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 subsh ell with spins aligned to the S = 3/2 core of Mn4+ (T-2g(3 up arrow)). We t ake the Hubbard and Hund energies to be effectively infinite. Our model Ham iltonian has two E-g orbitals per Mn atom, nearest-neighbor hopping, neares t neighbor exchange coupling of the S = 3/2 cores, and electron-phonon coup ling of Mn orbitals to adjacent oxygen atoms. We solve this model for light doping. Electrons are confined in local ferromagnetic (FM) regions (spin p olarons) where there proceeds an interesting competition between spin polar ization (spin polarons), which enlarges the polaron, and lattice polarizati on (Jahn-Teller polarons), which makes it smaller. A symmetric seven-atom f erromagnetic 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 elec tron are predicted. The possibility that two electrons will form a bipolaro n 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 dop ing x similar or equal to 0.045 where the polaronic insulator becomes unsta ble relative to a FM metal.