Dos casos paradigmáticos de excitaciones colectivas en la banda de conducciónmonocristales superconductores y nanoestructuras plasmónicas

Résumé

In this thesis, two different kinds of systems whose properties arise from a colective behavior of conduction band electrons have been studied. These systems are superconductors and plasmonic materials, both being paradigmatic examples of colective phenomena in the conduction band. These systems are of great interest in the fields of basic research and techonological applications. While for superconductors there is a mutual interaction between electrons, that is not the case for plasmons. Therefore, even when both effects emerge from the conduction band, the characteristic features of each system are quite different. The superconductors studied in this thesis are nickel-doped single crystals of the BaFe2As2 compound. These crystals belong to the most recently discovered group of superconductors: the iron pnictides. In these materials, superconductivity is achieved through doping of the parent compound (i. e. BaFe2As2 in our case), in a similar way than in other wellknown superconducting systems as cuprates. However, the presence of ferromagnetic atoms (such as iron in the parent structure or dopants like nickel and cobalt) in the iron pnictides does not suppress superconductivity, as it happens in cuprates...