Role of orbitals in the physics of correlated electron systems

Rich properties of systems with strongly correlated electrons, such as transition metal (TM) oxides, is largely connected with an interplay of different degrees of freedom in them: charge. spin. orbital ones as well as crystal lattice. Specific and often very important role is played by orbital degrees of freedom. They call lead to a formation of different superstructures (all orbital ordering) which are associated with particular types of structural phase transitions - one of very few examples where the microscopic origin of these transitions is really known: they largely determine the character of magnetic exchange and the type of magnetic ordering: they call also strongly influence many other important phenomena such as insulator-metal transitions (IMT). etc. In this comment I will try to shortly summarize the main concepts and discuss some of the well-known manifestations of orbital degrees of freedom, but will mostly concentrate on a more recent development in this field. More traditional material is covered in several review articles [Kugel. K. I. and Khomskii, D. I., Sov. Phys. Usp. 25, 231 (1982); Tokura. Y. and Nagaosa. N., Science 288, 462 (2000); van den Brink. J. et al., in Colossal Magnetoresistive Manganites, Kluwer, 2002]. Although I tried to cover the main new development in this area. the choice of topics of course is influenced by my own interests other people probably would have stressed other parts of this big field.