Bond ordering and molecular spin-orbital fluctuations in the cluster Mott insulator GaTa4Se8

For materials where spin-orbit coupling is competitive with electronic correlations, the spatially anisotropic spin-orbital wave functions can stabilize degenerate states that lead to many and diverse quantum phases of matter. Here we find evidence for a dynamical spin-orbital state preceding a T∗ = 50 K order-disorder spin-orbital ordering transition in the j=3/2 lacunar spinel GaTa4Se8. Above T∗, GaTa4Se8 has an average cubic crystal structure, but total scattering measurements indicate local noncubic distortions of Ta4 tetrahedral clusters for all measured temperatures 2<T<300 K. Inelastic neutron-scattering measurements reveal the dynamic nature of these local distortions through symmetry forbidden optical phonon modes that modulate j=3/2 molecular orbital occupation as well as intercluster Ta-Se bonds. Spin-orbital ordering at T∗ cannot be attributed to a classic Jahn-Teller mechanism and, based on our findings, we propose that intercluster interactions acting on the scale of T∗ act to break global symmetry. The resulting staggered intercluster dimerization pattern doubles the unit cell, reflecting a spin-orbital valence bond ground state.