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, \(\lambda(T)\), was measured in optimally - doped, \(x=\)0.35, as-grown (\(T_c \approx\)25 K, RRR=\(\rho(300K)/\rho(T_c)\)=4.5) and annealed (\(T_c \approx\)35 K, RRR=6.4) single crystals of SrFe\(_2\)(As\(_{1-x}\)P\(_{x}\))\(_2\) iron - based superconductor. Annealing increases the RRR and decreases the absolute value of the London penetration depth from \(\lambda(0) = 300 \pm 10\) nm in as-grown sample to \(\lambda(0) = 275 \pm 10\) nm. At low temperatures, \(\lambda(T) \sim T\) indicating 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\(_{1-x}\)P\(_{x}\))\(_2\), showing that isovalently substituted pnictides are inherently different from the charge - doped materials.