High-Tc Nodeless s_\pm-wave Superconductivity in (Y,La)FeAsO_{1-y} with   Tc=50 K: 75As-NMR Study
  We report 75As-NMR study on the Fe-pnictide high-Tc superconductor Y0.95La0.05FeAsO_{1-y} (Y0.95La0.051111) with Tc=50 K that includes no magnetic rare-earth elements. The measurement of the nuclear-spin lattice-relaxation rate 75(1/T1) has revealed that the nodeless bulk superconductivity takes place at Tc=50 K while antiferromagnetic spin fluctuations (AFSFs) develop moderately in the normal state. These features are consistently described by the multiple fully-gapped s_\pm-wave model based on the Fermi-surface (FS) nesting. Incorporating the theory based on band calculations, we propose that the reason that Tc=50 K in Y0.95La0.051111 is larger than Tc=28 K in La1111 is that the FS multiplicity is maximized, and hence the FS nesting condition is better than that in La1111. 
H. Mukuda, S. Furukawa, H. Kinouchi, M. Yashima, Y. Kitaoka, P. M.   Shirage, H. Eisaki, and A. Iyo
cond-mat.supr-con