Origin of $T_{\text{c}}$ Enhancement Induced by Doping Yttrium and Hydrogen into LaFeAsO-Based Superconductors: 57Fe-, 75As-, 139La-, and 1H-NMR Studies

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We report our extensive 57Fe-, 75As-, 139La-, and 1H-NMR studies of La0.8Y0.2FeAsO1-y (La0.8Y0.21111) and LaFeAsO1-yHx (La1111H), where doping yttrium (Y) and hydrogen (H) into optimally doped LaFeAsO1-y [La1111(OPT)] increases $T_{\text{c}}=28$ 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 $s_{\pm}$-wave model that the SC gap and $T_{\text{c}}$ in La0.8Y0.21111 become larger than those in La1111(OPT) without any change in doping level. In La1111H, the SC gap and $T_{\text{c}}$ 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 $T_{\text{c}}$ among these La1111-based compounds through the optimization of the Fermi surface topology.
2010-10-15