Non-trivial order parameter in Sr$_{2}$IrO$_{4}$

TL;DR

This paper uses advanced density functional theory calculations to reveal that in Sr$_{2}$IrO$_{4}$, a higher order magnetic multipole acts as the primary order parameter, explaining its magnetic properties and small magnetic moments.

Contribution

It identifies a non-trivial higher order magnetic multipole as the primary order parameter in Sr$_{2}$IrO$_{4}$, expanding understanding beyond the conventional $j_ ext{eff}=1/2$ model.

Findings

Electronic structure matches experimental band gap and magnetic moments.

The primary order parameter is a rank five magnetic multipole.

The model explains the small magnetic moments observed.

Abstract

Electronic structure calculations obtained with an approach with density functional theory with an enhanced local Coulomb interaction, DFT+$U$, are presented for the relativistic magnetic insulator Sr$_{2}$IrO$_{4}$. The results are in accordance with experiments with a band gap and a small moment anti-ferromagnetic ground state. This solution is thereafter thoroughly analyzed in terms of Landau theory where it is found that the ordered magnetic moments only form a secondary order parameter while the primary order parameter is a higher order magnetic multipole of rank five. It is further observed that the electronic structure in the presence of this order parameter is related to the earlier proposed $j_\mathrm{eff}=1/2$ model, but in contrast to that model the present picture can naturally explain the small magnitude of the ordered magnetic moments.