Field and Temperature Dependence of the Superfluid Density in LaO_{1-x}F_xFeAs Superconductors: A Muon Spin Relaxation Study

摘要

We present zero field and transverse field \muSR experiments on the recentlydiscovered electron doped Fe-based superconductor LaO_{1-x}F_xFeAs. The zerofield experiments on underdoped (x=0.075) and optimally doped (x=0.1) samplesrule out any static magnetic order above 1.6 K in these superconductingsamples. From transverse field experiments in the vortex phase we deduce thetemperature and field dependence of the superfluid density. Whereas thetemperature dependence is consistent with a weak coupling BCS s-wave or a dirtyd-wave gap function scenario, the field dependence strongly evidencesunconventional superconductivity. We obtain the in-plane penetration depth of\lambda_{ab} (0) = 254(2)nm for LaO_{0.9}F_{0.1}FeAs and \lambda_{ab} (0) =364(8)nm for LaO_{0.925}F_{0.075}FeAs. Further evidence for unconventionalsuperconductivity is provided by the ratio of T_c versus the superfluiddensity, which is close to the Uemura line of hole doped high-T_c cuprates.