Origin of Tc Enhancement Induced by Doping Yttrium and Hydrogen into LaFeAsO-Based Superconductors: 57Fe-, 75As-, 139La-, and 1H-NMR Studies

We report our extensive 57 Fe-, 75 As-, 139 La-, and 1 H-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 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 ± -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.