Cooperative Jahn-Teller Effect: Fundamentals, Applications, Prospects

Keeping in mind the pedagogical goal of the presentation the first third of the review is devoted to the basic definitions and to the description of the cooperative Jahn-Teller effect. Among different approaches to the intersite electron correlation in crystals the preference is with the most fundamental and systematic Hamiltonian shift transformation method. Order parameter equations and their connection to the crystal elastic properties and to the orbital ordering are considered. An especial attention is paid to the dynamics of Jahn-Teller crystals based on the coupled electronic, vibrational, and magnetic excitations which are of big interest nowadays in orbital physics. In the next part of the review some of the most important applications of cooperative Jahn-Teller effect and related new phenomena in materials are under discussion. Ferro- and antiferroelectricity are considered using a dielectric zircon structure antiferroelectric as an example. The relative roles of different phonon modes are analyzed. Giant dynamic and static magnetostriction and electrostriction as typical properties of Jahn-Teller crystals are discussed. Qualitative analysis of these properties and the results of numerical calculations and experimental data for different materials are presented. The new phenomena of metaelasticity and meta-magnetoelasticity and their experimental observation and theoretical description are demonstrated. These phenomena are discussed in connection with the giant magnetoelectricity found in Jahn-Teller crystals and the relation of these phenomena to the magnetoelectricity of modern multiferroics. The review is ending with some suggestions on possible development in the field.