Magnetic penetration depth of single-crystallineSmFeAsO1−xFy

Abstract

We report measurements of the in-plane magnetic penetration depth $\ensuremath{\lambda}$ in single crystals of ${\text{SmFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{y}$ $(x\ensuremath{\simeq}y\ensuremath{\simeq}0.2)$ with ${T}_{c}\ensuremath{\simeq}44\text{ }\text{K}$. We find that at low temperature $\ensuremath{\lambda}$ has an exponential temperature dependence which suggests that the Fermi surface is fully gapped. The magnitude of the minimum energy gap, ${\ensuremath{\Delta}}_{1}=1.1\ifmmode\pm\else\textpm\fi{}0.1{k}_{B}{T}_{c}$ 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 $({\ensuremath{\Delta}}_{2}=1.7\ifmmode\pm\else\textpm\fi{}0.2{k}_{B}{T}_{c})$ associated with $\ensuremath{\sim}80%$ of the total superfluid density.