Electronic structure and magnetic properties of the linear chain cuprates Sr_2CuO_3 and Ca_2CuO_3

Abstract

Sr_2CuO_3 and Ca_2CuO_3 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 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 Ca_2CuO_3 are shifted downwards by 0.7 eV compared with Sr_2CuO_3, pointing to different Madelung fields and different on-site energies within the standard pd-model. Both compounds differ also significantly in the magnitude of the inter-chain dispersion along the crystallographical a-direction: \approx 100 meV and 250 meV, respectively. Using the band-structure and experimental data we parameterize a one-band extended Hubbard model for both materials which can be further mapped onto an anisotropic Heisenberg model. From the inter-chain dispersion we estimate a corresponding inter-chain exchange constant J_{\perp} \approx 0.8 and 3.6 meV for Sr_2CuO_3 and Ca_2CuO_3, 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 Néel temperature T_N and the magnetization m_0 at zero temperature. Our estimate of $J_{\perp}$ gives the right order of magnitude and the correct tendency going from Sr_2CuO_3 to Ca_2CuO_3. In a comparative study we also include CuGeO_3.