Spin versus lattice polaron:  Prediction for electron-doped CaMnO₃

CaMnO₃ is a simple bipartite antiferromagnet (AF) that can be continuously electron doped up to LaMnO₃. Electrons enter the doubly degenerate E_g subshell with spins aligned to the S=3/2 core of Mn⁴⁺(T_2g^3↑). We take the Hubbard and Hund energies to be effectively infinite. Our model Hamiltonian has two E_g orbitals per Mn atom, nearest-neighbor hopping, nearest neighbor exchange coupling of the S=3/2 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 (Mn₇²⁷⁺) is the stable result, with a net spin S=2 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 U_c=0.98|t| (where t≈-0.75 eV) will destroy any simple bipolaron. Therefore we do not expect phase separation to occur. The model predicts a critical doping x≃0.045 where the polaronic insulator becomes unstable relative to a FM metal.