in precisely oriented magnetic field. Effect of controlled disorder*

A systematic evolution of thermal conductivity in magnetic fields parallel to [001] crystallographic axis (orbital upper critical field, H_c2) and parallel to [100] direction (Pauli limited H_c2) was studied in nodal superconductor KFe₂As₂. Low-temperature irradiation with relativistic 2.5 MeV electrons was used to shift from clean to dirty limit. The dirty limit response in the Zeeman case is in qualitative agreement with the predictions for linear quasi-particle density increase with magnetic field and continuous transition at H_c2, as opposite to the 1^st-order transition in the clean limit.