P23 pairing in neutron-star matter: Magnetic field effects and vortices

Using Ginzburg-Landau theory, we consider the effect of a magnetic field on the anisotropic (P23) neutron superfluid. In a strong magnetic field the pairing state has an angle-dependent energy gap with a node along the field direction. Methods of homotopy theory are used to determine the topologically stable line defects allowed by the P23 order parameters. Our free-energy analysis of line vortices gives a qualitatively different structure to the single vortex, outside of the core region, from that obtained by other authors.