Importance of dxy orbital and electron correlation in iron-based superconductors revealed by phase diagram for 1111-system

The structural flexibility at three substitution sites in LaFeAsO enabled investigation of the relation between superconductivity and structural parameters over a wide range of crystal compositions. Substitutions of Nd for La, Sb or P for As, and F or H for O were performed. All these substitutions modify the local structural parameters, while the F/H-substitution also changes band filling. It was found that the superconducting transition temperature Tc is strongly affected by the pnictogen height hPn from the Fe-plane that controls the electron correlation strength and the size of the dxy hole Fermi surface (FS). With increasing hPn, weak coupling BCS superconductivity switches to the strong coupling non-BCS one where electron correlations and the dxy hole FS may be important.