Hidden antiferro-nematic order in Fe-based superconductor BaFe2As2 and NaFeAs above T_S

In several Fe-based superconductors, slight C₄ symmetry breaking occurs at T^*, which is tens of 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 evidences of bulk phase transition have been accumulated. To explain this long-standing mystery, we propose the emergence of antiferro-bond (AFB) order with the antiferro wavevector q=(0,π) at T=T^*, by which the characteristic phenomena below T^* are satisfactorily explained [1]. This AFB order originates from the inter-orbital nesting between the dxy-orbital hole-pocket and the electron-pocket, and this inter-orbital 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. [1]