Abstract
One of the central questions about the iron pnictides concerns the extent to which their electrons are strongly correlated. Here, we address this issue through the phenomenology of the charge transport and dynamics, the single-electron excitation spectrum, and magnetic ordering and dynamics. We outline the evidence that the parent compounds, while metallic, have electron interactions that are sufficiently strong to produce incipient Mott physics. In other words, in terms of the strength of electron correlations compared with the kinetic energy, the iron pnictides are closer to intermediately coupled systems lying at the boundary between itinerancy and localization, such as V2O3 or Se-doped NiS2, rather than to simple antiferromagnetic metals like Cr. This level of electronic correlations produces a new small parameter for controlled theoretical analysis, namely the fraction of the single-electron spectral weight that lies in the coherent part of the excitation spectrum. Using this expansion parameter, we construct the effective low-energy Hamiltonian and discuss its implications for the magnetic order and magnetic quantum criticality. Finally, this approach sharpens the notion of magnetic frustration for such a metallic system, and brings about a multiband matrix t–J1–J2 model for the carrier-doped iron pnictides.
Highlights
Two decades after the discovery of the cuprate superconductors, we have a new and copper-free family of materials with high temperature superconductivity [1, 2]
A central question is how strongly correlated are the iron pnictides? It is meaningful to address the issue in the normal state, given that the energy scales for the normal state electronic and magnetic excitations appear to be considerably larger than those for superconductivity
It is adequate to focus on the 3d electrons of the Fe atoms, which represent the majority of the electronic states near the Fermi energy
Summary
Two decades after the discovery of the cuprate superconductors, we have a new and copper-free family of materials with high temperature superconductivity [1, 2]. We discuss how this placement of the iron pnictides at the boundary between itinerancy and localization serves as the basis to construct the low-energy effective Hamiltonian. The latter, in turn, allows a controlled analysis of magnetic order and magnetic fluctuations iron pnictides. When w is non-zero, a perturbative treatment in w will lead to an effective low energy theory that couples the local moments to the itinerant coherent electronic excitations. The incoherent excitations are non-perturbative effects that go beyond the Fermi liquid theory They represent a precursor to the lower and upper Hubbard in a Mott insulator, which itself is a phenomenon non-perturbative in interactions. The transition has been extensively studied in terms of the dynamical mean field theory (DMFT) [27]
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