Spin excitations in the excitonic spin-density-wave state of the iron pnictides

TL;DR

This paper investigates spin excitations in an excitonic spin-density-wave state relevant to iron pnictides, using RPA to analyze two-band models and compare their magnetic responses.

Contribution

It introduces a detailed RPA-based analysis of spin excitations in excitonic SDW states, contrasting two different models and relating findings to iron pnictides.

Findings

Spin-wave dispersions are derived for both models.

Magnetic responses show similarities despite different SDW states.

Comparison with Hubbard model highlights key differences.

Abstract

Motivated by the iron pnictides, we examine the spin excitations in an itinerant antiferromagnet where a spin-density wave (SDW) originates from an excitonic instability of nested electron-like and hole-like Fermi pockets. Using the random phase approximation (RPA), we derive the Dyson equation for the transverse susceptibility in the excitonic SDW state. The Dyson equation is solved for two different two-band models, describing an antiferromagnetic insulator and metal, respectively. We determine the collective spin-wave dispersions and also consider the single-particle continua. The results for the excitonic models are compared with each other and also contrasted with the well-known SDW state of the Hubbard model. Despite the qualitatively different SDW states in the two excitonic models, their magnetic response shows many similarities. We conclude with a discussion of the relevance of the excitonic SDW scenario to the iron pnictides.