Introduction

The superfluid transition in liquid 3He can be regarded as a Bose-Einstein condensation of the fermionic Cooper pairs (or a variation thereof) resulting in a dense condensate, at the other end of the scale from the tenuous cold-gas equivalents. While the cold gases provide interesting laboratories for looking at problems of wider relevance, their limited size and densities can be a disadvantage. The superfluid 3He condensate, while less straightforward to achieve, has a number of beneficial properties which make it ideal for many experiments.

The analogies between superfluid dynamics and the behaviour of pure Euler liquids have long been apparent. However, the superfluid case is complicated, at medium temperatures (0 < T < Tc), by the presence of ‘normal’ fluid, i.e., the residual gas of those unpaired particles not contributing to the condensate. Superfluid 3He is simpler in this regard than superfluid 4He in that the condensate (or superfluid fraction) can be identified with the Cooper pairs and the normal fluid fraction with the remaining unpaired 3He atoms. In the case of our experiments, we avoid this latter complication by working at the zero-temperature limit, where the normal fraction is negligible. In this region, the superfluid flow does indeed correspond to that of an Euler fluid and is irrotational. However, we can simulate rotation by introducing vortices, or line defects, along the cores of which the condensation is suppressed. Around these vortices, circulation is allowed, since the local liquid flow remains irrotational.