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