Orbital order, spin waves, and doping effects in the $(\pi,0)$ SDW state --- comparative study of the three band models for iron pnictides

TL;DR

This study compares two three-band models for iron pnictides to understand how microscopic parameters influence spin waves, orbital order, and magnetic moments in the $(c,0)$ SDW state, revealing the importance of filling level and doping.

Contribution

It provides a comparative analysis of two models at different fillings, showing how doping and filling affect SDW stability, spin excitations, and orbital order in iron pnictides.

Findings

Model at $n=4$ shows SDW instability and weak magnetic moments.

Model at $n=3$ produces a stable, gapped SDW state with magnetic moments matching experiments.

Spin wave energies and orbital order peak near half filling.

Abstract

Aimed at identifying the role of microscopic Hamiltonian parameters on spin wave excitations, orbital order, and magnetic moments in the $(\pi,0)$ SDW state, two different three band models for iron pnictides are compared --- one at two-third filling ($n=4$) and another at half filling $(n=3)$ --- both of which yield qualitatively correct Fermi surface structure in the paramagnetic state. Spin wave analysis of the model at $n=4$ shows instability of the SDW state, which is attributed to weakly developed magnetic moments and weak magnetic couplings due to overfilling, as inferred from the observed stabilization of the SDW state and enhancement of magnetic excitation energies upon hole doping. In contrast, the model at $n=3$ (half filling) yields a gapped SDW state with well developed magnetic moments and the calculated spin wave excitations are in excellent agreement with INS experiments. The sign of the orbital order ($n_{xz} - n_{yz} \approx +0.2$) is also in agreement with experiments. Both the zone boundary spin wave energies in the F and AF directions as well as the orbital order are shown to peak near half filling, highlighting the correlation between orbital order and SDW state stabilization.