Orbital degeneracy and Mott transition in Mo pyrochlore oxides

TL;DR

This paper presents a theoretical study of phase competition in Mo pyrochlore oxides using a two-band model, revealing complex spin and orbital orders and metal-insulator transitions driven by Coulomb and superexchange interactions.

Contribution

It introduces a comprehensive two-band double-exchange model incorporating orbital degeneracy, Coulomb repulsion, and superexchange, providing new insights into phase behavior in Mo pyrochlore oxides.

Findings

Identification of four dominant phases with distinct spin and orbital orders.

Discovery of a ferro-orbital ordering along the <111> axis in the insulating phase.

Phase diagram showing transitions driven by Coulomb and superexchange interactions.

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 $e_g'$ orbitals under trigonal field splitting, the interorbital Coulomb repulsion, the Hund's-rule coupling between itinerant $e_g'$ electrons and localized $a_{1g}$ spins, and the superexchange antiferromagnetic interaction between the $a_{1g}$ 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 $e_g'$ orbitals along the local $<111>$ axis, irrespective of the spin ordering.