Multiple Antiferromagnetic Spin Fluctuations and Novel Evolution of T^sub c^ in Iron-Based Superconductors LaFe(As^sub 1-x^P^sub x^)(O^sub 1-y^F^sub y^) Revealed by ^sup 31^P-NMR Studies

We report on ^sup 31^P-NMR studies of LaFe(As^sub 1-x^P^sub x^)(O1-yFy) over wide compositions for 0 ≤ x ≤ 1 and 0 ≤ y ≤ 0.14, which provide clear evidence that antiferromagnetic spin fluctuations (AFMSFs) are one of the indispensable elements for enhancing Tc. Systematic ^sup 31^P-NMR measurements revealed two types of AFMSFs in the temperature evolution, that is, one is the AFMSFs that develop rapidly down to Tc with low-energy characteristics, and the other, with relatively higher energy than the former, develops gradually upon cooling from high temperature. The low-energy AFMSFs in low y (electron doping) over a wide x (pnictogen height suppression) range are associated with the two orbitals of dxz/yz, whereas the higher-energy ones for a wide y region around low x originate from the three orbitals of dxy and dxz/yz. We remark that the nonmonotonic variation of Tc as a function of x and y in LaFe(As^sub 1-x^P^sub x^)(O^sub 1-y^F^sub y^) is attributed to these multiple AFMSFs originating from degenerated multiple 3d orbitals inherent to Fe-pnictide superconductors.