Magnetic penetration depth of single crystal SmFeAsO_(1-x)F_y: a fully gapped superconducting state

We report measurements of the in-plane magnetic penetration depth $\lambda$ in single crystals of SmFeAsO$_{1-x}$F$_y$ ($x\simeq y \simeq 0.2$) with $T_c \simeq 45$ K. We find that at low temperature $\lambda$ has an exponential temperature dependence which suggests that the Fermi surface is fully gapped. The magnitude of the minimum energy gap, $\Delta_1=1.1\pm 0.1 k_BT_c$ at T=0 K, is significantly smaller than the BCS weak-coupling value suggesting that the gap is either strongly anisotropic or varies significantly between the different Fermi surface sheets. Our data fits well a two gap model with an additional larger gap of magnitude $\Delta_2 = 1.8\pm0.2 k_BT_c$ which is associated with $\sim 80$% of the total superfluid density.