Orbital degeneracy near the itinerant electron limit in CoV2O4

Vanadium spinels, AV$_2$O$_4$ have both magnetic frustration and orbital degeneracy on the V$^{3+}$ sublattice, which lead to strong coupling of the orbital, lattice and spin degrees of freedom. Additionally, upon decreasing the V-V distance, the material is predicted to go from a Mott insulator to a metallic phase. Of all the materials in the AV$_2$O$_4$ series, CoV$_2$O$_4$ is closest to the predicted transition, and it's debated whether it may be fully described by either localized or itinerant electrons pictures. In all other studied vanadium spinels, there is a cubic to tetragonal transition associated with ordering of the degenerate V$^{3+}$ orbitals, consistent with a local orbital picture but, this transition is surprisingly absent from CoV$_2$O$_4$ despite being an insulator with local spins. In this talk we present recent high resolution neutron diffraction and inelastic scattering measurements by our group on powders of CoV$_2$O$_4$. Diffraction data show there is small but clear first order structural transition present which correlates with canting of the V$^{3+}$ spins, while inelastic data are well described by a local spinwave picture. We discuss how these results contribute evidence of a local orbital ordering phase in the region near electron itinerancy.