Jahn-Teller Magnets

A wide class of materials with different crystal and electronic structures including quasi-2D unconventional superconductors, such as cuprates, nickelates, ferropnictides/chalcogenides, ruthenate Sr2RuO4, and 3D systems, such as manganites RMnO3, ferrates (CaSr)FeO3, nickelates RNiO3, 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, spanning from non-magnetic and magnetic insulators to unusual metallic and superconducting states. The unconventional properties of JT magnets can be attributed to the instability of their highly symmetric Jahn-Teller "progenitors" with the ground orbital E-state with repect to charge transfer, 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 and effective Hamiltonians for single- and two-band JT magnets, concluding with a short overview of physical properties for actual JT magnets.