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The response of superconductors to controlled introduction of point-likedisorder is an important tool to probe their microscopic electronic collectivebehavior. In the case of iron-based superconductors (IBS), magneticfluctuations presumably play an important role in inducing high temperaturesuperconductivity. In some cases, these two seemingly incompatible orderscoexist microscopically. Therefore, understanding how this unique coexistencestate is affected by disorder can provide important information about themicroscopic mechanisms involved. In one of the most studied pnictide family,hole-doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ (BaK122), this coexistence occurs over awide range of doping levels, 0.16~$\lesssim x \lesssim $~0.25. We usedrelativistic 2.5 MeV electrons to induce vacancy-interstitial (Frenkel) pairsthat act as efficient point-like scattering centers. Upon increasing dose ofirradiation, the superconducting transition temperature $T_c$ decreasesdramatically. In the absence of nodes in the order parameter this provides astrong support for a sign-changing $s_{\pm}$ pairing. Simultaneously, in thenormal state, there is a strong violation of the Matthiessen's rule and adecrease (surprisingly, at the same rate as $T_c$) of the magnetic transitiontemperature $T_{sm}$, which indicates the itinerant nature of the long-rangemagnetic order. Comparison of the hole-doped BaK122 with electron-dopedBa(Fe$_x$Co$_{1-x}$)$_2$As$_2$ (FeCo122) with similar $T_{sm}\sim$110~K,$x=$0.02, reveals significant differences in the normal states, with noapparent Matthiessen's rule violation above $T_{sm}$ on the electron-dopedside. We interpret these results in terms of the distinct impact of impurityscattering on the competing itinerant antiferromagnetic and $s_{\pm}$superconducting orders.

npj quantum materials

Interplay between superconductivity and itinerant magnetism in  underdoped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ ($x=$ 0.2) probed by the response to  controlled point-like disorder