Superfluids :

A superfluid is the unusual properties of liquid helium when it is cooled below 2.18 K (-270.97 C), called the lambda point. The term was coined in 1938 by the Soviet physicist Pyotr Kapitsa following an extensive series of experiments showing that in this state of helium, called helium II (He II), there is an apparent enormous rise in heat conductivity, rapid flow through capillaries or over the rim of its containment vessel as a thin film, and that a number of other unusual properties also appear.

In order to account for such behaviour, the "two fluid" model, as proposed by Laszlo Tisza, described He II as a mixture of normal helium and superfluid helium. The normal component is attributed to helium atoms in excited energy states, whereas the superfluid component is attributed to atoms all in the ground state (having lowest or zero-point energy). As the temperature continues to be lower below the lambda point, more of the He II becomes superfluid. It is assumed that this superfluid component is able to move through its container without friction, thereby explaining most of the unusual behaviour of helium II.