Possible collective spin excitation in the spin-triplet superconducting state of Sr2RuO4. Multi-band theory

We investigate possible collective spin excitation by calculating the dynamical spin susceptibility in the spin-triplet superconducting state of Sr2RuO4. The effect of the fluctuating triplet-pairing condensate on the spin susceptibility is taken into account. The momentum and band dependences of superconducting gap structure are determined microscopically by solving the linearized Eliashberg equation for a realistic three-band Hubbard model within the perturbation theory in Coulomb interaction, and the evolution of gap magnitude below the transition temperature is determined by the standard BCS gap equation. The triplet pairing symmetry is assumed to be the most promising form: d(k) - (kx + iky)z. For small (maybe realistic) anisotropy of pairing interaction in spin space, we expect a possible low-energy spin-wave mode as a peak structure in the imaginary part of the dynamical spin susceptibility, isolated from the spin excitation continuum. The present work is a natural extension of previous single-band calculations to a more realistic three-band calculation. (author)