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 As-75 NMR studies are performed on heavily electron-doped LaFePnO(0.75)H(0.25) by controlling the pnictogen height (h(Pn)) from the Fe plane through the substitution at Pn(= As) site with Sb or P. The measurements of nuclear spin relaxation rate (1/T-1) and Knight shift (K) 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.