Orbital Ordering and Unfrustrated (π,0)(\pi,0) Magnetism from Degenerate Double Exchange in the Iron Pnictides

The magnetic excitations of the iron pnictides are explained within a degenerate double-exchange model. The local-moment spins are coupled by superexchanges $J_1$ and $J_2$ between nearest and next-nearest neighbors, respectively, and interact with the itinerant electrons of the degenerate $d_{xz}$ and $d_{yz}$ orbitals via a ferromagnetic Hund exchange. The latter stabilizes $(\pi,0)$ 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 double-exchange. A dramatic increase of the spin-wave energies at the competing N\'eel 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.Comment: 11 pages, 6 figures. Version accepted in PR

Orbitally and Magnetically Induced Anisotropy in Iron-based Superconductors

Recent experimental developments in the iron pnictides have unambiguously demonstrated the existence of in-plane electronic anisotropy in the absence of the long-range magnetic order. Such anisotropy can arise from orbital ordering, which is described by an energy splitting between the two otherwise degenerate $d_{xz}$ and $d_{yz}$ orbitals. By including this phenomenological orbital order into a five-orbital Hubbard model, we obtain the mean-field solutions where the magnetic order is determined self-consistently. Despite sensitivity of the resulting states to the input parameters, we find that a weak orbital order that places the $d_{yz}$ orbital slightly higher in energy than the $d_{xz}$ orbital, combined with intermediate on-site interactions, produces band dispersions that are compatible with the photoemission results. In this regime, the stripe antiferromagnetic order is further stabilized and the resistivity displays the observed anisotropy. We also calculate the optical conductivity and show that it agrees with the temperature evolution of the anisotropy seen experimentally.Comment: 10 pages, 9 figures. published version. references adde

Orbital order in iron-based superconductors

In this thesis, we propose that a ferro-orbital order, which breaks the degeneracy between the Fe $d_{xz}$ and $d_{yz}$ orbitals, is the effective cause of the structural and the magnetic transitions in the iron-based superconductors. We will discuss this orbital order in the framework of the local-itinerant dichotomy. First, due to the spatial anisotropy of the occupied orbitals that form the local moments, the magnetic exchange constants acquire dramatically different values along the two in-plane directions. Second, the itinerant electrons also undergo a nematic transition, causing the anisotropy observed in various experiments. Finally, combining orbital order in both the local moments and itinerant electrons, we find that the underlying magnetism is unfrustrated, consistent with the inelastic neutron scattering results. The thesis is organized as follows. We will first provide the necessary background knowledge of the iron-based superconductors in Chapter 1. As a preliminary, we discuss in detail three different theoretical approaches, namely the weak-coupling, strong-coupling and local-itinerant models. Chapter 2 serves as the motivation of the thesis. Various experimental results will be presented to demonstrate the existence of the in-plane anisotropy. We will introduce two distinct theoretical scenarios that account for the nematic order. We will argue that orbital order, instead of the spin-nematic order, is the underlying mechanism. Chapters 3, 4, and 5 are the main contents of the thesis. In Chapter 3, we will study the orbital order from the strong-coupling theories, with emphasis on the Kugel-Khomskii model. Chapter 4 deals with the orbital order in the weak-coupling theories and its experimental consequences. Finally in Chapter 5, we propose the degenerate double-exchange model, and show how the orbital order in the itinerant electrons leads to the unfrustrated effective spin model

Electron doping evolution of the neutron spin resonance in NaFe1−x_{1-x}Cox_{x}As

Neutron spin resonance, a collective magnetic excitation coupled to superconductivity, is one of the most prominent features shared by a broad family of unconventional superconductors including copper oxides, iron pnictides, and heavy fermions. In this work, we study the doping evolution of the resonances in NaFe$_{1-x}$Co$_x$As covering the entire superconducting dome. For the underdoped compositions, two resonance modes coexist. As doping increases, the low-energy resonance gradually loses its spectral weight to the high-energy one but remains at the same energy. By contrast, in the overdoped regime we only find one single resonance, which acquires a broader width in both energy and momentum, but retains approximately the same peak position even when $T_c$ drops by nearly a half compared to optimal doping. These results suggest that the energy of the resonance in electron overdoped NaFe$_{1-x}$Co$_x$As is neither simply proportional to $T_c$ nor the superconducting gap, but is controlled by the multi-orbital character of the system and doped impurity scattering effect.Comment: accepted by PR

Effect of Pnictogen Height on Spin Waves in Iron Pnictides

We use inelastic neutron scattering to study spin waves in the antiferromagnetic ordered phase of iron pnictide NaFeAs throughout the Brillouin zone. Comparing with the well-studied AFe2As2 (A=Ca, Sr, Ba) family, spin waves in NaFeAs have considerably lower zone boundary energies and more isotropic effective in-plane magnetic exchange couplings. These results are consistent with calculations from a combined density functional theory and dynamical mean field theory and provide strong evidence that pnictogen height controls the strength of electron-electron correlations and consequently the effective bandwidth of magnetic excitations