Helium-4 becomes a superfluid at a critical temperature of approximately 2.17 K, known as the lambda point.
Below this temperature, liquid helium enters helium II, a phase with extraordinary properties including near-zero viscosity, the ability to flow through extremely narrow channels, and the formation of a thin film that can climb container walls. Above 2.17 K, it remains helium I, an ordinary liquid phase. The transition occurs at about 1 atmosphere; the exact temperature varies with pressure.
The name “lambda point” comes from the shape of helium’s specific-heat curve near the transition. The sharp peak resembles the Greek letter lambda, λ. This transition is a quantum phenomenon: at very low temperatures, large-scale quantum behavior becomes visible in the liquid rather than being confined to individual atoms.
A common mix-up is 4.2 K, the approximate boiling point of helium-4 at atmospheric pressure. Cooling helium below its boiling point does not immediately make it superfluid; it must reach the lower transition temperature near 2.17 K. Helium-3 also becomes superfluid, but only at far colder temperatures.