La cooperación sur-sur como nueva dimensión de la política exterior brasileña en el gobierno de Lula: una aproximación constructiva

Over the last decade, we have witnessed a growing number of academic researches that have Brazil and its position in the international system as an object of study, which debate is open, because there is no consensus of its role and intentions: if it is to promote the development and cooperation, or, after all, if it also has international political interests, like greater weight in the international system, greater decision capability, and others typically economics like how to gain market for its companies. Because of its geographical size, its growing economic importance, its visibility for the Brazil 2014 World Cup and the Rio 2016 Olympics, and the political demonstrations in the streets, it is hard to deny the attraction that Brazil is gaining every day. This work was created with several objectives that are tried to be answered along the writing of this thesis: a general objective that generates secondary ones and, at the same time, subsequent collateral objectives. Among the general objectives, we have the purpose of understanding and analyzing how the South-South Cooperation (SSC) is in the wider debate of the International Cooperation for Development (ICD). In this sense, it is necessary to point out the difficulty to trace clear lines of differentiation between the traditional North-South Cooperation (NSC) and the latest SSC. This is because, among other questions, there is not a definition until this moment shared by all the countries participating in it, as well as the lack of database that indicates precisely which are all the 'development partners' emerging. This thesis introduces the debate about international cooperation analyzing the challenges that it faces, the global changes that have come to transform the 'new architecture to development' and to analyze the gaps that Brazil can take with other emerging powers to improve the current and weakened assistance system...

Variational mean-field approach to the double-exchange model

It has been recently shown that the double exchange Hamiltonian, with weak antiferromagnetic interactions, has a richer variety of first- and second-order transitions than previously anticipated, and that such transitions are consistent with the magnetic properties of manganites. Here we present a thorough discussion of the variational mean-field approach that leads to these results. We also show that the effect of the Berry phase turns out to be crucial to produce first-order paramagnetic-ferromagnetic transitions near half filling with transition temperatures compatible with the experimental situation. The computation relies on two crucial facts: the use of a mean-field ansatz that retains the complexity of a system of electrons with off-diagonal disorder, not fully taken into account by the mean-field techniques, and the small but significant antiferromagnetic superexchange interaction between the localized spins.