Dynamic spin fluctuations in the frustrated spin chain compound Li3Cu2SbO6

We report the signatures of dynamic spin fluctuations in the layered honeycomb ${\mathrm{Li}}_{3}{\mathrm{Cu}}_{2}\mathrm{Sb}{\mathrm{O}}_{6}$ compound, with a $3d S=1/2 {d}^{9}\phantom{\rule{4pt}{0ex}}{\mathrm{Cu}}^{2+}$ configuration, through muon spin rotation and relaxation ($\ensuremath{\mu}\mathrm{SR}$) and neutron scattering studies. Our zero-field (ZF) and longitudinal-field (LF) $\ensuremath{\mu}\mathrm{SR}$ results demonstrate the slowing down of the ${\mathrm{Cu}}^{2+}$ spin fluctuations below 4.0 K. The saturation of the ZF relaxation rate at low temperature, together with its weak dependence on the longitudinal field between 0 and 3.2 kG, indicates the presence of dynamic spin fluctuations persisting even at 80 mK without static order. Neutron scattering study reveals the gapped magnetic excitations with three modes at 7.7, 13.5, and 33 meV. Our density functional theory calculations reveal that the next-nearest-neighbor (NNN) antiferromagnetic (AFM) exchange (${J}_{\text{AFM}}=31$ meV) is stronger than the NN ferromagnetic (FM) exchange (${J}_{\text{FM}}=\ensuremath{-}21$ meV), indicating the importance of the orbital degrees of freedom. Our results suggest that the physics of ${\mathrm{Li}}_{3}{\mathrm{Cu}}_{2}\mathrm{Sb}{\mathrm{O}}_{6}$ can be explained by an alternating AFM chain rather than the honeycomb lattice.