We report our extensive 57Fe-, 75As-, 139La-, and 1H-NMR studies of La_{ 0.8}Y_{ 0.2}FeAsO_{ 1-y} (La_{ 0.8}Y_{ 0.2}1111) and LaFeAsO_{ 1-y}H_{ x}(La1111H), where doping yttrium (Y) and hydrogen (H) into optimally doped LaFeAsO_{ 1-y} (La1111(OPT)) increases T_c=28 K to 34 and 32 K, respectively. In the superconducting (SC) state, the measurements of nuclear-spin lattice-relaxation rate 1/T_1 have revealed in terms of a multiple fully gapped s_\pm-wave model that the SC gap and T_c in La_{ 0.8}Y_{ 0.2}1111 become larger than those in La1111(OPT) without any change in doping level. In La1111H, the SC gap and T_c also increase slightly even though a decrease in carrier density and some disorders are significantly introduced. As a consequence, we suggest that the optimization of both the structural parameters and the carrier doping level to fill up the bands is crucial for increasing T_c among these La1111-based compounds through the optimization of the Fermi surface topology.
Categories: cond-mat.supr-con