79,368 research outputs found
Origin of ferromagnetism in CsAgF: importance of Ag - F covalency
The magnetic nature of CsAgF, an isoelectronic and isostructural
analogue of LaCuO, is analyzed using density functional calculations.
The ground state is found to be ferromagnetic and nearly half metallic. We find
strong hybridization of Ag- and F- states. Substantial moments reside on
the F atoms, which is unusual for the halides and reflects the chemistry of the
Ag(II) ions in this compound. This provides the mechanism for ferromagnetism,
which we find to be itinerant in character, a result of a Stoner instability
enhanced by Hund's coupling on the F
Frustration of tilts and A-site driven ferroelectricity in KNbO_3-LiNbO_3 alloys
Density functional calculations for K_{0.5}Li_{0.5}NbO_3 show strong A-site
driven ferroelectricity, even though the average tolerance factor is
significantly smaller than unity and there is no stereochemically active A-site
ion. This is due to the frustration of tilt instabilities by A-site disorder.
There are very large off-centerings of the Li ions, which contribute strongly
to the anisotropy between the tetragonal and rhombohedral ferroelectric states,
yielding a tetragonal ground state even without strain coupling.Comment: 4 pages, 5 figure
Dislocation nucleation and vacancy formation during high-speed deformation of fcc metals
Recently, a dislocation free deformation mechanism was proposed by Kiritani
et al., based on a series of experiments where thin foils of fcc metals were
deformed at very high strain rates. In the experimental study, they observed a
large density of stacking fault tetrahedra, but very low dislocation densities
in the foils after deformation. This was interpreted as evidence for a new
dislocation-free deformation mechanism, resulting in a very high vacancy
production rate.
In this paper we investigate this proposition using large-scale computer
simulations of bulk and thin films of copper. To favour such a dislocation-free
deformation mechanism, we have made dislocation nucleation very difficult by
not introducing any potential dislocation sources in the initial configuration.
Nevertheless, we observe the nucleation of dislocation loops, and the
deformation is carried by dislocations. The dislocations are nucleated as
single Shockley partials.
The large stresses required before dislocations are nucleated result in a
very high dislocation density, and therefore in many inelastic interactions
between the dislocations. These interactions create vacancies, and a very large
vacancy concentration is quickly reached.Comment: LaTeX2e, 8 pages, PostScript figures included. Minor modifications
only. Final version, to appear in Philos. Mag. Let
Modelling of dislocation generation and interaction during high-speed deformation of metals
Recent experiments by Kiritani et al. have revealed a surprisingly high rate
of vacancy production during high-speed deformation of thin foils of fcc
metals. Virtually no dislocations are seen after the deformation. This is
interpreted as evidence for a dislocation-free deformation mechanism at very
high strain rates.
We have used molecular-dynamics simulations to investigate high-speed
deformation of copper crystals. Even though no pre-existing dislocation sources
are present in the initial system, dislocations are quickly nucleated and a
very high dislocation density is reached during the deformation.
Due to the high density of dislocations, many inelastic interactions occur
between dislocations, resulting in the generation of vacancies. After the
deformation, a very high density of vacancies is observed, in agreement with
the experimental observations. The processes responsible for the generation of
vacancies are investigated. The main process is found to be incomplete
annihilation of segments of edge dislocations on adjacent slip planes. The
dislocations are also seen to be participating in complicated dislocation
reactions, where sessile dislocation segments are constantly formed and
destroyed.Comment: 8 pages, LaTeX2e + PS figures. Presented at the Third Workshop on
High-speed Plastic Deformation, Hiroshima, August 200
Electronic Structure and Thermoelectric Prospects of Phosphide Skutterudites
The prospects for high thermoelectric performance in phosphide skutterudites
are investigated based on first principles calculations. We find that
stoichiometric CoP_3 differs from the corresponding arsenide and antimonide in
that it is metallic. As such the band structure must be modified if high
thermopowers are to be achieved. In analogy to the antimonides it is expected
that this may be done by filling with La. Calculations for LaFe_4P_12 show that
a gap can in fact be opened by La filling, but that the valence band is too
light to yield reasonable p-type thermopowers at appropriate carrier densities;
n-type La filled material may be more favorable.Comment: 3 pages, 3 figures, 1 tabl
Knowlesi malaria in Vietnam
The simian malaria parasite Plasmodium knowlesi is transmitted in the forests of Southeast Asia. Symptomatic zoonotic knowlesi malaria in humans is widespread in the region and is associated with a history of spending time in the jungle. However, there are many settings where knowlesi transmission to humans would be expected but is not found. A recent report on the Ra-glai population of southern central Vietnam is taken as an example to help explain why this may be so
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