Theory for the change of bond character in divalent-metal clusters

To determine the size dependence of the bonding in divalent-metal clusters we use a many-electron Hamiltonian describing the interplay between van der Waals (vdW) and covalent interactions. Using a saddle-point slave-boson method and taking into account the size-dependent screening of charge fluctuations, we obtain for Hg_n a sharp transition from vdW to covalent bonding for increasing n. We show also, by solving the model Hamiltonian exactly, that for divalent metals vdW and covalent bonding coexist already in the dimers.

The Brazilian Zero Hunger social program implications in achieving sustainable development

This article aims to analyse the progress taken by Brazil towards the accomplishment of a sustainable development, mainly highlighting the success of the Zero Hunger social programme in achieving the elimination of starvation. As one of the essential pillars to the sustainable development, the State’s social performance demands actions directed to the elimination of extreme poverty and hunger, establishing a basis for equitable growth. It is necessary to clarify the juridical and constitutional framework of Brazil, aiming to emphasise the Zero Hunger importance in achieving the Brazilian Republic’s goals and reaching an international pattern on sustainable development. Also, it will be stressed that the international order influences Brazilian domestic law and is one of the main aspects for the Zero Hunger program development.

Electronic origin of bond softening and hardening in femtosecond-laser-excited magnesium

Many ultrafast structural phenomena in solids at high fluences are related to the hardening or softening of particular lattice vibrations at lower fluences. In this paper we relate femtosecond-laser-induced phonon frequency changes to changes in the electronic density of states, which need to be evaluated only in the electronic ground state, following phonon displacement patterns. We illustrate this relationship for a particular lattice vibration of magnesium, for which we—surprisingly—find that there is both softening and hardening as a function of the femtosecond-laser fluence. Using our theory, we explain these behaviours as arising from Van Hove singularities: We show that at low excitation densities Van Hove singularities near the Fermi level dominate the change of the phonon frequency while at higher excitations Van Hove singularities that are further away in energy also become important. We expect that our theory can as well shed light on the effects of laser excitation of other materials.