-char for downstream char gasification
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