Triplet Pairing Superconductivity Induced by Short-Range Ferromagnetic Correlations in Sr$_2$RuO$_4$

The microscopic origin of triplet superconductivity in Sr$_2$RuO$_4$ is discussed, paying attention to the role of Coulomb interaction, $U_{pp}$, at the O site. It is shown on the $d$-$p$ model that $U_{pp}$ induces a ferromagnetic exchange interaction between "d-electrons" (molecular orbital with $d_{xy}$-symmetry) at adjacent Ru sites, leading to short-range ferromagnetic correlations and promoting Cooper pairing with ($\sin p_x\pm\ii\sin p_y$)-symmetry on the $\gamma$-band. The reason why such ferromagnetic correlations work effectively may be traced back to the fact that the level of 4d-electrons at Ru sites is relatively low and located near that of $2p$-electrons at O sites.