AUTHOR=Onari Seiichiro , Kontani Hiroshi 

TITLE=Diverse Exotic Orders and Fermiology in Fe-Based Superconductors: A Unified Mechanism for B1g/B2g Nematicity in FeSe/(Cs,Rb)Fe2As2 and Smectic Order in BaFe2As2

JOURNAL=Frontiers in Physics

VOLUME=10

YEAR=2022

URL=https://www.frontiersin.org/journals/physics/articles/10.3389/fphy.2022.915619

DOI=10.3389/fphy.2022.915619

ISSN=2296-424X

ABSTRACT=<p>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 <italic>B</italic><sub>1<italic>g</italic></sub> symmetry nematic transition in FeSe<sub>1−<italic>x</italic></sub>Te<sub><italic>x</italic></sub>, the <italic>B</italic><sub>2<italic>g</italic></sub> symmetry nematicity in AFe<sub>2</sub>As<sub>2</sub> (A = Cs, Rb), and the smectic state in BaFe<sub>2</sub>As<sub>2</sub> 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<sub>1−<italic>x</italic></sub>Te<sub><italic>x</italic></sub> (<italic>n</italic><sub><italic>d</italic></sub> = 6.0), each FS is very small and the <italic>d</italic><sub><italic>xy</italic></sub>-orbital hole pocket is below the Fermi level. In this case, the small spin fluctuations on three <italic>d</italic><sub><italic>xz</italic></sub>, <italic>d</italic><sub><italic>yz</italic></sub>, and <italic>d</italic><sub><italic>xy</italic></sub> orbitals cooperatively lead to the <italic>B</italic><sub>1<italic>g</italic></sub> nematic (<bold><italic>q</italic></bold> = <bold>0</bold>) order without magnetization. The experimental Lifshitz transition below the nematic transition temperature (<italic>T</italic><sub><italic>S</italic></sub>) is naturally reproduced. 2) In BaFe<sub>2</sub>As<sub>2</sub> (<italic>n</italic><sub><italic>d</italic></sub> = 6.0), the <italic>d</italic><sub><italic>xy</italic></sub>-orbital hole pocket emerges around the M point, and each FS is relatively large. The strong spin fluctuations due to the <italic>d</italic><sub><italic>xy</italic></sub>-orbital nesting give rise to the <italic>B</italic><sub>1<italic>g</italic></sub> nematic (<bold><italic>q</italic></bold> = <bold>0</bold>) order and the smectic [<bold><italic>q</italic></bold> = (0, <italic>π</italic>)] order, and the latter transition temperature (<italic>T</italic>* ∼ 170K) exceeds the former one (<italic>T</italic><sub><italic>S</italic></sub> ∼ 140K). 3) In heavily hole-doped AFe<sub>2</sub>As<sub>2</sub> (<italic>n</italic><sub><italic>d</italic></sub> = 5.5), the large <italic>d</italic><sub><italic>xy</italic></sub>-orbital hole pocket and the four tiny Dirac pockets appear due to the hole-doping. The <italic>B</italic><sub>2<italic>g</italic></sub> nematic bond order emerges on the <italic>d</italic><sub><italic>xy</italic></sub>-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.</p>