Charge and orbital ordering in transition metal oxides

Abstract

\nTransition metal oxides with strongly correlated d-electrons show an astonishing variety of\nproperties. This is largely determined by an interplay of different degrees of freedom: charge,\nspin, orbital, lattice ones. Often there appear in them various superstructures. In this talk I will\nconsider different types of superstructures in transition metal oxides, especially charge and\norbital ordering, willdiscuss the main mechanisms leading to their formation and consider\nspecific examples of superstructures in manganites, cobaltites and in some frustrated systems.\nRelative role of purely electronic mechanisms and of the electron-phonon interaction will be\ndiscussed. In particular, I will show that the elastic interactions can naturally lead to different\nsuperstructures, including stripes. Special features of charge and, especially, orbital ordering\nin frustrated systems, where frustrations may be caused both by the geometric structure of the\nlattice and by the special features of orbital interactions, will be considered, and it will be\nshown that the order-from-disorder mechanism can lead to a unique ordered ground state in\nmany of these cases..\n