Intertwined spin-orbital coupled orders in the iron-based superconductors

Abstract

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 non-symmorphic 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 electro-nematic effect, by which electric fields acts 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. netically ordered phases are described in terms of two magnetic vector order parameters:Formally, M a is the staggered Fe magnetization with mo-arXiv:1902.10831v2 [cond-mat.supr-con]

Mn-induced Fermi-surface reconstruction in the SmFeAsO parent compound

“…To describe the electronic structure and the magnetic instabilities in Mn-doped SmFeAsO we adopt the two-dimensional low-energy model proposed for the iron-pnictide superconductors 20 – 23 . It not only captures the electronic structure near the relevant high-symmetry points generated by the , , and orbitals of iron atoms, but also allows the correct description of several magnetic instabilities in these systems.…”

Orbital transmutation and the electronic spectrum of FeSe in the nematic phase

“…For theā andb excitations no orbital transmutation occurs because 1 − 3 − φ 1 − φ 3 in the denominator of Eq. (19) does not change sign.…”

“…In the nematic phase, the occupations of the xz and yz orbitals become inequivalent on both hole and electron pockets [18], and the occupations of the xy orbitals at X and at Y also generally become different (the latter gives rise to a hopping anisotropy in real space [19]). This changes both the shape of the pockets and the orbital composition of excitations along them.…”

Observation of an electronic order along [110] direction in FeSe

“…The evolution of these two orders share similar behavior with the Néel and stripe spin fluctuations. Recently, a calculation based on the group theory has predicted a similar charge order in iron-based superconductors 68 . Our work lays a foundation for further experiments and theories to probe the nature of various ordered states in FeSe.…”