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Spin-1/2 chains with alternating antiferromagnetic (AF) and ferromagnetic (FM) couplings exhibit quantum entanglement like the integer-spin Haldane chains and might be similarly utilized for quantum computations. Such alternating AF-FM chains have been proposed to be realized in the distorted honeycomb-lattice compound Na₂Cu₂TeO₆, but ambiguities remain as to the coupling strengths. Here we employ neutron scattering to study the spin dynamics in Na₂Cu₂TeO₆ and accurately determine the coupling strengths through the random phase approximation and density functional theory (DFT) approaches. We find the AF and FM intrachain couplings are the dominant terms in the spin Hamiltonian, while the interchain couplings are AF but perturbative, thus establishing Na₂Cu₂TeO₆ as a weakly-coupled alternating AF-FM chain compound. The hierarchy in the coupling strengths and the alternating signs of the intrachain couplings can be understood through their different exchange paths.

S. Gao et al. arXiv2010.06745 (2020)