Charge and orbital order in transition metal oxides

TL;DR

This paper reviews charge and orbital ordering phenomena in transition metal oxides, emphasizing the role of Coulomb interactions and competing instabilities in forming various ordered phases, including magnetic and stripe orders.

Contribution

It provides a comprehensive overview of the mechanisms behind charge and orbital order, highlighting the importance of Coulomb interactions and competition among different phases in transition metal oxides.

Findings

Charge and orbital orders arise from strong Coulomb interactions.

Stripe order in doped cuprates, nickelates, and manganites explained by competing instabilities.

Orbital order stabilizes specific magnetic phases in undoped insulators.

Abstract

A short introduction to the complex phenomena encountered in transition metal oxides with either charge or orbital or joint charge-and-orbital order, usually accompanied by magnetic order, is presented. It is argued that all the types of above ordered phases in these systems follow from strong Coulomb interactions as a result of certain compromise between competing instabilities towards various types of magnetic order and optimize the gain of kinetic energy in doped systems. This competition provides a natural explanation of the stripe order observed in doped cuprates, nickelates and manganites. In the undoped correlated insulators with orbital degrees of freedom the orbital order stabilizes particular types of anisotropic magnetic phases, and we contrast the case of decoupled spin and orbital degrees of freedom in the manganites with entangled spin-orbital states which decide about certain rather exotic phenomena observed in the perovskite vanadates at finite temperature. Examples of successful concepts in the theoretical approaches to these complex systems are given and some open problems of current interest are indicated.