Superfluid density and penetration depth in the iron pnictides

We consider the superfluid density ρs (T) in a two-band superconductor with sign-changing extended s -wave symmetry (s+) in the presence of nonmagnetic impurities and apply the results to Fe-pnictides. We show that the behavior of the superfluid density is essentially the same as in an ordinary s -wave superconductor with magnetic impurities. We show that, for moderate to strong interband impurity scattering, ρs (T) behaves as a power law Tn with n 1.6÷2 over a wide range of T. We argue that the power-law behavior is consistent with recent experiments on the penetration depth λ (T) in doped BaFe2 As2 but disagrees quantitatively with the data on LaFePO.