REFeAsO1−y (RE = La, Pr, Nd) and Ba0.6K0.4Fe2As2

We discuss the novel superconducting characteristics and unusual normal-state properties of iron (Fe)-based pnictid e super- conductors REFeAsO1−y (RE=La,Pr,Nd) and Ba0.6K0.4Fe2As2(Tc = 38 K) by means of 57Fe-NMR and 75As-NQR/NMR. In the superconducting state of LaFeAsO0.7 (Tc = 28 K), the spin component of the 57Fe-Knight shift decreases to almost zero at low tem- peratures, which provide firm evidence of the superconducti ng state formed by spin-singlet Cooper pairing. The nuclear spin-lattice relaxation rates (1/T1) in LaFeAsO0.7 and Ba0.6K0.4Fe2As2 exhibit a T 3-like dependence without a coherence peak just below Tc, indicating that an unconventional superconducting state is commonly realized in these Fe-based pnictide compounds. All these events below Tc are consistently argued in terms of an extended s±-wave pairing with a sign reversal of the order parameter among Fermi surfaces. In the normal state, 1/T1T decreases remarkably upon cooling for both the Fe and As sites of LaFeAsO0.7. In con- trast, it gradually increases upon cooling in Ba 0.6K0.4Fe2As2. Despite the similarity between the superconducting properties of these compounds, a crucial difference was observed in their normal-state properties depending on whether electrons or holes are doped into the FeAs layers. These results may provide some hint to address a possible mechanism of Fe-based pnictide superconductors.