 He was held in the NKVD's Lubyanka prison until his release, on 29 April 1939, after Pyotr Kapitsa, an experimental low-temperature physicist and the founder and head of the institute, wrote a letter to Joseph Stalin in which he personally vouched for Landau's behaviour and threatened to quit the institute if Landau were not released.After his release, Landau discovered how to explain Kapitsa's superfluidity using sound waves, or phonons, and a new excitation called a roton.Landau led a team of mathematicians supporting Soviet atomic and hydrogen bomb development.