Origin of diverse nematic orders in Fe-based superconductors: 45 degree rotated nematicity in AFe₂As₂ (A=Cs, Rb)

The origin of nematicity and their order parameters in strongly correlated superconductors have been attracting increasing attention. Both the orbital order without magnetization in FeSe and d-wave bond order in cuprate superconductors are satisfactorily explained based on the spin-fluctuation-driven mechanism [1-3]. Recently, new type nematic order has been discovered in heavily hole-doped (n_d = 5.5) compound AFe₂As₂ (A = Cs, Rb). The discovered nematicity has B_2g symmetry, rotated by 45 degrees from the B_1g nematicity in usual Fe-based superconductors. To reveal the origin of B_2g nematicity, we analyze the spontaneous symmetry-breaking in the self-energy [2] based on the multiorbital Hubbard model. We predict that the ``nematic bond order'', given by the symmetry-breaking in the d_xy orbital correlated hopping, is responsible for the B_2g nematic order in AFe₂As₂. It is concluded that not only conventional B_1g orbital order (n_xz ≠ n_xz), but also B_2g bond order with respect to d_xy orbital in AFe₂As₂ are caused by the same mechanism, that is, the charge-spin mode-coupling due to the Aslamazov-Larkin vertex correction. [1] S. Onari et al., PRL 116, 227001 (2016). [2] Y. Yamakawa et al., PRX 6, 021032 (2016). [3] M. Tsuchiizu et al., PRB 97, 165131 (2018).