Superfluid density and penetration depth in Fe-pnictides

We consider the superfluid density ${\rho }_s(T)$ in a two-band superconductor with extended s-wave symmetry ($s^{+}$) in the presence of non-magnetic 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 inter-band impurity scattering, ${\rho }_s(T)$ behaves as a power-law $T^n$ with $n\approx 1.6÷2$ over a wide range of $T$. We argue that the power-law behavior is consistent with recent experiments on the penetration depth $\lambda (T)$ in BaFe${}_2$As${}_2$, but disagree quantitatively with the data on LaFePO.