Structural distortions and model Hamiltonian parameters: from LSDA to a tight-binding description of LaMnO_3

TL;DR

This paper evaluates a two-band tight-binding model for LaMnO_3 by comparing it with detailed LSDA calculations, analyzing the effects of distortions and magnetic order, and determining model parameters from first-principles data.

Contribution

It provides a comprehensive validation and parameterization of a TB model for LaMnO_3 based on LSDA calculations, including effects of distortions and magnetic configurations.

Findings

Hopping amplitudes are independent of magnetic order and JT distortions.

Both JT distortions and Hund's coupling are essential for an insulating ground state.

GdFeO_3-type rotations reduce hopping amplitudes without invalidating the TB model.

Abstract

The physics of manganites is often described within an effective two-band tight-binding (TB) model for the Mn e_g electrons, which apart from the kinetic energy includes also a local "Hund's rule" coupling to the t_{2g} 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 "GdFeO_3-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 "GdFeO_3-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.