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.

$\endgroup$
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
    Feb 7, 2021 at 22:39

1 Answer 1

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"

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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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