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

Abstract

We examine magnon and orbiton behavior in localized O${}_2$ anti-bonding molecular ${\pi }^{\ast }$ 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 Néel temperature.

5 pages, 2 figures

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• Rubidium superoxide: a p-electron Mott insulator
• Mott physics in $2p$ electron dioxygenyl magnet : O${}_2$$M$F${}_6$ ($M$=Sb, Pt)
• Spin-Orbital Ordering in Alkali Superoxides