Cooperative Jahn-Teller Effect and Electron-Phonon Coupling in $La_{1-x}A_xMnO_3$

TL;DR

This paper develops a classical model for lattice distortions in La_{1-x}A_xMnO_3, revealing strong electron-phonon coupling and suggesting polaronic transport mechanisms, with a focus on the structural transition's nature.

Contribution

It introduces a mean field model for lattice distortions in doped manganites, quantifies the energy of local distortions, and explains the order-disorder nature of the structural transition.

Findings

Strong electron-phonon coupling (~0.6 eV per site) supports polaronic transport.

Structural transition is of the order-disorder type.

Rapid transition temperature change due to random fields from added holes.

Abstract

A classical model for the lattice distortions of $\lax$ is derived and, in a mean field approximation, solved. The model is based on previous work by Kanamori and involves localized Mn d-electrons (which induce tetragonal distortions of the oxygen octahedra surrounding the Mn) and localized holes (which induce breathing distortions). Parameters are determined by fitting to the room temperature structure of $LaMnO_3$. The energy gained by formation of a local lattice distortion is found to be large, most likely $\approx 0.6$ eV per site, implying a strong electorn-phonon coupling and supporting polaronic models of transport in the doped materials. The structural transition is shown to be of the order-disorder type; the rapid x-dependence of the transition temperature is argued to occur because added holes produce a "random" field which misaligns the nearby sites.