Sergio Sanchez

We employed our recent version of the Schwinger multichannel method, implemented with a model potential [F. F. Frighetto, A. S. Barbosa, and S. d'A. Sanchez, Phys. Rev. A 108, 012818 (2023)] to calculate cross sections for elastic positron scattering by a set of hydrocarbons. Initially, by comparing the model potential calculations for methane, ethylene, and acetylene with our best possible ab initio calculations to date, we obtained the cutoff parameter for the carbon atom. Our results show that this parameter depends on the hybridization of the carbon atom for each molecule. We then applied the same cutoff parameter for the model potential to calculate the cross sections for other saturated and unsaturated hydrocarbons with encouraging results. We also discuss the positron binding to these hydrocarbons and compare the present results with available data from the literature.

In the present article we have proposed and tested a new polarization scheme within the Schwinger Multichannel method based on a model potential to describe positron-molecule polarization and correlation. For this test, we have calculated integral and differential cross sections for the elastic scattering of positrons by pyrazine and compared these results with the existing theoretical and experimental data. Furthermore, in order to evaluate the possible bound state, we have also obtained the scattering lenghts. Our current results greatly improved previous {\it ab-initio} calculations showing a significantly better agreement between the theoretical results and experimental results, particularly at low angles for the differential cross sections.

This work reports a theoretical investigation on the elastic-positron collision with the 1,1-difluoroethylene molecule for energies up to 10 eV. We have employed the Schwinger multichannel method in the static-plus polarization level of approximation to obtain the integral and differential cross sections. In order to account for the long-range potential due to the permanent dipole moment of the target, the Born-closure procedure has been employed. Also, our present differential cross sections have been applied to estimate a correction to the total cross section measurements available in the literature.

This work reports on cross sections calculations for elastic scattering of low-energy positrons by acetone (C₃H₆O) molecule. For this purpose, we employed the Schwinger multichannel method to calculate integral and differential cross sections in the static plus polarization approximation for energies up to 10 eV. Acetone is a polar molecule, and the effect of the long-range dipole interaction was taken into account through the Born-closure scheme. Our integral cross section was compared with the experimental total cross section results available in the literature, which do not agree among themselves below 2 eV. Our results agree qualitatively well with the most recent experimental data available in all energy region. Particularly, below the positronium formation channel threshold, when the experimental data is corrected due to the angular resolution of the apparatus, the quantitative agreement becomes impressive.

We report calculated elastic integral, differential, and momentum transfer cross sections for electron and positron collisions with the formamide (HCONH₂) molecule, for impact energies up to 10 eV. We have used the Schwinger multichannel method in the static-exchange and static-exchange plus polarization approximation for collisions of electrons and the static plus polarization approximation for collisions of positrons. The Born-closure procedure was applied to account for the long-range potential due to the permanent dipole moment of formamide. We obtained the well-characterized π* shape resonance located at around 2.38 eV, which belongs to the A″ symmetry of the C_s point group. Our integral and differential cross sections for collisions of electrons were compared with the data available in the literature and showed a good qualitative agreement. To the best of our knowledge, no experimental and theoretical data are currently available for positron–formamide collision, so our present cross sections were compared with the cross sections of formic acid, which is also polar and is isoeletronic to formamide.

Although geographic isolation has been shown to play a key role in promoting reproductive isolation, it is now believed that speciation can also happen in sympatry and with considerable gene flow. Here we present a model of sympatric speciation based on assortative mating that does not require a genetic threshold for reproduction, i.e., that does not directly associate genetic differences between individuals with reproductive incompatibilities. In the model individuals mate with the most similar partner in their pool of potential mates, irrespective of how dissimilar it might be. We show that assortativity alone can lead to the formation of clusters of genetically similar individuals. The absence of a minimal genetic similarity for mating implies the constant generation of hybrids and brings up the old problem of species definition. Here, we define species based on clustering of genetically similar individuals but allowing genetic flow among different species. We show that the results obtained with the present model are in good agreement with empirical data, in which different species can still reproduce and generate hybrids.