Narrow Results

Samples of YBa₂Cu₃O_7-δ superconducting ceramic were prepared and characterized by resistivity measurements using the four probe method and structural analysis using X-ray diffraction (XRD). XRD-patterns show the presence of orthorhombic Y-123 phase with a small fraction of secondary phase, YBa₂Cu₃O₅. Samples were irradiated with gamma (γ) rays using Co⁶⁰ source with five successive doses of 2.5 Mrad. A monotonic increase in the T_c0 value was observed for radiation dosage up to 10 Mrad. When exposed to even higher γ-doses, the T_c0 value leveled off at a saturation value or decreased. XRD-analysis shows slightly changed values of the lattice constants and a mild shift of diffraction peaks towards a lower 2θ value. These observations indicate structural changes in YBa₂Cu₃O_7-δ induced upon gamma irradiation. The present work describes our experimental findings and attempts to offer a theoretical explanation for the effects observed.

When we consider structured populations with sexual reproduction, the distribution of characters among mating pairs may influence the population biology, namely its dynamics and genetics. In the present paper, we propose a general framework to estimate the population mating patterns taking into account individual mating preferences and mating system, thus taking into account the inter- and intra-sexual interactions such as mate competition and mate choice. According to our results, mating patterns are not a direct reflection of mating preferences, but also depend upon the average sex ratio between individuals that are ready to mate at any given time (i.e., the Operational Sex Ratio, OSR). Therefore, mating patterns should be assessed not only in function of preferences, but also of OSR. It is then possible to distinguish three OSR regions: (1) the equilibrium OSR, where there is a predominance of assortative mating patterns due to differential access to mates (inter-sexual interactions); (2) the slightly biased OSR, where there is a high diversity of mating patterns and no clear predominance of inter- or intra-sexual interactions; and (3) the highly biased OSR where there is a predominance of mating patterns corresponding to single-sex uniform preferences and an increased influence of intra-sexual interactions. We hope that this approach may allow to further explore the interaction between OSR and mate choice, namely how such interaction may affect sexual selection and mate choice tactics.