Spin-orbital physics for p orbitals in alkali RO_2 hyperoxides --- generalization of the Goodenough-Kanamori rules

TL;DR

This paper develops a spin-orbital model for alkali hyperoxides, revealing how lattice-induced orbital order and frustration influence magnetic properties, leading to generalized Goodenough-Kanamori rules and specific antiferromagnetic order.

Contribution

It introduces a realistic finite Hund's exchange spin-orbital model for alkali hyperoxides and explains their magnetic order through generalized rules accounting for lattice effects.

Findings

Orbital order induced by lattice dominates superexchange preferences.

Geometric frustration destabilizes spin and orbital order.

Predicted spin-wave dispersion lacks soft modes, guiding future experiments.

Abstract

We derive a realistic spin-orbital model at finite Hund's exchange for alkali hyperoxides. We find that, due to the geometric frustration of the oxygen lattice, spin and orbital waves destabilize both spin and p-orbital order in almost all potential ground states. We show that the orbital order induced by the lattice overrules the one favoured by superexchange and that this, together with the large interorbital hopping, leads to generalized Goodenough-Kanamori rules. They (i) lift the geometric frustration of the lattice, and (ii) explain the observed layered C-type antiferromagnetic order in alkali hyperoxides. This is confirmed by a spin-wave dispersion with no soft-mode behavior presented here as a prediction for future experiments.