Entropy entrainment and dissipation in superfluid Helium

TL;DR

This paper develops a variational multi-fluid framework for finite temperature superfluid Helium, connecting it to traditional models and extending it to complex systems like neutron star cores and quark superconductors.

Contribution

It introduces a hydrodynamic formalism that unifies superfluid Helium modeling with complex astrophysical and quantum systems, incorporating entropy entrainment and vortex effects.

Findings

Equivalence to traditional two-fluid superfluid Helium models.

Entropy entrainment encodes normal fluid density.

Reduced dissipation coefficients due to irrotationality constraint.

Abstract

Building on a general variational framework for multi-fluid dynamics, we discuss finite temperature effects in superfluids. The main aim is to provide insight into the modelling of more complex finite temperature superfluid systems, like the mixed neutron superfluid/proton superconductor that is expected in the outer core of a neutron star. Our final results can also (to a certain extent) be used to describe colour-flavour locked quark superconductors that may be present at the extreme densities in the deep neutron star core. As a demonstration of the validity of the model, which is based on treating the excitations in the system as a massless ``entropy'' fluid, we show that it is formally equivalent to the traditional two-fluid approach for superfluid Helium. In particular, we highlight the fact that the entropy entrainment encodes the ``normal fluid density'' of the traditional approach. We also show how the superfluid constraint of irrotationality reduces the number of dissipation coefficients in the system. This analysis provides insight into the more general problem when vortices are present in the superfluid, and we discuss how the so-called mutual friction force can be accounted for in our framework. The end product is a hydrodynamic formalism for finite temperature effects in a single superfluid condensate. This framework can readily be extended to more complex situations.