Spin versus lattice polaron: Prediction for electron-doped (formula presented)

(formula presented) is a simple bipartite antiferromagnet (AF) that can be continuously electron doped up to (formula presented) Electrons enter the doubly degenerate (formula presented) subshell with spins aligned to the (formula presented) core of (formula presented) We take the Hubbard and Hund energies to be effectively infinite. Our model Hamiltonian has two (formula presented) orbitals per Mn atom, nearest-neighbor hopping, nearest neighbor exchange coupling of the (formula presented) cores, and electron-phonon coupling of Mn orbitals to adjacent oxygen atoms. We solve this model for light doping. Electrons are confined in local ferromagnetic (FM) regions (spin polarons) where there proceeds an interesting competition between spin polarization (spin polarons), which enlarges the polaron, and lattice polarization (Jahn-Teller polarons), which makes it smaller. A symmetric seven-atom ferromagnetic cluster (formula presented) is the stable result, with a net spin (formula presented) relative to the undoped AF. The distorted oxygen positions around the electron are predicted. The possibility that two electrons will form a bipolaron has been considered. A fairly modest Coulomb repulsion (formula presented) (where (formula presented) eV) will destroy any simple bipolaron. Therefore we do not expect phase separation to occur. The model predicts a critical doping (formula presented) where the polaronic insulator becomes unstable relative to a FM metal.