Electronic Origin of Tc in Bulk and Monolayer FeSe

FeSe is classed as a Hund’s metal, with a multiplicity of d bands near the Fermi level. Correlations in Hund’s metals mostly originate from the exchange parameter J, which can drive a strong orbital selectivity in the correlations. The Fe-chalcogens are the most strongly correlated of the Fe-based superconductors, with d_xy the most correlated orbital. Yet little is understood whether and how such correlations directly affect the superconducting instability in Hund’s systems. By applying a recently developed ab initio theory, we show explicitly the connections between correlations in d_xy and the superconducting critical temperature T_c. Starting from the ab initio results as a reference, we consider various kinds of excursions in parameter space around the reference to determine what controls T_c. We show small excursions in J can cause colossal changes in T_c. Additionally we consider changes in hopping by varying the Fe-Se bond length in bulk, in the free standing monolayer M-FeSe, and M-FeSe on a SrTiO₃ substrate (M-FeSe/STO). The twin conditions of proximity of the d_xy state to the Fermi energy, and the strength of J emerge as the primary criteria for incoherent spectral response and enhanced single-and two-particle scattering that in turn controls T_c. Using constrained RPA, we show further that FeSe in monolayer form (M-FeSe) provides a natural mechanism to enhance J. We explain why M-FeSe/STO has a high T_c, whereas M-FeSe in isolation should not. Our study opens a paradigm for a unified understanding what controls T_c in bulk, layers, and interfaces of Hund’s metals by hole pocket and electron screening cloud engineering.