Electronic structure and magnetic properties of the linear chain cuprates Sr2CuO3 and Ca2CuO3

Sr2CuO3 and Ca2CuO3 are considered to be model systems of strongly anisotropic, spin-1/2 Heisenberg antiferromagnets. We report, on the basis of a band-structure analysis within the local-density approximation (LDA) and on the basis of available experimental data, a careful analysis of model parameters for extended Hubbard and Heisenberg models. Both insulating compounds show half-filled nearly one-dimensional antibonding bands within the LDA. That indicates the importance of strong on-site correlation effects. The bonding bands of Ca2CuO3 are shifted downwards by 0.7 eV compared with Sr2CuO3, pointing to different Madelung fields and different on-site energies within the standard pd model. Both compounds also differ significantly in the magnitude of the interchain dispersion along the crystallographical a direction: approximate to 100 meV and 250 meV, respectively. Using the band-structure and experimental data we parametrize a one-band extended Hubbard model for both materials which can be further mapped onto an anisotropic Heisenberg model. From the interchain dispersion we estimate a corresponding interchain exchange constant J(perpendicular to), approximate to 0.8 and 3.6 meV for Sr2CuO3 and Ca2CuO3, respectively. Comparing several approaches to anisotropic Heisenberg problems, namely the random-phase spin-wave approximation and modern versions of coupled quantum spin chains approaches, we observe the advantage of the latter in the reproduction of reasonable values for the Neel temperature T-N and the magnetization m(0) at zero temperature. Our estimate of J(perpendicular to) gives the right order of magnitude and the correct tendency going from Sr2CuO3 to Ca2CuO3. In a comparative study we also include CuGeO3.