Orbital Polarons in the Metal-Insulator Transition of Manganites

TL;DR

This paper investigates the metal-insulator transition in manganites, proposing that orbital polarons, caused by strong orbital polarization near holes, are key to understanding the doping-dependent phase diagram.

Contribution

It introduces the concept of orbital polarons as a mechanism for the transition, emphasizing the role of charge-orbital coupling beyond disorder effects.

Findings

Orbital polarons explain the phase diagram at low and intermediate doping.

Disorder-order crossover has minimal impact on charge mobility.

The model aligns well with experimental phase diagrams.

Abstract

The metal-insulator transition in manganites is strongly influenced by the concentration of holes present in the system. Based upon an orbitally degenerate Mott-Hubbard model we analyze two possible localization scenarios to account for this doping dependence: First, we rule out that the transition is initiated by a disorder-order crossover in the orbital sector, showing that its effect on charge mobility is only small. Second, we introduce the idea of orbital polarons originating from a strong polarization of orbitals in the vicinity of holes. Considering this direct coupling between charge and orbital degree of freedom in addition to lattice effects we are able to explain well the phase diagram of manganites for low and intermediate hole concentrations.