Superfluids and superfluid mixtures in atom traps

Superfluidity has become a topic of active interest in the field of atom tr ap Bose-Einstein condensates (BECs). We motivate this line of research by p lacing the BEC developments against the general backdrop of the theory of s uperfluidity and by highlighting the advantages offered by the atom trap te chnology. We review the standard description of the BEC dynamics, as well a s basic concepts of superfluidity and we present theoretical arguments for expecting the BECs to exhibit superfluid behaviour. In atom traps, as in co ndensed and nuclear matter, superfluidity can manifest itself in a manifold of guises. Moreover, some of its manifestations can even be undesirable, a s we illustrate by the poignant example of sympathetic cooling with condens ates. We also review recent experimental evidence for superfluid BEC behavi our and we suggest further experiments to probe superfluidity in atom traps . The low densities of the BECs allow optical imaging and imply that the re levant physics occurs on time and length scales that are much slower and la rger than in the traditional He-4 superfluid. Consequently, BEC experiments can study known superfluid phenomena and structures in a different and, pe rhaps, more detailed manner. In addition, the flexibility offered by existi ng techniques in atomic, molecular and optical physics is unprecedented in the field of low-temperature physics and suggests the prospect of creating superfluids of fundamentally novel structures. Finally, it is conceivable t hat new applications of superfluidity may arise from these studies.