High-Tc Superconductivity near the Anion Height Instability in Fe-Based Superconductors: Analysis of LaFeAsO1-xHx

The isostructural transition in the tetragonal phase with a sizable change in the anion height, is realized in heavily H-doped LaFeAsO and (La, P) codoped CaFe2As2. In these compounds, the superconductivity with higher T-c (40-50 K) is realized near the isostructural transition. To find the origin of the anion-height instability and the role in realizing the higher-T-c state, we develop the orbital-spin fluctuation theory by including the vertex correction. We analyze LaFeAsO1-xHx and find that the non-nematic orbital fluctuations, which induce the anion- height instability, are automatically obtained at x similar to 0.5, in addition to the conventional nematic orbital fluctuations at x similar to 0. The non- nematic orbital order triggers the isostructural transition, and its fluctuation would be a key ingredient to realize higher-T-c superconductivity of order 50 K.