Does every superfluid have a normal and a superfluid component?

Question

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?

Answer 1

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 $0$.

Answer 2

As the other answer stated, the answer to both questions is yes. A superfluid can be understood from a macroscopic perspective as a fluid with zero viscosity, or from a microscopic perspective as an ensemble of interacting particles that form a condensate. This means the number of particles in the quantum ground state (the lowest energy state) has become a finite fraction of the total number of particles. This is in contrast with most systems, where the number of particles in the ground state is vanishingly small. When people talk about the superfluid and normal fluid fractions, they are conceptually dividing the system into a mixture of two fluids. The superfluid component is the subset of particles in the ground state, and the normal fluid component is all other particles. Only the superfluid component has the special property of zero viscosity, though there are caveats to this claim when one considers the role of defects like vortices.

The reason why the ratio of the components changes with temperature is simply because reducing the temperature means lowering the average energy of particles by definition. This means the fraction of particles in the lowest energy state increases. Eventually, at T=0 (not that you can actually reach this limit), the superfluid fraction becomes 100% and the viscosity essentially approaches zero.

I could go on to talk about superfluid 3He, but this answer is too long already.

Reference: "Superconductivity, Superfluids, and Condensates" by James F Annett.