Effect of heavy-ion irradiation on London penetration depth in overdoped Ba(Fe1−xCox)2As2

Irradiation with 1.4 GeV ${}^{208}$Pb ions was used to induce artificial disorder in single crystals of iron-arsenide superconductor Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ and to study its effects on the temperature-dependent London penetration depth and transport properties. A study was undertaken on overdoped single crystals with $x=0.108$ and $x=0.127$ characterized by notable modulation of the superconducting gap. Irradiation corresponding to the matching fields of $B_{\varphi }=6$ T and 6.5 T with doses $2.22\times {10}^{11}$ $\mathit{\text{d}}$/cm${}^2$ and $2.4\times {10}^{11}$ $\mathit{\text{d}}$/cm${}^2$, respectively, suppresses the superconducting $T_c$ by approximately 0.3 to 1 K. The variation of the low-temperature penetration depth in both pristine and irradiated samples is well described by the power law $\Delta \lambda \left(T\right)=A{T}^n$. Irradiation increases the magnitude of the prefactor $A$ and decreases the exponent $n$, similar to the effect of irradiation in optimally-doped samples. This finding supports universal $s_{\pm }$ pairing in Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ compounds for the entire Co doping range.