  Superfluidity in fermionic systems originates from pairing of fermions, and
Bose condensation of these so-called Cooper pairs. The Cooper pairs are usually
made of fermions of different species; for example in superconductors they are
pairs of electrons with opposite spins. Thus the most favorable situation for
pairing and superfluidity is when the two species of fermions that form pairs
have the same density. This paper studies the possible superfluid states when
the two pairing species have different densities, and show that the resultant
states have remarkable similarities to the phases of liquid crystals. This
enables us to provide a unified description of the possible pairing phases, and
understand the phase transitions among them.
