We report muon-spin relaxation (${\mu }^{+}\mathrm{SR}$) measurements of copper acetate [$\mathrm{Cu}{\left({\mathrm{CH}}_3{\mathrm{CO}}_2\right)}_2·{\mathrm{H}}_2\mathrm{O}$], a model spin-1/2 Heisenberg antiferromagnetic dimer chain with an alternation parameter $\alpha$ = 0.001. Zero-field ${\mu }^{+}\mathrm{SR}$ data collected from 2 to 200 K revealed an oscillatory asymmetry that was analyzed using a model based on the muon-stopping site determined by density-functional theory $+\mu$ calculations. Below 50 K, the fitted parameters capture spin dynamics characteristic of the singlet ground state, while at higher temperatures, an additional relaxation was observed due to the thermally populated triplet state, affecting the local magnetic field around the muon-stopping site. The temperature dependence of the fitting parameters was found to exhibit characteristics similar to those of a bipartite entanglement measure, “distance between the states,” obtained from the magnetic susceptibility data. Longitudinal-field ${\mu }^{+}\mathrm{SR}$ measurements reveal field-dependent relaxation at field values much lower than the field values required to close the spin singlet-triplet gap, emphasizing the importance of quantum fluctuations in the spin dynamics of the dimerized copper acetate.

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• Molecular magnetism
• Quantum entanglement
• 1-dimensional spin chains
• Molecular magnets
• Density functional calculations
• Muon spin relaxation & rotation