High Tc in monolayers and intercalates of FeSe: role of incipient bands and orbital selectivity

High temperature superconductivity in monolayers and some intercalates of FeSe is puzzling because it appears to contradict the usual spin fluctuation pairing scenario for Fe-based superconductors, where the repulsive interactions between hole and electron Fermi pockets lead to strong pairing. In this picture, however, $T_c$ should be negligible in these Fe-chalcogenide systems since they are known to be missing $\Gamma$-centered hole pockets. I discuss two possible modifications of the conventional spin-fluctuation approach that may be relevant to this paradox. First, I consider the hitherto neglected role of bands away from the Fermi level (``incipient")[1], and argue that the incipient band contributes significantly to spin-fluctuation pairing in the strong coupling limit where the system is close to a magnetic instability [2]. As the incipient band extremum (or doping) is tuned, the competition between the paramagnon pairing bandwidth and interaction can lead to high $T_c$ with a dome-like dependence. In this context I further discuss the role of phonons and impurities [3]. Finally, I consider the effect of orbitally selective electronic correlations, which can strongly affect the anisotropy of the gap functions observed [4]. \vskip .4cm 1. X. Chen, S. Maiti, A. Linscheid, and P. J. Hirschfeld, Phys. Rev. B 92, 224514 (2015). \vskip .03cm 2. A. Linscheid, S. Maiti, Y. Wang, S. Johnston and P. J. Hirschfeld, Phys. Rev. Lett. 117, 077003 (2016). \vskip .03cm 3. X. Chen, V. Mishra and P.J. Hirschfeld, Phys. Rev. B 94, 054524 (2016). \vskip .03cm 4. A. Kreisel, P. O. Sprau, A. Kostin, J. C. Davis, B. M. Andersen, P. J. Hirschfeld, arXiv:1611.02643.