I hereby certify that the thesis I am submitting is entirely my own original work except where otherwise indicated. I am aware of the University's regulations concerning plagiarism, including those regulations concerning disciplinary actions that may result from plagiarism. Any use of the works of any other author, in any form, is properly acknowledged at their point of use. Lada PeksováFirst and foremost, I would like to sincerely thank both my supervisors, Branislav Jurčo and Dorothea Bahns. Thank you both for your guidance and support throughout my study.My wholehearted thanks are to Ján Pulmann for many fruitful discussions. I feel indebted to Jan for all his patience when explaining to me (not only) the physical aspects of the theory.Four years is a long time. And there were many friends and colleagues who helped me to understand the mathematical background and to the physical aspects of the following thesis. Many of them gave me essential insights and comments. I am deeply grateful to