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

摘要

We discuss the novel superconducting characteristics and unusual normal-stateproperties 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 ofLaFeAsO$_{0.7}$ ($T_{c}=$ 28 K), the spin component of the $^{57}$Fe-Knightshift decreases to almost zero at low temperatures, which provide firm evidenceof the superconducting state formed by spin-singlet Cooper pairing. The nuclearspin-lattice relaxation rates $(1/T_{1})$ in LaFeAsO$_{0.7}$ andBa$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ exhibit a $T^{3}$-like dependence without acoherence peak just below $T_{c}$, indicating that an unconventionalsuperconducting state is commonly realized in these Fe-based pnictidecompounds. All these events below $T_c$ are consistently argued in terms of anextended s$_{\pm}$-wave pairing with a sign reversal of the order parameteramong Fermi surfaces. In the normal state, $1/T_1T$ decreases remarkably uponcooling for both the Fe and As sites of LaFeAsO$_{0.7}$. In contrast, itgradually increases upon cooling in Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$. Despitethe similarity between the superconducting properties of these compounds, acrucial difference was observed in their normal-state properties depending onwhether electrons or holes are doped into the FeAs layers. These results mayprovide some hint to address a possible mechanism of Fe-based pnictidesuperconductors.