Spin versus lattice polaron:: Prediction for electron-doped CaMnO3 -: art. no. 064401

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 up arrow)). 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 (Mn-7(27+)) is the stable result, with a net spin S = 2 relative to the undoped AE 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 approximate to -0.75 eV) will destroy any simple bipolaron. Therefore we do not expect phase separation to occur. The model predicts a critical doping x similar or equal to 0.045 where the polaronic insulator becomes unstable relative to a FM metal.