The metal-insulator transition in manganites is strongly influenced by the concentration of holes present in the system. Based upon an orbitally degenerate Mott-Hubbard model, we analyze two possible localization scenarios to account for this doping dependence: First, we rule out that the transition is initiated by a disorder-order crossover in the orbital sector, showing that its effect on charge itineracy is only small. Second, we introduce the idea of orbital polarons originating from a strong polarization of orbitals in the vicinity of holes. Considering this direct coupling between charge and orbital degrees of freedom in addition to lattice effects we are able to explain well the phase diagram of manganites for low and intermediate hole concentrations.

• Received 3 June 1999

DOI:https://doi.org/10.1103/PhysRevB.60.13458