Orbital ordering and unfrustrated(π,0)magnetism from degenerate double exchange in the iron pnictides

Abstract

The magnetic excitations of the iron pnictides are explained within a degenerate double-exchange model. The local-moment spins are coupled by superexchanges J1 and J2 between nearest and next-nearest neighbors, respectively, and interact with the itinerant electrons of the degenerate dxz and dyz orbitals via a ferromagnetic Hund exchange. The latter stabilizes (π, 0) stripe antiferromagnetism due to emergent ferro-orbital order and the resulting kinetic energy gain by hopping preferably along the ferromagnetic spin direction. Taking the quantum nature of the spins into account, we calculate the magnetic excitation spectra in the presence of both, super-and doubleexchange. A dramatic increase of the spin-wave energies at the competing Néel ordering wave vector is found, in agreement with recent neutron scattering data. The spectra are fitted to a spin-only model with a strong spatial anisotropy and additional longer ranged couplings along the ferromagnetic chains. Over a realistic parameter range, the effective couplings along the chains are negative corresponding to unfrustrated stripe antiferromagnetism.

Antiferromagnetic order and spin dynamics in iron-based superconductors

“…Upon cooling to lower temperatures, the electrons in more itinerant orbitals can be driven into a true static AF ordered or superconducting state via Hund's coupling to the preformed localized AF state. This is analogous to the orbital-selective Mott transition, where itinerant and localized electrons in different orbitals may separate as independent degrees of freedom (Kou et al, 2009;Leong et al, 2014;Lv et al, 2010;Yang et al, 2010;You et al, 2011).…”

“…In this picture, the local moments interact with each other via J 1 and J 2 Heisenberg exchanges, and they are coupled to the itinerant electrons via Hunds rule coupling. Since itinerant electrons are only associated with d xz and d yz orbitals that break the C 4 rotational symmetry of the underlying x-y lattice plane due to their different occupancies, these orbitals can form a Hamiltonian that drive the in-plane magnetic anisotropy, producing unfrustrated collinear AF order and lifting the degeneracy of the (1, 0) and (0, 1) magnetic states (Chen et al, 2010a;Kou et al, 2009;Krüger et al, 2009;Lee et al, 2009;Lv et al, 2010;Yin et al, 2010). Here the magnetic anisotropy is due to purely electronic ferro-orbital order that spontaneously breaks the rotational symmetry of the underlying lattice and drives the observed magnetic and structural transitions without Fermi surface nesting or magnetic frustration (Krüger et al, 2009;Lee et al, 2009).…”

Electronic origin of structural transition in 122 Fe based superconductors

“…One such scenario is the spin-nematic transition whereby the spins of the two Fe sublattices phase-lock, this breaks C 4 symmetry, without developing any spontaneous magnetization, i.e., without breaking time reversal symmetry [18][19][20][21][22][23][24][25][26][27][28]. Another possible candidate in this scenario is ferro-orbital ordering [28][29][30][31][32][33], where below nematic transition either the occupations or the hopping matrix elements (or both) of the d xz and the d yz orbitals of Fe become inequivalent. Apart from the above two electronic scenarios, other possibilities include a d-wave Pomeranchuk instability [34], in which the Fermi surfaces undergo symmetry-breaking distortions due to interaction effects.…”

Spin dynamics of aJ1−J2antiferromagnet and its implications for iron pnictides

“…In a metallic system close to a Mott transition, quasi-local moments are expected to arise; this picture is further supported by the experimental observation of zone boundary spin wave excitations in the magnetically ordered state at low temperatures 23 . The inelastic neutron scattering experiments demonstrated the need for an anisotropic J 1 − J 2 model with J 1x = J 1y , which may reflect an orbital ordering [24][25][26] while pointing to the relevance of magnetic frustration from the extracted ratio (J 1x + J 1y )/2J 2 ∼ 1 23 . Therefore, results in the tetragonal, paramagnetic phase of the parent compounds are of great importance for understanding the relevance of an isotropic J 1 − J 2 model as well as the strength of the underlying magnetic frustration.…”