Mixed-symmetry superconductivity in two-dimensional Fermi liquids.

We consider a 2D isotropic Fermi liquid with attraction in both s and d channels and examine the possibility of a superconducting state with mixed s and d symmetry of the gap function. We show that both in the weak coupling limit and at strong coupling, a mixed s + id symmetry state is realized in a certain range of interaction. Phase transitions between the mixed and the pure symmetry states are second order. We also show that there is no stable mixed s + d symmetry state at any coupling.