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Interplay between the orbital and magnetic order in monolayer La1−xSr1+xMnO4 manganites

A two-dimensional model which describes eg electrons in a monolayer of an undoped and half doped manganite La1−xSr1+xMnO4 is studied using correlated wavefunctions. The effective Hamiltonian takes into account the kinetic energy, the crystal field splitting between x2−y2 and 3z2−r2 orbitals, and on-site Coulomb interactions for eg electrons. They interact with S = 3/2 spins due to t2g electrons, which are treated as frozen core spins. Furthermore, the model includes antiferromagnetic superexchange interaction between core spins, and the coupling between eg electrons and Jahn–Teller modes. The model reproduces the antiferromagnetic order in the undoped LaSrMnO4 compound, with occupied 3z2−r2 orbitals and elongated MnO6 octahedra along the direction perpendicular to the Mn–O plane. In half doped La0.5Sr1.5MnO4 manganite one finds robust chequerboard-like charge order using realistic parameters. However, the experimentally observed CE phase is more difficult to stabilize, and we discuss the necessary conditions to obtain it within the present model. Altogether, we conclude that the Jahn–Teller effect plays a crucial role in the entire regime of doping.