From 1937 until 1962, Landau was the head of the Theoretical Division at the Institute for Physical Problems. On 27 April 1938, Landau was arrested for comparing the Stalinist dictatorship to that of Hitler's, and he was held in the NKVD's Lubyanka prison until his release on 29 April 1939, after the head of the institute Pyotr Kapitsa, an experimental low-temperature Physicist, wrote a letter to Joseph Stalin, personally vouching for Landau's behavior, and threatening 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.