Structural distortions and model Hamiltonian parameters: From LSDA to a tight-binding description ofLaMnO3

Abstract

The physics of manganites is often described within an effective two-band tight-binding TB model for the Mn eg electrons, which apart from the kinetic energy includes also a local “Hund’s rule” coupling to the t2g core spin and a local coupling to the Jahn-Teller JT distortion of the oxygen octahedra. We test the validity of this model by comparing the energy dispersion calculated for the TB model with the full Kohn-Sham band structure calculated within the local spin-density approximation LSDA to density functional theory. We analyze the effect of magnetic order, JT distortions, and “GdFeO3-type” tilt rotations of the oxygen octahedra. We show that the hopping amplitudes are independent of magnetic order and JT distortions and that both effects can be described with a consistent set of model parameters if hopping between both nearest and next-nearest neighbors is taken into account. We determine a full set of model parameters from the density functional theory calculations, and we show that both JT distortions and Hund’s rule coupling are required to obtain an insulating ground state within LSDA. Furthermore, our calculations show that the GdFeO3-type rotations of the oxygen octahedra lead to a substantial reduction of the hopping amplitudes but to no significant deviation from the simple TB model.