Superfluids

Superfluids are quantum fluids that flow with negligible viscosity when cooled below a characteristic critical temperature. In this phase, a significant fraction of the fluid occupies a single quantum mechanical ground state, resulting in macroscopic quantum phenomena such as frictionless flow and quantized vortices. The best-known examples are liquid helium-4, which becomes superfluid below the lambda point at 2.17 Kelvin, and helium-3, which becomes superfluid at millikelvin temperatures after forming Cooper-like pairs. In helium-4, the superfluid phase is often described by the two-fluid model, consisting of a viscous normal component and an inviscid superfluid component. The model, proposed by Tisza and refined by Landau, explains phenomena such as second sound (a temperature wave) and very high thermal conductivity.

Superfluids exhibit several distinctive effects: zero viscosity in ordinary flow, the ability to climb and creep along surfaces (Rollin films) and through tiny channels, and the presence of quantized vortices whose circulation is fixed in units of h/m. The order parameter is a macroscopic wavefunction with a well-defined phase, yielding long-range coherence.

In addition to liquid helium, superfluid behavior has been observed in ultracold atomic gases, where Bose-Einstein condensates form superfluid states, and in paired fermionic systems such as ultracold helium-3 and superconducting analogs, though superconductivity is a separate phenomenon. Research into superfluids informs understanding of quantum hydrodynamics, turbulence at quantum scales, and potential applications in precision measurement and quantum simulation.