LAUSR

We describe the effect of nonmagnetic impurity on the superconductivity behavior of the NdFeAsO 0.8 F 0.2 iron-based superconductor. The resistivity measurements show that the superconductivity is suppressed upon increasing the low amounts of calcium impurity ( x  ≤ 0.05). Also, the T C decreases with the increase in the residual resistivity. Such behavior is qualitatively described by the Abrikosov–Gorkov theory and confirms that these impurities act as scattering centers. Moreover, we present the phase diagram of our synthesized samples for the various calcium dopings. We find that according to the increase in the calcium impurities and the decrease in the spin-density wave transition tempeθrature ( T SDW ), Fe ions are arranged stripe-antiferromagnetic at lower temperatures and also the superconducting transition temperature ( T C ) declines. Based on our results and in agreement with the available theories as is explained in the text, since the S ++ state has no effect on the impurity-doped samples, and for low amounts of calcium, the S ± state that is attributed to the spin-fluctuations causes the superconductivity suppression. So, it confirms the role of the spin-fluctuations as a dominant pairing mechanism in our synthesized samples.