We discuss the universality of the complicated ground states and the high-T_c superconducting states in various Fe-based compounds from a microscopic point through NMR techniques.

First we show systematic NMR study on [Sr₄Sc₂O₆]Fe₂(As_1-xP_x)₂(≡SrSc42622)[1], which has local lattice parameters of iron-pnictogen (FePn) layer similar to the series of LaFe(As_1-xP_x)O that exhibits the reemergent antiferromagnetic (AFM) order at x~0.6 (≡AFM2) separated from the parent AFM order at x=0 (≡AFM1)[2,3]. Although the static AFM2 order is not observed in SrSc42622, the dynamical low-energy AFM spin fluctuations are enhanced at around x~0.8, indicating that the onset of the static AFM2 is quite sensitive to the local lattice parameters of FePn layer[4]. The result points out that such reenhancement of low energy AFM spin fluctuations derived from AFM2 order phase play a significant role for enhancing the Tc (=17K) at x=0.8~1 of SrSc42622[5].

In order to elucidate the further universality and diversity, we also present the recent studies on Fe-based superconductors in heavily electron-doped regime, that occurs without the nesting of Fermi surfaces and apparent spin fluctuations at low energies. Here we discuss the universality and diversity on the Fe-arsenide LaFe₂As₂(Tc=12K)[6,7], LaFe(As_1-x(Sb/P)_x)(O_1-yH_y)(Tc~30K)[2], and intercalated FeSe high-Tc superconductors(Tc=44K)[8] in heavily electron-doped regimes.

[1] F. Sakano et al., Phys. Rev. B 100, 094509 (2019).
[2] S. Miyasaka et al., Phys. Rev. B 95, 214515 (2017).
[3] H. Kinouchi et al., Phys. Rev. B 87, 121101(R) (2013).
[4] H. Usui et al., Sci. Rep. 5, 11399 (2015).
[5] H. Ogino et al., Supercond. Sci. Tech. 22 075008 (2009).
[6] A. Iyo et al., J. Phys. Chem. Lett. 10, 1018 (2019).
[7] T. Kouchi et al., J. Phys. Soc. Jpn. 88, 113702 (2019).
[8] S. Nishioka, T. Kouchi et al. unpublished.

Keywords: Fe-pnictides, Antiferromagnetism, Spin fluctuations, high temperature superconductor