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

To elucidate the origin of the reenhanced high-$T_c$ phase in the heavily electron-doped Fe pnictides, systematic ${}^{75}\mathrm{As}$ NMR studies are performed on heavily electron-doped $\mathrm{LaFe}Pn{\mathrm{O}}_{0.75}{\mathrm{H}}_{0.25}$ by controlling the pnictogen height $\left(h_{Pn}\right)$ from the Fe plane through the substitution at $Pn(=\text{As})$ site with Sb or P. The measurements of nuclear spin relaxation rate $(1/T_1)$ and Knight shift $\left(K\right)$ reveal that the moderate spin fluctuations at high temperatures are suppressed toward low temperatures. Such characteristic spin fluctuations with gaplike feature at low energies are more enlarged in higher $T_c$ compounds with higher $h_{Pn}$, while those are totally suppressed in nonsuperconducting compounds with lower $h_{Pn}$. This implies that the contribution of the finite-energy part in the spin-fluctuation spectrum is crucial for enhancing $T_c$ in the heavily electron-doped regime. This is in contrast to many cases of typical Fe-based compounds with hole and electron Fermi surfaces of similar sizes, where the spin fluctuations at low energies develop significantly at low temperatures. The features in the heavily electron-doped states are discussed in relation with the characteristics of the faint hole Fermi surface derived from the $d_{xy}$ orbital that rises when $h_{Pn}$ is high, together with the enhanced electron correlation effects.