Superfluid density in the s_(pm)-wave state of clean iron-based superconductors

Based on a phenomenological model and the Kubo formula, we investigate the superfluid density $\rho_s(T)$ and then the penetration depth $\lambda(T)$ of the iron-based superconductors in the coexistence region of the spin-density-wave(SDW) and superconductivity, and also in the over-doped region. Our calculations show a dramatic increase of $\lambda(0)$ with the decrease of the doping concentration $x$ below $x=0.1$. This result is consistent with the experimental observations. At low temperatures, $\rho(T)$ shows an exponential-law behavior, while at higher temperatures, the linear-in-$T$ behavior is dominant before it trends to vanish. The evolution of $\Delta\lambda(T)$ can be roughly fitted by a exponential function at overdoped levels while in other doping range it is a power-law function with the exponent depending on the doping concentration. We show that the Uemura relation holds for the iron-based superconductors only at very low doping levels.