superfluiden
Superfluiden, or superfluids, are a phase of matter characterized by the absence of viscosity. In a superfluid, flow can occur without energy dissipation, yielding persistent currents and frictionless motion. The phenomenon is most famously observed in liquid helium-4 below its lambda point at 2.17 K and, at still lower temperatures, in helium-3. Ultracold atomic gases can also form highly coherent superfluid states known as Bose-Einstein condensates.
Two-fluid theory describes a superfluid as a mixture of a viscous normal component and an inviscid superfluid
Historical overview: Helium-4 was identified as a superfluid in 1938 by Pyotr Kapitza and by John Allen
Related systems include superconductors, where Cooper pairs form a charged superfluid, and ultracold fermionic gases that