Abstract
We discuss the novel superconducting characteristics and unusual normal-state properties of iron (Fe)-based pnictide superconductors REFeAsO 1− y (RE = La, Pr, Nd) and Ba 0.6K 0.4Fe 2As 2 ( T c = 38 K) by means of 57FeNMR and 75AsNQR/NMR. In the superconducting state of LaFeAsO 0.7 ( T c = 28 K), the spin component of the 57Fe-Knight shift decreases to almost zero at low temperatures, which provide firm evidence of the superconducting state formed by spin-singlet Cooper pairing. The nuclear spin–lattice relaxation rates (1/ T 1) in LaFeAsO 0.7 and Ba 0.6K 0.4Fe 2As 2 exhibit a T 3-like dependence without a coherence peak just below T c , indicating that an unconventional superconducting state is commonly realized in these Fe-based pnictide compounds. All these events below T c 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/ T 1 T decreases remarkably upon cooling for both the Fe and As sites of LaFeAsO 0.7. In contrast, it gradually increases upon cooling in Ba 0.6K 0.4Fe 2As 2. 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.
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