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After seeing the question Will a propeller work in a superfluid?, I remembered an old video by Alfred Leitner (1'50'' and 3'00'')where he experimentally shows that liquid Helium below the $\lambda$ point has two components, one superfluid with zero viscosity and the other normal with a small but non vanishing viscosity. So I have two questions (which might be related):

  1. Does every superfluid have a normal and a superfluid component?
  2. If the ratio between those components is temperature dependent (as one of the experiments on the video suggests) then does the normal component goes to zero as temperature goes to zero?
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  1. Yes.
  2. Yes.

It is a quantum probability thing. The He4 atoms in a superfluid are indistinguishable Bosons. At a much lower temperature, the He3 particles form pairs, and become indistinguishable Bosons. Indistinguishable because they are in the ground state.

You can't say which is which. But the temperature is not $0$, so a certain fraction of them will be excited enough to be in a normal state. The fraction is temperature dependent, and goes to $0$ as the temperature goes to $0$.

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  • $\begingroup$ I changed the second question (I apologize) since the first experiment on the video actually shows the ratio between the components is temperature dependent. So the natural question is whether this ratio tends towards zero along with the temperature. $\endgroup$ – Diracology Feb 10 '17 at 22:33
  • $\begingroup$ So superfluids don't really exist since 0 K isn't reachable? $\endgroup$ – MaDrung Feb 14 '17 at 12:13
  • $\begingroup$ Superfluids exist. That is, He atoms form a kind of [Bose-Einstein condensate][1]. The change in properties at the critical temperature is dramatic. But a superfluid without any normal component mixed in doesn't exist. So there is always some residual viscosity. It would be like saying a crystal doesn't exists because a perfect crystal doesn't. [1]: en.wikipedia.org/wiki/Bose%E2%80%93Einstein_condensate $\endgroup$ – mmesser314 Feb 14 '17 at 14:35

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