Spin-orbit coupling and symmetry of the order parameter in strontium ruthenate

Determination of the orbital symmetry of the pairing state in the spin triplet Sr2RuO4 superconductor is a challenge of considerable importance. Most of the experiments show that the chiral state of the z(k(x)+/- ik(y)) type is realized and remains stable on lowering the temperature. Here we have studied the stability of various superconducting states of Sr2RuO4 in the presence of spin-orbit coupling. Numerically we found that the chiral state is never the minimum energy. Alone among the five states studied it has < L center dot S >=0 and is therefore not affected to linear order in the coupling parameter lambda. We found that the stability of the chiral state requires spin-dependent pairing interactions. This imposes strong constraints on the pairing mechanism.