Spin versus Lattice Polaron: Prediction for Electron-Doped CaMnO3

Abstract

CaMnO3 is a simple bi-partite antiferromagnet(AF) which 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 Mn^4+ (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 7-atom ferromagnetic cluster (Mn_7^27+) is the stable result, with net spin S=2 relative to the undoped AF. The distorted oxygen positions around the electron are predicted. The model also predicts a critical doping x_c=0.045 where the polaronic insulator becomes unstable relative to a FM metal.

9 pages with 7 embedded postscript figures and 2 tables

References in corpus (2)

• Charge-orbital ordering and phase-separation in the two-orbital model for manganites: Roles of Jahn-Teller phononic and Coulombic interactions
• Franck-Condon-Broadened Angle-Resolved Photoemission Spectra Predicted in LaMnO3

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• Non-adiabatic small polaron hopping in the n=3 Ruddlesden-Popper compound Ca4Mn3O10
• High temperature susceptibility in electron doped Ca1-xYxMnO3: Double Exchange vs Superexchange
• Giant electro-thermal conductivity and spin-phonon coupling in an antiferromagnetic oxide
• Revealing polarons with high pressure on low electron-doped manganites
• Magnetic Polarons in the 1D FM Kondo Model
• Doping dependence of the exchange energies in bilayer manganites: Role of orbital degrees of freedom
• Spin-polaron formation and magnetic state diagram in La doped $CaMn{O}_3$
• Polaronic Signatures in Mid-Infrared Spectra: Prediction for LaMnO3 and CaMnO3