This paper was completed under the careful guidance of my tutor, Researcher Wang Lan. From the topic selection of the paper, the solution of difficulties, to the typesetting of the article, all the efforts of the tutor are devoted to it. I was deeply impressed by Mr. Wang's rigorous academic attitude, profound knowledge, generous demeanor and extreme responsibility for students' study. Here, I would like to extend my high respect and heartfelt thanks to Mr. Wang!
I am very grateful to associate researchers Dai Xianchun and Wang Tong for their selfless help in completing my project.
Thanks again to all the teachers, classmates and friends who helped me with my graduate school. Thanks to my family. I will always miss this wonderful time.