Jahn-Teller magnets
Abstract
A wide class of materials with different crystal and electronic structures from quasi-two-dimensional unconventional superconductors (cuprates, nickelates, ferropnictides/chalcogenides, ruthenate SrRuO4), 3D systems as manganites RMnO3, ferrates (CaSr)FeO3, nickelates RNiO3, to silver oxide AgO are based on Jahn-Teller 3d and 4d ions. These unusual materials called Jahn-Teller (JT) magnets are characterized by an extremely rich variety of phase states from non-magnetic and magnetic insulators to unusual metallic and superconducting states. The unconventional properties of the JT-magnets can be related to the instability of their highly symmetric Jahn-Teller "progenitors" with the ground orbital E-state to charge transfer with anti-Jahn-Teller d-d disproportionation and the formation of a system of effective local composite spin-singlet or spin-triplet, electronic or hole S-type bosons moving in a non-magnetic or magnetic lattice. We consider specific features of the anti-JT-disproportionation reaction, properties of the electron-hole dimers, possible phase states of JT-magnets, effective Hamiltonians for single- and two-band JT-magnets, and present a short overview of physical properties for actual JT-magnets.
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