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

Abstract

We report ab initio quantum chemical cluster calculations of the chain ( J a ) and the largest interchain ( J b ) Heisenberg exchange of the chain cuprates Ca 2CuO 3 and Sr 2CuO 3. We find that J a is comparable to the in-plane J in layered cuprates and J a / J b ≈250–400. Using recent theory we obtain close agreement with experiment for the staggered magnetic moments and critical temperatures. This implies that T N does not depend on the third parameter, J c ≪| J b |, and cannot be calculated using spin-wave theory. We propose an explanation of this fact in terms of a 1D→2D cross-over.