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Abstract

The spin-1/2 alternating Heisenberg chain system Na3Cu2SbO6 features two relevant exchange couplings: J1a within the structural Cu2O6 dimers and J1b between the dimers. Motivated by the controversially discussed nature of J1a, we perform extensive density-functional-theory (DFT) calculations, including DFT + U and hybrid functionals. Fits to the experimental magnetic susceptibility using high-temperature series expansions and quantum Monte Carlo simulations yield the optimal parameters J1a=−217 K and J1b=174 K with the alternation ratio α=J1a/J1b≃−1.25. For the closely related system Na2Cu2TeO6, DFT yields substantially enhanced J1b, but weaker J1a. The comparative analysis renders the buckling of the chains as the key parameter altering the magnetic coupling regime. Numerical simulation of the dispersion relations of the alternating-chain model clarify why both antiferromagnetic and ferromagnetic J1a can reproduce the experimental magnetic susceptibility data.