Hidden antiferro-nematic order in BaFe₂As₂ and NaFeAs above <i>T</i>_S

In some Fe-based superconductors such as BaFe₂As₂ and NaFeAs, C₄ symmetry breaking emerges at T*,which is tens of Kelvin higher than the structural transition temperature T_S [1,2]. In this "hidden nematic state" below T*, the orthorhombicity is very tiny. To explain this long-standing mystery, we propose the emergence of antiferro-bond order with antiferro wavevector q=(0,π) at T*, by solving the linearized density-wave equation based on the vertex correction theory [3,4]. This antiferro-bond order originates from the inter-orbital nesting between the d_xy-orbital hole-pocket and the electron-pocket, and naturally explains the pseudogap, band-folding, and tiny nematicity linear in T*-T.
[1] S. Kasahara et al., Nature 486, 382 (2012).
[2] R. Zhow et al., Phys. Rev. B 93, 060502(R) (2016).
[3] S. Onari, Y. Yamakawa, and H. Kontani, Phys. Rev. Lett. 116, 227001 (2016).
[4] S. Onari and H. Kontani, Phys. Rev. B 100, 020507(R) (2019).