Superconductivity in repulsive electron systems with three-dimensional disconnected Fermi surfaces

The idea of raising T_c in the spin-fluctuation mediated superconductivity on disconnected Fermi surfaces with the gap function changing sign across but not within the Fermi pockets, proposed by Kuroki and Arita for two dimensions, is here extended to three-dimensional (3D) systems. Two typical cases of 3D disconnected Fermi surfaces (stacked bond-alternating lattice and stacked ladder layers) are considered. By solving Eliashberg’s equation for Green’s function obtained with the fluctuation exchange approximation for the repulsive Hubbard model on these structures, we have shown that T_c can indeed reach O(0.01t), which is almost an order of magnitude higher than in ordinary 3D cases and similar to those for the best case found in two dimensions. The key factor found here for the favorable condition for the superconductivity on disconnected Fermi surfaces is that the system should be quasi-low-dimensional, and the peak in the spin susceptibility should be appropriately “blurred”.