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.