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 ${\mathrm{CaFe}}_2{\mathrm{As}}_2$. 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 ${\mathrm{LaFeAsO}}_{1-x}{\mathrm{H}}_x$ and find that the non-nematic orbital fluctuations, which induce the anion-height instability, are automatically obtained at $x\sim 0.5$, in addition to the conventional nematic orbital fluctuations at $x\sim 0$. 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.

• Received 7 December 2013

DOI:https://doi.org/10.1103/PhysRevLett.112.187001