Superfluid helium-4

Superfluid helium-4 is the low-temperature phase of liquid helium-4 that flows without viscosity and displays quantum behavior on macroscopic scales.

Lambda transition: The continuous phase transition in liquid helium-4 near 2.17 kelvin, marked by a sharp change in heat capacity. This transition marks the onset of helium-4’s superfluid phase.
Fountain effect: A pressure difference produced when superfluid helium is heated through a porous barrier. It demonstrates how heat flow and superfluid flow couple in helium-4.
Heike Kamerlingh Onnes: A Dutch physicist who pioneered low-temperature physics and liquefied helium in 1908. His helium liquefaction made later experiments on helium-4 possible.
Superfluid helium-3: A superfluid phase of helium-3 formed when its fermionic atoms pair at extremely low temperatures. Unlike helium-4, it becomes superfluid through pairing rather than condensation of individual bosonic atoms.
Bose–Einstein condensation: A state in which many bosons occupy the same lowest-energy quantum state. It provides a broad framework for understanding macroscopic quantum behavior in helium-4.
Rollin film: A thin helium film that creeps over surfaces because of superfluidity. Its spontaneous motion reveals helium-4’s ability to flow along container walls.
Pyotr Kapitsa: A Soviet physicist who discovered superfluidity in liquid helium-4 in 1937. His experiments established helium-4’s nearly frictionless flow.
Superconductivity: A state in which a material conducts electrical current without resistance and expels magnetic fields. Both are macroscopic quantum states, but helium-4 transports mass rather than electric current.
Two-fluid model: A model of superfluid helium-4 as interpenetrating inviscid and normal components. It describes how superfluid and thermal excitations coexist below the transition.
Second sound: A temperature and entropy wave that propagates through superfluid helium as a distinct mode. It shows that heat can travel as a wave in helium-4.

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