superfluidity

Superfluidity is a phase of matter in which a fluid can flow without viscosity. It arises in certain quantum fluids at very low temperatures when a large fraction of particles occupies the same quantum state, allowing macroscopic quantum coherence. In liquid helium, superfluidity emerges in helium-4 below 2.17 K (the lambda point); helium-3 requires much lower temperatures and, in that case, arises from Cooper pairing of fermions.

Experiments reveal several hallmark features. A superfluid has zero (or extremely small) viscous dissipation, can support persistent, frictionless currents, and exhibits quantized vortices with circulation multiples of h/m. The two-fluid model describes coexisting superfluid and normal components, with their relative fractions determined by temperature. A second sound, a wave of temperature and entropy, propagates through the liquid, in contrast to ordinary pressure waves (first sound).

The concept was developed in the 1930s and 1940s. Pioneering experiments by P. Kapitza and by J. F. Allen and A. Misener showed frictionless flow in liquid helium. L. Landau formulated the two-fluid theory and introduced the Landau criterion for a critical velocity, predicting dissipation when excitations, such as phonons or rotons, could be created. In helium-4 the superfluid phase is associated with Bose–Einstein condensation; helium-3 becomes superfluid through fermionic pairing at microkelvin temperatures.

Beyond helium, superfluidity has been realized in ultracold atomic gases, where Bose-Einstein condensates exhibit flow without viscosity and support vortices. It is also studied in other platforms such as exciton-polariton systems and in certain superconducting and quantum fluid contexts, highlighting the broader class of macroscopic quantum phenomena that underlie superfluid behavior.