Superfluids

[arfa brane]

We just got told that in ⁴He, the kinetic energy of each atom falls to zero at the critical point, and so the momentum of each atom is zero and there is zero pressure on the walls of a container, or F = dp/dt = 0 per unit area.

I asked the lecturer what happens in a sufficiently large container such that a pressure gradient should be seen due to gravity, and he said he would have to think about it.

Has anyone here thought about it? Is a superfluid compressible?

[AlphaNumeric]

Are you sure it's zero kinetic energy and not just the minimal ground state, which typically has some vacuum energy? If the atoms utterly stopped they would violate the uncertainty principle. Furthermore superfluids have the annoying properties of 'creeping'.

For most normal fluids if you pour them into an open beaker they will sit there. For superfluids they will actually climb up the sides of the container's walls, forming a very thin film, and then out over the lip of the container, eventually all flowing out. If the atoms didn't move AT ALL then they wouldn't do this.

As for compressibility I think they are. This is because they can carry oscillations in the form of quantised pulses known as phonons, which basically act like particles do normally. These can move through the interior of a fluid so the fluid should be undergoing the usual compression-expansion of longitudinal waves.

/edit

http://en.wikipedia.org/wiki/Rollin_film