The possibility of the D3d distortion of TiO6 octahedra is examined theoretically in order to understand the origin of the G-type antiferromagnetism [AFM(G)] and experimentally observed puzzling properties of LaTiO3. By utilizing an effective spin and pseudospin Hamiltonian with the strong Coulomb repulsion, it is shown that the AFM(G) state is stabilized through the lift of the t2g-orbital degeneracy accompanied by a tiny D3d-distortion. The estimated spin-exchange interaction is in agreement with that obtained by the neutron-scattering experiment. Moreover, the level-splitting energy due to the distortion can be considerably larger than the spin-orbit interaction even when the distortion becomes smaller than the detectable limit under the available experimental resolution. This suggests that the orbital momentum is fully quenched and the relativistic spin-orbit interaction is not effective in this system, in agreement with the result of a recent neutron-scattering experiment.
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