Irradiation with 2.5 MeV electrons at doses up to 5.2 ×10 19 electrons/cm 2 was used to introduce point-like defects in single crystals of Ba1−xKxFe2As2 with x = 0.19 (Tc = 14 K), x = 0.26 (Tc = 32 K) and 0.34 (Tc = 39 K) to study the superconducting gap structure by probing the effect of non-magnetic scattering on electrical resistivity, ρ(T ), and London penetration depth, λ(T ). For all compositions, the irradiation suppressed the superconducting transition temperature, Tc and increased resistivity. The low -temperature behavior of λ(T ) is best described by the power -law function, ∆λ(T ) = A(T /Tc) n . While substantial suppression of Tc supports s± pairing mechanism, in samples close to the optimal doping, x = 0.26 and 0.34, the exponent n remained high (n ≥ 3) indicating robust full superconducting gaps. For the x = 0.19 composition, exhibiting coexistence of superconductivity and long -range magnetism, the suppression of Tc was much more rapid and the exponent n decreased toward dirty limit of n = 2. In this sample, the irradiation also suppressed the temperature of structural/magnetic transition, Tsm, from 103 K to 98 K consistent with the itinerant nature of the magnetic order. Our results suggest that underdoped compositions, especially in the coexisting regime are most susceptible to non-magnetic scattering and imply that in multi-band Ba1−xKxFe2As2 superconductors, the ratio of the inter-band to intra-band pairing strength, and associated gap anisotropy, increases upon the departure from the optimal doping.

The doping-evolution of the superconducting gap structure in iron-based superconductor (Ca −x min c ). Both the decrease of the exponent and the increase of the absolute value of Δλ in the underdoped regime are similar to the observations in other charge-doped iron-based superconductors, such as doped BaFe 2 As 2 and NaFeAs, suggesting a universal behavior in iron-based superconductors.