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In several Fe-based superconductors, slight $C_4$ symmetry breaking occurs at$T^*$, which is tens of Kelvin higher than the structural transitiontemperature $T_S$. In this "hidden" nematic state at $T_S<T<T^*$, theorthorhombicity is tiny [$\phi=(a-b)/(a+b) \ll 0.1$%], but clear evidences ofbulk phase transition have been accumulated. To explain this long-standingmystery, we propose the emergence of antiferro-bond (AFB) order with theantiferro wavevector ${\bf q}=(0,\pi)$ at $T=T^*$, by which the characteristicphenomena below $T^*$ are satisfactorily explained. This AFB order originatesfrom the inter-orbital nesting between the $d_{xy}$-orbital hole-pocket and theelectron-pocket, and this inter-orbital bond order naturally explains thepseudogap, band-folding, and tiny nematicity that is linear in $T^*-T$. Thehidden AFB order explains key experiments in both BaFe$_2$As$_2$ and NaFeAs,but it is not expected to occur in FeSe because of the absence of the$d_{xy}$-orbital hole-pocket.

Hidden antiferro-nematic order in Fe-based superconductor BaFe$_2$As$_2$ and NaFeAs above $T_S$