42 research outputs found
Band structure model of magnetic coupling in semiconductors
We present a unified band structure model to explain magnetic ordering in
Mn-doped semiconductors. This model is based on the - and - level
repulsions between the Mn ions and host elements and can successfully explain
magnetic ordering observed in all Mn doped II-VI and III-V semiconductors such
as CdTe, GaAs, ZnO, and GaN. This model, therefore, provides a simple guideline
for future band structure engineering of magnetic semiconductors.Comment: 4+ pages, 5 figure
Electron correlation in the Si(100) surface
Motivated by the controversy between quantum chemists and solid-state
physicists, and by recent experimental results, spin-polarized
density-functional (DFT) calculations are used to probe electron correlation in
the Si(100) reconstructed surface. The ground state displays antiferromagnetic
spin polarization for low dimer inclinations indicating, not magnetic order,
but the importance of Mott-like correlations among dangling bonds. The lowest
energy corresponds to a higher dimer inclination with no spin. DFT energies,
however, should be taken with caution here. Our results together with
quantum-chemical findings suggest dimers with highly correlated electrons that
tend to buckle due to interactions with other dimers.Comment: 5 pages, 1 eps figure, 1 table; RevTeX v3.1. To appear in Surface
Science (proceedings of the European Conference On Surface Science, ECOSS-19,
Madrid, Sept. 5-8, 2000
Preservatives and neutralizing substances in milk: analytical sensitivity of official specific and nonspecific tests, microbial inhibition effect, and residue persistence in milk
Composição química e digestibilidade in vitro de lâminas foliares e colmos de gramíneas forrageiras, em função do nível de inserção no perfilho, da idade e da estação de crescimento
Degradação in vitro de tecidos da lâmina foliar e do colmo de gramíneas forrageiras tropicais, em função do estádio de desenvolvimento
Mass-rearing of Mediterranean fruit fly using low-cost yeast products produced in Brazil
Influência de práticas conservacionistas na umidade do solo e no cultivo do milho (Zea mays L.) em semiárido nordestino
FRUIT MORPHOLOGY AND PRODUCTIVITY OF BABASSU PALMS IN NORTHERN MINAS GERAIS STATE, BRAZIL1
Triterpenos pentacíclicos e esteróides da casca do uchi (Sacoglottis uchi, Humiriaceae)
The ethanol extract from stem bark of Sacoglottis uchi Huber (popularly known as \x93uchi\x94 in the Amazon Region) was submitted to chromatographic fractionation. The dichloromethane fractions provided the pentacyclic triterpene 3-oxo-friedelin (1). The dichloromethane:methanol fractions provided the pentacyclic triterpenes pseudotaraxasterol (2), lupeol (3), a-amyrin (4), betulin (5), and methyl 2ß,3ß-dihydroxy-urs-12-en-28-oate (6) and a mixture of the steroids sitosterol (7) and stigmasterol (8). Their chemical structures were determined by NMR spectroscopy and comparison with spectroscopic data from the literature. All compounds are described for the first time in this species.O extrato etanólico da casca do caule de Sacoglottis uchi Huber (conhecida popularmente como \x93uchi\x94 na Amazônia) foi submetido a fracionamento cromatográfico. As frações eluídas com diclorometano forneceram o triterpeno pentacíclico 3-oxo-friedelina (1). As frações em diclorometano:metanol forneceram os triterpenos pentacíclicos pseudotaraxasterol (2), lupeol (3), a-amirina (4), betulina (5) e 2ß,3ß-di-hidroxi-urs-12-en-28-oato de metila (6), além de uma mistura dos esteróides sitosterol (7) e estigmasterol (8). Suas estruturas químicas foram determinadas por espectroscopia de RMN e comparação com os dados espectroscópicos descritos na literatura. Todas as substâncias isoladas são descritas pela primeira vez nesta espécie