Multiple Antiferromagnetic Spin Fluctuations and Novel Evolution of Tc in Iron-Based Superconductors LaFe(As1−xPx)(O1−yFy) Revealed by 31P-NMR Studies

We report on P-31-NMR studies of LaFe(As1-xPx)(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 T-c. Systematic P-31-NMR measurements revealed two types of AFMSFs in the temperature evolution, that is, one is the AFMSFs that develop rapidly down to T-c 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 d(xz/yz), whereas the higher-energy ones for a wide y region around low x originate from the three orbitals of d(xy) and d(xz/yz). We remark that the nonmonotonic variation of T-c as a function of x and y in LaFe(As1-xPx)(O1-yFy) is attributed to these multiple AFMSFs originating from degenerated multiple 3d orbitals inherent to Fe-pnictide superconductors.