On the topology of the Fermi surface in dense neutron matter

The model of dense neutron matter has been considered, where the topological rearrangement of the ground state of the system of Landau quasiparticles, which is associated with the appearance of the second sheet of the Fermi surface, occurs through two different scenarios. The rearrangement scenario depends on the relation between the wave vector $q_c$ of critical spin-isospin fluctuations and the Fermi momentum $p_{\text{F}}$. Rearrangement at $q_c<p_{\text{F}}$ occurs continuously with vanishing of the topological rigidity, whereas rearrangement at $q_c>p_{\text{F}}$ occurs with the stepwise appearance of a bubble with a radius of about $\simeq0.5p_{\text{F}}$ in the filled Fermi sphere.