Magnetic penetration depth of single-crystallineSmFeAsO1−xFy

Abstract

We report measurements of the in-plane magnetic penetration depth $\lambda$ in single crystals of ${\text{SmFeAsO}}_{1-x}{\text{F}}_y$ $\left(x\simeq y\simeq 0.2\right)$ with $T_c\simeq 44\text{K}$. 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, ${\Delta }_1=1.1\pm 0.1k_B{T}_c$ at $T=0$, 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 are well fitted by a two-gap model with the larger gap $\left({\Delta }_2=1.7\pm 0.2k_B{T}_c\right)$ associated with $\sim 80\%$ of the total superfluid density.