Orbital degeneracy and Mott transition in Mo pyrochlore oxides
Abstract
We present our theoretical results on an effective two-band double-exchange model on a pyrochlore lattice for understanding intricate phase competition in Mo pyrochlore oxides. The model includes the twofold degeneracy of eg' orbitals under trigonal field splitting, the interorbital Coulomb repulsion, the Hund's-rule coupling between itinerant eg' electrons and localized a1g spins, and the superexchange antiferromagnetic interaction between the a1g spins. By Monte Carlo simulation with treating the Coulomb repulsion at an unrestricted-type mean-field level, we obtain the low-temperature phase diagram as functions of the Coulomb repulsion and the superexchange interaction. The results include four dominant phases with characteristic spin and orbital orders and the metal-insulator transitions among them. The insulating region is characterized by a `ferro'-type orbital ordering of the eg' orbitals along the local <111> axis, irrespective of the spin ordering.