Crossover in the magnetic response of single-crystalline Ba1â‹TMxKxFe2As2 and Lifshitz critical point evidenced by Hall effect measurements

We report on the doping evolution of magnetic susceptibility χ(T) and Hall coefficient RH in high-quality Ba1−xKxFe2As2 (0.13≤x≤1) single crystals. It is found that the normal-state magnetic susceptibility of Ba1−xKxFe2As2 compounds undergoes a crossover from linear-T dependence in the undoped and underdoped samples into KFe2As2-type magnetic response in the overdoped samples with increasing K content. Although magnetic susceptibility χ(T) of optimally doped samples (0.34≤x≤0.47) still follows a monotonic increase with increasing temperature, a big hump around 300 K emerges. As x exceeds 0.53, a broad peak forms in overdoped samples (0.53≤x≤1), which shifts toward 120 K for the end member KFe2As2. Above the peak temperature T∗=120 K, a Curie-Weiss-like behavior is observed in KFe2As2. The Hall coefficient RH of underdoped sample x=0.22shows a rapid increase above spin-density-wave transition temperature TSDW. Below TSDW, it increases slowly. RH of optimally doped and slightly overdoped samples (0.34≤x≤0.65) shows relatively weak temperature dependence and a saturation tendency below 150 K. However, RH of K heavily overdoped samples (0.80≤x≤1) increases rapidly below 150 K. Meanwhile, the Hall angle cotθH displays a concave temperature dependence within the doping range 0.22≤x≤0.55, whereas it changes to a convex temperature dependence within the doping range 0.65≤x≤1. The dramatic change coincides with the Lifshitz transition occurring around the critical doping x=0.80, where angle photoemission spectroscopy measurements had confirmed that the electron pocket disappears with excess hole doping in the Ba1−xKxFe2As2 system. It is suggested that the characteristic temperature T∗ at around 120∼150 K observed in susceptibility and the Hall coefficient, as well as previously reported resistivity data, may indicate an incoherence-coherence crossover in the Ba1−xKxFe2As2 system.