Antiferromagnetic Excitonic Insulator State in Sr3Ir2O7

TL;DR

This study provides experimental evidence for an antiferromagnetic excitonic insulator state in Sr3Ir2O7, supporting a theoretical model where magnetic excitons condense, distinct from traditional charge-based excitonic insulators.

Contribution

It demonstrates the realization of an antiferromagnetic excitonic insulator in Sr3Ir2O7 through RIXS measurements and a bilayer Hubbard model analysis, a scenario previously only predicted theoretically.

Findings

Identification of a magnetic mode in Sr3Ir2O7 via RIXS

Magnetic mode merges with electronic continuum at certain points

Material exhibits properties consistent with antiferromagnetic excitonic insulator

Abstract

Excitonic insulators are usually considered to form via the condensation of a soft charge mode of bound electron-hole pairs. This, however, presumes that the soft exciton is of spin-singlet character. Early theoretical considerations have also predicted a very distinct scenario, in which the condensation of magnetic excitons results in an antiferromagnetic excitonic insulator state. Here we report resonant inelastic x-ray scattering (RIXS) measurements of Sr3Ir2O7. By isolating the longitudinal component of the spectra, we identify a magnetic mode that is well-defined at the magnetic and structural Brillouin zone centers, but which merges with the electronic continuum in between these high-symmetry points and which decays upon heating concurrent with a decrease in the material's resistivity. We show that a bilayer Hubbard model, in which electron-hole pairs are bound by exchange interactions, consistently explains all the electronic and magnetic properties of Sr3Ir2O7 indicating that this material is a realization of the long-predicted antiferromagnetic excitonic insulators phase.