Fundamental Study of hole overdoped Ba_1-xK_xFe₂As₂

While the superconducting transition temperature of hole doped BaFe₂As₂ decreases from the optimally doped region, superconductivity does not completely disappear even for the fully doped KFe₂As₂ compound, in contrast to the cuprates. In fact, superconductivity is robust even through a Lifshitz transition where electron bands become hole like around the zone corner around x~0.7 for Ba_1-xK_xFe₂As_2. There are reports of Broken Time Reversal Symmetry in proximity to the Lifshitz transition as the pairing symmetry changes from nodeless to nodal. On the other hand, there is a lack of electronic structure calculations across doping levels with experimental benchmarks, which is a prerequisite for a systematic understanding. Angle Resolved Photoemission Spectroscopy (ARPES) is the tool of choice to bridge the gap and gain insights on the interesting anomalies on superconductivity. Thus far, most ARPES superconducting gap measurements of Ba_1-xK_xFe₂As₂ have focused in the vicinity of optimal doping (x=0.4). We use high resolution ARPES to study three doping levels (x=0.53, 0.78, 0.89) of Ba_1-xK_xFe₂As₂ above optimal doping and across the Liftshitz transition. Our data reveals the expected Lifshitz transtion but with a twist of added complexity of additional participation of the dz² band. Further, we found there is doping dependent renormalization of bands around the Lifshitz transition. We compare our results to calculated band structures, aiming to understand the origin of this change and its relation to the displayed physical phenomenon.