75As NMR studies on iron-based La1111 superconductors in lightly and heavily electron doped states

We report 75As NMR experiments in heavily electron-doped LaFePnO0.75H0.25 (Pn=As1−x Sb x and As1−x′ P x′ ) compounds with the maximum T c max ( ∼ 33.1 K ) , and compare with the previous results in lightly electron-doped LaFePn(O,F) compounds. The Tc of this series can be sensitively controlled by the pnictogen height (hPn ) through the substitution at Pn site, and the electron doping level through the substitution at the O site with H or F. In heavily electron-doped LaFePnO(La1111) compounds, we found that spin fluctuations at low-energies were moderately suppressed upon cooling in the Sb-substituted high Tc compounds with high hPn (x ≥ 0), although they are completely suppressed in P-substituted non-superconducting compounds (x′ ≥ 0.2) with lower hPn . This feature is largely different from that in the lightly doped La1111 compounds with the well-nested Fermi surfaces, where the spin fluctuations are critically enhanced upon cooling. Here, we present the characteristics of spin fluctuations over wide doping region of La1111-based compounds, and discuss the relationship with the superconductivity.