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  • Author: J. Schnell x
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An eigenleitendem Silicium wurden im Temperaturbereich zwischen 300 und 1000°K Leitfähigkeit und Ultrarotabsorption gemessen, letztere zwischen 1,0 und 3,5 μ. Die Wellenlängenabhängigkeit entspricht der Drude-Zener-Fröhlichschen Theorie für die Absorption freier Ladungsträger. Dieser Theorie entsprechend wird durch die Quotientenbildung beider Meßwerte die zunächst unbekannte Trägerzahl eliminiert und quantitativ die Elektronenbeweglichkeit und ihre Temperaturabhängigkeit gewonnen (bE[cm2/V sec] =1,85· 106 T-1.5). Auch die sich daraus ergebende Trägerzahl (NE [cm-3] =5,71 ·1016 T1,5 exp [ -1,16/2 k T]) stimmt mit der von anderen Autoren aus dem HALL-Effekt bestimmten überein. Die Theorie wird damit der experimentellen Absorption im Durchlaßbereich jenseits der Bandkante gerecht. Aus der Verschiebung der Bandkante mit der Temperatur wird die Temperaturabhängigkeit des Bandabstandes ermittelt.

Abstract

Deflections of semiconductor cluster beams in an inhomogeneous electric field deliver enhanced polarizabilities for cold isolated GaNAsM clusters with odd number of atoms n=N+M up to n=17. One speculated that the high polarizabilities result from their electronic open shell structure. This was qualitatively confirmed by optical absorption cross section measurements, but the enhancement of polarizability is too large to be explained through a Kramers–Kronig-relation as purely electronic effect. Moreover, quantum chemical calculations gave permanent dipole moments of some tenths of a Debye for certain compositions. The clusters are too heavy to identify these dipole moments by beam broadening effects, but we will show in the present analytic investigation within classical top theory, that a slight reversible-adiabatic alignment of the clusters dipole moment in the electric field increases the average beam deflection and the derived polarizability values. This effect was up to now not taken into account and it offers a consistent explanation of the GaNAsM polarizabilities by small permanent dipole moments of some tenths of a Debye and a reasonable rotational temperature between 2 and 12K.

Abstract

Density functional calculations on the series of clusters NaMSeN predict different polyanionic selenium fragments depending on the relative amount of sodium. Calculations for Na2SeN clusters give e.g. horseshoe shaped polyanionic selenium chains that are accompanied by two Na atoms. The geometries obtained for NaMSe6 clusters are changing from chains to Se2 and Se3 units highly dependent on the number of Na atoms. These clusters can be investigated experimentally by adding sodium atoms to selenium clusters in the reaction zone of a dual laser vaporization source and depositing them in a nitrogen matrix where they are investigated by Raman spectroscopy. An intense low frequency band between 165 and 225 cm−1 is observed that can be attributed by a simple model of dynamical charge to certain vibrational modes with considerable changes of Na–Se bond lengths. The model spectra allow a quite good description of the low frequency Raman band.

Achievements and Perspectives

Abstract

Inadequate knowledge of the population structure and diversity present often hamper the efficient use of germplasm collections. Using a high through-put system, twelve microsatellite loci were used to analyze genetic diversity and population structure in a national field genebank repository of 243 cacao accessions grouped into 11 populations based on their known sources. Based on multi-locus profiles, the Bayesian method was used for individual assignment to verify membership in each population, determine mislabeling and ancestry of some important accessions used in breeding program. A total of 218 alleles was revealed with a mean number of 18.2 alleles per locus. Gene diversity (He = 0.70) and allelic richness (4.34 alleles per locus) were highest in the F1 hybrid population. Differential mating system was suggested as responsible for the observed deficit and excess of heterozygotes observed among the populations. Analysis of molecular variance showed that within-population variance accounted for 63.0% of the total variance while the rest 37% was accounted for by the among-population variance. Cluster dendrogram based on UPGMA revealed two main subsets. The first group was made up of the Amelonado/Trinitario ancestry and the other of Nanay/Parinari ancestry. We found that Nanay and Parinari populations were the major source of Upper Amazon genes utilized while a large proportion of genetic diversity in the field genebank remained under-utilized in development of improved cultivars released to farmers in Nigeria. This study showed that the presence of alleles of the Upper Amazon Forasteros (Nanay, Parinari and Iquitos Mixed Calabacillo) genetic materials in the locally available accessions predated the formal large scale introduction of Upper Amazon materials in 1944. This is the first report of population structure of field genebank collections of cacao in Nigeria since more than seven decades of formal cacao breeding research.