Monte Carlo simulations applied to AlxGayIn(1-x-y)X quaternary alloys (X=As,P,N): A comparative study

We develop a different Monte Carlo approach applied to the A(x)B(y)C(1-x-y)D quaternary alloys. Combined with first-principles total-energy calculations, the thermodynamic properties of the (AI,Ga,In)X (X=As, P, or N) systems are obtained and a comparative study is developed in order to understand the roles of As, P, and N atoms as the anion X in the system AlxGayIn1-x-yX. Also, we study the thermodynamics of specific compositions in which AlGaInN, AlGaInP, and AlGaInAs are lattice matched, respectively, to the GaN, GaAs, and InP substrates. We verify that the tendency for phase separation is always towards the formation of an In-rich phase. For arsenides and phosphides this occurs in general for lower temperatures than for their usual growth temperatures. This makes these alloys very stable against phase separation. However, for nitrides the In and/or Al concentrations have to be limited in order to avoid the formation of In-rich clusters and, even for low concentrations of In and/or Al, we observe a tendency of composition fluctuations towards the clustering of the ternary GaInN. We suggest that this latter behavior can explain the formation of the InGaN-like nanoclusters recently observed in the AlGaInN quaternary alloys.712

Microscopic description of the phase separation process in AlxGayIn1-x-yN quaternary alloys

Ab initio total energy electronic structure calculations are combined with Monte Carlo simulations to study the thermodynamic properties of AlxGayIn1-x-yN quaternary alloys. We provide a microscopic description of the phase separation process by analyzing the thermodynamic behavior of the different atoms with respect to the temperature and cation contents. We obtained, at growth temperatures, the range of compositions for the stable and unstable phases. The presence of Al in InGaN is proven to "catalyze" the phase separation process for the formation of the In-rich phase. Based on our results, we propose that the ultraviolet emission currently seen in samples containing AlInGaN quaternaries arises from the matrix of a random alloy, in which composition fluctuations toward InGaN- and AlGaN-like alloys formation may be present, and that a coexisting emission in the green-blue region results from the In-rich segregated clusters.70

Phase stability, chemical bonds, and gap bowing of InxGa1-xN alloys: Comparison between cubic and wurtzite structures

Thermodynamic, structural, and electronic properties of wurtzite InxGa1-xN alloys are studied by combining first-principles total energy calculations with the generalized quasichemical approach, and compared to previous results for the zinc-blende structure. Results for bond-lengths, second-nearest-neighbors distances, and bowing parameter are presented. We observed that the wurtzite results are not significantly different from the ones obtained previously for the zinc-blende structure. The calculated phase diagram of the alloy shows a broad and asymmetric miscibility gap as in the zinc-blende case, with a similar range for the growth temperatures, although with a higher critical temperature. We found a value of 1.44 eV for the gap bowing parameter giving support to the recent smaller band gap bowing findings. We emphasize that other theoretical results may suffer from incomplete sets of atomic configurations to properly describe the alloy properties, and experimental findings. Moreover one must take into account a broad composition range in order to obtain reliable results.74

Theoretical prediction of ferromagnetic MnN layers embedded in wurtzite GaN

We studied, using the spin density functional theory, the manganese mononitride (MnN) grown on GaN in the wurtzite phase, forming the GaN/MnN heterostructures. We obtained a ferromagnetic ground state with a higher magnetic moment than the hypothetical wurtzite bulk MnN. This behavior can be explained in terms of the high magnetization of the MnN interface monolayers that have longer first and second neighbors bond lengths due to structure relaxation. We suggest that this system can be applied to the new spintronics technology by being able to provide spin polarized carriers in the important wide-gap nitride systems.88

Strain-induced ordering in InxGa1-xN alloys

The energetics and thermodynamic properties of cubic (c-)InxGa1-xN alloys are investigated by combining first-principles total energy calculations, a concentration-dependent cluster-based model, and Monte Carlo simulations. The search for the ground-state energies leads to the conclusion that biaxial strain suppresses phase separation, and acts as a driving force for chemical ordering in c-InxGa1-xN alloys. Ordered superlattice structures, with composition xcongruent to0.5 and stable up to T=1000 K, arises as the relevant thermodynamic property of the strained alloy. We suggest that the In-rich phases recently observed by us in c-GaN/InxGa1-xN/GaN double heterostructures are ordered domains formed in the alloy layers due to biaxial strain. (C) 2003 American Institute of Physics.82244274427

