Magnetic Excitations in the High Tc Iron Pnictides

TL;DR

This paper analyzes magnetic excitations in high-temperature iron pnictides, calculating neutron scattering intensities based on a two-sublattice antiferromagnetic model, highlighting differences from cuprates in excitation spectra.

Contribution

It provides a theoretical analysis of magnetic excitations in iron pnictides, distinguishing their spectral features from cuprates and exploring superexchange couplings J1 and J2.

Findings

No resonance peak at (pi,pi) as in cuprates.

Saddlepoints near (pi,pi/2) and (0,pi/2) in the spectrum.

Magnetic excitations are not related to Fermi surface nesting.

Abstract

We calculate the expected finite frequency neutron scattering intensity based on the two-sublattice collinear antiferromagnet found by recent neutron scattering experiments as well as by theoretical analysis on the iron oxypnictide LaOFeAs. We consider two types of superexchange couplings between Fe atoms: nearest-neighbor coupling J1 and next-nearest-neighbor coupling J2. We show how to distinguish experimentally between ferromagnetic and antiferromagnetic J1. Whereas magnetic excitations in the cuprates display a so-called resonance peak at (pi,pi) (corresponding to a saddlepoint in the magnetic spectrum) which is at a wavevector that is at least close to nesting Fermi-surface-like structures, no such corresponding excitations exist in the iron pnictides. Rather, we find saddlepoints near (pi,pi/2) and (0,pi/2)(and symmetry related points). Unlike in the cuprates, none of these vectors are close to nesting the Fermi surfaces.