I know that Bose-Einstein condensate is not always a superfluid. So what can cause viscosity in a BEC gas?
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This might help - Does every superfluid have a normal and a superfluid component? – mmesser314 Dec 15 '23 at 14:55
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A Galilean BEC (by which I mean a BEC of massive bosons whose kinetic energy $\sim p^2$) has quadratically dispersing low energy excitations, whereas a superfluid has linearly dispersing excitations. The linear dispersion in superfluids is crucial to zero viscosity -- see Landau's ingenious argument, discussed in most references on the topic . – Nandagopal Manoj Dec 17 '23 at 04:19