Superconductivity due to spin fluctuations originating from multiple Fermi surfaces in the double chain superconductor Pr2 Ba4 Cu7 O15-δ

The mechanism of superconductivity in Pr2 Ba4 Cu7 O15-δ is studied using a quasi-one-dimensional double chain model with appropriate hopping integrals, on-site U, and off-site repulsion V1. Applying the fluctuation exchange method to this model and solving the Eliashberg's equation, we obtain the doping dependence of superconductivity that is consistent with the experiments. The superconducting gap has a spin singlet symmetry with the nodes of the gap running between the disconnected Fermi surfaces, which gives a temperature dependence of the spin-lattice relaxation rate that does not contradict with the experimental results.