Hidden antiferronematic order in Fe-based superconductor <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>Ba</mml:mi><mml:msub><mml:mrow><mml:mi>Fe</mml:mi></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mi>As</mml:mi></mml:mrow><mml:mn>2</mml:mn></mml:msub></mml:math> and NaFeAs above <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>T</mml:mi><mml:mi>S</mml:mi></mml:msub></mml:math>
Seiichiro Onari, Hiroshi Kontani
Abstract
In several Fe-based superconductors, slight C 4 symmetry breaking occurs at T * , which is tens of degrees Kelvin higher than the structural transition temperature T S . In this "hidden" nematic state at T S < T < T * , the orthorhombicity is tiny [ = (ab)/(a + b) 0.1%], but clear evidence of a bulk phase transition has been accumulated. To explain this long-standing mystery, we propose the emergence of antiferro-bond (AFB) order with the AF wave vector q = (0, ) at T = T * , by which the characteristic phenomena below T * are satisfactorily explained. This AFB order originates from the interorbital nesting between the d xy -orbital hole pocket and the electron pocket, and this interorbital bond order naturally explains the pseudogap, band folding, and tiny nematicity that is linear in T * -T . The hidden AFB order explains key experiments in both BaFe 2 As 2 and NaFeAs, but it is not expected to occur in FeSe because of the absence of the d xy -orbital hole pocket.