Bond Ordering and Molecular Spin-Orbital Fluctuations in the Cluster Mott Insulator GaTa$_4$Se$_8$

For materials where spin-orbit coupling is competitive with electronic correlations, the spatially anisotropic spin-orbital wavefunctions 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 GaTa$_4$Se$_8$. Above T$^*$, GaTa$_4$Se$_8$ has an average cubic crystal structure, but total scattering measurements indicate local non-cubic distortions of Ta$_4$ 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.