Charge and Orbital Order in Transition Metal Oxides
Andrzej M. Oleś
Marian Smoluchowski Institute of Physics, Jagellonian University, Reymonta 4, PL-30-059 Kraków, Poland
Max Planck Institut für Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany
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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 oxides 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 (disentangled) 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.
DOI: 10.12693/APhysPolA.118.212
PACS numbers: 75.10.Jm, 75.30.Et, 03.67.Mn, 61.50.Ks