We have analyzed heat capacity and thermal conductivity measurements of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ in the normal and superconducting state and come to the conclusion that an order parameter with nodal lines on the Fermi surface is required to account for the observed low-temperature behavior. A gapped order parameter is inconsistent with the reported thermodynamic and transport data. Guided by a strongly peaked dynamical susceptibility along the diagonals of the Brillouin zone in neutron-scattering data, we suggest a spin-fluctuation mechanism that would favor the pairing state with the gap maxima along the zone diagonals (such as for a $d_{\mathrm{xy}}$ gap). The most plausible candidates are an odd parity, spin-triplet, f-wave pairing state, or an even parity, spin-singlet, d-wave state. Based on our analysis of possible pairing functions we propose measurements of the ultrasound attenuation and thermal conductivity in the magnetic field to further constrain the list of possible pairing states.

• Received 31 March 2000

DOI:https://doi.org/10.1103/PhysRevB.62.9697