Superconductivity and Antiferromagnetic Spin Fluctuations in LaFe(As1–xPx)(O1–yFy) probed by 31P-NMR

Abstract

31P-NMR study on LaFe(As1–xPx)(O0.86F0.14) unraveled that antiferromagnetic spin fluctuations (AFMSFs) emerge significantly at x=0.4 where the Tc is markedly enhanced, indicating that the AFMSFs are one of the important factors for raising Tc. From extensive comparison over wide compositions for 0 ≤ x ≤1 and 0 ≤ y ≤ 0.14, we revealed that there are two different types in temperature evolution of the AFMSFs: One is enhanced particularly at low energies that evolves only at low temperatures, which mainly derives from the two orbitals of dxz/yz. The other is distributed broadly at finite energies that appears up to high temperatures, which derives from three orbitals of dxy and dxz/yz. The highest Tc(=27 K) state in the present compositions appears at (x, y)=(0.4, 0.1) where two characteristics of AFMSFs merge, suggesting the contribution of the AFMSFs over wide energies to the onset of SC. The nonmonotonic variation of Tc in LaFe(As1–xPx)(O1–yFy) is attributed to the AFMSFs from degenarated multiple-3d-orbitals on iron-pnictide superconductors.