Field and Temperature Dependence of the Superfluid Density inLaFeAsO1−xFxSuperconductors: A Muon Spin Relaxation Study

Abstract

We present zero field and transverse field muon spin relaxation experiments on the recently discovered Fe-based superconductor ${\mathrm{LaFeAsO}}_{1-x}{\mathrm{F}}_x$ ($x=0.075$ and $x=0.1$). The temperature dependence of the deduced superfluid density is consistent with a BCS $s$-wave or a dirty $d$-wave gap function, while the field dependence strongly evidences unconventional superconductivity. We obtain the in-plane penetration depth of ${\lambda }_{ab}(0)=254(2)\mathrm{nm}$ for $x=0.1$ and ${\lambda }_{ab}(0)=364(8)\mathrm{nm}$ for $x=0.075$. Further evidence for unconventional superconductivity is provided by the ratio of $T_c$ versus the superfluid density, which is close to the Uemura line of high-$T_c$ cuprates.