The underdoped phase diagram of the iron-based superconductors exemplifies the complexity common to many correlated materials. Indeed, multiple ordered states that break different symmetries but display comparable transition temperatures are present. Here we argue that such a complexity can be understood within a simple unifying framework. This framework, built to respect the symmetries of the nonsymmorphic space group of the FeAs/Se layer, consists of primary magnetically ordered states and their vestigial phases that intertwine spin and orbital degrees of freedom. All vestigial phases have Ising-like and zero wave-vector order parameters, described in terms of composite spin order and exotic orbital-order patterns such as spin-orbital loop currents, staggered atomic spin-orbit coupling, and emergent Rashba- and Dresselhaus-type spin-orbit interactions. Moreover, they host unusual phenomena, such as the electronematic effect, by which electric fields act as transverse fields to the nematic order parameter, and the ferro-Néel effect, by which a uniform magnetic field induces Néel order. We discuss the experimental implications of our findings to iron-based superconductors and possible extensions to other correlated compounds with similar space groups.
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The authors are grateful to W. R. Meier for inspiring discussions on the group-theoretical description of iron pnictides. The authors further acknowledge fruitful discussions with C. Batista, E. Berg, P. Canfield, A. Chubukov, S. Kivelson, A. Kreyssig, P. Orth, J. Schmalian, and R. Valentí. This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award No. DE-SC0012336. R.M.F. also acknowledges support from the Research Corporation for Science Advancement via the Cottrell Scholar Award.
© 2019 American Physical Society.