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

TL;DR

This study uncovers a dynamical spin-orbital state in GaTa4Se8 with local distortions and phonon modes influencing molecular orbitals, leading to a spin-orbital valence bond order below 50 K.

Contribution

It provides evidence for a dynamical spin-orbital state and identifies a non-Jahn-Teller mechanism for the order-disorder transition in GaTa4Se8.

Findings

Local non-cubic distortions persist across all temperatures

Symmetry forbidden phonon modes modulate molecular orbitals

Spin-orbital order involves intercluster dimerization and symmetry breaking

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$^*$=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.