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AcknowledgementsThe work presented in this Ph.D thesis was made possible by support of the Physics of Nanostructuresand Advanced Materials Group at the Donostia International Physics Center (DIPC)and Centro Mixto CSIC-UPV/EHU (CFM), under the supervision of Prof. Pedro MiguelEchenique Landiribar. Many other colleagues in the group have greatly contributed to mywork and enriched my life. Words are not enough to express my gratitude of all the memorablemoments of my stay in DIPC and CFM. I would like to extend my sincere gratitude toall those that had given me a helpful hand in the past 4,5 years. Among them, I am especiallygrateful to:My supervisors Daniel Sánchez Portal and Andrés Ayuela Fernández for their supervisionsand scientific guidance. I would like to thank them for their support, encouragement, insight,hand-to-hand education and advices, as well as for their extensive feedback on my manuscriptsand presentations. They always allowed me to pursue my own ideas and treated me, since thevery first day on the same footing as a collaborator, listening to my suggestions and ideas, andhaving confidence in my potential. It has been a great pleasure and inspiration to work withyou!Pedro Miguel Echenique Landiribar, for his support and always present scientific view ofthe life. I am very grateful to him for sharing his wide physics wisdom with me during myfirst years in the Ph.D study as well as his good advices. I truly admire his optimism andpermanent positive attitude which represented to me a vast source of motivation and energy tokeep on track.Mads Brandbyge, for his hospitality at my stays at Technical University of Denmark(DTU), fall 2009 and spring 2010, and for his endless energy for doing fruitful and inspiringcomments. I very much enjoyed the pleasant working environment at DTU and the manyhours expend in the "theory room 022" in weekends, holidays, etc, either alone or in the nicecompany of Mads Engelund, Jing Tao Lu, Joachim Alexander Fürst, Tue Gunst. I thank MadsBrandbyge not only for the privileged opportunity to work in his group but also for discoveringsuch a kind and friendly person. I would like also to thank Prof. Antti-Pekka Jauho at DTUfor the introduction about transport methods and Green’s function formalism. His valuablecourse has given me the right tools to treat many problems in theoretical electronics as well asin scattering process.The DIPC and CFM staff, Ana López de Goicoechea, Marimar Alvarez, Amaia Etxaburu,Txomin Romero Asturiano, Belen Isla, Carmen Martin, Luz Fernandez, Elixabete MendizabalItuarte, Maria Formoso Ferreiro, Karmela Alonso Arreche, Iñigo Aldazabal Mensa andTimoteo Horcajo (In memoriam), for making everything to work so fluently.Sampsa Juhana Riikonen who helped me a lot in my first years with the SIESTA methodand to understand the real life of a PhD student. Thank you for the many excursions that we7