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Hidden antiferronematic order in Fe-based superconductor BaFe2As2 and NaFeAs above TS

S. Onari and H. Kontani, Phys. Rev. Research 2, 042005(R) (2020).

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 TS<T<T, the orthorhombicity is tiny [φ=(ab)/(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 TT. 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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