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 Tcmax(∼33.1K), 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.