Mechanism for orbital ordering in KCuF3.

The Mott insulating perovskite KCuF3 is considered the archetype of an orbitally ordered system. By using the local-density approximation+dynamical mean-field theory method, we investigate the mechanism for orbital ordering in this material. We show that the purely electronic Kugel-Khomskii super-exchange mechanism alone leads to a remarkably large transition temperature of T(KK) to approximately 350 K. However, orbital order is experimentally believed to persist to at least 800 K. Thus, Jahn-Teller distortions are essential for stabilizing orbital order at such high temperatures.