Ab initio determination of exchange integrals and Néel temperature in the chain cuprates

We report ab initio quantum chemical cluster calculations of the chain (Ja) and the largest interchain (Jb) Heisenberg exchange of the chain cuprates Ca2CuO3 and Sr2CuO3. We find that Ja is comparable to the in-plane J in layered cuprates and Ja/Jb≈250–400. Using recent theory we obtain close agreement with experiment for the staggered magnetic moments and critical temperatures. This implies that TN does not depend on the third parameter, Jc≪|Jb|, and cannot be calculated using spin-wave theory. We propose an explanation of this fact in terms of a 1D→2D cross-over.