Orbital order in vanadium spinels

Motivated by recent theoretical and experimental controversy, we present a theoretical study to clarify the orbital symmetry of the ground state of vanadium spinel oxides AV2O4 (A=Zn, Mg, Cd). The study is based on an effective Hamiltonian with spin-orbital superexchange interaction and a local spin-orbit coupling term. We construct a classical phase diagram and prove the complex orbital nature of the ground state. Remarkably, with our analysis we predict correctly also the coherent tetragonal flattening of oxygen octahedra. Finally, through analytical considerations as well as numerical ab initio simulations, we propose how to detect the predicted complex orbital ordering through vanadium K-edge resonant x-ray scattering.