Acknowledgements

I would like to thank my supervisor Dr. Maxim Lobashov for his patience and continuous guidance throughout my work. Moreover, I am very grateful to Dr. Ursula Kluwick, who helped me to correct misspellings and deficiencies in my English.

Finally, I would like to thank my wife Veronika, my children Anna, Maria and Christina and my parents, who supported me throughout my work and gave me the time I needed to complete my work.