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# Diverse Exotic Orders and Fermiology in Fe-based Superconductors: A Unified Mechanism for $$B_{1g}/B_{2g}$$ Nematicity in FeSe/(Cs,Rb)Fe$$_2$$As$$_2$$ and Smectic Order in BaFe$$_2$$As$$_2$$

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### Abstract

Rich nematic/smectic orders in Fe-based superconductors are important unsolved problem in the strongly correlated electron systems. A unified understanding for these rich orders has been investigated for the last decade. In this article, we explain the $$B_{1g}$$ symmetry nematic transition in FeSe$$_{1-x}$$Te$$_x$$, the $$B_{2g}$$ symmetry nematicity in AFe$$_2$$As$$_2$$ (A=Cs, Rb), and the smectic state in BaFe$$_2$$As$$_2$$ based on the same framework. We investigate the quantum interference mechanism between spin fluctuations, by developing the density wave equation with the self-energy correction. In this mechanism, the observed rich variety of nematic/smectic orders are naturally understood, depending on the characteristic shape and topology of FS of each compound. (i) In FeSe$$_{1-x}$$Te$$_x$$, each FS is very small and the dxy-orbital hole pocket is below the Fermi level. Then, small spin fluctuations on three dxz, dyz, and dxy orbitals cooperatively lead to the $$B_{1g}$$ nematic order without magnetization. The experimental Lifshitz transition below the nematic transition temperature $$(T_S)$$ is naturally reproduced. (ii) In BaFe$$_2$$As$$_2$$, the dxy-orbital hole pocket emerges around M point, and each FS is relatively large. Then, the strong spin fluctuations due to the dxy-orbital nesting give rise to the $$B_{1g}$$ nematic order and the smectic order, and the latter transition temperature ($$T^*$$) exceeds the former one $$T_S$$. (iii) In heavily hole-doped AFe$$_2$$As$$_2$$, the large dxy-orbital hole pocket and the four tiny Dirac pockets appear due to the hole-doping. Then, the $$B_{2g}$$ nematic bond order emerges on the dxy-orbital hole pocket due to the same interference mechanism. The present spin-fluctuation interference mechanism provides a unified explanation why the nematic/smectic orders in Fe-based superconductors are so rich, based on the well established fermiology of Fe-based superconductors.

### Author and article information

###### Journal
08 April 2022
###### Article
2204.03928
0346cc79-010e-4d20-b273-8fae479f31d7