2
$\begingroup$

I'm trying to understand superfluidity from these Caltech notes on Advanced Statistical Physics (Week 1, Section IV: Landau Criterion for Superfluidity) -

So far it is not clear why a moving superfluid doesn’t dissipate its kinetic energy. The spectrum of excitations is not gapped (which would be a sufficient condition for superflow), even though the number of low lying excitations is decreased relative to a non-interacting BEC.


Question - Why would a gapped excitation spectrum be a sufficient condition for superflow? (I understand a gapped excitation spectrum as simply a spectrum of allowed energy/momentum states that is NOT continuous but rather like a discrete band). I don't know where I am conceptually going wrong.

Improve this question
$\endgroup$
1
  • $\begingroup$ One way to see this is to continue reading. A gapped spectrum satisfies Landau's criterion and automatically gives a non-zero critical velocity. $\endgroup$
    – Thomas
    Commented Feb 7, 2021 at 22:39

1 Answer 1

Reset to default
0
$\begingroup$

A superfluid has an energy gap from strong interatomic repulsion at short distances that prevents viscosity until the superfluid reaches a critical value of the excitation energy of the atoms.

https://arxiv.org/ftp/arxiv/papers/1307/1307.4892.pdf "The state of atoms in the liquid 4 He is characterized by wave functions and the discrete energy spectrum resulting in formation of s – and p- zones corresponding to the ground and excited states of helium atoms, respectively, separated by a gap. The width of the gap in 4 He system equals ~8.5K at T=0"

Improve this answer
$\endgroup$
1
  • $\begingroup$ Not sure about this paper. Superfluid liquid helium is gapless, due to phonon excitations. $\endgroup$
    – Thomas
    Commented Feb 8, 2021 at 5:42

Not the answer you're looking for? Browse other questions tagged or ask your own question.