Generality of high-T_c superconductivity originating from coexisting wide and incipient narrow bands

An ideal situation for realizing high T_c superconductivity is to have light electron mass and strong pairing interaction at the same time, but usually the two are not compatible with each other. In ref.[1], one of the present authors proposed a way to overcome this problem ; in a system consisting of wide and narrow bands, light effective mass and strong pairing interaction is simultaneously realized when the Fermi level sits in the vicinity of, but does not intersect, the narrow band. The 2-leg Hubbard ladder with diagonal hoppings was studied as a system in which such a situation is realized, where a possible occurrence of extremely high T_c was suggested. Here, we extend this study, and show that this high T_c mechanism works in a variety of systems that consist of wide and narrow (of flat) bands, such as the diamond lattice [2] and the 3-leg ladder. We apply the FLEX approximation to the Hubbard model on these lattices, and show that superconductivity is strongly enhanced when the Fermi level sits close to the narrow band. The present study shows the generality of the high T_c superconductivity originating from wide and incipient narrow bands.
References: [1] K. Kuroki, et al., Phys. Rev B, 72, 212509 (2005). [2] K. Kobayashi et al., Phys. Rev. B 94, 214501 (2016).