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Abstract: ZrB2(0001) crystals grown by the rf-floating zone technique were characterized by X-ray photoelectron spectroscopy, reflection high-energy electron diffraction, and atomic force microscopy. These characteristics were investigated as a function of thermal cleaning temperature up to 1000°C in vacuum for as-received substrates as well as substrates treated ex situ in HF aqueous solution. The HF treatment process removed the ZrO2 native oxide layer present on as-received substrates and resulted in ZrB2(0001) surfaces exhibiting long-range order. Upon annealing the HF-treated surface in high vacuum, two types of reconstructions were observed: an incommensurate reconstruction from 650 to 900°C related to residual H2 gas, and n × n reconstructions at 1000°C, possibly related to oxygen. [Copyright &y& Elsevier]

Growth of group-III nitrides was carried out on zirconium diboride (ZrB2) (0 0 0 1) substrates for the first time. ZrB2 is a semi-metal compound and have a hexagonal crystal structure. The a-axis lattice constant of ZrB2 is 3.169 Å, which is almost lattice-matched to GaN and AlN. GaN and AlN were grown on mirror-polished ZrB2 (0 0 0 1) substrate directly by molecular beam epitaxy using elemental Al and Ga, and radio frequency plasma-excited active nitrogen. Epitaxial growth was confirmed by in situ reflection high-energy electron diffraction observation and X-ray diffraction pole-figure measurement. The surface morphology of 1.4 μm-thick GaN layer was rough. On the other hand, that of 0.9 μm-thick AlN layer was specular. [Copyright &y& Elsevier]

Abstract: Stimulated by the “anomalous mode” observed in the time-of-flight measurement of phonon signals [J.K. Wigmore, A.G. Kozorezov, H. bin Rani, M. Giltrow, H. Kraus, B.M. Taele, Physica B 316–317 (2002) 589], we have theoretically studied the group velocities and the phonon images in CaWO4. Comparing the phonon caustics of transverse modes in the low-frequency nondispersive and slightly dispersive 1 THz regions, we propose that the anomalous phonon mode is originated from the phonons near the cuspidal edge of the group velocity curve of the fast transverse branch. Also the low-frequency phonon images (two-dimensional maps of phonon group velocities) calculated from the two sets of published elastic-constant data of CaWO4 [J.M. Farley, G.A. Saunders, J. Phys. C: Solid State Phys. 5 (1972) 3021] and [M. Gluyas, F.D. Hughes, B.M. James, J. Phys. D: Appl. Phys. 6 (1973) 2025] are compared. A significant difference is found in the images in the plane owing to the variance of the reported values of of about 40%. [Copyright &y& Elsevier]

Abstract: Thin (∼25nm) zincblende GaN (111) epilayers have been grown on hexagonal ZrB2 (0001) substrates by plasma-assisted molecular-beam epitaxy at temperatures near 600°C. Layers grown in near-stoichiometric conditions exhibit high phase purity; no wurtzite inclusions are detected by high-resolution X-ray diffraction or photoluminescence. For growth in the presence of excess Ga the layers are predominantly zincblende phase, but contain some wurtzite inclusions. Reducing the growth temperature to 400°C results in a much-increased fraction of wurtzite inclusions. The growth of zincblende GaN with high phase purity on hexagonal ZrB2 implies the existence of an underlying physical mechanism rather than simply a result of “growth mistakes.” A possible surface energy-related mechanism by which zincblende GaN (111) can be preferentially nucleated on hexagonal substrates is discussed. [Copyright &y& Elsevier]

Abstract Ancient hybridization leading to plastid capture is a likely cause of cytonuclear discordance, although discordance may also be attributed to other mechanisms. Cytonuclear discordance is ubiquitous in a clade of southern African Oxalis known as the Hirta clade, despite a notable lack of documented field hybrids for this genus. The aims of this study were to determine whether hybridization is possible between different Hirta clade species, what the fitness effects on seed set of possible hybridizations are, and to determine the mode of inheritance of plastid DNA in Oxalis. We tested these aims using manual interspecific and conspecific crosses, as well as self-pollination control crosses of species within this clade, conducted in a research collection context. Roughly 10% of interspecific crosses produced seed, most of which were confirmed as unambiguous hybrid plants using DNA sequencing of the Internal Transcribed Spacer (ITS). Low levels of self-compatibility were recorded in several species, while one accession showing very high levels of self-compatibility (accession MO1218). Interspecific crosses produced significantly less seed per capsule compared to conspecific crosses with respective parents, suggesting decreased fitness in the establishment of the F1 hybrids. Selfing seed set was significantly lower than interspecific and conspecific seed set upon removal of MO1218. Using plastid fragment length polymorphisms, Oxalis was found to have predominantly or exclusively maternally inherited plastid DNA. This study establishes the viability of the earliest stages of plastid capture in Oxalis , and provides context relevant to the lack of documented field hybrids in the southern African members of this genus. Future studies should focus on F1 survival and fertility and the maternal fitness effects of subsequent backcrosses to the maternal parent, as well as on hybridization rates under field conditions, to assess whether hybridization occurs at rates sufficient to affect Oxalis evolution in southern Africa. Highlights • Produced the first, unequivocal confirmed Oxalis F1 hybrids • Confirm some of the hybrids to be heteroploid • First unequivocal evidence of self-compatibility in SA Oxalis • Show that plastid DNA is maternally inherited in Oxalis [ABSTRACT FROM AUTHOR]

Abstract: Contact zones of different cytotypes provide an opportunity to address evolutionary mechanisms underlying the origin, establishment and maintenance of karyological diversity at intraspecific level. We explored the fine-scale distribution of ploidy levels of Oxalis obtusa in seven mixed-ploidy sites in the Western and Northern Cape Provinces, South Africa, and searched for potential isolating barriers promoting assortative mating. Five different ploidy levels (2x, 3x, 4x, 6x and 8x) were detected among 336 samples collected at 112 microsites. The studied sites were inhabited by two (2 sites), three (4 sites) or even all five (1 site) different cytotypes. Despite their sympatric growth, different ploidies show some spatio-ecological segregation. The greatest differences among microsites hosting different cytotypes were found in precipitation parameters. There is a clear altitudinal gradient in ploidy composition in the most ploidy-variable site, Pakhuis Pass. Our results show that a combination of niche partitioning and clumping of same-ploidy individuals due to vegetative reproduction seems to be efficient reproductive barriers, which limit inter-ploidy gene flow in the zones of ploidy contact and contribute to the long-term maintenance of cytotype mixtures in O. obtusa. [Copyright &y& Elsevier]