First-principles study of light-element doping effects on iron-based superconductors

First-principles calculations are performed for hydrogen-doped iron-based superconductors, LaFeAsOH_x, which exhibits higher transition temperature than hydrogen-free LaFeAsO. We find that hydrogen atoms favor the positions near FeAs layers and induce FeAs₄ tetrahedrons to regular ones, which are considered to bring about higher transition temperature. However, hydrogen doping more than x ～ 0.25 breaks typical Fermi surface and therefore we estimate the optimal doping as x ～ 0.2.