ARPES investigation of heavily hole-doped Fe-based superconductor (Ba,K)Fe$_2$As$_2$

A Lifshitz transition occurs in the (Ba,K)Fe$_2$As$_2$ family upon K doping and electron pocket are absent in the heavily doped compounds, including KFe$_2$As$_2$. The pairing symmetry is argued to undergoes a phase transition due to the existence of gap node(s) reported in various experiments. In this work we present angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy studies of KFe$_2$As$_2$. We observe a van Hove singularity (vHs) in proximity of the Fermi level ($E_F$), which locates in the middle of the principal axes of the first Brillouin zone. The density-of-states at $E_F$ mainly comes from the vHs whereas it is non-gapped in the superconducting state. Our observation provides natural explanations for many novel behaviors in this material. In particular, it is consistent with our measurements of the gap structure in Ba$_{0.1}$K$_{0.9}$Fe$_2$As$_2$. All these results suggest that Cooper pairing is induced by a strong-coupling mechanism.