Efficient charge carriers induced by extra outer-shell electrons in iron-pnictides: A comparison between Ni- And Co-doped CaFeAsF

Abstract

A comprehensive study of the difference between CaFe1-xNi xAsF and CaFe1-xCoxAsF systems has been carried out by measuring the efficient charge carrier concentration, the valence states and the superconducting phase diagram. It is found that at the same doping level, Ni doping introduces nearly twice the number of charge carriers as Co doping. However, x-ray absorption near-edge spectroscopy measurements reveal that the valence state of Fe in both systems is close to 2, indicating that there is no valence mismatch. We suggest that the charge carriers in CaFe 1-xMxAsF (M = transition metal elements) are not induced by valence mismatch but come from the difference in the number of outer-shell electrons. We also suggest that with Ni and Co doping, the systems change from a multi-band material in the underdoped regions to a single-band state in the overdoped regions. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.