superconductivity in Ruddlesden-Popper compounds: Incipient narrow bands originating from “hidden ladders”

It has been suggested[1] that superconductivity should be strongly enhanced in systems with coexisting wide and narrow bands, as in the two-leg Hubbard ladder with the second-neighbor hopping when the Fermi level E_F is put near the narrow band edge. As a way to realize this situation in actual materials, we introduce a concept of a “hidden ladder” electronic structure in the bilayer Ruddlesden-Popper compounds. When E_F crosses the bands with t_2g orbital character, an electron in the d_xz/yz orbital selectively hops in the x/y direction as well as in the z direction normal to the bilayer. This means that the d_xz/yz orbital form a ladder with x/y and z directions being the leg and rung directions, respectively. We propose that Sr₃Mo₂O₇ and Sr₃Cr₂O₇ are candidates for the hidden ladder materials where E_F sits right in the vicinity of the narrow-band edge. Based on this electronic structure, we discuss possible occurrence of high-T_c superconductivity in these materials originating from the large number of interband pair-scattering channels between the wide and the “incipient” narrow bands[2].
References: [1] K. Kuroki, et al., Phys. Rev B, 72, 212509 (2005). [2] D. Ogura, et al., arXiv: 1706.01711 (2017).