Microwave Surface Impedance Measurements of SrFe$_2$(As,P)$_2$ Single Crystals

Various pairing symmetries have been proposed concerning Fe-based superconductors both theoretically and experimentally. It was reported that LaFePO[1] and BaFe$_2$(As,P)$_2$[2] have line nodes in their superconducting gap. It is in sharp contrast to other Fe-based compounds such as LiFeAs[3] and Fe(Se,Te)[4]. To confirm whether line nodes in gap function is a common feature among P doped systems, we measured the microwave surface impedances of SrFe$_2$(As,P)$_2$ single crystals ($T_c\sim30K$). \\ Single crystals were grown by self-flux method. The surface impedances were measured using a cavity perturbation technique. The imaginary part of surface impedance, which is proportional to London penetration depth in the superconducting state, shows a power law, $\lambda(T)-\lambda(0)\propto T^n$. The power law indicates low-energy quasiparticle excitation, and an exponent slightly smaller than 2 does not exclude the possibility of the existence of line nodes. \\ $\left[ 1 \right]$ J. D. Fletcher {\it et al.}, Phys. Rev. Lett. 102 (2009) 147001.\\ $\left[ 2 \right]$ K. Hashimoto {\it et al.}, Phys. Rev. B 81 (2010) 220501(R).\\ $\left[ 3 \right]$ Y. Imai {\it et al.}, J. Phys. Soc. Jpn. 80 (2011) 013704.\\ $\left[ 4 \right]$ H. Takahashi {\it et al.}, Phys. Rev. B 84. (2011) 132503.