Bond ordering and molecular orbital fluctuations in a cluster Mott insulator

When spin orbit coupling is on equal footing with electronic correlations, the spatially anisotropic character of d-orbitals in transition metal compounds can lead to many and diverse quantum phases of matter. The most celebrated example is the Kitaev model for j=1/2 Kramers doublets on the honeycomb lattice. However, coupling of spin and orbital fluctuations can result in many more possibilities, including multi-polar order, spin liquids, and/or spin orbitals liquids. In this talk, I will discuss recent measurements on the cluster Mott insulating lacunar spinels, where electronic correlations, spin orbit coupling, and orbital degeneracy act in concert to produce several interesting magnetic and orbital phases. I will present a set of neutron and x-ray scattering results on one member of this family: GaTa4Se8, a material that realizes an orbitally active j=3/2 model on the face centered cubic lattice. I will discuss the evidence for or a dynamical spin-orbital state preceding a T*=50 K order-disorder spin-orbital ordering transition. Inelastic neutron scattering measurements reveal the dynamic nature of local distortions through symmetry forbidden optical phonon modes that modulate j = 3/2 molecular orbital occupation as well as intercluster Ta-Se bonds. The resulting staggered intercluster dimerization pattern doubles the unit cell, reflecting a spin-orbital valence bond ground state. I will discuss these results in relation to the family of Lacunar spinels.