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

Abstract

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 $j=3/2$ 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 $j=3/2$ 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.

arXiv comment13 pages, 8 figures

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Cited by in corpus (4)

• Quasimolecular $J_{\mathrm{tet}}$=3/2 moments in the cluster Mott insulator GaTa${}_4$Se${}_8$
• Cluster rearrangement by chiral charge order in lacunar spinel GaNb${}_4$Se${}_8$
• Resonating holes vs molecular spin-orbit coupled states in group-5 lacunar spinels
• Jahn-Teller driven quadrupolar ordering and spin-orbital dimer formation in GaNb${}_4$Se${}_8$