Superfluid fraction in the crystal phase of the inner crust of neutron stars

Abstract

In the most extended layer of the inner crust of neutron stars, nuclear matter is believed to form a crystal of clusters immersed in a superfluid neutron gas. Here we analyze this phase of matter within fully self-consistent Hartree-Fock-Bogoliubov calculations using Skyrme-type energy density functionals for the mean field and a separable interaction in the pairing channel. The periodicity of the lattice is taken into account using Bloch boundary conditions, in order to describe the interplay between band structure and superfluidity. A relative flow between the clusters and the surrounding neutron gas is introduced in a time-independent way. As a consequence, the complex order parameter develops a phase, and in the rest frame of the superfluid one finds a counterflow between neutrons inside and outside the clusters. The neutron superfluid fraction is computed from the resulting current. Our results indicate that at densities above 0.03 fm$^{-3}$, more than 90% of the neutrons are effectively superfluid, independently of the detailed choice of the interaction, cluster charge, and lattice geometry. This fraction is only slightly lower than the one obtained recently within linear response theory on top of the Bardeen-Cooper-Schrieffer approximation, and it approaches the hydrodynamic limit for strong pairing. As a consequence, it is likely that the inner crust alone can provide a sufficient superfluid angular momentum reservoir to explain pulsar glitches.