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Abstract

The spin-orbital superexchange model is derived for the cubic (perovskite) symmetry of LaMnO3, whereas real crystal structure is strongly deformed. We identify and explain three a priori important physical effects arising from tetragonal deformation: (i) the splitting of e(g) orbitals alpha E-z, (ii) the directional renormalization of d-p hybridization t(pd), and (iii) the directional renormalization of charge excitation energies. Using the example of LaMnO3 crystal we evaluate their magnitude. It is found that the major effects of deformation are an enhanced amplitude of x(2)-y(2) orbitals induced in the orbital order by E-z similar or equal to 300 meV and anisotropic t(pd) similar or equal to 2.0 (2.35) eV along the ab (c) cubic axis, in very good agreement with Harrison's law. We show that the improved tetragonal model analyzed within mean field approximation provides a surprisingly consistent picture of the ground state. Excellent agreement with the experimental data is obtained simultaneously for: (i) e(g) orbital mixing angle, (ii) spin exchange constants, and (iii) the temperatures of spin and orbital phase transition.