During the drought in the university, Landau published his first academic paper on the spectrum of diatomic molecules at 1927. In the same year, he first used the concept of density matrix in his paper dealing with bremsstrahlung with wave mechanics, which played an important role in later quantum mechanics and quantum statistical physics. Two days before his 19 birthday, Landau graduated from Leningrad University and became a graduate student at the Leningrad Institute of Technical Physics of the former Soviet Academy of Sciences. After several applications, Landau was allowed to go abroad on 1929 and 10. In less than two years, Landau visited Germany, Switzerland, the Netherlands, Britain, Belgium and Denmark for further study. He once recalled that during this time, he met almost all quantum physicists except Fermi. In his contacts with these famous scientists, Landau fully demonstrated his talent and personality. In Copenhagen, Denmark, Landau was deeply influenced by the "Copenhagen spirit" and became an active member in Bohr's research class. Later, Bohr said of Landau, "His arrival left a deep impression on us. His insight into physical topics and strong insight into human life have improved the level of many seminars. " Although Landau has been in contact with countless physicists in his life, and he only stayed in Bohr for about four months, he worships Bohr very much and only admits that he has been a student of Bohr all his life.