London penetration depth in single crystals SrFe$_2$(As$_{0.65}$P$_{0.35}$)$_2$

In a contrast to a fully-gapped charge-doped (Ba,K)Fe$_2$As$_2$ and Ba(Fe,T)$_2$As$_2$ [1], isoelectron - substituted BaFe$_2$(As,P)$_2$ exhibit nodal superconducting gap [2]. To explore possible universality, low-temperature variation of the London penetration depth, $\Delta \lambda (T)$, was measured in optimally - doped SrFe$_2$(As$_{0.65}$P$_{0.35}$)$_2$ with $T_c$=29~K. $\Delta \lambda (T)$ revealed notable deviations from the exponential temperature dependence, expected for a fully-gapped superconductors. Instead the data are best fit with a power-law function, $\Delta \lambda =AT^n$. The analysis of the data below $1/3 T_{c}$ over a variable fitting temperature range produced exponents $n\le2$ which suggests the presence of nodes in the superconducting gap, similar to the P - doped Ba$122$ compounds. \\[4pt] [1] R. Prozorov and V. G. Kogan, Rep. Prog. Phys. {\bf 74}, 124505 (2011). \\[0pt] [2] K. Hashimoto, {\it et. al.} Phys. Rev. B {\bf 81}, 220501(R) (2010).