Signature of multigap nodeless superconductivity in fluorine-doped NdFeAsO

We investigate the temperature dependence of the lower critical field H-c1(T), the field at which vortices penetrate into the sample, of a high-quality fluorine-doped NdFeAsO single crystal under static magnetic fields H parallel to the c axis. The temperature dependence of the first vortex penetration field has been experimentally obtained and pronounced changes of the H-c1(T) curvature are observed, which are attributed to the multiband superconductivity. Using a two-band model with s-wave-like gaps, the temperature dependence of the lower critical field H-c1(T) can be well described. These observations clearly show that the superconducting energy gap in fluorine-doped NdFeAsO is nodeless. The values of the penetration depth at T = 0 K have been determined and confirm that the pnictide superconductors obey an Uemura-style relationship between T-c and lambda(ab)(0)(-2).