NMR investigations toward understanding the variety of ground states in iron-based superconductors

We report NMR experiments on heavily electron-doped Fe-based superconductor in comparison with the results on the parent Fe-based compounds. The typical parent Fe-based compound LaFe(As1-x'Px')O exhibits the re-emergent antiferromagnetic (AFM) order at x' similar to 0.6 (AFM2) separated from the parent AFM order at x' = 0 (AFM1). Systematic P-31-NMR study on [Sr4Sc2O6]Fe-2(As1-xPx)(2) (SrSc42622), which has local lattice parameters of iron-pnictogen (FePn) layer similar to the series of LaFe(As1-x'Px')O, also revealed that the presence of AFM1 order is universal for most of parent Fe-based compounds. In contrast, the static AFM2 order was absent in this series, however, the dynamical low-energy AFM spin fluctuations are enhanced at around x similar to 0.8, indicating that the onset of the static AFM2 is quite sensitive to the local lattice parameters of FePn layer. In order to elucidate the further universality and diversity, we have carried out Se-77-NMR measurement on Li-x(NH3)(y)Fe2-delta Se2 (T-c = 44 K) in heavily electron-doped regime. Although the spin fluctuations at low energies does not significantly develops upon cooling, the moderate spin fluctuations were extracted at high temperatures from comparison of the temperature (T) dependences of Knight shift and nuclear relaxation rate (1/T1T). We discuss the universality and diversity of the relationship between the T-c and the characteristics of the spin fluctuations in the Fe-based compounds from a microscopic point of the NMR measurements.