Origin of the Optical Gap in Half-Doped Manganites

We have analyzed the coexisting charge and orbital ordering in half-doped manganites using a model which includes Coulomb and Jahn-Teller orbital polarization interactions. Most surprisingly, the gap in the optical conductivity is reduced by both on-site and intersite Coulomb interactions, but increases and explains the experimental results when the Jahn-Teller terms with orbital polarization are considered. The origin of this behavior is explained within a molecular model which arises in the limit of extreme topological frustration, when single electrons are confined to molecular units consisting of three orbitals.