The Microscopic Nature of Orbital Disorder in LaMnO$_{3}$

TL;DR

This study uses machine-learning molecular dynamics to reveal the dynamic and mixed nature of orbital disorder in LaMnO₃, challenging static views and linking electronic instability to phonon anharmonicity at high temperatures.

Contribution

It introduces a machine-learning-based simulation approach to accurately model the temperature-dependent orbital disorder in LaMnO₃, providing new insights into its dynamic structural distortions.

Findings

Orbital-disordered phase is a mixture of distortions with and without inversion symmetry.

Distortions are highly dynamic with ~40 fs lifetime at 1000 K.

Electronic instability drives phonon anharmonicity at high temperatures.

Abstract

We present a revised atomistic picture of the order-disorder transition in the archetypal orbital-ordered perovskite material, LaMnO$_{3}$. Our study uses machine-learning-driven molecular-dynamics simulations which describe the temperature evolution of pair distribution functions in close agreement with experiment. We find the orbital-disordered phase in LaMnO$_{3}$ to comprise a mixture of differing structural distortions with and without inversion symmetry, implying a mixture of different orbital arrangements. These distortions are highly dynamic with an estimated lifetime of $\sim 40$ fs at 1,000 K, and their fluctuations converge with the timescales of conventional thermal motion in the high-$T$ phase - indicating that the electronic instability responsible for static Jahn-Teller distortions at low temperature instead drives phonon anharmonicity at high temperatures. Beyond LaMnO$_{3}$, our work opens an avenue for studying a wider range of correlated materials.