New light on the intriguing history of superfluidity in liquid4He

Abstract

Surprisingly, it was 30 years after the first liquefaction of4He in 1908 that thediscovery that liquid 4He is not just a ‘cold’ liquid was made. BelowT = 2.18 K, it is a ‘quantum’ liquid which exhibits spectacular macroscopic quantum behaviour thatcan be seen with the naked eye. Since the observation of superfluidity in liquid4He is one of the greatest discoveries in modern physics, we present a day-to-day chronology ofthe tangled events which preceded the seminal discovery of zero viscosity in 1938 byKapitza in Moscow and by Allen and Misener in Cambridge. On the theory side, Londonargued in 1938 that the microscopic basis for this new superfluid phase was theforgotten phenomenon of Bose–Einstein condensation (BEC) first suggested byEinstein in 1925. In 1941, Landau developed a very successful theory of superfluid4He, but it was not anchored in a microscopic theory of interacting atoms. It took another 20years for theorists to unify the two seemingly different theories of Landau and London.Experiments on trapped superfluid atomic gases since 1995 have shone new light on superfluid4He. Inthe mid-1930s, London had emphasized that superconductivity in metals and superfluidity in liquid4He were similar. Experiments on trapped two-component Fermi gases in the last fiveyears have shown that a Bose condensate is indeed the basis of both of thesesuperfluid phases. This confirms the now famous Bardeen–Cooper–Schrieffer–BECcrossover scenario developed for superfluidity by Leggett and Nozières in theearly 1980s but largely ignored until a few years ago. The study of superfluid4He will increasingly overlap with strongly interacting dilute quantum gases, perhaps openingup a new era of research on this most amazing liquid.