The London penetration depth $\lambda \left(T\right)$ was measured in single crystals of a ${\mathrm{SrFe}}_2\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right){}_2$ $\left(x=0.35\right)$ iron-based superconductor. The influence of disorder on the transition temperature $T_c$ and on $\lambda \left(T\right)$ was investigated. The effects of scattering controlled by the annealing of as-grown crystals was compared with the effects of artificial disorder introduced by 2.5 MeV electron irradiation. The low-temperature behavior of $\lambda \left(T\right)$ can be described by a power-law function $\Delta \lambda \left(T\right)=A{T}^n$, with the exponent $n$ close to one in pristine annealed samples, as expected for a superconducting gap with line nodes. Upon electron irradiation with a dose of $1.2\times {10}^{19}$ $e/{\mathrm{cm}}^2$, the exponent $n$ increases rapidly, exceeding a dirty limit value of $n=2$, implying that the nodes in the superconducting gap are accidental and can be lifted by the disorder. The variation of the exponent $n$ with $T_c$ is much stronger in the irradiated crystals compared to the crystals in which disorder was controlled by the annealing of the growth defects. We discuss the results in terms of different influence of different types of disorder on intraband and interband scattering.

• Received 27 May 2014
• Revised 30 June 2014

DOI:https://doi.org/10.1103/PhysRevB.90.020508