  We derive the energy of the surface between the normal and superfluid
components of a mixed phase of a system composed of two particle species with
different densities. The surface energy is obtained by the integration of the
free energy density in the interface between the two phases. We show that the
mixed phase remains as the favored ground state over the gapless phase in weak
coupling. We find that the surface energy effects emerge only at strong
coupling.

Superconductivity;Astrophysics;Strongly Correlated Electrons;High Energy Physics - Phenomenology;Nuclear Theory