Enhanced superconductivity and moderate spin fluctuations suppressed at low energies in heavily electron-doped La1111-based superconductor

To elucidate the origin of re-enhanced high-$T_c$ phase in the heavily\nelectron-doped Fe-pnictides, systematic $^{75}$As NMR studies are performed on\nheavily electron-doped LaFe$Pn$O$_{0.75}$H$_{0.25}$ by controlling the\npnictogen height ($h_{Pn}$) from the Fe plane through the substitution at\n$Pn$(=As) site with Sb or P. The measurements of nuclear spin relaxation rate\n(1/$T_1$) and Knight shift ($K$) reveal that the moderate spin fluctuations at\nhigh temperatures are suppressed toward low temperatures. Such characteristic\nspin fluctuations with gap like feature at low energies are more enlarged in\nhigher $T_c$ compounds with higher $h_{Pn}$, while those are totally suppressed\nin non-superconducting compounds with lower $h_{Pn}$. This implies that the\ncontribution of the finite energy part in the spin fluctuation spectrum is\ncrucial for enhancing $T_c$ in the heavily electron-doped regime. This is in\ncontrast to many cases of typical Fe-based compounds with hole and electron\nFermi surfaces of similar sizes, where the spin fluctuations at low energies\ndevelop significantly at low temperatures. The features in the heavily\nelectron-doped states are discussed in relation with the characteristics of the\nfaint hole Fermi surface derived from $d_{xy}$ orbital that rises when $h_{Pn}$\nis high, together with the enhanced electron correlation effects.\n