Novel Superconducting Characteristics and Unusual Normal-State Properties in Iron-based Pnictide Superconductors: 57Fe-NMR and 75As-NQR/NMR studies in REFeAsO_1-y (RE=La,Pr,Nd) and Ba0.6K0.4Fe2As2

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${}_{0.6}$K${}_{0.4}$Fe${}_2$As${}_2$($T_c=$ 38 K) by means of ${}^{57}$Fe-NMR and ${}^{75}$As-NQR/NMR. In the superconducting state of LaFeAsO${}_{0.7}$ ($T_c=$ 28 K), the spin component of the ${}^{57}$Fe-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${}_{0.6}$K${}_{0.4}$Fe${}_2$As${}_2$ 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${}_{\pm }$-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${}_{0.6}$K${}_{0.4}$Fe${}_2$As${}_2$. 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.

8 pages, 12 figures, Submitted to Physica C Special Edition on Superconducting Pnictides