Temperature-dependent orbital physics in a spin-orbital-lattice-coupled 2 p electron Mott system: The case of K O 2

We have investigated the temperature (T)-dependent evolution of orbital states in a typical spin-orbital-lattice-coupled 2p electron Mott system KO2, based on the electronic structures obtained by the dynamical mean-field theory as well as the density functional theory. We have shown that KO2 exhibits the orbital fluctuation feature at high T due to degenerate πg* orbitals. Upon cooling, the orbital fluctuation is suppressed by the Jahn-Teller-type crystal field that becomes stronger with the lowering of structural symmetry, and then the ferro-orbital (FO) ordering emerges at low T. This FO ordering feature distinguishes KO2 from RbO2 and CsO2 in that the latter two seem to have antiferro-orbital orderings at low T, indicating that the underlying physics is different between them. We propose that the suppression of the orbital fluctuation in KO2 can be observed by thermal-conductivity measurement, as observed in spin-orbital-lattice-coupled 3d transition-metal oxides such as LaVO3.