Superfluidity

Summary

Superfluidity is the ability of a quantum fluid to flow without viscosity. It occurs in liquid helium-4 below about $2.17$ K, in helium-3 at much lower temperatures, and in dilute Bose–Einstein condensates. The superfluid flow is described by a macroscopic wave function, so its velocity is the gradient of a phase. It is irrotational except at quantum vortices, and its circulation is quantised. Landau's criterion explains why such a flow cannot lose energy below a critical speed.