I would like to thank Prof. Sandro Stringari without whom this
thesis would not exist. He gave me the opportunity to join his
research group. Also I found invaluable his help in solving the
organizational problems.
I would like to gratefully acknowledge the enthusiastic supervision
of Dr. Stefano Giorgini and Prof. Lev P. Pitaevskii. They taught me
so many new things and were always ready to answer any of my
countless questions.
I would also like to thank Dr. Yuri E. Lozovik, my supervisor from
Moscow Institute of Physics and Technology,
who offered many insightful comments and suggestions
and always supported me.
I am grateful to many people whom I meet in the University of Trento
and who have assisted me so much in the course of this work.
