Preface Particles do interact and that interaction usually is not negligible. This is one of the very basic things that physics teaches us. However, to know the importance of human interaction, we don't really need physics. Our daily experiences tell it loud and clear. I highly value my constructive interactions during the period of my doctoral studies. It was not an easy journey to take it alone. I am very thankful to Professor Risto Nieminen for his supports and encouragements during the time

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... of my studies. I am also grateful to Prof. Martti Puska, Prof. Angel Rubio, and Prof. Robert van Leeuwen for our useful collaborations. Our discussions were, at many points, enlightening and fruitful and enhanced my understanding. I also learned a lot from my old friend and recent collaborator Dr. Ali Akbari; I am happy about and grateful for our collaboration and more importantly our friendship. Quite often, I feel very lucky and happy for having amazing friends. Those who stand by your side in failure and success. I am thankful to them, now and ever, for making my life better. During these years, I was working in a very relaxing atmosphere and I had very friendly and helpful colleagues. Moreover, we had an excellent lunch group that made up for the food quality. Many thanks to all of them. Special thanks to Pezhman Behjouyan for his creative work for the cover.