4 research outputs found
Electronic structure of -SiAlON: effect of varying Al/O concentration at finite temperatures relevant for thermal quenchin
-SiAlON is a prominent example of systems
suitable as hosts for creating materials for light-emitting diodes. In this
work, the electronic structure of a series of ordered and disordered
-SiAlON systems is investigated by means of
ab initio calculations, using the FLAPW and the Green function KKR methods.
Finite temperature effects are included within the alloy analogy model. We
found that the trends with the Al/O doping are similar for ordered and
disordered structures. The electronic band gap decreases with increasing by
about 1 eV when going from =0 to =2. The optical gap decreases
analogously as the electronic band gap. The changes in the density of states
(DOS) at Si and N atoms introduced by doping -SiN with Al
and O are comparable to the DOS at Al and O atoms themselves. The bottom of the
conduction band in -SiAlON is formed by
extended states residing on all atomic types. Increasing the temperature leads
to a shift of the bottom of the conduction band to lower energies. The amount
of this shift increases with increasing doping .Comment: 11 pages, 11 figures, 1 tabl
Birds girdling activity on exotic tree species as a form of adaptive behavior?
Four tree species in the Kostelec n. . l. arboretum (Czech Republic) have been repeatedly damaged by Dendrocopos medius. The unique aspect of this otherwise common behavior called girdling consists in regularly visiting the same trees every spring, although there are more than 1.200 tree species within the arboretum. We monitored transpiration, leaf phenology and the chemical composition of the xylem sap of girdled and nongirdled trees. Spectral analysis revealed slightly higher amounts of sugars, especially saccharose, in Cladrastis Raf. as the most regularly girdled tree among other conditions, comparing girdled to non-girdled trees. Higher transpiration rates were not confirmed in connection with girdling-quite the opposite-Cladrastis Raf. as the most highly favored tree for girdling showed the lowest transpiration rates (in average 6 kg water per day within spring months) compared to other non-girdled trees. We presume that the birds do not choose a particular tree on the basis of any visible or chemical traits but they examine many trees within their territory. Afterwards they probably remember the position of trees whose xylem sap starts to flow early in the spring compared to other trees, as their transpiration stream is enriched with sweet organic substances that represent an advantage for the forthcoming nesting period