Mass Enhancements and Band Shifts in Strongly Hole-Overdoped Fe-Based Pnictide Superconductors: KFe2As2 and CsFe2As2

The interplay of high- and low-energy mass renormalizations with band shifts reflected by the positions of van Hove singularities (VHS) in the normal-state spectrum of the most strongly hole-overdoped correlated AFe 2 As 2 (A122) with A=K, Cs is discussed phenomenologically from ARPES data and band-structure (GGA) calculations with full spin-orbit coupling. The big increase of the Sommerfeld coefficient γ from K122 to Cs122 is ascribed to an enhanced coupling to low-energy bosons in the vicinity of a quantum critical point to an unknown, yet incommensurate phase different from the commensurate Mott one. We find no sizeable increase in correlations for Cs122 in contrast to Eilers et al. (Phys. Rev. Lett. 116 , 237003 2016 ). The empirical (ARPES) VHS positions as compared with GGA predictions point even to slightly weaker correlations in Cs122 in accord with low- T magnetic susceptibility χ ( T ) data and a decreasing Wilson ratio ∝ χ (0)/ γ .