In several Fe-based superconductors, slight C₄ symmetry breaking occurs at T^∗, which is tens of degrees Kelvin higher than the structural transition temperature TS. In this “hidden” nematic state at T_S<T<T^∗, the orthorhombicity is tiny [φ=(a−b)/(a+b)<<0.1%], but clear evidence of a bulk phase transition has been accumulated. To explain this long-standing mystery, we propose the emergence of antiferro-bond (AFB) order with the AF wave vector q=(0,π) at T=T^∗, by which the characteristic phenomena below T^∗ are satisfactorily explained. This AFB order originates from the interorbital nesting between the dxy-orbital hole pocket and the electron pocket, and this interorbital bond order naturally explains the pseudogap, band folding, and tiny nematicity that is linear in T^∗−T. The hidden AFB order explains key experiments in both BaFe₂As₂ and NaFeAs, but it is not expected to occur in FeSe because of the absence of the dxy-orbital hole pocket.

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