Superfluid Density in the s±-Wave State of Clean Iron-Based Superconductors

Abstract

Based on a phenomenological model and the Kubo formula, we investigate the superfluid density ρs(T) and then the penetration depth λ(T) of the iron-based superconductors in the coexistence region of the spin-density wave and superconductivity, and also in the overdoped region. Our calculations show a dramatic increase of λ(0) with the decrease of the doping concentration x below x=0.1. This result is consistent with the experimental observations. At low temperatures, ρs(T) shows an exponential-law behavior, while at higher temperatures, the linear-in-T behavior is dominant before it trends to vanish. It is in qualitative agreement with the direct measurement of superfluid density in films of Fe-pnictide superconductor at x=0.08. The evolution of Δλ(T) can be roughly fitted by 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.

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