Spin-orbital order in LaMnO$_3$: $d-p$ model study

Abstract: Using the multiband $d-p$ model and unrestricted Hartree-Fock approximation we investigate the electronic structure and spin-orbital order in three-dimensional MnO$_3$ lattice such as realized in LaMnO$_3$. The orbital order is induced and stabilized by particular checkerboard pattern of oxygen distortions arising from the Jahn-Teller effect in the presence of strong Coulomb interactions on $e_g$ orbitals of Mn ions. We show that the spin-orbital order can be modeled using a simple \textit{Ansatz} for local crystal fields alternating between two sublattices on Mn ions, which have non-equivalent neighboring oxygen distortions in $ab$ planes. The simple and computationally very inexpensive $d-p$ model reproduces correctly nontrivial spin-orbital order observed in undoped LaMnO$_3$. Orbital order is very robust and is reduced by $\sim 3$ \% for large self-doping in the metallic regime.