Superfluid helium-4 is a phase of helium-4, a stable isotope of helium, that occurs at very low temperatures, typically below 2.17 Kelvin (-270.98 degrees Celsius). In this superfluid state, helium-4 exhibits remarkable and counterintuitive properties that differ significantly from those of normal fluids.
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Also sometimes called helium II, in contrast to helium I, which is the non-superfluid liquid helium phase.