Abnormal Occupation REvisited

It is demonstrated the Fermi surface of dense neutron matter may experience a rearrangement near the onset of pion condensation, due to strong momentum dependence of the effective interaction induced by spin-isospin fluctuations. In particular, the Fermi surface may take the form of a partially hollow sphere having a spherical hole in its center (a ``Lifschitz bubble''). Thus, a second (inner) Fermi surface may form as high-momentum single-particle states are filled and low-momentum states are vacated. The influence of this phenomenon on the superfluid transition temperature of the Fermi system is characterized with the help of a separation transformation of the BCS gap equation. This work may be viewed as a revival of the search for physical realizations of abnormal occupation in infinite, homogeneous Fermi systems -- plausible instances in which the quasiparticle distribution differs from that of an ideal Fermi gas and Fermi liquid theory breaks down.