Hidden antiferronematic order in Fe-based superconductor BaFe_{2}As_{2} and NaFeAs above T_{S}

Abstract

In several Fe-based superconductors, slight C_{4} symmetry breaking occurs at T^{*}, which is tens of degrees Kelvin higher than the structural transition temperature T_{S}. 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 d_{xy}-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 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.