We discuss a new mechanism of orbital ordering, which in charge transfer
insulators is more important than the usual exchange interactions and which can
make the very type of the ground state of a charge transfer insulator, i.e. its
orbital and magnetic ordering, different from that of a Mott-Hubbard insulator.
This purely electronic mechanism allows us to explain why orbitals in
Jahn-Teller materials typically order at higher temperatures than spins, and to
understand the type of orbital ordering in a number of materials, e.g.
K_2CuF_4, without invoking the electron-lattice interaction.Comment: 4 pages, 2 figure