Rubidium superoxide: a p-electron Mott insulator

Rubidium superoxide, RbO_2, is a rare example of a solid with partially-filled electronic p states, which allows to study the interplay of spin and orbital order and other effects of strong electronic correlations in a material that is quite different from the conventional d or f electron systems. Here we show, using a combination of density functional theory (DFT) and dynamical mean-field theory, that at room temperature RbO_2 is indeed a paramagnetic Mott insulator. We construct the metal-insulator phase diagram as a function of temperature and Hubbard interaction parameters U and J. Due to the strong particle-hole asymmetry of the RbO_2 band-structure, we find strong differences compared to a simple semi-elliptical density of states, which is often used to study the multiband Hubbard model. In agreement with our previous DFT study, we also find indications for complex spin and orbital order at low temperatures.