Superfuid-insulator transitions at non-integer filling in optical lattices of fermionic atoms

We determine the superfluid transition temperatures $T_c$ and the ground states of the attractive Hubbard model and find new insulating phases associated with non-integer filling at sufficiently strong pairing attraction $\mathrm{\mid }U\mathrm{\mid }$. These states, distinct from band and Mott insulating phases, derive from pair localization; pair hopping at large $\mathrm{\mid }U\mathrm{\mid }$ and high densities is impeded by inter-site, inter-pair repulsive interactions. The best way to detect the breakdown of superfluidity is using fermionic optical lattices which should reveal new forms of ``bosonic’’ order, reflecting ground state pairing without condensation.