On 27 April 1938, Landau was arrested for a leaflet which compared Stalinism to German Nazism and Italian Fascism. 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) and Bohr wrote letters to Joseph Stalin. Kapitsa personally vouched for Landau's behaviour and threatened to quit the institute if Landau was 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.