superfluide

Superfluid is a phase of matter characterized by the ability to flow without viscosity. The best-known example occurs in liquid helium-4 at very low temperatures, below about 2.17 kelvin (the lambda point). Helium-3 also becomes superfluid, but at much lower temperatures through a different pairing mechanism. In recent decades, ultracold atomic gases have allowed the creation of superfluid states in dilute Bose-Einstein condensates, enabling broader experimental access to quantum hydrodynamics.

The standard theoretical framework is the two-fluid model, introduced to describe how a liquid can support both viscous and frictionless flow. The liquid is treated as a mixture of a normal component, which carries entropy and dissipates energy, and a superfluid component, which flows without viscosity. The total density is the sum of these two parts, and their relative fractions depend on temperature. The superfluid velocity is determined by the gradient of the macroscopic phase of its order parameter. Because the phase is single-valued, circulation is quantized in units of h/m, leading to stable quantized vortices.

Key phenomena include the existence of frictionless flow, the formation of persistent currents, and the appearance of quantized vortices in rotating systems. In addition to ordinary sound, superfluids support second sound, a temperature or entropy wave distinct from conventional density waves. The concept of a critical velocity describes the maximum flow speed before excitations destroy the superfluid state.

Applications and research areas span quantum turbulence, vortex dynamics, precision measurements, and connections to other quantum fluids. Superfluidity remains central to studies of quantum many-body physics, Bose-Einstein condensation, and, in astrophysical contexts, the behavior of dense quantum fluids in neutron stars.