Re: What is the difference between a superfluid and a BEC?

Hi John, a simple google search will tell you that: Bose and Einstein considered the condensation of *noninteracting* bosons. This is a reasonable approximation for a dilute gas. In a superfluid the bosons are interacting with each other, and this makes things much more complicated. For example, you can get all sorts of interesting vortices in a superfluid. The original work by Bose and Einstein concerned ideal (noninteracting) bosonic quantum gases, which is enough to see the phenomenon of condensation as such. But the BEC theory that is relevant for today's famous BEC experiments treats interacting gases in various approximations. For instance a simple delta pseudo potential gives rise to the famous Gross-Pitaevskii equation for the condensate wave function. (e.g. http://www.scie nce.unitn.it/~astra/thesis/node6.html Also vortices in BECs are an interesting topic, both theoretically and experimentally, as a search for the relevant keywords verifies (incidentally, some nice pictures at http://cua.mit.edu/ketterle_group/Projects_2001/Vortex_lattice/Vortex .htm Even though the microscopic mechanisms of some superfluids and "modern" (e.g. alkali atom) BECs may be different, the fundamental principle is the same: A mode (ideally the single-(quasi)-particle ground state) is populated as a coherent state and hence the condensate or superfluid acts like a classical Schroedinger wave. In conclusion, you possibly could say any BEC is a superfluid (unless constrained otherwise, e.g. by trapping it in a periodic potential where it may go into a Mott-insulater state).

Hope this helps, from Keith, your mad Welsh scientist.