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 component. The normal part carries entropy, while the superfluid has essentially zero viscosity. The flow is irrotational except where quantized vortex lines appear. Phenomena such as second sound, a temperature or entropy wave, and quantized circulation around vortices are hallmarks. The superfluid fraction increases as temperature decreases.

Historical overview: Helium-4 was identified as a superfluid in 1938 by Pyotr Kapitza and by John Allen and Don Misener. Helium-3 joined as a superfluid at microkelvin temperatures in 1972, discovered by David Lee, Robert Richardson and Douglas Osheroff. In recent decades, ultracold atomic gases have enabled tunable superfluid systems and direct observation of Bose-Einstein condensation.

Related systems include superconductors, where Cooper pairs form a charged superfluid, and ultracold fermionic gases that pair to produce fermionic superfluidity. In astrophysics, models of neutron star interiors posit superfluid neutrons that affect rotational dynamics. Ongoing research explores quantum vortices, turbulence in quantum fluids, and potential applications in precision measurements and quantum simulations.