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.

13 pages, 8 figures

References in corpus (20)

• Quantum ESPRESSO: a modular and open-source software project for quantum simulations of materials
• Advanced capabilities for materials modelling with Quantum ESPRESSO
• Mantid - Data Analysis and Visualization Package for Neutron Scattering and $\mu SR$ Experiments
• PDFgetX3: A rapid and highly automatable program for processing powder diffraction data into total scattering pair distribution functions
• Orbital effects in solids: basics, recent progress and opportunities
• Magnetically Hidden Order of Kramers Doublets in $d^1$ Systems: Sr${}_2$VO${}_4$
• Valence bond crystal in a pyrochlore antiferromagnet with orbital degeneracy
• Band Jahn-Teller Instability and Formation of Valence Bond Solid in a Mixed-Valent Spinel Oxide LiRh2O4
• The Jahn-Teller effect and spin--orbit coupling: friends or foes?
• First-Order Insulator-to-Metal Mott Transition in the Paramagnetic 3D System GaTa4Se8
• Classical dimers and dimerized superstructure in orbitally degenerate honeycomb antiferromagnet
• Direct experimental observation of the molecular Jeff=3/2 ground state in the lacunar spinel GaTa4Se8
• Cooperative cluster-Jahn-Teller effect as a possible route to antiferroelectricity
• Lattice dynamics and electronic excitations in a large family of lacunar spinels with a breathing pyrochlore lattice structure
• Two-orbital degeneracy lifted state as a local precursor to a metal-insulator transition
• Dual Orbital Degeneracy Lifting in a Strongly Correlated Electron System
• Magnetic phase transitions and spin density distribution in the molecular multiferroic GaV${}_4$S${}_8$ system
• Localised Wannier orbital basis for the Mott insulators GaV4S8 and GaTa4Se8
• Novel $J_{\mathrm{eff}}$=3/2 Metallic Phase and Unconventional Superconductivity in GaTa${}_4$Se${}_8$
• Spin dynamics in the skyrmion-host lacunar spinel GaV4S8

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${}_8$