Field and temperature dependence of the superfluid density in LaFeAsO1-xFx superconductors:: A muon spin relaxation study

We present zero field and transverse field muon spin relaxation experiments on the recently discovered Fe-based superconductor LaFeAsO1-xFx (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) nm for x = 0.1 and lambda(ab) (0) = 364(8) 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.