Orbital polarons and ferromagnetic insulators in manganites

We argue that in lightly hole doped perovskite-type Mn oxides the holes Mn⁴⁺ sites) are surrounded by nearest neighbor Mn³⁺ sites in which the occupied 3d orbitals have their lobes directed towards the central hole (Mn⁴⁺) site and with spins coupled ferromagnetically to the central spin. This composite object, which can be viewed as a combined orbital-spin-lattice polaron, is accompanied by the breathing type (Mn³⁺) and Jahn-Teller type (Mn³⁺) local lattice distortions. We present calculations which indicate that for certain doping levels these orbital polarons may crystallize into a charge and orbitally ordered ferromagnetic insulating state.