Low-temperature electronic properties of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4.$  II.  Superconductivity

The body centered tetragonal structure of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ gives rise to umklapp scattering enhanced interplane pair correlations in the $d_{\mathrm{yz}}$ and $d_{\mathrm{zx}}$ orbitals. Based on symmetry arguments, Hund’s rule coupling, and a bosonized description of the in-plane electron correlations the superconducting order parameter is found to be a orbital-singlet spin-triplet with two spatial components. The spatial anisotropy is 7%. The different components of the order parameter give rise to two-dimensional gapless fluctuations. The phase transition is of third order. The temperature dependence of the pair density, specific heat, nuclear quadrupole resonance, Knight shift, and susceptibility are in agreement with experimental results.