Condensation of the ideal Bose-gas predicted by A. Einstein (1925) one hundred years ago went through more than a decade of strong doubt before it has been widely accepted in 1938 after a convincing elucidation and formal mathematical arguments due to F. London.
It is the discovery of superfluidity of liquid helium 4^He by P.L. Kapitza and J.F. Allen, A.D. Misener (1938) that renewed interest in Bose-Einstein condensation. In 1938 F. London wrote: "In discussing some properties of liquid helium, I recently realised that Einstein's statement has been erroneously discredited; moreover, some support could be given to the idea that the peculiar phase transition ("lambda-point"), ... very probably has to be regarded as the condensation phenomenon of the Bose-Einstein statistics, ... ."
The idea to scrutinise and to start experimental study of Bose-Einstein condensation in superfluid He II dates back to the 1970s. It supposed that the high-energy neutrons (with a very short de Broglie wave-length) scattered off of single helium atoms may supply information about the zero-momentum condensed atoms in superfluid He II. In 1975 (JINR-Dubna) this approach provided experimental evidence that zero-momentum condensation does exist and that it appears in liquid 4^He simultaneously with superfluidity below the lambda-point.
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