Nodal superconductivity in isovalently substituted SrFe 2 (As 1 − x P x ) 2 pnictide superconductor at the optimal doping x = 0.35

Temperature-dependent London penetration depth $\ensuremath{\lambda}(T)$ was measured in optimally doped $x=0.35$, as-grown [${T}_{c}\ensuremath{\approx}25$ K, $\text{RRR}=\ensuremath{\rho}(300\phantom{\rule{0.28em}{0ex}}\text{K})/\ensuremath{\rho}({T}_{c})=4.5$], and annealed (${T}_{c}\ensuremath{\approx}35$ K, $\text{RRR}=6.4$) single crystals of SrFe${}_{2}$(As${}_{1\ensuremath{-}x}$P${}_{x}$)${}_{2}$ iron-based superconductor. Annealing increases the residual resistivity ratio (RRR) and decreases the absolute value of the London penetration depth from $\ensuremath{\lambda}(0)=300\ifmmode\pm\else\textpm\fi{}10$ nm in the as-grown sample to $\ensuremath{\lambda}(0)=275\ifmmode\pm\else\textpm\fi{}10$ nm. At low temperatures, $\ensuremath{\lambda}(T)\ensuremath{\sim}T$ indicating a superconducting gap with line nodes. Analysis of the full-temperature range superfluid density is consistent with the line nodes, but differs from the simple single-gap $d$ wave. The observed behavior is very similar to that of BaFe${}_{2}$(As${}_{1\ensuremath{-}x}$P${}_{x}$)${}_{2}$, showing that isovalently substituted pnictides are inherently different from the charge-doped materials.