Model for Unconventional Superconductivity ofSr2RuO4: Effect of Impurity Scattering on Time-Reversal Breaking Triplet Pairing with a Tiny Gap

A model for unconventional superconductivity of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ is presented to resolve its puzzle. It is shown that the short-range ferromagnetic spin fluctuations give rise to the triplet pairing with $p$-like symmetry which is breaking the time-reversal symmetry and has a tiny gap due to the salient shape of the Fermi surface characteristic to ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$. The effect of nonmagnetic-impurity scattering in the unitarity limit is shown to fill up easily the tiny gap giving rise to an appreciable residual density of states, which explains consistently the puzzling properties observed so far.