Some people refer to a Phd or advanced research degree as a dream.Some other, as one of my best friends, called it "the dream that becomes a nightmare". In any case, I had the opportunity to pursue this dream some years ago. Undoubtedly, this fact would not has been possible without the support of many people, friends and family. I am deeply indebted to all of them. Specially, words are not enough to express my gratitude to Alberto Trasloheros Michel. Some years ago, Alberto was my teacher and mentor. Happily, now I can say that he is one of my best friends.Thanks to him, I was able to begin this wonderful journey. I am so grateful that he always trusts in me, even though results was not as good as expected. Of course, I have learned many valuable life lessons from him and I shall be eternally grateful for that.My deepest gratitude goes also to my supervisors Antonio Barrientos and Jaime del Cerro. Thank you very much for trusting in me and for providing me with the tools needed to achieve this goal. I will always appreciate the critical review that you always performed about my thesis and the insights you provided me. Undoubtedly, work with you made a huge difference in both this research and my personal life.