A rich variety of nematic/smectic orders in Fe-based superconductors is an important unsolved problem in strongly correlated electron systems. A unified understanding of these orders has been investigated for the last decade. In this study, we explain the B1g symmetry nematic transition in FeSe_1−xTe_x, the B_2g symmetry nematicity in AFe₂As₂ (A = Cs, Rb), and the smectic state in BaFe₂As₂ based on the same framework. We investigate the quantum interference mechanism between spin fluctuations by developing the density wave equation. The observed rich variety of nematic/smectic orders is naturally understood in this mechanism. The nematic/smectic orders depend on the characteristic shape and topology of the Fermi surface (FS) of each compound. 1) In FeSe_1−xTe_x (n_d = 6.0), each FS is very small and the d_xy-orbital hole pocket is below the Fermi level. In this case, the small spin fluctuations on three d_xz, d_yz, and d_xy orbitals cooperatively lead to the B_1g nematic (q = 0) order without magnetization. The experimental Lifshitz transition below the nematic transition temperature (TS) is naturally reproduced. 2) In BaFe₂As₂ (n_d = 6.0), the d_xy-orbital hole pocket emerges around the M point, and each FS is relatively large. The strong spin fluctuations due to the d_xy-orbital nesting give rise to the B_1g nematic (q = 0) order and the smectic [q = (0, π)] order, and the latter transition temperature (T^* ∼ 170K) exceeds the former one (T_S ∼ 140K). 3) In heavily hole-doped AFe₂As₂ (n_d = 5.5), the large d_xy-orbital hole pocket and the four tiny Dirac pockets appear due to the hole-doping. The B_2g nematic bond order emerges on the d_xy-orbital hole pocket because of the same interference mechanism. The present paramagnon interference mechanism provides a unified explanation of why the variety of nematic/smectic orders in Fe-based superconductors is so rich, based on the well-established fermiology of Fe-based superconductors.

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