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${}^{\ast }$=50 K order-disorder spin-orbital ordering transition in the $j\text{\hspace{0.17em}⁣}=\text{\hspace{0.17em}⁣}3\mathrm{/}2$ lacunar spinel GaTa${}_4$Se${}_8$. Above T${}^{\ast }$, 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 $j\text{\hspace{0.17em}⁣}=\text{\hspace{0.17em}⁣}3\mathrm{/}2$ molecular orbital occupation as well as intercluster Ta-Se bonds. Spin-orbital ordering at T${}^{\ast }$ 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${}^{\ast }$ act to break global symmetry. The resulting staggered intercluster dimerization pattern doubles the unit cell, reflecting a spin-orbital valence bond ground state.