Structural properties and Raman modes of zinc blende InN epitaxial layers

We report on x-ray diffraction and micro-Raman scattering studies on zinc blende InN epitaxial films. The samples were grown by molecular beam epitaxy on GaAs(001) substrates using a InAs layer as a buffer. The transverse-optical (TO) and longitudinal-optical phonon frequencies at Gamma of c-InN are determined and compared to the corresponding values for c-GaN. Ab initio self-consistent calculations are carried out for the c-InN c-GaN lattice parameters and TO phonon frequencies. A good agreement between theory and experiment is found. (C) 1999 American Institute of Physics. [S0005-6951(99)00503-3].74336236

Idiopathic central precocious puberty in girls: presentation factors

<p>Abstract</p> <p>Background</p> <p>It is sometimes difficult to distinguish between premature thelarche and precocious puberty in girls who develop breasts before the age of 8 years. We evaluated the frequencies of the signs associated with breast development and the factors influencing the presentation of girls with idiopathic central precocious puberty (CPP).</p> <p>Methods</p> <p>353 girls monitored 0.9 ± 0.7 year after the onset of CPP.</p> <p>Results</p> <p>The age at CPP was < 3 years in 2%, 3–7 years in 38% and 7–8 years in 60% of cases. Pubic hair was present in 67%, growth rate greater than 2 SDS in 46% and bone age advance greater than 2 years in 33% of cases. Breast development was clinically isolated in 70 (20%) cases. However, only 31 of these (8.8% of the population) had a prepubertal length uterus and gonadotropin responses to gonadotropin releasing hormone and plasma estradiol. The clinical picture of CPP became complete during the year following the initial evaluation.</p> <p>25% of cases were obese. The increase in weight during the previous year (3.7 ± 1.4 kg) and body mass index were positively correlated with the statural growth and bone age advance (P < 0.0001).</p> <p>There was no relationship between the clinical-biological presentation and the age at puberty, the interval between the onset of puberty and evaluation, or the presence of familial CPP.</p> <p>Conclusion</p> <p>The variation in presentation of girls with CPP does not depend on their age, interval between the onset and evaluation, or familial factors. This suggests that there are degrees of hypothalamic-pituitary-ovarian activation that are not explained by these factors.</p

Leishmanicidal Metabolites from Cochliobolus sp., an Endophytic Fungus Isolated from Piptadenia adiantoides (Fabaceae)

Protozoan parasites belonging to genera Leishmania and Trypanosoma are the etiological agents of severe neglected tropical diseases (NTDs) that cause enormous social and economic impact in many countries of tropical and sub-tropical areas of the world. In our screening program for new drug leads from natural sources, we found that the crude extract of the endophytic fungus Cochliobolus sp. (UFMGCB-555) could kill 90% of the amastigote-like forms of Leishmania amazonensis and inhibit by 100% Ellman's reagent reduction in the trypanothione reductase (TryR) assay, when tested at 20 µg mL−1. UFMGCB-555 was isolated from the plant Piptadenia adiantoides J.F. Macbr (Fabaceae) and identified based on the sequence of the internally transcribed spacer (ITS) regions of its ribosomal DNA. The chromatographic fractionation of the extract was guided by the TryR assay and resulted in the isolation of cochlioquinone A and isocochlioquinone A. Both compounds were active in the assay with L. amazonensis, disclosing EC50 values (effective concentrations required to kill 50% of the parasite) of 1.7 µM (95% confidence interval = 1.6 to 1.9 µM) and 4.1 µM (95% confidence interval = 3.6 to 4.7 µM), respectively. These compounds were not active against three human cancer cell lines (MCF-7, TK-10, and UACC-62), indicating some degree of selectivity towards the parasites. These results suggest that cochlioquinones are attractive lead compounds that deserve further investigation aiming at developing new drugs to treat leishmaniasis. The findings also reinforce the role of endophytic fungi as an important source of compounds with potential to enter the pipeline for drug development against NTDs