4,475 research outputs found
Electric-Field Gradient at Cd Impurities in In2o3. A FLAPW Study
We report an ab initio study of the electric-field gradient tensor (EFG) at
Cd impurities located at both inequivalent cationic sites in the semiconductor
In2O3. Calculations were performed with the FLAPW method, that allows us to
treat the electronic structure of the doped system and the atomic relaxations
introduced by the impurities in the host lattice in a fully self-consistent
way. From our results for the EFG (in excellent agreement with the
experiments), it is clear that the problem of the EFG at impurities in In2O3
cannot be described by the point-charge model and antishielding factors.Comment: 4 pages, 2 figures, and 2 table
Transient jets in the symbiotic prototype Z Andromedae
We present development of the collimated bipolar jets from the symbiotic
prototype Z And that appeared and disappeared during its 2006 outburst. In 2006
July Z And reached its historical maximum at U ~ 8.0. During this period, rapid
photometric variations with Dm ~ 0.06 mag on the timescale of hours developed.
Simultaneously, high-velocity satellite components appeared on both sides of
the H-alpha and H-beta emission line profiles. They were launched
asymmetrically with the red/blue velocity ratio of 1.2 - 1.3. From about
mid-August they became symmetric. Their spectral properties indicated ejection
of bipolar jets collimated within an average opening angle of 6.1 degrees. We
estimated average outflow rate via jets to dM(jet)/dt ~
2xE10-6(R(jet)/1AU)**(1/2) M(Sun)/year, during their August - September
maximum, which corresponds to the emitting mass in jets, M(jet, emitting) ~
6xE-10(Rjet)/1AU)^{3/2} M(Sun). During their lifetime, the jets released the
total mass of M(jet, total) approx 7.4x1E-7 M(Sun). Evolution in the rapid
photometric variability and asymmetric ejection of jets around the optical
maximum can be explained by a disruption of the inner parts of the disk caused
by radiation-induced warping of the disk.Comment: 31 pages, 9 figures, 2 tables, accepted for Ap
The problem of a metal impurity in an oxide: ab-initio study of electronic and structural properties of Cd in Rutile TiO2
In this work we undertake the problem of a transition metal impurity in an
oxide. We present an ab-initio study of the relaxations introduced in TiO2 when
a Cd impurity replaces substitutionally a Ti atom. Using the Full-Potential
Linearized-Augmented-Plane-Wave method we obtain relaxed structures for
different charge states of the impurity and computed the electric-field
gradients (EFGs) at the Cd site. We find that EFGs, and also relaxations, are
dependent on the charge state of the impurity. This dependence is very
remarkable in the case of the EFG and is explained analyzing the electronic
structure of the studied system. We predict fairly anisotropic relaxations for
the nearest oxygen neighbors of the Cd impurity. The experimental confirmation
of this prediction and a brief report of these calculations have recently been
presented [P.R.L. 89, 55503 (2002)]. Our results for relaxations and EFGs are
in clear contradiction with previous studies of this system that assumed
isotropic relaxations and point out that no simple model is viable to describe
relaxations and the EFG at Cd in TiO2 even approximately.Comment: 11 pages, 8 figures, Revtex 4, published in Physical Review
Understanding the effects of fungal spillover on tropical forest seedling communities
Deforestation rates of tropical forests have led to a massive increase in forest edge habitats globally. One of the foremost fragmentation types of concern is agriculture, which can introduce new pathogenic fungi into tropical forests via spillover. The long-term effects these spillovers will have on tropical forests are largely unknown but have the potential to influence overall community diversity. This study utilizes both a theoretical model that uses a modified Lotka-Volterra equation and an empirical study in Costa Rican forests to investigate how pathogenic fungal spillover will affect tree seedling diversity. Theoretically, spillover had various effects on plants, depending on a species’ competitive strength, palatability to the pathogen, and overall pathogen strength. In all cases, coexistence of species was inhibited by spillover at the forest edge, however it promoted more instances of coexistence at the interior and rescued weaker plant species when targetting the superior competitor. The study in Costa Rica found that fungal pathogens had no effect on survival but varying effects on overall diversity. This indicates that fungal spillover has the potential to weaken or strengthen mechanisms driving diversity and may be forest-specific, no longer defined by traditional diversity hypotheses. Overall, this study highlights the importance of understanding the effects of fungal spillover and how it may potentially influence the growth and survival of tree seedlings in fragmented tropical forests across the globe
Appearance of room temperature ferromagnetism in Cu-doped TiO films
In recent years there has been an intense search for room temperature
ferromagnetism in doped dilute semiconductors, which have many potentially
applications in spintronics and optoelectronics. We report here the unexpected
observation of significant room temperature ferromagnetism in a semiconductor
doped with nonmagnetic impurities, Cu-doped TiO thin films grown by Pulsed
Laser Deposition. The magnetic moment, calculated from the magnetization
curves, resulted surprisingly large, about 1.5 per Cu atom. A large
magnetic moment was also obtained from ab initio calculations using the
supercell method for TiO with Cu impurities, but only if an oxygen vacancy
in the nearest-neighbour shell of Cu was present. This result suggests that the
role of oxygen vacancies is crucial for the appearance of ferromagnetism. The
calculations also predict that Cu doping favours the formation of oxygen
vacancies.Comment: 4 pages, 3 figures, published in Phys. Rev. B (Rapid Comm.
A theoretical model for tellurite-sulfates NaCu(TeO)(SO)(OH) and KCu(TeO)(SO)(OH)
A theoretical model for two new tellurite-sulfates, namely
NaCu(TeO)(SO)(OH) and KCu(TeO)(SO)
(OH) is determined to be compatible with ab-initio calculations. The
results obtained in this work show that some previous speculations in the
literature about the couplings are correct, obtaining a model with a mixture of
ferromagnetic and antiferromagnetic couplings. We use a combination of
numerical techniques to study the magnetic properties of the model. Our
numerical calculations based on the density-matrix renormalization group method
reveal that the system presents Ising-like magnetization plateaux at rational
values of the saturation magnetization.Comment: 7 pages, 8 figure
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