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-xx (Ga1−x_{1-x}Inx_x)2_2O3_3 alloys

We study the electronic and local structural properties of pure and In-substituted $\beta$-Ga$_2$O$_3$ using density functional theory (DFT). Our main result is that the structural energetics of In in Ga$_2$O$_3$ 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