Mathematical modeling and investigation on the role of demography and contact patterns in social distancing measures effectiveness in COVID-19 dissemination.

In this article, we investigate the importance of demography and contact patterns in determining the spread of COVID-19 and to the effectiveness of social distancing policies. We investigate these questions proposing an augmented epidemiological model with an age-structured model, with the population divided into susceptible (S), exposed (E), asymptomatic infectious (A), hospitalized (H), symptomatic infectious (I) and recovered individuals (R), to simulate COVID-19 dissemination. The simulations were carried out using six combinations of four types of isolation policies (work restrictions, isolation of the elderly, community distancing and school closures) and four representative fictitious countries generated over alternative demographic transition stage patterns (aged developed, developed, developing and least developed countries). We concluded that the basic reproduction number depends on the age profile and the contact patterns. The aged developed country had the lowest basic reproduction number ($R0=1.74$) due to the low contact rate among individuals, followed by the least developed country ($R0=2.00$), the developing country ($R0=2.43$) and the developed country ($R0=2.64$). Because of these differences in the basic reproduction numbers, the same intervention policies had higher efficiencies in the aged and least developed countries. Of all intervention policies, the reduction in work contacts and community distancing were the ones that produced the highest decrease in the $R0$ value, prevalence, maximum hospitalization demand and fatality rate. The isolation of the elderly was more effective in the developed and aged developed countries. The school closure was the less effective intervention policy, though its effects were not negligible in the least developed and developing countries.

DFT-1/2 method applied to 3D topological insulators.

In this paper, we present results and describe the methodology of application of DFT-1/2 method for five three-dimensional topological insulators materials that have been extensively studied in last years: Bi2Se3, Bi2Te3, Sb2Te3, CuTlSe2and CuTlS2. There are many differences between the results of simple DFT calculations and quasiparticle energy correction methods for these materials, especially for band dispersion in the character band inversion region. The DFT-1/2 leads to quite accurate results not only for band gaps, but also for the shape and atomic character of the bands in the neighborhood of the inversion region as well as the topological invariants, essential quantities to describe the topological properties of materials. The methodology is efficient and ease to apply for the different approaches used to obtain the topological invariantZ2, with the benefit of not increasing the computational cost in comparison with standard DFT, possibilitating its application for materials with a high number of atoms and complex systems.

DFT-1/2 method applied to 2D topological insulators: fluorinated and hydrogenated group-IV honeycomb systems.

We present a computationally efficient and accurate methodology to computeZ2topological invariants for systems without inversion symmetry including quasiparticle (QP) effects within the density functional theory (DFT)-1/2 method. The Wannier charge center evolution is applied to compute theZ2topological invariant and investigate the topological properties of group-IV graphene-like systems, graphene, silicene, germanene and stanene, whose inversion symmetry is broken by simultaneous functionalization with hydrogen and fluorine atoms. Different atomic arrangements are studied. The systems are stable with cohesive energy decreasing along the row from carbon to tin. A similar trend is observed for band gaps. The resulting topological invariants are compared with values obtained within conventional DFT and using a hybrid exchange-correlation functional. The variation of the results with the treatment of exchange and correlation demonstrates the importance of QP corrections for the prediction of the topological or trivial character.

Efficient calculation of excitonic effects in solids including approximated quasiparticle energies.

In this work we present a new procedure to compute optical spectra including excitonic effects and approximated quasiparticle corrections with reduced computational effort. The excitonic effects on optical spectra are included by solving the Bethe-Salpeter equation, considering quasiparticle eigenenergies and respective wavefunctions obtained within DFT-1/2 method. The electron-hole ladder diagrams are approximated by the screened exchange. To prove the capability of the procedure, we compare the calculated imaginary part of the dielectric functions of Si, Ge, GaAs, GaP, GaSb, InAs, InP, and InSb with experimental data. The energy position of the absorption peaks are correctly described. The good agreement with experimental results together with the very significant reduction of computational effort makes the procedure suitable on the investigation of optical spectra of more complex systems.

Thermodynamic Stability and Structural Insights for CH3NH3Pb1-xSixI3, CH3NH3Pb1-xGexI3, and CH3NH3Pb1-xSnxI3 Hybrid Perovskite Alloys: A Statistical Approach from First Principles Calculations.

The recent reaching of 20% of conversion efficiency by solar cells based on metal hybrid perovskites (MHP), e.g., the methylammonium (MA) lead iodide, CH3NH3PbI3 (MAPbI3), has excited the scientific community devoted to the photovoltaic materials. However, the toxicity of Pb is a hindrance for large scale commercial of MHP and motivates the search of another congener eco-friendly metal. Here, we employed first-principles calculations via density functional theory combined with the generalized quasichemical approximation to investigate the structural, thermodynamic, and ordering properties of MAPb1-xSixI3, MAPb1-xGexI3, and MAPb1-xSnxI3 alloys as pseudo-cubic structures. The inclusion of a smaller second metal, as Si and Ge, strongly affects the structural properties, reducing the cavity volume occupied by the organic cation and limitating the free orientation under high temperature effects. Unstable and metaestable phases are observed at room temperature for MAPb1-xSixI3, whereas MAPb1-xGexI3 is energetically favored for Pb-rich in ordered phases even at very low temperatures. Conversely, the high miscibility of Pb and Sn into MAPb1-xSnxI3 yields an alloy energetically favored as a pseudo-cubic random alloy with tunable properties at room temperature.

Electronic properties of fluorides by efficient approximated quasiparticle DFT-1/2 and PSIC methods: BaF2, CaF2 and CdF2 as test cases.

Dialkali halides are materials of great interest from both fundamental and technological viewpoints, due to their wide transparency range. The accurate determination of their electronic, excitation and optical properties in bulk and low dimensional systems is therefore of crucial importance. Moreover, it is a challenge from the theoretical point of view to deal with quasiparticle band structure calculations for such large energy gap materials, requiring very expensive methods for achieving a desirable accuracy. Here we report electronic quasiparticle band structures for three representative bulk fluorides, BaF2, CaF2 and CdF2, calculated using two low computational cost methods, the DFT-1/2 and the PSIC schemes, which have been relatively little explored by the theoretical community so far. Our results, compared with both available experimental data and previous heavyweight DFT-GW self-energy calculations, demonstrate a satisfactory accuracy for the examined compounds, at a level comparable with the perturbative G0W0 approach. Remarkably, both our proposed methods scale quite similarly to standard local density functional approaches, thus resulting in a large saving of computational effort with respect to the computationally heavyweight GW. Our results open up the perspective of the computational exploration of much bigger fluoride systems. As a significant proof of concept of this capability, we also calculated the quasiparticle properties of the (1 1 1) surfaces of all the three systems under study. Very good agreement with experiment was found.

Deposition of topological silicene, germanene and stanene on graphene-covered SiC substrates.

Growth of X-enes, such as silicene, germanene and stanene, requires passivated substrates to ensure the survival of their exotic properties. Using first-principles methods, we study as-grown graphene on polar SiC surfaces as suitable substrates. Trilayer combinations with coincidence lattices with large hexagonal unit cells allow for strain-free group-IV monolayers. In contrast to the Si-terminated SiC surface, van der Waals-bonded honeycomb X-ene/graphene bilayers on top of the C-terminated SiC substrate are stable. Folded band structures show Dirac cones of the overlayers with small gaps of about 0.1 eV in between. The topological invariants of the peeled-off X-ene/graphene bilayers indicate the presence of topological character and the existence of a quantum spin Hall phase.