Orbital-dependent pairing effects in the nuclear spin-lattice relaxation rate of Sr2RuO4

The p-wave chiral superconductor Sr2RuO4 is a quasi-two-dimensional, highly anisotropic system with three bands crossing the Fermi energy. Low temperature power law behaviour is observed in temperature-dependent physical properties, including specific heat, penetration depth and NMR relaxation rate, which is consistent with the existence of a line of nodes in the quasiparticle gap. However, the interpretation of these experiments is complicated by possible temperature dependences arising from the multi-band or orbital-dependent paring. In this paper we have calculated the NMR relaxation rate 1/T-1 on the basis of a realistic orbital-specific, three-orbital, three-dimensional model with phenomenological coupling constants. The model leads to the ground state with chiral-gapless order parameter on the gamma sheet of the Fermi level and with nodes on the remaining two sheets. Our results compare well with existing experimental Ru-101 NQR data showing a relaxation rate 1/T-1 similar to T-3 and no visible Hebel-Slichter-like peak below T-c. On the other hand, the diagonal d(xy) orbital projected components of the relaxation rate show a more complex behaviour, including a Hebel-Slichter-like peak below T-c. We speculate that these orbital dependences might be related to the differences observed between spin relaxation rates for O-17 and Ru-101 nuclei in this material.