Molecular spin-orbital fluctuations in cluster J_eff-3/2 Mott insulator

Spin, orbit, and lattice dynamics are coupled in strong spin-orbit coupling systems. In such materials, a delicate balance of energy scales can achieve the novel quantum phase in condensed matter. Therefore, a systematic study on the role of spin-orbital coupling and exchange correlation is crucial for understanding the novel phases in materials. The lacunar spinels GaM₄X₈, M=(V, Nb, Ta), X=(S, Se) are a class of cluster-based Mott insulators that host various novel quantum phases. For M=V, a local Jahn-Teller effect quenches the orbital moment but is suppressed by spin-orbital coupling in M=(Nb, Ta) members. In addition, our inelastic neutron scattering uncovers the preceding phonon anomaly of the molecular J_eff-3/2 singlet ground state. In this presentation, I will discuss a systematic study of the entire family of compounds using neutron and x-ray total scattering measurements and inelastic neutron scattering on lattice dynamics. This work provides insight into the interplay between spin, orbit, and lattice dynamics in the molecular J_eff-3/2 Mott insulators.