Superfluid helium: physics and applications

The behaviour of liquid helium is dominated by quantum effects, which give rise to superfluidity at low temperatures. An account will be given of the observed properties of the superfluid phase of liquid 4He, and of the way in which these properties can be described by a two-fluid model, supplemented by the requirement that motion of the superfluid component be irrotational with a hydrodynamic circulation that is quantized in units of h/m4. The existence of superfluidity and the quantization of circulation will be related to the presence of a Bose condensate. The description of excited states of the liquid in terms of weakly interacting thermal excitations (phonons and rotons will be discussed, and the relation between these excitations and the normal-fluid component will be explained. The existence and role of quantized vortex lines in the superfluid component will be discribed. A brief account will be included of topics of current research interest.