Orbital order, stacking defects, and spin fluctuations in the p-electron molecular solid RbO2

We examine magnon and orbiton behavior in localized O-2 antibonding molecular pi* orbitals using an effective Kugel-Khomskii Hamiltonian derived from a two-band Hubbard model with hopping parameters taken from ab initio density-functional calculations. The considerable difference between intraband and inter-band 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 Neel temperature.