Orbital order, stacking defects and spin-fluctuations in the $p$-electron molecular solid RbO$_2$

TL;DR

This paper investigates the interplay of orbital order, stacking defects, and spin fluctuations in RbO$_2$, revealing how orbital degeneracy and defects influence magnetic properties and magnon behavior in a $p$-electron molecular solid.

Contribution

It introduces an effective Kugel-Khomskii Hamiltonian based on ab initio calculations to analyze magnon and orbiton interactions in RbO$_2$, highlighting the impact of orbital degeneracy and stacking defects.

Findings

Strong coupling between spin waves and orbital ground state.

Stacking defects lead to orbital domains disrupting magnetic order.

Orbital degeneracy reduces the Néel temperature significantly.

Abstract

We examine magnon and orbiton behavior in localized O$_2$ anti-bonding molecular $\pi^*$ orbitals using an effective Kugel-Khomskii Hamiltonian derived from a two band Hubbard model with hopping parameters taken from {\em ab initio} density functional calculations. The considerable difference between intraband and interband hoppings leads to a strong coupling between the spin wave dispersion and the orbital ground state, providing a straightforward way of experimentally determining the orbital ground state from the measured magnon dispersion. The near degeneracy of different orbital ordered states leads to stacking defects which further modulate spin-fluctuation spectra. Proliferation of orbital domains disrupts long-range magnetic order, thus causing a significant reduction in the observed N\'eel temperature.