Spectral properties of orbital polarons in Mott insulators

Since orbital symmetry is lower than SU(2), superexchange in Mott insulators with orbital degrees of freedom is typically not Heisenberg-like and hole propagation is highly nontrivial [1]. We investigate cases with Ising-like superexchange, where the hole cannot propagate by its coupling to spin fluctuations. We find that the usually neglected three-site hopping determines hole motion [2]. One realization of Ising superexchange is the Falicov-Kimball model, where only electrons with one orbital flavor can move, and the other ones are localized --- then a hole inserted into the Mott insulator either moves via three-site hopping processes, or remains trapped in a small polaron. In another case of Ising exchange, a class of $t_{2g}$ or $e_g$ orbital systems, renormalized three-site hopping leads to one-dimensional hole propagation, with its direction determined by the orbital flavor of the hole. \\[3pt] [1] J. van den Brink {\it et al.\/}, Phys. Rev. Lett. \textbf{85}, 5174 (2000). \\[0pt] [2] M. Daghofer {\it et al.\/}, Phys. Rev. Lett. \textbf{100}, 066403 (2008).