Effect of Electron Irradiation on isovalenly substituted SrFe$_2$(As$_{1-x}$P$_x$)$_2$, $x=0.35$

The effects of electron irradiation on the termperature-dependent London penetration depth, $\lambda$(T), have been investigated in annealed optimally doped single crystals of isovalently substituted SrFe$_2$(As$_{1-x}$P$_x$)$_2$, $x=0.35$, using the tunnel diode resonator technique. The low temperature behavior of $\lambda$(T) changes under electron irradiation from almost $T-$linear to practically exponentially saturated behavior, similar to the observations in another isovalently substituted 122 pnictide, BaFe$_2$(As$_{1-x}$P$_x$)$_2$. Furthermore, aluminum - coating technique was used to measure the absolute values of the London penetration depth, $\lambda$(0), which allowed calculations of the superfluid density, $\rho_s$. We conclude that, similar to BaFe$_2$(As$_{1-x}$P$_x$)$_2$, the superconducting properties of SrFe$_2$(As$_{1-x}$P$_x$)$_2$ are compatible with $s_{\pm}$ pairing with accidental line nodes that are lifted by pair-breaking disorder. This work was supported by the Department of Energy Office of Science, Basic Energy Sciences under Contract No. DE-AC02-O7CH11358.