1,945 research outputs found
First-principles calculation of the piezoelectric tensor d of III-V nitrides
We report direct first-principles density-functional calculations of the
piezoelectric tensor \tensor{d} relating polarization to applied stress for
the binary compounds AlN, GaN, and InN. The values of \tensor{d} are rather
sensitive to the choice of the exchange-correlation functional, and results are
presented for both the local-density and gradient approximations. A comparison
with experiment and with values predicted indirectly from the elastic and
e-piezoconstant tensors is also presented.Comment: RevTeX 3 pages, no figure
Structure and gap of low- (GaIn)O alloys
We study the electronic and local structural properties of pure and
In-substituted -GaO using density functional theory (DFT). Our
main result is that the structural energetics of In in GaO causes most
sites to be essentially inaccessible to In substitution, thus reducing the
maximum In content in thi to somewhere between 12 and 25 \% in this phase. We
also find that the gap variation with doping is essentially due to "chemical
pressure", i.e. volume variations with doping.Comment: 4 pages, 3 figure
Stability of Ge-related point defects and complexes in Ge-doped SiO_2
We analyze Ge-related defects in Ge-doped SiO_2 using first-principles
density functional techniques. Ge is incorporated at the level of ~ 1 mol % and
above. The growth conditions of Ge:SiO_2 naturally set up oxygen deficiency,
with vacancy concentration increasing by a factor 10^5 over undoped SiO_2, and
O vacancies binding strongly to Ge impurities. All the centers considered
exhibit potentially EPR-active states, candidates for the identification of the
Ge(n) centers. Substitutional Ge produces an apparent gap shrinking via its
extrinsic levels.Comment: RevTeX 4 pages, 2 ps figure
Beauveria bassiana strain ATCC 74040 (Naturalis®), a valuable tool for the control of the cherry fruit fly (Rhagoletis cerasi)
Naturalis® is a bioinsecticide based on living conidiospores of the naturally occuring
Beauveria bassiana strain ATCC 74040. The entomopathogenic fungus acts primarily by
contact: once attached to the insect’s cuticle, the conidiospores germinate producing
penetration hyphae, which enter and proliferate inside the insect’s body. The fungus
invades and feeds on its host, causing its death due to dehydration and/or depletion of
nutrients. Several years of laboratory, semi-field and field studies showed that also Tephritid
flies (Ceratitis capitata, Rhagoletis cerasi, Bactrocera oleae) are susceptible to infection by
B. bassiana strain ATCC 74040. The results of efficacy trials conducted in 2004-05 are
reported. Naturalis was tested both alone and in an integrated pest management strategy.
The product showed high efficacy in controlling R. cerasi, comparable to or higher than
that of the chemical reference treatment. The B. bassiana-based product Naturalis can thus
be considered an efficient tool for the control of the cherry fruit fly
The cosmological 7Li problem from a nuclear physics perspective
The primordial abundance of 7Li as predicted by Big Bang Nucleosynthesis
(BBN) is more than a factor 2 larger than what has been observed in metal-poor
halo stars. Herein, we analyze the possibility that this discrepancy originates
from incorrect assumptions about the nuclear reaction cross sections relevant
for BBN. To do this, we introduce an efficient method to calculate the changes
in the 7Li abundance produced by arbitrary (temperature dependent)
modifications of the nuclear reaction rates. Then, considering that 7Li is
mainly produced from 7Be via the electron capture process 7Be + e -> 7Li +
nu_e, we assess the impact of the various channels of 7Be destruction.
Differently from previous analysis, we consider the role of unknown resonances
by using a complete formalism which takes into account the effect of Coulomb
and centrifugal barrier penetration and that does not rely on the use of the
narrow-resonance approximation. As a result of this, the possibility of a
nuclear physics solution to the 7Li problem is significantly suppressed. Given
the present experimental and theoretical constraints, it is unlikely that the
7Be + n destruction rate is underestimated by the 2.5 factor required to solve
the problem. We exclude, moreover, that resonant destruction in the channels
7Be + t and 7Be + 3He can explain the 7Li puzzle. New unknown resonances in 7Be
+ d and 7Be + alpha could potentially produce significant effects. Recent
experimental results have ruled out such a possibility for 7Be+d. On the other
hand, for the 7Be + alpha channel very favorable conditions are required. The
possible existence of a partially suitable resonant level in 11C is studied in
the framework of a coupled-channel model and the possibility of a direct
measurement is considered.Comment: Final version, accepted for publication in JCA
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