Importance of $${{d}}_{{{xy}}}$$ 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 parameters over a wide range crystal compositions. Substitutions Nd for La, Sb or P As, F H O were performed. All these substitutions modify local parameters, while F/H-substitution also changes band filling. It was found that superconducting transition temperature $T_{c}$ is strongly affected by pnictogen height $h_{Pn}$ from Fe-plane controls electron correlation strength size $d_{xy}$ hole Fermi surface (FS). With increasing $h_{Pn}$, weak coupling switches to strong one where FS crucially important.